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How to Determine the Proper Gear Ratio

How to Determine the Proper Gear Ratio
Category: Comet Technical Karting Blog
Posted: 08-11-2015 16:57

The Comet Kart Technical Blog is back! Today we discuss gear ratio's and how to find the right gear ratio for your track!

Figuring out the proper gear ratio for a track is always a balance between top end, mid-range and bottom end. If you have too big of a sprocket on the back of the kart you might be fast through the tighter parts of the track and off the slower corners but once you hit the longer straightaways you’ll be a sitting duck. It’s like being in 4th gear on the interstate when your competition is in 6th gear. On the other hand if you have too small of a rear sprocket you might be a rocket at the end of the straightaway but you’ll lose way too much off the tighter corners and slower parts of the track to really benefit from the increased maximum mph.

So how do you determine what gear you should run on a given track? Like most things in racing you just have to try different sprockets, see where you lose time (and gain time) and work up an educated guess on the optimum ratio. There are many times where you will be fast in practice with (for example) an 11-80 sprocket combination but come race time you are pulled off the tight corners enough that you have to add a tooth or two so you can keep up with the competition. The amount of space you lose in the tight stuff is too great compared to the amount you gain back on the straightaway. Eventually the competitor builds up an insurmountable lead, you take two steps back in the tight parts of the track for the one step you make up on the straightaway. So in a situation like that you add a tooth (example in this is an 11-81) to minimize the amount of loss in the tighter sections with the hope that only adding one tooth still gives you an advantage on the straightaways.

The above can be reversed where you are fast through the tighter, slower parts of the track, can pull a gap but on the straights you lose too much ground. So in that situation removing a tooth (11-79) might give you the top end you need without sacrificing too much bottom end.

So let’s say you are on an 11-80 and you are getting beat out of the tight corners but you are not anything special on the straights either. Well you find out that are on the same ratio as the fast guy and you are now really stumped on how you can be on the same ratio, getting beat in the tighter parts and not making up time on the straights. Well this is a good time to point out that having the correct ratio is only one part of the equation. There are two other big factors that determine if the gear ratio will work correctly, driver ability and chassis setup.

(For this article we are going to ignore flex length on the exhaust but the rule of thumb on flex is the longer the flex length the more bottom end and the shorter the flex length the higher the top end speed. Flex works alongside gear ratio so you might take a tooth off to increase top end but add a little bit of flex to help the bottom end power and help offset removing a tooth from the kart.)

If the driver isn’t getting through the corners then the ratio is pretty irrelevant. You can have 3 more teeth on than your competitor and still lose off the tight hairpin corner if you over drive the entry or under drive the exit. Same thing with a kart that is too tight. If the kart doesn’t roll off the tight corner very well then you are going to lose time to your competitor every time you turn the steering wheel. Proper corner entry and exit and a kart that rolls free make a huge difference on what gear ratio you can run. If you maximize the karts ability to roll free off the corners you might be able to run a tooth or two less than your competition, carry the same or more speed through the tight corners and be unbeatable on the straights! That is why driver ability and chassis tuning are so important.

I’m of the opinion that you run the proper gear ratio and you work with the driver and the kart until you can run the proper ratio competitively. If the gear ratio for a given track is always an 11-80 for the fast drivers then put on an 11-80 and run that ratio until you are competitive. Adding a couple teeth because your driver isn’t getting through the corners very well or because the kart isn’t rolling through the corners as well as it should isn’t really a very good fix. I try to run the proper ratio and fix the real root of the problem if I can.

So what about gear ratio in the rain? Well as you might suspect more gear when the track is wet or slow can help. If you switch to rain tires and it is a monsoon then adding 3 to 6 teeth isn’t uncommon. As the track dries and the corner speeds increase, running less teeth (and getting closer to a dry track gear) becomes more and more important. What ratio you run in the rain is always an educated guess, the wetter the track the more teeth you add, the drier the track the less teeth you add.

Finally I’ll end this blog with a little controversy in gear ratios. The front sprocket size. Maybe you have heard this before and maybe you haven’t. On a “momentum track” (long straights without too many tight corners or a big oval) run a bigger front sprocket because it creates a flywheel effect and on a tighter track (lots of tight corner and short straights or a very small oval track) run a smaller front sprocket because it pulls harder off the bottom end because of increased leverage. The gear ratio will be almost exactly the same (11-80 = 7.27 ratio) (10-73 = 7.30 ratio) (12-87 = 7.25 ratio) but the mechanical advantages of the front sprocket will help on certain track types. Personally I think “a gear ratio is a gear ratio”. There might be advantages to adding or removing a half a tooth if the 11 tooth and 10 tooth ratio are a little different but I don’t think that any gain you receive will be from the actual front driver size.

Now I do think there is a definite advantage of running a smaller front sprocket on a tight track because it allows you to run a smaller rear sprocket for ground clearance. Sometimes the rear sprocket will become too large and if you drop a wheel off the side of the track you can hit the sprocket and lose the chain. So for example going from an 11-80 sprocket to a 10-73 will give you a much smaller rear sprocket and an increase in ground clearance. That in my opinion is the best reason to run a smaller front sprocket at a tight track. I run a bigger front sprocket on bigger tracks because with the sustained RPM’s on a longer straight you are helping minimize front sprocket wear with more teeth helping out. I’m in the “a gear ratio is a gear ratio” camp but if you disagree that’s fine too.

In conclusion, the gear ratio you run is dependent on many factors including track size, track conditions, driver ability and chassis setup. The best rule of thumb is to run as small a rear sprocket as you can to maximize top speed but without giving away too much on the bottom end. If you can keep up in the tighter sections of the track and driver by the competition on the straights it will make overtaking much easier and make it that much harder for the competition to pass you back. There is a balance between bottom end and top end speed.

Mark Dismore Jr.
Comet Kart Sales

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