By Tod Spaude
Of TS Racing, Inc.

One of the most asked questions on gearing over the phone or from our walk-in customers is the 9T drum vs. the 10T drum question. It usually goes like this; "My buddy George uses a 10T drum and he beats me by half a lap. I need a 10T drum instead of my 9T drum." So we here at the shop get some scrap paper and a gear chart and proceed to explain that George's gear of 10-70 is the same as his 9-63. Sometimes this is easily seen other times it's difficult.
Divided out, 70 / 10 = 7.0 or (7.0 to 1) and 63 / 9 = 7.0 or (7.0 to 1) are the same, clear and simple. Old math or new math, slide rule or calculator, it's still 7.0 to 1.
To continue, 7.0 to 1 is referring to 1 axle revolution to 7 revolutions of the crank. This represents a 7.0:1 gear ratio. Rule #1: Gear ratios, no matter how you arrive at them, are the same. The front sprocket with a 9 tooth or 10 tooth will still rotate 7 times with the above rear gears. There are slight exceptions to this rule #1, which we will get into later. This also applies to 4 cycle and enduro racing as well, a gear ratio is a gear ratio. Example: for a 4 cycle, 12-60 = 5.0 to 1, this is the same gear ratio as 15-75. In regards to, George had a 10 tooth drum and Fred had a 9T drum, both with a 35 pitch chain. If each had the same gear ratios (10-70 and 9-63, respectively), both would be rotating the axle at the same rpm. Notice I said axle. Think axle, not tire for now. Consider this: George, using his 10-70 gear, comes off the corner much faster than Fred, but Fred gathers up the distance lost at the end of the straight. Let's assume for the article all is equal except for one thing- can you guess? Tire size in circumference. Measure it with a thin bladed (1/4") tape measure, so the blade is wrapped snug against the outside of the tire. (photo A)
George comes off the corner faster because he has 34" circumference tires, and Fred is faster down the straight because he has 36" tires. So how many rear sprocket teeth does it take for Fred to add to equal George's IPR? Wait, what is IPR? Well, let me explain. First, we never had to worry much about tire sizes changing in the old days. When I say "old days", I mean before soft, sticky tires. The circumferences of our old Goodyears ("Fred Flintstone" tires, because they were so hard) very seldom changed sizes between one tire to the next. Remember, also that there weren't many different sizes or compounds. Generally you only bought new tires because you were going to the Nationals to race.
Now tire size and circumference between manufacturers, compounds, widths, etc. varies dramatically. We had to come up with a way to relate each other's gear ratio and tire circumference together. IPR translates to Inches Per Revolution, which means 1 revolution of the motor = "x" number of inches the rear tire will move the kart forward.
Example: George has 34" tires and 10-70 gear, and Fred has 36" tires and a 9-63 gear.
George's IPR = 34 x 10 / 70 = 4.857 " traveled forward.
Fred's IPR = 36 x 9 / 63 = 5.143" traveled forward.
Keep in mind gear ratio on both George's and Fred's karts are the same. So how many teeth does Fred need to add to equal George's IPR? Let's do some algebra.
Replace Fred's 63 with George's IPR.
It should look like this: 36 x 9 / 4.857 = 66.7. Fred would need to change to either a 66 or 67 rear sprocket.
So remember the next time you ask your buddy's gear, make it a point to ask him about his tire circumference also. And be ready for him to tell you the gear ratio and not his tire size. So carry your pocket ruler with you, so you can measure his rear tire.
Reprinted with permission from TS Racing catalog 3

About the author:

Tod Spaude is President of TS Racing, one of the leading kart shops in America. He has built over 80 National and World Championship engines. He may be contacted at the TS Racing web site TSRacing.com