Fixed gear conversion: chainline and tension

Will probably spoil the look, but you could always put an idler in the
chain line on a spring to take up the slack when necessary (similar to
the pair on a rear derailleur)

peter

naked_draughtsman wrote on 09/07/2006 09:55 +0100:

Quoted message said:

Will probably spoil the look, but you could always put an idler in the
chain line on a spring to take up the slack when necessary (similar to
the pair on a rear derailleur)

Won't work on a fixed because, unlike a freewheel system, the lower
chain run can come under as much tension as the upper and the upper
chain run can have all the slack on it. A derailleur style double idler
would get ripped off. I doubt a single idler wheel could apply enough
tension and you'd need two, one for the lower chainline and one for the
upper. The problems of the chain derailing and jamming are not pleasant
either and an idler would make the probability of that happening on
bumps and potholes much higher.

--
Tony

"Anyone who conducts an argument by appealing to authority is not using
his intelligence; he is just using his memory."
- Leonardo da Vinci

Mark came up with the following;:

Quoted message said:

Hi

I've just finished building an old Viscount frame into a fixer with a
new wheel built on a Miche hub, spaced to 126mm OLN. I've got (what I
think is) a Stronglight 99 chainset with a 42t ring bolted to the inside
with single ring bolts. The chainline looks about right and seems almost
spot-on with the straight-edge-ring-to-sprocket method and about 2mm
off (chainring further outboard than sprocket) by measurement. Is this
close enough?

Should be, empirical testing will tell you better ... 😉

Quoted message said:

I'm having trouble getting the chain tension right, probably becuase the
ring is old and no longer circular. Indeed, when adjusted so the chain
is taut at the tightest spot there is 10-15mm of total slack in the run
when at the loosest point. I've tried Sheldon's trick of loosening stack
bolts, tapping the chain and retightening them again but to no avail;
the ring is a firm fit on the spider so I doubt that's the cause of the
eccentricity, anyay. Time for a new ring, right?

It might be worth trying it out anyway, steadily at first, then if all seems
good, try a decent hill where you can get some pressure on the pedals. This
can help 'seat' chain rings, but if it doesn't then it's time for a new
chain ring.

Quoted message said:

I was looking forward to riding it tomorrow 🙁

See above .. 😉

--
Paul ...
(8(|) Homer Rules ..... Doh !!!

Paul - [censored] came up with the following;:

Quoted message said:

It might be worth trying it out anyway, steadily at first, then if all
seems good, try a decent hill where you can get some pressure on the
pedals. This can help 'seat' chain rings, but if it doesn't then it's
time for a new chain ring.

Sorry for the bad form and all that, replying to my own post, but ....

It occurred to me that it might also be worth checking the bottom bracket is
running true. I had a similar problem years ago with a damaged bearing on
one side of the BB. Everythign worked fine until really pressing hard, then
one of the bearing races would open out a bit, there was a crack in it, and
the BB would 'twist'. Easily cured, time and money is all ... it also cured
the 'ticking' noise I'd been hearing but couldn't pin down for ages ... 😉

--
Paul ...
(8(|) Homer Rules ..... Doh !!!

Mark said:

Hi
I'm having trouble getting the chain tension right, probably becuase the
ring is old and no longer circular. Indeed, when adjusted so the chain
is taut at the tightest spot there is 10-15mm of total slack in the run
when at the loosest point. I've tried Sheldon's trick of loosening stack
bolts, tapping the chain and retightening them again but to no avail;
the ring is a firm fit on the spider so I doubt that's the cause of the
eccentricity, anyay. Time for a new ring, right?

There might be benefit in remounting the ring in any
of the other 6 possible ways (rotational symmetry).

You *might* get an improvement.

But, in any case, 10-15mm of slack is tolerable.

BugBear