Why is everyone looking for a rear brake kit?
#76
Guest
Posts: n/a
STOPTECH on-line support
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
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
#77
Guest
Posts: n/a
2 more questions
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.
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.
#78
Guest
Posts: n/a
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
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
#79
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Posts: n/a
I thought I would post
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.
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.
#81
Guest
Posts: n/a
Pulp Friction
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
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
#82
Originally posted by GarySheehan
Pimp,
As most here know, we are using the Stoptech BBK (328mmx28mm 2-piece Aerorotors & ST-40 calipers) on the front and the stock rear set-up. We run Pagid RS-14 pads in the front and custom cut Hawk HT-10 pads in the rear. We use Castrol SRF synthetic brake fluid.
We have had no heat related issues with our rear brakes whatsoever.
Gary
Sheehan Motor Racing
www.teamSMR.com
Pimp,
As most here know, we are using the Stoptech BBK (328mmx28mm 2-piece Aerorotors & ST-40 calipers) on the front and the stock rear set-up. We run Pagid RS-14 pads in the front and custom cut Hawk HT-10 pads in the rear. We use Castrol SRF synthetic brake fluid.
We have had no heat related issues with our rear brakes whatsoever.
Gary
Sheehan Motor Racing
www.teamSMR.com
Gary what do you recommend on pads. I bought Porterfield street pads for fronts but haven't installed them yet.
It would be great to hear opinions from pros like this.
Isn't it about time pros posting threads and correct mis-infos and those who 'claim' to know a lot
#83
Guest
Posts: n/a
Originally posted by bLuImP
Good point.... I was about to change my brakes to H6 setup but I've changed my mind. Every pros tell me leave rear stock except lines and pads.
Gary what do you recommend on pads. I bought Porterfield street pads for fronts but haven't installed them yet.
It would be great to hear opinions from pros like this.
Isn't it about time pros posting threads and correct mis-infos and those who 'claim' to know a lot
Good point.... I was about to change my brakes to H6 setup but I've changed my mind. Every pros tell me leave rear stock except lines and pads.
Gary what do you recommend on pads. I bought Porterfield street pads for fronts but haven't installed them yet.
It would be great to hear opinions from pros like this.
Isn't it about time pros posting threads and correct mis-infos and those who 'claim' to know a lot
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.
#84
I didn't feel like reading through 6 pages, but like myself I have an L with rear drums.......enough said, hahaha!
-Joe
lots of interesting stuff about brakes in this thread, the thing I know the least amount about...Thanks guys
-Joe
lots of interesting stuff about brakes in this thread, the thing I know the least amount about...Thanks guys
#85
Registered User
iTrader: (2)
Join Date: Nov 2002
Location: Jersey City
Posts: 120
Car Info: Subaru WRXB wagon
Here is a scenario:
Take your Subaru to dry empty lot, disconnect ABS and at about 30, 40, 50 MPH lock up your wheels. Than get out from your car and (meassure before lengh between front and rear contact patch of your tires) meassure lengh between your front and rear tires skid marks than you will see where is your car brake bias is.
And than adjust your bias accordingly, wether with pads, proportioned valve or other means.
We did it before in my country when I was racing cars there 20 years ago where it was unheart about ABS, but mostly for a reason to make sure that your all brakes works properly.
Serge.
Take your Subaru to dry empty lot, disconnect ABS and at about 30, 40, 50 MPH lock up your wheels. Than get out from your car and (meassure before lengh between front and rear contact patch of your tires) meassure lengh between your front and rear tires skid marks than you will see where is your car brake bias is.
And than adjust your bias accordingly, wether with pads, proportioned valve or other means.
We did it before in my country when I was racing cars there 20 years ago where it was unheart about ABS, but mostly for a reason to make sure that your all brakes works properly.
Serge.
#87
Registered User
Join Date: Nov 2002
Location: Washington State, Car: 2000 Impreza Coupe RS-T T
Posts: 792
Car Info: 2000 Impreza Coupe RS-T Tec3/Vishnu turbo Color: B
Why does an RS have way smaller rear brakes than an STi?? because it must be better or they would have the same rears.
Looks better on 17" or larger rims too.
Looks better on 17" or larger rims too.
#88
My God this is an awesome thread! It's really made me reconsider my brake upgrades. I'll be sure to continue checking this thread out. I apologize for not being able to contribute, but again...an awesome thread!
#90
Originally posted by Kobayashi
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.
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.
ken