Still trying to find out a cheap way to give 13" rotors to the '84's
12-27-2010, 02:12 AM
#106
Rennlist Member
Thread Starter
I dont care how you "feel" this isnt a shrink talking to a patient, but a race car.
you weigh 3000lbs, at 1.2gs you will transfer near 1000lbs from front to rear.
that means the front weighs 2500lbs and the rear weighs 500lbs. It gets better, each tire has only 250lbs on it in the rear! Now, put a little turn into the equation, so you are trail braking into most turns. this 250lbs can very easily become 150lbs. Now, remember the front has now, 2500lbs on them, thats 1500lbs on each tire. what do you think the proportions should be???
It doesnt take a math major to see some intuitive limits and trade offs here in performance.
This is why you shouldnt spend a lot time looking for performance gains in the rear brake area. Ive driven cars with too much and the are un driveable on the track. fun to impress your friends on the street, but this is not "real " peformance you can use, but certainly feel.
you weigh 3000lbs, at 1.2gs you will transfer near 1000lbs from front to rear.
that means the front weighs 2500lbs and the rear weighs 500lbs. It gets better, each tire has only 250lbs on it in the rear! Now, put a little turn into the equation, so you are trail braking into most turns. this 250lbs can very easily become 150lbs. Now, remember the front has now, 2500lbs on them, thats 1500lbs on each tire. what do you think the proportions should be???
It doesnt take a math major to see some intuitive limits and trade offs here in performance.
This is why you shouldnt spend a lot time looking for performance gains in the rear brake area. Ive driven cars with too much and the are un driveable on the track. fun to impress your friends on the street, but this is not "real " peformance you can use, but certainly feel.
On my S4 I use the Turbo 911 rear bias bar and it is a HUGE HELP in braking. My rear brakes actually DO SOMETHING with that set up vs, before my back brakes were doing pretty much nothing. My S4 stops like an anchor has been thrown out. My race car (with S4 brakes but different remote vacuum master cylinders [two of them]) does not even stop CLOSE to as positively as the S4. I think it probably stops a bit better, ultimately, but it does not FEEL like it stops nearly as well.
12-27-2010, 02:33 AM
#107
Nordschleife Master
Mark,
I am not arguing with you here...and I don't know what the reason for it is, but with the 911 Turbo rear brake bias bar added my car stopped better than it did w/ the stock 928 bias bar. It is NOT how it feels (that was just a relative thing between the race car and the street car) it is a FACT that my S4 with the 911 Turbo bias bar on street or track brakes WAY better than it did w/ the stock 928 bias bar.
I trail brake in to most all corners and have never had a problem with rear lock up on either car (probably means I am not braking hard enough in the race car because it does not have antilock, the S4 does). Totally understand how little (relatively speaking) work the rear brakes do...I used to race motorcycles as well...
I am not arguing with you here...and I don't know what the reason for it is, but with the 911 Turbo rear brake bias bar added my car stopped better than it did w/ the stock 928 bias bar. It is NOT how it feels (that was just a relative thing between the race car and the street car) it is a FACT that my S4 with the 911 Turbo bias bar on street or track brakes WAY better than it did w/ the stock 928 bias bar.
I trail brake in to most all corners and have never had a problem with rear lock up on either car (probably means I am not braking hard enough in the race car because it does not have antilock, the S4 does). Totally understand how little (relatively speaking) work the rear brakes do...I used to race motorcycles as well...
12-27-2010, 04:36 AM
#108
Rennlist Member
Thread Starter
You have an automatic, correct? so, what you are seeing could make up fror the lack of engine braking. I noticed a lack of rear step out when racing Brians auto racer.
I havent even looked at the S4 vs 85 bias differnces, but i think the S4 feels like it has more rear brakes, plus i put on pagid blacks in the rear as well. only problem, you can never bed the rears properly with a weak bias set up. (well there is one way, but it incorporates jacking the car up and flooring it and applying the brakes. )
I havent even looked at the S4 vs 85 bias differnces, but i think the S4 feels like it has more rear brakes, plus i put on pagid blacks in the rear as well. only problem, you can never bed the rears properly with a weak bias set up. (well there is one way, but it incorporates jacking the car up and flooring it and applying the brakes.
) Mark,
I am not arguing with you here...and I don't know what the reason for it is, but with the 911 Turbo rear brake bias bar added my car stopped better than it did w/ the stock 928 bias bar. It is NOT how it feels (that was just a relative thing between the race car and the street car) it is a FACT that my S4 with the 911 Turbo bias bar on street or track brakes WAY better than it did w/ the stock 928 bias bar.
I trail brake in to most all corners and have never had a problem with rear lock up on either car (probably means I am not braking hard enough in the race car because it does not have antilock, the S4 does). Totally understand how little (relatively speaking) work the rear brakes do...I used to race motorcycles as well...
I am not arguing with you here...and I don't know what the reason for it is, but with the 911 Turbo rear brake bias bar added my car stopped better than it did w/ the stock 928 bias bar. It is NOT how it feels (that was just a relative thing between the race car and the street car) it is a FACT that my S4 with the 911 Turbo bias bar on street or track brakes WAY better than it did w/ the stock 928 bias bar.
I trail brake in to most all corners and have never had a problem with rear lock up on either car (probably means I am not braking hard enough in the race car because it does not have antilock, the S4 does). Totally understand how little (relatively speaking) work the rear brakes do...I used to race motorcycles as well...
12-27-2010, 07:43 AM
#109
Bump this one. Does anyone have those dims so I can search the P-car graveyard here and make sure I get a set of the Early spindles?
Also with the 330mm x 32 rotors, what 4-pot caliper (36-41-44mm) would be best suited for the Factory MC bore dia.?
Also with the 330mm x 32 rotors, what 4-pot caliper (36-41-44mm) would be best suited for the Factory MC bore dia.?
Last edited by xschop; 12-27-2010 at 07:59 AM.
12-27-2010, 08:09 AM
#110
Rennlist Member
Following this thread with interest but I am a bit puzzled as to why
racers like Mark et all seem to be trying to get good brake balance
by juggling with discs, pad type, fixed bias valves, etc.
Why not use an adjustable rear bias valve.
My old Triumph Dolomite replica period racer originally had a
rear brake antilock device operated by a link to the rear axle.
When you braked hard the rear of the car rose and a pivoted
lever cut off pressure to the rear brakes, crude but effective.
I removed this and put a Willwood rotary bias adjuster in the
line to the rear with the control on the tunnel just behind the gearlever.
I could adjust it to get the rear wheels starting to lock under
extreme braking then turn it back a little to prevent rear wheel lockup.
It was easy to fine adjust it to allow for different venues, weather
conditions and so on.
This is what these devices are specifically made for; why not use them.
racers like Mark et all seem to be trying to get good brake balance
by juggling with discs, pad type, fixed bias valves, etc.
Why not use an adjustable rear bias valve.
My old Triumph Dolomite replica period racer originally had a
rear brake antilock device operated by a link to the rear axle.
When you braked hard the rear of the car rose and a pivoted
lever cut off pressure to the rear brakes, crude but effective.
I removed this and put a Willwood rotary bias adjuster in the
line to the rear with the control on the tunnel just behind the gearlever.
I could adjust it to get the rear wheels starting to lock under
extreme braking then turn it back a little to prevent rear wheel lockup.
It was easy to fine adjust it to allow for different venues, weather
conditions and so on.
This is what these devices are specifically made for; why not use them.
12-27-2010, 06:44 PM
#112
Race Director
Following this thread with interest but I am a bit puzzled as to why
racers like Mark et all seem to be trying to get good brake balance
by juggling with discs, pad type, fixed bias valves, etc.
Why not use an adjustable rear bias valve.
My old Triumph Dolomite replica period racer originally had a
rear brake antilock device operated by a link to the rear axle.
When you braked hard the rear of the car rose and a pivoted
lever cut off pressure to the rear brakes, crude but effective.
I removed this and put a Willwood rotary bias adjuster in the
line to the rear with the control on the tunnel just behind the gearlever.
I could adjust it to get the rear wheels starting to lock under
extreme braking then turn it back a little to prevent rear wheel lockup.
It was easy to fine adjust it to allow for different venues, weather
conditions and so on.
This is what these devices are specifically made for; why not use them.
racers like Mark et all seem to be trying to get good brake balance
by juggling with discs, pad type, fixed bias valves, etc.
Why not use an adjustable rear bias valve.
My old Triumph Dolomite replica period racer originally had a
rear brake antilock device operated by a link to the rear axle.
When you braked hard the rear of the car rose and a pivoted
lever cut off pressure to the rear brakes, crude but effective.
I removed this and put a Willwood rotary bias adjuster in the
line to the rear with the control on the tunnel just behind the gearlever.
I could adjust it to get the rear wheels starting to lock under
extreme braking then turn it back a little to prevent rear wheel lockup.
It was easy to fine adjust it to allow for different venues, weather
conditions and so on.
This is what these devices are specifically made for; why not use them.
Adjustable regulator is the ideal solution....but not many 928 racers do it?
12-28-2010, 04:47 AM
#113
I dont care how you "feel" this isnt a shrink talking to a patient, but a race car.
you weigh 3000lbs, at 1.2gs you will transfer near 1000lbs from front to rear.
that means the front weighs 2500lbs and the rear weighs 500lbs. It gets better, each tire has only 250lbs on it in the rear! Now, put a little turn into the equation, so you are trail braking into most turns. this 250lbs can very easily become 150lbs. Now, remember the front has now, 2500lbs on them, thats 1500lbs on each tire. what do you think the proportions should be???
It doesnt take a math major to see some intuitive limits and trade offs here in performance.
This is why you shouldnt spend a lot time looking for performance gains in the rear brake area. Ive driven cars with too much and the are un driveable on the track. fun to impress your friends on the street, but this is not "real " peformance you can use, but certainly feel.
you weigh 3000lbs, at 1.2gs you will transfer near 1000lbs from front to rear.
that means the front weighs 2500lbs and the rear weighs 500lbs. It gets better, each tire has only 250lbs on it in the rear! Now, put a little turn into the equation, so you are trail braking into most turns. this 250lbs can very easily become 150lbs. Now, remember the front has now, 2500lbs on them, thats 1500lbs on each tire. what do you think the proportions should be???
It doesnt take a math major to see some intuitive limits and trade offs here in performance.
This is why you shouldnt spend a lot time looking for performance gains in the rear brake area. Ive driven cars with too much and the are un driveable on the track. fun to impress your friends on the street, but this is not "real " peformance you can use, but certainly feel.
Smiffy
12-28-2010, 09:54 AM
#114
Administrator - "Tyson"
Lifetime Rennlist
Member
Lifetime Rennlist
Member
I "doesn't take a math major" to work out that to get your quoted weight transfer of 1000lb at 1.2g, with the 928 wheelbase of 98.4 inches and a weight of 3000lb, the centre of gravity would have to be 52" off the ground. As a standard 928 is only 51.47" high, that is impossible.
12-28-2010, 01:07 PM
#117
Rennlist Member
Assumptions:
1.2 G decel
Decel force = 3000 lbs * 1.2 = 3600 lbs
weight of car 3000 lbs
WB 98.4"
CG Height = Y"
Weight distribution 50/50
X location of CG = WB/2 = 49.2"
weight on axles:
no braking F = 1500 lbs, Rr 1500 lbs
weight transfer off rear wheels during braking 1000 lbs
weight on axles:
Braking: F = 2500 lbs, R 500 lbs
Balance torque on chassis under braking. (using front contact patch as as fulcrum)
CCW torque = (decel force * CG Height) + (force on rear axle * WB)
CCW = 3600 * Y + 500 * 98.4
CW = weight * (WB/2)
CW = 3000 * 49.2
set CCW = CW and solve for Y
3600 * Y + 500 * 98.4 = 3000 * 49.2
3600 * Y + 49200 = 147600
36 * Y = 984
Y = 27.3"
So with a roof line of 52" a CG Height of 27" may be reasonable though logically the CG should lie below the 1/2 height point. So in reality it will probably take more than a 1.2 g stop to get 1000 lbs of weight transfer. if anyone know the actual CG height it would be very easy to calculate the actual weight transfer.
