Chain line issue

scotth1991

Hiya all I'm new around here, I have had mtbs for years but have just got myself a road bike to commute the 15 or so miles to work.
The bike is fitted with a single chain ring up front with 8 speed on the rear but I have found I only get the use of the top 3 gears without the chain coming off. Looking at it this appears to be caused by the angle of the chain in the lower gears. I just wanted to get some advice on what I could do to resolve the issue.

pilot_pete

When you say ‘top three gears’ do you mean the biggest cogs on the cassette, or the top three gears (as in highest ratios - the smallest cogs on the cassette)?

So, is the chain coming off the chainring at the front or off the sprockets at the back?

First thing I would do is adjust the rear derailleur, especially the limit screw adjustments to ensure it is shifting correctly.

Check the chain for wear, easiest method is with a chain checker, but you can measure it with a steel rule. If it is 3/4 worn or more then it will likely be jumping and / or starting to damage your sprocket/ chainring teeth.

Now check the chainrings and cassette teeth for excessive wear and/ or damage. Search “Sheldon Brown’s” website which has descriptions of tooth wear.

Now, I am assuming that amongst that list you will probably have found the cause. If not, other things to consider are;

Derailleur hanger for straightness/ alignment
Chainring bolts for tightness
Chain line (correct bottom bracket fitment)
Rear derailleur cage for twist
Rear derailleur pulley wheels for wear (teeth worn excessively, bearings/ bushings worn/ right) - bear in mind the top pulley will have some play
Shifter cable for smoothness of operation - check for kinks, fraying, corrosion or stickiness in operation
Check shifter for correct function

And that’s about all I can think of off the top of my head...

Pictures would help...

PP

scotth1991

Sorry I should have specified but it's the 3 smallest cogs I have the use of.

I have put on a new chain and rear cassette so that's not a problem, the chain ring isn't too worn and looks like it might have been replaced recently.

The next thing I'm going to check is the bottom bracket I feel like that might be the problem.

Thank you for you're help I will update if I get to the bottom of it.

g00se

I once set up a frankenbike 1x8 from an old frame. However, the frame was an old steel frame that needed the rear dropouts cold-set wider to allow for the wider wheel - and this meant the sprockets were further outboard than they should have been, so going to the smaller sprockets would have the chainring drop the chain.

Yours is doing the opposite, so it sounds like your chainset is too far outboard. If it's an older square taper bottom bracket? There are so many different versions (JIS and ISO standard tapers, widths for track, single, double and triple chainrings) that the wrong one will put the chainline out of whack.

Finally, is the mech hanger or the rear mech pulley cage bent? If it's really out, back-pedalling may ship the chain from the chainring too.

vegasfoster

Depending on the chainset and current position, you may be able to move the chainring from the inside to the outside of the crank arm, or vice versa, and that may help, but best bet is to get the correct sized bb as indicated. Those things said, even if the chainline is is dead on, in my experience the chain will still come off periodically unless you have a narrow-wide chainring or chain guard.

scotth1991

Thanks for the help everyone, I have solved the problem moving the chain ring to the opposite side and it worked a treat today.

My next plan was to switch to two chainrings in the front but the frame doesn't appear to have guides for a cable to a front mech, the 8 gears turned out to be more than enough though.

sungod

you can always add cable guides, choices range from plastic with a zip tie to the very pretty cinelli ones...

https://www.wingedstore.com/components/ ... s-355.html