Allen’s Tech Tips, “Calculating Gearing”

Mtn. Muncher

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Allen’s Tech Tips, “Calculating Gearing”

Playing with the gearing on your snow bike is a vey common modification that a lot of guys do. It’s been hard to set a standard for measuring gearing on a snow bike in comparison to the tires or to another snow bike. For me, I do not go off a calculated gear ratio. I measure the track inches of roll out to one revolution of the engine sprocket. This will show you how far you would travel with one revolution of the engine counter shaft sprocket. This does not compare transmission gearing. It is used only to set a standard for gearing snow bikes, so that we can all go off of the same thing when talking shop tech.


How to do this:
  1. Mark a line on a tooth of the engine sprocket and on the engine case that are in line with each other.
  2. Mark a line on the hifax and on the track that are also in line with each other.
  3. Rotate the track forward while looking at the engine sprocket. Stop when you get to 1 revolution.
  4. Measure the distance between the line on the track and the line on the hifax.<O:p</O:p
This is your gearing and now you can compare to other setups and other snow bikes or dirt bikes. I keep notes for myself so that I can compare apples to apples on different bikes and different kits that might have different driver sizes that would affect the gear.<O:p</O:p



For example:<O:p</O:p
Stock gearing on the ST Mtn. Horse kit using a 2.86 pitch 7 tooth driver with a 13 (engine) -17 (jack shaft) / 17-20 (chain case) = 13-1/8” of track roll-out.
<O:p</O:p
<O:p</O:p


Please post up the performance advantages of the gear setup you run along with the track roll out measurement and bike model. Also note how tight or loose your chain was and if you had to modify the length from the stock MH chain length.
 
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mtn-doo

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That makes a great baseline. I didn't measure roll-out on the KX500 "monster". The math was 14/17/16/17 to produce a .036 final. I wish I had that roll out measurement for folks to reference. Great topic. A lot can be gained through gearing. I shoot for 3rd and the "sweet spot". Max track speed just below max RPM. Gearing slightly up or down to produce that "sweet spot" will maximize your bikes potential. Gear to YOUR motor, YOUR bike, not other people. Rider weight, riding style, etc, will change for each person. Even riding the same make and model of bike. They all have a different "sweet spot", max track speed in the gear they can pull, for your riding style. I feel 3rd is a great gear to "dial in" the sweet spot. 2nd is a technical fall back gear, and 4th and 5th are for well,.... you know,...
 
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Nov 23, 2012
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I've been running a 2.86 pitch 137 by 16" Ripsaw with 1,25" luggs and a cross country with 1,75" luggs on my Super Enduro 950R. The Ripsaw had a rather high but still working 16-20/17-17 gearing wich gave a 17.8" roll out. It never reached the revstop and got up to just over 80mph. When I put the Cross country track on I decided to try a lower 16-20/15/17 gearing wich gave a 15.8" roll out and a top speed of 76 mph easily reaching the revstop. This season I will try something in between on my new "ufo/TEDEsled/Hans Hjort Evo 2" kit. I'll be driving a 2.52 pitch 120 by 16" cross country with 1,75" luggs with a 16-21/21-21 gearing resulting in just under 17" of roll out and a top speed of about 80mph. If it hits the revstop too easily I can allways try a 17 tooth sprocket on the engine witch should give an probably unreachable top speed of about 86mph :) :face-icon-small-ton


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tomk

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I am running a 98 kx500 with a 14-20-20-21 an I have a 16.25" roll out
 

jacobdewey

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I'm going to try a 14t on the front of my bike. Anyone know the roll out for a 14-17-17-20?

edit:
If my calculations are anywhere near accurate, (I wouldn't bet on it) It should add an inch of roll out. (14 1/8") roughly.
 
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Nov 6, 2012
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I've got a slightly different approach to optimizing my gearing.

I start with determining my max track speed in each gear. For my bike (stock 2013 KTM 500 XCW) I know that on the rev limiter in 6th gear I'm doing about 90km/h (max speed from my GPS). Knowing this and assuming my track is not spinning (or very little) on the trail I can determine my max track speed in everygear.

I start with the assumption that in the powder the ideal track speed is the fastest I can spin the track with the power I have. With my bike and stock timbersled gearing (13/17,17/20) I can pretty much spin the track at all times in 3rd gear and often can't keep up in 4th gear. This tells me I really want a track speed somewhere between my current 3rd and 4th gear speed. I going to try 14/20,17/17.

Attached is a spreadsheet to use if you're a geek like me. Easiest way to use it is only change the cells highlighted in yellow.
 

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tomk

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I am running a 98 kx500 with a 14-20-20-21 an I have a 16.25" roll out
At 1150 ft elevation here in mn (flat lander) on the trail I can top out 5th but in a foot of fresh snow it's working pretty hard in 4th to keep speed
 

Ryanexcr

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Ktm 500EXC 121 track 2013 kit, 12 tooth on motor 17/20/17 on kit, 16-5/8 inchs of rollout on track, I like this setup the best so far, 3rd and 4th are my sweet spot. But after first ride I tightened the chain and now it hits the track it worked fine on my 2011 kit. GHhhrrrrrr
 
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mtn-doo

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Ktm 500EXC 121 track 2013 kit, 12 tooth on motor 17/20/17 on kit, 16-5/8 inchs of rollout on track, I like this setup the best so far, 3rd and 4th are my sweet spot. But after first ride I tightened the chain and now it hits the track it worked fine on my 2011 kit. GHhhrrrrrr
Needs a 13T drive sproket. Chain length will be correct. Your "sweet spot" gears will be 2nd and 3rd. Then 5th will have some road speed.
 

Mtn. Muncher

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Allen what are you running on the 501's and how do you like it so far as gearing goes??? Thanks.
I run the stock
13/17/17/20.
I think it perfect for the extreme mountain climbing I like to do.
However if I was doing more trails and meadows I would run this.
14/16/17/20
I will probably run this when I turbo the 501.
 

Ryanexcr

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I run the stock
13/17/17/20.
I think it perfect for the extreme mountain climbing I like to do.
However if I was doing more trails and meadows I would run this.
14/16/17/20
I will probably run this when I turbo the 501.
I am going to try the stock setup 13/17/17/20 it was way to low geared on my 530excr that's how I ended up with the 12/17/20/17 it runs great all mountain and ran nice with the old kit but now it rubs the track. I ran the 14/16/17/20 on my 530 and it was ok but not perfect. I think it is just this goofy wide ratio tranny on the EXC that I'm dealing with. I will probably put a 14/17/17/20 after another ride or 2 it should fit. I do like blue might have to go track one down :peace: Thanks
 
Jan 14, 2004
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I just tried both 14 17 and 15 17 on my 08 KTM and you can't do either of those without a half link in your chain.

M5
 
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