Brake bias valves and T-pieces - Brindabella Motor Sport Club

Mark · 22-02-2006, 10:15 AM

I have a rally car. I want to put a brake bias valve in it. However it has standard brakes. Hence sticking in a brake bias valve can only restrict the pressure going via the rear linel, to the rear brakes. Obviously I could stick the brake bias valve in the front line but that's just all dirty and wrong.

I've had no problems before when upgrading an old car to much bigger and more effective rear brakes then wacking in a brake bias valve. So, yes, I know how to fit one thank you.

It was suggested to me that it is the rear T-piece that controls a fair bit of the pressure going to the rear brakes. I personally thought that the pressure front-to-back would have been a result of the sizing and plumbing in the master cylinder therefore a pain in the arse to fix, other than to get bigger brakes on the rear.

My T-piece is a very simple affair; there is no adjusting bolt etc.

1) I would like to know if anyone has any knowledge that the rear T-piece is a significantly restrictive device ?
2) Has anyone had any experience drilling these critters out? And does that sound just a bit dodgy/scary or a mere Group N hack?

On the pro-side of the argument, in my brain, it says that its just a block of metal with a right angle bend in two directions (left and right rear brake lines) that flow a liquid. It should be a matter of improving the flow i.e. over boring to allow greater pressure to the rear brakes.

Thanks.

Mark

Matt · 22-02-2006, 10:29 AM

In the case of the Datsun / Nissan, they have an "NP Valve". Basically, after the lines come out of the master cylinder, they go into this valve.

The NP valve has a poppet valve inside it which limits the amount of pressure that goes to the rear brakes. When the front brake pressure gets to 400 psi (or thereabouts) it opens the valve to the back brakes. As the rear pressure rises, it closes off. In effect, it just makes sure that there is 400 psi more pressure going to the front brakes than the rear.

This valve is present on both drum-rear and disc-rear Nissan / Datsuns that I have seen. It is not adjustable.

I merely removed the poppet valve from the NP valve block and put in a pressure-limiting valve (thank you Kev Gavin).

Perhaps you have one as well?

It does this by

tortfeaser · 22-02-2006, 10:40 AM

I don't get it. Its a system filled by an incompressible liquid - it doesn't matter what volume the system has (cf boring anything), if volume remains constant then pressure at one end = pressure at the other. While flow considerations will mean a time delay between building pressure with the pedal and the slave cylinder 'seeing' the same pressure there will be no difference in eventual pressure at both points. Given the diameter of brake lines I can't imagine the time delay would be noticeable. Thus I can't see any benefit in boring the t piece except in reducing the time delay which I doubt you'd be able to feel.

rowdytoot · 22-02-2006, 12:34 PM

I've been musing on this for a while today...

Pressure limiting valves are effectively a flow restriction though aren't they tort, Matty? I know we run back pressure reg.s on our instruments at work to help regulate the flow on a pump so that it has a constant pressure to push against to against.. all they are is a very very small orifice for a short length (given the ID of the normal tubing is 10 thou) so effectively the restriction increases the pressure upstream and therefore is lower pressure downstream.....

I recall reading about modding mini pressure limiting valves in a elcheapo bias valve by allowing for variable flow through.- ie varying the size of the restriction with the spring loaded screw or some such..

Hence I think if the T does pose signiificant restriction then boring it will allow more flow to the rear brakes which should allow more rear bias..

whether the restriction is significant enough for the change to increase the bias usefully is another matter..

Rowds

Spac · 22-02-2006, 01:08 PM

Quote:

Originally Posted by rowdytoot

...the ID of the normal tubing is 100 thou

Fixed.

rowdytoot · 22-02-2006, 01:58 PM

10 thou Spac..... I even went and checked the tags on the spare stuff we have just in case.. 20 thou is the biggest we use

Rowds

Mark · 22-02-2006, 02:09 PM

Spac,

Maybe you want to lend me a rear brake T-piece from your collection of cr.ap and we'll see if we can make the go, "spin, spin, spin...".

Mark

tortfeaser · 22-02-2006, 02:20 PM

Isn't Mark about a simple T piece? Not a limiting valve. Doesn't matter - a limiting valve (at least the mini one and the Tilton one in my car) doesn't restrict flow, it shuts off flow once pressure gets above a certain level - its a switch, not a restriction (as a turbo restrictor is).

In the Mini one, you adjust the preload on a spring which opposes a force acting on it by M/C pressure - once the pressure overcomes the spring pressure, the valve the spring is supporting closes.

rowdytoot · 22-02-2006, 02:35 PM

Tort I was thinking that the principle of a restriction was similar to a pressure limiting valve...anyway musing somemore......

I think I'm mostly right. The braking system is about displacement of fluid from the master cylinder displacing the pistons in the caliper. It seems to me that the pressure results because it's a closed system, and that the piston movement is limited by the disks.

My addled logic says if you place a significant restriction to the flow in a part of the system you can effectively limit pressure of the pad on the disk- reducing bias to that end, but probably increasing the relative flow/pressure in other parts of the system- so you'd have more pad-disc displacement in the non restricted part..

Hence if you removed the restriction in the rear T piece(if it exists), you'd likely end up with less front bias.... (more rear.. same difference)

Rowds

Bogan · 22-02-2006, 02:43 PM

Quote:

Originally Posted by rowdytoot

Some stuff...

But if by restriction you mean change in the cross section of the pipe/vessel/etc. you're still going to get the same effect at the caliper piston. Think about if you stretched (and at the same time made thinner) the metal brake line to the back brakes. They're still gonna work the same, the fluid flows faster through the thinner bit then slows down again once the pipe returns to normal size. You'd be better off getting calipers with different diameter pistons?

And anyway, don't most cars have separate front/back sections in the master cylinder these days anyway? Or am I off the track now?

T.

Mark · 22-02-2006, 02:54 PM

Quote:

Originally Posted by Bogan

And anyway, don't most cars have separate front/back sections in the master cylinder these days anyway? Or am I off the track now?

T.

Yep - correct. And then the single rear line is split (near where the rear seat should be in this case) in two to the rear left and rear right brakes via a T-piece.
The question is whether the T-piece is reducing significantly the braking effort, or whether the pressure difference front-to-back is controlled at the master cylinder end (which makes more sense, engineering wise).

Matt · 22-02-2006, 03:02 PM

Not having seen the system on the Lancer, how do the lines go? Two lines out of the master cylinder (F & R)? Where from there? Do they all go into a distribution block, or does the front have a T-piece as well?

Mark · 22-02-2006, 03:14 PM

Two lines definately separate coming out of the Master cylinder. Of course, at this point they go behind the engine - I can't say that I've honestly tracked each one their entire length. I haven't seen a sign of a distribution block.

I will have a look tonight while Frinky is welding the car back together.

Mark

Spac · 22-02-2006, 03:22 PM

Rowdy, ten thou is 0.25mm - barely enough to stick a human hair down.

rowdytoot · 22-02-2006, 03:25 PM

Boges.. I see your point... but for a given pressure the fluid will flow slower through narrower tubing, and and faster through more open tubing... but I know what you are getting at..

more musing....

under constant M/C displacement(pressure if you like) conditions I think tort's argument wins- the pressure in the fluid at the wheels should equalise... no matter the restriction- given enough time for the flow to happen..

If I had the time I'd go dummy something up out of our tubing leftovers...

Win to tort....

Mark... I suspect it'd be pedal box time

Rowds