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We Can Prove Why Extra Mass on Bike Wheels Is Your Worst Enemy

2020-07-13

Serious cyclists (and some who aren't so serious) obsess over every ounce on their bike. Yes, a lighter bike can save you some energy. And a rule of thumb states that mass on the wheel is like twice the mass on the frame.

But why? Let's look at some possible effects of extra mass on a bike.

Air Resistance
To ride at a constant velocity, the net force must be zero. Of course there is the gravitational force pulling down and the road pushing up. But there also is air drag force pushing against the direction of motion. You feel this same when you stick your hand out of a car window. The faster you go, the greater the force.

What happens to air drag force if I add mass to my bike? If the added mass doesn't change the shape or cross sectional area of the bike, the force remains the same. Air drag force doesn't depend upon the mass of the object. But what if the extra mass pushes down on the tires, causing the tires to stick out more and cause more air resistance? OK, that's technically possible-but it wouldn't matter if the mass was on the bike frame or the wheel.

Actually, in terms of air drag, extra mass can help. Suppose you have two bikes that look the same but have different masses. If they are traveling the same speed, they will have the same air resistance force on them. However, this force will produce a greater change in speed on the bike with less mass. Don't forget that a net force is equal to the produce of mass and acceleration. Same force but different masses means different accelerations.


cycling


Rolling Friction
If you get a bike going and coast, the bike slows and then stops. This would occur even without air resistance. As a wheel rests, the part of the tire touching the ground compresses and deforms. When the wheel turns, the section of tire being compressed changes. The constant compression and relaxation requires energy. This is called rolling friction.

What if you put a mass on the frame? The tire is compressed further, resulting in greater rolling friction. How about if you put the mass on the tire, not the frame? The same thing happens, but you could argue that the effect isn't as great. If the mass is evenly distributed around the circumference of the wheel, then a part of this mass will be at the contact point and not really push down on the bike. This might be true, but the effect would be tiny.

Internal Friction
When you pedal, you're working against friction in the wheel bearings, friction in the bottom bracket, friction in the chain and through the cogs and chainrings. This reduces your efficiency. But what about adding mass? More mass on a bike can increase the friction in the bearings-but again, this won't matter where the mass is located.


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