Originally Posted by
cyccommute
History is against you here. If wheel weight didn’t matter, why not just use steel wheels? They would be much stronger and less prone to the various failures that bicycle wheels experience. Every vehicle from bicycles to trucks have undergone significant weight reduction in the wheels over the last 50 to 80 years. Very few trucks run exclusively steel wheels anymore. Most all of them have aluminum wheels which reduces weight an insignificant amount compared to the weight of an over the road truck…80,000 lb (36,000 kg). An aluminum wheel on a truck is only going to save the truck a few pounds at most but the steel ones are worth replacing due to the weight difference.
I'll preface this by stating that far more competent people than me have figured this stuff out, so I'm really just parroting the fairly obvious conclusions. For example peterhski stated that in formula 1 racing the rotational weight of wheels and tires is considered but it's really only a very minor consideration. There was talk of orders of magnitude less important than other stuff. I don't have time to go look it up, but it's all there in the general discussion forum.
Anyway, there are a few reasons why one might consider it important to reduce automotive wheel weight. Though steel rims are still used and are really quite common. Probably half of the cars you see still use steel rims. At least where I live and specifically in the winter.
1) Unsprung weight. While it's relative to the overall weight, less unsprung weight means better working suspension. Considering how big of a challenge good suspension is on cars it's no wonder engineers would want to have less weight to work with.
2) inertial considerations in automotive speeds are pretty different from those in cycling. When you have a wheel and tire system that weighs in the region of 25+kg and you need to accelerate and decelerate it in traffic to speeds of 120+km/h, the weight is actually going to have a small effect on fuel mileage. But if we're being honest, when people choose aluminum rims for their cars, weight isn't typically a consideration. People choose the snazzy looking rims. Steel rims are also still used in trucks in the US. But aluminum rims don't corrode as readily so there's not as much of a risk of air leaks due to mating surface degradation.
So it's not all about weight.
In bicycles aluminum rims are and have always been better. I recently learned that aluminum started to get popular after the second world war and began replacing wood rims without a period of time in between with steel rims having a go as the top dog. Steel has always been the budget option and they're just not as good as aluminum rims so there's no real point in using them. Back in the day the braking surface was worse and had less friction especially in the wet so no real point even to consider them. And these days we have far better extrusion and welding methods for aluminum, and also carbon so steel is really going away fast.
So why not use steel rims in cycling? Steel has never been a serious contender. And it probably won't be.
Just to be clear, I’m not talking about the aerodynamic effects of the tire itself. I’m talking about the force needed to move the bicycle through the air and the impact that having heavier tires and wheels has on the work done. Heavier tires are harder to keep rolling because of the work needed to move them. Work in physics is defined as force acting over a distance. Force is related to mass. For more mass more force needs to be applied over a given distance. Less mass means less force needs to be applied over that same distance. Going back to that automotive example, alloy wheels are used because they require less work and, therefore, less fuel. If the small weight difference has such an impact on a heavy, highly powered vehicle, think of how much impact it has on a lighter, far less powerful vehicle. We are talking hundreds of horsepower (car) vs a fraction of a single horsepower for a bicycle.
Even with my limited knowledge of physics I know that the mass of an object is related most of all to accelerations (and decelerations, but aren't those kinda the same thing?). But once you get a heavier object going, it is going to travel farther than the lighter object, all other factors being equal. That's kinda how ballistics work (not an expert, even though I am a trained artillery section leader). So on a flat stretch of road heavier wheels or tires are not going to have any practical effect on how much energy it takes you to keep them going. On a hill the added weight is going to require more energy, but you need to compare that against the system weight. In accelerations it gets interesting. With my weight, an added kilogram in tires is going to require something in the region of 1 to 2 watts more to get to 25kph than if that same kilogram was on the frame. But once I'm up to speed it takes exactly zero watts more to maintain that speed, because once an object starts moving it keeps moving unless acted upon by an external force. And in this case the external forces acting would be wind resistance, rolling resistance and bearing friction.
It really was interesting to learn that wheel weight isn't really focused on much at all in F1 and that's the highest automotive racing category out there. The biggest effect it has on automotive racing applications is suspension functionality. But that's kinda how it should be in cycling too. I'm considering on building a set of carbon fatbike wheels not to make the bike lighter but to make the front suspension work better.
Far too much is made of rolling resistance. Yes, it is important but not as important as rotating mass and/or the work needed to move the bicycle. Small differences in rolling resistance might win a bicycle race but we aren’t talking racing here.
Well, it would turn out that rolling resistance is far more important than weight but also not as important than aerodynamics. But if we exclude aerodynamics since this is the touring forum and a four pannier bike is a sail, rolling resistance wins. So it's actually smarter to get the slightly heavier wider tire and be comfortable (and faster) than using a narrower tire in order to save weight. Then again the narrower tire can have a lower rolling resistance for the conditions, so it does get complicated. Thus, it's better to just choose width according to conditions and rolling resistance. Weight of the tire really isn't that important.