rear brake dragging

[richyrd5]

Since putting new pads in my 30 they drag when wheeling backwards, but are fine going forwards(yes I know I dont ride backwards!)is this because they are shite quality pads? Ive cleaned/overhauled caliper.but the 1 of the pad shims is missing(the small one at the front)and the shim/plate that the pad tangs go in is loose, are these available? Ta

[CRM]

Richard, i bet its possibly one of the pistons still a bit seized. did you use new seals or just clean it all up and clear out the seal channels ?
take it for a blast bed the pads in nicely and if it still drags i would be stripping the caliper again.,

[richyrd5]

Thanks for reply paul, no only cleaned pistons.didnt do new seals.they were fine.but the pistons are very free and going in and out lovely..it didnt do it with the other pads.but like I said, I think it may have something to do with the pad tang thingamie jigs??.this is the 4th time ive pissed about with it.when I take it apart and clean/bleed it all works fine doesn't drag on when pushing backwards(its always fine forwards)but when I get back from a ride, when I go to push it back to go out again, it drags!, doing my head in now, do you think its the piston still sticking??
hope cadwell went well chap.

[Neosophist]

Brake pads will always drag, does it drag so much its hard to push?

unless you get the seals out and clean behidn them and what not its a bit of a waste of time as lots of crap accumlates there

[richyrd5]

Yup, looks like your right neo,, need to replace seals,, thats what you get for trying to cut corners!!(dont worry though, , its all character building! )..

[willandrip]

You said the pistons slide freely.
You say it only does it when wheeled backwards.
You said it only does it with the new pads.

Spock would say ; "Logically its the new pads."

When travelling forwards they are being held in the correct position.
When reversed; the slight drag that is always present with a disc brake is causing the pad to cant in the caliper and one edge(end) to jam .
You have already answered your own question by reference to the shims/holders.
Some pads ; although they fit and work are a very bad fit in the caliper and allow the pad to move about too much or not at all even.
Compare the new and old pads ( be quick; its bin day on Fridays if you have already discarded them)

You might get away with a chamfer on the edges that are jamming, applied via a rasp.

[richyrd5]

Hmmm, bin days is wednesday round this ways..but chucked them weeks ago!! But your right that the pistons are free, no sticking at all...so...new tangs it is then..

[LostanFounds]

Got a similar issue myself at mo.
Anyone advice on where to get a set of seals.?
Don't want to end up with cheep Chinese rubber.!
And somewhere that won't break the bank..?
It's my NC30 not a Rolls Royce..!

[richyrd5]

Rick oliver is the best bet mate, price list on top page of general in v4 section.
£15.for the set,

[RickOliver]

I have brake seals in stock if you need them - phone or email me off forum if you want to order.

The root of the `dragging when going backwards` problem is the self servo effect - if any of you are old enough to remember drum brakes you`ll recall that the one to have was the TLS (twin leading shoe) type. On a conventional SLS (single leading shoe) drum brake with both shoes on the same pivot one side of the brake plate and a single operating cam the other, the rotation of the wheel tends to push one (leading) shoe harder into the drum (self servo effect) and the other (trailing) shoe away from it, so a better design is to have the shoes pivoted on opposite sides of the plate, each with its own operating cam so that they are both `leading` shoes whose effectiveness is increased by the wheel`s rotation.

Turn this principle through ninety degrees and apply it to a disc brake and what the self servo effect does is to press the rear end of the brake pads harder against the disc than the front end. Why are the pistons in your front calipers different sizes? That`s why - the bigger leading piston applies more pressure to the front end of the pad and thus evens out the wear.

Both the pistons in an NC30/35 rear caliper are the same size so the pads will always wear slightly `wedge shaped`. With the missing shims allowing the pads to move more than they should 1) the uneven wear will be accelerated and 2) rotating the disc backwards and forwards through the pads will have an action analogous to pulling a rope through cam cleat - one way it runs freely because the rotation is pushing the thick end of the pads away from the disc, the other way the thick ends are pulled inwards causing the disc to become (quite literally) `wedged`...