Brake proportioning valve help needed
12-19-2012, 09:36 AM
#32
Nordschleife Master
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(The following is meant good-naturedly...)
Engineers have fun with this kind of thing, only problem being that they will then allow these calculations get in the way of their driving - at the limit anyway.
At the edgy-edge of performance, far too many variables play a role in braking performance, the greatest of which is the driver. His skills and needs far outweigh any graphs and possible clamping variations.
Bill has outlined several brake combinations which work for 99.9999% of us. He has argued with several of us (most of us pointy end racers) regarding extreme rearward bias, but he is right for almost everybody.
I say pay attention to his stuff, learn on the track and make fine adjustments as your skill develops.
I do appreciate the work that goes into these numbers though, and have used them myself to help understand what happens when my car swaps ends violently...not that that has ever happened (tehe).
12-19-2012, 09:41 AM
#33
So here's the brake torques front and rear with and without pv for a few different combinations:
(which unlike the clamp force plots above this takes into account the changes in the rotor diameter, I think the maths is for each wheel but the NM figures may be out by a factor of 2
- but the ratios front and rear will be as per the charts)
Assumes friction coef of 0.5 (track pad)
Kai is running the combo in the top left - with the bias valve wide open the rears follow the green line, you can see why that might be hairy, but also fast. With 55bar pv though there's a reasonable margin for error.
Most 964s with "big reds" leave the stock 4 pot rears, which is bottom left, the rears are really just along for the ride, especially with the proportioning valve in place - all very safe.
Middle left is the next biggest you can go front and rear after Kai's setup without going custom adapters two piece rotors etc. Still quite a step down from Kai's. Maybe you could go with high mu rear pads to balance that up, and just watch the rear temps.
As ras62 points out the 2 pots offer more performance than the 4 pot rears, though unfortunately for only a short period of time before they overheat.
PS all made possible by Bill's excellent data http://members.rennlist.com/1976c38/brakes.htm, and inspired by this ready reckoner for Subaru's from user Legacy777 one nasioc.com http://www.main.experiencetherave.co.../brakemath.xls (forum thread http://forums.nasioc.com/forums/showthread.php?t=720871)
(which unlike the clamp force plots above this takes into account the changes in the rotor diameter, I think the maths is for each wheel but the NM figures may be out by a factor of 2
- but the ratios front and rear will be as per the charts)Assumes friction coef of 0.5 (track pad)
Kai is running the combo in the top left - with the bias valve wide open the rears follow the green line, you can see why that might be hairy, but also fast. With 55bar pv though there's a reasonable margin for error.
Most 964s with "big reds" leave the stock 4 pot rears, which is bottom left, the rears are really just along for the ride, especially with the proportioning valve in place - all very safe.
Middle left is the next biggest you can go front and rear after Kai's setup without going custom adapters two piece rotors etc. Still quite a step down from Kai's. Maybe you could go with high mu rear pads to balance that up, and just watch the rear temps.
As ras62 points out the 2 pots offer more performance than the 4 pot rears, though unfortunately for only a short period of time before they overheat.
PS all made possible by Bill's excellent data http://members.rennlist.com/1976c38/brakes.htm, and inspired by this ready reckoner for Subaru's from user Legacy777 one nasioc.com http://www.main.experiencetherave.co.../brakemath.xls (forum thread http://forums.nasioc.com/forums/showthread.php?t=720871)
12-19-2012, 10:01 AM
#34
Burning Brakes
At the edgy-edge of performance, far too many variables play a role in braking performance, the greatest of which is the driver. His skills and needs far outweigh any graphs and possible clamping variations.
learn on the track and make fine adjustments as your skill develops.
I do appreciate the work that goes into these numbers though, and have used them myself to help understand what happens when my car swaps ends violently...not that that has ever happened (tehe).
learn on the track and make fine adjustments as your skill develops.
I do appreciate the work that goes into these numbers though, and have used them myself to help understand what happens when my car swaps ends violently...not that that has ever happened (tehe).
12-19-2012, 10:12 AM
#35
Nordschleife Master
Engineers have fun with this kind of thing, only problem being that they will then allow these calculations get in the way of their driving - at the limit anyway.
At the edgy-edge of performance, far too many variables play a role in braking performance, the greatest of which is the driver. His skills and needs far outweigh any graphs and possible clamping variations.
12-19-2012, 11:33 AM
#36
Three Wheelin'
EDIT: obviously no one should ever take my advice on anything, ever... I'm only here for the spreadsheets
12-19-2012, 12:06 PM
#37
Nordschleife Master
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And the spreadsheets are wonderful!
Thanks.
We're I building up another street car, I'd run Bill's fav combo: Big Reds (Blacks) front and 993 rears, all over AP Racing rotors with nice hats. This is indeed the snizzle. For a race car, I'd do exactly what I've done.
BTW, Claudia is staying right here in Bugtussle where she belongs.
Thanks.
We're I building up another street car, I'd run Bill's fav combo: Big Reds (Blacks) front and 993 rears, all over AP Racing rotors with nice hats. This is indeed the snizzle. For a race car, I'd do exactly what I've done.
BTW, Claudia is staying right here in Bugtussle where she belongs.
12-19-2012, 01:12 PM
#38
Rennlist Member
Ciao alexjc4!
I have one question for you:
another common set is 322 front (with 965 3.3 turbo caliper) and 993 c2 rear. How much w and w/o 55 pv?
Cheers
Paolo
I have one question for you:
another common set is 322 front (with 965 3.3 turbo caliper) and 993 c2 rear. How much w and w/o 55 pv?
Cheers
Paolo
12-19-2012, 01:19 PM
#39
Three Wheelin'
12-19-2012, 01:21 PM
#40
Three Wheelin'
12-19-2012, 06:18 PM
#41
Rennlist Member
Alexjc4, thanks a lot!
Paolo
Paolo
12-23-2012, 09:52 PM
#42
Track Day
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Just to add- I've done similar calcs and graphs myself. When I upgraded to 4 pot rears, I chose to run with out a PV (running stock fronts and 4 pot rears). In quite a few hours on the track and just driving around daily I can honestly say that I still have more than enough front bias with normal high performance street tires.
Moving to wider front slicks may increase the weight transfer enough that I may need less rear bias, but with street tires I have never locked the rear end before the front- one of the front tires always locks first.
ok- I lied- there was one time when I overcooked the front brakes, got a bit of fade and ended up with a bit too much rear bite- but that's not really a good reason to go back IMHO and was easily solved with a bit of cool down and some better brake pads...
Moving to wider front slicks may increase the weight transfer enough that I may need less rear bias, but with street tires I have never locked the rear end before the front- one of the front tires always locks first.
ok- I lied- there was one time when I overcooked the front brakes, got a bit of fade and ended up with a bit too much rear bite- but that's not really a good reason to go back IMHO and was easily solved with a bit of cool down and some better brake pads...
12-24-2012, 08:40 AM
#43
Thats exactly what I was wondering.
I have a 1991 and changed out the rear calibers to 4 pot.
So I have stock front calibers, later 4 pot calibers in rear, cup 1 wheels with Conti DW's.
I use my car for street only and have yet to change the pv. I would like to eliminate it.
Whats your thoughts on this set up for me?
I have a 1991 and changed out the rear calibers to 4 pot.
So I have stock front calibers, later 4 pot calibers in rear, cup 1 wheels with Conti DW's.
I use my car for street only and have yet to change the pv. I would like to eliminate it.
Whats your thoughts on this set up for me?
12-24-2012, 10:14 AM
#44
I think the 3.3T caliper has the same piston sizes and discs size as the 993tt, just a smaller pad, the pad size won't make a difference to the brake torque (maybe a tiny one), so you should refer to the middle chart on the left (which according to Bill's tables, should be correct for the 3.3T also as that uses the same rears the 993c2)
12-24-2012, 03:54 PM
#45
Instructor
Thats exactly what I was wondering.
I have a 1991 and changed out the rear calibers to 4 pot.
So I have stock front calibers, later 4 pot calibers in rear, cup 1 wheels with Conti DW's.
I use my car for street only and have yet to change the pv. I would like to eliminate it.
Whats your thoughts on this set up for me?
I have a 1991 and changed out the rear calibers to 4 pot.
So I have stock front calibers, later 4 pot calibers in rear, cup 1 wheels with Conti DW's.
I use my car for street only and have yet to change the pv. I would like to eliminate it.
Whats your thoughts on this set up for me?
Somehow it is internet wistom that 4piston 964 calipers add more rear balance compared to 2 piston. Not realy...
Total piston area on 2piston calipers is actually bigger than 964 and smaller than 993
993 4 piston 3.14*(30^2+34^2)/4 = 1613.96
964 2 piston 3.14*44^2/4. = 1519.76
964 4 piston 3.14*(30^2+28^2)/4 = 1321.94
Pad is smaller on 2 pot, they look crappy, they get a bit hotter, but hydraulically they are more efficient than 4 pot
Reference:
Porsche brakes
http://members.rennlist.com/1976c38/brakes.htm
Brake bias calculator:
http://pws.prserv.net/usinet.Young/B...alculator.html
Oleg.
Last edited by perelet; 12-24-2012 at 05:18 PM.