Transmission slipping

[ChrisP100]

Hi all,

I think this could be a freehub issue, but I wanted to check first.

My 2x7-speed hybrid transmission is slipping occasionally under load (and heavy vibration). It's not slipping at the front chain rings, and I don't think it's the rear cassette as I only replaced that about 250 miles ago. I've checked the chain and it's less than 0.75% worn. Indexing is good and the gears shift well. Spinning the hub off load it doesn't seem to miss a beat, but it's difficult to tell. There is a tiny wobble on the freehub, but it's not excessive.

The ONLY thing that is throwing me off is that it's mainly happening on cogs 4 & 5, but that might just be coincidence.

I think the hub is SRAM (the cassette certainly is), but might be Shimano or some other cheap compatible.

Any ideas?

[Jamesh]

Perhaps your chain is too long and not putting enough tendon on the cassette to stop it jumping?
. cheers James

[Norman H]

I don't think it's the rear cassette as I only replaced that about 250 miles ago.

I would suspect a mismatch between the new cassette and the old chain.

I would always fit a new chain when fitting a new cassette. Its false economy not to, especially when 7 speed chains can be bought quite cheaply. I also change my chains at 0.5mm.

[axel_knutt]

I had a sprocket that occasionally used to let go suddenly without warning, usually when I was standing on the pedal, so it would dump me unceremoniously in the road each time. If the chain is slipping over the teeth it's obvious, because there's still some resistance, and you can hear it. If the sprocket lets go there's no resistance at all, and no sound.

[flat tyre]

If you still have the old cassette, put that back on and try it. If it wasn't slipping before you changed the cassette but only slips with the new cassette then the chain should be changed as well.

[ChrisP100]

If you still have the old cassette, put that back on and try it. If it wasn't slipping before you changed the cassette but only slips with the new cassette then the chain should be changed as well.

I don't think it's the rear cassette as I only replaced that about 250 miles ago.

I would suspect a mismatch between the new cassette and the old chain.

I would always fit a new chain when fitting a new cassette. Its false economy not to, especially when 7 speed chains can be bought quite cheaply. I also change my chains at 0.5mm.

The chain is not much older than the cassette to be fair, probably a couple of hundred miles at most. The 0.75mm end of the checker goes in about half way. It has been fine, but I had a week off riding, and it happened on my first ride back. I'll re index from scratch at the weekend I'm due a gear cable change before winter kicks in so I might just bring that forward and probably strip down and clean the rear mech while I'm at it. I'll see where we are from there.

Perhaps your chain is too long and not putting enough tendon on the cassette to stop it jumping?
. cheers James

Chain length is fine, but I'll check the B-screw adjustment when I re-index just to see if that has shifted since I last adjusted it.

[Norman H]

A new chain skipping on an old cassette typically occurs on the most worn sprockets and it's suspicious that yours is mainly misbehaving on cogs 4 & 5. If you still have the old chain refit it as flat tyre suggests and see if the problem persists.

It its a free hub issue then it could be down to a number of causes that can only be confirmed by stripping the freehub. Before you embark upon this course of action it would do no harm to flush the freehub with oil.

[531colin]

In my experience of slipping freehubs (2 SRAM Neos, one Shimano XT) there is quite a bang when the pawl re-takes up the drive....presumably there is just one pawl left functioning, and the freehub "freewheels the wrong way" until that pawl engages.

[ChrisP100]

Just a quick update.

I've done a full rear mech set up and re-index which seems to have done the trick. I took it on a cobbled climb on my commute home yesterday and it didn't skip a beat.

Hopefully it'll be good until I get around to changing cables.

[fausto99]

We all know what you mean, but it's % not mm on the 0.5 or 0.75 , for anyone new to chain checkers.