chain line.

[colly]

I have just tonight fitted a new compact chainset and now find that the chain line is out by around 4 mm or so.

The existing spindle is 111mm which according to the stuff that came with the chainset is the correct length and that should make the chain line something like 43.5 mm from the center of the BB. It is about 47 now.

Can I alter the position of the BB spindle to get it back in line? Or, if that isn't possible, could I get a ''floating'' jockey wheel which would maybe take up some of the mis -alignment?

 

[grimpeur]

If your wheel is QR then it is unlikely you will be able to move the rear axle(presume you mean by threading the cones further to one end) because the amount of threaded axle protruding at each end will then be different and the wheel will no longer sit in the dropouts.

Have never heard of a 'floating' jockey wheel but have to say that doesn't sound like the perfect component in a smooth running transmission system!

I would ask you to consider whether your present chainline is actually causing you any problems as such?Are you getting chainrub now that wasn't there before? Usually ,if the chainline has changed by only 3.5mm then minor adjustments to the derailleur limit screws should see you on your way.

If anything,chainlines tend to become more critical when you start moving chainsets closer to the bottom bracket rather than further away.

 

[colly]

I would ask you to consider whether your present chainline is actually causing you any problems as such?Are you getting chainrub now that wasn't there before? Usually ,if the chainline has changed by only 3.5mm then minor adjustments to the derailleur limit screws should see you on your way.

Many thanks for your reply.

The existing set up is fine , no rubbing at all.
What seems to be happening is that the chain is coming at the derailleur at an angle such that it doesn't quite seat on the jockey wheel and as a result it slips off every few links and hits the guides, then bounces back.
Adjusting the limit screws just puts the chain out of line with the sprockets.

As for the 'floating' jockey wheel, It's just that I recall reading something that referred to one some time ago.

Can I adjust the position of the axle on the BB?

 

[skwerl]

surely what you describe can't be caused by the new chainset. if the chain is threading into the rear-mech at a funny angle then how can chain-set position cause and issue. the chain line into the mech is determined by the rear cassette

BB axle can't be adjusted in a sealed unit. you'd need an old-school cup and cone job or one of the specialist adjustable units

ok. having re-read I'm guessing you mean the issue is to do with the chain leaving the bottom jockey-wheel. This must only be a problem in certain gear combinations - right?

 

[colly]

It seems that as the chain leaves the chainring it comes at the jockey wheel at an angle. The angle is such that the chain doesn't seat on the center line of the jockey wheel and the jockey wheel crawls onto the edge of the chain before after a few links it jumps back to the center of the chain and begins to creep out again.

I can't imagine the chainring is any different to the one I have just taken off.

I'm puzzled.

Of course I could take a hammer to it until it sees sense and works properly.

 

[PpPete]

I'm wondering if the difference is down to a difference between JIS and ISO standard square tapers?

 

[colly]

I'm wondering if the difference is down to a difference between JIS and ISO standard square tapers?

Hadn't thought of that. Both chainsets are Campag. though but I will have a look.

Thanks.

 

[grimpeur]

It seems that as the chain leaves the chainring it comes at the jockey wheel at an angle

But the jockey wheels move inwards and outwards with the rear mech.Adjustment of the rear mech would normally sort this out.
I take it you are only seeing this in certain gear ratios?

If the jockey wheels are out of line with the chainset in all gears then the jockey wheel cage itself must be slightly bent(out of line)
Maybe the new chainset has accentuated this.

 

[colly]

Sorry Grimpeur I see what you mean now.

Yes it is when the chain is on the largest sprocket at the rear and the large ring that it becomes evident.

I know you strictly speaking you should avoid that but up to press it hasn't even been a problem.

 

[grimpeur]

Yes it is when the chain is on the largest sprocket at the rear and the large ring that it becomes evident.

Well,there's you answer. This combination should be avoided at all times as the chain suffers too much deflection. If your transmission runs smoothly in all other gears then I would just ride and forget it.

If it continues to be a problem you could always fit a bottom bracket with a shorter axle.108 mm should be about right!

 

[Cranks]

The bottom bracket axle length is dictated by the chainset.

If you have replaced the chainset for a different model - be it generation or model number/name - then you need to find out what the manufacturer advises on bottom bracket axle length.

 

[grimpeur]

then you need to find out what the manufacturer advises on bottom bracket axle length.

He already has. See his initial post:

The existing spindle is 111mm which according to the stuff that came with the chainset is the correct length

 

[briank]

Well,there's you answer. This combination should be avoided at all times as the chain suffers too much deflection. If your transmission runs smoothly in all other gears then I would just ride and forget it.

If it continues to be a problem you could always fit a bottom bracket with a shorter axle.108 mm should be about right!

+1 to the first bit.

Out of academic interest
(a) where do you get your figues of 43.5mm from?
( have you replaced the chain? a new one would be less tolerant of a "bad" line.