Gear ratio for dummies

[haggard 2hundie]

Can someone dumb down the basics of gear ratio numbers ?what exactly do the numbers mean ? example my first gear ratio is 3.455 internal revolutions to one external,? does that refer to the sprockets one revolution ?
also primary reduction is 3.087 : 1
and final reduction is 2.293
what does all this mean and why do gears 4 5 and 6 only say 1.450 , 1.227 , and 1.083 without the : 1
thanks for your help.

[WilliamJ]

Hi, I'm new to trikes and this forum but I have done a lot of transmission work over the years (cars not bikes) and the principles are the same, so I'll try to give you a sensible answer.........but I'll have to work it through.

Looking at the Tri Z manual, the primary reduction is from the crank to the clutch. There is a small gear on the crank end and very big one on the clutch and that should give the 3.087:1 (This may be 23 : 71 teeth)

The clutch is mounted on the end of the mainshaft which is therefore turning at about one third of engine speed.

The other ratios on your post look sensible and are the individual gear ratios within the transmission. (But I have used the Tri Z ones in the calcs below)

First gear on the Tri-Z is 14:30 teeth (or 2.14:1) so the countershaft will be turning at about one sixth of engine speed. The drive sprocket is on the end of the countershaft.

The overall ratio is 6.606 : 1 - Engine to drive sprocket in 1st gear.

Your primary ratio doesn't change so to get the second gear ratio you multiply Primary (3.087) by Second (1.75 on a Tri-Z 5 spd) and it gives you 5.04:1 at the sprocket.

3rd is 3.087 x 1.44 = 4.45:1
4th is 3.087 x 1.2 = 3.7:1
5th is 3.087 x 1.0 = 3.087:1

Of course, there's a further reduction to to the rear wheels. The drive sprocket in the manual is 13 teeth . I don't know how many teeth the big rear sprockets generally have, but lets say this one has 39 to make the maths easy. The rear wheel will then turn at a third of the transmission drive sprocket. Multiply the above figures by 3.

So by the time you get to the rear axle, for every 1000 engine rpm, the rear wheels will turn:-
1st - 1000/19.8 = 50.5 revolutions
2nd -1000/15.12 = 66.1
3rd - 1000/13.35 = 74.9
4th - 1000/11.1 = 90.1
5th - 1000/9.26 = 108

Now you work out your speed. This gets a bit boring but stick with it.

With an 18 inch rear tyre we'll assume a rolling circumference of 18 x 3.142 = 56.56 inches
Lets just do top gear ...... it's getting late over here.
The tyre rotates 108 times a minute at 1000 engine rpm = 108 x 56.56" = 6,108 inches or 509 feet.
Multiply by 60 to get hours (for MPH) and you get 30,540 feet an hour / 5280 = 5.784 mph (at 1000 rpm)

So at 10,000 rpm in top gear you will be doing 58 mph with 18" tyres. Does this sound about right?

If you want I can work the speeds in all gears for you - what do you need to know?

Bigger rear tyres = higher potential top speed (but less acceleration)
Smaller rear sprocket = higher potential top speed

Bill

[haggard 2hundie]

wow thats alot of good info that pretty much covers it thanks man

[Grizzlypeg]

Great explanation William. Thanks.