Originally Posted by
Trakhak
Big rings on TT and triathlon bikes: that's a marginal-gains thing. It's not about muscling big gears.
The idea is that with a bigger big ring, you mostly use the sprockets toward the center of the cassette, rather than the smallest ones, on flat and rolling terrain.
The most important advantage is that the jumps between adjacent sprockets are smaller (in percentage terms) past the smallest sprockets, which facilitates staying close to your ideal cadence and power output
Also, the chain line is straighter, reducing the friction that results from transmitting power through a chain that's twisted sideways.
Finally, friction losses are greater with the chain on smaller sprockets than on larger ones.
REALLY good explanations.
Originally Posted by
VegasJen
I have a 52 on my Shiv, which has 165mm cranks, but I have a 54 on my P2 that has 170mm cranks. I wish I could say I landed on that after hours of testing but the honest truth is that's just the way those bikes came. I have not, as of yet, played with changing chainrings. I find it much easier and economical to test cassettes.
Interesting. So, I always look at gearing via gear-inches, because I know my needs there, and I'm always on the same crank length. However, IIRC(?), gain-ratio takes crank arm length into account, I think this was a favorite of Sheldon Brown and it's an option on their gear-calc page. Since you run different arm lengths, do you ever use gain-ratio? Or just pure on-bike testing?