Pedals Occasionally Slip Forward When In Top Gear

[woodbine]

Firstly, hello to all. I have lurked on here for a while, and decided to take the plunge and sign up. My main aim is to keep my ageing British Eagle MTB on the road.

BIKE DETAILS: Late 80s British Eagle MTB, 18 speed (6 gear Uniglide & 3 sprocket oval Biopace), Deore derailleurs and Deore SIS shifters.

The bike was laid up for 20 years and I re-commissioned it last year. Apart from a new Uniglide hub (previous one had worn noisy bearings and miraculously my local bike shop had a NOS one for £30) and crankset rebuild, most of the main mechanicals are original. I also fitted a new KMC chain as the old one was well stretched.

Anyway, when I am in top/18th gear and I apply lots of force to the pedals, especially when standing up on them, the pedals occasionally slip forward as if they have jumped a couple of teeth. It's accompanied by a bit of a clunk. This can be very disconcerting.

All other gears are fine. What could be causing this? Could it be the new KMC chain bedding in, or something else? I don't remember this happening before the bike was laid up.

Thanks for any advice.

 

[xerxes]

Re:

Once a chain is very worn (stretched), it will wear the freewheel or cassette sprockets to fit the worn chain and the chainrings as well. Once the sprockets and chainrings are worn to fit a worn/stretched chain a new, unworn chain will no longer fit properly and may even skip over the teeth. The top gear, the sprocket with the least teeth, is always the most prone to wear as the load/pedalling force is spread across fewer teeth.

It's best to change chains well before they get too worn in order to make the freewheel/cassette sprockets and chainrings last longer as once they are worn to fit a worn chain, you'll have to replace the whole drive train.

See here: http://www.bikeradar.com/road/gear/arti ... ned-46015/

 

[woodbine]

Thanks very much for your reply. Makes sense, especially as top gear is probably one of the gears that gets most use.

Is it true that on Uniglides, you can turn the sprockets around to get more use out of them? The problem sprocket will be the smallest outer one that is also the threaded lock ring that holds the sprocket set together. Is this reversible? Is the spacer between it and the next sprocket part of the outer/locking sprocket, or is it separate?

Finally, if all else fails - and I realise they're scarce - where would I start looking for new or good used sprockets?

 

[xerxes]

Re:

6 speed freewheels are still readily available, you can buy a new Shimano for under £8.00, so it's not worth messing about rotating the sprockets: http://www.ebay.co.uk/sch/i.html?_from= ... 5573.m1684

Lots of new bicycles are still sold with six speed freewheels, albeit cheap ones, freewheels are an older technology and started to be replaced with freehubs and cassettes in the 80s.

See here:
https://en.wikipedia.org/wiki/Freehub
http://www.sheldonbrown.com/free-k7.html

 

[woodbine]

Thanks,but there will presumably be a lot more to change if I change to a freewheel and different cassette?

As for the alternative 6 speed freewheels in your Ebay link, I wouldn't know which one to go for if I was to change the setup. Definitely want to stick with 6 speed though and would want quality like Shimano. Presumably, I would have to change the Uniglide freehub too? Then would a different freewheel setup work with my Deore SIS derailleur? Would I need to change the SIS index shifter too, and would a different freewheel setup fit in the frame dropouts? I'm not very knowledgeable in these things.

Definitely would want to keep 6 speed on the back though. Although a MTB, it is fitted with slicks and rarely leaves tarmac. 18 gears is plenty for my needs.

P.S. Some of the freewheel cassette prices for 6 speed Shimano stuff, eg. Tourney, on your Ebay link are for pennies. Wouldn't know where to start though. Even a complete wheel, freewheel and cassette for £30, but I would never be able to work out if it would be suitable for my application.

http://www.ebay.co.uk/itm/700c-Hybrid-S ... HXKQEQXzGw

 

[xerxes]

Re:

Thanks,but there will presumably be a lot more to change if I change to a freewheel and different cassette?

I'm assuming that what you have is a freewheel, as far as I know, there weren't any 6 speed cassettes. As such, replacing it means you just unscrew the one you have, and screw on a new one.

Can you take a picture of what you have?

 

[Silvergoat]

Re:

when I am in top/18th gear and I apply lots of force to the pedals, especially when standing up on them, the pedals occasionally slip forward as if they have jumped a couple of teeth. It's accompanied by a bit of a clunk. This can be very disconcerting.

I strongly agree – this scenario can be most disconcerting, especially when you're in the middle of traffic and need to get out of people's way suddenly.

There's a few factors which can make chains slip at the wrong moment. Ideally, starting with a new freewheel or cassette and a new chain together helps. Also, the 'correct' chain length – just wrap chain around biggest cog (1st) and biggest ring, leaving out running it through the rear mech. Hold the ends together, see where it meets/overlaps, then add say 3 links, before removing the unwanted length. Another factor is the rear mech and cabling...

Good to see you're no longer lurking on here and good luck with the Eagle.

 

[hamster]

60% of the time this is curable with a new chain...the other 40% it's a new cassette / freewheel.

 

[woodbine]

Re: Re:

Can you take a picture of what you have?

Hope these are OK.

Thanks to all for replies. New KMC chain already fitted - it was OK with the old, stretched one.

 

[Silvergoat]

Re:

Have to say, those cogs look in very good shape/well defined.

KMCs are pretty sound chains. Though the 'magic' link can be a bugger to undo when you need to remove the chain :facepalm: