Why do the little ones have 20 x 1-1/8 tires with 20 x 1-3/8 rims on their bikes? The common answers are, “smaller”, “lighter” and “less rotational mass”. Smaller tire, less weight, easier the pedal, Right?

Maybe!

This may have been true in the past but not anymore. When the smallest tire you could get for a 406mm 20” rim was 1.75” and when it weighed 400 grams or more putting a narrow 370 gram 451mm tire on your kids bike made sense. With lightweight 406mm tires and rims available today the 451mm option isn’t as much of an advantage any more.

I did some calculations to figure out the rotational resistance of two tire combinations. The first was a Sun CR18 20×1.75 rim with a Tioga Powerblock 20×1.4 tire. The other was a Sun CR18 20×1-3/8 rim with a Tioga Powerblock 20×1-1/8 tire. I didn’t include spokes and hubs because they would’nt do much to affect the results.

The weight of the standard BMX 406mm combination was 630g and the weight of the mini/jr BMX 451mm combination was 650g. Using the formula for moment of inertia below, it takes 32% more torque (power) to start the mini/jr combination rolling. It sounds huge, but really it’s not much of a difference.

The point is, the figures show exactly the opposite of what is expected! The 406mm combination only needs 85.43 inch/pounds of torque to start it rolling and the 451mm combination needs 112.77 inch/pounds of torque.

Not really worth changing your setup for but you never know, 406mm rims may give your little one a quicker jump out of the gate

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Combo 1 (Standard)

Sun CR18 20 1.75 rim / Tioga Powerblock 20×1.4

mass of tire and rim: tire 280g + rim 350g =630g
Diameter of tire bead: 406mm
Moment of inertia (force to start movement): 25.9g/m2 or 85.43 inch/pounds of torque

Combo 2 (mini/Jr.)

Sun CR18 20 1-3/8 rim / Tioga Powerblock 20×1-1/8

mass of tire and rim: tire 280g + rim 370g =650g
Diameter of tire bead: 451mm
Moment of inertia (force to start movement):33g/m2 or 112.77 inch/pounds of torque

Formula Used:

Moment of inertia = M * D2
Where,
M = Mass of the Rim and Tire
D = Distance between axle and tire bead (half the diameter of the tire bead)

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