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Technical FAQ: Does wheel weight matter?

Lennard Zinn answers more questions on bike weight and readers weigh in on previous columns

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Editor’s Note: Lennard Zinn’s regular column is devoted to addressing readers’ technical questions about bikes, their care and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn.

Dear Lennard,
I read your May 22 column (“Does bike weight matter?”) with interest. I have a follow-up question:  Does wheel weight really matter (especially when climbing)?

The links you included in your answer to the earlier question were helpful — it’s nice to be able to quantify how much you gain by trimming bike weight. I found it surprising, though, that it seemed to make no difference whether weight was trimmed from the frame or from the wheels. At the first link you provided, they ran separate tests adding four pounds to the frame and four pounds to the rims — and the performance hit in a 50-minute ride up Alpe d’Huez was the same!

This isn’t immediately obvious from the numbers they report. But once you notice the rider’s average power was lower in the “rim weight added” case and you adjust for that lower power, the time difference between the two cases (rim weight vs. frame weight) is only five seconds. Average power was 0.72 percent lower in the “rim weight added” case, so if you subtract 0.72 percent from the time in that case, you get 52:01 – 0:22 = 51:39, only five seconds slower than the “frame weight added” case.

I found this result quite surprising. I’ve always heard one of the surest ways to improve your climbing is to use lightweight wheels, and especially wheels with light rims. But this test seems to indicate that old maxim “just ain’t so”.

What’s your take on this question? Do lightweight wheels offer any significant benefit when climbing? Or is this result pretty much in line with other results you’ve seen — and wheel weight really is no more significant than frame weight when climbing?

Dear Lennard,
Interesting analysis. But doesn’t rotational weight count for more than static weight? I had heard that rotational weight has four times as much effect, so that taking 400 grams off your wheels/tires is the same as taking 1.6 kilo off the bike.

Also, if Miguel thinks a top-of-the-line carbon bike is 17 pounds, he’s a Neanderthal. A friend has a full SRAM Red Guru that tips the scales at 12.1 pounds. My Ultegra BH Cristal is in the 15-16-pound category.
— Tamar

Dear Dave and Tamar,
There is no question that if a rider climbs at constant speed, it doesn’t matter where the weight is located on the bike. Extra mass could be concentrated on the pedals, at the rims, in the frame, or in the hubs, and as long as the bike’s total weight is the same and it has otherwise the same characteristics, it will create the same resistance to the rider’s efforts.

That said, there is also no question that it takes more energy to accelerate the same amount of mass if it is located out on the rim as if it is located at the center of the wheel (or on the frame). This you can easily measure with a stopwatch on our Velo torsional pendulum we have been using for years to measure wheel rotational inertia: if you have two wheels of identical total mass but one of them has more mass out at the rim than the other, it will take it longer to twist back and forth once (i.e., the period of oscillation will be longer and the frequency of oscillation will be lower) on the torsional pendulum. But Tamar, if there is a circumstance in which “rotational weight has four times as much effect, so that taking 400 grams off your wheels/tires is the same as taking 1.6 kilo off the bike,” it is a very isolated circumstance under extremely high acceleration from a slow speed to a high speed. Perhaps in a standing start in a pursuit or short time trial…

The bike always has to accelerate at least once to get up to speed, and that will take more energy to do if the added mass is at the rim than if it has instead been added to the frame. One question is whether the extra energy required for this initial acceleration is trivial and can be ignored or not. After that, even if the rider speeds up and slows down the same way on each bike without using the brakes, it will not matter where the extra weight is located, at least in the “ideal, frictionless universe” used in elementary physics calculations of motion. If the rider stops pedaling, even on a climb, he will be carried further up the hill by the flywheel effect of the heavier rims than he will be on the bike with weight added to the frame. Then when he starts pedaling again, he will end up at the same point in the same amount of time on either bike. This is the principle that Ondrej Sosenka depended upon when he set the hour record with heavy rims; he reasoned that the heavy rims would carry him along and keep the speed more constant as he went through periods of weakness and strength. It seemed to work for him; I’m not going to argue with that result.

If the rider puts on the brakes, then the acceleration calculation comes into play again, and it again takes more energy to accelerate the bike with the added weight at the rims. Again, how trivial is this extra energy for two wheelsets of similar mass but different moments of inertia? We know the wheel with lower inertia will be faster in that situation, but how much faster? If you’re somebody who replaces the steel bolts holding your bottle cages on with aluminum or carbon bolts, then by all means get the low-inertia wheels — they will be faster. But if you’re not someone who spends money for very small performance advantages, then I’m guessing that the wheel inertia difference between two wheelsets of identical mass won’t be worth worrying about. But I can’t say for sure how much difference it makes until I see more data like this or until we test it ourselves.

Dear Lennard,
I read your Technical FAQ column this week, and see you used the link I sent to answer a related question, but not the question I asked. Well, um, … you’re welcome. Glad I could help out.

Got to thinking about it, and I wonder if the reason you chose not to discuss my question is that by doing so you might change the thinking (and thus the spending habits) of a lot of your readers — and some of the magazine’s advertisers might not be too happy about that.

By answering the question you did, you’re basically telling your readers that: Yes, you have to spend thousands of dollars on a lightweight bike if you want to stay on a level playing field — especially when climbing.

And by not discussing the question I asked, you’re NOT letting your readers know that: No, you do not have to spend thousands of dollars on a super lightweight wheelset to be on a level playing field when climbing.

I was a runner for years (high school, college), only discovered cycling 7-8 years ago. And though I love cycling, one of the things I’ve found appalling is how much money you have to sink into it to be competitive. (For running, you buy a good pair of running shoes, and you’re pretty much outfitted as well as the pros.) And of course, most equipment vendors do all they can to help encourage this appetite for the latest and greatest cycling gear.

But most competitive cyclists aren’t exactly wealthy — most of the ones I know are young, struggling financially, many just starting families, and pouring an awful lot of time and energy, and hard earned cash into their passion. I would think some of them might be glad to learn they don’t really need to sink another several thousands dollars into a super lightweight wheel set in order to remain competitive.

It seems to me you could help debunk a fairly widely held cyclist’s myth here, Lennard, and help out a lot of cyclists by doing so. (Help free us from the tyranny of the carbon pimps!)

Sorry if I’m off base here. If I’m misreading the situation and being too cynical, please let me know, I’d be glad to hear it. (And would apologize profusely!)

Dear Dave,
Actually, it was not my mission to extend “the tyranny of the carbon pimps” when I selected which questions to run in the column. Your question was essentially the same as the other Dave’s above, and I have now answered it. I have wanted for some time to repeat that water-in-the wheels experiment on our local Flagstaff Mountain and see if we get the same results they did on Alpe d’Huez. I have not gotten to it yet and do hope to in the future.

Feedback on last week’s bike weight column

Dear Lennard,
Not to muddy waters, but additional bike weight can feel heavier than additional rider weight because it is useless for propulsion. Rider weight isn’t just a force to be overcome, but a force that can be used to put power into the pedals. The bike weight, except the wheels with their “flywheel effect,” cannot be used the same way.

This relationship between the weight of the rider and the weight of the bike gets more pronounced as the rider’s weight decreases and less so as the rider’s weight increases. This may explain why recreational cyclists are less concerned with bike weight and skeletal racers obsess over it. Maybe this is a good thing for those of us that aren’t sponsored?

Of course, this line of thinking is less relevant when you are talking about a recumbent. In those cases, the rider’s position on the bike precludes them from using their weight to their advantage.

Dear Lennard,
Thank you for publishing my letter. To my horror, I found a mistake in my figures, albeit a small one: at 5 mph, the power difference is 10 N x 2.2 m/s = 22 W, not 32 W. Chalk it up to bad typing and proofreading.
— Lawrence

Dear Lennard,
You recently responded to a comment on this subject with:

“Actually, in a frictionless universe, that acceleration and deceleration due to varying pedaling efforts makes no difference, because you get the energy back.”

Given the restricted context of uphill cycling, this is not correct. Gravity constantly applies a counter-acting force so that the bicycle will decelerate and accelerate with the variance in pedaling power (presuming the bicycle always moves forward). The kinetic energy in the vertical plane that is lost when the bicycle decelerates due to gravity is lost for good (at least, in the upward direction — which is all we care about when we want to climb at least). You won’t get it back, at least not before you have crested the hill.

Friction just determines how much extra energy is needed to get to the top of the hill, energy that is not then available to be converted back to kinetic energy from descending. So when considering just the uphill case, all the energy appears “lost” — any friction just means you lose even more.

Dear Lennard,
Regarding Miguel’s bike weight question, you can do all the calculations and physics in the world, but a 29-pound bike versus a 17-pound bike will behave very differently in real world applications. The lighter, more expensive bike will handle better, stop better, shift better and generally just feel better and have a lovely snap to it that you’ll never find on paper because there are far greater difference than the mass.

You’re not talking apples and oranges here: this is not taking a 17-pound Cervélo and comparing it to the same bike with an additional 12-pound load. The 29-pound road bike will have inferior shifting, braking, handling and ride qualities. The lighter frame will be forgiving here and responsive where it needs to be, where as the 29-pounder will ride like a isotropic boulder. And generally the lighter bike will last longer with tighter tolerances and greater precision in the build.

If you’re talking $1000 savings between a 20-pound bike and a 17-pound bike, that is one thing; you’re talking near top of the line versus bottom of the barrel and that does not show on paper. A Ford Focus will get you up the mountain, but a Ferrari will not only do it faster, but with an entirely different experience.

Dear Lennard,
I have been following this discussion with interest and also have my $.02 worth to add.

The difference in weight between my road bike and mountain bike on a hill climb (regardless of gearing and/or tire rolling resistance) is very tangible. It usually leaves me wishing that I’d ridden my road bike and left the “windcatcher” at home. As far as I can see, bike weight is a concern, though not the end of the story. I agree with the comments about bike weight vs. body mass (i.e., that people who chose to spend thousands of dollars to save 800 grams here and 300 grams there would be better served to reduce their “static mass” as I like to call it).

Having lost over 30 kilos (that’s over 66 pounds in U.S. measure) in the last two years, I think I can comment on this without fear of criticism. When I bought my road bike 18 months ago I chose not to spend the extra $2000 to save 800g in weight and opted for an alloy frame over carbon, as I knew that I had more “static mass” to lose. I have seen more than one rider get off his multi-thousand-dollar carbon bike and walk it up a hill on a community ride (Tour Down Under Bupa Challenge, for example). At most levels rider fitness, condition and nutrition is probably more important than bike weight. Anyhoo, as I said, my $.02 worth.