Rotating Weight Doesn't Matter (GCN Content)

[FlashUNC]

Alright armchair engineer nerds. Have at it. GCN coming in hot with rotating weight not mattering:

https://www.youtube.com/watch?v=0QDnUkUaQfk

[Mark McM]

Not much to say. Basically, they're right. For all but the corner cases, rotating mass is either insignificant, or more than compensated for by other factors (such as improved aerodynamics with deeper rims).

This has been known for decades, but I guess some still haven't gotten the memo.

[Tz779]

absurd conclusion. GCN guy shld stick to his day job.

[4151zero]

silly 🙃

[vincenz]

Clickbait title.

If weight didn’t matter, why would manufacturers go to carbon instead of sticking with alu or steel? Make the wheels as heavy as possible for maximum aero and for durability.

Of course it matters. We don’t ride in a lab. We don’t pedal at one power consistently. Real riding and racing happens dynamically. If you have a 5kg bike with the lightest wheels possible and a 5.5kg bike with heavier wheels, the lighter one will be faster up the hill, everything else remaining equal. How would it not matter in that case? The guy in the video trying to sell his wheels confirmed as much. Weight is weight.

[oldguy00]

Its not that weight doesn't matter at all, its that 'rotating weight' doesn't matter more than static weight. At least not as it pertains to cycling.
And aero trumps weight as well.

Both these things have been known for a long time.

[Mark McM]

Originally Posted by vincenz

Clickbait title.

If weight didn’t matter, why would manufacturers go to carbon instead of sticking with alu or steel? Make the wheels as heavy as possible for maximum aero and for durability.

Of course it matters. We don’t ride in a lab. We don’t pedal at one power consistently. Real riding and racing happens dynamically. If you have a 5kg bike with the lightest wheels possible and a 5.5kg bike with heavier wheels, the lighter one will be faster up the hill, everything else remaining equal. How would it not matter in that case? The guy in the video trying to sell his wheels confirmed as much. Weight is weight.

You've missed the point. The question isn't whether weight matters. In the video they admit that on its own, it does. The question is, does it matter if it is rotating or not rotating? The second part of the issue is whether making wheels more aerodynamic, at the cost of extra weight, is a net benefit?

The answer to the first question is: In most cases, whether weight is rotating or not rotating is mostly insignificant. The answer to the 2nd question is probably the more important: In just about every case, the improvements in aerodynamics with deeper, heavier rims outweighs any deficit due to their extra weight. The Swiss Side fellow is hardly the first to conclude this. Many engineers and researchers (who have no wheels to sell you) have reached the same conclusion. Here's two of them:

https://www.slowtwitch.com/Tech/Why_...rtia_2106.html

http://www.biketechreview.com/review...el-performance

I myself put together a mathematical model to test this, and came to the same conclusions. I posted the analysis on the WeightWeenies web site 15 years ago:

https://weightweenies.starbike.com/f...e+kutta#p60824

The above analysis was in regard to the importance of rotating mass when climbing. I've also done similar analysis regarding whether heavier but more aerodynamic wheels matter when accelerating. I looked at the case of a criterium, with accelerations out of every corner, and the final sprint in a race. In both cases, the aerodynamics of the wheels were far more important than their weight (and rotational inertia).

Now, if you have some actual data or other evidence to provide, let's see it.

[ddtn]

I don't think there's anything new in that video. You want to have the lightest wheels just as you want to have the lightest whatever part, but saving 200g in rotating mass is the same as saving 200g in the frame. They didn't say saving weight on the wheels doesn't matter at all. This is consistent with what I've read since carbon wheels became mainstream.

[Ken Robb]

My personal feeling has been that I could imagine that heavier rims MIGHT be slower to accelerate their "flywheel" effect might compensate for that by making it easier to maintain speed in some conditions. This thinking just let me not poo-poo folks who had opinions on either side of the argument.
I didn't stick with aero rim-heavy wheels long enough to form an opinion of my own because so much of my riding has been along the coast where pretty brisk crosswinds are common and they would sometimes push me around a bit more when I was on aero rims.

[bicycletricycle]

if you think about it, nothing matters.

[Mark McM]

Originally Posted by oldguy00

Its not that weight doesn't matter at all, its that 'rotating weight' doesn't matter more than static weight. At least not as it pertains to cycling.
And aero trumps weight as well.

Both these things have been known for a long time.

This is completely true, except maybe in some extreme corner cases. For example, I have done the Mt. Washington Hill climb, which is a 7.6 mile climb that gains 4720 feet of altitude, at an average grade of 12%. I average about 5 mph, which is slow enough that aerodynamic drag matters little. Here, weight can be more important than wheel aerodynamics - although it doesn't matter whether that weight is rotating or not.

I've wondered about another corner case. As we know, rotational inertia only affects accelerations. We also know that aerodynamics makes less difference at slower speeds. And we know that the lower the total mass, the greater difference the mass of the bike (and the rotational inertia) matters. Are there any events in which the rider is spending a lot of time accelerating, and in which a lot of time is spent at low speed, and in which the ratio of bike-to-rider mass is higher? The even that comes to mind is the Women's 500m TT, which starts at dead stop, and then spends a spends a significant amount of time at lower speeds under high acceleration. Plus, women tend to weigh less, making the mass and inertia of the bike more important. I don't know how significant rotational inertial actually is in this event, but it would be interesting to find out.

[pasadena]

The difference between testing computer models and seperate factors with a lot of assumptions built in,
and reality.

On a bike, all other things being equal, lighter is always better for performance.

[Mark McM]

Originally Posted by pasadena

The difference between testing computer models and seperate factors with a lot of assumptions built in,
and reality.

Well, that's true of course. But the mathematical models indicate that the difference that rotational inertia makes is insignificantly small, which doesn't lend much hope that it will make much difference in real life.

We could try to do real life testing, but that adds many variables, some difficult to control. Giving that we are looking for an affect that is expected to be very small, it might be difficult to tease the meaningful values out of the data. Riders often say that they feel faster on lower inertia wheels; but we know from other cases, rider perception can be fooled. For example, I think it has been pretty well established that very high tire pressure can make a rider feel like they are going faster, when in reality they are actually faster at lower pressures.

Originally Posted by pasadena

On a bike, all other things being equal, lighter is always better for performance.

I don't think anyone disputes this. But the question being asked here is, how much better is it, and are there other factors which are more important?

[unterhausen]

On one hand, we have people saying it doesn't matter much at all. On the other hand we have people that say you can't mathematically model confirmation bias.

[vincenz]

Originally Posted by Mark McM

You've missed the point. The question isn't whether weight matters. In the video they admit that on its own, it does. The question is, does it matter if it is rotating or not rotating? The second part of the issue is whether making wheels more aerodynamic, at the cost of extra weight, is a net benefit?

The answer to the first question is: In most cases, whether weight is rotating or not rotating is mostly insignificant. The answer to the 2nd question is probably the more important: In just about every case, the improvements in aerodynamics with deeper, heavier rims outweighs any deficit due to their extra weight. The Swiss Side fellow is hardly the first to conclude this. Many engineers and researchers (who have no wheels to sell you) have reached the same conclusion. Here's two of them:

https://www.slowtwitch.com/Tech/Why_...rtia_2106.html

http://www.biketechreview.com/review...el-performance

I myself put together a mathematical model to test this, and came to the same conclusions. I posted the analysis on the WeightWeenies web site 15 years ago:

https://weightweenies.starbike.com/f...e+kutta#p60824

The above analysis was in regard to the importance of rotating mass when climbing. I've also done similar analysis regarding whether heavier but more aerodynamic wheels matter when accelerating. I looked at the case of a criterium, with accelerations out of every corner, and the final sprint in a race. In both cases, the aerodynamics of the wheels were far more important than their weight (and rotational inertia).

Now, if you have some actual data or other evidence to provide, let's see it.

I wasn’t even factoring in aerodynamics as this isn’t about that. The ~4s difference the guy quoted in the vid of two riders going uphill on a light vs heavy wheelset is only if they were at constant power the whole way up. No one rides, and especially no one races like that..

“Mostly insignificant” doesn’t cover all cases. I can think of a scenario like a very steep section of a long climb where two riders are playing cat and mouse on two bikes of the same total system weight, one with super lightweight wheels and the other with a heavy set. Aero aside, I think it’s obviously which one can out-accelerate the other and get away.