August: Gear Ratio Tech

A karter asks, "What is a gear ratio, and why do I care?"

Let's say you are new to a track, and your engine builder or fellow kart racer in your class recommends that you run about a 4.42 ratio. Your next question is, what clutch and gear makes a 4.42 ratio.

4.42= 53 divided by 12. It's just a mathematical equivalent. A ratio is just like a fraction but in decimal form so that it is easier to compare. example... 14T clutch, 56T rear gear. 56 (divided by) 14 = 4 This would be the same ratio as 15T (divided by) 60T = 4. 56/14 = 4, 15/60 = 4
Likewise 12/53 = 4.42. It's just 6th grade math and pre-algebra that many of us took for granted back then. Whoever said algebra was useless? Math is extremely useful in racing! Something you can try for yourself...lets say your gear chart only goes up to 19T, but a buddy of yours has been running a 20T clutch for street races or bigger tracks, (not uncommon). He is running a gear set of 20T clutch, 58T rear gear. You only have an 18T clutch with you.
What do you do?
A.) Pack up and go home cuz you can't buy a 20T clutch driver at the race track.
B.) Put the smallest gear you can find on your kart and hope not to overrev your engine.
C.) Grab a pencil and paper and do the math yourself. -- better yet, use a calculator.

Answer
C.) 58/20 = 2.90 so your buddy is running a 2.90 gear ratio even though it wasn't on your chart. Knowing that you want to run about the same as he,
Y/18 = 2.90 Y/18 X(times) 18 = 2.90 X(times)18.
Y=2.90 X(times)18.
Y=52.2 (pretty close to a 53 tooth gear.)
Therefore, all other things equal, engine, weight, chassis set-up, and driving style...you should just about the same rpm as your buddy with an 18-53 on.


The ratio chart just saves you all this time in doing the math is all.
It's really extremely useful.

The following is a gear ratio chart. You are welcome to copy this and print it out for future reference. A nice idea is to take it to a local copy shop and have it laminated and put it in the lid of your toolbox, or on the wall of your trailer next to your gear selection.

 

OK, so the chart may be somewhat dated, "TEN" years...LOL. We've been doing this for some TWENTY years now, but you see, the math stays the same. A ratio is a ratio.

You'll notice, looking at the chart, that for each tooth on any specific driver, you are approximately equal to four teeth on the rear gear. Keep that in mind when you are thrashing in the pit area to make quick gear changes. 1 tooth on the clutch = 4 teeth on the gear.

You ask, "but then why does karter "A" insist that you must run a 16 tooth driver at his track to be competitive? I have always run a 15 tooth and am looking for that "edge"." Aren't we all? Well, let me tell you this much. I have five years of math and physics from one of the best engineering schools in the country, Purdue University, and I'll argue 'til cows fly, that it just doesn't matter. Only one cow ever flew, and he went clear the heck over the moon! A ratio, is a ratio, is a ratio! If a guy can tell a difference on the track of a 15 tooth driver versus a 16 tooth driver, given the exact same final gear ratio, he's either awfully sharp, or been breathing too much tire prep. Here's the deal. Going from, say, a 12 tooth driver, to a 16 tooth driver, you'll definitely notice a difference. The smaller driver, along with smaller rear gear, shorter chain, etc, will always accellerate quicker than a larger driver / gear combination. Less weight, rotating mass, contact friction, drag, etc. However, a larger driver / gear combo will always out-pull the small gear set-up on top end. This is because the larger driver creates more torque to the rear wheels. Think about things in extremes: Instead of comparing a 15 to 16 tooth driver, consider a 2 tooth to a 400 tooth. The two toother has the chain going "click-click" around it, and is similar to a wrist twisting / screwdriver action. While the 400 tooth driver has the chain travelling a good distance around the crankshaft, almost like a breaker bar or torque arm - consider it like putting all of your forearm into it rather than just your wrist. Much more torque is produced to the rear wheels, and thus the driver will feel a slight difference at the end of the longest straightaway. Notice I say "slight." Some racers will exaggerate this profusely. Again, I dare to say that even the most veteren drivers can't feel the difference of 1 tooth on the driver, given the same ratio. I have what is probably the most accurate dynamometer system for kart engines available, a Davenport $15,000 dyno with full computerized data acquisition, and I cannot tell the difference in one tooth of the driver on it. Yet, I, and many karters with some real seat time will tell you, that you can notice "some" difference on the "big end" of the track as to how the engine is still pulling harder with the bigger driver. It is physics law and it can be debated forever, but the bottom line is this: If it's faster for YOU, then run it. Don't get caught up in what your competitors say, or what worked for the winner last week. Tracks change. Conditions change. Run what yields the very best lap times for your driver.
Words to live by: "A stop watch never lies."

 

 

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bcarlson@CarlsonMotorsports.com