Brake Bias

[cheekychimp]

If I have issues with brake balance, is simply getting an adjustable brake bias valve a band aid?

It has been so long, I don't even have all the material, PMs/documentation we used when calculating all the figures for these brake components but we clearly screwed up somewhere because the rears lock up well before the fronts which is obviously pretty dangerous. I am pretty sure this rear caliper is far too big.

So at present, the way I see it I have a few options. I can change out the rear caliper for a smaller one and/or reduce the rear rotor size. Alternatively I can try dialling in some more front bias. I have vague memories of information relating to cross linking of the brakes (front left linked to right rear etc). That may or may not have been something to do with the ABS system (which is now gone) but if anyone knows for sure if our brakes ARE linked and consequently whether I would need TWO brake biasing valves or just one please chime in.

For resident brake experts, can anyone tell me what I need to go back and measure to figure out where me messed up in sizing/pairing these components and ascertain how big a discrepancy we have? I am guessing that will go some way to helping us decide whether we just need to fine tune this setup with some better biasing or if we need to down size the rear brake components somewhere.

Paul.

 

[rdomeck]

Paul, A quick thought is to put a different pad in the rear that doesn't grip as well....Or put one in the front that grips better! I used to do this on stock Porsche cars that ran on the track. A cheaper alternative to my customers than putting in brake bias valves and all the plumbing to go along with it!

 

[cheekychimp]

Thanks. That might be an idea, I can certainly see if it makes a difference, but I really have a feeling I messed up on the calculation for the rear caliper. Irrespective of rotor size the front and rear calipers shouldn't have the same number of pots. The rears are generally always smaller! I am not sure why I got the idea this would work /ubbthreads/images/graemlins/dunno.gif/ubbthreads/images/graemlins/banghead.gif

Ken has a set of Nissan rear calipers (from an S33 I think) that he modified to bolt straight onto the GTO rear hubs/knuckles without brackets. They also fit the existing rotor. I'm thinking those might be another alternative to try with minimum fabrication.

I'm interested in the brake pad idea for further fine tuning however. How do you choose the correct material, what are the specs to look for etc in choosing a pad that will lock up later/sooner?

 

[kartorium]

My understanding is that the cross linking was because of the ABS system. Get rid of that and you essentially have a front line and a rear line. Put a proportioning valve in the rear and dial the pressure back. I look at it as a band aid, but that coupled with a new pad choice can give you a decent amount of tuning ability.

This has been working well for me for years now.

I can't remember hardly any facts about this cause I did it so long ago, but I'm pretty sure I researched for a long time and came up with this as the best option for my setup.

 

[FlyingEagle]

If going with the pad route, you can "effectively" cut the pads back until they have a percentage less area then contacting the rotor. So trim outer and inner edges, just less surface they affect friction to, per rotor revolution. Saw this on a show called "SETUP" where they were running Soltices in competition for a seat on Team Soltice; they cut the rear pads back, and it allowed the rears to stop locking up/too rear end biased/twitchy when up to temp. You can use the pads you currently have. /ubbthreads/images/graemlins/grin.gif

 

[rdomeck]

I'm not sure what calipers you have. If they are production car calipers you can buy really cheap pads from the local auto parts store for the rear and better one for the front from someone like Pegasus Racing? When I was heavy into Porsches I would run a performance pad on one end and a street pad on the other. Can't remember what brands I was using then. Generally a really cheap pad will work well the first month or two of street use and then grip less after it heat cycles some!

I would not be comfortable taking away part of the pad to reduce it's friction. I see how it would work, but that some very calculated risk you would be taking. Not to mention the bad rotor wear you would encounter doing something like this!

Edit: I would try an adjustable valve before doing to much other stuff. You see them in a lot of race cars....Permanently mounted...Working every time you hit the pedal!