gear ratios

Bryan Mappe · Posted: Thu Oct 25, 2001 3:30 am Post subject: gear ratios

"newbie" question just to verify what I am thinking
We have only raced on a short, tight track and are going to a longer track with less corners and longer straights.
To keep the max rpms the same which direction (no. of teeth / gear diameter) do I need to go with the gearing on my son's yamaha jr. sportman.
He currently has a 10/84.

Jimmy Moore · Joined: 26 Jul 2001 Posts: 664

You would go bigger on the engine gear (recomended) or smaller on the axle gear.

Bob Chiras · Joined: 30 Jul 2001 Posts: 198

.

Lets assume you are targeting a venue of a ¼ mile. The fast lap at the last event in your class was 16.5 seconds for a Briggs.

Your motor is at peak horsepower and peak torque at 5600 RPM's.

OK what is the gear?

The answer is simple. Convert the ¼ mile to feet. ¼ mile is 1320 feet, divide
the distance or 1320 feet by the desired time 16.5 seconds the result is 80.00
feet per second times 60 equals 4800 feet per minute. We next have to measure
our rear tires. Lets assume we have 600 X 5 tires and the diameter is 35 1/4
inches. Multiply the 4800 feet per minute times 12 so that we can get inches
per minute. Our result is 57,600 inches per minute. We divide 57,699 by our tire
diameter 35.25 inches and we get 1.634.04 that is the average RPM. Now divide
the optimum RPM range of the motor (5600) by 1,634.04 and the preferred gear
ratio is 3.43. That should get you in the ballpark.

Feet per mile / .25 miles = feet per ¼ mile
5280/.25= 1320

feet per ¼ mile / desired lap time = feet per second.
1320 /16.5 =80

Feet per second * 60 = feet per minute * 12 = inches per minute.
80 * 60 = 4800 * 12 = 57600

Inches per minute / tire diameter = average rpm.
57,600 / 35.25 = 1634.04

Average RPM / desired engine RPM =gear ratio
1634.04 / 5600 = 3.43

Now get your gear chart and select a gear combination for the kart. A 17 driver and a 59 rear gear could be a starting point

Roger Miller · Posted: Thu Oct 25, 2001 9:28 pm Post subject: gear ratios

Bob, you use the words tire *diameter* several times. You are speaking of circumference in your calculation.

Also, Bryan is working with a Yamaha, Junior Sportsman, probably a 3 hole can, so is probably looking at something near of above 8.5K for peak torgue and something higher than that for peak HP and generally shooting for the mid to hi 13s at the end of the longest straight.

So, if I want to be somewhere around there, I might use 12.5 or 13 as the target rpm and finsh the equations.

Using 13, (btw, this ratio needs to be inverted)
Desired RPM / "average" rpm = gear ratio
13/1634.04 = 7.95

(I quote average as a yamaha can't run at 1700 rpm but it is a math thing only)

And, for a jr. sportsman, we generally run something less that 8:1, so this is a reasonable result.

We usually run an 11 tooth front, and on the tightest tracks have run as big as 87 or 88 which is right at 8:1 and usually something in the more mid to lower 80s.

That's at 280 pounds.

Keith Archambeau · Posted: Thu Oct 25, 2001 10:54 pm Post subject: gear ratios

Bryan, the above formula will work if you know the distance of the longest straight, and the corner exit speed onto that straight.
The easiest way is to ask another competitor
in your class. This should get you close. If you donot have a mycron or other brand. I strongly recomend one. Makes gear & tuning a whole lot easier.
RACE!

Bob Chiras · Joined: 30 Jul 2001 Posts: 198

sorry I was in a big rush circumferance is correct.