S4/S6 rear brake hydraulics

[mcandmar]

This is probably one for UrS4boy, ....cue link to explanation

As shown in the ETKA pic there is a "brake force regulator" attached to the passenger side suspension arm to reduce braking force as the rear of the car rises under heavy braking. This much makes sense as it can reduce braking pressure to the entire back axle. From there the line runs into a T-piece mounted above the subframe which splits the circuit to the left and right brake caliper.

Now for the unusual bit, we discovered a device on the rear subframe which ETKA calls a "brake pressure regulator" which is on the line going to the driver side rear caliper. What we don't understand is why does it regulate one side and not the other?

Anybody got any ideas?

[UrS4boy]

I saw the "Bat Signal"

and went to the Bat Cave to retreive this photo (more comments under photo):



As I understand it, the UrS4 and non-UrS6 ABS system is a three channel system, i.e. left front, right front and rear wheels (one circuit for two wheels). It is my understanding that because the brake pressure regulator on the rear subframe/cross-member is only on the non-EDL UrS4/S6s with the non-EDL ABS system, it is a mechanical means of preventing both rear wheels from locking up under braking pressure. How exactly that is done, I do NOT know. (so only a partial answer).

The EDL cars have 4 channel ABS and do not need that regulator.

[mcandmar]

The plot thickens....

This subframe came out of a 92 S4 Avant, i bought it because it has the rear anti roll bar which i thought only S6's came with. Our 93 S4 Saloon does not have the anti-roll bar. ...yet :mischeif:

Anyhow i just climbed under our S4 and found it does NOT have this "brake pressure regulator" on the rear subframe, the brake line runs directly from the T-piece to the right rear caliper. So is it an avant only thing, or only on cars with a rear anti roll bar? Either way its existence makes no sense to me

[twoqu]

This system was pioneered on the 200Q. According to contemporary reports the valve is sensitive to cornering forces (lateral G) and limits pressure across the axle. Supposedly it is to prevent the inside rear wheel locking which is the one most likely to lift first and delays the point of ABS activation.
Info from ancient magazine article!

[mcandmar]

Sensitive to cornering forces, very interesting. Now that i think about it the main regulator is attached to the passenger side a-arm, so on hard left hand turns that wheel will extend closing the valve and reduce pressure to the entire rear axle. But on hard right hand turns its rendered useless as the passenger side wheel will be under compression holding the valve fully open.

So therefore that thingy on the rear subframe must only reduce brake pressure on the inside wheel while lateral g's are pushing to the left hand side of the car. I'm tempted to cut it open now and see whats inside, how do you make a mechanical valve sensitive to g forces in one direction. Only thing i can think of is a large weight/piston acting against a spring. As the g forces increase the piston will overcome the spring pressure and move to close a valve. I think a dissection is in order..

[UrS4boy]

This system was pioneered on the 200Q. According to contemporary reports the valve is sensitive to cornering forces (lateral G) and limits pressure across the axle. Supposedly it is to prevent the inside rear wheel locking which is the one most likely to lift first and delays the point of ABS activation.
Info from ancient magazine article!

Any chance you could scan that article and post the pertinent bit here? I can add it to an info post once we get more info, e.g. Dr. Mcandmar's dissection.

[UrS4boy]

This subframe came out of a 92 S4 Avant, i bought it because it has the rear anti roll bar which i thought only S6's came with. Our 93 S4 Saloon does not have the anti-roll bar. ...yet :mischeif: Anyhow i just climbed under our S4 and found it does NOT have this "brake pressure regulator" on the rear subframe, the brake line runs directly from the T-piece to the right rear caliper. So is it an avant only thing, or only on cars with a rear anti roll bar? Either way its existence makes no sense to me

Only the sedans/saloons with the 1BE sport suspension got the rear antiroll bar. This was the 92 MY spec cars and August 1992-built 93 MY UrS4s in NAmerica. Not sure about Europe, the UK, etc. because I think the 1BE might have been an option for you.

As for that brake pressure regulator, it was used for some time on the 3-channel ABS Type 44s and C4s, etc.

[twoqu]

In The Bentley for the later 200 it lists some pressures for the rear.
With the the car lifted 350 -450mm at the right rear and with the pressure on the left hand caliper at 40 bar, the right hand caliper should show 20 bar (+/- 5 bar). So that almost half!
It vaguely implies its to stop rear wheel lock during cornering.

[twoqu]

Any chance you could scan that article and post the pertinent bit here? I can add it to an info post once we get more info, e.g. Dr. Mcandmar's dissection.

It's a very poor copy in a collection of articles about the 200. I'll quote directly what it says and give the details of the article.

The brakes themselves are ABS as standard with the extra refinement that the rear brake pressure limiting device incorporates a valve which is sensitive to cornering force. Thus the rear braking effect is reduced in hard cornering which improves the distribution of breaking effort (since the inside rear wheel is always the one which is most likely to lift) and delays the point at which the ABS needs to come into operation.

Motor
A quiver of Quattros
22 Sept 1984

I was a bit confused about the inside rear wheel comment but I guess its based on LHD models. To us in the UK, the inside rear wheel would be the left!

[UrS4boy]

I was a bit confused about the inside rear wheel comment but I guess its based on LHD models. To us in the UK, the inside rear wheel would be the left!

Thanks for the quote (scan would still be better )

Hmm...in NAmerica, with LHD, the "inside" wheel isn't fixed, it varies from left to right. On a right hand turn, it is on the right hand side. On a left hand turn, it is on the left hand side.

I didn't realize that all your turns were left handers in the UK. Must be fun driving to Tesco.

[mcandmar]

Managed to get it off without breaking any of the lines, its pretty similar to what i thought. As you tilt the unit from side to side there is a large weight inside that moves back and forth. If the weight moves all the way to the end with the yellow label it blocks off the flow. With it tilted the other way i can blow though it freely.

Having said that it may not completely block flow, i was only blowing through it with a piece of hose in my mouth. Obviously brake pressure would be considerably higher pressures.

Note in the pic attached the bracket just below it sits perfectly level, should give you an idea of the angle the valve is mounted on the subframe. So basically as the car rolls on hard right hand turns the unit will be level allowing the weight to move to the end and cutoff/restrict pressure to the right hand rear wheel. (or inside wheel on a right turn)

Interesting solution, but i am probably not going to use it, i can swap subframes without having to disconnect any of the brake lines if i leave the setup as is. I do have both hyd lines for it so it would be a bolt on upgrade, but i just couldnt be bothered making a mess everywhere and having to bleed brakes when i'm done.

Also worth pointing out the old subframe dosent have a bracket to mount it, so there must be two different subframe parts numbers to differentiate them. I also tried to disassemble the valve as originally the end cap was threaded into the body, no chance its going to separate now without cutting it open so i didnt bother.

[twoqu]

Thanks for the quote (scan would still be better )

Hmm...in NAmerica, with LHD, the "inside" wheel isn't fixed, it varies from left to right. On a right hand turn, it is on the right hand side. On a left hand turn, it is on the left hand side.

I didn't realize that all your turns were left handers in the UK. Must be fun driving to Tesco.

Well whilst your version of inside varies, it is evident that the pressure restriction is only on the right hand rear wheel. In that sense it is fixed.

[mcandmar]

I also take "inside wheel" to mean whichever wheel is on the inside of the corner. The statement is true in the sense the main regulating valve will operate on left hand turns, i.e. left rear on inside. And the 2nd valve on the subframe on right hand turns, i.e. right rear on inside.

Only design flaw is on left hand turns the pressure is reduced on both rear wheels, where as on right hand turns the pressure is only reduced on the right hand wheel.

Its all irrelevant for me anyway as the lever for the main regulating valve rusted off many years ago

[UrS4boy]

...no chance its going to separate now without cutting it open so i didnt bother.

Oh go on, Dr. Mcandmar, brain surgeon. Get out the saws and let's have look in side. Please. (I am thinking the sliding weight is a piston and it might be two-way, not just one - prove me wrong)

[s2_wot]

Cut it open please, if you don't I will. I love cutting stuff open.