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MG MGF Technical - Brakes
|I have read some of the comments on here regarding brakes. I too am looking at upgrading mine, reading this though it sounds a bit of a nightmare? Can the car easily be upgraded to discs all round or is this not really needed in a standard road car?|
(1.8 F 1999)
I was going to get the Front Turbo grooved Disc set from Brown and Gammon - think its about 100 quid and get some pads too? I assume that the calipers wouldn't need changing with these.
Or if anyone else can suggest an alternative bearing in mind that I don't have David Beckhams budget!!
|I thought we had discs all round already, the upgrades are usually to the front discs and include larger discs/calipers and different grades of pad material.|
|>> Can the car easily be upgraded to discs all round or is this not really needed in a standard road car? <<|
Probably a typo there Jamie, but as you know, the MGF uses discs all round already - 240mm front (vented) and rear (solid).
Actually, if you are on a budget, then the market is full of excellent options. EBC pads and discs are cheap and effective (use the green stuff pads). These are available from the likes of B&G and Mike Satur for very reasonable money (often cheaper than buying replacement standard parts!)
If you intend to take your car on track days, then green stuff pads ought to be avoided. Use Mintex pads instead - but do go for grooved discs (EBC or MG Sport and Racing).
If you are looking for larger front discs then Mike Satur and PAG do some very reasonably priced big disc conversions.
Then, if you want better calipers (and we start to talk about a good deal more money here), then big discs and 4-pot calipers are available from Mike Satur. Similarly, if you want the MG-branded AP racing front discs, then the MGF Centre do these for reasonable money (but you'll need new wheels)
It's only really after you go past this point that things get complicated, and you have to start thinking about bespoke parts. Been doing some routing around in this area recently - but we're talking well over a grand for this kind of conversion, and probably towards 2k... :o(
|A summary of options available here: http://www.mgf.ultimatemg.com/brakes/index.htm|
|thanks - yep, sorry meant to say bigger discs all round!!!! thanks for the info|
|It all depends on which wheels you have on. 15" wheels seriously limit the ceiling of what you can do bar fit the excellent looking 4pot Billet version from HiSpec. The standard brakes are quite pathetic on a track day, not so long ago I could only do 4 laps albeit on a very fast hard track before I was driving through fade city. |
Mind you an Alfa 75 3 litre with tarox brakes got so hot the wheel centres melted out!!! I had him drive around in circles in the car park without touching the brakes to help cool it down/ stop it catching alight!
Mike Satur and Hi Spec do some very interesting big disc opions using the standard ( but dodgy) calipers for about £200 which is a good option.
Any one know HiSpec,s web site ??
i have a VVC
|Maybe this is the 1:|
|I understand the the OE pistons are 54mm and Hi spec 4 pots are 1.5ins so effective area is close to OE so would not effect M/C. The increase in braking is from using the larger disc both heat and torque wise.|
The Wilwood callipers above 1.5 is 1.75ins which increases effective area by prob 40% so may require a change of M/C.
EBC discs appear to be subject to warping.
|the fact the hispec is a 4pot caliper alone is advantageous to braking performance|
|Agreed a 4 pot increases effective radius and improves pad stability and prob lighter(compared to OE)improving unsprung weight. Still limited by tyre grip so advantage of heat loss is in disc|
|:o) I agree Paul. Not sure about caliper/ disc weights, but hopefully will be finding out more from MG sport and racing tomorrow. :o)|
The AP system must increase front braking efficiency by 25% so interested on how rear is balanced.
Information on disc weights is very hard to find despite the fact they are rotational weights.
The same comments are made re Greenstuff and Mintex on midgets and other cars.
|Paul, the Trophy, I understand, uses a different specification bias valve in the circuit which, presumably, moves the bias towards the rear.|
There a good number of standard MGFs around now that have been retrofitted with these AP front brakes - but using the original bias valve. In the most part, this doesn't seem to have a too detrimental effect upon braking efficiency - and as it is the front brakes that'll lock up first, it is at least 'safe' (or at least safer than having the rears prematurely locking up). If you make the point that this is not the 'best' way to achieve optimal braking performance, then I am in complete agreement with you.
Point taken on the lack of transparency on disc and caliper masses. But I think I know someone who'd be able to help us out ;o)
Because the overall braking of the car is improved there is going to be more rapid weight transfer to the front. Thus, the rear is lighter. Thus, the rears are more likely to lock up. I think.
The balance should still presumably be moved more to the rear ot actually reduce the braking effect of the fronts. If it were moved forward then the rears would be be braking less, but there would be more 'dive' and so the rear would be even more likely to lock?
I'm not sure.
Having said that, I've only had ABS kick in prematurely once (at the Donnington hairpin!) which increased my braking distance. That may be more down to the 'grippy' nature of the 1177s I have on the rear.
I can only remember locking the brakes on the Trophy (no ABS)once, that was going into a bend on the 'ring' which appeared to be a 90 left as I came over a crest, only the fronts locked up, no instabilty or rearend wander as I recall and yes I did make the bend without taking any of the rather large kerb with me.
Regards Mike M2 VJN.
|Paul, got this pinched from the SELOC forum (credits to one Simon Scutham esq):|
If you change the brakes at one end of the car, then you have to consider the effect it has on balance.
A bias valve (or limiter valve) can only reduce the effect of the brakes, not improve them, and they st only ever be fitted to the rear brakes, NEVER the front.
Thus a bias valve can only shift the balance of the brakes forwards. (It is possible to fit a balance bar pedal box, this will work both ways, but it is not easy and very expensive).
If you go and fit huge great cailpers to the front of the car, you HAVE to do something about the rears, otherwise you overall braking eicency will be dramatically reduced as the fronts will reach locking point way before the rears are doing anything useful.
If you go the Lotus Motorsport route, they put a 4 pot up front and move the 2 pot fronts to the rear, this is OK, but not great as the rear uprights have to be changed and you also loose the handbrake, also both the AP4 pot and the std 2 pot are hardly lightwieght calipers.
changing the diamiter of the disk will change effective braking force (for the same pedal pressure) thus just sticking a bigger disk on the front also has a bad effect on balance.
Bottom line is that to get the maximum benefit from the front brakes, you have to balance up the brake force generated by the rear. Your 1177s will go some way towards this, but better still would be to fit HiSpec's 285mm discs to the rear as well, whilst retaining the standard caliper... that way, as with the original set up, you have the same leverage ratio on the rotor front and rear.
|Just been on the phone regarding the brakes as fitted to the TF at the Autosport show. Unfortunately, as predicted, this car was wearing standard TF160 brakes (304mm AP front, 240mm rear), albeit with X-Power green calipers and cross-drilling of the discs. Pitty, as I was hoping for more. :o( In fact, I was so disappointed, I forgot to ask about rotor and caliper weights!!! D'oh!|
On basis no MC change then piston diams would be useful re Lotus Motorsport route and TF160.
Can you fit 285mm at rear and retain same effective radius and why go to 285 if TF 160 is using 240mm.
A big disc but reduced piston diams should not effect balance but you have more cooling.
Into areas of black magic and balance - help
The simple bit is tyres stop the car - you will not stop quickly even with Huge brake upgrades with bald tyres or on ice - the only thing that stops the car is the tyre and road surface interaction generating a force.
If you can lock your the brakes then you are past maximum grip. However, a tyres grip increase with weight until they let go, so tyre grip, weight transfer,and balancing front are rear need to be considered.If the fronts obtain more grip the rears can unload due to weight transfer and lock, and if the fronts lock well before rears then the rear grip is not at maximum. You could also try lessening weight transfer.
The main reasons for upgrading brakes is heat and simplest answer may be brake cooling ducts. Another reason is pedal feel driver preference and SS hoses will help re compliance and pedal feel.
The problem with heat is the pads coefficient of friction reduces with excess heat or being too cold, so apart from a pad change if its too hot then larger vented discs are needed but too large and it could be too cold for normal road use!
Would be interested in any data on stopping distances with the various upgrades including multiple stops.
Lastly Greenstuff have a high coefficient of friction of .45!
|From Brembo FAQ"s|
Where can I find test data on stopping distances?
At the speeds that stopping distance is generally measured from (60 to 70mph), the test is primarily testing the tire's grip on the pavement. As delivered from the manufacturer, nearly all vehicles are able to engage the ABS or lock the wheels at these speeds. Therefore, an increase in braking power will do nothing to stop the vehicle in a shorter distance. For this reason, we do not record stopping distances at this time. The Brembo systems will show their greatest advantages when braking from higher speeds, or when tasked with repeated heavy braking. The increased braking torque provides for maximum deceleration at speed, and the ability to absorb and quickly dissipate the intense heat generated during repeated braking insures that the braking system will perform at the same high level each time.
|Utterley hammered the LabRat this morning exploring rev limiter to an extent that I wouldnt dare on the F. Ok we had the usual scrabbling for grip and weird handling sensations...but blow me Brake Fade again!|
and when I got out the usual brake pad smells!
Is it MGR policy to underbrake all their cars ?
Brakes are probably the second biggest subject for F ownership after HGF's ....really
|Paul, you are raising some very valid points there - and I definitely appreciate that.|
To clarify, car currently uses standard sized 240mm diameter rotors: they are grooved MG Sport and Racing discs, vented at the front, solid at the rear. Pads used are Mintex 1177. Wheels are 16" 'Cup' wheels shod with Goodyear F1 GS-D3 215/40 ZR16 tyres. Hoses are of the braided type. The master cylinder has been braced, although there is still some residual movement (thankfully, David Mottram has given me an idea as to how to address the final degrees of movement). Essentially, I think that I have more-or-less optimised braking performance for the system within the limitation of having to use MSA group 1A tyres.
I am reasonably happy with the performance of this set up, which is entirely adequate for road and sprint/hill climb use. But for track days (particularly tracks like Donington), heat becomes a serious issue, and pad fade starts to become a problem.
Clearly there is a question of cooling to be addressed. Moreover, the discs most severely affected are the solid rears, and indeed the rear pads suffered from severe taper wear which might well have been a consequence of gasing.
So, no, I am not entirely happy with 240mm rear brakes.
Most brake upgrades are for the fronts - particularly with reference to MG's own 304mm AP racing set up. For me, that clearly doesn't address the problems being experienced with the rear brakes. Moreover, installing larger front rotors is going to introduce an imbalance to the braking system, as the improved leaverage of the 4-pot caliper on a 304mm disc is not matched by that on the rear. In fact, the front brakes would need to be limited to prevent premature front lock up (a well known problem with certain large front disc conversions on the F). In doing this, one negates any braking advantage the AP set up potentially offers.
In reference to multipot calipers - there may be additional advantages of using a 4-pot caliper over a single pot sliding caliper - not least better pad location and stability, but also much greater caliper rigidity that will have benefits to brake performance and 'feel'. So I can see this as an attractive option.
So what this boils down to is "great, I'd like to think about using larger discs for their superior cooling properties, for their stronger initial 'bite' and for their superior feel, but I'm not going to be able to make full advantage of their performance until there is a reasonable balance in performance from front to rear"
Of potential interest is the observation that other mid-engine roadsters, such as the Lotus Elise and the Porsche Boxster have very similar front and rear disc diameters with calipers that externally at least, appear of broadly similar size. Clearly, a mid-engine car with up to 60% of it's mass over the rear wheels puts more load on the rear brakes than does a more conventional front-engined car...
|>> However, a tyres grip increase with weight until they let go, so tyre grip, weight transfer,and balancing front are rear need to be considered. <<|
BTW Paul, this statement is a little misleading. To the accumilative groan of everyone in the BBS as Rob yet again mentions the phrase "Traction Circle", the maximum rate of retardation (deceleration) is not dependent upon the vehicle's weight, but rather just the coefficient of friction...
There's plenty more on the net regarding this fascinating subject of tyre grip - which will either have you glued to your computer monitor for the rest of the week or soundly asleep within 5 minutes... LOL
Having said that, second-order effects upon tyre loading also apply, so factors such as weight transfer will have an impact upon grip and therefore maximum retardation... ;o)
|Is it possible to fit a large 280mm, drilled and ventilated disk to the rear? Say similar to the kit MS produces for the front?|
In order to fit a larger diameter disk it would need either a spacer like the front or a replacement capiler callier.
However the thickness would be an issue: The Front disk are 22mm whereas the rears are 10mm. I don'd know much additinal space is available. But I suspect that 10mm would be pushing it!
|That should read "capiler carrier".|
Hispec do 280mm dia front ar rear.
|I had a chat with the guys at HiSpec and they went stright into the improvement of the rear brakes. They know their stuff and it a dollar to a doughnut that fitting bigger discs at the back is likely to make a huge improvement. Weight distribution and all that.|
The wikid looking 4pot system for the fronts is £400 just going for big discs is £200 keeping the wonky std calipers.
Fitting bigger discs at the back using the original calipers is £200 so it might be a good idea to just keep whatever wheels you have got and go for a £400 set of huge discs all round!
|The Hispec one is not badly priced at £183.75+VAT = £215.90 a pair. http://www.hispecmotorsport.co.uk/mgf.htm|
1) They are drilled and not grooved like the Rover sport ones.
2) They are 12mm whereas the origional are 10mm so I suspect that they are solid not ventilated
Anybody using them?
|Steve, there's no way you could fit a vented rear disc to the rear standard calipers. It is simply too thick a rotor. Of course, you could fit larger rear calipers to compensate (from say, an MG ZT) - but whether this is necessary? A larger - albeit solid - rear disc would, in any case, increase cooling. :o)|
The 285mm HiSpec front and rear disc conversion is probably one of the most cost effective on the market - no question. Plenty of good reports that a quick search of the internet revealed last night.
But I also heard some slightly negative reports too - not least the experience of our very own Mr Speed-Meister Nothard who's front pads suffered marked taper wear following a number of track days last year. I'm sure that Paul can fill in the details, as despite this, he seems very happy with them.
But Paul only has the 285mm discs on the front, not the rear.
|£430 all in and a jolly weekend's tinkering|
sounds like a very cost effective way of getting the brakes right
I just hope that the braided hoses still reach the calipers!
|Mike, that was Adenauer-Forst :-)|
OK weekend reading looks interesting
braking gives the front tire more stiction and the rear tires less.
On basis that tyres etc are same for F as a FWD car which stops quicker?
Another thought road cars brake from say 70-0 and track from say 140-70 but rarely to 0. The amount of heat involved from the diff stopping requirement is immense.
Detachable Brake Ducting may be useful on change between road and track.
There pain in changing the rear is the handbrake, but uprating rear even if by too much can be sorted with proportional valve.
Any piston diams?
|Any comments on bias valves re ABS or EBD|
How the front rear bias is adjusted for reduced fuel load or wet conditions.
What do grooves actually do and how many? Interesting article in retro car a few months back.
What is X drilling all about?
You're right about the interesting reading! :o) It's been a while since I last read it - so I might have to do some revision! LOL
>> On basis that tyres etc are same for F as a FWD car which stops quicker? <<
They'll stop in the same distance (remember maximum deceleration is independent of centre of gravity or weight over the wheels). That is assuming that the brakes are capable of capable of generating retardation that is equal to that of the tyre's grip.
>> Detachable Brake Ducting may be useful on change between road and track. <<
I guess that this is an option - might be possible to get Mr Heath Robinson to lend a hand in cobbling something together - but getting cool air to the rear brakes isn't quite as easy as it is for the fronts... we'll see!
>> There pain in changing the rear is the handbrake, but uprating rear even if by too much can be sorted with proportional valve. <<
Yeah, the handbrake is a real issue. AP's suggestion to me was to use an Escort Cosworth rear caliper in conjunction with their disc upgrade... Well, that's one option I guess!
>> Any piston diams?<<
For the rear? No - I'd have to go out and measure 'em (yuk! and it's raining! LOL)
>> Any comments on bias valves re ABS or EBD <<
In what regard? In essence, you want a well balanced braking system to start with - or you risk premature operation of both ABS or EBD - which is likely to be detrimental to braking distances...
>> How the front rear bias is adjusted for reduced fuel load or wet conditions. <<
No need remember: maximum retardation is independent of weight over the wheels. If consistently wet, then the coefficient of friction is going to be equally lowered on all four wheels. Problems only start when you have different coefficients of friction between wheels - exactly the situation that ABS was designed for.
>> What do grooves actually do and how many? <<
Grooves are present to clear pad dust and gas from the friction surfaces - a big advantage especially on a track. But they don't improve brake performance per se (there must be a slight degradation because, after all, your reducing the surface contact area) - but they do help reduce performace deterioration. As to how many are required... erm, no idea!!!
>> What is X drilling all about? <<
Supposedly for additional cooling - but I suspect more for cosmetics ;o) There are reports of discs tearing along the 'dots' like a perforated edge of a stamp - so this is one disc mod I'd personally avoid ;o)
|Don't they use a fluid lock on the cup cars to hold them in place in leu of a handbrake, put foot on brake, engage fluid lock, brake fluid is trapped with brakes on car goes nowhere?|
BTW techspeed have concluded that with the Hi-spec setup on my fronts my 1166 and roversport grooved on the rear will be fine
|Fluid locks are not legal for the purposes of a handbrake on a road car: the mode of operation MUST be different from that of the main footbrake.|
The option, in this case, is to fit an auxillary handbrake caliper that is cable operated.
>> BTW techspeed have concluded that with the Hi-spec setup on my fronts my 1166 and roversport grooved on the rear will be fine <<
This is the same set up that Paul uses, and yes it is fine.
But the disc size mismatch still annoys me... ;o)
Wilwood do sell seperate handrake calipers, never checked if road legal but Citrons operate this system in past? but would be OE.
When wet the coefficient of friction is going to be equally lowered on all four wheels, but will decelleration be the same and weight transfer? or what happens when you add downforce?or very sticky tyres at both ends?
Fuel reduction will reduce static weight at one end of car opposite to loading a car or van in this case bias valves can be load sensing- as ride height decreases with loading rear the pressure to rear brakes is increased.
Re Grooves I think AP recommend 4, agree with your comments - I find some designer discs may be a little excessive a bit like heavy alloys.
X drilling on bikes ss discs helps clear water, so may be very useful in those circumstances, but generally act likes grooves(removes some unsprung weight)etc - interesting that EBC use dimples perhaps avoiding your stamp example, may be useful if disc is very oversized.
Also minteresting is thae pad choices as perhaps another type of pad may reduce wedging.
Diff pads at each end is interesting as with heat the performance may be totally diff, should be OK if rear is less grippy but in wet cold conditions this may be reversed with a rear lock.
Had a look at web site and found these in Part 2
So, to keep your tires stuck to the ground, be aware that accelerating gives the front tires less stiction and the rear tires more, that braking gives the front tire more stiction and the rear tires less, and that cornering gives the inside tires less stiction and the outside tires more.
Magic Circle - Geeee this was confusing.
The basis for this was same type of tyre slips at say 1g full stop this is independant of weight so same tyre slips at 1g for Caterham as Volvo.
Newton has a formluae Force = mass x acceleration (or deceleration)
Deceleration = Force/ mass
With Caterham 1g = 500 (force)/500 (light car)
Volvo 1g = 2000 (force)/ 2000 (Heavy Car)
So statement works (prob very ball park for tyre specialist)
If we consider front and rear tyres seperately.
Lets say in dry
Front 1g = 1500(F)/1500(M)
Rear 1g = 500(F)/500(M)
In wet due to reduction in tyre coefficient g is now only 0.8g so less weight transfer from rear to front
Front 0.8g = 800 (F)/1000 (M)
Rear 0.8g = 800 (F)/1000(M)
If the brakes are set up to apply a tyre force of 1500 on front and 500 on rear for perfect stopping in dry (just before wheel lock), in the wet rears can only apply sufficient force to apply 500 pounds of force.
So in reality (but maybe not real world as lots of other factors come into play)
Front and rear combined (800+500)(F)/ 2000(M) = 0.65g
The above was in a bit of a hurry, perhaps worth mentioning that in Wet the Fronts will also lock early.
The F and FWD position (assuming brakes set up correctly) perhaps bean counters and whats in parts bin lead to not the best system for the car!
The F Fronts say 1g = 1000/1000
Rears 1g = 1000/1000
FWD Fronts 1g = 1500/1500
Rears 1g = 500/500
So should stop in same time.
There is a footnote in that the relationship between load, force and the dreaded slip angles are not linear, you do not get the same increase in force for a given increase in load or slip angle so its a lot easier to start with a balanced car.
I have not worked out how large brake improvements effect ABS and EBD (intelligent bias valve) apart from disabling them which does not sound to clever if a road car.
Apologies for batch posting and its time now for the really odd bit.
I recollect that you are not happy with just uprating the fronts but wish to uprate rears as well.
Taking a step at a time a really nice big front brake kit is added, this adds bite and the brakes come on immediately so stoping the car more quickly!
Lets now say with weight transfer the front to rear weight balance is F1200 R800
The factory set the car up with 60% front brakes 40% rear.
The front big brake kit now splits braking 70% Front 30% rear but the tyres do the stopping and they were already at limit at front on the point of locking before the kit, so adding the kit reduces the braking effect at the rear and your stopping distance has increased.
Can it get odder,
Lets lower car and therefore CG weight transfer is less so Front Rear Split is now F1000 R1000.
Without piston diams and effective radius assuming you do not change M/C, you cannot tell what changes are being made, but to me it looks like you could well be right, until you add sticky tyres because the weight transfer increases again (black magic and balance).
|It's enough to give one a headache isn't it? How much lower would the car need to be to significantly alter the brake distribution I wonder??? |
I've dropped my car's ride height from 368mm to approx 330mm: I wonder if this has had a significant impact upon the braking distribution? I suspect probably not *that* much.
In my simplistic way of thinking, if one can maintain similar proportion of leverage front and rear, then one shouldn't need to alter the standard bias valve. Is this fundamentally incorrect?
I thought traction circle was your favourite subject!
The early articles deals with weight transfer, so should be able to plug in a number.
Another variable is that you use wider tyres on front than OE, which may add some front end grip.
The logic would be that the rear is underbraked in order to avoid rear wheels locking first, this may not be true with a EBD system unless as a safety measure if the system fails.
So the starting point should be the rears are underbraked in first place and for a road car you must maintain that situation.
The enemy is heat so adding wider vented discs would not change the position. Larger discs can be used by reducing piston diams which again can be made not to increase leverage or uprate both ends by same amount. The pad idea could work but you would prob need a large diff in the coeffiction and you are adding grip at rear so temp changes could give probs.
A bias valve or brake balance can dial out front rear mismatches.
|But remember that the standard OE discs have the same diameter front and rear (240mm) - so why not fit two new rotors (say 285mm) front and rear? This would effectively give you the same proportional braking effort if all other factors are the same.|
Hmm - interesting point made regarding piston size and thus braking effort - something to bare in mind if considering a change to larger rear calipers... :o/
|Hmm. Just had a chance to look through those pages and digest what it says a little more.|
In essence we're both right - namely that the maximum retardation that the tyres can generate is independent of weight and weight transfer.
BUT the force that the brakes need to apply to achieve that maximum retardation IS critically dependent upon both factors - a bit of a no-brainer really. But this has some interesting implications:
The greater the mass of the vehicle, the greater the overall decelerative force that needs to be applied.
The more weight transfer, the greater the braking force at the front to reach maximum retardation, whilst the force required at the rear is commiserately less to reach that same level of deceleration. How this varies depends proportionally to the vertical height of the centre of gravity of the vehicle.
And the more force that the front brakes decelerate the car, the more weight is transfered, and the less effort the rears need to undertake... the dynamic situation is interesting to try and model!
Hmm. So I thought I do a couple of quick calculations to work out how the weight distribution changed under maximum retardation.
Let's make a couple of assumptions first:
1. The car's weight is 1100kg
2. max retardation is 0.9g on road tyres. F=ma, hence this approximates to a braking force of 1100*0.9*0.981 = 971.19Nm
3. centre of gravity is approximately the third of the vehicles height (ie, about 0.4 meters above the ground)
4. Weight distribution is 0.45 front, 0.55 rear
5. Wheel base is 2.375m
Using the equations quoted, you get a maximum front braking force of 658.6kg and a maximum rear force of 441.4kg - changing the basic weight distribution from 45:55 to something much closer to 60:40 at maximum retardation.
That figure looks very close to that factory figure on brake bias you quoted Paul! ;o) A fluke I suspect; this probably means that my "guestimate" for the height of centre of gravity is on the high side! LOL
[It is also interest to read your comments regarding wet versus dry - and the way in which this influences weight transfer... I must look up the way in which the bias valve works: is it a proportioner of hydraulic fluid, or does it simply limit the maximum force the rear calipers can apply?]
From http://www.wrc-cosworth.org/tech/brakes/brakecalculation.html there is an equation that calcuates the force at the brake caliper from the force at the wheel. Assuming that the effective radius of a 304mm AP disc rotor is 133.76mm (88% of actual disc radius) - and the similar measure for the standard 240mm rotor is 105.6mm. From this, it is easy to calculate that the 304mm AP brakes need only generate 79% of the force for a given retardation than the standard rotor and caliper. Thus, if you apply maximum retardation, the brake "bias" is now 64:36 F:R... so I guess that explains the increased propensity for front locking?
So I am assuming that if the rear brakes were similarly upgraded, (ie they too were 21% more 'powerful') the original proportionality F:R would be restored???
The above assumes similar hydraulic piston volumes... HiSpec calipers with 4 1.5" pistons are going to have double the area of a single pot 54mm diameter piston as fitted as standard in the F... Is this going to half the rate of force application? Nuts. Time to go away and do some more reading!
I will need time to take that in, just check that once at point of lock up on front no more rear pressure can be applied.
I just guessed the numbers to represent an F, and effective radius 1inch from disc edge.
The Hi Spec pistons should be appox (I did check this)the same as OE F, so its just increase in effective radius.
In essence the MC end stays the same unless you want the expense of a change, so if you keep piston areas (piston diams used to calculate this) same are increase pro rata, more fluid longer pedal and MC may only take in general say 20% per Wilwood, and increase both ends effective radius and a bias valve, you should get a well balanced system which can cope with a lot of heat, but rear upgrades are expensive and you have your wheel unspung weight issue.
|Paul, I suspect that we're talking to ourselves here! ROFL! ;o)|
Effective radius is 1" less than disc radius? I guestimated it differently, so the differences may be underestimated - but not by too much.
Regarding HiSpec caliper piston areas, I calculated the standard 54mm piston to be pi*27^2 or pi*729mm2, whereas the HiSpec calipers with four 1.5" (where 1" ~ 25mm, so 1.5" = 37.5mm) pistons, where each piston has an area of pi*18.75^2 or pi*352mm2 - so the area of four would be pi*1408mm2.
So effectively, the HiSpec calipers have double the standard caliper piston area. So either the HiSpec calipers actually use smaller pistons, or HiSpec are wrong to assume that their caliper has a similar piston area to the standard caliper...?
I think you may be right!
The Sliding caliper arrangement should be treated as 2 pistons for the calcs.
Under hard driving with pad fade the OE system performance will drop drastically, the increased front disc works in these situations, but maybe not for normal road use- I've not tried to calculate potential increase in stopping distance for road use.
The cynical view may be that they have discs and calipers to fit not that are matched to braking system of car, or the potential increase is not worth worrying about.
|>> The Sliding caliper arrangement should be treated as 2 pistons for the calcs. <<|
Ah - I did wonder about that: I presume this is because the sliding caliper has to move twice the distance to apply pressure to the disc than does each of the four pistons in a 4-pot caliper? Thus the hydraulic displacement for the sliding caliper and HiSpec's caliper are going to be pretty much the same... makes sense.
>> The cynical view may be that they have discs and calipers to fit not that are matched to braking system of car, or the potential increase is not worth worrying about. <<
Sorry, probably me being dense - but do you mean why did MG specify such small brakes in the first place? I think that's easy to answer: they had the parts on the shelf already, and although not optimal, the performance of the 240mm discs was 'adequate' for the road and the performance of the cars...
... bigger discs are probably only of any significant advantage on the track...
I was thinking of aftermarket suppliers rather than just parts bin syndrome.
Very interested in finding out about spec of AP.
Also for Track could use Slicks, but its that rare emergency stop on road which is the possible concern, which is not theory.
|I'll see what I can find out - but I suspect that the displacement volume must be similar to standard otherwise more people would complain of premature lock up?|
BTW, the use of slicks on a stock MGF is a sure way to destroy the rear wheel bearings within 50 miles... :o(
Its surprising what slicks can do!
The B is about same weight as F and the front discs (solid) are 270mm even with V8 they only increased width the OE car was approx 150bhp.
The roadster (not V8 until RV8)also had smaller pistons on rear drums than GT.
With upgraded brakes you can get a on/off situation which some don't like, however the reaction time of applying pressure may save say a tenth of sec which could be say 5ft reduction.
The F is designed as a road car - a road/track car would of course cost a lot more to start with.
It was TG the other night had the BMW with tyres you could only use in dry!
|>> It was TG the other night had the BMW with tyres you could only use in dry! <<|
Yup, I saw that - not particularly practical eh? ;o)
Interesting to hear your experiences with your B. BTW, are you going to North Weald on the 30th with MGoT?
My daughter is moving into new flat that weekend - I think I have a few jobs to do.
Just trying to follow piston area logic
If they stay same then pressure is same as normal front and rear, but you have introduced a larger disc so more effective stopping power up front and therefore if not at limit more weight transfer and so unloading rear. There is now a need for bias valve to keep weight transfer in balance as less grip at rear.
If increase front piston area but same size discs the fronts have more stopping power but there is less pressure to rears so rears are not unloaded as less effective stopping power balances the system.
Both methods reduce grip at rear increasing braking distance at the limit unless the rears produced too much grip in first place.
|The first method may be more promising if the OE brakes are not at maximum balance the OE equipment has acted like a bias valve although perhaps no margin of error. Tyre grip is at maximum at say slip of 10% in dry 5% in wet?|
Also the brakes will be sharper reducing response time and in high speed or multiple stop situations the large disc will lose heat quicker preventing fade at front.
The race pad at rear in cold conditions prob has less COF and again acts like a bias valve.
In heat situations the COF returns to normal and now same as front so both front and rear have better heat capabilities.
|This is the most interesting which has been running since I have departed the BBS for a new 'proper' job. It pretty much echoes what I have been thinking for a couple of months. As Rob knows I had a set of AP brakes fitted to my car in June (and very nice they are too), However, I do feel that the braking bias it a bit out. Most noticably this is at lower speeds, those usually encounters when heading for an impending grounding on a speed bump, or approaching a roundabout, junction ...... As Paul says with a larger front brake kit the weight transferance is greater/quicker than with the OE small items. As such I have a bit less confidence around town now than with the original calipers, though this is different on track.|
I was considering having a bias valve fitted to remedy this, however from the piece pinched from the SELOC site this would appear to have little effect. So, the options appear to me to be the Hispec rear brake kit for £200 which essentially is just a set of discs (the caliper set I suspect is considerably more, and they still look like sliding calipers). Or, what about creating the bias through a choice of discs and pads, would this be an option.....?
Can you repost SELOC info?
Do you have AP spec re piston sizes?
I'm not a great fan of pad option as to me you want the pads being consistent front and rear but it may be cheapest solution.
The alternative appear to be larger rear disc with the handbrake hassle, Balance bar M/C or the bias valve (see Robs link to cosworth above) or sticky tyres perhaps Yoko's or Avon's.
The puzzle is these AP brakes are on TF so should be matched?
|I am 3 inches when flacid, and 6 inches erect.|
|Horace J. Bender|
I have talked myself into bias valve from Cosworth site and the pads.
The Fronts with large discs may apply more effective braking but needs to be held at the slip level for maximim grip - the rears cannot achieve the maximum level and provide less effective braking so a bias valve would only reduce the effectiveness of rears more.
A race pad in rear may provide less braking on road but more effective under heat conditions.
It is opposite you need to increase rear line pressure
to maximise braking at both ends of car.
Not very tall then, perhaps a post on Midget board
|>> If they stay same then pressure is same as normal front and rear, but you have introduced a larger disc so more effective stopping power up front and therefore if not at limit more weight transfer and so unloading rear. There is now a need for bias valve to keep weight transfer in balance as less grip at rear. <<|
Paul, the rate of weight transfer is going to be greater, but the maximum weight transfer is independent of the size of discs / pads / pistons etc - this is purely dependent upon maximum retardation (and therefore tyres - approx 1.0G) and the height of the centre of gravity. Therefore increasing the power of the front brakes would mean you could increase the amount of braking force to the rear brakes to try and balance the mismatch in brake torque front to rear. However, if the rear brakes are too small, then the retardive torque that they'll deliver will fail to match those of the fronts, and therefore there will always be a tendency to lock the front wheels - simply because there is too much force being applied through the front wheels than the tyres can cope with...
... in fact, this appears to be the case even, in my experience,with the TF with these brakes fitted with standard - although they are almost certainly better than cars, like Tim's where the APs have been fitted with a OE bias valve.
Tim, you need to optimise your rear brake performance. That probably means installing a TF160 bias valve. Or fitting larger rear discs. ;o)
|"Tim, you need to optimise your rear brake performance. That probably means installing a TF160 bias valve. Or fitting larger rear discs. ;o) "|
So in the light of the SELOC comment will the bias valve actually be able to balance load FROM the front TO the rear. Otherwise it seems like the bigger disks.....
|Not exactly Tim - as Simon Scutham says, the bias valve has no impact upon the front brake's operation, but rather limits the hydraulic line pressure to the rear caliper. A bias valve can only ever reduce rear braking force. You can remove the bias valve entirely, BUT if the force generated by the front brakes proportionately exceeds that generated by the rear brakes, then frankly there isn't much you can do. Except increase the diameter of the rear rotor to balance out the braking torques of the front and rear calipers.|
Balance is all about proportionality - which I think Paul has already mentioned is 60% front and 40% rear. Given that the standard set up was designed to maintain this between two discs of the same diameter, and assuming that the piston areas are unaltered, then this standard balance is best maintained by increasing the rear disc size to match those of your new fronts (that's assuming that you can obtain 304mm rear rotors)
|As I thought then...... Bigger discs to be ordered in due course, after the imminent expenditure.|
The weight transfer is the product of front and rear, therefore if rears less effective you have less weight transfer!
At low speeds with more effective front you could unload rears!
If you use smaller pistons up front the rear pistons become more effective, the bias valve could be balancing this?
|>> The weight transfer is the product of front and rear, therefore if rears less effective you have less weight transfer! <<|
Yes, I agree. So the force that the front brakes would need to generate to get maximum deceleration -a(max) - would also be reduced due to the reduced mass over the front wheels. So if the front were over-braked, would lead to a situation where the fronts lock even earlier. To get the most out of an uprated front brake, you need to throw as much weight over the front wheels as possible during braking... and having effective rear brakes are as much a part of that equation as the front.
>> At low speeds with more effective front you could unload rears! <<
I'm not sure what you are getting at here Paul?
|I haven't had a chance to read it yet, but it looks as though this article covers what we are all talking about best: http://www.turnfast.com/tech_brakes/brakes_balancing.lasso|
|This would certainly explain the phenomenon which I have been experiencing. Brakes are fine when decceration from higher speed. ie more weight transfer, but lock as slower speeds, less weight transfer.... Interesting.|
Nicely put Rob & Paul.
Your foot controls the front brake keeping at maximum grip for loading- thus with less weight transfer your foot obeys the laws of physics - these feet are clever!
At high speed you have a lot of rotating wheel.
At low you do not and feet by now are twitchy!and the wheels do not have much energy left although the tyres and pads could cook bacon!
Thoughts about Progressive Braking.
Initial phase: Apply the brakes gently, causing the transfer of forces onto the fronts to be gentle and not break the grip of the tyre. As you begin to slow down, it is then possible to increase braking forces without fear of causing a skid.
Main phase: You can now brake harder until nearly at a complete stop. The forces stopping the wheels from turning are now not strong enough to cause a skid because the forces pushing down on the tyres have increased.
Final phase: As the car comes to a halt, start to reduce braking forces again. The wheels will be rotating quite slowly now, so the force you are applying to the brakes may now be strong enough to break the grip of the tyres on the road an induce a skid. When you brake hard as the car comes to a halt, the rear wheel (which still has lots of braking force being applied) but little downward force locks up right at the end.
The meagre OE equipment should cause less problem with this, but fit some really large discs both ends and there is a lot of power in the brakes so the final phase is at a lot higher speed - F1 cars (not very large discs (the carbon is less likely to explode as they have to use small discs) will prob lock a wheel at
apprx 50mph. (its about 4g now but with those old sticky tyres about 6g!)
Also having on/off brakes may cause problems with feeling all this (poor old feet).
Racing 'F driver tip given to me years back:
At speeds over 50mph you can stamp on the brakes like
there's no tomorrow as there is so much momentum that there is no way you'll lock the brakes.
Also interesting to think that with stiffened suspension - a little less weight transfer - there may be worse braking?
What would the effect on braking be if you had some funky linked suspension front to back? Oh, like the hydrogas 'F !! Hmmmm....?
The objective is to be smooth
I would disagree re springs - lowering car and CG reduces weight transfer. Sticky tyres increases.
Springs only resist the body of car moving the transfer of weight is passed on to wheels and contact patch. Usually by lowering car you increase spring stiffness so it may seam that because you have added increased spring the weight transfer is now less but its the CG.
you really should get out more.
|>> Also interesting to think that with stiffened suspension - a little less weight transfer - there may be worse braking?|
What would the effect on braking be if you had some funky linked suspension front to back? Oh, like the hydrogas 'F !! Hmmmm....? <<
Paul, I think that you are referring to suspension pitch under braking? So long as there is adequate suspension movement and the wheels stay planted to the ground, the effect upon weight transfer should be minimal (unless, of course, the pitch is so great that the height of the centre of gravity increases!)
>> Your foot controls the front brake keeping at maximum grip for loading- thus with less weight transfer your foot obeys the laws of physics - these feet are clever!
At high speed you have a lot of rotating wheel.
At low you do not and feet by now are twitchy!and the wheels do not have much energy left although the tyres and pads could cook bacon! <<
Paul (other Paul! LOL), if you brake from high speed, you need to apply more brake force than you would if applying brakes from a crawl. If traveling at 20 mph, to decelerate to 10 mph will require less force than decelerating from 80 mph to 40 mph in the same time frame (good old Newton's second law again). Using the same amount of pedal effort to achieve both scenarios will surely lead to the front wheels locking at low speed for a whole host of reasons - including weight transfer (which effects the amount of work the front brakes need to perform) - but primarily because you'd be asking the front tyres to decelerate beyond that theoretical 1.0G maximum.
Does this answer reflect the your thoughts? I remain unsure as to the point you're making??? :confused: Help?
For maximum braking the wheels should be kept rotating at around 5-15% slip. If you press the brake pedal even harder will take you beyond the 15% range, so you will need to cadence brake. These wheels slip so in what is say the distance of 10 wheels circumferce the wheel rotates 9.5 revolutions at lock up it is 0. I'll see if I can find a graph of an accelerating wheel.
If you brake hard you will get nosedive, and you will prob lock the brakes. Initial phase then build up pressure to build up load gradually, the difference between this and stamping on brakes is prob a fraction of a sec.
The third is to release the brakes smoothly. This means that the nose of the car comes back up to ride height smoothly, rather than abruptly in whic case it could perhaps even go past normal ride height as a result of the compressed suspension springs being suddenly released.
This is a graph of reverse braking, understanding black magic often appears double dutch. IMO the braking force applied to wheel has nearly the same effect at any speed re tyre grip or slip (tyre temps pressures etc) the dynamic bit is the loading.
The other black magic is the more you load a tyre the more grip it has but its not a linear relationship, eventually the tyre runs out of grip despite increasing the loading.
|This whole thread (which BTW is also running on other subject areas, also started by Paul), reads like a gigantic wind-up. Insofar as each time somebody comes up with a reasoned and scientific approach to a particular issue, Paul changes the point of emphasis. Now, what exactly was the question?|
Hvaing read all of this and noe having a somewhat sore head I thought I'd try to throw something else in.
What effect do the tyre sizes have when comparing the
braking efficiencies . . .
The trophy does have large front ap disc and calipers but
it also has narrower fron tyres. Would this have some
baring on the front rear bias that is being found.
What size tyres are being used with retro fit Trophy brakes 195 fronts or 215 front.
Or is this just a red herring
|>> For maximum braking the wheels should be kept rotating at around 5-15% slip. If you press the brake pedal even harder will take you beyond the 15% range, so you will need to cadence brake. <<|
Ah, Okay, I see where you're coming from now. Talking about tyre slip angle is the same as talking about the maximum rate of deceleration or tyre grip: they're all the same thing essentially - albeit different ways of expressing the same concept :o)
I need to read those links about braking technique! This is an area that I need to work on...
>> What effect do the tyre sizes have when comparing the
braking efficiencies . . .
The trophy does have large front ap disc and calipers but
it also has narrower fron tyres. Would this have some
baring on the front rear bias that is being found.
What size tyres are being used with retro fit Trophy brakes 195 fronts or 215 front. <<
Wider tyres DO have an impact - although not on the coefficient of friction, but rather on the development of slip angles, and hence grip generation... If I get a chance, I'll look this up on the 'net and see if I can get an eloquent explanation for you :o)
Wide tyres are another interesting subject. IMO the contact patch stays the same if using same tyre pressure, if you know car weight and psi you can calculate contact patch which should be same-never figured out theory behind larger diameter tyres.
With wide tyre the contact patch is wider but not as long, so cornering wise this is much better and they can lower slip angles.
The tyre graphs that I've seen for cornering force to load also indicate more grip.
What that means braking wise is more a mystery especially when you consider traction circle!
At the moment IMO they would have same braking grip and a compound change is needed say to a soft Yoko which are not hard wearing.
Back to the other subject the weight transfer follows braking, so at start the fronts are still lightly loaded although process is rapid.
I cannot see any diff if you slam brakes on at 50mph or 100mph still depends on tyre grip and weight transfer. Where there is a diff if at 100mph (travelk+ling at faster speed and longer distance) down your brakes start to fade then more pressure will have to be applied but then a viscious circle.
|I don't pretend to understand all this, but you may also find these articles interesting: http://autopedia.com/stuttgart-west/StuttPhysics.html (I hope the link works). My O-level physics gets stuck, particularly as I learned it in IS units and formulae, whereas this American guy talks in old money (and blames the Brits!). For eg he says that "slugs" is a British unit of mass.....!|
I will have a look but I'm only A level physics!
My view, which may be wrong is that at high speed the braking force is not sufficient to overcome the rotating mass of tyre.
If you brake too hard you go from ideal 5-15% slip to say 15%-80% perhaps you still have some grip but perhaps adhesion not tyre interlock ?
As the speed drops eventually the braking force will lock the wheel.
With OE equipment brakes this actual lock may occur at say 10mph with big discs all round say 30mph so the larger the braking force you can apply can physically lock the wheels at a higher speed, but you still need to hit sweat spot.
If you change to a sticky tyre then the grip availbale has increased the OE brakes will not be able to lock wheels perhaps, so upgrading the brakes is necessary.
|>> My view, which may be wrong is that at high speed the braking force is not sufficient to overcome the rotating mass of tyre. <<|
How significant does rotational inertia become I wonder? Probably more so on a heavy wheel with a heavy tyre (coming back to unsprung weight again! ;o) )
>> http://autopedia.com/stuttgart-west/StuttPhysics.html <<
Curious Chaz - these articles are the same as those presented in http://www.miata.net/sport/Physics/ - not surprising given that it's all by the same author! LOL
Brian Beckman explains the concepts pretty well, even for someone like me who only progressed to Physics 'A' level :o)
In theory the car with a heavy tyre will have brakes that produce a larger force so prob balances out, some being better than others. As far as stopping distance is concerned the rotational weight should not enter equation its just takes time for a spinning object to come to a stop it keeps slipping longer
Unsprung weight is much more of a problem with lighter cars, but adding unsprung weight to any car is not a good idea unless its being put to use by improving cornering or braking.
This thread was discussed between 09/01/2004 and 28/01/2004
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