How to get more rear brake bias?

[fredmeister]

Originally Posted by NathanUK

I'm sure it does, but it will not increase pressure to start with.

Correct! The Tilton valve and any other like it simply reduces the maximum available original pressure in the line by varying increments according to the position of the handle.
If you need more rear braking hydraulic line pressure than originally designed your only choice is to limit the front line pressure in relation to the rear and then step harder on the brake pedal to get overall line pressure to rise.
The OEM 930 brakes proportion line pressure by sizes of the piston diameters within the caliper themselves. Even though dimensionally the calipers are the same housings, the pistons bores are different front and rear. The fronts being larger and therefore exert more force on the front pads when squeezed.
You need to understand hydraulic theory to really make educated decisions regarding redesigning the system which is what you are doing when changing out calipers and installing reduction valves like the Tilton.
Some of the later 911 had a small pressure limiting valve on the rear brakes but I think it was 87 or later Carrera but not sure. This was installed to prevent rear lockups and the resulting spins that would accompany it. It did not help in any way for someone to maximize rear brake potential as it was not adjustable.
Try changing pads as this is easiest but not infinitely adjustable like a Tilton valve. More trial and error.....

[NathanUK]

I have an early 3.2 and I've only had the fronts lock up. I'd like a little more to the rear but it doesn't bother me.

[spuggy]

Originally Posted by NathanUK

+1 The valve cannot increase pressure.


I don't think that was what Spuggy meant. I think he was referring to the stock 3.2 part which IIRC is a limiting device, not a bias valve.
Hopefully Spuggy will advise us.

The 3.2 part (and most factory brake bias valves from most manufacturers) is a non-adjustable bias valve with a pre-set "knee" pressure.

These permit full pressure to the rears until a pre-set "knee" pressure, then starts to limit the pressure at a fixed ratio above that.

Porsche actually used 4 different factory bias valves IIRC, with knee pressures of, I think, 33, 45, 55 or 65 bar- of which only the first two are of any interest for road use).

If you need more rear braking hydraulic line pressure than originally designed your only choice is to limit the front line pressure in relation to the rear and then step harder on the brake pedal to get overall line pressure to rise.

This would be potentially very dangerous IMHO. The behavior of such a system would be that you got too much front bias at lower pressures, until the pressure reached the "knee". And then, as system pressure got progressively higher, pressure would start to rise faster in the rear until the rears locked.

Don't take my word for it. StopTech do this for a living, two quotes from them:

If you have the deeply-rooted need to install your own adjustable proportioning valve, be advised that they should NEVER be installed in-line to the front brakes. The effect would be to make your vehicle rear-biased before you could say “terminal oversteer.” Front brake line pressure should always be left alone – only the rear pressures should be considered for proportioning.

So how does one select the right kneepoint and slope? Without the test and measurement resources of a major automotive manufacturer, it’s next to impossible to say. Of course, you could trial-and-error your way into a solution that you believe to be appropriate, but without testing under all conditions of loading, speed, and road conditions there might be one operating condition just waiting to bite you.

In short, if you find yourself thinking, “I wonder how I would pick the right proportioning valve for my car?” you probably shouldn’t be changing it yourself.

From this: http://www.stoptech.com/tech_info/wp_proportioning_valves.shtml

[billjam]

Originally Posted by Craig 930 RS

930s use the Pagid size 1203 F&R -

Some folks use a higher friction rear (Black) with Yellow up front to get the rears to work more; typically I see this with 996 GT3 customers.

Reference: www.Rennstore.com

Thanks everyone.
Looks like Craig's suggestion is the most practical approach. I guess some yellow Pagids for the front have found their way onto the shopping list.

I presume I'm not pioneering here. Can anyone running this pad combination comment?
Wouldn't the F-R balance change as pads got really hot?
If the yellows are a higher performance pad, I would expect under hard use or track use they would work better than the black rears and so increase the bias to the front? (which is better than going the other way )

[911st]

My thoughts for what it is worth.

Verify the front and rear pads are the same mfg and type.

If just one front is locking, check corner balance and lean toward equal weights on each fronts instead of 50% diagonals.

I asunm you do not have extream negitive camber.

Next, if possable I would lean toward larger front tires. Use the fronts braking potental to your advantage. I believe you do not want the front tires to be more than 1" less in width than the rears. 255's are about 10.4" and 225's are about 9.1".

If you have a pressure limiter for the rear it will be under a little board cover smaller than a playing card that is retained with two bent over tabs that hide it. It is inline with a brake line that comes from the master cylinder in the front trunk and snakes down into the sunglass compartment. It is a bout an inch in diameter and about 2" long. I do not know if the 930's have these. The SC's did not. The Carrera 3.2's did. If you have one, you can replace w a different one. Or plumb in a adjustable bias valve (tilton) in its place. You could use SS flex brake lines and put it on or under your dash.

A best practices method of gaining control over your brake bias is an adjustable dual master set up. Smart Racing makes a conversion unit that works with your boosted brakes and bolts up in the trunk area where the existing brake cylinder is located. The SC/RS unit that goes behind the pedal bos is for a non bosted brake system I belive.

So check pads first, larger front tires second, play w different pad compounds third, adjustable brake bias last.

[rs27carrera]

Some rare time, having old rubber hoses, they may get clogged (layers will depart and they close tube). So you may check/replace old rear (and front) rubber hoses and use OEM or some better aftermarket products. If hoses are trouble, you may spend a lot of money on other stuff and still do not find cure to trouble.

Anyhow, it makes sense to replace old rubber hoses. Just recently I purchased all new fuel hoses as it's cheap insurance against burst fuel hose and fire (years ago I got fuel hose to burst, luckily no fire).

[NathanUK]

Originally Posted by spuggy

http://www.stoptech.com/tech_info/wp_proportioning_valves.shtml

I had no idea those Tilton devices worked like that.

Thanks for the link Spuggy.

I don't see the need for the 'knee' operation at all.
Surely it would make driving on ice tricky? Or is the idea to make the car more manouverable on ice/snow?

[georgeinhere]

I had the same issue...my mechanic installed new pads in front and the car basically would lock up and slide.
We ended up putting new pads in the back and the issue has been solved.

[spuggy]

Originally Posted by NathanUK

I had no idea those Tilton devices worked like that.

Thanks for the link Spuggy.

Welcome mate. I'm pretty sure that ALL brake bias valves, industry-wide, work like that - if fitted.

Originally Posted by NathanUK

I don't see the need for the 'knee' operation at all.
Surely it would make driving on ice tricky? Or is the idea to make the car more manouverable on ice/snow?

All brake bias is a compromise and a stock, street-driven car has different requirements at 30 MPH and 100MPH. Are the pre-computerized, non-ABS, factory brake bias ratios perfect for all circumstances? Probably not, and probably cannot ever be. So it errs on the side of safety in most (all) scenarios, even in the hands of someone who's never driven a rear-wheel drive car before.

Front bias = understeer as the fronts lock first. Mostly safe.
Too much rear bias = terminal oversteer if the rears lock first, which might only happen under "panic" braking...

I'm told for very fast track work, you probably want more front bias, as you're mostly hauling it down from high speeds anyway.

For town, rally or AX, more rear bias can be good for various reasons (better initial "bite", maintain steering control on loose surfaces, start car rotating faster).

This is why pro setups tend to use dual master cylinders, one for each circuit, with an adjustable (sometimes driver/in-car adjustable) balance bar, so the bias ratio can be set according to conditions, usage, preferences, fuel load etc.

The idea behind a bias valve on a factory car is that you can have relatively oversized rear brakes do more work at the onset of braking (when they're most effective because they have the most weight on the rear wheels), but the amount of work the rears do is controlled/reduced as braking force increases and more weight is thrown forward onto the front wheels, thereby reducing traction at the rear.

The 3.2 is the only non-ABS 911 with a bias valve (necessary due to the disproportionately large single rear pistons) - every other car - including the '78-up 930s - achieved the factory-desired front/rear bias (1.6) from the ratio of front caliper piston area versus the rear caliper piston area and doesn't use one.

The fronts are SUPPOSED to lock before the rears except for very specialist applications with skilled drivers. That's what should happen if they are correctly set-up and maintained.

Porsche brakes have pretty much always been the industry yardstick for their time. Even the relatively puny early car brakes will lock up the fronts under hard braking, they just take more pedal pressure/provide less feel and don't have the thermal capacity of the later/bigger brakes.

[NathanUK]

I'm quite happy for the fronts to lock up first, it's just that in my 3.2 it seems excessive.

I've never locked the brakes at high speed. I'll have to try it when I go to Brands Hatch in June

On a smooth surface, barely covered with small gravel at say 5mph the fronts lock very easily and can be a little frustrating, not to mention embarrassing. Same as braking at say 20mph while cornering, the front inside wheel has very little weight and just locks up... You wouldn't do this at a track, but on the road in real life with real people it can happen...

Now I'm thinking, I wonder if a policeman would consider this to be driving dangerously. We know why it happens but how does it appear to the average person...