Rear wheel or frame flex ??

Tjgoodhew

I have recently noticed that when suddenly putting pressure on the pedals (i.e standing start sprint getting away at lights, hitting a sharp climb in a high gear) My rear wheel seems to rub against the brake.

The brakes arent super close to the rim so something is obviously flexing quite considerably for this to happen

The bike is a Caad8 and has about 2500 miles on it and the wheels are Zondas with about 2000 miles.

Before i take a trip to the LBS i just wanted to know what it might be ? Could the wheels be the cause and if so is there anything i can do myself to check and or fix ? If its the frame is it likely this is being caused by damage and if so where and what do i need to look for ?

This has only started being a problem in the last 200 miles.

Thanks

ugo.santalucia

Most likely the tension in your wheel has dropped and it's now flopping around. Alternatively you do keep your pads too close even if you don't think so. Most people do these days as wheels got stiffer, somehow 2 mm of clearance has become the norm

term1te

Could it be that the wheel has slipped slightly in the dropouts? If it wasn't dead centred it might require less movement or flex to hit the pads? You said it started 200 miles back, did you take the wheel out then? I'd also check whether the brakes were centred properly too. Might have got knocked off centre, and again the original flex is now causing it to hit the pads. I'm sure you've checked these, but could save some time at the LBS.

debeli

There are a billion possible causes for this, some benign and others less so. Nobody can really have a go at giving a definitive answer on a web forum, but the problem will not be unfamiliar to many members here.

At the crappy end of the scale is a crack in the frame. Unlikely, but possible.

If your brakes are adjusted quite far from the rim, then frame flex seems fairly unlikely - and if you have some pretty stiff rims and the spokes are tight and everything is straight then even wheel flex seems unlikely.

If it's happening away from the lights... (i.e. from rest), then is there a possibility that the fulcrum of your rear brake calliper is sticking? This would mean the rear brake stayed in contact with the rim after stpping and this may become apparent only after you start to pedal.

There may be even more than a billion causes for this ill, but few of them require a trip to the LBS. I'd pop it on a workstand (or a turbo, or hang bars and saddle from ropes in the rafters) and try to replicate the cause.

While you're at it, give the bike a quick service - ping the spokes, check the wheels are true and straight and the QRs are appropriately tight, see that the brakes are coming on evenly and releasing as they should... and if all else fails, look very hard for a crack in the frame. It might even be a bent frame...

Whatever it is, do post the cause on here.... It may save someone else having to sod about looking for the same thing.

Good luck finding it!

Tjgoodhew

Thanks Debeli,

I will hopefully have soem time tonight to have a look at these things and will update if i find anything.

When you say ping the spokes what do you actually mean by this ? Am i simply just checking to see if any are loose ?

darkhairedlord

pluck them like guitar strings. Should all sound the same. There's an app for that!