BIG rear Callipers

[marcus 0]

BIG rear calliper conversion???

 

Im guessing this isnt straight forward as ive hardly seen any cars with impressive rear brake upgrades......so whats basically involved if, for example, i wanted a set of AP Racing or Brembos and larger discs adapted to fit the rear????

Thanks.

[vr6storm 0]

waste of time tbh..........especially as the bulk of the braking is done by the fronts,all you'll do is upset the balance

[marcus 0]

oh i though that if you had outstanding front brakes and rubbish rears, then that would also upset the balnace...but im just looking into it at the mo.

[alex303 0]

my rears are std at the mo Marcus and i've had no problems so far and i'm pretty aggressive with my braking at times, we're going to be running the same setup on the front with the 4 pots so think you'll be fine mate. Must admit though i'd like some slightly larger rear discs just to get rid of the weedy look of them behind the OZ's, seem some bigger 300mm discs somewhere you can run with the std calipers but its really more visual than practical!

 

oh i though that if you had outstanding front brakes and rubbish rears, then that would also upset the balnace...but im just looking into it at the mo.

[Blue_Joe 0]

waste of time tbh..........especially as the bulk of the braking is done by the fronts,all you'll do is upset the balance

 

Tis true - I thought the same when I put my engine in that I'd need to uprate the back to match the fronts. Approached AP Racing to see if could get a front and back setup - they told me to leave as is as if uprated the back would only end up with teh rear overtaking me everytime I braked hard!

[marcus 0]

Well i guess that solves my mystery then - cheers.

Defo gona look into the 300mm rear discs though, rears look crap as anything with the fronts beefed up.

[dinkus 10]

Careful about upgrading the rear brakes, as Blue_Joe says... if the rear brakes are working harder than the fronts then the back will step out and potentially throw you into a bush/hedge.

 

For that reason, if your rear brakes lock before your fronts, it's an MOT failure.

[coolrado 0]

if you increase the size and power of the front brakes the rear becomes even lighter under hard braking, so in theory smaller rear brakes would be better with big front brakes, the only time you would need to increase the size of the rear brakes is if you had an engine in the back.

they do look crap under big wheels though.

[alex303 0]

Thats all true but i think where Marcus is going with this is upgrading the size of the rear discs but keeping the same / existing calipers thus keeping the correct balance e.g:-

http://www.godspeedbrakes.co.uk/Rear_Discs/reardisc.htm

[Crasher 3]

Increasing the size of the rear discs would increase the lever arm effect (moment of force) so even with the original pads/discs, effective braking effect would be increased. An easy example of this is the 256 to 280-mm front conversion. Both these discs use the same pad and caliper, effectively anyway as both use 54-mm pistons. The problem with the discs looking puny is a problem and quite involved to cure.

[Blue_Joe 0]

Plus that big disc conversion wouldn't work on the C anyway as the rear disc also integrates the bearings etc. Too involved!!!!

 

The rears may look puny but when people see a BIG set of proper brakes up front - they know it means business and therefore the rear look becomes irrelevant!

[Kevin Bacon 5]

Pure conjecture.

 

The effect of larger rear brakes would not be anywhere near as bad as portrayed here.

 

Intelligent application of the correct parts should not see your rear end over taking the front. If it does, you shouldn't be driving on public roads, get a retest.

[mgbv8mike 0]

Am I missing something here ?

 

My VR6 has a brake balance valve and ABS, I don't see how increasing the size of the rear calipers would over-ride either of these, can you explain please ?

[corozin 0]

But surely one of the main reasons you don't see mega-rear disc brakes on Corrados is because the rear discs and stub axles are an integrated part? The complexity and cost of getting some suitable stub axles machined onto which you could hang bigger discs certainly put me off.

 

People have already highlighted the minimal braking effort going to the rear, but for track use it would be nice to be able to run larger discs to improve the cooling if nothing else. When you start to run bigger fronts, the brake bias is automatically pushed rearwards slightly and this does seem to result in the rears cooking the pads under hard use.

 

Big rears have been done though... I certainly recall that Jeroen Dik had fitted big rear discs to his G60 about 2-3 years ago when I last saw a picture of it on show at Essen. It can be done... just a question of time & money (and a CNC machine!)

 

John

[Toad 0]

I agree with John and Kev here. The rear brakes do have an affect on the effectiveness of the front brakes, as there is weight transfer to the front wheels from the back, in the same way that the fronts take weight off the rears.

 

I had no rear brake effort for a while and the front would lock and skid far too easily. Currently my bias valve is set with more effort going to the rear brakes than is recommended by VW, and I've not attacked a hedge *yet*.

 

As John says, a bigger rear brake disk would mean that the rear brake had more cooling and that the disk would last longer. I've seen a kit for bigger rear brakes. Peanut sold one recently I believe.

[Kevin Bacon 5]

Yep, that's mainly it..... trying for improved heat dissipation.

 

Despite having uprated front brakes, my rear discs still warp very easily and that's just from road use, so there's clearly a defficiency in cooling from the inadequately sized discs. Solid 239mm discs with tiny pads may have sufficed in 1984, but now, MOTs are stricter, tyres are stickier, cars are faster and there's more people on the roads, so we need the brakes more than ever before.

 

How many of you have had MOT 'advisory' notes on the rear brakes? Every year I get "Play in rear bearings" and "Rear brakes ineffective, just about passes, but needs improvement".

 

Now, I know for a fact the rear bearings are setup precisely to VW specs and the calipers are MK4, with MK4 pads and young discs, so they may have been fit for purpose (just) in the 80s, but not any more it seems. To be honest, I can't believe VW fitted the front and rear brakes they did to the VR6. 146mph sports car weighing over 1300Kg (weigh bridge confirmed)!!

 

Roll forward 13 years and we have 2 more 146mph cars from VW. The GTI and the R32 MK5s. The R32 admittedly is 1600kg nearly, but it has 280mm rear discs (vented) and 340mm front discs. The MK5 GTI has 280mm solid rotors, or they could be vented, can't remember, and 312mm fronts.

 

Even the MK4 FWD GTIs got 256mm vented rear brakes and 288 fronts and they weigh the same as a Corrado VR.

 

Both the MK4 and MK5 have proper rear hubs aswell.

 

Neither MK4 has locking rear brake problems.

 

The worst that will happen with bigger brakes on the back of our Rados is the ABS will kick in sooner.

 

Given the pedal feel is largely dependant on the rear brakes working properly, I think larger rears would work well.

 

Wilwood do some nice 4 pot rear calipers with handbrake mechanism built in.

 

Another way is to fit the Ford Focus rear hubs, but they'll need drilling to 5 x 100 for VRs, then you can slot on 256mm venteds and use the Focus carriers (same calipers as MK4).

 

It's just finding the time to experiment with it all!

[Toad 0]

Interesting post Kev.

 

I've had a few problems with my rear wheel bearings recently (odd wear has meant that adjustment doesn't take the play out of them), but I'm not sure If I should just replace the bearings as one disk is slightly warped, and the MOT is only a few months away.

 

It's rather frustrating that the disks are knackered but have not worn down that badly. I'm putting the warping dow to a sticky caliper on one side. A larger, slower wearing part would be better, but I suppose it could potentially lead to more problems with the caliper carrier bolts seizing. They are bad enough without being left untouched for an extra year or two.

 

O/T Do you find your rear bearings need adjusting quite regularly (every couple of months?) I adjusted mine up again a couple of weeks ago (6 weeks since the last time) but cannot take the play out of them without over tightening them. I assume that where they were let go out of adjustment for a while, the races and rollers have worn unusually.

[Kevin Bacon 5]

I changed the rear discs on my mate's Focus a while ago, which is how I found out the calipers are the same as ours and the bearing carriers / hub etc looked like they'd fit our trailing arms without too much work.

 

Yeah my rear discs are warped too, I can hear the Scrape.....scrape.....scrape noise as I reverse up the drive!! They do seem to warp long before the pads die don't they?

 

I check the rear bearing play any time it's on a ramp or on stands, which isn't that frequent believe it or not, LOL! The front is always in the air though....anyway...tbh, since I've used VW rear bearings (made by SKF or FAG usually), they've needed adjustment twice and that's it. It's hard to guage how much cold play you need as it's all taken up by heat expansion as you drive. This is why they fail the MOT (unnecessarily so) because they're cold when tested.

[Toad 0]

My disks aren't that badly warped, but you can feel it as you try to spin the disk. The slight rumble seems to go away as you drive, so I guess that's the thermal expansion.

 

I had to change the pads on the rear because of the one side sticking and the pad material crumbling. The annoying thing is, that it's all genuine VW kit too.

 

I tend to get hold of the rear wheels and give them a good shake whenever I'm looking around the car.

 

Speaking of rubbish unneccessary MOT fails, my VR failed on top mounts last week, despite there being no real problem with them when the car was on the ground, yet the leaking rear shock, no ABS and slightly rusty rear brake pipe didn't get a mention... I even got an advisory on the spare wheel, despite the fact that there isn't one...

[marcus 0]

my rears are std at the mo Marcus and i've had no problems so far and i'm pretty aggressive with my braking at times, we're going to be running the same setup on the front with the 4 pots so think you'll be fine mate. Must admit though i'd like some slightly larger rear discs just to get rid of the weedy look of them behind the OZ's, seem some bigger 300mm discs somewhere you can run with the std calipers but its really more visual than practical!

 

Have to track down these 300mm discs, sounds good enough.

I havent had any problems with the rear brakes, but asa as i do, id defo be up for playing around with callipers and discs etc e.g Focus idea.

[dinkus 10]

I wish I had a better understanding of the physics involved when you're braking, as I'm sure it has a lot to do with the suspension geometry and the weight distribution in the car before you start braking too.

 

It's worth noting that brake disks don't warp just because they get hot, they warp when they get hot and then you sit stationery with the brakes on. The hot pads keep that area of the disk warm while the rest of the disk cools down in the air. This uneven cooling casues the disk to distort.

 

Obviously if you don't get the disks that hot to start with then you're less likely to have that issue, but if you have been pressing on and put some heat into the brakes - don't sit on the brakes in traffic and the disks shouldn't warp.

[marcus 0]

DINKUS - im liking that tip of not holding brakes on during a thrash and ibriefly stopping in traffic :) .

[dinkus 10]

Heh heh - notice the use of "shouldn't" :lol:

 

I've been told that's why they warp and it seems to make sense, but dunno how true it is.

[Toad 0]

That is the reason I believe... Apparently reace cars are left with the brakes off when they are parked after a lap, but obviously this is an issue when driving in real life. I'd try slowing to almost a complete stop before a junction/traffic lights, and rolling slowly upto them...