Cranks and chain slip when applying torque pedalling hard

Seems to be a problem in gearing on my sons Mtrax
When applying a lot of torque or pedalling hard the cranks slip, it seems as if the chain is slipping on some of the cassette cogs. This happens only at certain gears and especially when the rear mech is extended.
And I think on smaller cogs

It's a case of pedal pedal slip slip pedal slip and its annoying.
The rear mech is an LX long cage mech, about 3 years old (I think re previous gen LX)
LX cranks
9 speed HG cassette
It has a 9 speed Sram chain

Any ideas what's happening? Or how to solve this? As lifting the bike up or turning it upside down and pedalling and changing gears is all perfectly normal, only happens when torque is applied and there is resistance ie wheels are on the ground.

Re: Cranks and chain slip when applying torque pedalling har

If chain, rings and sprockets are basically OK (not worn to ridiculous extent - they look fine on the pic) and rear mech alignment has no obvious horrible misalignment, it sounds like the rear freehub ratchet mechanism is slipping under load. Load on the freehub is highest when pedalling hard on small rear sprocket so your explanation fits. Me thinks knackered freehub.

Re: Cranks and chain slip when applying torque pedalling har

if it is all set up correctly (ie indexing, cassette lockring is tightened down etc) then it sounds like either the chain/drivetrain is worn, or as above the freehub mechanism is dying.

Have you only just fitted a new chain or cassette?

Re: Cranks and chain slip when applying torque pedalling har

The freehub sounds smooth and good the cassette chain and cranks are brand new
Shit maybe I haven't tightened the cassette - could this be a prob?

Re: Cranks and chain slip when applying torque pedalling har

Measure the chain, each link is 1/2", when measured over 12" what does the next pin measure?

Re: Cranks and chain slip when applying torque pedalling har

ibbz":2i7ptysk said:

The freehub sounds smooth and good the cassette chain and cranks are brand new
Shit maybe I haven't tightened the cassette - could this be a prob?

Click to expand...

Highly unlikely as the cassette is fitted on huge splines perpendicular to chainline so no way the cassette/sprockets can slip around the hub. The freehub sounding smooth doesn't mean anything really, it can still slip under load. Good job mountain bikes have low toptubes otherwise your son's voice might rise two octaves next time it slips. :shock:

Re: Cranks and chain slip when applying torque pedalling har

have a play about with the cassette/freehub, move it back and then forwards, it should engage within a very small distance (not sure on the specifics, probably out there if you Google though), I once had a freehub that turned about a quarter of a revolution before it would engage (obviously the pawls inside it were knackered) , it's more likely to slip under load though, so annoyingly a workbench test doesn't always show the problem up.

And I have seen cassettes spin on the freehub body before, had a freehub body that had been mangled by the cassette wearing down and gouging the locking splines on the freehub itself, under load it was enough to let the entire cassette move around on the freehub body.

Re: Cranks and chain slip when applying torque pedalling har

seeing as its got a new chain, cassette and cranks, it definitely sounds like the freehub imo
replacement springs and pawls are in order i'd say, if you're wanting to keep the hub, if not you can usually pick up wheels/hubs on here pretty cheap

EDIT: what hub is it?

Re: Cranks and chain slip when applying torque pedalling har

The chain looks a bit slack in the pic - too many links?

Re: Cranks and chain slip when applying torque pedalling har

Too many zip-ties on your chainstay protector. Remove the blue ones and you will find that your chain stops slipping.

On a less unhelpful and more serious level.... which hub is it?