Fault diagnosis - Loss of drive

[chip*]

Hi,

We went out to Epping Forest today for a nice muddy MTB ride, and my gf's bike had an intermittent issue. One minute she's pedalling normally and the next the pedals just spins but there's no drive through to the chain. It was annoying for her especially on the uphill section! I tried her bike on the flat and it also happened too.

Been a relatively newbie on the MTB scene, I have no idea what is causing this. She will be taking the bike back to the LBS for a check up anyway, but it would be useful to see if anyone know the cause for this issue?

Thanks

[Rouleur]

Sounds like it's the freehub, which is within the rear hub and allows you to freewheel. If the ratchet inside is worn or damaged it won't always engage and will just spin freely.

It shouldn't cost much to fix.

[bigandclever]

But, if the pedals are just spinning and the chain isn't being driven (ie the chainring isn't moving) then doesn't that suggest worn crank arms / bottom bracket? I think the description is a bit open to interpretation

[XG332]

Sounds like the ratchet system in the hub is playing up.
Assuming the chain and the rear sprockets are turning but no drive to the wheel.

[khushy]

Most likely freehub - c£20 fix if you are lucky!

[Paul Drawmer]

Or..the gears are misaligned and the chain is running over the top of a sprocket.

[rhinochopig]

Almost definitely pawls in the freehub. Was it very cold by any chance as they can freeze in very cold weather. I've seen it happen a few times in winter pedalling on the moors.

[paranoid airbag]

you say no drive to the chain - does that mean, when she tries to pedal, the chain doesn't move? In which case, erm, make sure it's meshing with the chainrings? Even very worn chainrings would give intermittent drive I'd've thought, unless the chain has fallen off or snapped, in which case you are . My money's on a knackered freehub (freewheel on cheaper bikes) with the others though.

[chip*]

Thanks for the useful response (good to know possible causes and get to know the bike's mechanics)

In reference to above questions: Temp was between 3-5 degree on the day, cold but not freezing. Yes, it's a budget introduction MTB Marin Trail Coast 2011 (rough guess ridden for ~1000km). The chain was sitting fine / aligned ok to my inexperienced eyes so it doesn't mean much!

Thanks again anyway. Will get the bike to the LBS this Saturday for a check over + hopefully get fixed.

Tom

[P-Jay]

The shop will be able to work it out, but if she was pedalling and the chain wasn’t moving chances are it's hopped off the chainring and it resting on the BB.

If the chain was moving ether the chain has hopped off the back against the spokes or against the frame, but it makes a hell of a horrible noise when that happens.

Or the hub has died

If it's intermittent I would put my money on the chain moving, happens with new bikes, they get set-up beautifully at the shop, then the cables stretch a bit (can't be helped) and the chain gets pushed off.

[shalmaneser]

It'll be the freehub, the chain skipping will make a lot of noise whereas the freehub going will be silent. Cheaper ones can be sticky when cold or the bikes not been used much.