The Big Brake Mistake

[Rob Bell]

I thought I'd share this with you guys before putting the finishing touches (ie graphics) to it and putting on the site:

www.mgcars.org/carclub/mgfregister/temp/Big_Brakes.htm

If you get a chance, read it through and let me know whether it makes sense or is a load of old cobblers

Cheers!

[ScarletFever]

>> we see that the weight distribution following a 0.9G braking manoeuvre 60:40.

That means that for our 1100kg MG, 660kg will be loaded over the rear wheel* (up from 495kg under static conditions - that's like sitting a 165kg elephant* on the bonnet!), whilst the rear wheels are loaded with only 440kg. <<

* = This should say front wheel?
* = 165kg? Elephant foetus maybe?

>> 2-tonne Bentley Continental GT, <<

For the record, my old man's 1995 Bentley Turbo R weighs in at 2950kg! And it'll wheelspin in 3rd if pushed!!

All very interesting indeed Rob, i didn't check you rmaths, but understood the theory pretty well so i'd say it's pretty well written mate. Would be nice to see some animated gifs or something showing weight transfer, an F with an arrow and a percentage above each wheel pitching forwards and the numbers altering would be good. Also, it needs to continue onto the brake options available and how they stack up against the theory - in so far as rear wheel options to correct the balance phenomenon. In my case i have, as you know, MS 4 pots and 280mm discs on the front and drilled and groved discs + Mintex 1155s on the rear so i have loked at the rear brake situation, but i have no idea if this rear upgrade helps much to restore the balance - do you see where i'm going with this? At the moment the article is interesting but doesn't give any solutions, therefore isn't very useful - maybe we need to sort out a testing session (if it is possible).

Other than that well done, i wouldn't have wanted to sift through all those maths!

[Rob Bell]

Thanks for the positive critical feedback Andy ;D

I've corrected the errors you spotted. And it now reads as a 165kg baby elephant! LOL

>> At the moment the article is interesting but doesn't give any solutions, therefore isn't very useful - maybe we need to sort out a testing session (if it is possible). <<

You're absolutely right, I don't. Mainly because I haven't - if you know what I mean. The best way that I can imagine to maintain the balance of the standard brakes (that I assume that some clever engineers have already spent considerable time agonising over) is to scale up everything in the original proportions. So if you fit 285 front discs, fit 285 rear discs. The same applies to piston areas in what ever calipers you fit. This way you can make use of previous development effort, and not have to carry out your own time-consuming and potentially expensive development. This is one of the attractions of HiSpec's kit.

Defintely would be worth while performing some testing. I was thinking of a skid pan session - if only to preserve tyres! LOL ;D

BTW the up-dated version of the above is now on-line on www.mgf.ultimatemg.com/

[ScarletFever]

Looks good Rob - nice use of a 'stock' image too!

[dave]

Well I've ploughed through it and even followed the links back to your source and read his stuff. I've got the general idea, but wouldn't say I could reproduce the mathematical underpinnings if asked

It is of course, as Andy says, all very theoretical. That isn't just a tautology - we don't know how the car is actually set up to begin with. OK, it's a reasonable assumption that someone at MGR understood all this stuff and designed the system for optimum performance, but then again ....

So, understanding the theory is one thing - applying it to an unknown start point is another!

If I hadn't read the stuff I'd have jumped at the mention of the 'real world' tests that Andy mentions. However, all the test cars would have to wear the same boots wouldn't they!

I have to say though that I read the stuff easily and followed the logic even if I'm a bit out of practice with the maths. Much easier to understand than that horrendous thread on the BBS

Well done Rob!

[ScarletFever]

dave said:

So, understanding the theory is one thing - applying it to an unknown start point is another!

Hmmm, are we giving MGR engineers too much credit here i wonder? We know the brakes the MGF wears are parts bin items from other cars (R800 and Metro IIRC, could be wrong here), therefore it is reasonable to assume that the components were designed for the original cars weight bias, rather than the F/TF. This being the case then it could very well turn out that the F's OE brakes are not set up correctly from the word go. After all, despite the fact i don't know the overall weights of the Metro and R800, it is reasonable to assume that they, being front engined, FWD vehicles, would have a different weight bias to the F/TF.

[Rob Bell]

ScarletFever said:

Hmmm, are we giving MGR engineers too much credit here i wonder? We know the brakes the MGF wears are parts bin items from other cars (R800 and Metro IIRC, could be wrong here), therefore it is reasonable to assume that the components were designed for the original cars weight bias, rather than the F/TF. This being the case then it could very well turn out that the F's OE brakes are not set up correctly from the word go. After all, despite the fact i don't know the overall weights of the Metro and R800, it is reasonable to assume that they, being front engined, FWD vehicles, would have a different weight bias to the F/TF.

Now there's a challenging statement!!! I like a challenge, so I'll play defendent of MG here for a moment.

Andy, the points you raise are all absolutely right. The brakes really are a parts bin special. The front are straight off a Metro GTi (but also the Maestro and Montego turbos used these exact same brakes). The rear brakes are indeed Rover 800 - or at least, the rear calipers are, the discs themselves MUST have been modified to fit the MGF - even if we're only talking about a new PCD and bell dimensions.

If we take the example of the Metro GTi, then using this car's front brakes is - at first glance - rather alarming. After all, the Metro is an 850kg car, which is at least 200kg lighter than an MGF, and 300 less than a TF. But, although it is a FWD with its engine over the front wheels, I can't imagine that the weight supported by the front wheels will significantly exceed 5-600kg (60-70% front weight bias). Under braking, perhaps as much as 700kg will be loaded over the Metro's front wheels - maybe more. But as we've calculated, the weight over the MGF's front wheels is in the order of 700kg under a 0.9 to 1.0G stop.
So in fact, the weight loading over the MGF's front wheels may not be all that different (perhaps even less than) that seen on the Metro GTi. I think that you can be confident that the Maestro and Montego were stressing the 240mm vented discs far more than an MGF would.

So it would appear that the 240mm front discs - at a stretch - are fit for the purpose on a road going MGF.

The rear discs, if for an 800 (as I say, I'm not entirely sure that they are), then again, because the 800 is a FWD with a front weight bias and a high centre of gravity, then despite the presumably near 2 tonne weight, the discs are unlikely to be taxed in the way that we see that the MGFs are.

SO what I am saying is that although the 240mm rotors are certainly a compromise, they are actually a rather clever one. Afterall, it is entirely possible to lock the front wheels with standard discs even in the dry (if you try hard enough! LOL), so they generate enough force to exceed maximum retardation that the tyres are capable of. That the rear pads are usually the first to wear out indicates that the rear brakes are working as they should (the unevenness of the wear suggests that the pads at least ought to have been a little larger - but that's another story) - and it is usually impossible to lock the rears... So yes, I'd suggest to you that MG's engineers did a good job with the hardware that they had to hand.

The AP 304mm front conversion I feel was more marketing lead. Whether the engineers have actually got the full potential from the larger discs and four-pot calipers I am not sure: I think a skid pan stopping distance test would shed light on this (assuming that we could get a consistent technique going). However, I bet those of us running larger front discs but with a standard bias valve haven't got the perfect balance. To revive the balance, use better rear pads, as both Andy and Dave have done. But this must limit the choice of pads at the front... but I am going way off topic here.

Just to recap: yup, I'd say that the MG engineers who put the car together knew what they were doing. But it was a pitty that the bean counters never saw fit to allow larger discs all round - probably not such a surprising over sight given that in 1995 track days were unheard of, and if you wanted a racing MGF, you bought an MGF Cup car!

[ScarletFever]

Rob said:

The front are straight off a Metro GTi (but also the Maestro and Montego turbos used these exact same brakes).

Hmmm, well from my experience of owning 3 Montegos, the brakes really were shocking and there were no OE upgrades available (i know because i tried to upgrade my MG Montego 2.0 EFi brakes to 2.0 Turbo standard only to be told by my dealership that the Turbo used the same set up! Must be true, since when have you known a dealership to turn down trade!! LOL)

So, my faith in MGR designer's abilities are a little shaken in this regard - especially as i put my blown engine in the MG Monty down to excessive use of engine braking - to compensate for the rubbish brakes!!!

Furthermore, there seemed to be some confusion in the press about the Fs braking ability, at first all the reviews were rosy, but later on, when it became popular to slag the F off, the brakes came in for some pretty heavy criticism. This i feel is the reason for the AP calipers fitted to the Trophy - an attempt to silence the critics.

From my own point of view, i've been less than 100% happy with the OE F brakes from the word go. At first i put this down to not having ABS, but having driven an F with ABS fitted at speed i was surprised to find it didn't make that much of a difference. So, i started looking at the options, as you know i've tried lots of combinations over the years, starting with Green Stuff pads (as everyone seems to!), progressing through vertually the entire EBC catalogue and finally ditching the lot in favour of 4 pots and Mintex. I am now happy with the brakes - from my point of view i feel they have made a difference. I would like more, but recognise that anything further will only increase the risk of locking up the wheels (which is easier to do now), but the brakes work well, earlier if you know what i mean. Maybe i should invest in 215 tyres all round?

Rob said:

I think that you can be confident that the Maestro and Montego were stressing the 240mm vented discs far more than an MGF would.

Yep, this would accord with my experiences (most of them hair raising!)

So, your are not actually arguing that the Fs brakes were engineered for the F, rather you are arguing that the brakes chosen from the parts bin were selected because they were more than adequate? If so then this throws into question the brake bias point which started this thread - or am i reading you wrong?

i.e.
Metro GTi front brakes on an F work at say 90% design capacity
R800 rear brakes work at say 70% design capacity

Therefore brake bias front to rear on the standard car is out before we even start looking at aftermarket items!

Rob said:

The AP 304mm front conversion I feel was more marketing lead. Whether the engineers have actually got the full potential from the larger discs and four-pot calipers I am not sure

This i agree with in general (see above), but i understood the AP conversion was designed for the Cup Cars and spun off onto the Trophy later? If this is the case then the Cup Car engineers would have started with a bog standard car and then worked it up to Cup Car spec - i would therefore argue that the AP conversion probably has had more engineering work done on it than the OE items - basically the Cup Car engineers had a free hand to spec what they wanted, whereas the OE design team were stuck with the parts bin.

[Rob Bell]

ScarletFever said:

So, my faith in MGR designer's abilities are a little shaken in this regard - especially as i put my blown engine in the MG Monty down to excessive use of engine braking - to compensate for the rubbish brakes!!!

Yup - I'd agree that the brakes on the Monty were indeed widely regarded as being shocking. But with a weight load of up to a tonne over the front wheels, that aint all that surprising! The MGF is a good deal more friendly towards its front stoppers thanks to the weight distribution

ScarletFever said:

Furthermore, there seemed to be some confusion in the press about the Fs braking ability, at first all the reviews were rosy, but later on, when it became popular to slag the F off, the brakes came in for some pretty heavy criticism. This i feel is the reason for the AP calipers fitted to the Trophy - an attempt to silence the critics.

You're probably right there Andy

ScarletFever said:

I am now happy with the brakes - from my point of view i feel they have made a difference. I would like more, but recognise that anything further will only increase the risk of locking up the wheels (which is easier to do now), but the brakes work well, earlier if you know what i mean. Maybe i should invest in 215 tyres all round?

Or invest in better rear brakes Andy - although, again, striking the balance is the tricky bit.

ScarletFever said:

So, your are not actually arguing that the Fs brakes were engineered for the F, rather you are arguing that the brakes chosen from the parts bin were selected because they were more than adequate? If so then this throws into question the brake bias point which started this thread - or am i reading you wrong?

i.e.
Metro GTi front brakes on an F work at say 90% design capacity
R800 rear brakes work at say 70% design capacity

Therefore brake bias front to rear on the standard car is out before we even start looking at aftermarket items!

Basically, yes: the discs were choosen because a) they were available and b) they were adequate for the job (but no more than that).

The brake bias would then have been calculated on the basis that the front and rear brakes had the same 240mm diameter, and that the caliper piston areas were a known quantity. Now, when we come to modify brakes, we pay scant regard to piston areas (although the HiSpec, and presumably also the AP and MS calipers all have a similar area to the standard caliper or the pedal travel would be altered), and just as bad, scant regard to the leverage on the disc. Fit a larger disc on the front, and the leverage at the front is considerably larger on a 280 or 304mm disc when compared to 240mm. Thus, the OE balance compromise BASED on equal leverage on the front and rear is upset. Which means that to compensate, you either need to increase the force that the rear discs are capable of producing such that it is in line with the new fronts OR you try to modify the bias valve - but there is only so much you can do in this regard...

ScarletFever said:

This i agree with in general (see above), but i understood the AP conversion was designed for the Cup Cars and spun off onto the Trophy later? If this is the case then the Cup Car engineers would have started with a bog standard car and then worked it up to Cup Car spec - i would therefore argue that the AP conversion probably has had more engineering work done on it than the OE items - basically the Cup Car engineers had a free hand to spec what they wanted, whereas the OE design team were stuck with the parts bin.

Ah, now, there is an interesting story behind the development of those discs! But I'll save that for another time. Suffice to say that these brakes bear little relation to those fitted to the MGF Cup car, other than the fact that they're made by the same people (AP).
The AP big brakes are the usual style of aftermarket modification: big brakes on the front - but pay little attention to the rotors at the rear. The difference being that the brake engineers will have altered the bias of the hydraulics to suit the new arrangement, and will have specified appropriate pads so as to maintain balance - R&D that may be peanuts to a car company - but out of reach of most after market tuners.

I hope that some of this makes some sense?

[ScarletFever]

Rob said:

Ah, now, there is an interesting story behind the development of those discs! But I'll save that for another time. Suffice to say that these brakes bear little relation to those fitted to the MGF Cup car, other than the fact that they're made by the same people (AP).

OK, i'm intrigued! ;D

[Rob Bell]

Goto the Secret Squirrel Zone...

[Rob Bell]

I've just made some more corrections on the Big Brake Mistake site, tidied up some other bits, and have added that section regarding ABS, and the potential detrimental effects that replacement calipers and rotors can have on ABS function.

Dave, how do you find your ABS with the AP racing calipers? I know that Paul doesn't seem to have too many problems with his HiSpec kit...

[dave]

I don't have any problem with the ABS. It still rarely ever functions - even on track. Maybe I still don't brake hard enough

[ScarletFever]

When the car brakes it is pitched forwards resulting in more weight being over the front wheels. The tyres on the front are therefore being worked harder than the rears.

Front tyres on a normal F have a smaller contact area with the road than the rears, therefore they are capable of generating less friction.

Would fitting larger width tyres to the front improve the stopping distances?

Or have i missed something?

[Neil Loud Fer and Labrat]

amax = mfamg/fam = mg

convinces me

so its std discs (disposable item) and 1177s