Brake Bias Valves

[Griff]

I've just fitted Mintex 1144 pads to the back and they're obviously a lot more sticky than the Lucas pads on there before as rear end locks pretty early. I have a bias valve which I haven't fiddled with yet, but checking this seems to be connected to the front brake circuit whereas I would have thought this would be on the rear circuit

 

Q1 - is it OK to have the bias valve on the front circuit?

Q2 - which way should the knob be turned to increase front braking? It's about midway at the moment.

 

Cheers,

Mike

 

Edited by - griff on 1 May 2003 22:32:11

[Julian Thompson]

Mike

 

Sorry. Edited this because I am not sure I'm right here.....

 

There are clever people contributing to this thread now....

 

Edited by - Julian Thompson on 1 May 2003 22:48:06

 

Edited by - Julian Thompson on 2 May 2003 19:28:55

[Jason Plato]

I would suggest you remove the bias from the front brake circuit altogether .

 

My car originally had the bias fitted by the factory to the front circuit - Car was factory built for first owner . I was told a story about a Caterham fitter with poor hearing who was tasked with fitting bias valves in '97

 

The valve even when fully unwound still reduces front braking by more than 10% .

 

It should be possible to take the valve out and reconnect the front brake pipes without the use of any addiional piping if like mine the valve is located next to the master cylinder . Obviously you will need to bleed the brakes after .

 

This alone made a big improvement to my ballance when using 1144's all round on big brakes .

 

dave

[Chelspeed]

The superlite R race cars from 2 or 3 years ago had the brake limiter valve in the front circuit, never really worked out why.

 

If the rear brakes are rubbish making the front brakes rubbish (with the limiter) doesn't seem the right way to restore some balance!

[Peter Carmichael]

*The device is not a limiter*.

 

It applies a hydraulic gearing ratio to the line pressure after a preload is overcome. To get more braking in the circuit in which it was fitted you need to wind it up so that the preload is less easily overcome.

 

Edited by - Peter Carmichael on 2 May 2003 20:47:53

[Rob Walker]

I have been advised that fitting a limiting valve in the front circuit is dangerous. Something to do with hysteresiss and this can hold on the front brakes slightly after the peddle pressure is removed. Mine is fitted in the rear circuit and works very well.

[Tight fart]

My car was built by Caterham and also has the bias valve on the front circuit, I was told that srewed fully in (clockwise) gave full power to the fronts i.e. as if no bias fitted, and to screw out gives the rear bias.

Now I'm not saying this is correct as I have heard different stories.

 

SLR for sale

T.F@O.F.

www.griptv.com

[Number31]

To quote from the AP Competition Catalogue

 

(See it here)

 

"Installation

To obtain the best performance using these valves, the brake balance should be biased towards the rear so that with the valve piped in to the rear line and set in position 7 (or the cap screwed right in) where virtually no reductions occurs, the balance is as much to the rear as will ever be needed. Moving the control lever to a lower setting (or screwing the cap outwards) will progressively reduce the rear line pressure giving more bias to the front, for use when more grip is available."

 

If you look at the spec sheet that comes with the valves, the 2003 lever type "fully in" still reduced the output pressure by about 20%, while the screw cap type get down to about 5%. When fully out both reduce the output pressure by about 66%.

 

From experience, the AP valve appears to suffer less hysteresis than some of the cheaper valves which have a simpler needle valve construction which is more prone to stick.

 

The SLRs and Roadsports A racers were allowed to run valves in the front circuit up to 2003. This is now specifically excluded in the regs. From chats I've had with several people, the front valve was fitted to allow a side to side adjustment to allow you to reduce the line pressure to a locking front wheel. A back valve was also fitted to adjust the classical front to back bias.

 

Hope this helps.

 

Number31

[westy]

They must be fitted to the front if a car is under braked at the rear.

[Griff]

Thanks to all for the comments. Now correct me if I'm wrong:-

a) To increase front braking (with the bias valve in the front circuit) I need to screw it right in? (Clockwise)

b) But to further reduce rear braking I might need to plumb it into the rear circuit and screw it out? (Anticlockwise)

c) Or perhaps fit 1144 pads to the front as well?

It concerns me that a bias valve in the front circuit could lose front braking efficiency (irrespective of the balance front-rear) when the fronts do most of the hard work. Strange that Catherham should recommend this, or is it? If I'm wrong with the above, please put me straight.

Cheers,

Mike

 

Edited by - griff on 2 May 2003 11:43:58

[Jason Plato]

No matter if it changes bias , limits , imparts hydraulic gearing ratio , ballances or goes hysterical you are better of without it in the front circuit where we are looking for maximum braking power - it simply reduces the braking force in the circuit its fitted to , even when fully screwed in .

I had a lot better braking when I removed it .

 

If you have one front locking before another , I would suggest getting the car flat floored .

 

Dave

[Peter Carmichael]

Doh! I made a mistake in my posting - now corrected. Winding up the proportioning valve gives more braking in the circuit to which it is fitted.

[Tight fart]

Can I just say on behalf of every body here, it's about time you got something wrong Peter. 😬

[Paul Gibb]

Right now I am totally confused.

A brake bias valve has literally just been installed into the rear circuit of L7FUN to allow damping of the now uprated rear brakes. I had assumed that screwing home the valve (ie turning it clockwise) would reduce rear braking effort - but 31 & Peter's posts suggest otherwise.

Could somebody please explian in plain english what is going on (Dave K, I know, I know, don't say you told me so!)

Paul

 

L7 FUN

 

Edited by - paul gibb on 2 May 2003 22:42:11

[Peter Carmichael]

Definitive: "The more you wind up the valve, the less it decreases the braking in that circuit."

 

I've got the graphs and an internal drawing around here somewhere...

[F355GTS]

Paul

 

if it helps I turn mine clockwise to reduce rear braking effect on a bias valve in the rear circuit

 

Mark

[Paul Gibb]

Thanks guys. Unfortunately..............

 

unless I am misunderstanding something basic here (I am assuming wind up means turning clockwise as in turning off a tap) these opinions seem contradictory ????

Paul

 

 

[David Hopkins]

.

[David Hopkins]

There are two types of valves .

 

1. Shut off type ie when line pressure reaches a pre determined value the valve closes and no more pressure increase occurs in the circuit the valve is fitted to . This type is primarily suited to short wheel base where there is a lot of weight transfer during braking such as a mini .

 

2. Reducing valve , this is probably whats fitted to your car . Basically inside the valve there is a piston which has diffrent diameters at each end , acting on that piston is a spring . When you adjust the valve , screwwing it in increases the preload and hence the point or pressure at which the valve starts to limit the line pressure . Screwing it out obviously gives the opposite effect . The differential areas relating to the piston gives you a ratio after the valve cuts in so a 2:1 ratio valve after it cuts in will give you half as much increase in line pressure in the circuit its fitted in compare to the one that doesnt have it .

 

Any more questions give me a ring 07785 287712.

[Jason Plato]

I think there is some confusion between us all and the numerous different types and designs of equipment . I know for instance that Peter has a different one to me ...

 

On the valve which I had fitted as std from the factory into the front circuit , there is a knurled knob with a bar coming out and around it . It also has the markings - OZ SA3 & 0197 on the casting .

 

When this knob is screwed in full braking force is available ( or around 90% in my experience ) , when the knob is screwed out or even fully unscrewed from its threads , the braking force is far less .

 

Of course the simple way to check is to screw it in - and do a brake test and screw it out - do a brake test .The difference was noticeable .

 

On mine I found that the rear brakes locked up much sooner when the knob was fully unscrewed out - remember it was fitted on the front circuit . On complete removal the fronts locked up before the rears and all was well

 

The ones pictured in the AP link mentioned earlier are different to the one I had fitted .

 

Dave

[Peter Carmichael]

Hmmm. Dave you are describing exactly the type of valve I use and the behaviour I have described.

 

"The more you wind up the valve, the less it decreases the braking in that circuit."

 

Rearranging your words:

DJ - when the knob was fully unscrewed out

PC - the more it decreases the braking in the circuit to which it is fitted

DJ - it was fitted to the front circuit

PC - so fully unwound means maximum bias in favour of the rear

DJ - I found that the rear brakes locked up much sooner - ie. not much front braking

PC - QED

 

I think we now have everybody agreeing apart from Mark. Perhaps you would like to check under the bonnet and make sure which circuit it is fitted to...

 

[Griff]

What a can of worms....

I think I understand where to go from here - a quiet road on a damp day to avoid flat spotting tyres and try both extremes. If I understand everyone, maximum front braking will be achieved with the valve fully screwed in (clockwise) when fitted to the front circuit.

The issue of whether or not to fit the bias valve to the rear circuit remains inconclusive, though a loss of 10% or more of front braking efficiency with it fitted to the front circuit sounds like a big penalty and a contradiction compared to the expense of four pot calipers and associated costs.

Are there any opions on whether it's worth uprating the front pads to 1144s to match the rears?

Thanks again for all the advice.

Mike

[Jason Plato]

I run 1144's all round - without the valve fitted .

 

Brakes are excellent .

 

Dave

[Rob Walker]

Griff,

 

The bias valve should be fitted into the rear circuit. With the valve screwed fully in gives least restriction and maximum rear braking. Not sure were you got the 10% loss in braking from ??? When the valve is screwd fully out giving most restriction this will only reduce the restriction to 95% giving 5% rear line pressure. This is designed in safety margin to prevent the rear brakes being unwittingly turned completey off.

 

I would not fit a valve into the front circuit.

[westy]

you would only fit a bias valve if the the car is overbraked at the front or rear, and it must go on the circuit that is causing the problem. If the rears are not getting enough pressure it's no good putting the valve on the rear circuit.

 

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