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MG MGB Technical - Axle location.
|I do quite a lot of trackdays in my V8mgb gt and at present have a Panhard rod fitted which is horizontal to and in the centreline of the axle when suspension is in a neutral position. I also made the rod as long as possible.|
Over the last 10 years I have tried just about every combination of front/rear suspension setup and the car has always had a tendency to oversteer all too easily. I have come to the conclusion that it is to do with the rear roll centre being too high.(compared with the front being very low).
I was planning to make and fit a Mumford linkage to lower the rear RC but after making a full size model I realise that this will not work with leaf springs as the axle moves sideways when the car rolls.
Other than making a coilover 4 link rear end I could lower the existing panhard rod by 50mm which will lower the RC.
Just wondering if its worth the effort to remake the panhard brackets.
Anyone played with the rear roll centre.
As I'm sure you already know, dropping the roll centre will give you better drive from the rear out of corners and lifting it will make your car turn in to a corner sharper
The centre line of the diff housing sounds a bit high to me - 50mm lower sounds more like where it should be.
If you are going to change it why not go for an adjustable Watts link - then you will have something that is quick and easy to play around with
Build it with the links out on the diff tubes and a vertically slotted or multi holed bracket on the centreline of the body to mount the pivot. Then it's simple to lift the pivot (roll centre)up and down to find that magic spot you're after
What part of the corner is the oversteer happening
Have you got plenty of travel in your rear shocks
As a starting point, with your car at it's normal ride height, if you run a string line from the centre of the front spring eye of one of your rear springs through to the cenre of the rear spring eye of that spring - the point at which the string passes the centre of the diff housing will be the car's natural roll centre and a good height to work from. It would be interesting to know how far away from this height you are now. Willy
|How stiff are your dampers? If these are still compressing through the corner than the attitude of the car will be changing. Stiff dampers seem to be what some people go for, surely compliance is the thing, stiff springing is something else.|
Thanks, this is new territory for me. For now I am lowering the Panhard by 50mm but still keeping it horizontal, just to see if there is any noticeable difference.
Rear shocks have plenty of travel, had them made specially.
My car turns into the corner like the front is on rails but mid corner and on the exit it is just always trying to step out even with very moderate acceleration (326bhp).
I would say you are right about compliance for road use but on track you have extreme weight transfer which is happening very quickly.
Have done a lot of experimenting and the car is so much more stable at speed with the dampers turned up pretty hard. Going too far definateley makes the back end break away specially going into a corner.
I also have a custom rear anti roll bar (adjustable)which is very compliant but again without it the car is unstable at very high speed 120mph+ and if anything it improves the cornering performance.
I have a trackday at Silverstone arena circuit on 3rd Dec so with the long corners will be able to evaluate the new panhard position.
|I know an RV8 owner who has the Holye Jag based IRS. He loves it for road use, even considering the cost. It might be worth researching this for track use, with wishbones you can set your roll centre wherever you like although Im sure the standard set up is for comfort in road use. It seems the car does not have to comply with a race series formula, just be set up to behave well on track days.|
Try your new Panhard position first as I am sure it is a step in the right direction and maybee all you need to do - but also another thought is that your front spring rates/ swaybar might be a tiddle soft or the rear a bit stiff Have you got anti tramp bars on the rear,if so maybee their design is binding up the rear as the body rolls. I do a bit on a friend's V8 and we ended up throwing all the rear addons away and starting fresh. - ended up with a Watts link and an extra half leaf in the front half of the back springs to stop the spring winding up under power, no tramp rods and even went back to revalved lever arm shocks after trying all sorts of shockies - the thing grips,
Please let us know the result of your lowered Pan.
|I guess you get the oversteer as you try to put the power down after apexing. Have you thought about bigger section/lower profile tyres at the back. This of course will make the car understeer power off but will let you get more power down before yuo use up all the slip angle at the back.|
I have had a ride on track in a high spec 4.6 roadster fitted with the Hoyle front and rear setup.
To be honest i was not particularly impressed, (maybe it wasnt set up properly) probably great for the road.
Im using the Composite rear springs that are very compliant and have an even spring rate and yes I have custom Anti tramp bars with a softish rubber bush one end and a Rose joint at the other. Suspension does not bind at all in fact with shock absorbers removed the rear seems excessiveley soft.
Front has uprated springs and a 1 inch anti roll bar which has rose jointed drop links.
I am sure that the lowered Panhard will be an improvement but will report back with findings.
If you want to see the car in action goto youtube and tap in v8bloke as I have about 6 onboard clips.
Yes I can have power oversteer virtually anytime I want but even on fast turns with light acceleration the back drifts out.
I have sebring rear wings and 235/50/15 toyo R888 tyres which i think is as wide as I can go and grip really well from a standing start. 0 to 60 is about 4 secs.
|Mark, Not sure what it says about me but I have already watched some of your Youtube clips before realising you used this board. I really enjoyed them esp watching you go past a lot of machinery 40 years younger than the MG as if they were in reverse :-). I got the imoression that your car was set up so you could wring pretty much everything it had got out of it without it getting twitchy, i also noticed for a powerful V8 it seemed quite kind to its tyres and brakes, often the achilles heel og a big powerful race car.|
|Thanks Stan. |
I have spent a lot of time trying to get the handling as good a possible.
Considering the performance it is very economical on brakes (vented discs all round).
Rear tyres dont last long though.
I have no experience of anything like what you are talking about, but I know a man who does. Malcolm Beer who runs Beer of Houghton (mg specialists for decades) has won the fully mod V8 class in BCV8 racing many times & has a very practical & down to earth view of mods. Might be worth a call.
|lowering the pan hard will increase the oversteer, decrease the roll.|
I fitted a 7/8ths (22mm) front antisway bar which completely cured my oversteer issues. I was left with a tiny controllable little bit of ineducable over steer with the throttle.
From what I have read it should reduce the oversteer but you know what they say about the proof of the pudding.
Will let you know.
|I dont consider myself a chassis desing expert, but, since the CG is above the roll centre as you lower the roll centre the moment becomes greater. This would seem to mean that a lower roll centre will increase weight transfer at that end of the car. In practice its not that simple and depending on springs dampers tye pressures etc etc it may actually help. A car that is fidgety at speed is oversteering, and the fact the rear tyres are wearing faster shows they are working harder, but then with that much power thats a given. So its not impossible that actually raieing the rear roll centre may help. The amont of room under the back of sn MGB may make this a rather theoretical point. I do know with my car (not in your league tweaked 1950 with 93 HP at the wheels) putting the 3/4 Front ARB on and ST spoiler transfomed the car at speed. You csn pull the tacho into the orange in o/d 4th (in Germany of course) and it just goes where its pointed, interstingly I still put 2 sets of back tyres on for one at the front.|
Yeah roll centre heights can be a little confusing but basically yes the car pivots around the roll crntre and you would think that a lower centre would mean more weight above it to transfer but there is also the sideways force on the car in the corner to be considered and looking at that on it's own the weight pushing sideways on a high roll centre will transfer more weight to the outside tyre than what would be transfered with a low one
An example is the v8 supercars that race here have adjustable Watts linkages and if a car is a tiddle twitchy mid corner and coming out of the corner like Mark's appears to be in his videos they would drop the roll centre probably three turns and it would be fixed
Three turns would be about 1/4 of an inch
I think Mark is heading in the right direction but there is only one way to find out for sure - But remember the golden rule - one change at a time- Willy
|Car all back together after New axle bearings and setup (by Helical gearboxes London). Panhard now lowered by 50mm. Drove it today and first impressions are positive, still very planted feel like its on rails. Need to test it on the limit ( 3rd December Silverstone).|
Its wonderful to have a quiet axle again.
|Mark, you say you have rear disc brakes? How did you do it? My GT has 7/8" anti-roll bar, 30% stiffer lever arm shocks at the front, fully poly bushed. I have the wilwood big brake conversion and standard rear apart from polybushed. I find it grips incredibly well (yokahama Eco-Conti tyres) but when you do hit the limit there isnt much warning and if i counteract the oversteer it will either spin out completely or very suddenly stop oversteering. I'd love to power slide but it never lets me :( . I too have some vids on youtube of the car under the username itsawindupuk .|
|A P New|
50mm is a fairly big jump, it should be a lot different
I'm quite interested to know how you get on It should be good
|A P |
I have lost count of how many brake upgrades ive done. An easy setup for the mgb is to use Rover 800 rear discs. I used a 10mm spacer between the disc and hub then used sierra calipers.
I am happy with my brakes now with 298 x 32 wilwood vented discs and superlite calipers up front and 273 x 20.6 wilwwod vented discs and cosworth 4x4 calipers at the back. All connected to my own twin master balance bar setup.
Yes to my mind it should be good, will let you know.
|Yes really looking forward to some new video clips and hearing how it drives . Getting the power down sooner should make yoor car even more of a missile. Of course you will then feel it can handle more power :-)|
|Brake upgrades are all very well but it is the tyre contact patch with the road that stops you, and you should always be able to lock the front wheels of an MGB with steadily applied (not banged on suddenly) pressure. Rear brakes are deliberately low-pressure compared to the fronts to stop them locking-up. If you upgrade the calipers such that less pressure is required to lock the fronts, then unless you make changes to the rear as well or fit a brake balancing system you have actually *reduced* overall braking effect as the rears similarly have less pressure and hence less braking effect.|
I agree with what you are saying and the standard setup would stop me ok on track for maybe 2 or 3 bends but after that they would have no hope because of the heat generated. The extreme temperature causes the solid disc to crack and pads wear very fast.
All upgrades have been done to overcome various problems all relating to heat management.
I set the brake bias by applying gradual pedal pressure and adjusting so that the fronts just lock before the backs.
|Well not surprisingly my Trackday on 3rd At Silverstone has been cancelled due to the bad weather.|
This means that I wont be testing the alterations to the Panhard Rod until spring time.
|We have the final round of the Tasmanian hillclimb championships here on Saturday. I'll spare you a thought while I'm standing out in the sun. Being nice and warm I should be able to remember. ha ha ha|
This thread was discussed between 11/11/2010 and 02/12/2010
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