Brake proportioning valve help needed
12-24-2012, 05:23 PM
#46
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+34^2)/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.
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+34^2)/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.
comparing 4 piston rear to 2 piston rears @70bar line pressure
4 > 1974/857 w/ p/v 1974/1217 w/o
2 > 1974/707 w/ p/v 1974/1309 w/o
4 uses 55bar p/v 2 uses 45bar p/v
the 4 has 54% more pad area which means they last longer
12-24-2012, 06:36 PM
#47
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ok- a few more numbers: at 1974 Nm of brake torque, the tire is reacting 6247 N against the road. If you consider a coefficient of friction of 1.5 (high for street tires), the front load would have to be around 850Kg to keep the tires from locking- which is around 63% of the car weight (a lot of weight transfer). In this case the rear tires would be braking with 3850 N against the road- or needing 525kg to keep them from locking.
In this case at 70 bar line pressure there is not enough friction to go around and you would have one end locked up (total load required is 1375kg for a 1350kg car..). You would also be decelerating at 1.5g- which is not possible on street tires and probably not even the best R-compounds. A more likely number is around 1g. Doing some estimates and testing (photography at braking zones plus spring rate) and I am very comfortable in the knowledge that I can't get enough forward weight transfer to lock the rears.
I don't have actual test numbers, but I think the maximum brake line pressure for street tires will be somewhere around 50 bar.
I didn't put the 4 pots on the back to get more braking- I did it because you can't find many pad compounds for the 2 pots. OEM pads just don't cut it on the track...
Great discussion! I love reading and thinking about this stuff...
In this case at 70 bar line pressure there is not enough friction to go around and you would have one end locked up (total load required is 1375kg for a 1350kg car..). You would also be decelerating at 1.5g- which is not possible on street tires and probably not even the best R-compounds. A more likely number is around 1g. Doing some estimates and testing (photography at braking zones plus spring rate) and I am very comfortable in the knowledge that I can't get enough forward weight transfer to lock the rears.
I don't have actual test numbers, but I think the maximum brake line pressure for street tires will be somewhere around 50 bar.
I didn't put the 4 pots on the back to get more braking- I did it because you can't find many pad compounds for the 2 pots. OEM pads just don't cut it on the track...
Great discussion! I love reading and thinking about this stuff...
12-25-2012, 01:53 PM
#48
The #s I posted are correct, using a modest Mu of .5
Check out Stoptechs Tech page for details
12-27-2012, 03:35 AM
#49
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Hi Bill,
I'll have a look- I must be missing something. I wasn't doubting your numbers- it just seems the friction limit of a front tire will be met before it can react 1974Nm of brake torque. I'll have to figure out where I went wrong...
I'll have a look- I must be missing something. I wasn't doubting your numbers- it just seems the friction limit of a front tire will be met before it can react 1974Nm of brake torque. I'll have to figure out where I went wrong...
