Losing chain tension completely in lowest smallest gears.

jeepie

On the commute in this morning I realised that I am losing all chain tension in the lowest, smallest gears. So the smallest chain ring at the front. When I come to start off from the lights the crank simply spins but once I get moving the chain tension seems to return. I have Shimano 105. The bike has done thousands of miles, and about 300 on this chain.

Any ideas as to what could be causing this. I'd really appreciate it as it makes starting off at the lights very tricky. Let me know any ideas or tests I could do to try to diagnose the problem further.

-liam-

Is your freehub spinning freely ?

jeepie

Thanks for the reply Liam - Do hope I can get it fixed to cycle home...

not sure...Could I test it by lifting up the rear of the bike. Spinning the pedals. And then looking for the cassette turning - similar to how you put the chain back on after it has come off....How would you test the freehub?

roger_merriman

i assume you mean the chain is slipping? what happens with steep hills? strange that it recovers. could be the rear derailer if the spring has gone/going.

if you look at it what happens i' have a good look at all the drive chain. another possiblity is the chainring, though i'm puzzeld as to why it recovers.

-liam-

Just lift up the wheel and spin the cassette backwards. It should spin a few times round without stopping. You can leave the chain in situ. If it comes to a halt fairly quickly or barely moves at all then that is probably your problem especially as you would relax any tension from the pedals at the traffic lights and so would the chain, but as the free hub is a little sticky it isn't' allowing the slack chain to pass resulting in a good half a turn of not alot when you come to set off

jeepie

Thanks for the replies. Chain is not slipping at all. Changes are deep and crisp and even. Top notch.

The only problem is when you freewheel to the lights or to a stop, you'll completely lose chain tension which makes it very hard to clip in and get away....

Just been out to the bike sheds to test out the freehub and I span the back wheel backwards. Because I'd freewheeled to park the chain was completely slack. Now although the wheel did spin backwards quite well (possibly a tiny bit sticky at the start, possibly...) I noticed that spinning the wheel backwards completely resolved the problem of the chain tension.

So does this help diagnose the problem. Is it the freehub?

Thanks a lot for the advice. I really appreciate it.

-liam-

You need to spin the cassette not the wheel. The cassette sits on the freehub or if you ride your bike and just pedal backwards while moving you should notice the slack in the chain appear and the chain will sag over the chain stay.

jeepie

Hi Liam,

Cheers for the help.

OK. If I spin the cassette it doesn't move at all. If I push it very hard the wheel and cassette move together.....

So sounds like I'm riding a fixed bike! Does this confirm the freehub is the problem? I take it I need a freehub service to fix the problem....

http://www.parktool.com/repair/readhowto.asp?id=45

Thanks

Jeepie

-liam-

Sounds like it has probably gone a little past a service but you can try it.

I did exactly the same to mine a couple of weeks ago as it was a little sticky and it apears to have worked

They aren't expensive though. About 15 to 20 quid plus what ever the lbs charge you for 15 minutes work or you can do it yourself if you have a cassette removal tool and a chain whip.

itboffin

If you lift the rear wheel and pedal the cranks backwards does the chain go loose?

jeepie

OK. Yeah - you were right guys. Tried pedalling backwards and the chain went totally slack. Bit of a shocker getting home but only 7 miles or so luckily. Took it to a chap I know down the road as at the moment I don't have the right tools and he is gonna replace the freehub and apparently the cones are loose too.

Thanks very much for the ace advice. I hope to be able to do this stuff in the future myself but at the mo don't have the correct tools! It was good to know what was happening so I could get home and get it sorted as initially it seemed a real mystery!

itboffin

I had the same problem but it wasn't the hub it was the bottom jockey wheel on the rear derailleur I loosened it off and normal service resumed.

itboffin

The very same jockey wheel is now fubar - it's the curse :roll:

jeepie

Sorry to hear that ITB....I agree it's a royal pain. I took my bike to the bike hospital in the end i.e. the bloke I know down the road who has big spanners and cool tools. Good luck getting it sorted. It's particularly annoying given the gorgeous weather....I need to go out on my speedy bike.

itboffin

Jeepie wrote:

Sorry to hear that ITB....I agree it's a royal pain. I took my bike to the bike hospital in the end i.e. the bloke I know down the road who has big spanners and cool tools. Good luck getting it sorted. It's particularly annoying given the gorgeous weather....I need to go out on my speedy bike.

Ta but no fear I have seven bikes, now which to choose today is the biggest issue