Balance Bar vs Proportioning Valve

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chimpvalet
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Balance Bar vs Proportioning Valve

Post by chimpvalet »

Just underway with development of an upgrade to brakes on an early Lotus Esprit, planning to ditch the servo, go larger on rotors all around. In regard to overall balance I am unclear on how twin M/C on balance bar compares with a proportioning valve for the rear circuit. Thoughts?
dannobee
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Re: Balance Bar vs Proportioning Valve

Post by dannobee »

The twin M/C setup with a balance bar lets you choose the optimal master cylinders for the size of the calipers. You'd use the balance bar adjustment to fine tune the F/R balance. Proportioning valves, OTOH, aren't linear in their function, they have a "knee point" where the pressure gets increasingly limited to the rear. That is less than ideal for track use. But in a panic braking street scenario, it helps prevent the rear wheels from locking up.
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Re: Balance Bar vs Proportioning Valve

Post by chimpvalet »

Agreed on those points, Dannobee. For street use I'd considered biasing toward the rear while using both B-B and P-V, thinking that overall effort might be reduced in mild to moderate braking, then having the balance come into range as threshold is approached. Concerned that could cause loss of stability in situations where traction is sub-par. For example, needing to brake suddenly and finding sand on the pavement, particularly if on approach to a corner.
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Re: Balance Bar vs Proportioning Valve

Post by David Redszus »

chimpvalet wrote: Tue Mar 09, 2021 4:16 pm Just underway with development of an upgrade to brakes on an early Lotus Esprit, planning to ditch the servo, go larger on rotors all around. In regard to overall balance I am unclear on how twin M/C on balance bar compares with a proportioning valve for the rear circuit. Thoughts?
Many. We have been engineering brake systems for race cars for many years.

First rule: bigger is not better.
Larger rotors provide very little benefit, if any. If the size of rotors, caliper pistons and number, are altered from the
original design, the brake balance may be affected. This may be good or bad, depending.

Pad size also has no effect on brake stopping power, although larger pads may reduce contact pressure so pads last longer. Pad compounds may be mixed, front and rear, to alter brake balance.

Twin master cylinders with a balance bar can be set to provide proper brake force balance, but only under one
braking G force scenario; it will be wrong everywhere else. That's where a brake force regulator is needed.

The best brake set up is to have the correct size MC (with properly set beam), rotor and tire size for the
vertical forces under braking. The requires the additional use of a properly selected brake force regulator.

Can you lock up all four wheels on a smooth rod surface? If so, you're done. If not, there is work to be done.

For a complete brake engineering analysis, the following information is required:
corner weights with driver and liquids
wheelbase length
tire diameter
number and size of master cylinders
pedal ratio
rotor mean diameter, front
caliper pistons, number and size, front and rear
regulator pressure curve
pad coefficients, estimated

With the above information, a proper brake system can be constructed that will allow braking to
the full tire grip limit, under all track conditions, wet or dry. Brake bias can also be adjusted from
the drivers seat while on the track. A slight rear bias helps corner turn-in; a front bias provides
straight line stability.

When properly designed, a significant increase in braking G force is accomplished that will
frighten your competitors.
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Re: Balance Bar vs Proportioning Valve

Post by chimpvalet »

The comprehensive synopsis is appreciated. We have all but the corner weights inputs in view from the start, will have those as soon as the first of 2 cars is made ready to go. I note the absence of rear rotor mean diameter and remarks on the pointlessness of larger rotors. These cars are designed for rear tire diameters 10% larger than front, roughly 23" - 25" fore-aft. Much of the imperative is in discarding the servo, thus we seek to offset loss of assist in some measure via larger rotor diameters, while renewing components optimally.
Outline: M/C's 0.625 x 2, calipers 1.375 4 pot all around, rotors 12.19 x .81 all around, pedal ratio 4.5/1. Curb weight expected around 2300 lb, static distribution roughly 43/57. Likely looking to Tilton for a rear circuit PV. Perhaps we overshoot in fully matching rears to front, better to use larger M/C for that?
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Re: Balance Bar vs Proportioning Valve

Post by chimpvalet »

Pads coefficient peaking near 0.47 500-600 F., near 0.45 from 300 - 900.
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Re: Balance Bar vs Proportioning Valve

Post by David Redszus »

As presently configured your brake bias is 52%F and 48%R.
Using a target braking of 1.3G, you will need 64.9%F and 35.1% rear bias due to vehicle weight
transfer under braking.

The present MC can be used with the balance beam set at 60F, 40R (percent).
A larger MC will reduce brake line pressure, not increase it.
The front caliper pistons should be increased to 38mm, the rears left alone.

Using a brake prop valve set at 70% and 150 lbs pedal force, the bias will be 64.7%F and 35.3%R.
And you will have the necessary brake force at the tire to produce a 1.3G deceleration, which
should be very close to your tire grip limit. If a 150 lb pedal force is found to be excessive, larger
caliper pistons can be used; but the proper bias must be maintained.

Do not use a lever type proportioning valve; they are quite inaccurate. Use the knob adjustment type.
An adjustment of brake bias from the cockpit, will allow adding more real bias to improve turn-in into
a tight corner.

It is important to ascertain the regulating pressure slope and the knee point for the regulator.
One size does NOT fit all.
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Re: Balance Bar vs Proportioning Valve

Post by chimpvalet »

Outstanding! Thanks for that.
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Re: Balance Bar vs Proportioning Valve

Post by j-c-c »

"Can you lock up all four wheels on a smooth road surface nearly simultaneously at a typical needed braking speed?If so, you're done. If not, there is work to be done."

I would also add, brakes stop the wheels, tires stop the car.
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Re: Balance Bar vs Proportioning Valve

Post by chimpvalet »

Acknowledged, thanks.
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Re: Balance Bar vs Proportioning Valve

Post by David Redszus »

j-c-c wrote: Thu Apr 08, 2021 12:55 pm "Can you lock up all four wheels on a smooth road surface nearly simultaneously at a typical needed braking speed?If so, you're done. If not, there is work to be done."

I would also add, brakes stop the wheels, tires stop the car.
That is a very good point that should be emphasized to all begining race drivers; perhaps to all drivers.

ALL vehicle dynamic forces: acceleration, braking, and cornering must pass through the tires.

Only money does not. :lol:
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