GEARING UP FOR TIRE CHANGES by Mark Bergfelt
It happens pretty often. The track
that you race on regularly might be one of those that the clay gets baked
by the sun, and hot racing tires, to the point that it is almost as hard as
asphalt. The tires that usually get the hot shots the best lap times
may even be compounded for blacktop as well. Then along comes Mother
Nature to make things interresting. She decides to dump tons of rain
water on the track at intermission. Not enough to totally rain out
the program, but just enough to delay the show so that the promoter and his
crew can run it back in. You know, or at least will shortly find out,
that those harder tires are probably not going to work. The seasoned
racer, at this point, is changing to softer rubber, probably alot softer.
Is that the only change that he needs to make? If the guy knows what
he is doing he may also be making a gear change. Why? The gear
he had might have been perfect. The kart pulled good through and off
of the turns and was fast down the straights. He also knows that if
the new tire has a different circumference, if he does'nt change the gear,
he did anyway. What? He only changed the wheel and tire. How
can that be the same as making a gear change?
When comparing the circumference of two tires, the
larger one will travel a greater distance than the smaller one. The
distance the tire travels, and moves the kart in one revolution, is the circumference
measurement of the tire, that is, if the tire is'nt spinning or slipping.
For this discussion, we will assume that the tire is gripping enough that
the tire is not spinning and all of it's effort is propelling the kart forward
The larger tire is requiring the engine to do more work. My old physics
book reminds me that work = force x distance moved. Your engine makes
the force, we call it torque and measure it at a particular rpm. The
rolling tire determines the distance. If you increase the distance,
you directly increase the amount of work that will be done. If you
were switching to a smaller circumference or diameter tire the opposite is
true. Please note that for this discussion, we are referring to the
tire size as diameter or circumference and not width. Although the
width does affect the grip, it does not determine the distance the tire travels
in one revolution.
The next question that comes up is, well, just how much
of a gear change should a karter make? The most accurate way to calculate
that is to use the following formula;
Previous tire circumference
New tire circumference
______________________
=
___________________
Previous axle sprocket
New axle sprocket
For example, suppose your original tire has a circumference of 35" (you
have to get out your tape measure and measure around the outside of your tire),
and your new one measures 36 1/2 " (36.5"). Suppose
your original axle sprocket had 66 teeth. What sprocket
will you need to wind up with the same overall performance? The
math will look like this;
35"
36.5"
______________ =
____________
66 s
s = what we want to find out, a sprocket size
The next step is to cross multiply so that we end up with;
35s = 36.5 ( 66)
which is the same thing as;
36.5 (66)
s = __________
35
Now just punch the numbers into your calculator and you will discover that we are looking for a 68.8 tooth sprocket. The closet size to that calculation is 69. So that's what you are looking for in your parts box.
WOW! A 1.5 INCH DIFFERENCE IN CIRCUMFERENCE EQUALED A 3 TOOTH SPROCKET CHANGE!
Perhaps a better way to remember the mathematical relationship is;
( new tire circumference) x (old axle sprocket)
new sprocket size = _______________________________________
old tire cumference
From the above you probably have figured out that a tape measure and a calculator should be included in every kart racers tool box.
To summarize the above, if you put on a taller or larger circumference tire you will also need to add teeth to the axle sprocket to stay the same. If you are changing to a smaller diameter, ie., smaller circumference tire, you will need to switch to a smaller axle sprocket.
Perhaps you are fortunate enough to have an extensive collection of mounted tires featuring a variety of tire compounds. There's a good chance that many of them carry the same size marking. Don't be mislead into believing just because they are marked the same, that they all measure the same. Many factors affect the circumference of a tire. Some of these factors include, the rim width and style, tire wear, pressure, what chemicals the tire has been exposed to, temperature, how much the tire has been heated by weather conditions or use and probably a few others. You must use your tape measure.
Most oval track drivers run some stagger from time to time. Usually the right rear will be the larger of the two rear tires and that is the one you should use for gear calculation purposes.
Some of you reading this have probably figured out by now that you can change your tires when you really want to change the gear ratio. I find it much easier to remove three nuts to change a wheel than it is to take off six to change a sprocket. Not to mention loosening the engine to remove, replace and adjust the chain. This is fine if you have the variety of tires needed. If you desire the effect of fewer teeth on the axle sprocket, switch to a larger diameter tire, If you want the effect of more sprocket teeth, install a smaller tire. How much larger or smaller? Do the math! Even if you don't have a warehouse full of wheels, you just may have what you need and did not realize it. Really knowing what you have available can certainly be a valuable asset.
To contact the author, email; bergfelt@verizon.net