SLR Brake Bias Valve

[gustosomerset]

Apologies if this has been covered elsewhere (no doubt it has, but I couldn't find it).

Having recently acquired SLR No2 I was delighted to be contacted by the man that built it at Caterham who mentioned that he remembered fitting a brake bias valve under the brake master cylinder at the factory and was it still there? Indeed it is - see pic. As far as I know these were not standard on SLRs? 

What little I've read so far suggests that it should be left well alone unless for serious track use - which I won't be doing - but does anyone have any other advice? There don't seem to be any markings on the adjustment so it's not clear how it is set (other than it could obviously be screwed in further).  I'm intrigued to know why Caterham thought it a good idea to add this to what was the press car...

[Gridgway]

I think it is a rear brake limiting valve rather than a true balance adjuster.

In race cars when we want to know where on earth the balance is we do this...

Get car off the ground

One person in the driver's seat, one outside to turn the wheels

The driver slowly presses the brake pedal until the wheel turner just can't turn the front wheel they are trying to turn

Then with the brake pedal held in the same place/pressure, the wheel turner should be able to turn the rear wheels "just"

It's an acquired taste to get right, how much the rear should turn with the front just locked.  But it's a start.

[gustosomerset]

Thanks very much for that. So as a limiting valve, presumably the more you screw it in, the more it limits the rear brakes or, in effect, moves the bias towards the front?

[Gridgway]

Yes essentially that's it.  It just removes the rear and the front stays the same give or take.

It's not obvious what the point is though as normally the car is going to be ok in dry driving.  What you mostly want is to move the bias backwards in the wet.  So really you need a front limiter rather than a rear one!

[Richard Price]

"I think it is a rear brake limiting valve rather than a true balance adjuster."

Looking at the photo, it looks just like a JPE I did a few jobs on a few years ago, with a limiting valve plumbed into the front brake line, as can be seen in the photo, where the valve is fixed on the back edge of the pedalbox.

I really dislike any of these valves. They're a bodge to try and improve front to rear bias. They're normally plumbed in the rear brakes, but, like this car, some times in the front.

Ideally, the rear brakes will lock up just after the fronts, but, what works on one car will not necessarily work on another car.... Wheel and tyre size and type, weight distribution, camber angle, all have an effect on each axles stopping power. Adding to that, different friction materials with different temperature to coefficient of friction characteristics, further complicate issues. Particularly where large ventilated discs are used on the front (that have to work less hard, and easily loose whatever heat is generated due to the discs being ventilated, and out in the fresh air), and standard solid rears are used (that have to work hard if front/rear bias is optimized, and, the solid discs build heat quite rapidly, but retain that heat longer due to being enclosed). 

[Richard Price]

"So as a limiting valve, presumably the more you screw it in, the more it limits the rear brakes or, in effect, moves the bias towards the front?"

I would suggest that the valve would operate in the opposite direction. The screw applies a preload to a spring inside the valve. The more preload applied, the less it limits pressure.

[Richard Price]

"So really you need a front limiter rather than a rear one!"

Or, dual master cylinders with a bias bar.

I'd agree, when I originally modified my car to accommodate dual master cylinders with a bias bar, I was looking to improve rear braking. Since doing this modification, I've been free to change all of the factors mentioned above, knowing that I can still optimize bias without having to experiment with different friction materials.

[gustosomerset]

Ah...OK - so on that basis, if it was fully screwed in there should be no limit applied to the rear...and the brakes would work 'normally' - i.e. as if there were no limiting valve? That said, the brakes work just fine so I think I'll leave well alone...

[Richard Price]

gustosomerset,

I'm pretty sure your valve is plumbed into the FRONT brakes.

Look at where the pipe goes to!

[Gridgway]

If it is on the front circuit it kind of makes more sense to me.  If it's there to give a fine adjustment to the conditions that's ok.  If it's there to correct an inherent inbalance then it's poor engineering.