Correct gear ratio
02-28-2005, 02:01 AM
#1
Senior Member
Thread Starter
Join Date: Apr 2004
Location: Sacred Heart,
MN
Posts: 114
Likes: 0
Received 0 Likes
on
0 Posts
Correct gear ratio
I read a different thread telling how to figure out the correct gear ratios to use and understood that pretty well. But when I figured out what I was using its making me a little bit confused. One part of the mathematics was to take the diameter of the tires and multiply by pi. So help me if im wrong but i have the Proline Dirt Hawgs and they measured 4 inches tall so I took 4 x 3.14 and got 12.56.
Then i took 84/15 and got 5.6. Looking up the specs on my rustler I think the transmission ratio is 2.72 so I multiplied 5.6x2.72 and got 15.232, so 12.56 - 15.232 is -2.672. which means I am undergeared. But I have read other places that you should use a pinion gear with 2 less teeth then there are turns in the motor. I was running this with a 17x3 motor but now am switching to a Jade 15x motor. But with the 17 if i went bigger with the pinion the motor would run burning hot.
Sooo. does someone know if Im doing some numbers wrong somewhere or does the fact that Im using the Rustler make a big difference because I know the more race worthy rc transmissions are probally better and have a smaller ratio.
Really need sum help on this one. Thanks!
Then i took 84/15 and got 5.6. Looking up the specs on my rustler I think the transmission ratio is 2.72 so I multiplied 5.6x2.72 and got 15.232, so 12.56 - 15.232 is -2.672. which means I am undergeared. But I have read other places that you should use a pinion gear with 2 less teeth then there are turns in the motor. I was running this with a 17x3 motor but now am switching to a Jade 15x motor. But with the 17 if i went bigger with the pinion the motor would run burning hot.
Sooo. does someone know if Im doing some numbers wrong somewhere or does the fact that Im using the Rustler make a big difference because I know the more race worthy rc transmissions are probally better and have a smaller ratio.
Really need sum help on this one. Thanks!
02-28-2005, 08:46 AM
#2
Senior Member
Join Date: Oct 2004
Location: Liberty,
MO
Posts: 3,172
Likes: 0
Received 0 Likes
on
0 Posts
RE: Correct gear ratio
The old rules of thumb don't always work - For instance, if you have a 6 turn motor, you'll have a hard time finding a 4 tooth pinion! 
I'd try a larger pinion, and check for overheating. As long as the esc and motor run cool, that's what you need.
I'd try a larger pinion, and check for overheating. As long as the esc and motor run cool, that's what you need.
02-28-2005, 09:46 AM
#3
Senior Member
Join Date: May 2004
Location: Pittsburgh, PA
Posts: 2,093
Likes: 0
Received 0 Likes
on
0 Posts
RE: Correct gear ratio
The correct starting point to gear your truck is to have your motor turn XX number of times, for 1 revolution of the tire. In your case, with the 84/15, your motor is turning 15.232 times for every revolution of the tire. At 1.21:1, this ratio is undergeared. It will allow for really fast acceleration, and a greater multiplication of torque, but limits the top rpm of the tires, and thus cuts your top end. It is a desireable setup for short track or loose track racing, where acceleration is the key to winning.
The best balance of top speed to acceleration is to gear the motor as close to the diameter of the tires as possible, in otherwords X number of revolutions to 1 revolution of the tires. This is regardless of the motor, to a point. RTR motors do not gernerate anywhere near the RPM of modded, or even 27t stock motor. Therefore, they are deliberately undergeared to compensate.
The dirt hawgs that you are running are almost 1" taller than the stock tire that came on your rusty. Therefore, the ideal starting point for your truck would be 84/17. However, since you are running a larger tire, you will see better torque and acceleration by replacing the 84t spur with an 87t. Your optimum gear ratio would be 87/18. You will lose an MPH or two, but it won't be noticeable. The motor should not 'run hot' as long as you keep the overall load to a minumum. Overgearing the truck is what causes heat, and thermal related problems, especially wear.
The best balance of top speed to acceleration is to gear the motor as close to the diameter of the tires as possible, in otherwords X number of revolutions to 1 revolution of the tires. This is regardless of the motor, to a point. RTR motors do not gernerate anywhere near the RPM of modded, or even 27t stock motor. Therefore, they are deliberately undergeared to compensate.
The dirt hawgs that you are running are almost 1" taller than the stock tire that came on your rusty. Therefore, the ideal starting point for your truck would be 84/17. However, since you are running a larger tire, you will see better torque and acceleration by replacing the 84t spur with an 87t. Your optimum gear ratio would be 87/18. You will lose an MPH or two, but it won't be noticeable. The motor should not 'run hot' as long as you keep the overall load to a minumum. Overgearing the truck is what causes heat, and thermal related problems, especially wear.
