Balance bars... for road cars ot not??

[cps13]

I've ben reading a lot about setting up the brake system on a seven style car.

I have been reading that the balance bars are on really for track cars. Is this because they are unsafe for road use or just because the benefits are lost due to road surfaces and cambers constanly changing etc?

I was intending on having a brake pedal with two single master cylinders and a balance bar. From what I have read I don't like the idea of a proportioning valve.

Any suggestions greatfully received...

[GreenV8S]

The balance bar is required if you have twin master cylinders. Unless you're fabricating it yourself, it will be adjustable. The adjustment is required to set the static balance i.e. the hydraulic/mechanical leverage of the two circuits to match the static weight distribution. If the weight distribution changes or the characteristics of the brakes change, you can adjust the balance bar to compensate.

The bias valve does a completely separate job, and regardless of whether you have a single master cylinder, tandem or twin+balance bar, you *always* need a bias valve. The bias valve varies the front/rear brake distribution according to the amount of braking, to compensate for weight transfer. The balance bar doesn't and can't do that.

[jagracer]

GreenV8S said:

The balance bar is required if you have twin master cylinders. Unless you're fabricating it yourself, it will be adjustable. The adjustment is required to set the static balance i.e. the hydraulic/mechanical leverage of the two circuits to match the static weight distribution. If the weight distribution changes or the characteristics of the brakes change, you can adjust the balance bar to compensate.

The bias valve does a completely separate job, and regardless of whether you have a single master cylinder, tandem or twin+balance bar, you *always* need a bias valve. The bias valve varies the front/rear brake distribution according to the amount of braking, to compensate for weight transfer. The balance bar doesn't and can't do that.

Nooooooo. A brake bias valve is used with a tandem master cylinder set up and does pretty much the same as an adjustable balance bar on a dual master cylinder set up only in a different way. The balance bar is adjustable so you can (if you attach a balance cable and knob) adjust to different surface conditions while you are driving.
With a bias valve you can only reduce or increase braking force to one system, normally the rear, with an adjustable balance bar you transfer the force from one to the other.

Edited by jagracer on Sunday 16th August 16:53

[Sam_68]

As GreenV8S says, you're effectively going to have to have a balance-bar set-up if you want to use twin master cylinders.

But once you'e got the bias set about right, you might as well leave it where it is: road tarmac is simply too variable (even if you only drive the car in warm, dry weather) to be able to tune the brake balance to the Nth degree. For safety, you've really got to balance the brakes to ensure the fronts lock up a decent margin before the rears, whereas on a circuit you can tune them so that the fronts lock only just before the rears for that day's tarmac, weather, temperature, tyre pressures, etc., etc., so that you're working them equally hard.

[GreenV8S]

jagracer said:

Nooooooo. A brake bias valve is used with a tandem master cylinder set up and does pretty much the same as an adjustable balance bar on a duel master cylinder set up only in a different way. The balance bar is adjustable so you can (if you attach a balance cable and knob) adjust to different surface conditions while you are driving.

I disagree.

A balance bar and a bias valve do completely separate jobs. The balance bar is used to establish the static balance between the two circuits i.e. the total mechanical/hydraulic leverage between your foot and the road. This determines the front/rear balance under light braking.

The bias valve adjusts this static front/rear balance to compensate for weight transfer. Without this compensation, you can get the brake balance right under light braking, or heavy braking, but not both.

In order for the braking system to work correctly you need to get the static balance right (and a balance bar is one way to achieve that) but you must also have a bias valve.

[jagracer]

GreenV8S said:

jagracer said:

Nooooooo. A brake bias valve is used with a tandem master cylinder set up and does pretty much the same as an adjustable balance bar on a duel master cylinder set up only in a different way. The balance bar is adjustable so you can (if you attach a balance cable and knob) adjust to different surface conditions while you are driving.

I disagree.

A balance bar and a bias valve do completely separate jobs. The balance bar is used to establish the static balance between the two circuits i.e. the total mechanical/hydraulic leverage between your foot and the road. This determines the front/rear balance under light braking.

The bias valve adjusts this static front/rear balance to compensate for weight transfer. Without this compensation, you can get the brake balance right under light braking, or heavy braking, but not both.

In order for the braking system to work correctly you need to get the static balance right (and a balance bar is one way to achieve that) but you must also have a bias valve.

Fair enough, we can agree to disagree.

[cps13]

My understanding of the valve is that you can set it to a certain pressue and it only effects the brake set it is attached to, i.e. front or rear. Therefore attaching it to the rear you can set a higher pressue to ensure the front wheels would lock first. This is also quite a precise method.
Also you do not need dual cylinders for this setup, just a tandem cylinder.

And the bias bar essentially does the same thing, however you can increase/decrease the pressue to both the front and rear brake cylinders, rather than just the set which includes the bias valve? With this option you can only do it with dual MCs?


Is this wrong?

[cps13]

[quote=GreenV8S]The adjustment is required to set the static balance i.e. the hydraulic/mechanical leverage of the two circuits to match the static weight distribution. quote]

I would have to disagree with this, I think this is simply the geometry of the brake pedal setup up. If the pedal is too vertical when it is at rest then by the time the fluid pressue builds up from applying pressue with your foot the pedal has already passed the vertical, and therefore the mechanical advantage of the pivot is lost.

[GreenV8S]

cps13 said:

GreenV8S said:

The adjustment is required to set the static balance i.e. the hydraulic/mechanical leverage of the two circuits to match the static weight distribution.

I would have to disagree with this, I think this is simply the geometry of the brake pedal setup up. If the pedal is too vertical when it is at rest then by the time the fluid pressue builds up from applying pressue with your foot the pedal has already passed the vertical, and therefore the mechanical advantage of the pivot is lost.

You've lost me there - I don't see what the geometry of the pedal has to do with it unless you have separate front and rear brake pedals.

If you have twin master cylinders you need a balance bar. It will inevitably be adjustable, and you'll need to adjust it to get the front-rear static balance about right.

The bias valve is not intended to get the static front/rear balance right, it is to compensate for weight transfer. A balance bar can't compensate for weight transfer (unless you plan to twiddle the adjuster continuously as you vary the pressure on the brakes), you need a bias valve to achieve this.

[cps13]

GreenV8S said:

The bias valve is not intended to get the static front/rear balance right, it is to compensate for weight transfer.

Could you explain this a bit more, I'm struggling to get what you mean...

thanks

[GreenV8S]

cps13 said:

GreenV8S said:

The bias valve is not intended to get the static front/rear balance right, it is to compensate for weight transfer.

Could you explain this a bit more, I'm struggling to get what you mean...

thanks

Imagine a car with 50:50 weight distribution and identical brakes front and rear. Under very light braking, you want equal line pressure in the front and rear brake circuits so that the braking effort is distributed 50:50 between the front and rear of the car, matching the weight distribution. This is the static balance. The static balance determines what happens when you are cornering very close to the limit of grip and gently apply the brakes. If you have the balance right it won't provoke oversteer or understeer.

Now imagine that same car with somebody standing on the brakes in an emergency stop. Because of the weight transfer under braking, you'll have much more weight on the front wheels and much less on the rear - typically 60:40 or 70:30 rather than 50:50. If the brake balance stayed at 50:50 you would now have far too much braking at the rear and the rear would lock up well before the front. So the front:rear distribution of brake effort needs to vary according to how hard you are braking.

Imagine taking this to an extreme case where there was enough grip to get the back wheels off the ground (a stoppie) then you would need 100% braking at the front and none at the rear. But only under hard braking - under light braking you still want a 50:50 distribution of effort.

The job of the bias valve is to vary the front/rear distribution according to how hard the brakes are being applied. Some of them are based on an inertial valve, some are based on a hydraulic restrictor, but they all have the characteristic that they restrict the rear circuit (preventing additional pressure from being applied) more and more as the front line pressure increases.

[cps13]

So then...

am i right in thinking that the pressure you set the bias valve too, is infact the maximum amount of pressure that the fluid in the rear brake lines could amount to?

I.e. if it was set to 10psi for arguments sake, no matter how hard you slammed your foot on the brake pedal the fluid could only reach a maximum of 10psi. Thus reducing the braking force on the rear wheels, and, stopping them from locking before the fronts?

thanks for your advice.

[GreenV8S]

cps13 said:

am i right in thinking that the pressure you set the bias valve too, is infact the maximum amount of pressure that the fluid in the rear brake lines could amount to?

I.e. if it was set to 10psi for arguments sake, no matter how hard you slammed your foot on the brake pedal the fluid could only reach a maximum of 10psi. Thus reducing the braking force on the rear wheels, and, stopping them from locking before the fronts?

Typically how they work is switching from 1:1 pressure upstream/downstream of the valve, to e.g. 3:1 upstream/downstream. The aftermarket ones usually have an adjuster which determines the pressure at which this switchover occurs. So once you get beyond the switchover point, pushing harder on the pedal might produce an extra 300 psi at the front callipers, but only 100 psi extra at the rears.

[Mr2Mike]

GreenV8S said:

In order for the braking system to work correctly you need to get the static balance right (and a balance bar is one way to achieve that) but you must also have a bias valve.

Absolutely untrue; there are plenty of cars driving around without bias valves that have excellent brakes. You can get a suitable balance simply by sizing the master and slave cylinders appropriately. Inertia or load compensating bias valves are more important on production cars where the weight and front/rear bias could vary significantly between unladen and laden states. Kit cars tend not to take many passengers and much luggage.

Bias bars are not a particularly good idea for road use unless fixed. Giving the driver the ability to fiddle with the braking bias on a public road is an invitation to an accident, which is why bias bar systems are no longer acceptable for the IVA (new SVA) unless welded up to make them completely non-adjustable.

[GreenV8S]

Mr2Mike said:

GreenV8S said:

In order for the braking system to work correctly you need to get the static balance right (and a balance bar is one way to achieve that) but you must also have a bias valve.

Absolutely untrue; there are plenty of cars driving around without bias valves that have excellent brakes. You can get a suitable balance simply by sizing the master and slave cylinders appropriately. Inertia or load compensating bias valves are more important on production cars where the weight and front/rear bias could vary significantly between unladen and laden states. Kit cars tend not to take many passengers and much luggage.

Absolutely untrue.

Without a way to dynamically adjust the front/rear brake balance to compensate for weight transfer, it is fundamentally impossible to get equal grip utilisation front and rear under light and hard braking. You can get it right for light braking, or hard braking, but not both. Can you name a single modern production car that doesn't have a bias valve? (By modern, I mean type approved within the last few decades.)

[Mr2Mike]

GreenV8S said:

Absolutely untrue.

Without a way to dynamically adjust the front/rear brake balance to compensate for weight transfer, it is fundamentally impossible to get equal grip utilisation front and rear under light and hard braking. You can get it right for light braking, or hard braking, but not both. Can you name a single modern production car that doesn't have a bias valve? (By modern, I mean type approved within the last few decades.)

Absolutely untrue.

Both myself and the OP are talking about kit cars, specifically 7's. If you re-read my post I clearly stated that it's more important in production cars where the front/rear weight bias can change significantly (and higher COG means more weight transfer to front wheels). If it makes you happy, Caterham and Westfield have type approval, and they don't use inertial/load sensing bias valves.

[GreenV8S]

Mr2Mike said:

Absolutely untrue.

Both myself and the OP are talking about kit cars, specifically 7's. If you re-read my post I clearly stated that it's more important in production cars where the front/rear weight bias can change significantly (and higher COG means more weight transfer to front wheels). If it makes you happy, Caterham and Westfield have type approval, and they don't use inertial/load sensing bias valves.

Not sure whether by load sensing you mean vehicle weight sensing, but I'll assume you mean hydraulic line pressure sensing.

I don't know whether there are any locaterfield type cars without a brake bias system. I'm surprised they would be able to get type approval without it. Whether they have or not, the fact remains that bias valves and balance bars serve a different purpose; you cannot have a properly balanced brake system unless you have a system to vary the brake distribution to compensate for weight transfer. A balance bar doesn't do it; you need a bias valve of some sort (there are several types available).

[Sam_68]

Mr2Mike said:

GreenV8S said:

Can you name a single modern production car that doesn't have a bias valve? (By modern, I mean type approved within the last few decades.)

If it makes you happy, Caterham and Westfield have type approval, and they don't use inertial/load sensing bias valves.

The Lotus Elise certainly doesn't use a bias valve. And I can't think of a single, proper, competition car that does, either.

GreenV8S said:

...you *always* need a bias valve.

This is plain and simple wrong. Bias valves are far from essential - sure, as you said elsewhere, you won't get perfect brake balance throughout the whole braking range without one, but guess what? It doesn't really matter!

So long as the balance is right when you're braking hard, to the point where the fronts lock up just before the rears, it really doesn't matter that the rears are doing less than their fair share of work when you're only braking lightly. You might slightly increase the rate of wear on the front pads and tyres compared to the rear, but that's about it. There can actually be handling advantages in the rears not doing much when braking lightly, as it means you can trail-brake into corners with less worry about braking stability.

As Mr2Mike suggested, they're only really necessary where you get large variations in load under braking, such as are typical on modern front wheel drive cars with high CoG.

You certainly don't need one on a 'Seven' type car, either practically or to satisfy IVA legislation.

Edited by Sam_68 on Wednesday 19th August 13:00

[GreenV8S]

Sam_68 said:

So long as the balance is right when you're braking hard, to the point where the fronts lock up just before the rears, it really doesn't matter that the rears are doing less than their fair share of work when you're only braking lightly.

It matters any time you are approaching the limit of grip, which could be under very light braking in slippery conditions or when cornering.

[Sam_68]

GreenV8S said:

It matters any time you are approaching the limit of grip, which could be under very light braking in slippery conditions or when cornering.

But under such conditions, you're better off with heavy front bias to the brakes (which is what you will have if you don't have a bias valve) than risking rear-end lock-up.

The trend for fitting bias valves, you will notice, has greatly increased with the introduction of ABS and stability control systems, because this isn't quite such a critical problem any more - if the arse end does lock up under light braking in slippery conditions, or entering a corner, the computers sort it out....

I say again: bias valves have no place on a Seven.