Does rotating weight really matter?
 

Weight matters no matter if it rotates or not. However the effect is greatly exaggerated on the forums and the reasons why is not generally understood by the forum "experts".

When starting from still, sure.
When trying to accelerate, sure.
When JRA, not really.

I dont need to watch a GCN 'experiment' though.

FWIW it wasn't a GCN experiment. Some Formula 1 tech guy ran some tests with a computer program is what I took from it. Apparently he's the founder and CEO of this company: https://www.swissside.com/?locale=en

Does rotating weight really matter?

it depends

Is that the video where the conclusion was "no, unless you're accelerating"? Isn't climbing a whole series of micro accelerations, once it gets steep enough for inertia to be negligible for a given speed? Also, what about crit racers constantly accelerating out of corners? It seems to me even casual cyclists are accelerating quite a lot.

Mass in motion and more than that, Harmonic Mass in motion. Not so much easily attainable but a little easier for bicycle riders. Looking at the technical aspects bicycle riders attempt this physically, mechanically, and metaphysically on a good ride...

https://cimg0.ibsrv.net/gimg/bikefor...8bbdfd4760.jpg

Yes. Technically speaking, power variation across the pedal stroke creates speed oscillations in all riding, although this is much less pronounced in situations with more inertia.

The thing is, rotational inertia works in both directions. Heavy wheels won't accelerate as quickly at the peak of your power stroke on a climb, but they also won't decelerate as quickly when the power isn't flowing.

I don't know much about racing, but I thought very small differences in acceleration can make or break a break away attempt. That video completely ignored that aspect of wheel weight.

People use the rotating mass argument as a justification tool when trying to explain why they are riding multiple thousand dollar wheel sets while they are 20 or 30 lbs overweight.

The peloton is what makes or breaks most break away attempts.

Yes, especially if you are out of shape like me, freewheeling now and then to catch my breath.

It may not manifest on the stopwatch, but it certainly affects handling feel. Bikes with lighter rims/tires feel much more fun and responsive to changes in orientation (less weight, lower angular momentum)

It matters for acceleration since you not only have to accelerate the wheel mass forward with the rest of the bicycle, but you also have to add rotational energy to the wheels. And the rotational (angular) energy grows with the square of the rotational speed (angular velocity). (For the technical people, there is an w^2 factor where w=omega.) Forward kinetic energy only grows linearly with speed.

This isn't true at all. Kinetic energy for linear acceleration is quadratic with velocity.

https://en.wikipedia.org/wiki/Kineti...f_rigid_bodies

The inertia of mass at the tire requires double the kinetic energy to reach a certain forward velocity than mass at the frame.

Disclaimer: All of the following may only have taken place in my head and is not based on anything scientific.

Purely anecdotal, but I didn’t like when I had lighter wheels, I’m not a heavy person so I definitely felt I had to work more to keep them going on flats, or on any road terrain in general. I felt like if I stopped pedaling the bike stopped. I did definitely notice they felt more touchy/responsive in a way that I actually didn’t like, which lines up with what whyfi said above. I understand I am the outlier here though lol. I feel more planted and comfortable with heavier wheels. I may just be crazy though. There’s also probably other variables I’m too dim to understand here. Long story short, I sold my lighter wheels.

All that said, the above qualities mean on a mtb lighter wheels make sense for me because I’m not rolling as much (inertia matters less) and I can appreciate the quickness and lighter feel more.

Yup. I was a skinny, light, long limbed climber with all slow twitch muscles. The "flatties" hated me when the race wasn't flat. But on the flat, all those little accelerations that they did so easily took a much bigger percentage of my max each time. Then there was getting on the wheel going by. Closing a gap. Lets say when you are at your max, super light rims and tires mean you are two feet behind that wheel and heavier rims put you at 5 feet back. 3' at 28 miles per hour comes out to 0.07 seconds. Who cares? In a 5 hour race, totally meaningless, right?

But, you are at your max. 2 feet means you are on that wheel, 5' means you aren't. You may not be able to close that extra 3 feet or it may take you minutes to do it. If you do make it, you may be cooked for the rest of the race. But getting that little extra jump to grab that wheel, that extra 0.07 seconds up the road? You're on! You're in the break. You may will be finishing 10 minutes ahead of that other you on heavier wheels who didn't quite make it.

In my hardest race ever, I was half way up the deciding hill 15 miles before the finish, in about 30th place with an unbroken line of rider in front of me. I was hurting bad. The race had been very fast and I'd been an instigator in an ill fated break. Looked over my shoulder to see how far I could fade back and still be with the leaders (being fully aware this might cause a field split that might never come together again). Next rider was 25 yards back. That's about 4 seconds at the speed we were going. I knew instantly the wheel I was on was going to beat that guy and everybody else back there by 10 minutes. I stayed on and was well rested and had waited in line, then got my drink at the water fountain before anybody else finished.

That was at steady speed and rims didn't matter. It was about holding a wheel at any cost. I paid a LOT to stay on that one. But at the top of the hil, I was there and the guy behind me wasn't. (I was on 270 gm average rims and 250 gm tubulars so they WERE very light and helped me to an unknown degree over the previous 3 hours.) Point being - the little things in races matter far more than any analysis of seconds gained or lost, etc. will ever tell you. If you are strong enough to simple clobber everyone with the raw power of your legs, those light rims might not matter. If you have to race with all your smarts and all your resources to be there at the end, rims can be make or break items.

Yes, this is scientific; you were after the flywheel effect. Time trial bikes have solid rear wheels for that (and aerodynamic) reasons. This is not a phenomenon that is unique to cycling. Those who have installed aftermarket flywheels in manual transmission cars also go back and forth on this issue.

I will also say that this effect is much more apparent on fixed gear bikes. I started riding on a fixie with 3000+g 50mm deep alloy wheels. When I switched to a road bike, my legs would literally stop (not exaggerating) at the top of the pedal stroke because I was so used to the pedals carrying my feet through the deadzone.

I get confused when people say riding a fixie gives you a better pedal stroke LOL

My take away thus far is that there has been a lot of money spent for not much gain. I am willing to be convinced otherwise but so far that's where I am.

Ride with some slack in your chain and go down fast hills (keeping the top run of the chain tight). Also ride up seated as much as possible. And use some discipline. The vets in my club told me to ride fixed 45 years ago. I've never regretted it. When my pedal stroke needs work, I get back on it. (But I've never ridden heavy wheels outside of big tired gravel.)

You say that like it's a bad thing.

So riding a fixie doesn't give you a better pedal stroke. Riding a fixie with the intent to specifically work on your pedal stroke gives you a better pedal stroke. Sure, I can get with that.

In other words, a guy who makes heavy aero wheels is trying to convince us that rotating mass doesn't matter very much.

Sure, yeah, I trust him.

Having more than one road bike?