Chain skipping

muddybootson

Hi, I bought the Shimano ultegra 6800 bottom bracket the other day for my 2014 charge plug 5 frame (68mm shell width), running a Shimano R565 crank and 50t single chainring (I removed the granny gear), the bike has 135mm rear hub spacing. When running on the highest gear on the cassette the chain skips on the chainring, but if I shift down one gear it's fine. With it having 135mm rear hub spacing the chain line isn't great when in the highest gear, could it be that causing the chain to skip and if so could I use a spacer on the bottom bracket to push the drive size out a couple of mm to improve the chain line? I realise that this bottom bracket doesn't require spacers, but would it hurt to put one in? Thanks a lot for your help.
Ash

The Rookie

More likely the teeth or chain are worn to be honest. The smallest gear obviously has a higher load/tooth.

muddybootson

I'd like it to be that, but it's all brand new, just built the bike up for a winter commute. Bought the frame and components separate. Cheers though.

arlowood

Why not try putting the 34t chainring back on the crankset and see what happens then. I presume you are running without a front derailleur so the presence of the extra chainring will be immaterial.

keezx

Give the chain some time to wear in and it will be fine.

seanorawe

Yeah as above. I find new chain can take time to wear in. It seems to happen using sram chains for me.

Bobbinogs

When you say the "line isn't great" when in the "highest gear" and this is the gear that skips...do you mean the biggest cog at the back or the smallest??

The line for the big front/big rear will always be pretty cack and may well explain your issue and the solution (as in don't cross the chain). If it is big front/small rear then the line should be pretty good so look for something else. If it skips all the time then look carefully when you rotate the drive chain and see why the chain is skipping off, such as a the RD guide wheel/RD alignment, etc.

muddybootson

Hi, cheers for the replies. It's happens on the small cog on the cassette and only that one, the other gears are fine. It happens when i power through, accelerate, power up slopes, sprint etc. I've built a few bikes up from new and I've never had this problem, I understand about sometimes having to let things run in, but I've put close on 200 miles already. I've never built a road bike with a 135mm rear hub, the others have always been 130mm so the chain line has been more favourable toward the highest gear/smallest cassette cog (whatever you want to call it), which is why I'm thinking the drive size of the BB may need a spacer. Cheers.

keezx

Most likeley ist has nothing to do with chainline, but rather a small cog that's not quite perfect to sizes.

muddybootson

Sorry Keezx, what do you mean? Cheers

keezx

Well, these cogs are made from sheet metal and hardened/coated after cutting and the final result should match the standards but sometimes something less perfect leaves the factory.
I bet you pick a new cassette it'll al be fine.
I have assumed it happens with the big ring in front?
It is possible that you may have to adjust the B-screw on your rear mech so more teeth are engaged.

muddybootson

Cheers, I'll have a play with the b-screw. Just to clarify, it's skipping on the chainring, not the cassette, the only way I can describe it or compare it is its like when a front derailleur is slightly off and every now and again the chain sounds like it clicking/snapping over to the other chainring but doesn't, except it's happening when I put the power down. Thanks :-)

The Rookie

Sounds like you need a new chainring then!

ben-----

Or could it be chain length?; I don't know.

bbrap

Smallest cog at the back will mean least chain tension which could be just slack enough to allow the front to skip. Depending on how tight the chain is on your largest cog I'd either lose a link or tighten the rear mech tension. Good luck.

svetty

Could be that slight flexing when you 'put the power down' is enough to cause the chain to skip. Have you checked the mech hanger is aligned properly - new ones often need adjusting?

Bobbinogs

Chaps, this is the important point:

muddybootson wrote:

...Just to clarify, it's skipping on the chainring, not the cassette...)

Hence, the skipping is nothing to do with B stops and all that, or the manufacturing tolerances of the cassette. I think it could be a few things that I hadn't considered when I thought the issue was skipping at the back not front, although chainline could still be an issue. Might be worth checking the chainring teeth, as mentioned, although the tailored profile of modern chainrings mean most folks get a false reading of worn/deformed teeth. I would also look at chain length, as mentioned. Might also be worth looking at the RD tension although I have a 30 year old RD with hardly any spring left and don't have any issues.

OP, when you say "it skips" do you mean dropping on the outside or inside of the big ring?

Other thoughts?? Ideas to keep the OP busy

keezx

Garbage in=garbage out.......

topgunsteve

135mm hub spacing was traditional to MTBs with MTB BBs being spaced a bit wider to compensate for the chainline. You might be able to stick a spacer in behing the BB to offset the chainring slightly or add some spcers behind the chainring bolts for the same effect. This would give you a better chainline to the smallest sprocket.

That being said, I'd check the chainring teeth for wear before I did the above

muddybootson

Thanks very much for all the replies, it's much appreciated! To be honest I think it's too do with chain tension, I added a spacer to the BB drive side and it's made little difference, however with the chain in the highest gear there is possibly a tad too much slack, but when in the lowest gear the mech is close to its limit and the chain is tight. I do have a big cassette on (11-36), and I never use all the gears, ive been meaning to stick on an 11-28, I think once that's there that will sort it, it's all I can think it is. Thanks very much for the replies :-)

Bobbinogs

If you are running an 11-36 on a single big chainring then it is no wonder you have blinking issues! You might want to consider moving the Low stop at the back to prevent you from shifting to the last 2 or 3 bigger cassette rings...and then size the chain appropriately. The chainline on the 50/36 combo will be really gash and the chain tension across the range of the cassette is far too varied for most normal road systems to cope with.

muddybootson

Hi yeah as I was typing my previous comment I realised how stupid it was! still, we learn from our mistakes! But top be honest all but the highest gear worked like a charm, but then I never went lower then the 28t cog. Anyway, new cassette on order, I'll do the walk of shame and embarrassment now! Cheers

muddybootson

It's still a bit weird how its skipping from the highest gear though, anyway....

muddybootson

Sorry, I'm not explaining myself very well today, it's been a long 24 hours! Basacily the drive train is all compatible, and I had limited the rear mech to not extend past the 28t cog on the cassette. So really, it's still a bit puzzling, but maybe when I get some sleep I'll be able to figure it out!