Brake lining glue

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember Adrian saying something like:

I'd be utterly amazed if they used bonded linings from new - given that rivetting was commonplace right up to the end of the 60s.

Somebody's fitted bonded linings, which might now have dropped off the catalogue ranges, but it means the original rivetted shoes are possibly kicking around somewhere in a NOS kind of way and even in a got-a-set-of-knackered-old-ones that can be rebuilt with new rivetted linings kind of way too.

I'd look for a set of used rivetted ones and pay to have them re-lined.

Yes, me too. But these are bonded and there's no sign of rivet holes in the shoes.

Rob

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember "Rob graham" saying something like:

At some point somebody made shoes for it using the new technique and as the cars dropped off in number they ceased to be worthwhile to make shoes for. If you have a good spare set, it'd be worthwhile taking them to a riveter for relining. It'd also be worth finding out what other Mercs (or whatever) had the same shoes. I'd be very surprised if only that model had them fitted.

I'd bet you could find some in Germany. There must be a boxhead mailing list.

Grimly Curmudgeon ( snipped-for-privacy@REMOVEgmail.com) gurgled happily, sounding much like they were saying :

You could almost certainly get 'em through your friendly local Three- Pointed-Star-vendor... and quite probably not that expensive, either.

Merc have a damn good rep for supporting their old stuff.

Andy Dingley presented the following explanation :

As I recall, it was a DIY task to reline them. The linings came pre-drilled with a countersunk hole for the rivet and (from memory) some small copper rivets. You chiselled the old heads off, then simply flared the new ones over after fitting the new lining.

Ben C formulated the question :

True, but some small pressure is still needed to overcome the return spring plus a little extra to force the lining against the drum. It is just that the leading shoe servo action 'assists' your effort and pressure on the brake, rather than fighting against it as does a trailing shoe.

Speaking as one who once had a bike with twin leading shoes on the front - which when moist first thing on a morning, if you unthinkingly applied your usual braking effort - would throw you over the top.

on 26/01/2007, Rob graham supposed :

There are two effects here and often both arise at the same time.

The drum heats up and expands, requiring you to press the pedal down further (not harder) to achieve the same braking effect. I have experienced that several times and it was common knowledge. The other effect is actual fade where the effectiveness of the brake becomes less for the same pressure, so you have to put even more pressure on which makes the fade even worse.

Conor presented the following explanation :

It would still need some small amount of pressure to keep the lining against the drum, or it would return for exactly the same reason that a pad on a disk return without return springs...

Small imperfections in the disk and run out force the pad away from the disk when the brakes are released. Stretching and deformation of the rubber seals of the piston, return that a little way back to permit that action.

Do you know how handbrakes with disks work?

Typically on rear disks with a handbrake on them the caliper piston needs to be turned as well as pushed in order to retract it. I don't understand how that helps the handbrake-- the piston must be able to back away from the pad enough to release the brake in normal use (in spite of the need to twist it), so how does this mechanism keep the pads firmly gripping the disk even when it contracts as it cools after you've parked?

Ben C wrote on 03/02/2007 :

Er, yes! A piston on a thread, which gradually winds itself out to take account of wear each time the brake is applied.

I have had problems with a few in the past.

Agreed.

The springy tension in the cable and the linkages of the handbrake help take up any small amount of contraction as the disk cools down.

I don't quite understand the point you are trying to make.

Just why are the rear pistons on threads?

The front pistons also push their way out gradually as the pads wear, and just sit there, without any threads, allowing the pads to back off as you describe under the influence of run out and small imperfections. Why wouldn't that work just as well on the back?

I was assuming that the threading on the rear pistons must have something to do with the handbrake, since that's the obvious difference between the front and rear. But I don't see how the threading helps the handbrake in any way. If it's not anything to do with the handbrake, then what is it for?

It happens that Ben C formulated :

It does work just as well on the back. The thread is there only for the handbrake auto adjust system.

Ah, sorry you were asking....

The thread is just there for the handbrake, to make the system auto adjusting. Everytime you apply the brakes, it winds itself out taking up any slack - hence the need to wind them back in to fit new pads. It doesn't have to be on the rears, some cars have a handbrake operating on the front brakes (SAAB).

From memory, the SAAB handbrake operated by turning the thread an extra bit to actually apply handbrake. Ford use the thread, combined with a lever cam to push on the threaded end I think.

...as I posted almost word for word eight days ago!

Chris

Yes - pros had a special tool which flared the end over neatly - but it was possible to DIY it with care.

That would explain the purpose of the thread.

But in that case I don't really see why the cam couldn't just push out a normal piston without threads.

And the threads are not like normal threads. When trying to retract the piston, it's not enough just to twist it-- it will go round and round without retracting. You have to push and turn at the same time.

Ben C submitted this idea :

The cam would have to be both course and quite large to achieve the movement needed to take up the slack as the pad wore, which would mean less leverage and less braking effect from the handbrake. Probably just as important, we have become used to these things being self adjusting to minimise the service intervals.

They have a courser thread and with more free play to allow them to move freely to take up the free play easily. You have to push and turn simply to make the threaded part stay still as you try to screw it back in - otherwise both piston and thread will go around. The small amount of extra friction as you push, is enough to hold the thread from moving. It is fairly obvious if you ever strip one of these down, how they work - though I was quite mystified by them the first time I came across them on the SAAB.

...and which only appeared on Tiscalis newserver after I posted my own installment - I had headers, but several bodies missing from the thread. If someone agrees with me, I don't usually take issue with them....

WOW! And I thought NTL's newsserver was poor!

Chris

I can assure you NTL's server is/was much worse :-)

Tiscali had a bit of a problem all round for a while, but at the moment all does seem well.