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Radius arm rubbing on caliper

john milner

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I have noticed that there is a sign of very light rubbing between my radius arms and calipers. By very light I mean more of a discolouration of rust and grime and not actual shiny metal on shiny metal. The marks are similar on each side so it is unlikely to be a sign of a serious mechanical failure and more likely to be normal wear and tear.

I have read that the calipers move towards the arms with pad wear. Is this likely to be the cause? The pads have done nearly 73,000 miles but have loads of life left in them. My other thought is if bushes have worn and the arms need to be replaced or rebushed.



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It is an SV with standard calipers.

There is some discolouration all over but each side has about a 1/4" patch where the discolouration has been rubbed slightly. It must be that the caliper and arm are just about touching occasionally. I mainly manage my speed using the throttle and the brakes mostly only get used when I stop.

I may be wrong on this but I think that as the outside pad wears the inside caliper moves towards the centre of the car to compensate for pad wear. The pads have a lot of life left in them so it just seems a bit odd that anything would touch that shouldn't.

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There is a different drive flange which moves the disc out. The caliper spacers are increased in length to move the caliper further out to match the distance the disc has moved. This increases the gap between the caliper and the radius arm.


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  • 2 weeks later...

I have now fitted new discs and pads and there is now a nice gap so brake wear was the culprit. I'm still surprised at the difference as pad wear was minimal when compared with new. The outside pads were about 1mm worn, inside pads 2mmish and discs almost 1mm on each face. I would also have thought that the caliper would only move inwards by the amount of outside wear but something must be going on that I haven't factored in. My advice is don't just check pad wear for deciding when to replace them but also check if the inside of the caliper is getting close to anything it shouldn't.

The socket needed to get the four disc bolts off is 9/16" and NOT 14mm as some have said. I did actually get three bolts out with a 14mm socket before thinking it shouldn't be this hard and switched to 9/16". I heated each bolt for at least a minute with a propane butane blowtorch and then basically laid my weight on top of the wrench and they undid using a regular 12 point chamfered socket. WD40 was very good at removing rust from the bolts and discs which allowed the socket to sit cleanly when weight was applied.

The 200lb/ft hub nuts undid a lot easier than I thought they would although the 41mm impact socket seemed a bit loose so maybe it should be imperial. I loosened the nuts with the wheels on the ground first. To tighten the offside I set my torque wrench to it's maximum (155lb/ft) and finished off by standing on the breaker bar a foot from the fulcrum (I am about 200lb UK). The torque wrench nearly right idea didn't work for the nearside as it doesn't click in reverse which I only found out when I nearly lifted that corner of the car off the stands.

The hubs came off easily so a puller was not needed.

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That is poor especially when you consider I bought it secondhand from Caterham in 2005. It was only two years old but they should have known about it and recalled. It's not such a big deal to me as I am a light braker but someone who was heavy with their right foot could need new pads a few times a year or worse break something due to contact. 

From looking at the build manual I think the part that was fouling may be the forward arm of a Watts link assembly. It's the longer of two bars that connect to the De Dion via a see saw thingy.

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