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Old 01-20-2009, 12:06 PM   #1
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Default 1/12 pan car quick questions.

1. What gear ratio should be used on 4 cells and 27t motor if I have only 100t spur?
2. The same only with usage of 5 cells.
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Old 01-20-2009, 12:41 PM   #2
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Quick answer: anywhere from a 30 toa 35 depending on the motor, tire size, and layout.

Long answer: You'd do better to think in terms of rollout than gear ratio for a foam tire car. The tires get smaller every run, the gear ratio needs to change to keep the car the same.

Rollout is the distance the car moves forward with one revolution of the motor. The good thing about rollout is that the tire size figures into the calculation.

The formula for rollout is:
rollout = (tire diameter*3.14) * (pinion/spur)

Once you're liking the way a particular motor is running in the car, figure the rollout and write it down. Next time you put some new tires on the car, use the following formula to determine what pinion to use to get that same rollout.

The formula to get pinion size is:
pinion = (rollout * spur) / (tire diameter*3.14)

For the Trinity Monster series stock motors (green can) I liked to run around a 41 to 43mm rollout. For the CO27 (blue can) I was running around 43 to 45mm.
Hope that helps some.
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Old 01-20-2009, 12:48 PM   #3
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Also depending on what car you have 100T may be too big of a spur to fit the proper pinion along with it. Most people are running a smaller spur like 88T for stock and 19T classes.
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Old 01-20-2009, 02:19 PM   #4
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I`ll fit some pinion with 100t spur, it`s handmade and has quite wide range of motor movement positions. The car is still in progress of making, everything will be like on a normal 1/12 but made out by myself.
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Old 01-20-2009, 02:24 PM   #5
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Sounds like a cool project. Post some pictures when you have it ready...
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Old 01-20-2009, 02:25 PM   #6
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BTW. Can someone please give me width for 1/12 with tires on, rear and front?
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Old 01-20-2009, 02:37 PM   #7
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typicaly 172mm rear, 164mm front
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Old 01-20-2009, 03:21 PM   #8
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172mm is the maximum legal width. most people are running at or near the maximum in the rear, a bit less up front.

My Gen X is set up at 170mm in back, 166 up front.
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Old 01-20-2009, 05:31 PM   #9
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Quote:
Originally Posted by Logika View Post
1. What gear ratio should be used on 4 cells and 27t motor if I have only 100t spur?
2. The same only with usage of 5 cells.
No one uses 5 cells in 1/12.
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Old 01-20-2009, 09:31 PM   #10
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Quote:
Originally Posted by Trips View Post
172mm is the maximum legal width. most people are running at or near the maximum in the rear, a bit less up front.

My Gen X is set up at 170mm in back, 166 up front.
and you do this w/ axle shims behind the wheels and hubs?
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Old 01-20-2009, 10:10 PM   #11
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Yes, with shims between the hubs and bearings.
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Old 01-20-2009, 10:33 PM   #12
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BMI makes a real nice machined 3mm spacer
http://www.bmiracing.com/webstore/ca...7294576100679d
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Old 03-05-2009, 07:52 AM   #13
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Ok, the handmade car is done, have yet to run it. I`ll post some pictures tomorrow.
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