Gear Ratio Question

[grasshopper]

What does the number represent for a gear ratio?

For example, with a 38 rear and a 13 front the gear ratio is 2.92.

You can get damn close with different sprocket combinations.

For example a 44 rear and a 15 front gives me a 2.93 gear ratio.

What does the 2.92 and 2.93 #'s mean?

Thanks ALot

I also understand that going up one in the front is like going down 2.5 or 3 in the rear and going up 3 in the back is like going down 1 in the front, I'm just curious from an engineers stand point what the # means.

[tzracer]

I prefer gearing numbers as rear sprocket/front sprocket (some do it the opposite way). The 2.93 tells you how many times the front sprocket rotates for one rotation of the rear sprocket. A larger number is shorter gearing, shorter gearing means more rpms for a given speed. Shorter gearing gives better acceleration, but less top speed. Tall gearing does just the opposite. How big of a change is noticeable depends upon the bike. Gearing for an RC51 is not quite as critical as for a TZ125.

I use gearing charts to pick a ratio and change wheelbase. For my TZs I have 4 front sprockets and 6 rear sprockets, as you noted, some combinations overlap (same or close to same ratio), these can be used to alter gear ratio and wheelbase. I tend to experiment quite a bit with gearing.

Things get a bit more complicated when you also have different internal gears from which to choose.

[Jeff]

It means the front sprocket must make 2.92 revolutions to equate to 1 revolution of the rear sprocket.

As for different combos equaling almost the same ratio, remember, you have a chain on the bike.  Different combos may not fit that chain.  And it can/will dramatically affect wheelbase which will affect handling as well.

[grasshopper]

Thanks for the explanation, that helps me.

How does changing the gear ratio alter the wheelbase.  ??? Pease explain.

[grasshopper]

It means the front sprocket must make 2.92 revolutions to equate to 1 revolution of the rear sprocket.

As for different combos equaling almost the same ratio, remember, you have a chain on the bike.  Different combos may not fit that chain.  And it can/will dramatically affect wheelbase which will affect handling as well.

AH HA!!! So if you put a larger sprocket on the rear the chain brings the rear wheel closer to the front causing a shorter wheel base.

I get it!  ;)

But, how big of a difference can it possibly make on a SV650. I know you don't have much to play with when it comes to moving the rear wheel foward or back. How big of a difference does an inch or two make?

[Jeff]

It can play a HUGE difference.  The biggest difference you will notice is turning.  A short wheel base will turn quicker (too short will be very twitchy), and a longer wheelbase will seem to be more difficult but smoother to turn.

For Road A, I try to have a longer wheelbase than any other track.

[roadracer162]

"How does changing the gear ratio alter the wheelbase.  ??? Pease explain."

I currently have a 15/42 sprocket combination yielding a 2.8  ratio. I wanted a 16/45 sprocket combination to gain more clearance between the chain and the swingarm pivot(1/4") The 16/45 yields a 2.81 ratio. With the 16/45 sprockets it caused the rear wheel to move forward by approximately 1/2", but I didn't have enough chain length to allow the fitment of the different sprockets.

When you ride a motorcycle that has limited horespower there are times that the gearing won't affect the top speed. Case in point, my stock GS1100 back in the 80's had 15/45 sprockets, I changed to 16/43 sprockets and the result was a slower top speed with nothing else done. It did however allow me to run a better 1/4 mile time due to the one less shift needed. I would match the ET of the then current GSXR1100.

[/quote]

[Eric Kelcher]

Also many bikes if you set up the rear end geometry to ideal then you have a radical downslope to the swingarm and need a larger front sprocket to keep chain from riding on the swingarm.