By moving weight rearward, the brake bias will ceratinly shift forward. Does this mean that an upsized 290 rear rotor package is "OK" under these conditions? Well, it should add rear bias, but your guess is as good as ours as to how "OK" it really is.

Will the car spontaneously butst into flames when the brakes are applied? Probably not.

Will the car have optimized stopping distances? Probably not.

Would we recommend trying it? Not without actual data...

james walker, jr.
teamscR@flash.net
http://www.teamscR.com

 

James 2 more topics if you could.

I have been pondering the effect on front to rear bias in a straight-line vs turning situation as well.

In your experience will turning tend to increase the likelihood of premature rear brake lock-up vs braking in a straight-line. I know the inside rear tire is gonna want to lock bigtime, but at the same time the additional weight applied to the outside rear tire would seem like it would stave off lockup.


Secondly(this may be much more complicated than I realize) is there a formula I could use to calculate how much I have changed the front to rear bias on my car. Assume I have piston and rotor diameters for before and after and nothing else was changed in the system except for the lines. The differences in pad size are almost negligible and I guess for dicussions sake we could exclude pad friction coefficient(cause that can be varied as well.)

You stated earlier an approximate 10% decrease in front brake torque and of course a constant rear brake torque. I was just wondering if I could figure how much inherent overall increase in rear torque after upgrading both front and rear brakes.


Here were my ideas on examples:

Stock f to r bias 90% front 10% rear
Stock front 90 lbs/ft stock rear 10 lbs/ft

Stoptech 10% drop in front torque to 81 lbs/ft stock rear 10 lbs/ft
Stoptech f to r bias now 89% front 11% rear

Is that approximately correct or is there a non-linear relationship?

Thanks
k.

 

Optimum bias in a straight line will always be 'more rear' than during cornering. Because of the forward-tilted roll axis of most production cars, weight is transferred from the inside rear to the outside front during a turn. Ever see a FWD car lift the inside rear at an auto-x? Ever notice the outside is totally tucked at the same time and the car sort of pitches forward?

Anyway, while there is some inside rear to outside rear weight transfer as well, you still end up with the tire being able to generate less braking force because you are now asking it to sustain lateral (cornering) load as well. We could go into the friction circle concept and such, but for all of these compounding reasons, less rear braking can be sustained when cornering.

As for your sizing question, the torque is simply a function of the brake line pressure times the piston area times the rotor effective radius times the pad coefficent of friction. If you know the piston area and effective radius (not to be confused with the advertised "size" of the rotor) before and after, then you can quickly calculate the % torque increase/decrease for any given pressure assuming pads remain at the same coefficient.

For example, if your piston area goes up 10% and your effective radius grows by 5%, you have a torque increase of 15.5% (1.1x1.05). Just note that the effective radius is NOT just the OD of the rotor divided by two - it's measured at the centerline of the pistons.

Do this calculation at both ends of the car and you can determine the shift in bias...sort of. The proportioning valve throws a discontinuity in the equation, but it will point you in the right direction.

Hope this answers your questions.

james walker, jr.
teamscR@flash.net
http://www.teamscR.com

 

some updated math on my front and rear upgrades as well as the "H6" upgrade in general:

My upgrade - stock to 2001 GT-B setup

Line pressure = constant
Piston area = constant
Pad coefficient of friction = constant
Effective rotor diameter(measured at the center of the pistons) = only variable


My car: (front 277 to 294mm) (rear 266 to 290mm)

Est. eff. original front rotor diameter 258mm

Est. eff. new front rotor diameter 275mm

Est. eff. original rear rotor diamter 250mm

Est. eff. new rear rotor diameter 274mm


Front torque increase 6.6%

Rear torque increase 9.6%

Net loss of front vs rear 3%(quite a bit lower than Stoptech's 10% but a step in the right direction)



Now let's consider the "H6" upgrade on a WRX already equipped with a 294mm front

Front torque increase 0%

Rear torque increase 9.6%

Net loss of front vs rear 9.6%(almost dead-on where Stoptech is with their kit)

I am hoping that the math is essentially correct, james?

Again all of this assumes no caliper or pad changes, just rotor changes. These are relatively rough numbers, but they should be pretty close.

And, of course, the last loose number is pad coefficient of friction manipulation. I will be trying slightly more aggressive pads in the rear to see if I can improve bias even more.

k.

 

and James if you are the one responsible for the "Pulp Friction" article you are definitely not a "******."

Great piece, I refer back to it regularly now.

k.

 

Yes, that was my article. It is a watered-down version of a brake seminar I developed as part of my lecture-tour duty at my alma matter, GMI. We stripped out some of the math, threw in the racer's perspective, and there you are. Glad you liked it.

Your calculations are accurate, but that's not typically how it's done. Effective diameter, while correct in principle, is not a term that is used in the braking world. Effective radius is really the factor we use, but in your example - because you were only looking at relative shifts - the percentages come out the same. When you start talking about torque and the like, however, you need to use the radius figures if for no other reason than to maintain consistency in your discussions.

In short: good math. Only an **** brake engineer would know the difference

james walker, jr.
http://www.teamscR.com
teamscR@flash.net

 

Uh did you read the entire thread? We have come to several conclusions I think with this thread.

A. Ths Stoptech kit is a WRX exclucsive unit that is designed to be used with the stock rear brakes and has a an ~10% drop in front brake torque vs the WRX 2pot stockers. If you have the Stoptech kit do NOT upgrade the size of the rear rotors. Upgrading the rears with the SToptech kit could reduce braking performance and be dangerous. The kit is a complete solution.

B. If you are not getting the Stoptech kit(I have a Legacy GT btw, and its not an option to me) measured upgrades can push a little of the bias rearward, just like the Stoptech kit does. Meaning if nothing else has changed going up to an "H6" rotor upgrade and keeping the stock front brakes on a WRX lands you in the same bias range as the Stoptech kit does. In my case, because I upgraded fronts and rears, using a 290mm rear rotor only slightly pushed the bias rearward. I am going for more aggressive pads next.

Perhaps you have the Stoptech kit. But for those who dont rear upgrades are a real option.

k.