Chain Line on a Specialized Shiv TT
 

Hi,

I just assembled a 2012 Specialized Shiv TT. The bike comes as a "module" with the crankset, brakes, aero bars, etc. I added 2008 Campagnolo Record-10 derailleurs and 2012 Campagnolo Record-10 Bar End Controls (back-to-zero position). I am not a professional mechanic but I have been doing all of my own work for over 40 years (unless there is a task that requires a tool that I don't own, such as pressing bearings into PF30 bottom brackets). However, this problem has me stumped.

It seems like there is a chain line problem. When I have the bike shifted on the small chainring (39T), I can't shift the rear derailleur past the 7th cog (counting largest to smallest) without having the chain rub on the large chainring. Actually, in the 7th cog there is just a hint of rub on the large chainring.

I double-checked how I installed the crankset and I think I did it correctly. There is no lateral play in crank.

For reference, I am using Record-10 cassettes on a Zipp 900 wheel.

I've only experienced something like this back when I was using Record-8. I would get this kind of chain rub on the large chainring on the 7th cog (of 8) on one of my bikes. Fixed that problem by adding a thin spacer between the fixed cup and frame and that worked. Obviously, you can't do anything like that with BB30.

Any thoughts? TIA for any help you can provide. I might take the bike in to the local Specialized superstore.

this is normal

As Reptilezs says it's normal, but IMO avoidable.

But first you have to decide what's important to you. I've never been a fan of 39/53 chainrings for most non-competitive riders. The problem is that it makes many of mid-range gears crossed combinations. So before dealing with the chainline ask yourself some questions.

1- what rear sprockets do you most use the 53t with?
2- do you routinely use the 53t with the smaller and smallest rear sprockets?
3- do you need to use the 39t with outer sprockets to get a gear that you use normally?

If you use the 53t with middle sprockets, and rarely or never use it with the outermost sprockets, you might be better off with a smaller outer chainring, or with what they now market as compact drive.

OTOH, if you're using the higher gears and want to keep them, you have another decision. You can leave it alone and live with the limitation, and find that mid gear with the 53t and an inner rear. Or you can move the crankset out a bit (assuming there's a spacer on the left to switch to the right) and buy yourself the ability to clear one more sprocket with the 39t. This doesn't always work, because the FD may not have enough travel to reach, so check this before you do anything.

So while the 39t to outer cassette limitation is normal, it's up to you whether to live with it, or make a change.

The difference in radius between a 50 and 53 ring is ~6mm. At the the ~45 degree point on the big ring where the chain crosses going from the small ring to the cassette, it comes to little more than 4 mm along the chain. IMO, that's not going to significantly affect the rubbing all else being equal.

I didn't suggest the smaller ring to improve the clearance issue. I suggested it to bring outer chainring gear range down and eliminate the need to use the 39t with outer sprockets.

If your FD has enough throw you might be able to shim the outer ring outwards somewhat to gain some more clearance. Search on "chainring spacers" or check your LBS; 1.2mm is a common size and might do the trick for you, or at least yield a few more usable cogs.

This solves the clearance issue and might allow the use of one more rear sprocket off the inner. But there's a possible downside, in that you might create enough room for the chain to get snagged between the inner and outer chainrings when you downshift. Chainring separation is very carefully calculated to be the widest possible while allowing reliable shifting. If the manufacturer thought there was room to go wider, you can rest assured that they would have.

Hi All,

Thanks for the advice.When you say this is normal, do you mean it's normal with S-Works Carbon Cranks? I don't have this problem with my Record-10 cranks. It's not a matter of specific gear ratios for me. When I race time trials I am primarily on the large ring but when I train I am on the small ring typically until the 39 X 12 for tempo training. If anything, if the course has a significant downhill, I can use something larger than a 53 X 11.

If this is normal for the S-Works crank I'll figure out a way to deal with it or go to a different crank.

The problem has nothing to do with particular bikes. It's one of simple geometry. When the chain comes from the outside of the cassette it's starting to the right of the outer ring and passing by it as it comes to the inner ring. Moving the outer ring closer, reduces the possible angle and so can create the problem. Things like shorter chainstays worsen the problem, but the difference in stay length these days is petty small. Moving the chainrings out more improves the situation by altering the angle favorably.

In your shoes, I'd start by mrasuring the actual chainline. Then seeing if you move the crank out by swapping a spindle spacer. But before doing anything make sure you have enough FD travel to the outside. You might also spread the rings as suggested in post 6, but be careful how far you spread them.

Another option if the outer ring has an inside shoulder is to file the shoulder off at a long shallow angle to create more clearance near the inside rim, but be careful because this can affect thing like the pickup pins.

BTW- one thing that might help is going to a larger inner ring. This improves the clearance angle by raising the chain, and has the benefit of not needing to use the outside of the cassette with the smaller ring. This isn't an option if you need the low gears the 39t provides when climbing.

TT bikes have shorter chainstay lengths so it makes the rubbing issues more prominent. most modern cranks don't really have any chainline adjustment. it can be moved(not a lot) sometimes but one must be familiar how the bearings are preloaded or how the slack gets taken up

OK, so it sounds like this is something with which I have to live. I hadn't thought about chain stay length and when I checked the Specialized web site it showed 390 mm. This compares to my road bikes which have 407 mm and 405 mm chain stays. My prior TT bike had 400 mm chainstays and did not have a hint of chain rub on the large chain rings (Campagnolo Ultra Torque Record-10 crank) with either 53/39 or a 55/44 chain rings combinations. Live and learn. Thanks to all who provided recommendations.