Wierd problem with my bike

lewis123

Hi,

Firstly thanks for anyone reading this, any help much appreciated.

Bike set up.

Specialized S works FSR
XT/XTR 9 speed transmission
Hope Mini's
Mavic crossmax wheels

The problem

When going fast down hill the chain comes off from 3 to 2 ring. normally no problem with that but it makes this crazy whirring sound, coming from the back wheel. ive never heard anything like it. It only happens when going down hill, never when free wheeling on the flat.Its freaking me out quite frankly. to be bombing down a hill and look down and see the chain swaying side to side and this awful noise coming from the back wheel. Has anyone got any ideas? could it be the bearings in the rear wheel, or something to do with the rear mech?

any help welcome.

Thanks

LEwis

supersonic

Sounds like the freehub mechanism isnt quite doing its job at speed - sticking slightly, causing slack in the chan and for it to drop down a chainring at the front. Spin the freehub by hand, How does it feel?

ride_whenever

Smooth downhills, or rough?

What rear mech exactly?

nicklouse

chain rubbing on the tyre?

lewis123

hey chaps.

both kind of downhills. soft and smooth. its not catching on the tyre at all. i think the first reply has it. ill check how it spins after the football. if its the hub, do i need a rebuild or a new hub?

lewis123

the rear mech is a 9 speed xtr....not the newest one but the one before.

skyliner

That'll be the M40850 freehub on the Crossmax wheel.
It has a nylon bushing on the inside closest to the hub body, which needs to be lubed with light oil periodically.
The noise is the bushing oscillating against the pawl carrier, and the friction is pushing the cassette forward and derailling the chain.
Take the wheel to a Mavic dealer, and get the hub serviced. It shouldn't cost you much more than £10-£20 to get it done. But if the freehub bushing is badly worn and you've got play at the cassette, you may need a new body at a cost of £45 -£50.