There is a pretty large debate out there in the cycling world about whether the outer rotational weight of your wheels matters or not. The answer is, it depends.
The parts that can have the biggest impact on the outer rotational weight of your wheel are the tires, rims, spoke nipples, and spokes in that order with the tires and rims being the biggest factors. Why? Because those two parts of the wheel are at the outermost rotational mass. Spoke nipples come next because they are just inside the rim and then spokes after that.
Hubs, brake rotors, cassettes and so on all do have a small impact on rotational mass, but because they are closer to the inside of the wheel they cause less of an effect.
I won’t get super detailed into the physics of rotational mass. That is something you can easily look up. I am going to talk about how adding or reducing outer rotational weight on your wheels affects how a bicycle rides. The basic idea is that more weight at the outermost portion of the wheel makes the wheel not want to change its rotational speed as easily. The farther away from the center the weight is (the larger diameter of the wheel) the harder it is to get the wheel to change speed. This affects the acceleration and deceleration of the wheel. It takes more force as the diameter increases and/or the weight of the outermost portion of the wheel increases. This is shown in the diagram above.
Bicycles have a wide variety of wheel and tire sizes. A smaller diameter wheel, such as a 20″ wheel on a BMX bike, will be able to spin quicker than a larger diameter wheel, such as a 29″ wheel on a mountain bike, given that the outer mass is the same between the two. This is simply because the outer mass of the tire and rim is much closer to the center of the wheel on a BMX bike. As the diameter of the wheel increases, so does the effort to get the wheel to change speed. The difference between a 20″ wheel and a 29″ wheel is pretty significant. In mountain biking and other forms of cycling, there are two common wheel sizes, 650b (27.5) and 700c (29). There are of course many more than that, but they are much less common so I am going to stick with those two sizes for comparison. For mountain biking, we refer to 650b as 27.5 inches and 700c as 29 inches. This is because once a tire is mounted on the rim the outer diameter measurement comes out to be really close to those inch measurements.
Being that there is a 1.5-inch difference between the two most common size wheels, does the small change in the size of the wheel make much of a difference in outer rotation mass? It actually does believe it or not. It’s subtle, but a rider can definitely notice a difference. I will explain some of the reasons why later, but just know it’s not just because of the 1.5 inches of difference.
For now, I want to talk about how different tire weights and rim weights can affect how your bike rides and when it matters or not. On a bicycle, tires are the easiest part for someone to replace, so let’s start there. There are a large variety of tires and tire sizes. The larger volume of the tire, the heavier it will be given that the casing and tread are similar. A thicker and more heavy-duty casing will add weight to the tire, but also make it more durable. These factor in on how much a tire will weigh and thus greatly affect outer rotational mass. It’s important to pick the correct tire for the type of riding you are doing as well as the terrain you will be riding on. Using a larger volume more durable casing tire for riding typical flow style singletrack will be more of a disadvantage than a benefit.
Rims also contribute to outer rotational mass. A carbon rim is generally lighter than an aluminum rim as long as the carbon rim isn’t overbuilt for enduro or downhill. A wider rim will be heavier than a narrower rim, although a wider rim does help support larger volume tires better. Too narrow of a rim for the tire and the tire can easily fold to the sides in corners. All of this needs to be factored in when trying to reduce the outer rotational mass on your wheels.
So why does all of this matter? Well, adding rotational weight has some negatives when it comes to acceleration and even for riders who want to maneuver the bike around. The heavier the wheel and the larger the diameter create a gyro effect. This essentially makes the bike want to stay upright and go in a straight line. This is why when a bicycle is rolling it’s much easier to balance and when it is stopped or going really slow, it’s much harder to balance. All good except for when a rider wants to move the bike around, let’s say in the air while going over a jump. If you watch closely, many riders will tap the rear brake when catching air to get the rear wheel to stop moving so they can lay the bike out into a tabletop or do a whip or something. Heavier outer rotational weight takes a lot more effort to move the bike around. Smaller diameter wheels help make bikes more agile, which is why many riders prefer 27.5-inch wheels over 29-inch wheels.
If you are looking for a bike that can accelerate quickly, especially out of corners, and is easier to maneuver around, easier to climb and less work riding longer distances you want smaller diameter wheels with lighter tires. Although that is from a physics perspective. A slightly larger 29-inch wheel and tire combo has a slight roll over advantage on rougher terrain, so keep that in mind when choosing a wheel size.
So when would you not care about the added weight to the outermost part of your wheel? Well, if you want durability and stability when going higher speeds, likely downhill. Also, if you ride over a lot of harsh terrain like sharp rocks or other things that can easily puncture or tear a lighter-weight tire, you might not have a choice to go with lighter tires and/or rims. The benefits of more outer rotation mass come into play when you are going at a higher speed and downhill. The wheels will create stability and help the rider keep the bike under control.
So earlier I mentioned that a rider will notice the difference in feel between a 29-inch and 27.5-inch wheel. This is because not only are you reducing the diameter by 1.5 inches, you are reducing the weight of the rim and tire because less material is needed for the smaller size, provided that the same rim and tire brand, type, and size are being used. So not only are you reducing outer rotational mass by reducing the diameter, but you are also reducing the weight of the tire and rim. That is why a rider will notice the difference between wheel sizes. This isn’t saying 29-inch wheels are bad, they just have different advantages, such as the ability to ever so slightly roll over things more easily than a smaller diameter wheel.
In conclusion, whether it matters or not is completely up to the rider to determine. That is often decided based on how a rider likes to ride as well as the type of terrain they are riding on. Ideally, you want the right tire, rim, and wheel setup for the style of riding you are doing and the terrain you will be riding. Some people, if they can afford to, will have 2 sets of wheels they can swap out depending on where they plan to ride. Others even have completely different bike setups. Those people tend to like to joke about needing n+1 bikes. For people who only have one bike, it’s generally best to have it set up for the majority of riding you will be doing and just leave it at that.