MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGF Technical - Results of massive amounts of research into brakes

And now the results of our research into the most cost efective improvement into MGF brakes, especially those of you with 15" wheels and Metro brakes.

Go and get a set of proper brake pads for the rear!
Mintex 1177's probably. It's as simple as that!

Much effort has been spent and money parted in the acquisition of large discs and multiple pot calipers. Truth of the matter is that the brakes on an old F are... let's be generous...adequate.

If you get bigger discs on the front you MUST do the same to the rear or risk throwing brake balance right out.

So the tweak is...when you upgrade the pads for the fronts but dont forget the rears, do all four wheels. Also think of the discs as consumable items, they are dirt cheap anyway.
Neil

How much is Mike charging for 1177s Neil?
Rob Bell

Neil

What was the research?

Paul
Paul

A few ramblings

Race car has sticky tyres and may use grooved and drilled large front brakes.

A road car may be underbraked at rear, lowered, grooves increase pad wear and drilling could increase any heat problem!

Paul
Paul

I Always thought they were more than adequate. I noticed the discs on a TDI passat the other day and to be honest they were no bigger than the F's. Last year I replaced all discs and pads at around the same time along with caliper seals and brake fluid. Assuming this brought them back to some where like new then even with standard pads they were pretty good.

So having said one nice thing about this car I fully expect my brakes to fail on the way home ( I kid you not ). Let's face it just about everything else has :-)
S Laithwaite

You can't brake any more than the tyres will let you.

This is the overriding rule before you think about upgrading the brakes. There is no point in fitting massive discs and calipers if you don't look at the tyre situation, all you are doing is making the wheels lock up earlier.

To be F/TF specific, given that the majority of braking happens at the front wheels, where the tyre size is narrowest, my advice is to fit larger width, softer compound tyres to the front to make the best use of your larger calipers/discs. Afterall, it is possible to lock up the front wheels even with standard brakes.

Once this is done, then you can look into the brake bias and upgrade the rears as necessary.

For instance, i have 4 pot calipers and 280mm discs on the front, therefore i have 205 spec tyres all round to help transmit the extra braking force of the 4 pots and discs to the road. Because of this my brake balance front to rear is now out, so to compensate i have drilled and grooved discs at the rear with Mintex 1155 pads.

The result is that i am pretty happy with the way the car brakes now, but will probably go for 215 spec tyres all round next time for the reasons above.

Rob has some excellent stuff on his website about this, check this out:

>> http://www.mgf.ultimatemg.com/brakes/big_brakes/Big_Brakes.htm <<

SF
Scarlet Fever

>> A road car may be underbraked at rear, lowered, grooves increase pad wear and drilling could increase any heat problem! <<

LOL Paul - you've hooked me: what's the background to this "ramble"?
Rob Bell

Spanner in works time!

Wide tyres do not have any more contact patch available, so although more cornering force available (patch wide but short) no more braking force - true of false?

For road tyres a soft compound perhaps a Yoko will help but tend to wear quickly and maybe not so good in wet?
Rob I'm hoping you can run some numbers on these comments

Lets say a road car is underbraked at rear by 10% to be safe (may not be true on all cars), similar to making car understeer.

The average enthusiast will lower car reducing weight transfer and then drives in wet conditions which further reduce weight transfer, say from 0.9g to 0.7g

A race car will be lighter have sticky tyres say 1.5g and not often run in wet but will have a bias valve anyway to readjust brake balance.

They will have large discs which are heavier so perhaps some drilling will help and a few grooves to keep pad bite at maximum, but reduces pad disc contact, swings and roundabouts!

Then look at discs available for road car, multi grooved with dimples or holes is this helping! It may help but the cost is certainly more pad wear and noisy brakes.

Just to go back to SF post, the main point is that the driver is happy and confident with brake balance.

Paul

Paul Wiley

Road test proves HUGE difference to braking but as SF says you need to keep it all in perspective. It never ends...The darkside will always get you in the end
Performance parts just like hard drugs they cost loads you get a little euphoria and then you pay a second time with the come down!
Neil


High speed scenario:

Brake hard with stock brakes... you slow to 50mph, but not that much happens.

Brake hard with "improved" brakes... you slow *quickly* to 50mph.

The brakes in both situations cannot get close to providing the forces necessary to either lock your wheel or exceed the forces the tyre is able to provide.

In all the discussions over the past month or so people seem to have forgotten the amount of energy within a moving car at high speed.

I would be willing to state that most brake upgrades to the front only will most probably improve high speed braking.

I would also agree totally with the original post and state that a balanced upgrade will improve braking at high, medium and lower speeds.
That's why I upgraded my rears to 1177s when I firstly upgraded my fronts to 1177s and then latterly fitted HiSpec kit to the front.

I do completely agree with all the arguments when they are applied to either lower speed situations, or situations where the surface is not good. eg. wet roads.

Summary:
Agree with the original post... go with a *BALANCED* upgrade.
I disagree with some of the stated facts as they apply to slower speeds only.

P.
Paul Nothard

>> Wide tyres do not have any more contact patch available, so although more cornering force available (patch wide but short) no more braking force - true of false? <<

True - and false. True because the force required to over come friction to make an object slide over a surface is independent of the contact patch. But false because of the way that tyres develop their 'stiction' through development of slip angle (yup, our old favourite again) - and slip angle IS effected by the tyre size - and a whole host of other factors too...

>> In all the discussions over the past month or so people seem to have forgotten the amount of energy within a moving car at high speed. <<

Paul, I am glad you raised this. Newton's second law of motion, F=ma.

So to decelerate from any speed at any given rate of deceleration will require the same amount of braking effort.

The lost kinetic engery of the whole car will be dissipated as heat: quite clearly, there will be more heat generated with the more speed that is shed.

But you've mentioned the impact of the rotational inertia of the wheel Paul - but I am not sure how this feeds into what we know about braking properties? I'm sure that the physics guys on the New Mini forums will be able to furnish us with some equations...?
Rob Bell

>>>Wide tyres do not have any more contact patch available, so although more cornering force available (patch wide but short) no more braking force - true of false?
===false. Wide tyre=wide contact patch. Larger wheel diameter=larger contact patch also.
>>>drilling could increase any heat problem
===care to elaborate Paul?
David S

You're right Rob.

F=ma does not take into account the force from road to car.

How much effort do I have to put in at 20mph to lock the wheel?

How much effort do I have to put in at 70mph to lock the wheel? Lots more.

I'll look into the physics of the rotational inertia etc... but I've not seen anything as deep as our discussions on the Mini board.

Paul (Mini-less now <sob>)
Paul Nothard

>> F=ma does not take into account the force from road to car. <<

But surely it does??? The force acting at the road/tyre interface is going to equal that acting at the centre of gravity (as shown in figure 2, http://www.mgf.ultimatemg.com/brakes/big_brakes/background_part1.htm)

"Effort" is another expression of energy - not force. So the force required to decelerate is independent of the velocity. But the energy consumed in this process clearly is.

I'd have thought that the rotational inertia of the wheel is linearly related to velocity as well - so why should braking from high speed require any more force than a similar deceleration from low speed?

There are bound to be 'second order' effects - such as what happens at the pad surface as heat causes it to vapourise - is this what you're referring to? Might certainly explain why it appears to be harder to lock wheels from high speed than from a lower speed.
Rob Bell

The reason it is harder to lock the wheels from a high speed than from a low speed is because the wheel is spinning that much faster, therefore the brakes have to work that much harder to bring them to a complete halt. It's all about the speed of the wheel at the initial point the brakes are applied, the faster the wheel is spinning the more it has to decelerate before it locks - 'an object in motion tends to want to stay in motion'

Wider tyres have a larger contact are with the road, larger wheels result in a larger contact area with the road. Therefore 16" wheels fitted with 215mm width tyres have a larger contact area with the road than 15" wheels fitted with 185mm width tyres. This menas that they have a lower slip angle and therefore are more capable of transferring the forces applied by the brakes to the road surface. This is particularly relevant on the F/TF with narrower tyres fitted to the front wheels, which is where the majority of the braking occurs.

I'll say it again - doesn't matter if the car is fitted with 50 pot calipers or bycicle rim blocks, the brakes will only work within the limitations of the tyres and the road conditions. Therefore to improve braking performance you need to look at the tyres as well as the brakes (front and rear) and upgrade them in harmony with each other.

Harmony, that's a good word for it. :-)

Some good points raised on this thread, nice to see a good discussion going on on here, it seems like ages since there was something to get your teeth into. :-)

SF
Scarlet Fever


To slow a car more than standard brakes you need to be able to increase F. This means exerting more of a force on your brake disks. There is more energy dissapated as heat. That is as a result of more effort being put into the braking.

But why does it seem nigh on imposible to lock the brakes at high speed?

The force required to produce a certain deceleration seems to be different at different speeds. ie. More energy in a body, the more force is required to produce the same deceleration.

Now... how to prove that on paper....
:-)

P.
Paul Nothard

I don't think that the retardive force applied by the brake calipers alters according to road speed - but I agree that *pedal pressure* does.

There is something that we've not yet discussed: the friction properties of the pad materials themselves - and how they behave when heated up.

The simplest model of pad behaviour is that the coefficient of friction DROPS with heat.

Deceleration from higher speed involves the shedding of much more energy than from lower speed (remember that kinetic energy = mv^2 where m is the mass of the vehicle, and v is the forward velocity).

So a car travelling at 120 mph has 4 times the engergy of a car travelling at 60 - and this additional energy is dissipated as heat.

So let's decelerate our car by the same speed - 30 mph.

The difference in energy between 120 to 90 mph is m(120^2 - 90^2) = m*6300j

The difference in energy decelerating from 60 to 30 is m(60^2 - 30^2) = m*2700j

The difference is approx 2.3 in this example - ie, decelerating from the higher speed you need to dissipate more than double the amount of energy.

That energy converts to heat.

That heat drops the coefficient of friction.

We know that the force required to make an object slide over another surface is equal to the coefficient of friction * the force pushing that object onto that surface, which in the case of a brake pad is proportional to the pedal pressure/hydraulic brake pressure.

So what if the coefficient of friction drops by 50%? In order to maintain the same braking force, we'd need to double the force applied to the brake pad - which translates into a higher pedal pressure.

So we percieve that we are applying more force at the pedal, yet the force at the brake rotor is actually the same...

Hope that this makes more sense?

The good thing about big brakes of course is that they dissipate heat that much better, meaning that the pads don't heat up so much and therefore don't suffer quite such a dramatic drop in friction coefficient...

Hmm - might be time to look up some brake pad references off google! LOL
Rob Bell

A bit of further reading regarding the properties of pads:

http://www.rpmnet.com/techart/pad_term.shtml

and

http://www.bath.ac.uk/~en0wgam/pad.htm

Thing is, neither really go into any detail on the phenomenon that Paul mentions: ie higher pedal pressure required to get retardation from higher velocities...

Any mechanical engineers out there who'd fancy explaining this for us?
Rob Bell

I'll need to read above in detail.

Tyre width - consider weight of vehicle and pressure in tyres, from this you can tell contact patch size. assume same pressure and tyre circumference.

Brakes apply a force which is sufficient to overcome tyre grip.

Maximum tyre grip is at 15% slip- I've been informed that after this point lock is quick but checked with accident graphs.

So if brakes can achieve this a slow speed why not at high.

The slip is a measure between car and wheel so say at 200mph a 15% slip is 30mph at a 100mph the diff is only 15mph?

If you look at pad sites you will find that the pad is effected by the disc velocity as well as heat but this is what all the R&D is for to give a pad a consistant COF for wheel velocity as well as heat range, sp pad graphs show this COF as constant, should be on Mintex race pad site.

So "big brakes" can deal with all that heat of high speed stops and also overcome tyres at low speed causing early lock unless brake force is reduced?


Paul
Paul

http://www.mintexracing.com/press/editorial.htm
Paul

-- Brakes apply a force which is sufficient to overcome tyre grip.

Yes. Unless your brakes are *really* bad. :-)

-- So "big brakes" can deal with all that heat of high speed stops and also overcome tyres at low speed causing early lock unless brake force is reduced?
Yes. I think so. But it's not the full story.
Big brakes can deal with the heat... and that's an advantage for sure.
I agree that at lower speeds (or wet roads) that big brakes can probably lock the wheel more if not balanced correctly. (Even if balanced correctly???)

None of this explains why I am unable to lock the wheels at, say, 70mph while standing on the brakes as hard as I physically can.
Why is that?

I can't yet explain it myself... but it seems to contradict some of the other theories does it not?


The best piece of advice I was ever given by a driver (some bloke called XX I believe) was to stand on the brakes very very hard to start with as "you're never going to lock them at high speed" and then to ease off as you get closer to the corner to get your entry speed just right.

P

ps. Has anyone lese noticed that some of the better explained suspension/brake modelling on the net is from games designers in search of reality? :-) Should we be worried by that?
Paul Nothard

-- None of this explains why I am unable to lock the wheels at, say, 70mph while standing on the brakes as hard as I physically can.
-- Why is that?

Friction - I think.

Friction *is* velocity related. At high speeds the friction exerted on the wheel by the road far greater than the friction able to be exerted on the wheel by the brakes.

There is a cross-over point on the graph where the brakes can exert more friction than the road can and thus the wheel locks.

ie. Over a certain speed you cannot lock your wheels.

NB. Ideal lab situation. etc. etc. etc.

Obviously F<->R brake balance will make a difference on the friction between the road and the rear tyres... which means that unless your brakes are balanced properly then your rear wheels will be able to lock up sooner than your fronts.
(This agrees with my experience in a race prepared 944 who's balance was out and I had rear wheel lock up into most bends! Eeek!)

Assuming that this isn't shot down in flames, then I think I'm a happy bunny having explained why big breaks really do stop you better(*).

(*) At hight speed. ;-)

P.
Paul Nothard

http://www.ferodo.co.uk/ferodo_home/high.html

Right site this time, which shows that brake force will with right pad will be consistant at various speeds.

In world of physics tyres are black magic, but the black stuff under weight fills the gaps in road surface and provide additional grip, and the more weight the further down the gaps are filled but it becomes more difficult, so grip levels do not increase at same rate as the load put on them, but not speed related and the weight transfer does not happen immediately to give the increased grip.

The rest is balance maximum stopping power is when front and rear brakes are at maximum or in harmony, the next modulation or ability to keep at maximum slip, on off brakes are not great in wet weather and too muck leg work is just a pain.

The variation in slip means that once starting to slip to magical 15% the COF of a tyre is a lot higher than past the point of no return and locking.

For a 15% slip the wheel will travel 8.5/10ths the distance of car past this point braking grip reduces quickly so this applies to 5/10-ths and 0/10ths locked.

I had to use very high speeds to illustrate earlier and Brembo on FAQ's do not say their big brake kits when used for road reduce stopping distances.

So in race mode the braking is from 140 to 70, loads of heat, on the road 70-0, but multiple stopping will warm things up.

Balanced brakes will stop you quicker bar one or prob several points (heat big front brakes in extreme conditions may at least give some braking rather than both front and rear going into smoke) reaction time - the quicker the brakes come on even by a 1/10th at high speed could be 8 feet even if not balanced.

Paul

Paul

Interesting from where OE may start front bias

http://www.stoptech.com/technical/balancedchart.htm
Paul

David,

Contact patch is about getting it as flat and verticle as possible whilst cornering, so I use neg camber on V8. Larger diam alloys mean less sidewall movement to keep that contact patch flat but decrease suspension provided by tyre, OK not comfy but more grip but the other downside is more unsprung weight - rotating at that so x say 4, so always a trade off balance.

The drilling aspect was to prevent gassing, should be less of issue now esp ferro carbon type pads and also to increase bite, but can form stresses within disc and less disc less mass re heat- vented and grooved re pad bite and debris are the better options IMO.

Rob has mentioned this with Alloys- look cool but never tell you the weight - with classics weber must be better than SU (prob only racing not road) - and then look at these multi grooved and drilled disc options- perhaps what the customer wants and looks cool but does it really work?

A quick cut and paste job

Pressure=weight/area.

That's about as simple a physics equation as you can get. For the general case of most car tyres travelling on a road, it works pretty well. Let me explain. Let's say you've got some regular tyres, as supplied with your car. They're inflated to 30psi and your car weighs 1500Kg. Roughly speaking, each tyre is taking about a quarter of your car's weight - in this case 375Kg. In metric, 30psi is about 2.11Kg/cm2.
By that formula, the area of your contact patch is going to be roughly 375 / 2.11 = 177.7cm2 (weight divided by pressure)
Let's say your standard tyres are 185/65R14 - a good middle-ground, factory-fit tyre. That means the tread width is 18.5cm side to side. So your contact patch with all these variables is going to be about 177.7cm2 / 18.5, which is 9.8cm. Your contact patch is a rectangle 18.5cm across the width of the tyre by 9.8cm front-to-back where it sits 'flat' on the road.
Still with me? Great. You've taken your car to the tyre dealer and with the help of my tyre calculator, figured out that you can get some sw*nky 225/50R15 tyres. You polish up the 15inch rims, get the tyres fitted and drive off. Let's look at the equation again. The weight of your car bearing down on the wheels hasn't changed. The PSI in the tyres is going to be about the same. If those two variables haven't changed, then your contact patch is still going to be the same : 177.7cm2
However you now have wider tyres - the tread width is now 22.5cm instead of 18.5cm. The same contact patch but with wider tyres means a narrower contact area front-to-back. In this example, it becomes 177.7cm2 / 22.5, which is 7.8cm.

Paul
Paul

Paul, I'm sorry but your grammar, sentence construction, use of abbreviations and mis-spelling is so bad that I simply cannot unscramble your latest thought processes. Your assumptions on contact patch are too simplistic - you haven't taken into account the construction of the tyre and the deformation of the tyre walls which all have a great bearing on matters. However it may not be relevant - now what was your question again? Exactly what point are you trying to make?
David S

-- Right site this time, which shows that brake force will with right pad will be consistant at various speeds.

Nope. It shows the coefficient of friction.
The coefficient of friction is the ratio of the limiting friction to the normal reaction between the sliding surfaces.
In this case it means the relationship between how hard you push on the pad and the limiting friction provided by the pad is constant.

Anyway, I was mainly talking about the friction between tyre and road.

Yes, this is very very complicated physics to take everything into account. I realistically do not think we can get close to "solving" the problem with our combined and, frankly, lame understanding of the physics involved.
However.... :-)
With tyres (in a simplistic world which is all we can realistically model here!!) you increase the rolling friction when you brake. If this rolling friction increases until it is greater than the starting friction then you skid and the tyres changes to sliding friction... which is much much less than the rolling friction you had initially. In addition, if this sliding friction stays smalled than the friction that you are able to exert via the brakes, then the wheel will stay lock up.

Your talk is good Paul.... but you have yet to agree or disagree with my question on locking wheels at high speed.
Are you saying it can be done?
Can you tell me why with formulae why it can or cannot be done?

I think I'm going to be be content with the fact that I have consitently proven on track that my brake setup is easily better than OEM and, as far as I can tell, is easily up there with the very best fitted to an 'F. (Only with 15" rims too! <grin>)

I do generally agree with you on the balance issue though. Always have done. I think that I have managed to maintain most of the balance that was designed into the car. Possibly luck, but it seemed like a good idea at the time. :)

P.
Paul Nothard

(Too many Pauls!!! <grin>)
Paul Nothard

Paul

Second graph on Ferodo site shows no change in COF re speed sensitivity

The following site will show the braking force is therefore the same

http://www.teamscr.com/grmbrakes.html

This graph shows tyre COF as wheel slips, although reverse braking

http://www.racelogic.co.uk/techtrac.htm

So the cars speed is not the issue.

Rob's "Physics of racing site" part?? shows time compared to distance.

So all the above happens in a short time frame.

As no degree in Physics the next bit is guesswork, although may be some of the maths on that Physics site

A spinning object such as a wheel has velocity and at low velocity stopping it dead is easy but at high velocity it will slow before stopping dead but it is still being braked.

David,

Road car weight stays same
Same Road tyre only wider but same circumference
Same tyre pressure
ie everday motoring

Question contact patch size the same?

Tyre companies can for oval racing change tyre construction totally but not really relevant.


The tyre force at a given slip and brake force remain constant at all speeds and we know the brake force is sufficient to lock the wheels at low speed.

The other Paul without PHD

I think we are just trying to establish the best value for money set up.

Paul
Paul

>>>I think we are just trying to establish the best value for money set up.
=== a pretty vague objective ! what parameters, ie intended style / mode of use, and what's the budget? The variables are so huge I suggest there's no such thing as the 'best VFM setup'.
David S

David,

Neil started this with a pad upgrade, and from other threads
Braided hose - movement on system Rob's web site shows details of problem.

Rob has light alloys and prefers not to change these.
I think Neil has already produced a good brake set up (spec on another site).

I think Paul N has a HiSpec set up - which obviously works well

Robs site also deals with weight transfer and big brakes for F.

Throw into the debate the "Physics" which is intersting, compared to reality - Neil did you capture any data - this gives opportunity for others to size up their own system for their own use.

The catalyst for this was Rob's concern with not upgrading rear.

Any answer on pressure weight question?

As an experiment in safe circumstances, anyone tried left foot braking?

Paul
Paul Wiley

>> Friction *is* velocity related. At high speeds the friction exerted on the wheel by the road far greater than the friction able to be exerted on the wheel by the brakes. <<

Paul(N), how does this work? [Probably me being dense LOL!] Why should a wheel that is rotating faster have a tyre that generates more grip (or coefficient of friction) than a tyre moving more slowly? It doesn't make too much sense to me, since we know that a tyre will generate, at max, 0.9-1.4G. I believe that this is *irrespective* of road speed.

Paul(W) - I've tried LFB, but I still haven't got my brain around the physics of this one!!! A conversation for another day I think! LOL
Rob Bell

Rob

I agree re COF of tyre but I do not think it is relevant.

The tyre decelerates by the force applied to road (we know that OE brakes can apply sufficient force to lock wheels at low speed) so they can apply sufficient force at high speed. The deceleration of wheel compared to car is governed by "striction" so it begins to slip - but at high speed it needs longer to achieve a 15% slip (only dependent upon tyre striction) so the wheel will not lock until the wheel has decelerated enough to go past that amount of slip. This is all dependent on tyre "striction" not brake force. At low speed the 15% slip is very easy to achieve and lock up is easy.

With big brakes with more available force, then at low speed it is very easy to apply too much brake force and lock tyres and take chunks out of tyre, but at both high and low speed both OE and big brakes are capable of maximum retardation of tyre, apart from heat factor.

All down to tyres not brakes!

Paul
Paul

Thanks Paul - I need to do a little more reading about tyre stiction to get my head around this one! :o)
Rob Bell

Rob,

A few bit of info

The longest skid mark recorded on a public road was made by a Jaguar on a German autobahn, the wheels locking at 130mph.

The longest not a public road was 10Km until brakes burned out from a speed of about 500mph (Bonneville).

When the F1 regs changed to grooved tyres the braking g reduced from a peak of 6g to 4g (I am not aware of any brake changes at that time), also takes 3secs from 200-50mph, and the brakes are in 13in wheels so not huge in dia, Ferrari use DOT4 fluid!

Around town with 10ins rotors the disc temp may get to around 385deg with about same for one 80mph stop. 4 back to back stops from 60mph could increase it to 550deg.

One 140mph stop could generate 1025F, now well above road or road race pads. At these temps the COF of pad will drop considerable and at this stage larger discs are required or at least race pads and fluid but this may find weak spots in bearings etc.

Testing with TR6
Front Caliper uprated
Excluding min distance
Stock 235ft
Front uprated 233ft
Front and Rear uprate 198ft
Excluding max distance (Multiple Stop situation)
Stock 277ft
Uprated Front 258ft
Front and Rear uprated 228ft

Paul
Paul

I think when the F1 regs went over to grooved tyres, carbon brake rotors were also outlawed - but point taken: tyres dictate brake performance.

Also interesting to see that imperical data demonstrating how much shorter braking distances can be achieved by upgrading both front and rear discs! :o)
Rob Bell

Rob,

Carbon is still used although test were carried out by D Hill with cast iron and a sintered pad and there was not much in it, but cast iron would need to be larger diam to deal with heat.

As the laws of physics apply equally to F1, the torque from say a 300mm disc and 6 pots can give 4g braking performance even if the cars are lighter than F.

Paul

Paul

Paul, Surrey, UK
>> All down to tyres not brakes! <<

Rob wrote:
>> but point taken: tyres dictate brake performance. <<

Which is where i came in! A nice circular argument :-)

I wrote:
>> You can't brake any more than the tyres will let you.

This is the overriding rule before you think about upgrading the brakes. There is no point in fitting massive discs and calipers if you don't look at the tyre situation, all you are doing is making the wheels lock up earlier. <<

and

>> I'll say it again - doesn't matter if the car is fitted with 50 pot calipers or bycicle rim blocks, the brakes will only work within the limitations of the tyres and the road conditions. Therefore to improve braking performance you need to look at the tyres as well as the brakes (front and rear) and upgrade them in harmony with each other.

Harmony, that's a good word for it. :-) <<

CONCLUSIONS:

The F/TF on normal spec wheels/tyres has a smaller contact area under the front wheels than the rear wheels. This is to induce lower slip angles and thus induce understeer on a car with a rearward weight bias (natural oversteer). This is the reason the rear steps out unexpectedly easily in wet/greasy conditions, when normally it is very tricky indeed to get the car to oversteer.

Because of this, under braking conditions the centre of gravity of the car moves forwards in the car, producing a load bias onto the front wheels (with the smaller contact area). This is counter-productive to the goal of stopping the car in as short a distance as possible.

Yes, there is a brake bias issue with the fitment of large spec brakes to the front and not looking at the rears, but the real issue is tyres.

The majority of your braking is transmitted to the road via the smallest tyre contact area, this is not a good set up IMO.

SF
Scarlet Fever

SF

The size and shape of a contact patch creates different characteristics of grip/traction. A wider contact patch will produce more cornering grip, and a longer contact patch will produce more accelerating/braking traction.

The area of the contact patch is a function of the load on the tyre and the tyre pressure alone - almost exactly. In other words, if you make a tyre wider, but keep the weight of the car, and the tyre pressure the same, the contact patch area will not increase. It will simply get wider and shorter. (See calculations above).

To improve grip its necessary to use a sticky tyre or non grooved tyre which is not useful in wet.

A wide tyre has more cornering force at lower slip angles which is one reason why F1 cars do look like they run on rails, or less slip angle than front on a F.

Paul

Paul

Hmmm, load distribution.

Basically you are saying that the load remains a constant (which is untrue, but for the sake of argument), and the tyre pressures remain a constant (which is again untrue, but to go into this also would cloud the argument), then the contact area between the tyre and the road will also remain a constant regardless of the width of the tyre (what you gain in width, you loose in length).

Logic says that this is correct, but Hypothetically if you put on tyres with a vastly increased width, there will come a point where the diameter of the wheel will dictate the minimum length contact patch. For example if your tyres were infinitely wide, the length of the contact patch would not be infinitely short, it would be proportionate to the wheels' circumference.

Does this extreme argument apply to the F/TF? It's simply here to illustrate a point, the point being that although load distribution is a factor, it is not key to the issue.

I shall apply this to the point i made in my previous post above:

>> Because of this, under braking conditions the centre of gravity of the car moves forwards in the car, producing a load bias onto the front wheels (with the smaller contact area). This is counter-productive to the goal of stopping the car in as short a distance as possible. <<

If we apply the load distribution theory to this scenario, this says that as the CofG of the car moves forwards and there is more load on the front tyres, the tyres will deform more resulting in a larger contact area with the road. Whilst this is undoubtedly true, the key word here is 'deform'. A deformed tyre is more likely to produce higher slip angles and therefore, despite the contact area being the same the actual stopping distance of the car will increase.

Larger wheels, with wider tyres will deform less and thus will be more effective.

SF
Scarlet Fever

SF,

For road cars the assumption is that circumferance will stay same, otherwise tyres will be like F1.

So assumption contact patch stays same area.

Load transfer at macro level when cornering - inner wheel will lift, now apply to micro level, the inner of outside tyre lifts, with a wide tyre this effect is reduced and the objective of larger diam alloys and camber changes try to maximise footprint.

Under braking the front middle part of tyre lifts leaving the outside edge in contact, the longer the tyre contact patch the better grip. The rears the outside edges lift.

Paul
Paul Wiley

Jeez look what I started!

would anybody care to precis the thoughts here!!!
what is the collective wisdom?

Yes the 1177's are viciously better than the originals and green stuff,
But they knarl up your discs much quicker which is ok as they cost very little, but without ABS you will have to watch sliding into things as speeds below 30! so you will have to remember cadence braking which is a bit of a palarver!...

I might be that 1177 should only really be used on track days
Neil

>> Under braking the front middle part of tyre lifts leaving the outside edge in contact, the longer the tyre contact patch the better grip. The rears the outside edges lift. <<

The amount of lift at the macro level is:
a) Proportionate to the size of the contact patch
b) Dependant on the stiffness of the side walls (and tyre construction) - lower profile tyres perform better than higher profile tyres, less deformation in the sidewall = more surface area of the tyre working efficiently in contact with the road.
c) Proportionate to the width of the tyre (middle of tyre cambers upwards, but the wider the tyre, the shallower the camber, meaning more tyre tread blocks are in contact with the road).

Larger wheels do not affect the rolling circumference (or at least they shouldn't do if you want to retain the speedo calibration), what they affect is the stiffness of the side wall and thus, the deformation of the tyre during cornering and under heavy braking.

The key issue is slip angles and how to reduce them. Fitting wider, lower profile tyres will reduce slip angles and they do so through the reasons given above.

It really is very simple, under braking load the front tyre walls collapse inwards, producing the upward camber across the tread blocks. The greater/more flexible the side wall, the greater the camber resulting in less tread blocks in contact with the road and more sidewall. This means higher slip angles and a longer stopping distances.

SF
Scarlet Fever

I have read the major part of this thread.... but didn't find too much about the fact that bigger brakes add more weight. As adding more weight results in more energy (kinetical energy=1/2 mv²) it will be harder to lock (or better almost lock) the front wheels and getting the most out of the braking process.
Or maybe that's equalled out (or kind of) by moving the callipers more out of the center.

keep on rolling and braking ;o)
Erik

BTW, bit worried to order a set of 1177 for the rear, as those in the front make a hell of a noice.
Erik

SF

How do you calculate a slip angle with braking in a straight line?

If you do not increase diam of alloy and rolling circumferance same, with a wide tyre the aspect ratio decreases but not height of side wall. If you increase diam of alloys you start increasing rotational unsprung weight for the benefit of grip another balance issue.

http://www.wrc.com/en_GB/Features/Content/2003_R_I_TarmacTyreTalk.htm

? lower tyre pressure

With regard to slip angles (tyre companies do not release this type of info) a wide tyre runs a lower slip angle but is likely to stall at a lower slip angle, F1 tyres stall at 5% slip!

Wide tyres have ability when traction is applied not to increase slip angles so much as a narrower tyre!

Neil,

Are you finding a decel hole in the abs with 1177?

I'm sure if we ever reach a conclusion Rob will sum it up on his web site.

My summing up is its a question of balance and normally no free lunch.

Paul
Paul

Erik,

The additional unsprung weight is taken into account on low braking circuits and even solid discs may be used in these circumstances. Calipers have been moved to bottom of disc to try to improve CG

The only forces involved with locking wheels at moment is the clamping force and tyre grip, with a big? re the part played by rotational force but pad speed sensitivity will play a part.

Paul
Paul

This thread was discussed between 18/02/2004 and 23/02/2004

MG MGF Technical index

This thread is from the archive. The Live MG MGF Technical BBS is active now.