Help me with a wheels/gt500 rear rotors/spacer/front brakes dilema

[Duane Black]

So from another thread, someone mentioned getting the gt500 rear rotors on my s197. I did! They really fill the wheel well nicely, hopefully theyll get more torque and manage heat better, which leads to another dilemma...

I'll be on GT stock rotors/calipers up front (not brembo) which is a 13.25" rotor, and running that with a 13.75" rear rotor. Obviously theres more to it ( I know tires, abs module, caliper, pad, surface area, etc mean something too) but am I creating a dangerous situation running a larger diameter rear rotor than front? It looks like the brembo is "only" a half inch larger anyway rotors.

Second, I have new 18x10 apex wheels which obviously fit beautifully. Amazing room. Do recommend. my old track wheels are 18x10 lmr bullitt style wheels. Those require a 3mm spacer minimal up front, and will require that in the back to fit and it's still DAMNED CLOSE! back there.

The lmr wheels are cheap. I doubt they'd sell as they arent major wheels to our market anyway. I paid $125 each new, and they're not shiney for the show car market anymore, but they are something. I also know at least one is bent and they all have minor scuffing. I doubt they will sell for more than $200 as is. I know they have value as spars . I might want to put toyo rr takeoffs on one day just for play, or perhaps if I run into the money, buy some proper autocross tires again and serve double duty.

So, long story short, what would you do? Buy a $60 spacer to accommodate the wheels? Sell the wheels for what ever I can get for them?

Or, keep the wheels, and forget the whole gt500 rotor anyway? Is that a problem with it being a touch bigger than the fronts?

 

[Opolis]

I just decided to allow the LMR wheels to collect dust and run the Apex's 24/7.
One developed a crack on the inner lip and I have no motivation to try and sell 3 wheels for next to nothing.

I also started out with stock GT brakes and, even with track pads, decided Brembos would be my first big upgrade.
While you're at it, add cooling ducts. I read everyone mentioning this as one of the first things to do for the track and of course they were correct!
Didn't take much track experience for me to figure that one out first hand.

 

[Coz]

I wouldn't run the bigger rotors on the back without upgrading to the Brembos up front. In addition to to the larger diameter rotor - ¾" bigger, not ½" - I believe the Brembo 1001 pad surface area is bigger than the OEM 1463 pad.

In the meantime, at the very least, I would put the weakest formula pad on the back so as not to overpower the fronts.

 

[Duane Black]

Yeah I'm really torn about these LMR wheels now.

I've got the brake cooling - could see the difference in the wheel staying black instead of turning gray.

 

[Grant 302]

I think you're proficient enough to safely test the larger rear rotor on your current setup and adding the spacer. I wouldn't normally suggest for that to be done. And certainly not for anyone on stock springs, because of the way they transfer weight under braking.

 

[Duane Black]

Ja, I'm on stock springs and stock abs which I know doesn't put much effort on rear braking. I also know the pad area is smaller and the effort applied is smaller soooo we will see. Does anyone have experience doing this and can say hell no dont do it?

I spent money and got h&r spacers last time. They're $60 . Is there a compelling reason not to use the $20 Amazon brand I never heard of spacer?

 

[Grant 302]

Nobody else is running a V6/auto ...so I don’t think anyone else would have relevant data. You have almost 100 lbs less over the nose. Should work in your favor over all the V8 cars.

 

[Eric62]

Putting the big rear rotor on the car means for the exact same brake pedal force, there will be more brake torque on the rear than before the change IF you use factory pads. The brake balance is towards rear lockup. Without ABS, that would mean you'd more likely spin the car while on the brakes. With ABS, the rears will go into ABS cycling before the fronts so that's good! Don't run without ABS!

It also allows you to add front brake torque by using a more aggressive front brake pad that can take more heat before fading to shift that torque bias back to the front. Also good. And it looks better.

I have 14 inch Brembo fronts with the big rears but I run an automatic trans - meaning essentially no engine braking. I have run the same compound pad front and rear and also a less aggressive pad in the rear. I like the control of each in different situations. Test to find which pads you like best.

 

[chicagobob]

As everyone is pointing out there is a lot that goes into thinking about changes in the brake bias. Take into consideration the pads front/rear and also keep in mind that the test results can change with the track conditions. There was a GT500 that had increased the rear bias with brake changes, but lost it on a damp track going into a corner, coming off a high speed straight due to the rear end coming around quicker than expected. Good Luck!

 

[blacksheep-1]

Go drive the car, just be careful, then figure out what size front brakes you need to accommodate the rears. Keep in mind you'll prolly be hosing up the anti lock criteria and until you install the fronts, the car will be tail happy on trail braking, and prolly undriveable in the rain.
With regards to the wheels, we throw spacers at the car all the time, but we also use ARP nuclear studs.
Those LMR wheels are inexpensive, but are not race acceptable, whatever wheel you decide to use, they need to be inspected often, very popular performance wheels also crack with age and abuse.
I'm a big CCW endurance racing wheel fan, but they also weigh 60 pounds with the tire, the Apex are about half the price, so if you get over half the longevity out of them, (as the CCWs) that's a win. If I had a choice between BBS and Apex, IMO, I'd go Apex all day long.

 

[Duane Black]

They have a SFI and DOT rating, always figured with "just 200tw" tires, I'd be fine. Maybe SFI means something and maybe it doesnt honestly. certification might be easier than I think. But, when I think "not suitable for racing" I really think "mounting hoosiers would be a bad idea" so correct me if I'm wrong on that.

Why does more rear bias make the car more tail happy? I know it does, I'm just not understanding the physics of why. I do love some trail braking so I may come to not like this "upgrade."

Also - stock ABS module vs Boss 302 means it has very little rear bias anyway, correct? As I have a stock module?

 

[blacksheep-1]

Why does more rear bias make the car more tail happy? I know it does, I'm just not understanding the physics of why. I do love some trail braking so I may come to not like this "upgrade."

because the rear of the car unloads and it can lead to brake lockup, this is why, in the rain, a lot of guys crank in more front bias, then brake in a straight line, and then turn the car.
With regards to the anti lock module, I have no idea, I don't think any of the cars that I'm familiar with ran anti lock brakes and the bias adjustment was via the prop valve.

 

[Boone]

Duane,
I still run stock brakes in the rear, so you're going exactly the opposite direction of my setup. I really don't like the thought of snap oversteer under braking. That said, I'm thinking of moving to 15" 6P Brembos. If so, I may have a set of 14" 4P Brembos available for you when I make the move. Lots of rotors and pads sitting in the trailer as well.

 

[Apex Wheels]

The larger rear rotor really doesn't add that much braking power to the back axle. It's still being clamped by the same small single piston caliper. Just make sure you don't use a rear pad of the same compound as the front. It needs to be less aggressive.

As for the wheel fitment issues, well lol. I'm glad you have a set of EC-7s for track work! They will easily clear the 4 piston and PP 6 piston Brembos if you decide to go that route, along with many other big brake kits.

- Cory

 

[Coz]

The larger rear rotor really doesn't add that much braking power to the back axle. It's still being clamped by the same small single piston caliper

I respectfully disagree to some extent. While the pad size remains small it has a greater effect due to the increased torque afforded by the longer arm (aka diameter).

How much more effective it seems may be subjective but I think the difference is at least noticeable.

In addition, the greater heat dissipation will mitigate any brake fade caused by fluid boil or overheated pads. To the extent that the smaller diameter setup was stressed due to heat, the effect of the larger diameter may provide an even greater effect.

 

[Apex Wheels]

I respectfully disagree to some extent. While the pad size remains small it has a greater effect due to the increased torque afforded by the longer arm (aka diameter).

How much more effective it seems may be subjective but I think the difference is at least noticeable.

In addition, the greater heat dissipation will mitigate any brake fade caused by fluid boil or overheated pads. To the extent that the smaller diameter setup was stressed due to heat, the effect of the larger diameter may provide an even greater effect.

I should have been more clear, I do definitely agree they will provide more brake torque and heat dissipation, but (at least to me) it sounds like some people are over exaggerating how much extra torque is added. The longer pad life is noticeable however.

On my car I'm using the Baer 2 piece 14" rear setup with the adpater brackets. ST41 rear pads. Up front is the Essex/AP 372 6 piston kit. It certainly stops well!