MOT parking brake performance

And also on the type of brake itself. Drums have a "self servo" effect that discs don't, which is really handy when you're trying to design a handbrake with a nice light lever that doesn't have to move too far.

Could be. They probably went all out for a light lever to give a "quality" feel. Actually working is of secondary importance...

Cheers,

Colin.

I regularly park on hills steeper than that. I guess that's why the leave in gear and turn wheels to kerb is such a good idea.

So you openly admit you don't maintain your daughters car properly?

The self servo effect only occurs when the vehicle is moving. It's no advantage to the handbrake.

Steve

It is when the car is parked facing downhill.

/John

Are you sure? Doesn't sound right to me. I thought the self servo effect was all about forces, not movement.

I'll have a Google...

Cheers,

Colin.

Or uphill. One shoe will be leading, whatever happens.

Cheers,

Colin.

Pretty well all I've seen have one leading and one trailing shoe, so the 'servo' effect should work equally in both directions.

But it will apply if the vehicle starts to move...

Colin Stamp ( snipped-for-privacy@stamp.plus.com) gurgled happily, sounding much like they were saying :

Unless the drum's a twin-leading-shoe design.

Which would be a bit curious, for a handbrake-only drum.

Cheers,

Colin.

If the vehicle, which is parked with the handbrake applied, starts to move, then the handbrake has failed.

And if it started to move with the handbrake applied, the handbrake isn't going to stop it.

Steve

The effect then cancels itself out.

You ever seen a twin leading shoe drum brake that includes a handbrake?

It could be hit by a moving vehicle?

It's *just* possible the self servo action might cause the shoes to grip. And then release when it stops. And grip again. ;-)

AFAIK on most vehicles the handbrake only operates one shoe in each drum, even if two are fitted.

/john

: The self servo effect only occurs when the vehicle is moving.

It applies any time there is a torque on the braking arrangement. Whether the car is moving is irrelevant.

Ian

The message from "John Kenyon" contains these words:

Really? The ones I've taken apart always have a lever which operates on a spreader to push both shoes out at the same time. At first glance they appear to only act on one shoe till you work out what's happening.

Then the handbrake would be much more efficient in one direction than the other due to the self servo action.

Think if you look more carefully, you'll find it does operate both shoes. Although at first glance it may be only one that moves. Try restricting the movement of this one and you'll find the other does in fact move.