R33 Gtr Rear Brakes Vs R33 Gtst Rear Brakes

[Al]

I have just upgraded my R33 GTSt front brakes to R33 GTR Brembos with DBA5000 rotors and Greenstuff pads. My rears remain R33 GTSt with DBA4000 rotors and comp3 pads. On a couple of occations, under hard braking, i have found the the rear is getting "floaty" and starts to come round. Thus the car is getting unsettled. Never had this problem b4 the upgrade and can only put it down to a possible incorrect brake bias; the fronts are overpowering the rears.

Just would like to know if there is any differance between R33 GTR and GTSt rear brake set-ups?

If placing GTR rear brakes, on my GTSt, will correct the bias issue and improve braking?

If i also need to change the brake master cylinder?

Thanks

Alan

[Duncan]

Yes, the GTR uses slightly larger 2 spot calipers and discs, all the dimensions are on the DBA website.

Re the master cylinder.....not really, the difference in piston volume isn't huge. just try it as is and change the cylinder if you are getting too much pedal movement.

[GTRgeoff]

Well for one mate, greenstuff are not an upgrade over the DS2500s I think you were using, or the RB74s but I use them on the Soarer to keep dust down.

Yes there is a ratio difference in braking area.

R32/33 = 2.2393:1 F/R

GTR Brembos = 2.1125:1 F/R

You now have 2.3187:1 F/R so the rear is going to pinch earlier as you try to get braking to start on the fronts. I hear too often, and have tried a couple of Brembo front and GTSt rear to confirm, that this setup is pinchy in the rear so get the brembos and Brembo GTR Master.

Going a step further the GTR has 324 and 300 mm discs F/R while GTSt has 296 and 297mm F/R so the torque arm is changed as well to further complicate the matter.

Finally the pedal will probably feel soft as the 33GTSt has 23.8mm (444.88mm^2) master diameter while a Brembo GTR has 26.9mm (568.32mm^2) so would have the shortest pedal travel of all models for the same braking force = harder pedal feel. Ratio is 1.277:1 Brembo:GTS25t so the area of the Brembo master is about 30% bigger. enough to actually think about.

Jack has tried the pedal in my GTSt Monday when he put it in the garage for rebuild and it is still very firm after 18 months sitting in the street. Just a good setup I guess

Come around sometime and sample the differences.

[djr81]

If you are suffering from too much rear brake bias the cheapest short term fix is either a higher coefficient of friction pad on the front or a lower one on the rear. Nothing like doing the job properly, however....

[SLY33]

Yes there is a ratio difference in braking area.

R32/33 = 2.2393:1 F/R

GTR Brembos = 2.1125:1 F/R

Sorry Geoff, could you please explain a bit further as im getting lost here!

the above ratio's are 32/33gtst vs 33GTRbrembo's correct?

So the GTR brembo setup has less front bias than the gtst setup? or are you just quoting area and nothing to do with bias?

Once i upgrade my GTR brakes to AP/Alcon or whatever, i planned to swap the GTR brembo setup front and rear onto my GTSt. Will this be ok?

regarding the master diameter, does this just mean that my GTSt with GTR brembos will have a softer pedal feel with longer travel? (unless i change to a GTR master, is this necessary?)

thanks

[Al]

I am a little confused myself Maybe because i initially thought my front brakes were clamping more strongly, shifting weight too quickly to the front, thus getting the rear light and twichy.

What i am reading (and please correct me if i am wrong) is that the rears are too pinchy and are locking up, causing the ass end to slide round.

I have just recently bought the Ferado's DS2500, but have not put these on as i was thinking that they will make the situation worse. But you are saying it will probably improve the situation.

Either was it looks like i am better off buying R33 GTR rear calipers, with DBA4000 GTR slotted rear rotors, to match the front. What rear pads shall i use?

Thanks to ALL for your advise, and keep it coming

PS: Anyone wrecking a 33GTR with rear Brembos and Master slave cylinder and booster?

[GTRgeoff]

Those ratios are front to rear area, or the volume of fluid units it will take to displace the pads the same distance per caliper, not the whole setup bias which has to account for pad size and torque arm effort. The actual areas of the Brembo calipers' pistons are about 200mm^2 per caliper larger than GTSt so for the whole setup you are looking at 800mm^2 extra area to push, which means the R33 master will be using a fair amount of travel to do the job.

One day I'll finish the document I'm working on with all the pedal arm dimensions and pedal travels as well as full bias ratios from pad sizes and disc diameters, but if anyone wants to measure R32 and R34 brake pedal arm lengths from the pivot to end and pivot to master cylinder pushrod in mm I'd be grateful.

I'm pretty picky about brakes, and once I have the pedals setup I like to heel-toe, but if the brake pedal falls too much it makes it hard for me as I don't have the leg flexibility to do it anyother way so hit the gas pedal when braking.

[Duncan]

but if anyone wants to measure R32 ...brake pedal arm lengths from the pivot to end and pivot to master cylinder pushrod in mm I'd be grateful.

can do just remind me on the w/end will be interesting to see all the data....

[RAZOR32]

I have got R34 GTR Brembos, Ferado's DS2500 and DBA 5000 on the front of my R32 gts-t and the pedal is a bit to soft-mushy at the top for my liking so changing to GTR master and rear brakes will fix this?

I was going to put the R34 GTR rear brakes on but I can not find any and they never come up for sale, any I dears where I could get some from or know any one that’s got some?

Cheers

Edited September 28, 2006 by RAZOR32

[GTRgeoff]

Razor, there is zero difference between R32, R33 and R34 Brembos front or rear, except the colour. Hope you didn't pay a premium.

I recommend pull everything down and service the seals, use braided brake lines, get a good master cylinder stopper (I make mine from 10mm steel plate) and regularly flush the lines. Also don't use crap pads like Bendix Ultisquish but the Ferodos should be fine.

[SLY33]

I am a little confused myself Maybe because i initially thought my front brakes were clamping more strongly, shifting weight too quickly to the front, thus getting the rear light and twichy.

I was running some mismatched pads, and normally they were fine, but when driving hard and getting the heat up in the rears, the rears would bite more when coming into a corner hard on the brakes, and in fact were JUST locking up the rears, and that would cause the rear to step out! it was a bit unnerving! This could be whats happening to you. even if you have only front brakes working and no rears, on hard straight line braking i dont think the rear would step out. at worst if you have over powered fronts youll just lock up your fronts more often, the rear should simply follow correct?!

[RAZOR32]

I fully Rebuilt the front Brembos when I first got them, and I only paid $1200 for them which I thought was a good price?

I already run a Cusco Brake master cylinder stopper and braided brake lines and RBF 600 brake fluid

And out on track my car also gets "floaty" and comes around abit under hard braking. And yes, Never had this problem b4 the upgrade

So should I look at upgrading the brake master cylinder and rear brakes to GTR Brembo items?

Edited September 28, 2006 by RAZOR32

[Roy]

There are so many different things that could cause a car to float around under brakes or lock rear brakes.

The best advise i think you can get is speak to a few performance brakes specialists. Get a feel for the ones who you think know what they are on about. Take your car to them and get them to find the problem.

It could be pads, suspension, ride height, tyres... a lot of things before you have to worry about changing master cylinders, installing bias valves etc.

[Abo Bob]

None of these guys sounds stupid though. You know if you make one change and then the car behaves differently, roughly what the cause is. The forum is about chasing down the answers.

I have run my car with stock pads on the rear and Ultimates on the front and noticed rear end squirminess. I always assumed that the front braked more and the rear sort of tried to overtake the front.

But this same sort of thing seems to be being described by a few people and Geoff seems to be saying it's the rears that are biting more??

I'm confused.

While we are talking about brembos etc. If I wanted to do this upgrade do I just bolt the rotors and calipers onto what I have or is it more complicated than that?

I found the information about pedal feel and travel very interesting thank you. I am just about to put an upgraded master cylinder on my car so am curious to see the difference it makes.

[Roy]

True, but with brakes the cost of the gear and the dramas if you get it wrong…well I sometimes think its easier and cheaper to get it handled by the experts

That all said. You increase the front piston area you increase the braking force available. Not the same thing as braking pressure. (its important to remember P=F/A). So you now have more braking force at the front, ditto if you increase the front rotor size. So normally about this time you will throw a better pad up front which typically has a higher coeffecient of friction. The better pad again increase the brakign force at the front. So now you have done all these things to the front which improves the brakign force and this changes the natural bias

So now the car is locking rears? Not really expected when you have increase your front brake bais. You can put better pads out back. Actually, I need to be working…ill get back to this later tonight, or Geoff may beat me to it...i also have a spreadsheet with all the caliper piston sizes and master cylinder sizes

[Al]

If i upgrade the master cylinder, do i need to upgrade the booster?

[Abo Bob]

I'll tell you after this weekend once I've fitted it. hehe. Although it would be good of someone knowledgable would tell us.

[Roy]

The question is why upgrade the master cylinder?

Assuming the leverage will be the same (unless you change your pedal etc it will be) then putting a bigger Master cylinder in the car will give you less brake pedal pressure. LESS. What is an R33s Pedal ratio anyway?

If you are going to play with master cylinder sizing you would want some pretty good feedback from trusted ppl you know, or know a lot of things about your car including front weight, rear weight, expected braking Gs..hell too much info.

Remember that Pressure = Force / Area. So if you increase the area by increasing the diam of the piston in the master cylinder. You are reducing the pressure for the same force on the brake pedal. If you want more braking force then you can generally either increase piston area, brake line pressure or pedal ratio. So going bigger in the master cylinder gives you more volume for bigger caliper pistons, but actually gives you a longer brake pedal. May not be a bad thing, but something to be aware of.

I would also hazard a guess that changing the master cylinder wont change the fact that you are locking rear brakes. And considering a GTSt is a lot lighter over the front axle, i wouldnt automatically assume that a bias setup intended for a front heavy GTR will be optimum on a GTSt, so perhaps throwing all the GTR gear on your GTSt isnt the answer either????

So getting back to your problem and my comment about getting someone to look at the car. Sit back and consider that your car could be diving excessively under brakes. The shitter the tyres the more front brake bias you want. The better the tyres the more rear bias you can generally run.

I would go as far as asking does it happen only on your 17"s, or 19"? Both? From a physics aspect your rotor is rigid to the hub that is rigid to the wheel, which is rigid to the tyre. So increasing the wheel diameter also has an effect

There are bunches of general rules...sure there are equations on paper but how they translate to your car? Its not really that complicated unless you try to make it...but the fact that my std R32 GTSt with std 280mm front rotors, locked rear brakes when i put new pads in the front. Put new pads in the rear it was worse (I dont know what the mechanic put in the car)

Then i killed those front pads at the track which ate my std rotors. Got new GTR sized rotors and better pads again. From that point on the car stopped really well but pinched rear brakes if they were cold. On the track they came good after a few laps. So figured it was a temperature co-efficient of friction thing. I lived with it until i had got some life out of the rear pads and then tried another compound. SPOT ON. Recently tried DS2500s and the balance again feels great. Its probably not a coincidence that my car brake bias was always good on semis...any problems i had was on road rubber.

So thats all the confusion i have had with locking brakes without even changing calipers...and lol, throw in when i dropped my cars ride height about 25mm over what i normally run and the car handled bad and locked up rear brakes excessively. Put the ride height back and back to not locking rear brakes.

So im confused. I have thoughts over what works on my car...but hell, i wouldn’t be convinced that increasing the piston area in the front caliper is cause enough for the problem you are having.

Are you running the same pads as you used to? I know Al has also gone bigger in the rotor at the same time as changing caliper. So???? LOL if you can understand the above ramblings then you are doing well, im sure if i re-read it then it would make no sense

Brake performance is affected greatly by tyres and suspension. If your car is brakign badly/poorly or misbehaving under brakes...sure look at what has changed recently but it could very well bne highlighting a chassis / suspension shortcoming

LOL, that's why i suggested someone with more experience across different cars, setups and applications

[Al]

Damn Troy you really know how to confuse a man, now i really don't know what to do

I refuse to take the car to Racebrakes, as they are a total bunch of knobs, so who else can sort out my issue?

Sounds like i am better off putting my GTSt calipers and rotors back on, as from you are saying any upgrade will give me hassles .

So am i better off trying the DS2500 at the fronts? Wouldn't this cause the car to dip harder at the front?

[GTRgeoff]

In a nutshell, yes P=F/A, but if you put larger area brakes on the pedal has to travel a lot further to do the same braking.

but I just did some more quick calcs and the total volume difference between Brembos and GTSt are 0.5% so the master cylinder doesn't need to be changed unless you want a harder pedal, however I have already demonstrated you can get a massively hard pedal in the GTS25t, even with the smallest master cylinder of the lot.

Just throw in the DS2500s in the front and see how it goes for now, and maybe go to Supercheap for some el-cheapo superstop pads. I did the 1.31 (should have been low 1.30 except for some old tool in some pommie piece of junk) on street rubber at Sandown with them in the rear and balance was superb.