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Old 01-13-2004, 02:29 AM   #1
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rollout what is it

I have a 12L3 and I am running a monster stock with 4 cells the motor has 20,000rpm
I am using a 72t 48p spur 1.93 rear tires,what is a good starting pinion. the track I run on has 22 feet straights and a lot of curves.
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Old 01-13-2004, 03:00 AM   #2
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Rollout is how far your car moves with every revolution of the motor. The formula is,

Tire diameter x 3.14 / Overall gear ratio(with a 1/12th scale it's just spur/pinion)

Sorry, not sure what pinion you should use.
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Old 01-13-2004, 03:58 AM   #3
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I would run a rollout in the high 30s. With a spur that small it might be tough to get there.

I would try 18 to 20

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Old 01-13-2004, 06:50 AM   #4
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thank you that is a start so what spur would you use or think is better for me.
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Old 01-13-2004, 09:31 AM   #5
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Just be careful with 'rollout'... you need to keep track of your units. None were given above in the formula or the recommendation of 'high 30's'. Rollout has to have units, or it makes no sense (unlike 'ratio', which is unitless). Saying a 'rollout of 30' is like saying '30 long', which doesn't mean anything unless you say '30 feet long' or '30 inches long' or '30 mm long'.

The recommendation of 'high 30's' is probably 'distance in mm that your car should travel per motor revolution'. The measurement in the original question was taken in inches, not mm, so a conversion needs to be made in the tire diameter (from inch to mm) or the recommendation has to change to inches traveled per motor revolution.

I actually don't like using 'distance per motor rev' for my rollout calculations. I find it confusing that 'rollout' (when using these units) gets smaller as ratio (gennerally given as spur/pinion) gets bigger.

Instead, I like to use "number of motor revs per distance" (and you can choose your distance). At least that way, higher numbers mean the same thing (less top speed, more run time) whether you are talking 'ratio' or 'rollout'. I use 'number of motor revs per 10 inch traveled.' That gives me 1/12th scale 'ratios' of around 5:1 and rollout numbers around 7 or so, and both get bigger for more run time and smaller for less run time.

Just want to make sure that any new guys reading about rollout don't get confused when their numbers don't make sense.
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Old 01-13-2004, 03:52 PM   #6
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Default Yeah, some measure it in inches

I use a 98 or 100 64 pitch spur. This gives me a wider range of ratios to choose from. For a beginner you are fine or better.

With 64 or 48b pitch gears, you figure rollout the same. Inches or Millimeters is your choice. I find Millimeters easier to work with.

Formula:

Ratio: Spur divided by pinion. Example 72 Divided by 19
= 3.789.

Tire diameter, Like posted earlier, 3.14 times 1.93 Inches = 6.06 inches around the tire.

Divid the distance around the tire by the Gear ratio, and you get Rollout. 6.06 Divided by 3.789 = 1.5994 Inches per revolutiojn of the motor. Or 1.5994 Times 25.4 = 40.6 Millimeters.

The idea is to keep the rollout the same as your tires wear. 48 Pitch gears will have a bigger jump in ratio or rollout with one tooth change of the pinion than 64 Pitch.

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