Sprocket selection?

[KawiRider]

I now have a 13 front with a 50 rear. I ride trails and race hare scrambles. I want my lower gears to be taller. I was thinking about either the 13/47, 13/48, or the 13/49 combo. Does anyone have experience with these combos? If you have any advice, I would really appreciate it.
-Thanks

 

[marcusgunby]

A 13/47 combo is a big change if you get into tight situations it may be too much-i would try a 13/48.

 

[KawiRider]

 

[KawiRider]

I ride a 6 speed YZ 125. Would the 13/48 combo still work good? Will it be good in the tight stuff and still be able to open it up on the fast stuff?
-Thanks

 

[marcusgunby]

The should will easily pull 13/49 but it all depends on how much low speed responce you need compared to balls out speed in the open.1 tooth on the back will only make a small difference-do you want a small difference or a large one-its down to your needs.

 

[KawiRider]

Thanks Marcus

Thanks for the reply. So your saying that 13/49 will have taller gears? Or do you think I should get the 13/48. I mean that race is won in the woods, so I need a mostly woods gearing. I need the taller gears for 1st, 2nd, and 3rd. What do you think is the best combo for what I am looking for?
-Thanks

 

[snaggleXR4]

Hey,
In my limited experience, changing one tooth on the rear makes a quite noticeable difference. On my old XR200, I geared it down one tooth on the rear, along with airbox/pipe/jetting mods which did help with throttle response, I went from bogging in the sand in 3rd, to pulling all the way through to 5th and 6th after the gearing change. A friend of mine, with an RM125 also geared down one tooth for the tight woods we ride, and he also noticed quite a difference. Neither bike lost much at all on top end, just gained alot in the bottom and midrange.

Ideally, you would only change your front sprocket, they are cheap, and easy to swap back and forth for quick and easy gearing changes for different tracks, trails etc. However, one tooth on the front =~3 on the rear. Knowing your current gearing ratio, calculate a ratio using your current 13T and a 14T front sprocket with some unknown rear. You should be able to find a rear sprocket, that when using either the 13 or 14 front will give you gearing slightly lower and slightly higher than what you are currently running. Granted, you might still have to buy a new rear ( if you think the 14/50 will be too tall) and an extra front but it will be worth while afterwards when you can just swap out the front for a quick gearing change. Good luck.

 

[marcusgunby]

snaggle sums it up well-if you want a noticable change 1 tooth on the rear may be right,but say you found you were tapped out in top gear in the open sections in a few hundred yards then 2 teeth would be the best option.Its not possible for me to say whats best for you,i would try one tooth off the rear 1st as a 125 may struggle in the tight stuff.

 

[KawiRider]

So your saying buy a 48 or 49 rear tooth sprocket?
-Thanks

 

[marcusgunby]

buy both and try both and see which one YOU prefer.

 

[MX-727]

Like Marcus said, try both. Buy two of the inexpensive steel ones and after you decide, buy the brand/quality of sprocket you want to run.

 

[Pantaz]

Sprocket change calculations

Changing only one or two teeth on the rear sprocket makes very small changes.
Here are some numbers for a 1987 YZ125, assuming 10,000 rpm, and a 24" rear tire diameter:

Sprockets = MPH in 1st, 2nd, 3rd, 4th, 5th, 6th gears
50/11 = 18 mph, 24 mph, 29 mph, 35 mph, 40 mph, 44 mph
50/12 = 20 mph, 26 mph, 32 mph, 39 mph, 44 mph, 48 mph
50/13 = 22 mph, 28 mph, 35 mph, 42 mph, 47 mph, 52 mph
49/13 = 22 mph, 28 mph, 36 mph, 43 mph, 48 mph, 53 mph
48/13 = 22 mph, 29 mph, 36 mph, 44 mph, 49 mph, 54 mph
47/13 = 23 mph, 30 mph, 37 mph, 44 mph, 50 mph, 56 mph
50/15 = 25 mph, 32 mph, 40 mph, 48 mph, 54 mph, 60 mph
47/15 = 26 mph, 34 mph, 43 mph, 51 mph, 58 mph, 64 mph

As you can see, countershaft sprockets changes are much more significant.

FORMULA FOR CALCULATING MPH @ ENGINE RPM

mph = ((rpm * tire_diam) / (primary_ratio * gear_ratio * drive_ratio * 336.1352398))

Note: Rounding off the constant to "336" makes things easier and does not significantly change the output.

Example:
Engine RPM = 10,000
Tire diameter = 24 inches
Primary transmission ratio = 3.333:1
Transmission gear ratio = 2.583:1
Drive ratio (50 tooth rear sprocket / 13 tooth front sprocket) = 3.5:1

((10000 * 24)/(3.333 * 2.583 * 3.846 * 336)) = 21.6 mph

I have this already plugged into a spreadsheet. Just enter your bikes gear ratios (should be in your manual), sprockets, and tire size.
You can download it from <http://home.pacbell.net/pantaz/motorcycles/index.html>

 

[KawiRider]

That is a good idea to buy both the 48 and 49 rear sprocket, but I don't have enough money to buy both. If I could I would. But, which would you think I would like better in the woods?
-Thanks

 

[marcusgunby]

Round and round it goes -where it stops nobody knows.

 

[Camstyn]

You get get both sizes in steel for close to the same price as one of them in aluminum.

 

[a454elk]

IMO, I say it's easier to clutch it a bit more and still have the top end. If you go to a larger rear sproket, you're options are a bit less. You have good pull down low but you max out fast. My .02cents
Elk

 

[KDX220rm]

Here is a good ratio chart.

http://www.renthal.com/resources/ratios.htm