Technical FAQ: Drivetrain friction test, steerer-damping headsets
Great article on drivetrain friction. I never considered the difference in torque on chain tension. But I do want to point out one thing. While you correctly pointed out that smaller cog and chainring combinations have greater chain articulation, it is also true that the number of chain links that experience this is smaller. For an example: A 53-tooth chainring has about 10% more teeth than a 48-tooth chainring. During one full cycle of the crank, 53 links of the chain are articulated at the top of the chainring. (53 x 8 for the whole drive train.) Now for that same cycle, 48 links are articulated for a 48-tooth chainring. So about 10% less links are articulated by about 10% more in chain articulation. That should turn out to be a wash.
I have pulled apart quite a few SRAM’s pulleys, and their upper pulleys generally have a lot of friction in them. Unfortunately, they generally can not be replaced, as they are custom and not a standard design. Also, the 1X drivetrains use significantly more cage tension. I suspect that these are the real culprits for the average difference.
Regarding narrow wide chainrings and pulleys, you got it right – it is a wash. Also, SRAM’s “two surface” versions are not the only shapes. I made the original ones years ago, and they had smoother shapes than SRAM’s. BTW, I hope nobody is paying SRAM royalties, as I presented the idea to SRAM before they filed IP and came out with them. (About the time Shimano switched to 9-speed and 11-tooth pulleys.) You can look up my application at the USPTO. SRAM knew this prior art and did not disclose it, as my application is not cited as prior art. The idea was actually done years before on farm equipment, as I found out.
— Will Temple
I discussed your points with Jason Smith, who ran the test on his equipment at Ceramic Speed USA.
Regarding your contention about chain articulation, you are forgetting about the increased chain tension with a smaller cog. The chain tension with a 48T chainring will be higher than with a 53T ring. Assuming the same rider torque and cadence, because the radius of the 53T is greater, the chain tension will be less. (Chain tension decreasing with increasing chainring size is explained in the article, if that is not clear.)
Splitting up the frictional contributions when going from a 48T to a 53T chainring:
1. There is an increase in friction due to the increased number of articulations.
2. There is a decrease in friction due to the smaller angle of articulation for each link.
3. There is a decrease in friction due to the decrease in chain tension.
So, instead of it being a wash as you contend, it’s one increase, and two decreases. Therefore, the net is a decrease in friction. This is what we see experimentally, not just on paper.
As for your contention about pulley-wheel friction, granted, different pulleys and rear derailleurs were used, so this indeed introduces two variables between tests. However, Jason and I are inclined to believe that the SRAM pulleys and/or SRAM rear derailleur are not the reason the 1x produces greater friction than the 2x. We say this for two reasons:
1. Firstly, the data in this test agrees with data from previous cross-chaining and ring-size testing. The trends in the data demonstrating the effects of smaller rings and cogs on frictional losses, given the same final drive ratio, are repeatable, regardless of the brand of derailleur.
2. Secondly, when the pulley wheels and rear derailleur contribute friction to the drivetrain, it is contributed in a relatively linear fashion through all of the gears. For example, say a pulley wheel and rear derailleur contribute 2 watts of friction to a drivetrain at a final gear ratio of 2. If the gear ratio is doubled (to 4), the pulley wheel and rear derailleur would still add close to 2 watts of power loss. However, in our 1x vs. 2x friction test, the change in friction between the 1x and 2x increases in a non-linear fashion as the gear ratio increases (a power loss of under 2 watts at a gear ratio of 2 jumps up to 6 watts at a gear ratio of 5). This (significant) non-linear increase is due to the chainring size and cross-chaining effects.
The frictional difference does not appear to be due to the brand of the pulley wheels and rear derailleur.
Followup question on steering damping:
Is there anything like the Cane Creek Viscoset that can work with a frame that needs an integrated headset, or some adapter that will let me use a Viscoset? The full S.H.I.S of my frame is IS 41/28.6 IS 52/40 in case it matters; I’ve got plenty of spacers under the stem so I can afford to lose 25mm.
The only idea I’ve had so far is to break the seals on my headset bearings and re-pack them with heavy grease: do you think that would be a safe thing to do, and is it likely to work?
Cane Creek’s marketing director Luke Bukoski said this about your question: “Unfortunately we don’t have an IS version of the Viscoset. As for the second part of the question, I asked Jim Morrison, our Principal Design Engineer and all-around headset guru and here is what he gave me:
‘Repacking a normal headset bearing with heavy grease will add damping, but really not that much. It’s a low-risk proposition, though, so it would be worth trying. I’d try to source some Nye Lubricants Fluorocarbon Gel 868H or VH (heavy or very heavy), carefully pop the seals, clean out as much grease as possible with a solvent, then pack as much damping grease in there as possible. The bearing will feel damp-y in your hand, whether that’s what you’re looking for at the handlebar is a personal preference kind of thing.’
Also, I know that Tim Hopey does make an IS-headset adaptor for his steering damper, but I don’t have any direct experience with it and can’t speak to its performance.”
Lennard Zinn, our longtime technical writer, joined VeloNews in 1987. He is also a custom frame builder and purveyor of non-custom bikes for big and tall riders. A former U.S. national team rider, co-author of The Haywire Heart, and author of numerous cycling books including Zinn and the Art of Mountain Bike Maintenance available also on DVD as well as Zinn and the Art of Road Bike Maintenance, Zinn and the Art of Triathlon Bikes and Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists. Zinn holds a bachelor’s degree in physics from Colorado College. Readers can send brief technical questions to firstname.lastname@example.org.