Fleck wrote:to mitigate chainline friction loss.
Always surprised to find when I go riding with triathletes, how many DON'T seem to understand this and ride on completely crossed up chain-lines!!
I kindly try and help out explaining that forget gears, power and all the stuff and think about running as straight a chain-line as you can in every situation when riding!
Do you (or someone else) mind explaining this to me now? On my tri bike, I have a 50/34 and 11/28. I ride in the big ring about 98% of the time. Is the idea that I should adjust my cadence to increase speed/power instead of shifting rear gears in order to keep the chain straight? Also, I guess I would assume keeping the chain straight would mean keeping the rear gear towards the middle? Or am I totally misunderstanding this?
The idea is that you should select your gearing so that you can spend as much time as possible with a straight chainline, ie closer to the middle of the cassette, at your preferred cadence and power output.
This is why world class cyclists run massive front rings in time trials. Not because they need the 58-11 gearing, but so they can push 58-15 on the flat (assuming the course is flat).
But yes in general, stick to the middle of the cassette if you can. If you're nearing the big-big combo, you'd be more efficient in a small-mid combo.
We seem to spend a lot of time worried about cross chaining and its corresponding friction losses here. While the concept makes sense, I wonder if it's a holdover from back in the day. Chains were wider, stiffer (laterally), and there was more space between cogs when we only had 5 cogs in the back on a 126mm hub vs. having 11 on a 130mm (rim brake) hub. I would love to see a test that shows what the wattage loss is for cross changing.
There was a test a few years ago (posted on slowtwitch, iirc) showing that for an equivalent gear-inch and cadence, it was more efficient to be on the big ring. I believe the theory was that it reduced friction in the chain because the links didn't bend as acutely, similar to running larger pulleys. So, small ring and smaller cog was not more efficient.
I'd be at least mildly surprised to find that world-class time trialists spend much time in a 58x15 on a flat course. A look at a gear, cadence and speed chart shows when using a 58 big ring with a 23 tire, that for 34mph with a 15 cog, a cadence of 112 is necessary. For a 14 cog, the cadence is 105. For a 13, its 97-98 . For a 12, it's 90. For an 11, it's 82-83. Changing to a 25mm tire changes the required cadence to 111-112, 104, 97, 88-89, and 82. I don't believe we see cadence over 100 very often anymore, with the exception of descents. In his heyday, he-who-shall-not-be-named was in the 102-105 range. Tony Martin looks more like low 90s to me. Using 34mph and a cadence of 95, the rider is spending a lot of time in the 13. Not a big/small, but not likely to be a big/15, either.