Brake Bias On 850s With The Stoptech Big Brake Kit

[The Stig]

Hi guys, I was chatting with Carson about this, and we thought we'd throw it out to the general membership. One request however - not to be mean, but please only post if you know what you're talking about regarding brakes (seriously, there's enough misinformation out there as it is). I'm a novice on this topic myself, so I'd like to learn more about this (as we all would) from the experienced guys on the board.

As you know, there is a Stoptech 330mm front Big Brake Kit out there for the 850s. In particular, according to Stoptech the upgrade is calibrated to achieve optimum brake bias utilizing the stock cylinder and stock rear brakes. However, I've noticed some people upgrading their rear brakes after doing their front brakes with Stoptechs, particularly with 330mm ones grafted from the S60R.

Now this begs the question, wouldn't this effectively throw off your brake bias (I would imagine rear-biasing it) and make for poorer braking times and potential instability? I know that there are a lot of factors to be considered (e.g. changes to the car's weight distribution, or an aftermarket suspension for instance, which might change the amount of weight transferred to the front under heavy braking), but I'm wondering the rationale is of adding the S60R rear brakes? Has there been any actual subjective testing to support their use with the Stoptech kit?

Thoughts?

[Aequitas_Veritas]

i would say it would, i know stoptech did alot of researching with the OEM rear brakes, and matched it to that setup. Making the rear brakes alot better, in a setup not tested by stoptechs, would def have some consequences, and would probably decrease braking performance. The best rationale ive seen is that the R rear brakes look better, but that dosnt matter when your car is totaled because of it. Carbon_R is the person that tested the original stoptech kit and would be able to give alot more infomation on how much testing they did, but speaking with him he agreed that the upgraded rear brakes would probably be a detriment to how the car stopped.

[towerymt]

I'm no expert. Compare piston size and pad area. Is there a bias valve used for the rear brakes that could be removed or changed?

[volvo4lif3]

Well, front brakes are used much more than the rears. I believe its somewhere around 70-80% front, and 20-30% for the rear. This being true, even though the stoptech kit is designed to use stock rear brake setup, shouldn't adding a better setup in the rear techically help in braking?

[socal850tuner]

I think what your saying has a very good point. There is only so much pressure/fluid the master cylinder could put. So what I'm saying is that yes it would be a problem, but could be easily solved with an upgraded master cylinder (a little goes a long ways, so not too much bigger) and brake bias adjustment valve (summit has them cheap).

[jmd]

I know the Stoptech addresses this on the '93-'95 RX7s by offering their standard bbk that matches up to your factory rear brakes, and a 'race' bbk for aftermarket rear brakes. I know because i was looking into getting the Stoptech kit for my FD (aka 3rd gen RX7) and had to figure out if i wanted to go with bigger rear brakes later on or not. But if they don't make two models of bbk for the Volvo you'll have to find out whether it's the 'race' kit or not. I would assume not unless otherwise noted on their site.

[Che'_Moderator]

You need to better define brake bias. Are you going by straight line pressure. If so bias remains unchanged no matter what calipers you have. If you are talking clamping force thats most determined by total piston size. If you are talking true bias you have to factor in rotor size, clamping force, and rotating mass of the axle. Back on point though. From memory the R calipers have the same piston volume as the base calipers. If that is the case they would have the same clamping force with increased leverage and friction. None of this matters though as these cars have ABS and 99% of you cannot drive without it. Carson is case in point.