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Technical Q&A with Lennard Zinn: Rotate this

Published: Apr. 11, 2006

Dear Readers,
Well, I just got back from Sea Otter, and I’m sure you’ve read my reviewof the SRAM road group. There were lots of other cool things there, includingpieces of the new XTR and lots of new mountain bike stuff from SRAM, Avid,Hayes, RockShox, Fox, Magura, Cat Eye, and many others. But one thing remainedthe same – my mailbox filling up with letters on the subject of wheelrotational weight and what difference it makes when climbing.The first time around, I published a letter saying it made almost nodifference. The next time we revisited, I published letters saying it wasa big deal. Once again, there is a broad mix pouring into my e-mail box.I am going to run a few letters and call it good; I don’t want this togo on ad infinitum, and I don’t think anybody other than a diehard or twowould read all of these letters if I posted one gigantic slew of them (andthat would never happen – editor). So I selected a few, mostly of the “itmakes little difference” ilk. As I mentioned last column and in my reporton it in the print VeloNews, I went to Mavic Tech Days in March – a two-dayseminar all about testing methods. One thing I think you’ll all find interestingis that Mavic performs lots of timed tests, trying to distinguish performancedifferences between wheels, using some of its 650 test riders. Mavic testengineers said that when testing wheels of different weights on a straightperformance test – numerous riders going full-out uphill, working to maintainconstant power output through all tests, that it “never” comes out as itshould, with the lightest wheel being fastest for all riders, all of thetime. So there you have it – test results many of you have been askingfor. That’s a far cry from towing a Hummer up a hill.
LennardDear Lennard,
I'm so sick of hearing convoluted arguments and ridiculous comparisonsconcerning the effect rotational inertia of the wheel of a bike. The simplest way to explain the added momentum of mass at the rim (rims,tubes, nipples, tires, whatever) can be proven easily a number of wayswith simple physics.  Skipping over the details of the derivation,the result is that mass at the rim counts exactly twice that of non-rotatingmass when accelerating on flat ground.  So getting a rim that is 20grams lighter will feel the same as losing 40 grams from the frame whenyou're accelerating on the flats.  This is, of course only true whenyou are accelerating and doesn't apply if you are climbing at a constantspeed.  If you are going up a hill at a constant speed, 20 grams is20 grams no matter where it is.  That's a fact.  In my personalopinion, arguments regarding energy lost from "micro-accelerations" overthe course of one's pedal stroke are convoluted and sketchy at best.
JonDear Lennard,
I'm sure that you've gotten lots of feedback on your recent VeloNewsposting on rotational weight.  The physics of this is quite simple,and your correspondent David is completely off in his understanding ofwhat is going on.  In fact, you don't even need to do any calculationsto understand this issue.  Given the principle of "conservation ofenergy" one can easily deduce that although it DOES take more effort tospin up a heavier wheel, that energy is returned when you coast/back offon the pedals.  The only time this is not true is when you are ina crit where you're braking in the corners - the energy is then lost toheat in the brake pads.  So when climbing, the added weight of heavierwheels is just that - added weight - and is no different than any otherweight on the bike or rider.The only forces that the climbing rider has to overcome are gravity,friction, and aero drag.  Once the wheels are spun up, there is noextra energy to keep them going, even if you speed is surging with eachpedal stroke.  Any extra power required to surge up in speed is immediatelyreturned as the speed drops with each pedal stroke.If one chooses, you can do a bunch of calculations to easily prove this,but a basic understanding doesn't require that degree of precision. It's good to air these discussions so as to educate riders, but lettingbad information out there without challenging it is not the way to increaseknowledge.
KerryDear Lennard,
In a crit, you pull the brakes to slow down. You burn off both theexcess linear and rotational velocity with the friction of shoe on rim.*Poof* The energy you used to spin up the wheel is lost. You don't getit back on the other side of the turn. Heavy wheels are a liability.Ona hill, the extra rotational velocity helps push you up through the weakpart of the stroke. As was noted at great length in the article, the humanbody does not generate power evenly through the stroke. Heavy wheels canactually make your stroke smoother by "pulling" you through the weak partsof the cycle.
CristDear Lennard,
In the debate about the importance of light wheel rims, attention hasbeen focused on the energy needed to impart rotational acceleration. Butwhat about the energy cost of vibration, on ordinary (imperfectly smooth)road surfaces?Many people have the impression that an ounce of weight saved on thebike--let along an ounce saved on the rim--counts for more that an ouncesaved on the rider's body. The effects should not be difficult to assessdirectly. I find that on a short steep climb, which I did several timesin alternation on a classic 531 bike (with as- new running gear) and ona carbon fiber bike, the modern bike called for similar or slightly lesspedaling force even when running a 10 percent higher gear ratio. The savings in total weight (bike plus  rider) was far below 10 percent,in fact less than 3 percent.  But the savings on the bike and on therim are each around 20 percent, and I wonder whether the proportionatelyreduced energy cost of jiggling rim and bike are contributing to the largereduction in effort.
Don

Technical writer Lennard Zinn is a frame builder (www.zinncycles.com), a former U.S. national team rider and author of several books on bikes and bike maintenance including the pair of successful maintenance guides " Zinn & the Art of Mountain Bike Maintenance" and "Zinn & the Art of Road Bike Maintenance."Zinn's regular column is devoted to addressing readers' technical questions about bikes, their care and feeding and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn. Zinn’s column appears here each Tuesday.