ABS & Proportioning valve pressure reduction 2

[murpia]

How does an ABS system or a rear brake proportioning valve reduce pressure in a caliper line without causing a 'soft' pedal?

All the mechanisms I can think of require some hydraulic flow, which would eventually result in the pedal hitting the floor...

Thanks, Ian

 

[ivymike]

maybe like this?

http://en.wikipedia.org/wiki/Proportioning_valve

 

[dgallup]

You'll get a much better explanation here:

http://www.stoptech.com/tech_info/wp_proportioning_valves.shtml

 

[GregLocock]

An EBD system does it withut causing a soft because the ABS pump isolates the pedal from the hydraulic circuit.

So far as I understand it an ABS system (rather than EBD) doesn't do proportioning until it detects wheelslip. Therefore you do you proportioning the old fashioned way, which is also cheaper.





Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[dashrr]

A fixed proportioning valve merely reduces the pressure out of the valve relative to the pressure into the valve by way of a piston/spring/seal arrangement. Proportioning by way of a vehicles ABS system essentially does the same thing by restricting pressure to the wheel by way of an isolation valve. The valve closes, restricting pressure to the rear brakes. The only pedal drop that would occur is when the valve is opened to increase pressure to the rear brakes. Limmiting the time the valve is open will limit the pedal drop that occurs.

 

[murpia]

"Proportioning by way of a vehicles ABS system essentially does the same thing by restricting pressure to the wheel by way of an isolation valve. The valve closes, restricting pressure to the rear brakes."

How does this work exactly? Does the isolation valve cycle rapidly switch between open and closed, so the rear line pressure 'steps' up and down tracking the front line pressure?

Thanks, Ian

 

[dashrr]

Ian--Proportioning doesn't really attempt to track the front pressure. The purpose of any proportioning system is to achieve optimum brake balance. Optimum balance or 100% braking efficiency would be when all four wheels skid at he same time. Brake proportioning is necessary because the rear wheels have a tendency to lock before the front wheels often due to the lack of weight or normal force over over the rear tires.

Brake proportioning methodology by way of the ABS system can be based upon decel or %wheel slip. Decel can be measured directly with a g-sensor or inferred from the information recieved from the vehicles wheel speed sensors. Wheel slip can also be calculated from this information. The proportioning system can compare the wheel slip occuring at the front an rear wheels and limit the pressure to the rear wheels/brakes if a certain wheel slip or decel threshold is exceded. The pressure is limited by closing the normally open isolation valve--preventing further pressure increase or wheel slip from occuring. While the valve can be closed or opened very quickly (within msec.), the actual time that it is held closed can be tuned. The valve can be opened and closed to create pressure steps that approximate a conventional proportioning valve upon reaching a certain decel threshold or pressure can be held upon sensing a wheel slip difference between the front and rear wheels. The system can be made to do whatever is desired. Electronic brake proportioning by way of the antilock system operates at low levels of wheel slip (2% - 3%) while full ABS actrivity would occur at wheel slip levels of 15%-20%. Full wheel lock is equivalent to 100% wheel slip. Hopefully I've answered your question.