Brake Bias - opposite problem to most!

[Fishy Dave]

Hello,

 

My car has the AP brake bias lever pictured HERE (ignore the overspray )

 

I have previously left the position the same as when I bought it as I was happy with the balance. Then when preparing the car for its (and my) first seasons racing I made some changes. I changed the front pads from standard to Mintex, new standard front discs and new standard rear pads on old discs (not perfect i know).

 

I did plenty of miles to bed in the rear pads to the old discs, but found the fronts now lock up, leading to a couple of scary moments during wet practise and on the road. I have tried the bias lever in all positions but can't honestly notice any difference.

 

I have tried locking up on a slippery surface with a spotter as well as jacking the car up so all wheels are off the floor in the manner described in one of my racing books but still can't get the rears to lock at the same time as the fronts.

 

A good bit of pressure on the pedal does lock all 4 wheels, with roughly equal power to both rear wheels, so I know that fluid is passing through the system. Oh, and fluid has been changed within the last few months (with no air in the system).

 

So, firstly, which way do I turn the lever to reduce or increase bias to the rear?

 

Assuming the problem has been caused by the uprated front pads should I uprate the rears to match, this time on new discs?

 

I have read in archived threads that some people have had the bias lever fitted to the front circuit, should I consider this?

 

I'm always on a tight budget, so any cheap tips would be great.

 

Thanks, Dave

 

David Smitheram, Wiltshire (South) AR, 07718 368173.

1400 supersport race car

[Mike Molloy]

Surely the lever just restricts the fluid flow to the rears, allowing you to adjust the rear lock up point (esp. in the wet).

If the fronts are locking then you need a more delicate boot!

 

The McMillan Motorsport solution to balance my Academy => Supergrad car was Mintex 1177 in the front, 1155 in the back. Standard (un-vented) discs and calipers.

Needed a bit of a warm up, but great apart from that!

[jonboylaw]

As Mike said, the Bias valve only reduces pressure to the rear so you can't get any more than equal pressure to the fronts.

 

Only adjustment I know of is to reduce the MC bore to the front (needs dual cylinder set up), but that seems rather extreme, as suggested, try braking more gently as the fronts probably have a lot more stopping power now.

 

 

Jon

[Richard Price]

Dave,

If you have standard calipers with Mintex 1144 pads in the front, and standard calipers with standard pads in the rear, then the bias will be too much to the front.

 

Using the Caterham rear pads that are specified for use with the big front brakes brings the balance closer to neutral.

 

However, making the rears work as they should introduces another problem!

The rears get very hot, and the design of the rear calipers transfers quite a lot of heat into the fluid rather than the caliper body, so fluid boil (giving a soft pedal after a few laps) is much more of an issue.

Changing the fluid to castrol SRF will cure this! (but SRF is four times more expensive than standard fluid!)

 

Edited by - Richard Price on 29 Jul 2009 17:38:34

[Fishy Dave]

Yes, that makes sense, probably me calling the lever a bias lever is wrong? I know it restricts the flow of the fluid to the rear, but unsure which way to turn the lever to restrict more or less? If anyone could look at the photo and tell me I'd be grateful.

 

I do brake firmly when racing of course, it's just unsettling that I now have loads of front wheel braking (or that's what it feels like).

 

Thanks for the tips on pad choice for the rear. I'm hoping that the combination of slower top speeds compared to the bigger engine Sevens and sub 15 minute races will limit any fluid boiling scenarios.

 

 

David Smitheram, Wiltshire (South) AR, 07718 368173.

1400 supersport race car

[Richard Price]

Dave,

I guess your valve is not that new.

The current AP valve is here, with instructions on this drawing here.

[Fishy Dave]

Thanks Richard, that's a big help.

 

Will order some new uprated pads for the rear.

 

David Smitheram, Wiltshire (South) AR, 07718 368173.

1400 supersport race car

[Mike Molloy]

The reason I switched to the Mintex race compounds front *and* rear was precisely the problem above. The standard rear pads overheated to the point where they crumbled! I was left wondering if the rear brakes need some cooling ducts!

[Jason Plato]

try taking the valve out of the rear system Dave ?. As I understand the valve even when "fully open" still has a reduction effect .

 

Then try being more progressive on the pedal 😬

 

here is my Duratec R .... C7 TOP

Taffia joint AO with Al

 

[h_____]

McMM usually recommend 1177s front and 1144s rear (not 1155s) although they recommend different for academy tyres (CR322s still?).

 

Certainly on 48s - the setup works perfectly. Remember you really want the fronts to lock first, albeit, just before the rears.

 

Unless you are super quick, I'd be tempted to take the thing off. Plenty of race winnings cars without bias valves including in the wet.

 

It may be that your pads are making it hard to get the best out the bias valve. what are you running?