Binding rear discs

[andy couchman]

Changed the (Sierra type) rear pads on the 7 the other day then went and did some hard braking to bed everything in.

 

Trouble is the brakes are now binding a bit on the back end.

 

My understanding of the self-adjusting mechanism is p*ss poor so might I have been a bit too enthusiastic and caused the pistons to come out too far for some reason? If so, should I strip the calipers again and wind back the pistons again or is there some other magic I should be aware of?

 

Thanks.

 

Andy

[susser]

Andy

Are you sure it's not the trouble I had with grinding from rear discs, ie rubbing at the periphery ?. Goes away as soon as the disc cools. Stands to reason that there should be no witness marks on the edge of the disc. Have you got some?

Paul.

 

[andy couchman]

Don't think it's that - the car is harder to push in the garage, which suggests the brakes are binding a bit - also the rear wheels are quite a bit hotter than the fronts after a run.

 

Andy

[Billyboy]

When you fitted the pads did they fit easy or was it a little difficult to get the caliper back over them?

 

Never changed pads on my cat but plenty of sierras - IIRC the sliding mechanism is either side.......Did you lubricate them with some copper slip / under the rubbers If Not when pulled back the are now operating in a place they have not and it may be all gunked up inside hence the binding - usually from the pad that is actuated by the mechanism NOT the piston....so IIRC it will be the pad on the outside.

 

Although the theory is that one pad is pushed onto disc by piston and the other is pulled

onto the disc by the mechanism and be released at the same time the sliders used to stick. Infact on the front they would seize altogether and new sliders could be bought - on the sierras that is - never changed rear sliders though

 

Need to go out to my car to look at the caliper.......

 

Edited by - Billyboy on 23 Aug 2006 17:25:23

[andy couchman]

Thanks Billyboy - yeah, I copperslipped the sliding mechanism but also found it quite difficult to get the pads in - but put that down to my cackhandedness. Certainly the 2nd one went together much faster than the first.

 

Think I might have a look later in the week and see if there's anything obvious.

 

Andy

[susser]

Are you sure you wound the piston fully back. ?

There should be room if you did. I still can't understand why the piston has to be rotated though. (ie why is it so tight ?)

[andy couchman]

No, I've never understood how the thing works either.

 

Anyone got a PDF from a suitable Haynes type manual?

 

Andy

[oldbutnotslow]

OK there something that may be worth checking. There is a locating tab on the metal back of the pads that fits into the X cut out in the piston. Are you sure that the X alignment on the piston was vertical/ horizontal. If the tab is not located properly it will cause the brake to bind.

 

 

Grant Taylor

OBNS Motorsport

www.OBNS.co.uk

 

😬 183 BHP of Black and 'Stone Chip' excitement. 😬 here

 

 

Edited by - oldbutnotslow on 24 Aug 2006 21:19:09

[susser]

Grant

I suppose the X cut out and the tab on the back of the pad is to stop the piston from rotating. Why ?.

I want to find out how the handbrake/self ajuster works.

Now is not a good time as iv'e just realised that my home brew (half nelson) is as strong as I hoped.

(managed to type this ok but I'm starting to struggle

Smashin,

 

[Myles]

I suppose the X cut out and the tab on the back of the pad is to stop the piston from rotating. Why

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You can wind the piston back in - so if there was nothing to stop it rotating in normal use, pressing the brake pedal *could* (theoretically) just spin the internal mechanism without exerting any force.

 

 

 

Project Scope-Creep is live...

 

Alcester Racing 7's Equipe - 🙆🏻™

 

Alcester-Racing-Sevens.com

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[susser]

Myles

Sorry but that doesn't stack up;

I imagine that the piston is pushed forwards by the hydraulic pressure behind it. There can ? be no rotational force applied by the fluid.

However, the handbrake mechanism could well be a screw type contrivance. I imagine that the piston is like a nut and the handbrake actuator is like a screw. rotating the screw would cause the nut to move along and press the piston. If the nut (piston) could rotate, then there would be no relative movement between the two and so, no force applied. Hence the tab and the X.

I may have to get hold of one and rip it apart unless someone can explain it to me. here is some of the way there.

What do you think. ?

 

Edited by - susser on 25 Aug 2006 08:45:18

[andy couchman]

Excellent stuff - and more to check 😬

 

Thanks all.

 

Andy

[shn7]

Maybe off the mark but no one else has mentioned it. Have you checked that the handbrake cable is going fully slack when off? Maybe that's simply adjusted a little too short with the new pads.

 

Steve.

Not forgetting Percy the Polar Bear

[Billyboy]

*thumbup* Good point Sir

[andy couchman]

Update

 

Checked various suggestions, then did a dry fit bolting just the U-shaped metal thingy to the axle (it's a James Whiting disc brake conversion on the Ital axle) not the caliper body itself.

 

That showed that the U shaped thing was not parallel to the disc rotor, so nor would be the pads. What was needed was some shims (washers) to bring the thing reasonably parallel and to get both pads roughly the same distance from the disc rotor. When I removed the calipers the shims had fallen out and I clearly did not reassamble the calipers correctly onto the axle 😳. Indeed, I'm not sure they ever were 100% as I have suffered from 'warped brake discs' which look now to be much better.

 

The dry fit enabled me to get a pretty good fit and, when the calipers were then attached the binding disappeared.

 

Yesterday drove about 140 miles including the sprint at Curborough and the binding had gone. On returning home the car can again be pushed easily back and forward in the garage and the 'warped' discs (warping seems to be one of those urban myths used to explain when discs take on uneven deposits from brake pads and explains how the brakes feel, not what has actually happened AIUI) seemed to have straightened themselves up again...

 

Steve's website gives a helpful explanation of the theory of these brakes - tho I still don't understand how the rotating piston mechanism works

 

Thanks again all.

 

Andy