Does ABS need a servo?

[saaby93]

re: discussions in another thread about loss of braking on full throttle i.e. no vaccum for the servo
I thought there were cars with electric pumps to pressurise the servo - is the latest Prius one of them?
What about the electric Berlingo?

If it's the case that cars could manage without a servo, relying on bigger friction material area, does ABS etc need a servo to operate?

[saaby93]

why's no-one replied

[Vidal Baboon]

saaby93 said:

re: discussions in another thread about loss of braking on full throttle i.e. no vaccum for the servo
I thought there were cars with electric pumps to pressurise the servo - is the latest Prius one of them?
What about the electric Berlingo?

If it's the case that cars could manage without a servo, relying on bigger friction material area, does ABS etc need a servo to operate?

Cars never used to have Servo assited brakes- al la Mini, MGs of the 50s & 60s, possibly even into the 70s. Instead they would have relied upon the self-servo action of Drum type brakes.

I would very much doubt a ABS pump needs a Servo to operate. Why would you think it would?

Don't forget Diesels (well, the older ones- not sure about the latest types) used to have a vacuum pump, usually driven from the camshaft to depressurise the servo diaphram.

Edited by Vidal Baboon on Wednesday 24th February 14:51

Edited by Vidal Baboon on Wednesday 24th February 14:53

[saaby93]

Vidal Baboon said:

I would very much doubt a ABS pump needs a Servo to operate. Why would you think it would?

Checking out various elises vs vx220s. Add ABS and it has a servo too Think of the vacuum pipe running the length of the car, and all that weight

[Huntsman]

Vidal Baboon said:

Instead they would have relied upon the self-servo action of Drum type brakes.

What?

[Motown Junk]

Why would you be braking on full throttle?

[Nee_Naw]

I'm sure the latest BMW bikes have non-servo ABS brakes. Maybe other bikes have the same, not sure...

[Herman Toothrot]

Yes the ABS on the VX / Elise needs servo assistance, people have even found in extreme applications the assistance isn't great enough and runs out so the pedal goes floppy and so modify the system so theres a larger volume available so I've read.

[fido]

Motown Junk said:

Why would you be braking on full throttle?

Because you drive a Toyota?

Edited by fido on Wednesday 24th February 16:55

[Graham]

Im fairly sure you can have ABS without a servo, after all, the servo is only there to reduce the pedal pressure you need. without a servo you just need more pressure or a stronger pump.

I'd have thought with a servo you need to make sure there is more servo capacity so it isnt all discharged by the abs unit, leaving you with no assistance..

[Zad]

ABS works by releasing the hydraulic pressure in the braking system (either per circuit or individually on each wheel). Once that pressure has been released, it needs a method of adding pressure back in, otherwise you have no brakes. That is usually provided by an electric pump + reservoir. It makes sense to use this pump to pressurise the system for normal use. I know this is how my old Sierra's system works.

[p1esk]

Huntsman said:

Vidal Baboon said:

Instead they would have relied upon the self-servo action of Drum type brakes.

What?

Drum brakes with a leading shoe arrangement used to have a self-servo effect. AIUI what happened was that the shoes were pivotted at the leading end and the wheel cylinders applied pressure at the trailing end. This caused the shoes to (sort of) pull themselves more tightly against the inner face of the drum when the brakes were applied, which meant there was no need for extra pressure to be applied from a hydraulic servo. That's not a very good description, but I know what I mean.

Sometimes you'd get an arrangement whereby a drum would have one leading shoe and one trailing shoe, which meant that only one double-ended brake cylinder was needed to operate both shoes.

There was a suggestion that braking systems using leading shoe arrangements were more prone to brake fade, i.e. a temporary loss of friction properties due to overheating.

Anyhow I expect somebody who knows what they're talking about will be along later. Let's hope so.

Best wishes all,
Dave.

[GiantCardboardPlato]

Motown Junk said:

Why would you be braking on full throttle?

To slow down, obviously.
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