GEAR CHARTS AND GEARING

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GEAR CHARTS AND GEARING

Postby mauriceandgrandad » 17 Jun 2011, 10:07

Here are gear charts for the most popular wheel sizes

These charts represent 'gear inches' ie the distance travelled with one revolution of the chainwheel.


Gear Chart for 20" X 1.75" wheels
Chainwheel....34----35---36---37---38----39---40----41---42---43----44---45----46---47
Rear 14........47.4-48.7-50.2-51.6-53.0-54.4-55.7-57.1-58.5-59.9-61.9-62.7-64.9-65.5
Rear 15........44.3-45.4-46.0-48.1-49.4-50.7-52.0-53.3-54.6-55.9-57.2-58.5-59.8-61.1
Rear 16........41.5-52.7-43.9-45.1-46.3-47.5-48.8-50.0-51.2-52.4-53.7-54.9-56.1-57.3
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
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

Gear Chart for 20" X 1.50" wheels
Chainwheel....34 -- 35 -- 36 --37 --38 -- 39 -- 40 --41 -- 42 --43 -- 44 --45 -- 46 --47
Rear 14........46.8 48.1 49.5 50.9 52.3 53.6 55.0 56.4 57.5 59.1 60.5 61.8 63.2 64.6
Rear 15........43.6 44.9 46.2 47.4 48.8 50.1 51.3 52.8 53.9 55.5 56.5 57.7 59.1 60.2
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
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
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

Gear Chart for 20" X 1.3/8" wheels
Chainwheel...34 .. 35 ..36 .. 37 ..38 .. 39 ..40 .. 41 ..42 .. 43 ..44 .. 45 ..46 .. 47
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
Rear 15........47.6 49.0 50.4 51.6 53.2 54.6 56.0 57.4 58.8 60.2 61.6 63.0 64.4 65.8
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
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
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

Gear Chart for 20" X 1.1/8" wheels
Chainwheel...34 .. 35 ..36 .. 37 ..38 .. 39 ..40 .. 41 ..42 .. 43 ..44 ..45 .. 46 ..47
Rear 14.......48.6 50.0 51.4 52.8 54.3 55.7 57.1 58.6 60.0 61.4 62.8 64.3 65.7 67.1
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
Rear 16.......42.5 43.8 45.0 46.2 47.5 48.7 50.0 51.2 52.5 53.7 55.0 56.2 57.5 58.7
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
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

Gear Chart for 24" X 1.75" wheels
Chainwheel...34 .. 35 ..36 .. 37 ..38 .. 39 ..40 .. 41 ..42 .. 43 ..44 .. 45 ..46 .. 47
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
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
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
Rear 17.......48.0 49.4 50.0 52.2 53.6 55.1 56.5 57.9 59.3 60.7 62.1 63.6 65.0 66.2
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

Gear Chart for 24" X 1.50" wheels
Chainwheel....34 ..35 .. 36 ..37 .. 38 ..39 .. 40 ..41 .. 42 ..43 .. 44 .. 45 ..46 .. 47
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
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
Rear 16........48.4 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
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
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

Gear Chart for 24" X 1.3/8" wheels
Chainwheel....34 ..35 .. 36 ..37 .. 38 ..39 .. 40 ..41 .. 42 .. 43 ..44 .. 45 ..46 .. 47
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
Rear 15........54.4 56.0 57.6 59.3 60.7 62.4 64.1 65.5 67.2 68.9 71.3 72.0 73.7 75.1
Rear 16........51.1 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
Rear 17........48.0 49.4 50.8 52.2 53.6 55.1 56.5 57.9 59.3 60.7 62.1 63.3 65.0 66.2
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

Gear Chart for 24" X 1.1/8" wheels
Chainwheel....34 ..35 .. 36 ..37 .. 38 ..39 .. 40 ..41 .. 42 ..43 .. 44 ..45 .. 46 .. 47
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
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
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
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
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


It is possible to do it very accurately, by measuring an individual wheel, but these charts are good enough for most purposes.

This is for information only. I am not competent to suggest which is best for a particular person or situation, but here are some pointers; (thanks to Rich Wielkiewicz)

One topic that is sure to get a lively discussion going at any track is gearing. Some racing families set the gear at the beginning of the racing season and never change it. Others, will try two or three different gears on the same day looking for the perfect set-up. Some racing familes even disguise the markings on their gears so others can't tell what gears they are using. This will be a simplified lesson in gearing that will get you started.

Let's start with the back gear or freewheel. Most racing bikes should begin with a 15 or usually 16-tooth freewheel and not change it, although it's possible to have a wider range of gearing available by having a "flip-flop" rear wheel that has two freewheels that can be changed by simply turning the back wheel around. I think the top riders shy away from the extra weight so you don't see these in use very much.

Thus, keeping the back wheel at 16-teeth means that the gear changes are made by changing the front gear or chainring .

If the rider struggles to get over the obstacles and there are lots of them on the track, switch to a lower gear (fewer teeth).

If the rider gets over the obstacles but loses ground to other riders in the straights, switch to a higher gear (more teeth).

A lower gear means the top speed of the rider is slower, but it is easier to get going.

A higher gear means that the top speed of the rider is faster, but they can have difficulty getting going.

The accepted wisdom seems to be that you have 16 teeth in the rear and be ready to switch among 36, 37, 38, 39 for the little ones and 42, 43, 44, 45, and maybe 46 teeth chainwheels for older riders. Never use a new gear for the first time at the track. Do a practice gear change at home to make sure the chain length and dropout (the slot where the rear wheel slips in) have enough room. You should consider having more than one chain set up for different chainrings (use zip ties and key tags because all chains look alike when you're in a hurry). For instance, going from 40/16 to 37/16 is two links on a chain, and you could run out of slot on the dropout. Imagine taking links out of a chain in a field in the rain, no fun. Modern half link chains make it easier to make accurate adjustments. Always carrry a couple of split links, they could save a race!

Changing the gear usually requires adjusting the tyre, shifting the brake pads, and sometimes relengthening the chain. Many racing bikes also have chain tensioners that hold the rear wheel in place. Naturally the chain tensioner will need adjustment when the gear is changed.

Most of the time a range of three gears is adequate to cover most track conditions.

Obviously as young children grow and gain strength and skills, they will be able to push progressively bigger gears before spinning out.

If you want to try changing gears, either set it up at home or practice until you can get the change done in about 15 minutes or less. You don't want to be working on the gear change when your racer should be in the pen!

There are times when a gear change can make all the difference in the world. Thus, I believe it is one of the skills that a newbie BMX mechanic should master. A gear change can be just what is needed for a big race on a strange track.
Last edited by mauriceandgrandad on 28 Jun 2011, 16:24, edited 2 times in total.
If it ain't broken, keep fixing it until it is!
http://www.bournemouthbmx.co.uk/
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Re: GEAR CHARTS AND GEARING

Postby rrb41 » 30 Jun 2011, 11:45

Some more gear ratios for those looking for smaller rear sprockets

http://www.profileracing.com/media/manu ... ergear.pdf
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Re: GEAR CHARTS AND GEARING

Postby elevensies » 15 Apr 2012, 15:29

The profile gear chart is for 24" wheels
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Re: GEAR CHARTS AND GEARING

Postby incey » 14 Jun 2013, 15:13

The only accurate way to measure your gearing is to measure the rollout of your tyre inflated at normal race pressure, convert that to diameter multiply by front chainring and divide by rear sprocket.

Gear charts will always only give you approximations.

The difference between my 1.5 dth tyre @ 80psi on a 1.5" rim and the same tyre on a 1.75" rim was 52.3 vs 51.6. Doesn't sound like much til you are riding. Most gear charts don't go into enough detail. Once you start using the above calculation you need to stick with it otherwise it will get really confusing. The charts can still be used for determining next up and down gear combos. I use excel formulas to make it all easy.

Also different tyres rollout to different lengths, some charts acknowledge this..some don't

To calculate rollout i run the tyre through a thin strip of water and measure the middle of the marks left on the concrete, at least twice. There a probably better ways but this works for me. remember to use inches or convert mm to inches

The full calculation is rollout (circumference) divided by 3.1416 (PI) {this give diameter} multiplied by front divided by back.

Low ratios 51,52 etc are good for getting out of the gate, recovering from incidents, and sneaking pedals... long gearing >54 is good for long pedalling sections

Hope this helped
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