Brake proportioning valve question

[ChrisBennet]

I'm hoping Bill Verburg will jump in here but perhaps someone else knows.
I am running 928S4 calipers up front and 930 calipers in the rear of my '86 911. I've been running this way for years (track and street) but when I wore out my front rotors (Colemans) my rear rotors (Porsche) measured new though my pads seem to wear out about the same time front and back.
Q: Do I need to remove or change my brake proportioning valve?
-Chris

[addictionMS]

there is a real simple test, get up a head of speed, and apply an ever increasing pressure on the brakes until something locks up, if the fronts lock up what seems to be way too soon, you might need a bias adjustment. If the rears lock up first then you also might consider on, but since all the weight is thrown on to the front, the rears will have less traction, If the test in inconclusive, and you stop like hell, then you are fine.

Adjucting bias is a tricky thing, and a very personal one.

good luck


Jim

[ChrisBennet]

Originally posted by addictionMS
there is a real simple test, get up a head of speed, and apply an ever increasing pressure on the brakes until something locks up, if the fronts lock up what seems to be way too soon, you might need a bias adjustment. If the rears lock up first then you also might consider on, but since all the weight is thrown on to the front, the rears will have less traction, If the test in inconclusive, and you stop like hell, then you are fine.

Adjucting bias is a tricky thing, and a very personal one.

good luck


Jim

The car stops fine now (on a dry track) but could it stop better? I need it to work on rainy streets as well so I am hesitant to start messing with things without learning more.
-Chris

[ChrisBennet]

Come on Bill V. I know you want to answer this!
(shameless bump)
-Chris

[dickster]

chris

have you read my thread?930 calipers (for '86 carrera) have arrived! what other parts do i need?

bill mentions removing it.

[Bill Verburg]

Sorry for the delay in responding, I went sailing yesterday, 25 knots from 320&deg was irresistable

The S4 calipers(44/36) front and the 930 calipers(30/30) rear have an hbr of ~1.796 slightly more front bias than factory(~1.596). My car with no p/v at all has ~1.72 and exhibits just the tiniest bit of front bias always locking the right front just a tad before anything else. As far as I'm concerned thats perfect.
Suspension stiffness also comes into play, a stiffer ride will want more rear bias.

The Colemans are notoriously soft and will wear quicker than most so whats happening on your car is very normal.

I don't remember what year you car is but the 84-89 have the proportioning valve in an easily accesible spot off the m/c. I would try removing it and do some experimenting. Due to the existing front bias I think you will be happy w/o it.

[APKhaos]

My 85 is eating rear pads - happened twice now at the track, most recently after three days at Watkins Glen.

Brakes are stock, and pads were new. Given the front bias of the stock PV, I'd expect the front pads to wear first, but the rears are down to the ebacking with better than half the front pad material remaining.

Braking appears normal - fronts will lock up a tad before the rears. What the heck is wrong here - any ideas?

[Bill Verburg]

84-89 have a basic hbr of ~1.306(much more rear bias than the pre 84 which had ~1.596). The p/v is 33bar, .46 reduction, this means that if the line pressure is below 33 bar the brakes remain rear biased and will wear rear pads more than the earlier cars. Above the 33 bar knee, line pressure to the rear is cut 46%, 100bar of front line pressure will have 54bar at the rear which is an effective ~1.8 bias(more front)

[ChrisBennet]

Originally posted by Bill Verburg
Sorry for the delay in responding, I went sailing yesterday, 25 knots from 320&deg was irresistable

The S4 calipers front and the 930 calipers rear have an hbr of ~1.796 slightly more front bias than factory(~1.596). My car with no p/v at all has ~1.72 and exhibits just the tiniest bit of front bias always locking the right front just a tad before anything else. As far as I'm concerned thats perfect.
Suspension stiffness also comes into play, a stiffer ride will want more rear bias.

The Colemans are notoriously soft and will wear quicker than most so whats happening on your car is very normal.

I don't remember what year you car is but the 84-89 have the proportioning valve in an easily accesible spot off the m/c. I would try removing it and do some experimenting. Due to the existing front bias I think you will be happy w/o it.

Bill,
Thanks so much. Off to Lime Rock tonight so no time to mess with it but I intend to try removing itthis weekend in time for track days Monday and Tuesday. Thanks!
-Chris

[APKhaos]

Thanks Bill, you really made the pv operation clear, and that really casts a different light on things. The non-linear nature of the pv probably means that high rear pad wear rates suggests low braking effort - ie below the 33 bar knee - is the cause.

Thought I'd been braking hard, but maybe not hard enough?
Could restrictions in the front circuits be contributing to this? When flushing new fluid through the system last weekend, I noticed that the bleed flow rate from the fronts was much lower than the rears [power bleeder, so the input pressure was constant]. Maybe I should replace the lines and rebuild the calipers before jumping to too many conclusions??

Many thanx,

[jluetjen]

Brakes are stock, and pads were new. Given the front bias of the stock PV, I'd expect the front pads to wear first, but the rears are down to the ebacking with better than half the front pad material remaining.

Before you start to spend a whole lot of money redesigning your braking system, you might want to get some temperature sensative paint and apply it to the edges of the calipers, pads and calipers. I suspect that you will find that the rear brakes are running much hotter then the fronts due to the difference in air flow. Brakes which are too hot will often eat the pads. It's most likely worth spending the $20 on the paint before you spend 200 redoing your system.

[Bill Verburg]

The flow rates should be substantially the same, but this is a difficult observation to make. The rubber hoses can sometimes collapse internally restricting fow. Its odd that both front hoses would collapse at the same time though.