Monday, April 27, 2015

cross-chaining on SRAM 1×11: another example

Last time I analyzed some data from DC Rainmaker, probably riding around Paris, where I was surprised to find that he would have experienced less lateral chain deflection on a 1×11 set-up with a 44-chainring and an 11-25 cassette than he did with a compact 2×11 setup with 34-50 in the front and 11-25 in the back. This is because the 34-50 is a relatively wide chainring range, and one ends up having to chose between cross-chained options on the front: big ring with large cog os small ring with small cog, neither of which is a good choice, just to hit the mid-range of gears. Meanwhile the 1×11 is optimized for the mid-range.

But what about rides with extreme terrain, climbing and descending? Here mid-range may be hardly used, spending most of the ride either in low climbing gears are large descending gears. Fortunately the second example I found, this time on the Di2Stats sample page, was highly representative. It's epic: a 60 mile ride with 9600 feet of climbing and descending. Here's those data:

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This is an interesting data set and it's worth taking some time to look at it. Of course it's just one ride and hardly represents the full spectrum of rides from this rider. But taking it on its own it seemed this rider would have been much, much better off with a compact crank. Climbing in the 39/28, he averaged only 60 rpm, while in the huge top end gear of 53/11, he averaged only 70 rpm with a maximum cadence of only 111 rpm. That's not even close to spinning out. Riders should still be applying near peak power at 120 rpm, and if not peak certainly enough to accelerate out of corners on a descent. So a 50-11 top gear would have brought peak cadence up to 117 rpm, and even a 50-12 top gear would have brought top cadence up to only 128 rpm. That shouldn't have been any sort of serious handicap, remembering this is peak cadence, not sustained. Meanwhile on the bottom end, a 34/28 versus that 39/28 would have allowed average cadence to come up from 60 rpm to 69 rpm.

None of the other gears are really significant, as average time spent in a gear was no more than 5.8 seconds for any other.

But he had what he had. I can map his gears onto a 44 single ring with an 11-32 rear. Obviously 44/11 isn't as big as his 53/11 but with the 44/11 he'd need to spin up to 133 rpm on that descent for at least one second. That's still well within reasonable cadence range. If you want he can use a 10-32 cassette and then peak cadence would be only 121 rpm. Whichever is chosen the analysis stays the same.

I'm assuming a progression on the 11-32 cassette of 11-12-13-14-15-17-19-22-25-28-32, which is what SRAM uses. I generally like SRAM progressions. Shimano uses 11-12-13-14-16-18-20-22-25-28-32 which results in a much larger fractional 14-16 jump compared to the 20-22 jump. Of course if you spend more time in the 18-25 range this smaller jump there might be nice, but SRAM generally tries to even out the fractional jumps across the range. If you want you can swap that to a 10-32, swapping the 11 cog for a 10, which provides a big "descending gear" where fine tuning cadence 10 vs 11 probably isn't useful: you're not going to have much acceleration on either of them so being able to shift into the middle-cog range from the 10, skipping the 11, is probably an asset.

Here's the result:

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Not surprising, with the 1×11 lateral chain deflection is larger, by 1.1 cog positions. With the rider tending to be in small-big or big-small and with each chainring assumed to be 1.2 cog positions off center it had to be close to 1.2. So using those FrictionFacts data this is going to come at a bit of a cost. On the high end range it will be a bit larger since the switch is from a 53 to a 44 ring in front, but then the important thing is the cog size since that's where most of the chain bending occurs, and if the switch was made from an 11 cog to a 10 cog that will be significant. On the low side, which is obviously more important from a power efficiency point of view, the increased drivetrain loss from cross-chaining of approximately 0.4 watts will be partially compensated by less chain tension and less chain bending going from a 39/28 low gear to 44/32.

So it may be a wash. But the key point is that when you're spending most of your time at one versus the other extreme of the shifing range the 1-by system is going to increase your lateral chain deflection by on order one cog spacing.

I have to say these Di2stats tables are fascinating and they alone would be motivation to go to an electronic system like the forthcoming SRAM electronic. You can get gear ratio by taking the ratio of cadence to speed (adjusting for wheel size) but this doesn't always tell you if you're on one versus the other similar gears, one for a big ring the other for little ring. For example, if I ran a 48-36, a nice chainring combination, a 48-20 would be the same as a 36-15.

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