Rotax Gear Ratio

ok so from what i know, you can combine the front sprocket and the rear sprocket to give you a gear ratio… i have heard that as the front sprocket increase (11 to 12) this is equivalent to a 6 tooth increase on the rear. so my question is if i have multiple rear sprockets at your disposal, do you ever need to change the engine sprocket? if i am at a track with a 12/78 and someone else is using a 11/72 is this the same? if i stick on the 12 engine sprocket and buy every rear sprocket from 70 - 90 say… do i ever need to change the engine sprocket??

and clarification appreciated…

Hi yes you are right in that when you raise the front sprocket by one tooth it is equivalent to running the lower sprocket with around 6-7 teeth more. You can punch in google kart gearing sprocket calculator and charts will come up showing what is equivalent to what. Once the gear ratio is equivalent then there really isn’t any performance benefit to running one over than the other…the benefit might be more a clearance of rear sprocket to track thing (the smaller rear will give more clearance for curbs etc).

As for you question on if you ever need to change, the short answer is yes. It is all dependent on the track, chassis setup, engine limiter and much more. The shorter you go on the gearing (going bigger with the sprocket axle and leaving the engine sprocket unchanged or going down on the engine sprocket and leaving the axle sprocket unchanged) you will gain acceleration potential but at the expense of top speed. The converse is true if you do the opposite. How do you know which you need to be on? You gotta test it but gearing is something that is also driver dependent. What i mean is, the best drivers will always be geared longer than novice drivers, meaning they can still roll the same or more speed than the novice guys in the slow bits but then get a huge advantage in straightline speed vs the shorter geared guys. This is where chassis setup comes in as well. If you dial the kart in to roll through the tight stuff really good, you can go longer on the gearing and gain straightline speed potential. Hope I didn’t confuse you too much

You’ll probably run the same front driver gear at almost every track, so I wouldn’t worry about that one too much. Having a big selection of rear gears will cover you for most situations, but like Chris said, sometimes you may want to change the front gear if you end up getting really large on the rear gear and risk bottoming out.

Going to a variety of tracks might require such significantly different gearing that you might need to change the driver to get the right ratio.

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From a gearing point of view it is the ratio, 'axle sprocket
teeth/engine sprocket teeth ‘which is important.
In your example 78/12 is a ratio of 6.50 -1…84/11 is a ratio of 7.64-1. Vastly different.
To keep the same or similar ratio with a smaller engine sprocket you need less teeth on the axle sprocket as well so to keep the 6.50-1 ratio needs 6.50x11=71 teeth at the axle.
Although 6t axle sprocket to 1t engine sprocket is a valid equivalence for a Rotax or similar , it varies with the gear ratio.
On my engine of choice ( Briggs v twin) gear ratio around 3.0-1. equivalence 3 to1.
At the other end of the scale a screamin’ 100cc might be runni
ng 10-1gearing on a tight track, equivalence 10 t axle sprocket= 1t engine sprocket.

Just read this ratio chart and decide for yourself.