How to get more rear brake bias?

[billjam]

My 930 has standard brakes which are more than adequate for road use and the occasional club event, however I find it too easy to lock up the fronts so I’d like to get a bit more braking out of the rears by adjusting the bias.
I have searched the board for info on this but got seriously bogged down for hours today in a number of threads about brake upgrades and didn’t really find what I was looking for.

What I really want is the easiest (and cheapest?) way to add in an adjustable bias control. It doesn’t even need to be readily accessible. After some testing, I’d be quite happy to set and forget.

I can’t be the first one down this path – can someone show me how they did theirs or point me in the right direction.
TIA
Bill

[purplehaze]

Hi Bill,

Easiest way is probably to use softer compound pads in front, racier/grippier blend in back.

I added that adj bias control you mentioned for the rears. I've never had to throttle it closed to restrict the pressure to the rear.

Share a bit more of your setup as well please.

Too small a front tire/contact patch and it'll lock. I run 275's in front and it grips much better, locks less than 245 fronts.

What pads?

Cheers-

Glenn

[fredmeister]

Be careful going towards more rear bias as soon as you get too close and lock the rears before the fronts you will be going for a spin. Unless it is blatently locking fronts try what was suggested with different pad compounds between front and rear.
Only other way is to install the bias valve....Tilton makes them but be careful as getting it adjusted properly will take you to the edge I mentioned and locked rear brakes are very hard to overcome.
Are you sure the car is corner balanced and you are not just locking one front tire (lightly loaded one) unintentionally?
Assuming you have the stock 917 calipers front and rear? The factory set up should be close to balanced.

[A930Rocket]

+930 on the pads.

With my Pagid yellows, I can easily lock up the fronts on the street (and the track when cold).

[spuggy]

Originally Posted by billjam

My 930 has standard brakes which are more than adequate for road use and the occasional club event, however I find it too easy to lock up the fronts so I’d like to get a bit more braking out of the rears by adjusting the bias.
I have searched the board for info on this but got seriously bogged down for hours today in a number of threads about brake upgrades and didn’t really find what I was looking for.

What I really want is the easiest (and cheapest?) way to add in an adjustable bias control. It doesn’t even need to be readily accessible. After some testing, I’d be quite happy to set and forget.

I can’t be the first one down this path – can someone show me how they did theirs or point me in the right direction.
TIA
Bill

Odd, never heard anyone else complain the fronts on a 930 lock up too soon? Maybe there's a maintenance issue starting you haven't found yet?


Bias valves - a la the one fitted to the 3.2 - are a really bad idea to fitted to the front circuit, according to Bill V. Because these valves are intended to be fitted to the rear circuit, to provide unimpeded braking to the rear at low pedal pressure and then, as you stomp hard on the pedal, the "knee" limits the pressure to prevent locking.

Which is what exactly you want on the 3.2 with the disproportionately oversized 42mm rear caliper - but you NEVER want to limit the pressure to the fronts - because that's where most of the braking should be happening.

I think most people do what you're asking for with a dual master-cylinder setup which allows you to adjust the front/rear bias.

There's even a factory part for that as the SC RS used one:

http://books.google.com/books?id=eeLfBLR_iZ8C&pg=PA222&lpg=PA222&dq=SC+RS+master+cylinder+dual&source=bl&ots=i9Z1rpWPzF&sig=lrlbylZRPO_5WRWM0VUAfBRSY74&hl=en&ei=tR_pSdmJEeKwtge-k7DMBQ&sa=X&oi=book_result&ct=result&resnum=2#PPA225,M1

Dunno if it's big enough for 930 or larger brakes, off hand. I think there's a fair chance - looks like two stock M/C's bolted into the .

You might find that an asymmetric LSD helps too - you can run more rear brake bias with a 40/65 LSD that you can with a factory LSD or an open diff.

[kycarguy 935]

My DP935 came with the Tilton brake bias valve. I left it alone so not sure how well it works.

[billjam]

Originally Posted by purplehaze

Hi Bill,

Easiest way is probably to use softer compound pads in front, racier/grippier blend in back.
I added that adj bias control you mentioned for the rears. I've never had to throttle it closed to restrict the pressure to the rear.
Share a bit more of your setup as well please.
Too small a front tire/contact patch and it'll lock. I run 275's in front and it grips much better, locks less than 245 fronts.

What pads?

Cheers-
Glenn

Wheels and tyres (Goodyear Eagle F1) are fairly conservative ...
- front tyres are 225x45x17 on 7.5x17 wheels
- rear tyres are 255x40x17 on 9x17 wheels
Brake pads are what I inherited with the car. The last time I had them out I checked and they were Pagid brand but were black, so I assume they are not one of the higher spec grades.
Standard LSD is fitted.

Glenn, do you have details, pics, of your valve?

Fred,
I have only recently changed the suspension setup. This was initiated by a front right locking problem.
When I delved into the front suspension I found both TBs were rubbing inside the A-arm. Caught a dose of while-you're-in-there-itis and ended up fitting new 22mm front TBs, new bushes, Bilstein sport shocks. At the same time I did new 28mm rear TBs and bushes to go with the existing rear Bilstein sport shocks. On top of that, I lowered the ride and balanced it within 10kg left-right.
Ride is a little firm, but I still drive it daily and it handles like a dream out in the country.
This thread needs a picture to liven it up ...


My only complaint now is a tendency for either front wheel to lock up under hard braking. I just feel that there is more braking still "on the table" at the rear.

Playing around with different pads may help but I'd prefer to try something a little more incremental as I may not be far off the optimum balance with current setup.
Changing pads is a rather large step change. If I was going to change pads, I'd prefer to do the same thing at both ends (unless there's good evidence to the contrary) and then still try and get best balance with a bias valve.

So, I guess I'm still looking for a simple bias valve solution if there is one.

[A930Rocket]

They could be Pagid blacks for the track or street. You can't always go by the color of the backing plate. Any numbers you can read on the pads and compare to the pagid chart?

[purplehaze]

Originally Posted by billjam

...
Glenn, do you have details, pics, of your valve?

Bill,

Thanks for the photo. Too much text-only and I glaze over.

Here's where I put my valve for the rear. Again, it's never needed to be throttled closed at all. My rear pads and rotors also hardly wore, so I too swapped to softer front pads and firmer rears to get more rear bite.



I've seen found a dual M/C unit to be installed in an upcoming project. For now, the 2 pad compounds do the trick.

[billjam]

Originally Posted by A930Rocket

They could be Pagid blacks for the track or street. You can't always go by the color of the backing plate. Any numbers you can read on the pads and compare to the pagid chart?

Last time I had the pads out I took a photo and tried to identify what they were.
The agent in Australia couldn't make sense of the numbers so I gave up and just assumed they were a Pagid OE equivalent.

How do these fit into the "softer / firmer" debate?
Give me some hints. How about some examples of which pads are softer, and which are harder?

Can someone decipher these numbers for me ...

[billjam]

Originally Posted by purplehaze

Bill,

Here's where I put my valve for the rear. Again, it's never needed to be throttled closed at all. My rear pads and rotors also hardly wore, so I too swapped to softer front pads and firmer rears to get more rear bite.

Glenn, I don't see how a valve in the line to the rear brakes can do anything other than reduce rear braking. I want to increase it.
I was sort of hoping that there was an existing valve in the system somewhere that was limiting rear braking and that I might be able to adjust it, bypass it or replace it with an adjustable one like yours.

Am I correct in assuming that the amount of rear braking is managed by different piston sizes in master cylinder and calipers and not by an external valve?
This would indicate why people are playing with different MCs and pads.

I could change the bias by putting a valve in the front brake line, but there is no way you would want to limit pressure to the front (as spuggy noted earlier).

[billjam]

Originally Posted by kycarguy 935

My DP935 came with the Tilton brake bias valve. I left it alone so not sure how well it works.

Can you explain or show where it fits into the brake circuit?

[A930Rocket]

I installed a Tilton rear brake bias valve in my car when I went to Big Reds. Thought I might need it, but the car has been great using Pagid oranges, blaks and now yellows all around.

To install one, you have to find the circuit you want to install it in and find a line you can splice in the valve. For the rear, there is a line that goes into the smugglers box you can remove, and with a new line on each end of the valve, install it. I mounted mine in the cockpit centerd under the dash kneeebar.

[gumba]

Read the Tilton instruction sheet. It doesn't just decrease the pressure to the rear brakes.

[purplehaze]

Bill,

You're right - that valve only reduces. I was just showing how/where to add in an adj bias valve as you asked.

You're also correct that "the amount of rear braking is managed by different piston sizes in master cylinder and calipers and not by an external valve" - hence our compound suggestions.

Hopefully Craig, or Steve @ Rennsport will chime in about your current Pagid pads. Or PM them.

The idiots guide to Pagid, in terms of soft to harder biting compounds (and ability to endure heat) are yellow, orange, black, to keep it simple.

The Orange compound left deposits on my front rotors (as they have for others) and the rotors needed to be turned to eliminate serious vibration. I tossed them. I use either Blacks or PFC 97's in the rear for better bite, and yellows in the front for a little less aggressive bite.

If you have stock pads, you're at the point where you'd benefit from race pads.

Good luck!

Glenn

[gumba]

Tilton installation notes: A typical handle setup has the handle oriented in such a way that when the handle is pushed towards the front of the vehicle it REDUCES the rear brake pressure. When the handle is moved toward the rear of the vehicle in INCREASES the rear brake pressure.

[WERK I]

Have you thought of going softer in the front? (softer torsion bars/struts) Or wider front tires?

[NathanUK]

Originally Posted by billjam

Glenn, I don't see how a valve in the line to the rear brakes can do anything other than reduce rear braking. I want to increase it.

+1 The valve cannot increase pressure.

Originally Posted by billjam

I could change the bias by putting a valve in the front brake line, but there is no way you would want to limit pressure to the front (as spuggy noted earlier).

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.

[NathanUK]

Originally Posted by gumba

Tilton installation notes: A typical handle setup has the handle oriented in such a way that when the handle is pushed towards the front of the vehicle it REDUCES the rear brake pressure. When the handle is moved toward the rear of the vehicle in INCREASES the rear brake pressure.

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

[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