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Insight into chainless WC DH runs
We all recall some famous chainless runs, like Gwin's 2015 Leogang winning run and Mulally's 2014 Hafjell World's run. Many theories have been put forth, often focusing on pedal kickback and derailleur clutch force.
Kickback has been debunked, as we now know kickback is nearly impossible while coasting and, if it does occur, it's only a tiny fraction of the total possible kickback.
Derailluer clutch force has also been debunked, as the force is so much smaller than bump impact force.
So ... what is it?
I've always assumed it was due to careful riding and the smoothness that comes from knowing errors cannot be corrected with a few hard pedal strokes. Another factor may have been introduced by Matt Miller in the latest Performance Advantage podcast (time: 51:00). He mentions a test in which riders were instructed to "race" or "coast" and both strategies led to similar speeds. Combined with insights from Lewis Kirkwood, it seems possible the reduced physiological demands from not pedaling and/or (presumably) riding more smoothly could leave the rider with more energy available to manage vibrations and hold onto the bar more securely.
If the vibration issue was a factor, this highlights the performance potential of a system that transmits less vibration to the rider.
Kickback has been debunked, as we now know kickback is nearly impossible while coasting and, if it does occur, it's only a tiny fraction of the total possible kickback.
Derailluer clutch force has also been debunked, as the force is so much smaller than bump impact force.
So ... what is it?
I've always assumed it was due to careful riding and the smoothness that comes from knowing errors cannot be corrected with a few hard pedal strokes. Another factor may have been introduced by Matt Miller in the latest Performance Advantage podcast (time: 51:00). He mentions a test in which riders were instructed to "race" or "coast" and both strategies led to similar speeds. Combined with insights from Lewis Kirkwood, it seems possible the reduced physiological demands from not pedaling and/or (presumably) riding more smoothly could leave the rider with more energy available to manage vibrations and hold onto the bar more securely.
If the vibration issue was a factor, this highlights the performance potential of a system that transmits less vibration to the rider.
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Member since Dec 15, 2009
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I invite you to do the math on it for yourself - I have. You'll see it takes a spectacular impact for there to be a realistic chance of any kickback while coasting, let alone problematic kickback.
The rear wheel is either locked or turning at full speed - there's very little in between. It's true the rear wheel is sometimes locked, but it's not a large fraction of the time. I even spent a day testing kickback without coasting last year by intentionally locking my rear wheel on rocky terrain, on a bike with above-average kickback. Rarely even noticed it.
A few years ago I tested a wheelset that came with 2 different freehub star ratchets. One was the standard, and the other was higher engagement - with half the engagement rotation of the standard one. I was testing on an older Norco Range, which had a fairly rearward arcing axle path (i.e.: higher likelihood of pedal kickback). I absolutely noticed the difference in suspension performance between the two star ratchets. I stuck with the lower engagement ratchet. I'm not riding a trials bike - I don't need high engagement.
I would LOVE to see some telemetry of a WC DH racer's rear wheel rotation in the course of a DH run. It would have to be really sensitive to pick up the effects we're talking about, but it wouldn't be impossible.
I am equally incredulous at your assertion the wheel spends significant time at speeds between locked and turning at the speed of the bike. Lift the bike off the ground and spin the rear wheel up to speed. Drop the bike. The wheel stops nearly instantly, even without a 150+ lb rider forcing it against the ground. The inertia of the rear rim and tire are small compared to that of the rest of the system.
Have a look at this footage: www.youtube.com/watch?v=3_a-BA8PqRU There are moments where the wheels show stick-slip behaviour, but even when sticking and slipping, the wheels start and stop in a fraction of a rotation. It really is almost a binary state system. Clearly the wheel is occasionally locked, but even in these especially technical sections of trail, where wheel locking is more likely than average, the time spent locked looks to be in the single-digit percentage range.
It is possible you noticed differences in kickback between your star ratchets, but it's also possible you perceived what you expected to perceive. A blind test would be ideal.
In particular, a high virtual pivot location leads to kickback and to a more rearward axle path. The same parameter causes both something believed to be detrimental to descending performance and something believed to be beneficial. My opinion is that the axle path is more beneficial than the kickback is detrimental, so bring on the kickback! (Or the high pivots and idlers, but that's a whole other blog post.)
"Yup. At this point without sensitive telemetry, we're both just voicing our opinions."
Well, not entirely. I did create a spreadsheet to calculate kickback from various virtual pivot location paths, sprocket combinations, bump shapes, riding speeds, and hub properties. That's more than just an opinion.
I've often wondered how effective it would be to mount a bike computer to each wheel and compare the difference in "distance" recorded by each. This could be a crude, but very cheap, way to estimate how much time the rear wheel spends locked. I would actually be more interested to do this on a climb to test the slip rate of the rear wheel, but again, that's a whole other blog post.
www.youtube.com/watch?v=SN06xwkku-g
Take a close look at some of the recent Aaron Gwin clips by Clay Porter. The slow-mo shots give you a better idea what the rear wheel is doing in these scenarios. Then consider that this is one of the fastest guys on the planet. I promise you that you and I and everyone else is experiencing this effect even more, because we are worse at modulating our brakes.
For this reason, I purposely run a low engagement freehub body on my bike. It means there is a lower likelihood of pedal kickback affecting my suspension when I’m close to lock-up.