Brake bias valves.
Discussion
I'm thinking of fitting an adjustable brake bias valve in my Griff over winter. The reason is I'm going to be upgrading the rears to match my current monster Compbrake 4 pots on the front 
Is there any reason why it should be fitted towards the end of the brake line? The reason I'm asking is the rear line is a pig to get to, but there's a fairly accessible point in the engine bay before it runs along the chassis.
I'm not looking to adjust it on the move, once I have it set how I like I'll forget it's there.
Is there any reason why it should be fitted towards the end of the brake line? The reason I'm asking is the rear line is a pig to get to, but there's a fairly accessible point in the engine bay before it runs along the chassis.
I'm not looking to adjust it on the move, once I have it set how I like I'll forget it's there.
HRG said:
I'm thinking of fitting an adjustable brake bias valve in my Griff over winter. The reason is I'm going to be upgrading the rears to match my current monster Compbrake 4 pots on the front
Is there any reason why it should be fitted towards the end of the brake line? The reason I'm asking is the rear line is a pig to get to, but there's a fairly accessible point in the engine bay before it runs along the chassis.
I'm not looking to adjust it on the move, once I have it set how I like I'll forget it's there.
No reason at all, in fact mine is at the front close to the m/c.Is there any reason why it should be fitted towards the end of the brake line? The reason I'm asking is the rear line is a pig to get to, but there's a fairly accessible point in the engine bay before it runs along the chassis.
I'm not looking to adjust it on the move, once I have it set how I like I'll forget it's there.
Bear in mind that the brake bias valve is there to deal with the effects of weight transfer under braking and you will have to set the static brake balance first by other means.
I believe the Griff has an inertial bias valve hiding somewhere in the rear line. How will an adjustable proportioning valve affect this?
My thinking is:
Larger rear brakes will help the car to squat less at the nose under braking, the new proportioning valve will allow me to fine tune the static pressures etc. and then the inertial valve will reduce rear pressure under weight transfer.
Is my train of thought correct?
My thinking is:
Larger rear brakes will help the car to squat less at the nose under braking, the new proportioning valve will allow me to fine tune the static pressures etc. and then the inertial valve will reduce rear pressure under weight transfer.
Is my train of thought correct?
HRG said:
I believe the Griff has an inertial bias valve hiding somewhere in the rear line. How will an adjustable proportioning valve affect this?
My thinking is:
Larger rear brakes will help the car to squat less at the nose under braking, the new proportioning valve will allow me to fine tune the static pressures etc. and then the inertial valve will reduce rear pressure under weight transfer.
Is my train of thought correct?
I assumed the OP would remove the original bias valve and replace with the new adjustable one.My thinking is:
Larger rear brakes will help the car to squat less at the nose under braking, the new proportioning valve will allow me to fine tune the static pressures etc. and then the inertial valve will reduce rear pressure under weight transfer.
Is my train of thought correct?
Changing the rear brakes doesn't affect the amount of weight transfer under braking, it just changes how much braking effort comes from the rear wheels.
A bias valve is not the right way to set the static balance. You set this by changing the hydraulic/mechanical leverage of the front versus rear brakes. The bias valve is only there to compensate for the weight transfer that occurs under braking.
Ah, I'll take a step backwards... Originally the Griff is fitted with 260mm fronts and 273mm rears, both single pot off the top of my head. I now have 300mm four pots on the front and standard rears.
I want to increase the rear braking effort by upgrading to 300mm four pots, but I don't want the bias to be as far towards the rear as it originally was.
In theory it should be about right with the additional mechanical force at the rear, I was considering an additional bias valve to fine tune the line pressure while leaving the inertial valve in situ to reduce rear braking effort under heavy braking.
If it helps, the car is predominantly used on the road, I'm not looking for a track biased setup.
I want to increase the rear braking effort by upgrading to 300mm four pots, but I don't want the bias to be as far towards the rear as it originally was.
In theory it should be about right with the additional mechanical force at the rear, I was considering an additional bias valve to fine tune the line pressure while leaving the inertial valve in situ to reduce rear braking effort under heavy braking.
If it helps, the car is predominantly used on the road, I'm not looking for a track biased setup.
HRG said:
In theory it should be about right with the additional mechanical force at the rear, I was considering an additional bias valve to fine tune the line pressure while leaving the inertial valve in situ to reduce rear braking effort under heavy braking.
I think I see where you're coming from. You've substantially upgraded the fronts and now have too much braking on the front relative to the rear, so you're going to increase the amount of braking at the rear by fitting bigger discs and/or calipers. That's a very sensible approach to get the static brake balance right by changing the mechanical / hydraulic leverage.Alternative approaches would be to install twin master cylinders with a balance bar, or fit a linear bias valve with a suitable constant attenuation. Most aftermarket bias valves are adjustable with no attenuation (1:1) up to an adjustable threshold, then changing to a fixed but very high (around 3:1) attenuation above that. These are designed to approximate the theoretically ideal curved response corresponding to the weight transfer varying with total braking. You may find a bias valve with a fixed attenuation closer to 1:1 but I think you will struggle. Unless you fancy the hassle of an adjustable balance bar you would be sensible to aim to get it about right based on the physical geometry and just acknowledge that the balance will never be absolutely perfect as the vehicle weight distribution changes and pad and tyre temperatures change. It only has to be about right.
You only want a single bias valve to compensate for weight transfer. It is unlikely that the original inertial valve will be correct since your new calipers will presumably have a different mechanical leverage and piston area so they will run at a different line pressure. So I suggest you ditch the original one, fit an adjustable one somewhere convenient and adjust set it to suit your new setup.
You can test the static balance by finding somewhere quiet and setting the car up in a corner close to the limit of grip. It will probably understeer. If you very gently brush the brakes you should find the car slows and drifts wide without any dramatic understeer or oversteer. If it oversteers your static balance has too much braking to the rear - if it understeers, you have too much to the front. The point here is to be close enough to the limit of grip under cornering that you don't need to brake hard to feel the car start to slide.
Once the static balance is correct you can test the dynamic balance by braking much harder with the car almost in a straight line on a grippy surface. By having the car very slightly cornering you will feel whether the back is trying to step out or the front is tending to push on under hard braking. You can then increase of decrease the amount of braking to the rear by your bias valve to get the balance you want. Usually you want to aim for slight understeer under braking. It's very reassuring as you pile into a corner to know that things are going to tend to straighten out as you hit the brakes rather than get worse! You only want *slight* understeer though, you don't want to get terminal understeer when you hit the brakes.
Very good advice, thanks. My major point of concern was that by upgrading the rears to the same size as the fronts I'd return to the tendency towards dramatic understeer as was present with my original setup.
In theory the static balance should be closer than standard as I've effectively reduced the mechanical ratio slightly on the rear compared to the stock setup i.e. 273mm rear and 260 front vs 300mm rear, 300mm front.
I may have to reduce the master cylinder ratio a fraction as these puppies really bite! but the original objective was to increase feel rather than overall stopping power which thus far has been a big success.
In theory the static balance should be closer than standard as I've effectively reduced the mechanical ratio slightly on the rear compared to the stock setup i.e. 273mm rear and 260 front vs 300mm rear, 300mm front.
I may have to reduce the master cylinder ratio a fraction as these puppies really bite! but the original objective was to increase feel rather than overall stopping power which thus far has been a big success.
I'm not telling anyone to suck eggs here but just want you to be sure you understand how these proportioning valves work. We are talking about the in line mounted screw top or lever adjustable valves right? (If you are discussing something different ignore the below and reply to me with some entertaining colourful language!)
I think the AP Racing website has a good graph showing how these things work and the important thing to remember is that they DO NOT work the same or give you the same adjustability as a balance bar. The adjustability basically changes the preload on a spring sitting behind an internal plunger. As you brake the pressure builds in the brake lines and as GreensV8 says the mechanical/hydraulic leverage diff front to rear gives the brake balance front to rear. As the rear brake pressure increases further it will overcome the spring preload in the valve and part of the braking effort will now move this plunger as well as pushing the pads against the disc. This results in a change in relationship between pedal effort and rear braking effort meaning that during heavy braking, high weight transfer, we can prevent the rear brakes from locking as the become lightly loaded. What we can't do is increase the front braking effort with these valves only limit the rear above a given threshold. Have a look at the AP Racing chart and you can see this effect and the effect of changing the preload setting.
A twin master cyliner setup does allow for some front to rear adjustment however it should only be used for fine tuning and as again GreensV8 has said you should always try to optimise your hardware first to achieve your desired brake balance and use the balance bar only as a refinement. Over reliance on the balance bar will result in the bar operating through some strange and severe angles and will introduce braking ineffiencies and can even cause front the rear balance variation on badly deisgned or setup balance bars.
I think the AP Racing website has a good graph showing how these things work and the important thing to remember is that they DO NOT work the same or give you the same adjustability as a balance bar. The adjustability basically changes the preload on a spring sitting behind an internal plunger. As you brake the pressure builds in the brake lines and as GreensV8 says the mechanical/hydraulic leverage diff front to rear gives the brake balance front to rear. As the rear brake pressure increases further it will overcome the spring preload in the valve and part of the braking effort will now move this plunger as well as pushing the pads against the disc. This results in a change in relationship between pedal effort and rear braking effort meaning that during heavy braking, high weight transfer, we can prevent the rear brakes from locking as the become lightly loaded. What we can't do is increase the front braking effort with these valves only limit the rear above a given threshold. Have a look at the AP Racing chart and you can see this effect and the effect of changing the preload setting.
A twin master cyliner setup does allow for some front to rear adjustment however it should only be used for fine tuning and as again GreensV8 has said you should always try to optimise your hardware first to achieve your desired brake balance and use the balance bar only as a refinement. Over reliance on the balance bar will result in the bar operating through some strange and severe angles and will introduce braking ineffiencies and can even cause front the rear balance variation on badly deisgned or setup balance bars.
That's how I understand it too. I want to potentially limit the amount of rear braking available as I have plenty at the front!
I will be going up from 273mm single pots to around 300mm four pots at the rear and if it moves the balance too far back then I want to be able to tame it down a little.
As stock was larger at the rear anyway I think matching disc sizes should get the mechanical ratio closer to ideal.
I will be going up from 273mm single pots to around 300mm four pots at the rear and if it moves the balance too far back then I want to be able to tame it down a little.
As stock was larger at the rear anyway I think matching disc sizes should get the mechanical ratio closer to ideal.
I have a feeling that the Griffith only had a very simple pressure limiting valve in the line to the rear brakes. If it looks like a steel cotton rel with on pipe going into the centre of one end and another pipe coming out of the centre of the other end, I think it just reduces the line pressure to the rear brakes - regardless of deceleration level.
I think the "S" models (and maybe some very early Griffiths) had a much larger cast iron afair under the bonnet angled upwards towards the front. This is a "G" valve and there is a ball bearing inside that (at a particular level of deceleration) rolls up the slope and (essentially) closes off a port, limiting any further increase in rear braking effort.
I think the "S" models (and maybe some very early Griffiths) had a much larger cast iron afair under the bonnet angled upwards towards the front. This is a "G" valve and there is a ball bearing inside that (at a particular level of deceleration) rolls up the slope and (essentially) closes off a port, limiting any further increase in rear braking effort.
Avocet said:
I think it just reduces the line pressure to the rear brakes - regardless of deceleration level.
I think it is simply a non-adjustable version of the usual aftermarket attenuators. It will allow the full pressure through to the rear brakes until the line pressure exceeds a threshold, then it will restrict the rear circuit so that the rear line pressure goes up slower than the front line pressure (the usual ratio is about 3:1). Gassing Station | Suspension, Brakes & Tyres | Top of Page | What's New | My Stuff