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MG MGF Technical - Big Rear Brakes!

Time to put something up on the BBS about this - as about 3 months and 4000 miles ago, I went up to Wayne Nation's (of VHS) place and had a set of 266mm rear rotors fitted to my beloved F.

4000 trouble-free miles later, and the time has come to report that they are really superb.

At the beginning of the year, I finally fitted the larger 304mm AP Racing front discs, replacing the venerable 240mm standard set up using Mintex 1177 pads. I had little problem with this combination - although the rear brakes always seemed to suffer much more than the fronts - possibly a heat thing.

The problem with fitting a larger front rotor is the possibility of over-braking the front axle - and I wrote about the 'great big brake mistake' before (here: http://www.mgf.ultimatemg.com/group2/brakes/big_brakes/Big_Brakes.htm)

I have found this a particular problem on any F fitted with the AP front rotors, without ABS, on heavy braking: the fronts would always lock prematurely - and compared to my 240mm standard set up on more aggressive pads on the front and rear, these larger front brakes were not apparently capable of the same ultimate retardation.

A partial solution proved to be the fitment of 1177 rear pads to the 1144 pads to the AP fronts - in other words redress the balance by using more aggressive pads on the rear than on the front. A number of the trackday fraternity have done this and report a good result - as did I on my car in the months that I had the standard rear discs.

But the problem of rear brake heat dissipation remained, along with accelerated wear of the rear brake components. Wouldn't it be nice to have a rear set up that offered the same benefits, but used less aggressive pads, was capable of shifting more heat, last longer as a result - and ultimately be cheaper to run in the long term?

I had looked at a number of options, including floating rotors which offered less unsprung weight as one of their many benefits. But cost proved to be prohibitive - not only in terms of initial set up, but also in terms of replacement rotors as they wear out.

A better solution would be a more 'mass production route' - and to this end, Dave Livingstone and I approached the VHS chaps for a solution.

It has taken quite some time, but I think that the results are well worth it. The combination of 304mm front and 266mm rear, using 1144 Mintex pads all round is simply awesome. There is very little in the way of premature front lock up. There is absolutely no evidence of rear lock up either, whatever the weather conditions.

Sadly, my work and exam commitments have stopped me from going on track so far, but I know that Dave has consistently found himself out-braking himself into corners - an annoying situation if you find yourself having to get back on the throttle having come to a halt some way ahead of the corner! LOL

I have to say this kit comes very highly recommended! And that it stops the rear brakes looking quite so weedy behind those nice alloys of your is something of a bonus... see picture attachment :o)

I gather that the kit is available for DIY fitment - I am going to put some instructions on line soon for those wanting to brandish their spanners - will let you know when they're up :o)

Go on, you know you want to!

(And no, I don't have any financial connections of any sort with VHS! Me just happy punter :o))

Rob Bell

Hi Rob,

great solution. I think the look alone is worth it !
;)
Dieter

I kind of wish for 280mm rear discs for exactly that reason Dieter ROFLMAO - but the 266s look pretty good and a little subtle :oD
Rob Bell

I've still to try my 280 fronts (courtesy Mike Satur, with 1144 all round and the grooved rears, no ABS). I'm hoping that the extra front bias will let me brake further into the corner without losing the back end (F1-style in my dreams). The situation is also subject to how my front spoiler works, and replacing the standard F bootlid with the TF version I've just acquired. I'll be looking for a TF centre brake light before next spring - anyone got one for sale?
Mike Cunningham

Last year, I did some calcs and I derived following biases:

304 front/240 rear: 55%/45% (higher clamping force at the rear so pads heats up faster)
304 front/280 rear:59%/41% less clamping force required because of greater leverage

304 front/266 rear:58%/42%.


I don“t know if these calcs are any good.
there are a lot of variables involved, such as deceleration, hight of c.o.g., etc etc. so might be worthless alltogether.

How did they work around the mechanical handbrake Rob? 13mm outwards might not have posed a big problem?
David Peters

Mike, you're highly unlikely to lock the back end of an F with the weight distribution that our cars have - and the presence of a brake bias valve, not to mention the relative piston sizes in the calipers front to rear. Far, far more likely to lock the front end, particularly with a larger front rotor.

David, I had a stab at estimating all those variables - and the calculations can be found here: http://www.mgf.ultimatemg.com/group2/brakes/big_brakes/background_part1.htm

You can see the assumptions I have made - but in essence, the higher the CoG and the greater the braking deceleration, the greater the load over the front wheel (and therefore load on the front brakes...)

Have to say that the current set up is excellent. VHS used Automotive engineer's software to estimate the brake force developed by a larger rear brake. I got the impression you could go up to 280mm, but then it gets marginal in terms of the risks of locking the rear wheels before the fronts (the latter being far safer of course)

Rob Bell

These look most impressive Rob and i am sure the difference is huge. Wish my rears were not in so good condition, cannot justify changing.
Yigit

That's great Rob! It has obviously been a while since I browsed your site *LOL*
What do you estimate µ to be?
I used µ tire-road:.9 (probably to high)
µ pad-disc:.485 (front and rear)
decel 0.85G
cog 40cm
i used unladen + driver (1270) in stead of unladen weight (1095-1150kg).
so things should match pretty well,
cheers,
enjoy the setup
david.
David Peters

Hi Rob, long time no hear.

It looks like you have managed to retain the existing calliper and pads. Is that correct and how is it spaced out further?

Any idea of price for the disc's?

Cheers
Steve

Steve Ratledge

HOw does the kit work? Does it use a spacer braket for the caliper?
Dave

David, I was struck by how similar our figures were, so I think that we're probably on the same wave length :o)

Hi Steve - indeed it's been ages! Hope you and family are well!

Dave, Steve, yes, the calipers are spaced out - and retain the original mechanisms and cables. As David infers, the handbrake can be a bit of an issue, as the spacing out of the calipers does mean that any slack in the cable is taken up, but my brakes so far haven't required adjusting, although Dave's did.

The spacer is pictured below:

(PS discs are standard Peugeot rotors, but re-drilled with the MG PCD - cost I think 80 quid a pair at current production levels, would get cheaper if uptake became a lot higher of course ;o))

Rob Bell

Hi Rob,

Looks very interesting, the Custard Tart cuold do with a little help at the rear as the fronts have locked up on occasions :-) a certain corner at the 'ring' comes to mind.

Look forward to seeing instructions on your site.

Regards

Mike
Mike

Very interesting Rob, what model Peugeot rotors as I have a set for a 405 just sitting in a box!. I have forgotten how much damage the ravages of an English winter can do to a vehicle.
Andrew W Regens

Judging from the thickness of the spacer bracket in picture, the hight of the disc is a lot lower than MGR“s OEM disc.
So I bet he“s got peugeot“s 405 discs,
total hight 27mm (oem 43mm) thickness 10mm (oem 10mm), hub diameter 143mm (oem 140mm)

I“d be interested in knowing the metal specs of the bracket. Any idea Rob.

Before I bought AP“s (couldn“t resist the cheap price they go for nowadays), I did some research for ohter OEM discs. So I“ve gathered quite a list. Including some adaptor bracket designs (intended for front use with Willwood calipers and fit behind cups though)

I“m quite happy to see the hanbrake cable can still be mounted. Very happy actually!


David Peters

I am very pleased to hear that Rob is happy with the performance of our rear brake kit. Looking at the various comments above, I can confirm that the disc comes from PSA group and is machined to suit the MGF/TF. We also provide a spigot ring which ensures that the disc is correctly located radially on the drive flange. The adaptor brackets reposition the caliper radially to accomodate the bigger disc.
The kit is now available at £340 + VAT for DIY fitting. Replacement discs are £80 + VAT for a pair. I am also pleased with the performance on my mgtf sprint car - see www.vehiclehandlingsolutions.com for details.
Cheers,
Andy
A Kitson

When I looked into getting all the parts together, I found I couldn't beat the VHS price on my own! It's good value IMO, and I am very pleased. :o)

Andrew - just surface rust mate! ;o) LOL That's the 13th English winter my car's seeing... Overall, the old gal is holding up well :o)
Rob Bell

This thread was discussed between 10/11/2007 and 17/11/2007

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