Discussion
GreenV8S said:
The brake balance bar/dual master cylinder only sets the static balance, you still need a bias valve to compensate for weight transfer under braking.
How does a bias valve work?
Unless it varies pressure to the rear dependant on applied brake pressure or varies by sensing body position or G then I can't see what it does over that of the bar and different size master cylinders.
Steve
steve_d said:
How does a bias valve work?
Unless it varies pressure to the rear dependant on applied brake pressure or varies by sensing body position or G then I can't see what it does over that of the bar and different size master cylinders.
Steve
You can get all sorts, some fancy ones are controlled by suspension height, some use an inertial sensor, but the most common ones just use a pressure sensitive hydraulic valve to restrict the rear line pressure once it exceeds a threshold. This means that the rear line pressure goes up 1:1 with the front line pressure up to the threshold, and goes up at say 1:4 after it. This only gives you a crude approximation to the ideal pressure distribution curve, but it's close enough to work.
bigmack said:
Do you really? ... Ultima's also don't have a great deal of weight transfer.
It comes down to weight distribution, CoG height and wheelbase. For example my TVR has 50:50 weight distribution and the CoG height is roughly 15% of the wheelbase. That takes the brake balance from 1:1 under light braking to about 1.85:1 under heavy braking. That's a big difference, and you can't get the balance right under heavy *and* light braking without a bias valve. You'd need to do similar calcs for the Ultima to see how much effect the weight transfer had. I'd be surprised if it was negligeable.
Here's a good article crafty: www.stoptech.com/tech_info/wp_brakebiasandperformance.shtml
Some parts especially relevant to the hydraulic brake-bias valves vs. the type of brake-bias dial that adjusts the mechanical leverage on the front-versus-rear master-cylinder pistons, are:
- the paragraph below figure 5 (i.e., hydraulic valves don't do a *perfect* job for light and heavy braking either -- although they help more than the mechanical-leverage type -- which is probably why just about everyone in professional racing uses a dial to adjust the mechanical-leverage paced on each mstr-cyl's pistons: they want to get it perfect, for racing or track-weekends, and
- below figure 3 (i.e., if you plan to be adjusting your Ultima's brake-pads, tires, suspension perches, etc., then either type of brake-bias adjuster will need to be adjusted each time you change your car's set-up! e.g., on a race-weekend, if you change any of things, you need to adjust your settings).
Given that a 2000-lb car's brake-wear during light stops isn't much, compared to brake-wear during cadence-braking, when this article mention brake-pad life as one of the benefits of using a hydraulic valve, I had to question the value (cost and ESPECIALLY in terms of the time to set it up on a one-off car, compared to a driver-adjustable dial that adjusts your brake pedal's mechanical leverage on the mstr-cylinders). I wonder if greenV8S can tell us any other advantages? ...other than the obvious advantage for soccer-moms: getting decent stopping each time it rains, *without* needing to *manually* adjust a mechanical type of dial each time it rains (so that all four wheels would then lock-up nearly-simultaneously on wet pavement).
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...aside from the above stuff, which that article reminded me of:
IMHO, two things are more important than whether you get your brake-bias using a hydraulic valve downstream of the rear brakes' master-cylinde vs. the mechanical leverage on each master-cylinder:
#1. With either brake-biasing system, if you want to sacrifice a tiny bit of performance for more safety ("safety" = more stability when braking, i.e. less tendency for the car to try and spin-out whilst you brake), then you'd want to lock-up the fronts slightly *before* the rears lock instead of "all four tires locking-up simultaneously". This is often done (rather than setting the brakes to lock-up *exacty* at the same time on all 4 wheels) so that when your car gets onto pavement with a slightly different Cf than what you were on when you set-up your brake-balance, you won't lock your rear tires first. Just think of a motorcycle braking with only its front brake, or even too much front-brakes, and that's what happens when you have more traction on your front-tires than your rears and suddenly lock the rears, without locking the fronts first.
Most people here probably know the above aspect of setting up brake-balance, but just in case, so no one spins their Ultima if that's their first car with front/rear brake adjustments... ;-)
#2. The ability to fine-tune it on-the-fly, from the driver's seat, which is easier to have with the mechanical system, AFAIK. This is just a personal preference of mine. Since you can also do more than fine-tuning from the driver's seat (e.g. manually setting your wet vs. dry brake-balance), I wonder if a hydraulic valve would do anything on a car which already has a driver-adjustable knob, other than save a little brake-pad wear? (if GreenV8s could note any other benefits, or if one-off racing cars typically use the hydraulic type, I'd be interested)
>> Edited by jeff-texas on Tuesday 14th March 17:42
Some parts especially relevant to the hydraulic brake-bias valves vs. the type of brake-bias dial that adjusts the mechanical leverage on the front-versus-rear master-cylinder pistons, are:
- the paragraph below figure 5 (i.e., hydraulic valves don't do a *perfect* job for light and heavy braking either -- although they help more than the mechanical-leverage type -- which is probably why just about everyone in professional racing uses a dial to adjust the mechanical-leverage paced on each mstr-cyl's pistons: they want to get it perfect, for racing or track-weekends, and
- below figure 3 (i.e., if you plan to be adjusting your Ultima's brake-pads, tires, suspension perches, etc., then either type of brake-bias adjuster will need to be adjusted each time you change your car's set-up! e.g., on a race-weekend, if you change any of things, you need to adjust your settings).
Given that a 2000-lb car's brake-wear during light stops isn't much, compared to brake-wear during cadence-braking, when this article mention brake-pad life as one of the benefits of using a hydraulic valve, I had to question the value (cost and ESPECIALLY in terms of the time to set it up on a one-off car, compared to a driver-adjustable dial that adjusts your brake pedal's mechanical leverage on the mstr-cylinders). I wonder if greenV8S can tell us any other advantages? ...other than the obvious advantage for soccer-moms: getting decent stopping each time it rains, *without* needing to *manually* adjust a mechanical type of dial each time it rains (so that all four wheels would then lock-up nearly-simultaneously on wet pavement).
------------------------------------------------------------------
...aside from the above stuff, which that article reminded me of:
IMHO, two things are more important than whether you get your brake-bias using a hydraulic valve downstream of the rear brakes' master-cylinde vs. the mechanical leverage on each master-cylinder:
#1. With either brake-biasing system, if you want to sacrifice a tiny bit of performance for more safety ("safety" = more stability when braking, i.e. less tendency for the car to try and spin-out whilst you brake), then you'd want to lock-up the fronts slightly *before* the rears lock instead of "all four tires locking-up simultaneously". This is often done (rather than setting the brakes to lock-up *exacty* at the same time on all 4 wheels) so that when your car gets onto pavement with a slightly different Cf than what you were on when you set-up your brake-balance, you won't lock your rear tires first. Just think of a motorcycle braking with only its front brake, or even too much front-brakes, and that's what happens when you have more traction on your front-tires than your rears and suddenly lock the rears, without locking the fronts first.
Most people here probably know the above aspect of setting up brake-balance, but just in case, so no one spins their Ultima if that's their first car with front/rear brake adjustments... ;-)
#2. The ability to fine-tune it on-the-fly, from the driver's seat, which is easier to have with the mechanical system, AFAIK. This is just a personal preference of mine. Since you can also do more than fine-tuning from the driver's seat (e.g. manually setting your wet vs. dry brake-balance), I wonder if a hydraulic valve would do anything on a car which already has a driver-adjustable knob, other than save a little brake-pad wear? (if GreenV8s could note any other benefits, or if one-off racing cars typically use the hydraulic type, I'd be interested)
>> Edited by jeff-texas on Tuesday 14th March 17:42
I don't know how important the bias valve would be on an Ultima. It would be possible to work it out from the CoG height, wheelbase and weight distribution. Anybody have these figures to hand? What this would tell you is how much the brake balance needs to vary between gentle and heavy braking. If you don't have a bias valve then it is possible to adjust the static balance to get the brake balance correct under light braking, or heavy braking, but not both. So if you set it up so that the braking is perfect in a straight line in the dry, then when you brake gently while you are cornering hard, or in the wet, there will be far too much braking at the front. It's a mistake that seems very common among the TVR community where people only upgrade the front brakes because those are the only ones that are overheating, and take the bias valve to get enough braking to the rear in the dry. And it's fine when you're showing off to your mates in the dry. Completely different story when you have less grip available.
Ive done the math - its a sliding scale dependent on decelleration but if recall correctly you get an equivalent 50-50 weight distribution @ ~0.7g braking effort (0g is 60 rear 40 front or thereabouts). 1.4g reverses the 0g condition (to 60 front and 40 rear). However, in practice you need more rear bais than the figures might suggest. I'll see if I can dig up the calcs.
The reason I was asking this question is really about brake setup.I'm not expecting to adust my brakes all the time.
Ultima owners: have any of you touched your brakes after setting them up initially. Did you find that the initial setup worked.
I can see that trying to adjust anything in the footwell once the car is built is going to be a pain in the ass.
I was thinking of either installing the bias cable, or putting an inspection cover over the pedals as Jack Rosen has done.
Ultima owners: have any of you touched your brakes after setting them up initially. Did you find that the initial setup worked.
I can see that trying to adjust anything in the footwell once the car is built is going to be a pain in the ass.
I was thinking of either installing the bias cable, or putting an inspection cover over the pedals as Jack Rosen has done.
If you have the correct bias valve fitted, there is one correct setting which is determined by the geometry of the car and brakes and is right for all conditions. Having done this, the only reason to change the bias is to change the stability of the car under braking.
If you don't have the correct bias valve fitted then the brakes will only be balanced correctly for one particular amount of braking, and the car will tend to lock the fronts under less braking and lock the rears under more. This might mean you have to keep adjusting the static balance as conditions change, but it will never really be right.
If you don't have the correct bias valve fitted then the brakes will only be balanced correctly for one particular amount of braking, and the car will tend to lock the fronts under less braking and lock the rears under more. This might mean you have to keep adjusting the static balance as conditions change, but it will never really be right.
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