The MGF Register Forums

There have been many posts highlighting the need to improve rear braking on Fs/TFs.
This has been largely driven by the observation that MGR didn't increase rear braking performance when they upgraded the front brakes to the AP 4 pot callipers with the larger 304mm disks (from 240mm)
Many have elected to upgrade their rear brakes by fitting larger discs, either the 266mm Maidstone Sports Cars conversion or the 280mm ones available elsewhere.

I've always felt that increasing the disk size offers only a limited improvement. For instance, using a 280mm disk instead of a 240mm disk will give a 280/240 or 17% improvement in braking, all other things being equal. Whereas choosing your pads carefully and selecting those with a greater Coefficient of Friction (stickiness) could do the same, and much more, but with no other changes.

For instance..last year I had a set of Mintex 1202 pads fitted to my LE500..The CoF of those is documented as 0.25 to 0.35 (say 0.30). I had those checked as part of the MOT.
This year I changed the rear pads to EBC Greenstuff 2000s..The CoF of those is documented as 0.55, a nominal improvement of 83% over the Mintex. I had those checked as part of the MOT a few days ago at the same garage.

At the same time I changed the front pads to EBC Ultimax2 and they had a slightly lower CoF of 0.46 compared to 0.50.

I attach the figures here for interest but the main findings were:

1. Overall braking has improved by 6%
2. Rear braking has improved by a massive 40%
3. Curiously the front braking has decreased by 16%, although the new pads were only 8% less sticky. I can't explain the results for the front.
4. ..but confirming the 83% increase in CoF of the EBC Greens my handbrake performance has improved by 81%.

Above post edited due to change in EBC Ultimax2 CoF

Interesting, the decrease at the front has evened out the front/rear balance.

EBC used to have a poor reputation, looks like that is no longer deserved.

I would have thought to calculate the improved braking area of a larger disc you should subtract the internal diameter of the swept area of the pads.

I don't have the numbers, but for example of a standard 240 mm disc has a swept area of say from 200 to 235, that's 35 mm,
and a 280 mm disc has a swept area of say 200 to 275, that's 75 mm. So that's 75/35 = 214%.

I'm also assuming these larger discs come with pads that are longer as well as wider. The physics student in me thinks the improvement in
braking would simply be the ratio of the area of the upgraded and original brake pads.

I'm not knocking your idea, in fact it's awesome and so easy to just change your pads for the ordinary owner.
But I think larger discs offer a bigger improvement than your calculation suggests.

Don't forget that at some point all this retardation force must be transmitted to the ground, so once you get to the point
where the ABS is activated at the rear tyre then that's all the stopping you will get without bigger tyres or some down force.

Surely the physical surface area of the pads remains constant.. As the original rear calipers are used with the larger disc conversions?

Or did I miss something?

Interesting results David

RobboMC wrote:I would have thought to calculate the improved braking area of a larger disc you should subtract the internal diameter of the swept area of the pads.

I don't have the numbers, but for example of a standard 240 mm disc has a swept area of say from 200 to 235, that's 35 mm,
and a 280 mm disc has a swept area of say 200 to 275, that's 75 mm. So that's 75/35 = 214%.

I'm also assuming these larger discs come with pads that are longer as well as wider. The physics student in me thinks the improvement in
braking would simply be the ratio of the area of the upgraded and original brake pads.

I'm not knocking your idea, in fact it's awesome and so easy to just change your pads for the ordinary owner.
But I think larger discs offer a bigger improvement than your calculation suggests.

Don't forget that at some point all this retardation force must be transmitted to the ground, so once you get to the point
where the ABS is activated at the rear tyre then that's all the stopping you will get without bigger tyres or some down force.

It's very easy to get fooled into thinking this is a really complicated issue..it really isn't..Physics tells you otherwise..

Larger diameter disks offer better braking in two respects..the major one being the extra leverage due to the larger disks...The caliper is placed further away from the centre so can exert a greater anti-turning force on the wheel for the same amount of effort at the brake pedal...the improvement is calculated by comparing the difference in radius because the pads are located further out..As diameter is a function of radius the sum is a simple 280/240. It's actually a bit more refined than that as the comparison should be between where the centre of the pads is on the larger disk compared with the same on the smaller disk...but 280/240 is a good approximation. I have the measurements for the pads and piston diameter so may do that refined calculation later. I'll post the results if I do.. The other advantage is that the bigger disk acts like a better heat sink (more mass of metal) so can disperse more heat so that brake fade is less. Swept area has no other effect on brake performance.

The brake pads are the same size because the same calipers are used. You can't fit bigger pads. Your calculation above attempts to describe a greater swept area by increasing the size of the pads. The swept area is bigger because the diameter of the disk is bigger and the pads cover a greater area as they sweep the disk, not because the pads are bigger.

Bigger tyres don't help...bigger tyres just disperse the same amount of downforce over a bigger area so that the pressure per inch/cm on the road is lighter...You are right that the tyres and road surface are the limiting factor...that's why braking systems are only really designed for one set of conditions...so we theorise that the maximum g that can be accommodated before the wheel breaks traction for a particular road surface and tyre is 0.9g or 1g or some other figure...fortunately we have ABS which kicks in whenever a wheel stops turning.

As a follow up to my original post where I postulated that 280mm rear disks offered only a 17% greater braking force than the standard 240mm disks. I was wrong...It's 3% better than that

The major difference in braking force that comes as a result of using a bigger disk is due to the greater leverage effect of having the callipers located further out from the centre of the disk.

I said that the improvement is 280/240 where 240 is the diameter of the original disk and 280 is the diameter of the upgrade disk.

Actually the improvement is 120.5/100.5 or 20%

The radius of the respective disks is 140mm and 120mm respectively..The pads are 39mm (I measured them) thick/deep/wide, whatever you call it..and we assume they are located right to the edge of the disk...so the centre of the pad where the centre of the caliper piston is pressing is located 19.5mm from the edge..so the difference in effective radius is 140mm - 19.5mm = 120.5mm and 120mm - 19.5mm = 100.5mm.

So.... a 20% improvement in braking from the 280mm upgrade...still much less than is available by using sticker pads..

As you have driven with the mintex pads how do the green stuff feel too you? could you put your hand on your heart and sell me them as an improvement?

Well they are definitely better than the Mintex....the figures speak for themselves...not sure I can add anything more objective...I've now got the Greenstuff 2000 pads on front and rear so I'm sold...what would sell them to you?

Presumably, the downside is that stickier pads wear out faster?

I use Greenstuff on my 200bhp Vectra. Swear by them and I've tried various pads in the 11 years I've had the car. Cost a bit more, but are low dust and give a massive increase in braking.


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Figures can say all sorts of wonderful things, but as you have started out with the same pads I have, and I have replace almost everything on the brakes, I am more interested in your actual findings of how they feel, after all you have no reason to put up BS so I thank you for posting your findings and Green stuff is on the shopping list. When the brakes are "adequate" every little helps.......may even still look at 280's in the future but the pads would be a cheaper first step.

Hi Steve...well I've just started bedding in the front greens driving up to MGF20 and already I feel they are better than the Ultimax2s....braking now feels more powerful..can't speak to the dust issue yet..or fade on repeated hard braking because I'm bedding them in for a couple of hundred miles....so far they feel better than previous and certainly better than the OEM Mintex...more later...

Nice work! I am a later responder, but I have been interested in the "Big Brake Mistake" for years. One of the tricks to compensate for big front 304mm discs and standard 240mm rears was to use Mintex 1177 at the rear, and 1144s at the front - which balanced things out very nicely indeed. I confess this was a wet finger in the air exercise and not worked out so nicely with empirical calculations - but this is really nice.

Look forward to hearing how the car feels and performs!

Using 280mm discs all round seems to have resulted in a nice balance on the 'Shed - whether there are further improvements to be had by selecting appropriate pads for the front and rear would be a very interesting exercise

I'm desperately trying to get my "tame" corner weight buddy to agree a Centre of Gravity measurement...that's the big missing piece of the equation...then we can calculate exactly how the braking needs to be apportioned at the point of maximum braking..at present we are all working on the 60/40 rule...which so far hasn't been ratified...At the moment I'm hopeful..

I have a feeling that the original "guestimate" is not far from the reality. Further, it may be a moving target insofar as the CoG will alter according to fuel load, number of passengers, weight of luggage etc. etc. An approximation may be as good as it gets, but having a measured approximation would be best!

Rob Bell wrote:?...
Using 280mm discs all round seems to have resulted in a nice balance on the 'Shed - whether there are further improvements to be had by selecting appropriate pads for the front and rear would be a very interesting exercise

Looking forward to the results of this solution , bought me a set of 280's at the back last weekend, to support the 280's at the front.
Now The quest is for the right choice of pads for the optimal result. Hoping the rear wheels will be less covered in brake dust as before.

All that for the friendly black ghost, no alterations on green Lizzie although an upgrade on that car would be welcome too.

I think you will be pleased with the result of fitting the 280mm rears Kasper. Presently I am using completely bog-standard pads front and rear on the Shed (pads purchased off ebay!!!) and the brake performance is more than adequate (given the car's weight is now around 995kg). I am also thinking of fitting a brake bias valve - but this would be more inclined towards competition use - but there may be something to be said for altering the standard brake bias set up on Mk1 MGFs (later cars with Bosch 5.1 ABS use an electronic brake force distribution algorithm is is, I think, superior to the mechanical solution found on earlier cars).

Rob Bell wrote:I have a feeling that the original "guestimate" is not far from the reality. Further, it may be a moving target insofar as the CoG will alter according to fuel load, number of passengers, weight of luggage etc. etc. An approximation may be as good as it gets, but having a measured approximation would be best!

My intention is too measure the worst case..so maximum load and hence maximum weight transfer so that at the point where the rear is most unweighted we can calculate the brake effort to just not lock up the rears...You may be right about the guesstimate..maybe we should run a book on how far out it is to the worse case?..

Remember that this will be based upon my TF LE500...dunno how applicable it will be to the F with a different spec of steel undercarriage..

Probably not massively different - but would be happy to join you in an MGF if your chap is happy to help?

You know I would love to do that but his arm is so far up his back at the moment that I dare not "persuade " him any further...