Gear Ratio

[TrialKid32]

I was just wondering, does a 18:12 ratio ride the same as a 24:18? I know it might sound like a stupid question I don't really understand why there is all these ratios you can have.

[Matthew_Gibson]

Easy way to work that out. Divide them

24/18 = 1.3333333333333333333333333

18/12 = 1.5

To youd need either a 13 or 15

the 15 will give you a 1:2 ratio.

make sence??

[TrialKid32]

Not really can you explain it differently please.

[NVWOCI WVS]

[patronising_girly_voice] aaaaah, bless him!... he's only 14, must of not done dividing at school yet! [/patronising_girly_voice]

[TrialKid32]

i get the division bit but how will it give you those ratios

[cai]

i get the division bit but how will it give you those ratios

Well you obviously don't get the dividing bit then!

You divide the front cog by the rear - and the higher the number you get, the higher the gear.

So if you currently run 24/18 which equals 1.333

And you wich to run 18/12 which will equal to 1.5

You'll notice that 1.5 is higher than 1.333, so 18/12 will be a higher/harder gear than 24/18!

Make sense now?

[TrialKid32]

Yes, thanks!

[Netherking]

[patronising_girly_voice] aaaaah, bless him!... he's only 14, must of not done dividing at school yet! [/patronising_girly_voice]

HAHAHA

[Ladd]

I had 24 : 16 at one point felt mint

[sstein]

It doesn't really matter what gear ration you use as long as you feel comfertable using it. I personally use 22:12 or 1.83 gearing and I find this fine, before you choose a gear setup think about these things :

-How far will you ride inbetween spots;

-Can you do the tricks you want easily with the setup you use;

-What do you feel comfertable with.

Never ride with gearing that you are unused to just because someone told you it is right.

[psycholist]

As for what the ratios mean, this line of thinking works for me:

A bike chain has pins 0.5" apart --> every sprocket tooth carries 0.5" of chain.

An 18 tooth sprocket that makes one full turn will move the chain forward 9".

This 9" chain movement must drive the back cog 18 teeth forwards.

If the rear cog is 18T it will make one full turn.

If it's smaller than 18T it will make more than one full turn (The ratio will be greater than 1 - that is one turn of the pedals causes more than one turn of the wheel).

If the driven sprocket is bigger than 18T the sprocket won't make a full turn, giving a gear ratio less than one.

A guideline I've made up for picking a gear (And most trials gears seem to be on or about this figure) is that half a turn of the pedals should move the bike forward between a wheelbase length and a wheelbase plus a half tyre length. This means that starting with your good foot to the back and turning the cranks a half revolution to your preferred pedal position, will either get you from having the front tyre on an edge to being on the back wheel on the edge or from having the front wheel against a vertical rise to being on the back wheel on top of it.

For a 26" bike, wheel circumference is 26(pi)" = 82" (Rounded here, not in my calculation), take wheelbase as 42" (1067mm) + 13" for the front wheel radius, so for the bike to move to the correct spot the wheel has to turn 55/82 of a turn or 0.67 of a turn. This 0.67 of a turn is driven by a 0.5 turn of the cranks, so the gear ratio from this is 0.67/0.5 = 1.347, which is a little on the high side, or for a wheelbase movement 42/82 = 0.51 turns of the wheel = 0.51/0.5 = 1.028, which is a little on the low side.

I'm pretty sure the same rule holds for 20" and 24" bikes too. Are mods usually geared between 1.337 and 1.655 and 24" bikes between 1.114 and 1.432. There are always going to be outliers, but I've always run a gear around 22:17 (1.294) on a freehub and currently use freewheel with 18:14 (1.286) gear, which is a little over the average of the values I've calculated (1.187), but I think I run a slightly higher gear than average anyway. For a 1080 wheelbase 26" bike the average of these figures is 1.2002. This matches surprisingly well with the 18:15 (1.2) ratio a lot of people like...

If you know your wheelbase and wheel diameter I'd be interested to hear whether your gear is within the range calculated anyway - I've never heard anyone else describe trials gear choice in this way, so it'd be nice to see if practice and my theory match .

[forteh]

blah blah blah

Theoretically: -

Using a wheelbase of 42.9" (1090) and 18/16 (1.125:1) a half crank turn gives you 45.95" (1167) forward movement. This falls between the wheelbase (1090) and wheelbase+half (1420)

Using a wheelbase of 42.9" (1090) and 18/15 (1.2:1) a half crank turn gives you 49.01" (1244) forward movement.

You would have to go to 18/13 to give more than a wheel+half forward movement.

18:16 seems alittle light to me but then Ive just come from 22:18 whereby a half crank turn gives you 56.16" (1426) forward movement.

The theory appears to hold up for stock