Bike Tech

Crank Size

Crank Sizing Chart
Riders Inseam
<=19"
20"
21"
22"
23"
24"
25"
Crank Length
135mm
140mm
145mm
150mm
155mm
160mm
162mm
Riders Inseam
26"
27"
28 to 29"
30 "
31"
32"
34"
Crank Length
166mm
171mm
175mm
180mm
182mm
185mm
190mm
Note: Everyone is different, inseam length as well as riding style should be considered. If your hips sway side to side and you zigzag while pedaling, your cranks are to long. Running too short of cranks do not give you enough leg extension, therefore not enough power.

Gearing

What gear should you run? Only you can answer that question because everyone is different. Typically BMX racers run a 53-55 inch rollout. However, a few younger smaller riders may run a 51-52 inch rollout. Likewise, a few older stronger riders may run a 56-57 inch rollout. Riders with faster legs and shorter cranks tend to run a lower gear. Riders that have stronger legs and longer cranks tend to run a higher gear. Shorter cranks have less leverage but take less distance to make a full revolution where as longer cranks have more leverage but take more distance to make a full revolution. If you are spinning out down the first straightaway and everyone is pulling away from you, you may need to take your gear up or pedal faster. If you can never get a good spin going at all and everyone is pulling you down the first half of every straight, you may need to gear down or pedal harder.

To calculate your rollout here is the formula:
(Sprocket / Freewheel * Tire Diameter = Rollout)
Example: 44 / 16 * 19.5 = 53.625

Approximate True Tire diameter
20" X 1.1/8" = 20 inches inflated 20" X 1.3/8" = 21 inches inflated
20" X 1.50" = 19.25 inches inflated 20" X 1.75" = 19.5 inches inflated
24" X 1.1/8" = 22.62 inches inflated 24" X 1.3/8" = 24 inches inflated
24" X 1.50" = 23 inches inflated 24" X 1.75" = 24 inches inflated
Note: Tire brands will vary in inflated diameter. Your "True" outside tire diameter might be slightly larger or smaller than the tire diameter used in the gear charts below!
To find you true inflated tire diameter: First measure around the rear inflated tire to find the circumference with a cloth tape measurer then divide it by Pi.
(Tire Circumference / 3.14159 = True Tire Diameter)
 

Gear Charts

Gear Chart for 20" X 1 1/8" wheels
Chainwheel 34 35 36 37 38 39  40 41 42 43 44 45 46 47 48
Rear 14 48.6 50.0 51.4 52.8 54.3 55.7 57.7 58.6 60.0 61.4 62.8 64.3 65.7 67.1 68.6
Rear 15 45.3 46.7 48.0 49.3 50.7 52.0 53.3 54.7 56.0 57.3 58.7 60.0 61.3 62.7 64.0
Rear 16 42.5 43.8 45.5 46.2 47.5 48.7 50.0 51.2 52.5 53.7 55.0 56.2 57.5 58.7 60.0
Rear 17 40.0 41.2 42.3 43.5 44.7 45.9 47.0 48.2 49.4 50.6 51.8 52.9 54.1 55.3 56.5
Rear 18  37.7 38.8 40.0 41.1 42.2 43.3 44.4 45.6 46.7 47.8 48.9 50.0 51.1 52.2 53.3
Gear Chart for 20" X 1 3/8" wheels
Chainwheel 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48
Rear 14 51.0 52.5 54.0 55.5 57.0 58.5 60.0 61.5 63.0 64.5 66.0 67.5 69.0 70.5 72.0
Rear 15 47.6 49.0 50.0 51.8 53.2 54.6 56.0 57.4 58.8 60.2 61.6 63.0 64.4 65.8 67.2
Rear 16 44.6 45.9 47.3 48.6 49.9 51.2 52.5 53.9 55.2 56.5 57.8 59.1 60.4 61.7 63.0
Rear 17 42.0 43.2 44.5 45.7 47.0 48.2 49.4 50.7 51.9 53.2 54.4 55.6 56.9 58.1 59.3
Rear 18 39.7 40.7 42.0 43.2 44.4 45.5 46.7 47.9 49.0 50.2 51.4 52.5 53.7 54.9 56.0
Gear Chart for 20" X 1.50" wheels
Chainwheel 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48
Rear 14 46.8 48.1 49.5 50.9 52.3 53.6 55.5 56.4 57.5 59.1 60.5 61.8 63.2 64.6 66.0
Rear 15 43.6 44.9 46.2 47.4 48.8 50.1 51.3 52.6 53.9 55.2 56.5 57.7 59.1 60.2 61.6
Rear 16 40.9 42.1 43.3 44.5 45.7 46.9 48.1 49.3 50.5 51.7 52.9 54.1 55.3 56.5 57.7
Rear 17 38.5 39.6 40.8 41.9 43.1 44.1 45.2 46.4 47.5 48.7 49.8 51.0 52.2 53.1 54.3
Rear 18 36.4 37.4 38.5 39.6 40.6 41.7 42.8 43.9 44.9 46.0 47.0 48.1 49.2 50.3 51.3
Gear Chart for 20" X 1.75" wheels
Chainwheel 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48
Rear 14 47.4 48.7 50.2 51.6 53.0 54.4 55.7 57.1 58.5 59.9 61.3 62.7 64.1 65.5 66.9
Rear 15 44.3 45.4 46.8 48.1 49.4 50.7 52.0 53.3 54.6 55.9 57.2 58.5 59.8 61.1 62.4
Rear 16 41.5 42.7 43.9 45.1 46.3 47.5 48.8 50.0 51.2 52.4 53.7 54.9 56.1 57.3 58.5
Rear 17 39.0 40.2 41.3 42.5 43.6 44.7 45.9 47.0 48.2 49.4 50.5 51.7 52.8 53.9 55.1
Rear 18 36.8 37.8 39.0 40.1 41.2 42.3 43.3 44.4 45.5 46.6 47.7 48.8 49.9 51.0 52.0
Gear Chart for 24" X 1.1/8" wheels
Chainwheel 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48
Rear 14 54.9 56.5 58.1 59.7 61.3 63.1 64.7 66.3 67.9 69.4 71.0 72.6 74.4 76.0 77.5
Rear 15 51.3 52.8 54.3 55.8 57.3 58.8 60.3 61.8 63.3 64.9 66.3 67.9 69.4 70.8 72.4 
Rear 16 48.1 49.5 50.9 52.3 53.7 55.1 56.6 57.9 59.5 60.8 62.2 63.9 65.1 66.5 67.9
Rear 17 45.2 46.6 47.9 49.3 50.6 51.9 53.2 54.6 55.9 57.2 58.6 59.9 61.3 62.4 63.8
Rear 18 42.7 43.9 45.2 46.6 47.7 49.1 50.3 51.5 52.8 54.0 55.3 56.6 57.9 59.0 60.4
Gear Chart for 24" X 1.3/8" wheels
Chainwheel 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48
Rear 14 58.3 60.0 61.7 63.3 65.0 66.9 68.6 70.3 72.0 73.7 75.3 77.0 78.9 80.6 82.3
Rear 15 54.4 56.0 57.6 59.3 60.7 62.4 64.1 65.5 67.2 68.9 70.3 72.0 73.7 75.1 76.8
Rear 16 51.1 52.5 54.0 55.5 57.0 58.5 60.0 61.4 63.1 64.5 66.6 67.4 69.1 70.5 72.0
Rear 17 48.0 49.4 50.8 52.2 53.6 55.1 56.5 57.9 59.3 60.7 62.1 63.6 65.0 66.2 67.7
Rear 18 45.3 46.5 48.0 49.4 50.7 52.0 53.3 54.7 56.0 57.3 58.5 60.0 61.4 62.6 64.1
Gear Chart for 24" X 1.50" wheels
Chainwheel 34   35  36 37 38 39 40 41 42 43 44 45 46 47 48
Rear 14 55.9 57.5 59.1 60.7 62.3 64.2 65.8 67.4 69.0 70.6 72.2 73.8 75.7 77.3 78.9
Rear 15 52.1 53.7 55.2 56.7 58.2 59.8 61.3 62.8 64.4 65.9 67.4 69.0 70.6 72.0 73.6
Rear 16 48.8 50.3 51.8 53.2 54.6 56.0 57.5 58.9 60.3 61.8 63.2 64.6 66.2 67.6 69.0
Rear 17  46.0 47.3 48.7 50.0 51.4 52.8 54.1 55.5 56.8 58.8 59.5 60.8 62.3 63.5 64.8
Rear 18 43.4 44.7 46.0 47.2 48.5 49.8 51.1 52.4 53.7 54.9 56.2 57.5 58.9 60.0 61.4
Gear Chart for 24" X 1.75" wheels
Chainwheel 34 35 36  37 38 39  40 41 42  43 44  45 46 47  48
Rear 14  58.3 60.0 61.7 63.3 65.0 66.9 68.6 70.3 72.0 73.7 75.3 77.0 78.9 80.6 82.3
Rear 15 54.4 56.0 57.6 59.3 60.7 62.4 64.1 65.5 67.2 68.9 70.3 72.0 73.7 75.1 76.8
Rear 16 51.0 52.5 54.0 55.5 57.0 58.5 60.0 61.4 63.1 64.5 66.0 67.4 69.1 70.5 72.0
Rear 17 48.0 49.4 50.8 52.2 53.6 55.1 56.5 57.9 59.3 60.7 62.1 63.6 65.0 66.2 67.7
Rear 18 45.3 46.5 48.0 49.4 50.7 52.0 53.3 54.7 56.0 57.3 58.5 60.0 61.4 62.6 64.1