Lombard

If you had been dieting alone without increased training, you probably would have also lost some muscle mass and therefore your weight savings would have been offset by being weaker overall.

3alarmer 

...it probably was. If you introduce more mass into the wheel, it has a tendency to stabilize the system, once it has been brought to speed. This is the essential fallacy that's been argued in this unnecessarily rancorous exchange. The basis for it is that modern wheel rim choices for bicycles are all relatively light. The essential advances in materials technology that allow us these choices was accomplished some time back, and the result is that the momentum equations for rotating mass versus simple linear acceleration have very small differences in the values input to get the results.

Thus, with the advent of new materials technologies, aerodynamic design has become more and more of a factor in bicycle wheels made to go fast, with wider profile rims and fewer spokes, even though the additional mass of the wider (AKA "deeper") profile adds mass at a portion of the wheel where it is least desirable, in terms of rotating mass equations.


But that does not change the nature of the average bike ride, which consists of many accelerations and decelerations, into something like a train with 7 linked engines and 500 loaded cars, travelling along a track at speed. The fallacy lies in the modeling of the problem. Were it a simple problem, perhaps there would not be so much misunderstanding of it. Weight is one factor. Certainly as speed on a bicycle increases, aerodynamics is another important consideration. The sportscar people pay more attention to reducing rotating weight of the wheels (often called "unsprung weight" in the literature), because they have been more limited in wheel and tire materials development due to the larger forces placed upon their wheels and tires. And their designs have also paid more attention to aerodynamics over the years. Usually this is accomplished using shrouding and body modifications. I'm sure the ideal triathlon bike would probably be one with a fairing. But I'm also pretty sure that's against the rules. So they are left with aero bars and body positioning, and searching for a light bicycle with a fast set of wheels, as their alternative for faster.

As a practical consideration, my Nissan electric car would not have alloy wheels and a strangely futuristic body design, only as a sales gimmick. They are present because they save some energy, and therefore serve to extend its range at speed. Likewise the use of composites in constructing the body although much of the chassis is still made from steel. Bicycle frames and components have been so reduced in weight, that it is easy to miss the reasons for it, and the shear magnitude of it, if your sole frame of reference is the past 20 years. They predated most cars in this regard, because they are human powered, and it's hard to add horsepower.

Chapter 4 of Bicycling Science is entirely devoted to the various factors that apply to Power and Speed on a bicycle. I think you can probably find it as a pdf download using a search engine, but I don't have a link for you. And there are subsequent chapters, one on aerodynamics and one entirely devoted to wheels and tires with regard to frictional losses and resistance to rolling. Again, in our lifetimes, rolling resistance has been much reduced in available tire choices. And modern bearing technology makes the values for these losses so small as to be inconsequential. Everyone in a race is using similar bearing technology.

Atlas Shrugged

What do you esitmate the diffence would be saving 500 grams at the wheel vs 500 at the frame 0 to 30 kmh acceleration.

3alarmer 

...if the difference is negligible, it is because modern materials technology has so reduced these weights on bicycles, that the variable values input to the equations is negligible, not because it's all bs. I realize that there is a human compulsion to simplify every problem like this, to a bs / not bs answer format. Here's one result quick Google search on sprung versus unsprung weight.
Someone will now chime in that these are cars and car wheels, which have no relationship at all to bicycles. No more than freight trains, in fact. I can only respond that the physics of all these various things that move forward on wheels are more or less the same. The practical limits set for them differ because of the real world circumstances of their use. So it will be a while before we see CF wheels and lightweight tires on cars. And rules, of course, if they are used in sporting events, like train racing.

3alarmer 

...I'm not doing the math homework for you. The equations for rotational mass and linear acceleration are readily available, if you are interested. My estimate is that 500 grams is not very much weight (about a pound, to those of us who still believe in American values. ) But you can certainly set up the experiment easily enough on a bicycle of your own, and report back on your results. I honestly do not know too many people who ride regularly who would willingly add a pound to their wheels/tires. I know plenty of people who think nothing of carrying a pound of extra crap in a front bar bag....me, for example

Koyote

"The whole debate regarding unsprung vs. sprung mass often casts a massive shadow on another vital aspect of unsprung mass reduction — rotational mass or rotational inertia. By reducing the unsprung mass of a vehicle, you also reduce the amount of weight that your engine needs to be defeated by your engine to accelerate the car.

As a result, lower unsprung mass means higher rates of acceleration and shorter braking distances."


3alarmer , I'm not really sure that this quote (the part in bold) makes sense...? Did something get lost in translation?

tomato coupe

If you do a quick back-of-the-envelope calculation, the difference in acceleration is something like 0.3%. To put it another way, if you're racing a crit and accelerating out of every turn at 600 W, your twin brother with 500 g heavier wheels but a 500 g lighter frame will have to produce 602 W to keep up with you. (Assuming aerodynamic drag, rolling resistance, etc. are the same.)

Branko D

I wouldn't add 500g of stuff on the bike for a climb, at least not if it's a race.

I'd add 500g to the wheelset if it makes sense, the deep front and the rear disc on my TT bike are heavy but that does not really matter.

Of course, our perception of heavy is relative, 1950g is like a boat anchor in the cycling world. If you hand it over to a non cyclist they'll remark how light it is.

Lombard

in case you didn't realize it, the train giffy was in response to this thread becoming a train wreck. It wasn't meant to have anything to do with the actual topic of rotating mass.

Trakhak

I have a copy of Bicycling Science tucked away somewhere that I haven't looked at in decades, along with the big blue C.O.N.I Italian training bible. (The part of your post that I deleted was also superbly expressed, by the way.)

Atlas Shrugged

Awesome. In the world of marginal gains, lightweight wheels are in aerodynamic sock territory. I find it interesting that even within a community (Bike Forums) of engaged hobbyists, the same worn-out myths never die even though the evidence is clearly contrary. Narrow, high-pressure tires being faster in real-world riding and the so-called magical dampening properties of titanium are my other bs triggers.

3alarmer 

...in case you haven't yet realized it, my responses to you in this thread have been thinly veiled sarcasm, ever since you responded to this:

...with this non sequitur:

Somehow, you are spring loaded on this issue. You see it everywhere, lurking in dark corners, like some conspiracy theorist.
It's entertaining to witness the continuation of it at this point. The train gif was just a high, hanging pitch, right over the plate.

Koyote

I think about this occasionally when I get a Strava KOM on nice smooth pavement while riding my gravel bike with 44mm knobby tires, or while riding my 27 lb fendered light touring bike.

3alarmer 

...it's one of those unfortunate sentences with double or triple negatives, that sometimes show up in the car literature. Translated, it says the engine acceleration works better with lighter wheels and tires. The engine works harder to defeat extra unsprung weight, so less is available to produce overall acceleration.

...did you calculate it out cumulatively over 112 miles of rolling terrain , with 20-30mph wind gusts ? Because that was the Ironman course here on the day of the race. And then you have to hop off your bike at the end, and run a marathon.

vespasianus

This site (https://www.omnicalculator.com/sports/cycling-wattage) can calculate power needed for certain speeds based upon bike and body weight. No wheel weight but it gives you an idea of how little difference 2-3 lbs makes. A 190lb rider, on a 20 lbs bike, going up a 5% grade would need 328 watts to go 12 mph. If the bike was 16 lbs, it would be 323 watts. Bike weight, for the non competitive hill climbing racer is waaaaay over-rated.

Koyote

Occam would probably say that it's a typo, since it doesn't say what you say it says.

tomato coupe

It doesn't change anything. Every time the rider accelerates, the extra 500 g wheel weight vs. 500 g frame weight costs him 0.3% in acceleration. (Assuming, again, that aerodynamic drag, rolling resistance, etc. are the same.) Of course, a rider doesn't accelerate every foot of the 112 mile course, so the overall effect is actually much smaller than 0.3%.

Koyote

And no one ever wants to talk about the effect of larger mass on descents.

Lombard

So in other words, you've been trolling the whole time. How entertaining. And I fell for the bait. Be proud.

3alarmer 

...I inserted a comma at the appropriate place. I presumed on reading it, that it references the ability to use a smaller, thus lighter, engine to drive the same car. Important in classes where engine displacement is limited. But yes, automotive stuff is not always up to Strunk and White standards.

3alarmer 

....seriously now, you are going to try to troll-shame me ? OMG

Atlas Shrugged

There is a popular forum called SlowTwitch for triathletes where they spend a lot of time discussing the challenges and out-and-out awesomeness of triathletes and triathlons, I am sure they would benefit from your inciteful insights.

3alarmer 

...not certain, but I think you missed out on the whole "rotating weight acceleration at the wheels," versus "linear acceleration weight on the frame" thing. No matter. It's a big part of all the online arguments about how deep wheels are heavier, but it's OK because they are aero and faster. And there are enough of those to fill a shipping container. Sometimes, when I close my eyes in this thread, I think I must be in the road forum.

It's been really interesting to me, personally, to have watched the crowd opinion switch from "lighter is better", to "I don't care how much they weigh, I need those deep aero wheels to reach my full human potential." I have no explanation for it, other than most roadies are slaves to fashion.

3alarmer 

...just between us, I am Lance Armstrong's Bike Forums sock puppet account. But it's a secret, so don't tell anyone.

3alarmer 

...you like that .3% number I see. Stick with what you believe, sir. There are far too many people willing to abandon their belief systems, that are supported by small numbers, in order to risk calculation of total energy expense over a long course in windy conditions. Do not be that person.