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I'm planning to buy a TCX Advanced SX, this one: https://www.giant-bicycles.com/us/tcx-advanced-sx-2017
All is cool, I simply would like a 10/42 instead of the stock SRAM 11/42. What is the less expensive way to get about it? Can I simply swap the 11t for a 10t?
Thanks!!
Changing the front chain ring to a 40t is easy and requires only to swap out one SRAM part for another.
A 10t sprocket is too small for a regular freehub. A special hub body that is compatible with SRAM XD cassettes is needed.
There are many hubs that allow you to swap the driver body. For example, the DT 350 hubs on my bike can be converted to XD standard with a part that costs about €40.
That may or may not be the case for the hubs in Giant's PX wheels. AFAIK these are, or were, available for mountain bikes with XD hubs as well. I couldn't find anything in a brief search on the web however. Perhaps I simply confused something.
If you plan to replace the stock PX wheelset, which isn't all that great anyway, it will be an easy conversion to a XD driver. Then you could mount any SRAM XG 11 speed cassette. A 10-42 XG-1150, for instance.
As Criggie remarked, you may consider buying a bike that fits your specs better. A similar Canyon Inflite has a 40t chainring, and DT380 hubs. For these a switch to a XD driver is easy. Otherwise the specs and geometry are very similar.
If you get a 44t chain ring at the front, you will have the same transmition ratio to a 11t sprocket as with a 40t to a 10t. (A Focus Mares, for example, comes with 44t and has nearly the same frame geometry.) You might then simply choose an humongous last cog. For example, up to a 11-42 cassette instead of the 11-32 that comes with the TCX (which is geared for cyclo-cross where you carry and run up slopes when it is too steep for 32/42).
A 10t cog is by principle inefficient (chord losses). In a nutshell the chain doesn't wrap properly around a circle when the radius is small compared to a link's length. Thus 40/10 is less desirable than 44/11.
