alistairolsen
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| posted on 29/6/09 at 10:18 AM |
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can someone explain why I should fit a pedal box rather than a twin piston master cyl and bias valve?
What are the pros and cons?
I understand the need to adjust bias, just not why one method is better than the other.
Cheers
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britishtrident
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| posted on 29/6/09 at 10:47 AM |
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Unless you are using the car on the track you don't need or want a bias bar pedal box.
Also you probably won't need a pressure limiting valve.
What you should use is a smaller bore tandem dual circuit master cylinder to bring the pedal effort down acceptable levels.
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yellow melos
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| posted on 29/6/09 at 12:11 PM |
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Well my dutton ( based on a MK1 Escort ) has a single cylinder feeding all 4 wheels and no bios valve... nver had any problems.....
just mean if it fails i have to resort to a harder pull on the hand brake !!!!
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alistairolsen
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| posted on 29/6/09 at 12:20 PM |
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I will be using the car on track.
I do want dual circuits (by law)
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andkilde
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| posted on 29/6/09 at 12:33 PM |
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A tandem is essentially two masters of fixed bore built onto a common actuator shaft -- adding a bias valve is a bit of a crutch as it limits the
performance of one of the two factory circuits in order to get the balance "closer" to ideal. Also, single circuit limiters don't
actually limit pressure, they limit flow so if you hold your foot on the pedal, pressure starts low and climbs, causing lockup to occur later.
Additionally, if the master cylinder bore diameter is too far from ideal you'll get a "too soft" or "too hard" pedal
with no real way to fix it.
Duals with a bias bar usually allow you to pick specific bores for each circuit, and the bias bar adjust front to rear pressure bias more
"cleanly" by adjusting the mechanical pressure each master sees in relation to the other. Think of the bias bar as two variable length
levers if you will.
If you could find a factory tandem with the correct bore sizes and correct bias out of the box, that would probably be the best "Locost"
choice.
There are Excel files kicking around the net which can help you select the ideal master and caliper cyl diameters based on vehicle weight, front to
rear weight bias and disc diameter -- last place I saw it being discussed in anger was on <www.fsae.com>.
Cheers, Ted
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alistairolsen
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| posted on 29/6/09 at 12:47 PM |
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Cheers!
From what I could see, the beneifit was only that a valve will limit the pressure to the rear and the extra effort is lost whereas in the bias bar the
power taken from the rear is applied to the fronts.
I see what you mean about limiting flow, Ive been reading lots of stuff about them this morning and some have a crack point offering two different
slopes, some dont. I assume the ones with a crack point lust offer no restriction to s certain point and then start to restrict flow?
problem you have is, without knowing the curves of tyres and track suface friction against normal weight applied and the same again for the pads, I
dont ee how you can draw meaningful theoretical conclusions.
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MikeRJ
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| posted on 29/6/09 at 03:11 PM |
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quote:
Originally posted by andkilde
Also, single circuit limiters don't actually limit pressure, they limit flow
I don't think this is true; bias valves aren't just simple in-line restrictions, they are proper pressure regulators.
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Bob C
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| posted on 29/6/09 at 06:08 PM |
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Hi Alistair,
yeah the bias valves with a "crack point" generally restrict above a specified pressure & are called "proportioning
valves".
The bias lever & 2 master cyls is essentially an easy fix, you can guarantee to be able to achieve just about any fr/rr bias you like by adjusting
it. If you use a tandem cylinder & a bias valve (proportioning valve) you have to design it right, but I would argue you actually end up with
better brakes because they keep the fr/rr bias right over a wide range of conditions. Very few folk take the trouble to do it,- the guys at SVA had
never seen a system like mine & were convinced it would fail!
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britishtrident
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| posted on 29/6/09 at 06:46 PM |
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quote:
Originally posted by alistairolsen
Cheers!
From what I could see, the beneifit was only that a valve will limit the pressure to the rear and the extra effort is lost whereas in the bias bar the
power taken from the rear is applied to the fronts.
I see what you mean about limiting flow, Ive been reading lots of stuff about them this morning and some have a crack point offering two different
slopes, some dont. I assume the ones with a crack point lust offer no restriction to s certain point and then start to restrict flow?
problem you have is, without knowing the curves of tyres and track suface friction against normal weight applied and the same again for the pads, I
dont ee how you can draw meaningful theoretical conclusions.
No power involved brakes work on hyrostatics.
Proporting valves have advantages in that they are to an extent "smart" in slippery conditions at light pedal pressures they apply a
greater proportion of the braking effort to the rear brakes. In dry conditions the pedal (and hence hydrualic) pressure is higher and they stop or
reduce the rate of increase in pressure going to the rear brakes.
The two types of interest are the simple shut off valve used in early Minis or the dual rate proportioning valve used in some other cars.
However if using Cortina/Escort or Sierra brakes generally premature rear wheel locking isn't generally a problem because the brakes come off a
tintop with a 60%-40% and are fitted to a car with a 50%-50% weight distribution..
The main problem with Locost style cars is because of space restriction the available pedal leverage is half that of a Sierra and and
usually no servo so if a Sierra mastercylinder is used even allowing for the much reduced weight to get a standard 0.9+ g emergency you have to push
the pedal at least twice as hard to stop the car.
Both Caterham and Westfield employ 17mm bore tandem master to reduce the pedal effort to acceptable levels.
[Edited on 29/6/09 by britishtrident]
[Edited on 29/6/09 by britishtrident]
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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