Chain jumps off large chainring under high torque

[cameronpack]

You can get a 127mm BB (Shimano UN54, pretty cheap and reliable)

http://www.chainreac...x?ModelID=17473

This is a silly question but, will this BB move the position of the chainrings / cranks 14mm further out (than my 113mm BB) then? I know it sounds silly but when I changed the 110mm to a 113mm BB it didn't seem to do an awful lot. Was that because it was only 3mm involved?

Regards
Cameron

 

[MacB]

we need Recordacefromnew for the BB/chainset info...by the way I had the cassette bit arse about face, I meant it wasn't on far enough not on too far, maybe there is a spacer on that shouldn't be.

 

[MartinC]

This is a silly question but, will this BB move the position of the chainrings / cranks 14mm further out (than my 113mm BB) then? I know it sounds silly but when I changed the 110mm to a 113mm BB it didn't seem to do an awful lot. Was that because it was only 3mm involved?

Regards
Cameron

Most BB axles are symmetrical so changing a 110mm to a 113mm will shift the chainline 1.5mm outwards.

If the chain's coming off the the outside of the big ring it's almost certainly a misaligned front mech (too high, stop screw to far out or cage not straight).

 

[cameronpack]

Right, managed onto the home computer while the Mrs feeds the dogs.

Left crank to centre seat post = 62mm
Right crank to centre seat post = 61mm (so cranks ok)
Inner chainring to centre seat post = 41mm

Chainline inner chainring to about 7/8 on the rear cassette (1 being small sprocket, 9 being large sprocket)
Chainline outer chainring to about 5 on the rear cassette

I checked the front derailleur and it was 3mm clear of the outer chainring and parallel.

I tried again in large chainring to small sprocket and the chain rubbed on the derailleur. If I adjusted the derailleur any further out it would rub and hit the right crank arm.

I looked at Sheldon Brown's pages for info and found one entry for an advised size for a Campagnolo double crankset BB to be 116mm. I am not sure that extra 1.5mm would be enough to straighten the chain up and stop it rubbing on the front derailleur. I think I will purchase a larger one as suggested earlier. It would also help centralise the front chain rings to the rear cassette. I guess I really want the inner (parallel) chainline to be on about 6 on the rear cassette and the outer (parallel) chainline to be on about 4 on the rear cassete?

Off to measure the distance I need to move the chainset out by 1.5 to 2 rear sprockets.

Do you guys reckon this is the case and I am heading down the right road?

Thanks

Cameron

 

[cameronpack]

Sheldon Brown says Campag 9 speed cassette sprockets spacing is 4.55mm. So not going to be able to push the chainrings over that much. However, should be able to move them 5-7mm with a larger BB.
Groovy??
Cameron

 

[MacB]

I don't have any experience of Campag but do know that 41/46mm are the standard spacing for a Shimano road double which gives a 43.5mm chainline. If the big ring is in line with the middle sprocket then you should be able to hit either end of the cassette, maybe not ideal, without the chain coming off.

Exactly what parts are you running together here, do you have the details or can you take some pics?

 

[cameronpack]

Xenon 9 speed front and rear derailleurs, mirage 53/39 double crankset, 113mm square taper bottom bracket, campag 9 speed chain (sorry never took length), veloce 9 speed cassette 13-26 and mavic aksium rear wheel with mavic ed10 freehub.
Cheers
Cameron

 

[MacB]

Ok, so it's all Campy, which I have no experience of so you'll need to research any detailed FD setup instructions yourself. However certain things should hold true, front shifting and chainrings are, after all, about as simple a system as you get.

Firstly if the FD is allowing the chain to unship on the outside then I'd look to outer limit screws or bottom bracket length. Given what we've already discussed then BB length is looking favourite but there's also FD alignment. I know with the Shimano ones I've done it can be finicky and visually deceptive especially as, no matter how you align it the act of tightening the clamp bolt can move it.

I think I'd still want to go with a longer BB due to where your big ring is aligning on the cassette, but I'd also have a good look at the FD and the limit stops. You could start by releasing the cable and adjusting the limits to give the maximum movement, the limit screws do what they say on the tin, just placing upper and lower limits on that range of movement. Then note where the FD sits with no tension and then manually move it out to the limit and note how far that is. That'll give you an understanding of the full range of movement available. If you're happy that your new BB is giving a better chainline then set the lower limit screw while the chain is in small at the front and big at the back. Get it close but not so close that it will rub under load. Then put the cable into the clamp, I find pulling it finger tight is enough. You should then be able to shift to the large ring and set the outer limit to whatever the recommendation is beyond the chainring.

Of course this could all be bollocks as I haven't touched a Campy FD, but I'd expect to be able to get at least 8 of the 9 cassette cogs from the big ring and at least 7 from the small. but I wouldn't be surprised if all 9 were avialable from each ring without rubbing or losing the chain.

 

[cameronpack]

Thanks MacB,
I'm certain it isn't the front derailleur limit as when I was setting up before running the bike the chain was being thrown of mid shift. I corrected this at the time by adjusting the limit screw.
Ordered the bottom bracket last night, so should hopefully have it for the weekend when I can trial it for 20 or 30 miles.
I think you're right, I don't think they'll be any difference between the two brands. The principles are the same, just different shift ratios.
When I install the new bottom bracket I will set the lower limits (as you say) without tension in the gear cable (I just remove it from the clamp to be sure). Then finger tighten it and lock the cable in position. Something I hadn't done but will do is ensure the front derailleur cage specifically accesses the most rear sprockets from the largest down without rubbing or having a bad chainline. After all, these will be the ones I will be using from this front chainring. Then I'll shift up onto the large chainring checking the limit screw to ensure the chain isn't thrown off in the shift. Then I'll double check the derailleur allignment to ensure I can access the most rear sprockets from the smallest up without rubbing or having a bad chainline.
Thanks again for all your help guys. Hopefully this will fix it...
Kind regards
Cam
p.s. I will let you know how it goes

 

[MartinC]

Cameron, the information about your chainline's interesting. Your big ring is well towards the "wrong" end of the cassette so you'll have had to adjust the front mech to not have chain rub with this bad chainline - so the front mech cage outer plate will be well outside the chainring allowing the chain to escape off the ring. The bad chainline will make the chain want to do this.

A Campag double chainset should run on a 111mm ISO BB giving a 43.5mm chainline (IIRC). Campag don't make a 113mm BB. My guess is that you're running a JIS (different standard to ISO for the axle tapers) BB and the cranks are moving to far in on the tapers giving you this poor chainline.

I'd suggest buying a Campag 111mm BB and then adjusting the front mech to match the new chainline.

 

[MacB]

Cameron, the information about your chainline's interesting. Your big ring is well towards the "wrong" end of the cassette so you'll have had to adjust the front mech to not have chain rub with this bad chainline - so the front mech cage outer plate will be well outside the chainring allowing the chain to escape off the ring. The bad chainline will make the chain want to do this.

A Campag double chainset should run on a 111mm ISO BB giving a 43.5mm chainline (IIRC). Campag don't make a 113mm BB. My guess is that you're running a JIS (different standard to ISO for the axle tapers) BB and the cranks are moving to far in on the tapers giving you this poor chainline.

I'd suggest buying a Campag 111mm BB and then adjusting the front mech to match the new chainline.

This is the bit I'm not clear on as, if Cam's measured correctly and it's fairly easy to do, his inner chainring is 41mm, that would indicate an outer at 46mm and the 43.5mm chainline you refer to. If this is the case then, even if it isn't the right BB he's using, he's getting the expected chainline from it.

 

[MartinC]

This is the bit I'm not clear on as, if Cam's measured correctly and it's fairly easy to do, his inner chainring is 41mm, that would indicate an outer at 46mm and the 43.5mm chainline you refer to. If this is the case then, even if it isn't the right BB he's using, he's getting the expected chainline from it.

Yes, I'm not clear either. If his chainline is about right then the big ring won't align with the middle sprocket - but he says it does. Something is inconsistent here. The mavic hub with an ED freewheel should put the cassette where it should be

A perfect chainline will put the big ring between his 3rd and 4th sprockets. Even with his bad chainline I find it hard to see the chain being dragged down outside the big ring if the front mech's adjusted properly. But the information is that the outer plate is 3mm (half a 9 speed chain) outside the ring which is right.

Cameron - when the chain comes off how do you get it back on? The reason I ask is that pedalling might generate enough force to force the chain through the gap between outer plate an ring but it'll be really hard to get it back through by hand if the mech's adjusted properly.

Check that the outer plate is only 1-2mm above the big ring when they're level - if the gap here is too big then it leaves more room for the chain to escape.

I think there are (at least) 2 problems here. The chainline's bad and should be fixed. There's also something wrong with the front mech too - adjustment or is it a compact not a standard front mech (somewhere inside the cage it'll be stamped CT, STD or STD/CT - if it's CT it's wrong for the chainset)?

 

[cameronpack]

Because I was getting some rub from my bad chainline I over adjusted the front mech with trim (away from the frame). I know this because my right crank sometimes rubbed past it ever so subtly. I think I also had the front mech slightly angled towards the small rear sprocket. I did alter these last night and was getting some rub again on the front mech due to the bad chainline. Prior to altering it, there was enough space for the chain to wedge itself then force itself between the cage and chainring. As mentioned, I have now altered this.
Martin, you say the standard campag bb is 111mm but I found info on Sheldon Brown's page that said (for one particular model of campag double) the ideal bb was 116mm.
I am using the shimano un54 bb's. Would a campag 111mm one not let the crank arm slide on as far as the shimano one then? Thus keeping it the magic offset from the seat tube?
Not sure about the front mech, will check, but I know it's a xenon which I don't think catered for compact.....
Thanks for all your expert help guys. I appreciate it.
Cameron

 

[cameronpack]

Yes, I'm not clear either. If his chainline is about right then the big ring won't align with the middle sprocket - but he says it does. Something is inconsistent here. The mavic hub with an ED freewheel should put the cassette where it should be

Hey Martin, I think the chainline lines up 1 off the middle sprocket, not with it.

Cameron

 

[MartinC]

Hey Martin, I think the chainline lines up 1 off the middle sprocket, not with it.

Cameron

That sounds better, I think I had a triple in mind when I said the big ring should align between 3rd and 4th sprockets. With a double it should be between 4th and 5th.

Campag make a 115.5mm BB for use with triples where the seat post is wider than 28.6mm. This isn't to get the chainline right but with a fat seat tube the triple sometimes can't move inwards far enough without fouling the seat tube. So in effect the 115 BB moves the chainline outwards to allow the mech to room to work. In practice I've never found it necessary to use the wide BB on triple on a 'fat' seat tube frame - the 111mm has always worked for me.

ISO (Campag, Miche, TA, some Stronglight) and JIS (Shimano et al) tapers are similar but not the same. They're both 2 degree square tapers but one (and I can never remember which) starts off at the end .2mm smaller than the other. This means that if you mix and match the cranks will either slide to far or not far enough along the taper. This will mean that the chainline will be not as designed and if you're unlucky either the crank bolt will foul on the taper end before it's tight enough or there won't be sufficient contact between the crank and the taper.

If you're happy that the cranks fit properly and the chainline is where you want it then mixing ISO and JIS is OK. A Xenon 111mm BB is only about a tenner and they're pretty reliable so I's just fit the right one and be done with it.