Fitting a 39t inner on a compact crank
 

Hello,

I have been riding my 50/39/30 Tiagra triple on some pretty serious hills recently, and I am loving what I can do in the 39. My primary bike is a 50/34 x 11sp, and I only use the 34 on the worst of the worst climbs, and I never use the 34x28. I want to put a 38 or 39 tooth chainring on the inside of that 50/34 compact, but I am having a ton of trouble finding a 110 bcd chainring with 38 or 39 teeth. The crank is a Shimano 105 5700 5 arm with 110 bcd. Could I put a SRAM 110 38t on that crank without shifting issues? I'd rather upgrade to a 50/39 than buy a 53/39 because I ride a lot more hills than flats, and can get a new inside chainring for a lot cheaper than I can get a whole new crank!

Thanks for any/all advice.

With the exception of Campy's 110 patterned rings pretty much all 110 rings interchange WRT the fitting on the crank arms. Various brands have offered 38T rings in a 110. Sugino comes to mind first (QBP's part number is CR1157 and is in stock, likely under $30). The SRAM one quite likely will also fit. Shifting issues on the front are very user technique dependent BTW. Andy

Before spending any money do yourself a favor.

Take the time to calculate and chart every single gear inch ratio. Decide how much overlap you need between chain rings. I suspect what you'll find out is the only thing that happens is you'll eliminate the gear ratios that you're not using now. You can do that now for free by just not shifting there.

Here's a link to a nice gear comparison calculator: kstoerz.com | visual drivetrain comparison tool

If you just want tighter spacing between the rings, go for it. You've got nothing to lose but the cost of the chainring. Nothing else will have to change. Look on ebay, you should be able to find one.

There is something to be said for how much nicer a 50/38or39 can shift than a 50/34... but by that point, I consider standard cranks so that the rings are supported as close to the edge as possible, though. Don't listen to me, I'm not good for budgets. ;)

Check your front derailleur's inner cage plate for clearance over the 38 or 39 tooth chain ring. I made this switch on a Campy Record equipped set-up and found a lot of interference between the inner plate of the derailleur and the 38-tooth chain ring. (Couldn't shift the chain to the 50-tooth chain ring. Solved the problem with a Shimano 105 derailleur)

Specialites TA makes 110mm BCD rings in any tooth count you'd want.

Note however you'll spend much less importing from Europe than buying in the US.