Rear sprocket #50 to #48

14 replies to this topic
  • Team_Towrope

Posted April 05, 2001 - 11:54 AM


Does anybody know what top end speed difference there would be from the stock 50 tooth to a 48 tooth? I know it will be faster but how much.

  • Jemtec

Posted April 05, 2001 - 10:43 PM


If I remember correctly, there is a 6% difference in top speed for each tooth removed. (taller ratio..Lower numerically) and visa versa. Lets say you went 100Mph In 5th top speed with a 50 tooth sprocket.. you then change and move to a 49.. your speed will be right about 106 Mph. And if you went to a 51, it would change to 94 Mph.. Etc etc..
If you change your Engine Sprocket, it has a similar effect, but is alot harder on the teeth of the spocket if you move down (1 tooth isnt bad, but anymore is. In particularly if you move down instead of up.) Say from a 14 to a 12.. Oww.
What you do to find out a ratio, is Divide the Rear Sprocket. Example: Lets use the stock Wr 426's Numbers for this.."50" for the rear Sprocket and 14 teeth on the Engine spocket.. Result?. 50/14=3.571. Also..3.571 TURNS of the rear wheel Per engine Revolution . Now, this changes with the transmission gears, but you are working with what they call "final drive Numbers"... so its only the final output of how Fast or how slow Turning of the rear wheel commences (depending on what sprocket you decide to use).
Of course, as you reduce the Spocket teeth on the rear, you will increase speed, but loose bottom end power with each change. And its pretty noticable.
Sorry about the Lesson, but You asked. :)
Hope this helps.

  • samohT

Posted April 06, 2001 - 02:24 AM


It seems like its you, Jemtec, who need a lesson. :)
(I'm doing my best Rondo Talbot mpersonation.)

Youre saying "Also..3.571 TURNS of the rear wheel Per engine Revolution." is not correct.
If the engines primary and gear ratio gave a 1:1 reduction, then 3.571 turns of the engine would turn the wheel 1 turn, not the other way around.

To calculate the change in speed when going from 50 to 42 rear sprocket you take the difference, 2, and divide with what you had in the first place, 50, to get the change in percent. 2/50 = 0.04 = 4%. So at any gear and any rpm you will be going 4% faster than before.
If you change the front sprocket one tooth you will get: 1/14 = 0.0714 = 7,14%

European 2000 WR400

[This message has been edited by samohT (edited 04-06-2001).]

  • Dan_Lorenze

Posted April 06, 2001 - 06:32 AM


Dax, Don't kill yourself man! You desert guys are all the same! :) Im running stock gearing and I can't imagine wanting to go any faster out there, I bend rims as it is. Are you gearing up for Pahrump or Tonopah? If you do change gearing let me know what you did ok? See Ya, Dan

  • Clark_Mason

Posted April 06, 2001 - 06:57 AM


You guys have got your percentages messed up it goes as follows:

For every thooth change on the rear from the stock 50T sprocket there will be a 2% change in the overall gear ratio. So a 48 will be 4% taller and 52 4% shorter.

For evey tooth change on the countershaft sprocket there will be and approx 7% change in the overall gearing. So a 13T will be 7% lower and a 15T 7% taller.

On my WZ 420 I will run one of the following gear ratios depending on conditions:

mostly tight trails jeep roads and single track 13/49--5% lower than stock

Dual sporting with mainly dirt roads and very little trail 15/49---9% taller than stock

A mix between the two above 14/49--2% taller than stock--This gearing is my normal gear setup. I use this most of the time. When I head for the Sierras I go the the 13/49 which is 5% lower than stock 14/50.


  • Ron_in_SoCal

Posted April 06, 2001 - 07:27 AM


I believe the "general rule of thumb" of gearing is one tooth on the countershaft sprocket equals three teeth on the rear sprocket, ie one tooth larger on the c/s sprocket is approximately the same as three teeth smaller on the rear sprocket. This roughly coincides with Clarks percenages.

I've run a 15/50 combo in a hare scrambles and cleened up because of it, and tried the same in an enduro and it killed me. Big mistake on my part (actually I was too lazy to change it).

Most of the riding I do is a combination of twisty single track and jeep/fire roads that connect the single track. I think I'm happy with the stock 14/50. 14/49 may be in my future...

[This message has been edited by Ron in SoCal (edited 04-06-2001).]

  • Jemtec

Posted April 06, 2001 - 10:36 PM


I thought I might have missed something..
The 3.571 ratio is based on Final Drive gearing which is based on a 1/1 ratio as you mentioned..but, because the transmission can change ratios, wouldnt that mean 1/1 ratio or 1.00 to 1? Since 4th gear is this on the WR that means that it turns the same number of rotations as the Engine in rpm.. Correct?... Well at .840 to 1 in 5th gear, that means that the engine is no longer turning as fast as the Speed of the Countershaft Sprocket which is now moving faster than engine Rpm. Thats where I moved to Final Drive.. That means, it takes that Rpm..and divides that speed by the number of sprocket teeth on the Primary..and the Number on the Secondary sprockets. As I mentioned 3.571. That means that the ratio has been altered to your mentioned 3.571 turns of the Sprocket to make 3.571 turns at the Primary spocket to complete 1 full wheel revolution, and that I agree I made an error. :)
However, transmission gearing has equally as much to do with speed as the Sprocket size, so in essance Im also wrong in saying that the "average" would be 6%.. Therefore..all of us here are basically right. The determination could be done by taking each gear.. determining its maximum allowable Rpm Mechanically and then taking its turning Rpm and dividing it with the Final drive Gearing.
A better example would be a Truck with a rear end of 3.571 Now, where Im thinking I might have messed up is that On ALL trucks(and rear wheel drive cars that are almost non-existant now).. this ratio is based on the turn of 1 revolution of the Driveline on the rearend and 3.571 TURNS at the Rear Wheel. A ratio of 4.57 to 1. (notice I said to 1 which is one turn) would be 1 more full revolution of the rear Wheel. And it would be pretty low geared. Why? Because it will produce less maximum speed because it will take alot more rpm out of the driveline (or on a bike,the primary sprocket) and the maximum engine rpm will be reached much sooner.
So that being said, My divison was correct to find Final drive ratio. However, adding into account the speeds of the transmissions output through each gear.. that might cause a few percentage points to vary from bike to bike. Alot of Shop guys said its around 5 to 6 %. per tooth. But maybe they ment Primary change?..since the primary is the the "driveline" so to speak and rpm there is more sensitive to changes to drive the rear sprocket. I think the rear would realistically be more like 3-4%.
I cant believe Im talking about this so much.. :D
I honestly think all of you are right..but yet..we are all wrong too.
(unquestionably since we dont design these gear ratios). Anyway its still fun to talk about.

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  • Bruce_in_Phoenix

Posted April 07, 2001 - 09:17 PM


99 WR ran 87mph with stock 14/50 gearing on pavement.
Ran 99mph with 15/48 gearing on pavement and 89mph on a gravel road.
I would imagine 14/48 would get you in the neighborhood of 92 or 93 on pavement.

My 2001 seems to have taller stock internal ratios than the 99 did. I have not had my GPS out with the new bike yet. Waiting to get to 300 miles before I top end it on pavement. Ooo take that back I did top end it already but I got hammered by a 2000 with a pipe on it and stock gears. :)

I loved 15/48 gearing on that thing, first was still low enough for the slow rocky switchbacks and 5th well lets just say it went real well at 80+ mph for a long time without being on the limiter or near it.

  • Hick

Posted April 08, 2001 - 03:29 PM



You type pretty fast :)

Anyway, you are forgetting the primary reduction, so, even though 5th is overdriven (countershaft spins faster than main shaft) that doesn't mean it is spinning faster than the motor. On most (all?) cars of course the input shaft will spin at crank speed (more or less, torque converter and clutch slippage notwithstanding).

I'm at a friends house or I'd post the primary ratio, it may also be in the manual, just the number of teeth on the main drive gear divided by the number of teeth on the clutch gear, which is much larger. I have a hard time calculating top speed for a dirt bike, I get some obviously wrong numbers using the tire size as given.

One of these days I'll measure a dang tire (mounted of course) and then I'll know how fast I've been going all this time :D

  • Taffy

Posted April 09, 2001 - 01:25 AM



your not the first to say that the new bike 'feels' taller geared. the big piston lugs harder & you probably change gear earlier.

the gearing hasn't changed.


  • Jemtec

Posted April 09, 2001 - 09:41 PM


Yeah that would be cool hick.
Id be curious myself. All I know right now is that the Wr is "supposed" to go 103 Mph for 2001. (US model). And as fast as ive taken the bike.. I would seriously believe it.
I for some reason didnt take that into additional Account.. The Wr Uses a 18 inch rear wheel instead of the YZ 19 inch. That would mean (unless the tire height averages out between the skinnier 19 tire height and the 18s higher tire height. That the wheel on the Wr Spins faster than the 19.
From what I remember in reading magazines, alot of times they dont write "correct Speeds" of the Bikes because of insurance claims! So, If they say the bike only runs say "90 Mph".. and it really does "100 Mph".. would they be covering it up?.. I guess in a sense yes. I thought that was pretty odd that they did that, but Its America, what else is new? :)
Anyway, It would be interesting to calculate real wheel Speed in each gear.
Thanks Hick

  • Hick

Posted April 10, 2001 - 06:16 AM



Yeah, that is what I get when I calculate 21/25 (.840) 5th gear @ 11,500 rpm w/ 110/100-18 tire & 14/50 final gearing (equals 102.98 mph). I didn’t think it really went that fast.

That is with using 62/21 as the primary ratio. I can’t remember where I got that, somebody probably posted it here, I’ve never actually counted the teeth.

So a YZ w/ stock tire and gearing I get 93.19 mph. Does my bike really go that fast? I thought I saw a top speed of like 88 mph published in Dirt Bike once. At 11,000 rpm my formula returns 89.14 mph. Plus I’m thinking the tires may actually be LARGER than the given size, so I don’t trust my formula.

Can anybody verify the primary ratio? Anybody ever measure a knobby? Does a WR really do a hundred?

Questions for the ages, I’m sure…

BTW, as I’m calculating it, the stock WR tire (110/100-18) is 26.66 inches, YZ tire (110/90-19) is 26.80 inches.

(Aspect ratio x size x 2) + rim diameter = tot. diameter

Speed in mph at 11,500 rpm in each gear then is:
1 – 35.80
2 – 49.91
3 – 65.91
4 – 82.39
5 – 102.98

YZFM (new ratios in ’00)
1 – 48.06 :)
2 – 59.14
3 – 69.42
4 – 81.32
5 – 93.19

The ratios I’m using indicate YZ first is a tad shorter than WR second. That is why I wonder about guys who buy these things for the dez, first gear is truly tractor-like on the WR (although a WR 5th would be a nice swap for my YZ). Life is full of compromises, I guess…

  • Team_Towrope

Posted April 10, 2001 - 06:34 AM


Thanks everyone! I love this place. I don't know how many times I've got the "uhh, I don't know" answer from my local bike shop. Thanks again. I may have to do something really geeky and right a program to calculate gear ratios and speeds someday.

My wife likes your answer the best. I don't think she understands that there's always room for more speed in the desert. :) You also missed a great race in Pahrump. It rained about 140 miles out of 280 on me. It was awesome. There was no dust, a little mud, but lots of traction. My pit crew wasn't real happy about it but they still had fun. There making me ride Tonopah wether I have the money or not -- so hopefully I'll see you there!



  • Team_Towrope

Posted April 10, 2001 - 06:39 AM


Looks like Hick beat me to it. Hick must be a faster geek then me. :)

  • Team_Towrope

Posted April 10, 2001 - 05:58 PM


OK...I've topped Hick's geekness. I wrote a program that calculates your
speed at a particular RPM for each gear. Here's a screen shot of the
program. If anybody is interested in getting it then let me know and I'll post a link to
the install exe.


Posted Image

Also, let me know if you think my math is wrong. I think I grasped the concept but I could be wrong.

??? I can't seem to get this image to work. I'll figure out a different way so atleast you all can see it.

[This message has been edited by Team Towrope (edited 04-11-2001).]


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