Brake bias knob - wrong type ?

morgan · Post by **morgan** » Fri Jan 17, 14 9:23 am

Chaps -

Gonna cut my gorgeous brake pipes about to pop a bias valve in. Deal with the old 'rear end lockup'. Bought the right size fittings and a screw type valve tou mount in the floor and route my pipes into - so far so good as a plan.

Just reading the operation instructions and it is describing something different - which doesnt sound right to me. I was expecting it to be a variable one-way valve i.e. the harder its screwed down, the tighter it limits one-way flow, whilst maintaining normal backflow (i.e. brake release is not impacted).
This thing reads that its fixed as a 3:1 pressure reduction and the knob merely changes the point this happens.

That doesnt sound right. (read red ringed text)

Thoughts ? I think it might have to go back.

Dave-R · Post by **Dave-R** » Fri Jan 17, 14 9:40 am

I THOUGHT the type I had just reduced pressure as you turned it.

http://www.summitracing.com/int/parts/s ... /overview/

Maybe I was mistaken?

GJUK · Post by **GJUK** » Fri Jan 17, 14 12:21 pm

You install them on either your rear line, or your front line. Depending on which ones lock up first...

So if your rears lock up, install it on that line, turning in will reduce the pressure to the rears so then you will not have the lock up issue.

You can also wind it right in to do burn outs (100% braking to front)

I have one on my mk1 escort. great fun.

Jon

autofetish · Post by **autofetish** » Fri Jan 17, 14 12:52 pm

I think there describing the two end of the spectrum in the red box

Eg 1 or 10

Does not mention 2-9

morgan · Post by **morgan** » Fri Jan 17, 14 1:59 pm

Hmm. What Dave described was what I was expecting, Wil, your interpretation sounds positive - but the more I read the more I think its not right.

Here is the link to the item - sounds fine - cant see any mention ? http://www.ebay.co.uk/itm/380660503012

See graph below.
This implies that at a certain pressure in the system the valve merely flicks to a reduced flow rate of 3:1. The graph shows the point of that transition changing, but the transition is fixed. This makes no sense to me, why would I want a fixed degredation of 3:1 at a certain pedal pressure ?
What I want is effectively a variable restrictor in the pipe. In fact I cant think of ANY reason I would want a fixed 3:1 reduction ?!?

The graph should be a variable curve, not a series of straights.

There is enough here for me to think this aint right. I reckon its gotta go back. I might give them a call to clarify.

Dave999 · Post by **Dave999** » Fri Jan 17, 14 2:07 pm

it just limits flow

so if you want it to limit flow all the time do it up tight and it will mean that you reduce rear breaking action by 3 every time.

in my book thats too harsh

if you want it to limit flow just before lockup, to avoid lock up undo it a bit do some emergency stops and find the place where it kicks in just before the lockup

i.e all brakes work as before until it kicks in like a limiter on the rears

it achieves what you want its just not how i thought it would work.

Dave

Dave999 · Post by **Dave999** » Fri Jan 17, 14 2:15 pm

can't remeber if the relationship between a restrictor diameter and the flow of a liquid through it is linear

i'm inclined to think its not due to the strange effects of the wall ad restrictor on velocity of the liquid

hence don't trust the graphs

looks like the thing is working much like a basic fuel pressure regulator

and they probably all work like this.....

OEM VW van one uses a weight on a lever

as you brake the weight swings forward and reduces the rear brakeing

Dave

Charger · Post by **Charger** » Fri Jan 17, 14 2:21 pm

the second paragraph below the red box appears to explain why it works the way they describe, reading that it makes sense to me, maybe not what you originally wanted

with that valve you have there it only reduces the flow under heavy braking, the point at which that reduction happens is variable by you by turning the doins, under light braking it has no effect, as in there is no need to reduce the flow under light braking as that’s not the scenario where the rears lock, seems ok to me

Pete · Post by **Pete** » Fri Jan 17, 14 2:32 pm

GJUK wrote: ..................so then you will not have the lock up issue.

You can also wind it right in to do burn outs (100% braking to front)

Jon

For drag racing you really need a line lock that holds the fronts on; not shuts the backs off.

Having a "shut off" valve on the rears is a recipe for forgetting to turn it back on in the heat of battle, and having no rear brakes at the top end.....

morgan · Post by **morgan** » Fri Jan 17, 14 2:40 pm

The ebay description looks fine.
The instructions that came with it are what caused alarm. Gonna contact them...

Dave999 · Post by **Dave999** » Fri Jan 17, 14 4:10 pm

i personally think it sounds like a better solution than something that has an impact on the rear brakes all the time

GJUK · Post by **GJUK** » Fri Jan 17, 14 4:32 pm

Pete wrote:
GJUK wrote: ..................so then you will not have the lock up issue.

You can also wind it right in to do burn outs (100% braking to front)

Jon
For drag racing you really need a line lock that holds the fronts on; not shuts the backs off.

Having a "shut off" valve on the rears is a recipe for forgetting to turn it back on in the heat of battle, and having no rear brakes at the top end.....

Yeah I agree. But if you have time you can twizzel the nob

Pete · Post by **Pete** » Fri Jan 17, 14 4:40 pm

GJUK wrote:Yeah I agree. But if you have time you can twizzel the nob

Yeah...if you run that slow....

Jon Connolly · Post by **Jon Connolly** » Fri Jan 17, 14 5:05 pm

http://www.demon-tweeks.co.uk/motorspor ... ning-valve

Morgan

I would rather use something like this ( there are cheaper brands )

Dave999 · Post by **Dave999** » Fri Jan 17, 14 5:25 pm

yes but it most likley works the same way.
just gives you 2 restrictors for 2 pipes not 1
and a dingle dangle lever

other than isolating the rear of the rear circuit completely from the front of the rear circuit and usuing a piston or diaphraghm to alter or adjust the pressure applied to the rear end
and we know these brake bias things don't do that. if they did you'd never be able to bleed the brakes
the only way to make a change to to rear brakes without swapping wheel cyinders is to slow the rise of pressure in the rear circuit beyond the bais valve so the braking comes in slower

with any restriction that allows some flow of brake fluid the pressure will equalise eventually across the whole rear circuit i.e master cylinder to bias valve and bias valve to wheel cylinder will eventally be the same pressure if you hold the peddle down with the same force for long enough.

Long enough needs to be longer than it takes you to get the car into a controled stop. too short and you lock the rears

Dave