Which proportioning valve gives more pressure to the rear?

[DodgeZ]

Which proportioning valve gives more pressure to the rear? Rear disc or drum?

[Bossman429]

Rear disc would apply more pressure than drum. Takes more presure to push a piston on a caliper than push out a small wheel cylinder.

[DodgeZ]

Rear disc would apply more pressure than drum. Takes more presure to push a piston on a caliper than push out a small wheel cylinder.

thanks

[cordes]

Would the disks need more pressure, or would they need more volume of fluid? Would cylinder's size in comparison to the size of the piston need more or less pressure, or would it be strictly a volume issue?

[Bossman429]

I think the pressure, and volume go hand in hand. Higher volume will require more pressure than smaller. Consider something like a medical Serenge (Sp? the thing that you would give a shot with)
If you have two of them, one twice the size of the other, and you pull the plunger out at the same rate. They will suck out the same amount of fluid. Therefore, if you put disc brake calipers (big plunger) in a system designed for a wheel cylinder (small plunger) the discs will not work properly because they are not recieving enough volume, or pressure of fluid. I hope this analogy makes sense.
Let me take this route:
The reason that a prop valve is on a front disc/rear drum car in the first place is the difference in pressure required for braking on the two systems.
If you had a none restrictive prop valve in a disc/drum setup. As soon as you feathered that pedal, the rear drums would LOCK because if the pressure needed to clamp the front caliper was sent to that wheel cylinder, the pressure would be far too great for the system at hand.
Thus, a disc/ drum prop valve allows the front discs to operate at the high pressure that they need to clamp that rotor enough to stop, while restricting the rear fluid enough to eliminate the rears from locking up from too much pressure.
So a Disc/disc prop valve would still likely reduce the pressure to the rear calipers, because the front calipers are still larger than the rears, so the same rules apply, but the pressure can be increased from the original disc/drum setup because the rear discs will require more pressure than a wheel cylinder, but still not as much pressure as the fronts will in either setup.
Feeeeeeeeeeeeeeeeewwwwwwwwwwwwww.....That was a mouthful!

[HotRodDodge]

Drum would apply more pressure to the rear.

[Bossman429]

Drum would apply more pressure to the rear.

How do you figure?

[BIG PSI]

Drum would apply more pressure to the rear.

Yah I thought it is harder to push on a piston than expanding a disc? What chu' talkin' 'bout Willice?

[turbovanman²]

Drum would apply more pressure to the rear.

He might be right, think about it, you have small cups for wheel cylinders so they would require alot more pressure to activate so if you swap to discs, you now have too much pressure due to the larger piston and now, you need to tone it down.

To get more rear braking with drums, you put on bigger wheel cylinders so yeah, he's right.

[Bossman429]

The purpose of the prop valve is to turn down the pressure on the rear drums. If rear drums/wheel cylinders ran at the same pressure as discs do, they would lock up instantly. It takes significantly less pressure to operate a drum than to operate a disc caliper. Drum brakes use pivot points and such to work almost as a pulley system to reduce the amount of pressure needed to activate the brakes. Whereas the disc system requires a direct clamp of the rotor which requires MUCH more pressure than the drum brake system.
Also take a look at most master cylinders, the reservoir for the rear is smaller for a reason.
Ughh. I give up.

[Bossman429]

Be sure to read the second sentance under "PROPORTIONING VALVE":
http://auto.howstuffworks.com/master-brake2.htm

[cordes]

Be sure to read the second sentance under "PROPORTIONING VALVE":
http://auto.howstuffworks.com/master-brake2.htm

While that site provides us with a great sonopsis of how a proportioning valve works, it does nothing for anyones argument one way or another in regard to the question at hand.

Can we get a physicist on this thread?

[Wink]

What is the area of the drum brake cylinders?

What is the area of a disk brake piston?

That will tell you what needs less pressure to lock up the back brakes.


In my experience, I have swapped disks into the back of a factory equiped drum brake system. The backs locked up almost immediately with the stock brake biasing. The disk brakes required less pressure to have the same braking effectiveness of the drums at higher pressure.

If you ever get lost with this, just remember one thing.

Force = Pressure x Area

Force is the total braking effectiveness
Pressure is the fluid pressure within the system
Area is the surface area of the piston within the caliper or cylinder.



Wink

[DodgeZ]

The reason I asked is because we were installing a SLH upgrade and didn't have time for the P-valve. We were going to run it for the night (so he could get home) and then do the MC and P-valve the following day. Then I remembered I put the 11" disc and vented rears on my 85 daytona. It has the stock p-valve and MC. It stops like a mad man.

[Dave]

Yah I just always figured it this way... drum brake equipped vehicles always have a SMALLER master cylinder, whereas a disc brake equpped vehicle has always had a LARGER master cylinder... coincidence? I don't think it is, it requires more pressure, and more mass in the master cylinder allows for more vaccum, and more pressure to be distributed.

[MiniMopar]

According to Ed Peters, the small disc/drum setup on the early L-bodies had no prop valve, just the cross-over block. The small drums were small enough that it worked out as a well-balanced system. The prop valve as added later when they went to the bigger drums, and even more so for rear discs.

My 1987 CSX is pretty tail-happy when I have to hit the hooks hard. There are a lot of contributing factors: solid rear swaybar, short wheelbase, etc. I've spun it on the highway twice. Once I got lucky. Once I didn't. Need more track time.

[turbovanman²]

My 1987 CSX is pretty tail-happy when I have to hit the hooks hard. There are a lot of contributing factors: solid rear swaybar, short wheelbase, etc. I've spun it on the highway twice. Once I got lucky. Once I didn't. Need more track time.

I did that-sort of, after lowering my van, forgot about the adjustable prop valve in the back that changes with load, road was wet, hauling ---, light went amber, hit the brakes and went sideways, doh! Scared the ---- out of me.

[DodgeZ]

dang!

[J&H Ryan]

Assuming non-power boosting brakes (this done from memory only, so feel free to correct):

driver pushes pedal with 100lbs of force.
5:1 mechanical advantage on pedal
=500lbs on master
master cly has area of one inch squared
creates 500psi of force

If rear cyl (drum) has an area of .5 in squared, it will exhibit 250 lbs of force.

If front caliper (disk) has an area of 1.25 in. squared, it will exhibit 625 lbs of force.

Therefore, a disk/drum prop valve has more force on the rears to make up for the small cylinder size. The disks however require more flow due to the increased size and the fact that discs require more fluid as they wear, unlike drums. This combo is why you have to use the right prop/master combo.

Now, those numbers are theoretical. Disks likely require even less pressure to create as much force because they rely on their square cut seals to return the piston (caliper if free-floating) while drums use heavy springs that the force has to counteract before braking occurs.


So basically, sounds like your going to fishtail kevin

[cordes]

Assuming non-power boosting brakes (this done from memory only, so feel free to correct):

driver pushes pedal with 100lbs of force.
5:1 mechanical advantage on pedal
=500lbs on master
master cly has area of one inch squared
creates 500psi of force

If rear cyl (drum) has an area of .5 in squared, it will exhibit 250 lbs of force.

If front caliper (disk) has an area of 1.25 in. squared, it will exhibit 625 lbs of force.

Therefore, a disk/drum prop valve has more force on the rears to make up for the small cylinder size. The disks however require more flow due to the increased size and the fact that discs require more fluid as they wear, unlike drums. This combo is why you have to use the right prop/master combo.

Now, those numbers are theoretical. Disks likely require even less pressure to create as much force because they rely on their square cut seals to return the piston (caliper if free-floating) while drums use heavy springs that the force has to counteract before braking occurs.


So basically, sounds like your going to fishtail kevin

Sounds like we have a non-voodoo answer. Thanks.