Servo assisted brakes

[asymptote]

Hi Automotive Forum.
Can any body offer an explanation to an Electrical Eng the reason for the following issue. The servo brakes when "pumped" with the petrol engine off become very solid with very little pedal movement no matter how heavy the pedal pressure ie without servo assistance. With the engine running, the same action yeild a much softer brake feel with much longer pedal travel. I understand the basic principles of the servo system but can't get my mind around why the extra pedal travel. The pedal push rod into the master cylinder is only getting assitance from the servo so where is the extra brake fluid going? This has been a common issue on the last three new automatic petrol cars we have had, indeed in one the pedal had even further movement when the gear was selected from P to D.

 

[patprimmer]

I expect that the calliper pistons travel further as the calliper flexes and the pads compress under the extra force applied by the servo.

Also, higher line pressures will expand the lines.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
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[asymptote]

patprimmer.
Thanks for that comment, as no other explanation has been offered I guess yours is correct. However the cc of oil displaced seems a lot of component yeild in a system that is designed to have only the brake pot yeilding!
Regards

 

[rmw]

I don't know the answer, but I think the answer lies with how a brake servo works.

First understand one thing. Don't confuse pedal pressure with braking effect with the servo system with no vacuum on the system. I once lost an alternator belt on my diesel PU truck which meant that my vacuum pump was not being turned, and my first application of the brakes expended what vacuum remained in the system.

Without the servo, literally standing on the brakes with both feet, (as in panicked) wouldn't stop the truck. It wouldn't even overcome the idling torque of the engine. I had to shift it onto neutral (auto trans) at traffic lights, (after creeping up on them,) because I couldn't apply enough pedal pressue to hold the truck stopped.

So, the pedal pressure you are feeling is not necessarily indicative of the pressure in the brake hydraulic system.

Now, about servos, any type of a servo. They take effort and multiply it by means of the operationg mechanism of the servo, in the case of our discussion, vacuum operated brakes.

A slight amount of pedal motion produces low multiplication, and slight braking pressure to the wheel cylinders or calipers. Heavy pressure, as in a panic stop is multiplied many times, to the point of locking the brakes up, if not careful. And, all points inbetween.

As an electircal engineer, imagine having a boiler draft control system that will produce a 4-20 ma output signal. How big of a damper drive can you operate with those few milliamps. However with a servo device, the 4-20 is converted to AC or DC amps sent to a motor driving a gear mechanism, and enough torque is developed through the output HP of the drive motor, and the gear train to move the damper, sometimes in the tens of thousands of foot lbs.

That is what the vacuum servo is doing. You are putting a slight amount of pressure on the pedal, and the vacuum diaphram is using the pressure over area principle to multiply your effort a small amount. Heavy pressure, big output.

Now, I have never taken a booster apart, and seen just how this is accomplished. Maybe someday I will.

So, your pedal resistance is not necessarily indicative of fluid displacement, system pressure, and/or braking force at the wheels, just resistance to pedal pressure.

rmw

 

[GregLocock]

Pat's right.

The total compliance in the system is significant. Calliper and pad stiffness is a large part of the equation.

Fluid displaced is proportional to pedal travel, corrected for linkage.

Cheers

Greg Locock

 

[Rob45]

Between Pat and rmw, you've got the total answer; there's not much fluid compression - if any - but the pressures are high enough that hose expansion and unfortunately caliper flex can be quite substantial. Since the mastercylinder piston is quite small, any expansion or flex resulting even a small fluid displacement means a lot of pedal travel.
- R

 

[asymptote]

Hi Forum. Many thanks for taking time out to go through this issue. It seems I had the answer but just could not accept items like the caliper expansion. Next time I have a wheel off I shall measure the difference with and without servo assistance.
Kind regards to all

 

[rd400guy]

For those that are interested in a pretty darn good explanation (with pictures and animation) check out this link:

http://auto.howstuffworks.com/power-brake.htm