Setting up Bias valve help

21st April 2015, 15:06

On my track VTS I have removed the load valve and replaced all the brake lines.

I have plumbed in a bias valve so I can adjust the rear brake pressure to stop the rear brakes locking up too easily.

I am in the process of setting it up however I am not 100% sure which way I should be turning the knob.

Am I right in saying that if I turn the knob clockwise adding more pressure (according to the sticker on top of the knob) this will reduce the braking performance to the rear brakes.

I was chatting to a friend and he thinks its the other way round and I am now questioning myself.

I am currently using the car with the bias valve fully clockwise and the rear of the car seems a bit lively when braking from 60/70mph.

Thanks

22nd April 2015, 14:02

Craig

I know this isn't a response to the question you asked but saves making another thread...
would just like to know more about what you did and how you did it?

Had a new axle on my saxo last year at which point it was lowered 40mm.
The brake lines were seized into the old axle a good'un so i chopped them off and made new lines. Kris did advise me the bias valve was no good but i didnt really understand as to why and what was wrong with it.

Admittedly, through the winter and even now i've not been under that end of the car so i dont know what state it is in, the last i can remember the spring was very slack, i dont think the hook end of it was attached to anything....

Subsequently, under hard braking or any sort of braking at speed, the car is VERY skittish, i nearly died yesterday coming through the twisties when someone decided they would park on a bend - as such a had to slam on my brakes. as you can expect the back skipped out, i actually know how to drive lol and managed to correct it but it was literally a 75 degree skip.

when it happens, the nose always swings to the left (pavement) so i dont know if its a bad line or what as it seems to be left/right as opposed to front/back, it always skips the same way.

anyway, i've had a look online and cant seem to find the brake bias valves anywhere to buy new, i'd rather not buy a second hand one.
this axle is also on its way out, so i'm thinking while its off it may be worth making a custom bias valve as you have done, rather than sourcing a second hand one that may not be any good anyway.

parts are cheap enough, i can imagine its just the valve itself and quite a length of brake pipe?
Would you start with the valve inside the car and work your way outwards? i.e valve near handbrake or somewhere tidy, then thread the pipes through like you would sub wiring for example, then to the front/back as necessary?

i didnt really want to touch the front brakes as they are spot on, had the pads off the donor saxo, as did the back!
Either way, if you're braking in a straight line, even if the backs did lock up you should just brake straight?

Any suggestions etc are welcome, especially with regards to how to JUST fix the back and leave the front alone.

really just want to get it back to a safe standard!

23rd April 2015, 11:58

On my VTS I removed the ABS system and changed to a 4 port 23mm master cylinder instead of the 2 port master cylinder. If your car is a VTR then you will already have a 4 port master cylinder as you don't have ABS fitted.

2 of the ports are for the front brake pipes but for the rears I blocked off one of the ports and from the other port, I ran a brake line through the bulk head and inside the car along the transmission tunnel (the car is fully stripped out). Just behind the handbrake the brake line goes into the bias valve so I can adjust as you are driving.

A single brake lines comes out the rear of the bias valve and then into a T piece further back. I routed the 2 brake lines through a rubber bung above the fuel tank. the load sensing valve was removed and the brake pipes connect to some braided lines/flexible brake piping and then into the calipers.

Hope this makes sense. You problems could be down to a few things including

Flexible brake hoses not allowing full flow
brake caliper seized so only one side works
master cylinder issue

The standard load sensing valve controls the flow to both rear brakes, so if one side brake is doing more than the other causing the back end to steer under braking then I wouldn't have thought the load valve is at fault.

28th April 2015, 09:25

thanks craig that makes perfect sense.

266 conversion is on the cards so i may aswell get some new hoses for the backs as well, can't second guess your braking system.
i had another vtr (which was down as an S on logbook, no abs or pass airbag though) which i took the entire axle off and swapped over, brakes etc the lot, they were in good condition when it went on in... november? brand new pads and calipers seemed ok, so i think i can rule that one out.

as standard, what is the m/c layout?

you've gone front/back like i will, as standard is it lf/rr + rf/lr or one each corner?

i think it's more than likely a restricted hose like you said, that would make perfect sense as the bias is left/right
What issues could i have with m/c and how would i diagnose this?

as i'd quite like to keep my interior, i supposed i could do the same as you have done as far as the m/c, then put in the bias valve in the engine bay between the m/c and rear brakes.

Thanks again Craig, given me something to think about, never even contemplated the hoses or m/c being an issue, always the little things you overlook!

I'll give you an add, once upon a time i was actually quite adept at motorsport prep but it's been a few years since i've had anything to do with a track.
Cheers buddy!

26th October 2015, 14:19

lefty loosy - righty tighty

27th October 2015, 14:36

the only sensible place to mount the bias valve is next to h/brake .so you can adjust it when driving /testing -just keep trying full power stops and adjusting it till you get rears just !! locking before fronts lock -for best track use
as you can still steer with backs locked --but not so the fronts .
having under bonnet will piss you off as it will take forever to set it up and you cannot alter to suit road conditions --its one of the things you set on the warm up lap as every day will be different and so it will need "slight" adjustment to get the perfect balance .

car will then "squat" when you brake not nose dive and cause over heating of front brakes