Allen’s Tech Tips, “Calculating Gearing”

Mar 9, 2017
stillwater MN
This was a great thread and i was sorry to see it die. With that said here is my dilema. yz450fx with 17 timbersled, first is useless, second seems to low for boon docking and cant quite pull third in the pow at elevation. I would like to set up second gear as my sweet spot boon docking gear. Currently running 13-17 and 17-17 If you look at the chart of the wide ratio fx vs the more narrow f second on the f is right in between second and third on the fx. I feel like that is where i need to be. Could someone with a f reverse engineer there role out so I could get close to the f??? I think this would work?? Put the bike in second gear, role out 1 revolution on the counter sprocket and give me the number on the track role. I thought a 15-17 would be really close but from my quick test looks like I would need to add a half link to the chain to get that to work. Thinking of trying 14-17 and going to 18-16 in chain case. Yes 18 on top . Problem there is 14 sprocket and 17 chain is very close to frame which I think was disused above. The chart I found that compares the f to the fx says it is a 8 tooth difference on the rear sprocket (DIRT WHEELS) between the two bikes. Hope I am making sense. For those of you with a f do you feel like second is your sweet spot or would you like to change it one way or the other??


Well-known member
Premium Member
Nov 29, 2007
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,...
Im looking at your math , I dont get a finale ratio of .036 at all , your finale ratio is 1.289. care to explain how ewe derived at this ?
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