Ok, so my [url= http://www.singletrackworld.com/forum/topic/more-new-old-bike-content-principia-mac-b-resurrection ]thread[/url] about my bike build wasn't interesting enough to warrant any comments 🙁 but I still need to get an answer about the chain jumping off issue please!
As I started the first uphill the chain jumped off the inside of the rear cog - my guess is the tensioner moved slightly and the loosened chain was then able to ride off the cog - but is there a sure way to check chainline?
Would it be advisable to change the rear cog for a proper ss one?
Or is it just a matter of getting a good, tight chain?
cheers!
Chain looks far too slack. Take a link out and tension upwards.
A proper ss cog will help though
[i]tension upwards[/i]? Chain runs over the top of the tensioner roller?
Lay a long straight edge (like a steel ruler) on the chainring, and make sure the other end goes back to the sprocket on the rear. the front and back should be pretty much inline.
easy to check chainline by sight.
But yes shorten chain and use proper cog.
And OMG get rid of blue cable and hideous forks - they must weigh more than the frame!
What sprocket is it? A SS specific one or one from a cassette?
the chain does look a bit slack - take a link out if you can.
on the tensioner you have there Dez, yes you can have the chain over the top. Advantage is your chain wraps round a couple more teeth of the rear cog, and you lose a very few grams in having a chain which is a link or two shorter.
Although I found on my old frame (kinesis maxlight) that the relationship/distance/angle between dropout and mech hanger hole meant even with a half link I couldn't get that particular tensioner to go upwards enough (the longer curved slot needed to be eben longer if you see what I mean).
Oh and +1 for a proper singlespeed cog. Taller teeth and no shifting bevels = chain more likely to stay put. You may have had so ,little tension in the chain cos the roller was so noisy. Accept the noise and have half to three quarters of an inch of 'up and down' in the chain. I hear (well, don't hear really!) that tensioners with a toothed wheel are quieter.
[i]And OMG get rid of blue cable and hideous forks - they must weigh more than the frame![/i]
Aesthetics (and weight) are really not an issue! Forks were a tenner and the blue cable was attached to the brake, was about the right length, so stayed!
The whole thing is still lighter than my road bike.
Its a sprocket from a cassette.
Thanks folks - and well explained Julian. Can get it sorted pretty easily now.
check the chainline - measure from centre of seattube to tip of front ring - will be 45-53mm approx
now measure rear sprocket tip to inside face of dropout, ie at end of axle nuts. take this measurement (15-25mm i expect) and deduct from 67.5 (ie 135mm rear / 2 = 67.5mm, the centreline of bike)
the 2 should match within 1-2mm if using an 8-9spd chain, or within a mm if using a track chain. if it comes off the inside rear, i expect your rear sprocket is too far from the bikes centreline, or less likely, the chainring is too close to the centreline.