Bikefan73

Hi,

I am looking for advice/solutions to a problem I have at present. I have a 2009 Specialized Tarmac SL3 which uses an external threaded bottom bracket. I am running a Shimano Ultegra 6700 chainset with a Shimano SM-BB6700 bottom bracket. I have an issue with the chain crossover when in a 53x24 gear. I used a digital calipers and followed the correct way to measure front and rear chain line (Sheldon Brown website), my rear chain line is fine at 42.5mm but my front is 49.3mm. Shimano recommend a 43.5mm front chain line with the Ultergra 6700 chainset.
Does anyone have a suggestion on how I can reduce this chain line to closer to the 43.5mm target. I need it to move closer to the frame which is impossible at present as the external bearing cup is preventing this. It is almost like I would need a BB with thinner cups to reduce this distance. It's a bit frustrating as it is a super bike to ride.

Appreciate any feedback.

FMB42

Take a look at this link for information on determining what you might need to correct your chain line problem: https://wheelsmfg.com/products/bottom...-adapters.html

DaveSSS 

I'd double check your measurement. The chainring spacing of 5mm that Sheldon mentions is not correct. The tooth to tooth centerlines are in the 7.5-8mm range. I have a drawing from Campagnolo that shows the spacing on an 11 speed crank - it should be a narrow as any and it's 7.7mm.

Use a precision machinist's rule and measure from the side of the seat tube to the tip of a tooth on the little ring. From this measurement, add half the ST diameter, to get the ST centerline, then add 4mm to get the midpoint between the two chainrings. That should be close to 43.5mm.

For example, a bike with a 32mm ST would measure 23.5mm from the side of the ST to the tip of a tooth. Add 16mm and 4mm to get 43.5.

Bikefan73

Thanks for your feedback. I looked at this again and it is difficult to measure. I had measured from the side of the ST to the inner side of the big ring, then added half the width of the ST and minused half the distance between the 2 chainrings. The trouble is the ST is hugely tapered directly across from the small ring, so I guess it is possible that it measures the 43.5mm or at least less than my original calculation, however the chain crossover problem still exists. I will remeasure per your recommendation.

Thanks again.

gyozadude

What specifically is the "chain cross-over" problem you're experiencing? Typically it's not recommended to cross chain. However, the noise and grinding might be aggravated by RD alignment. It might be rotated inward through the vertical axis and create really lousy alignment on the Big-Big cross chain, but be okay for small-small cross chain. check B-screw setting and chain like length. You might to too short by half a link or something for your chainstay length or be using too large a cog on the cassette.

But 49mm is a big diff from 43mm in relative terms. Either way of measurement should get you roughly to the same number. A quick check is to see if your frame's chain stays will still give you decent and even clearance on either side of the bike. With older JIS square taper spindle BBs, I've caught quite a few bikes that came in with spindles reversed and it was easy to spot with the asymmetric spindles on those bikes. With newer embedded spindle/cranksets, and symmetric spindles, there's less problem. It's rare but you might see if it came out of the factory with some asymmetry. Hopefully not.