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When measuring rollout should you be measuring the largest tire on the car or just the RR tire not matter wat the size of the other tires?
 

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I'm a little confused about this rollout thing. Say I wanted to find my rollout then I need to know my pinion size which I'm not sure which to use. If I want to find my pinion size I need my rollout which I don't know either. Do guys know the rollout automatically by the size of the track or what.
 

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Ask someone at the track what their rollout is. (preferrably someone fast) Most will tell you or get you close. Then you can figure a gear ratio from there.
 

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To measure roll out, You need to know what tooth your pinion and your spur is, it should be stamped on the pinion and spur, and you need to know what your tire diameter is. the rest is easy.... spur divided by pinion= gear ratio, tire diameter divided gear ratio= roll out. (96 tooth spur divided by 37 tooth pinion = 2.594. (tire diameter 7 inch) 7 divided by 2.594 = 2.698 roll out.
 

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Al left out calculating tire circumference which is diameter x 3.14. His 7" tire is 2.229 in diameter. 7 divided by 3.14 = 2.229

Formula I use is: Tire diameter x 3.14 divided by spur x pinion.
 

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But what if I was doubtful about what pinion to use Al then I couldn't figure out my rollout.
Easy, just start throwing gears at the car until it's fast then do your rollout. I never actually calculate my rollout until the car is fast, then I do it just so I have record of what I ran.
 

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When you ask what rollout that fast guy is running, make sure to ask what ESC and Timing boost being used. Oh and what timing the motor is set at.........If you believe him, try it!!!!!!! Not that anyone would ever be untruthful. LOL
 

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OK, I guess I should have been more spusific... Yes when I say 7 inch tire, I mean circumference not hight. LOL. (Thats right man, I run tractor tires on my pan car.....)
(Tire hight 2.125)
 

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I'm a little confused about this rollout thing. Say I wanted to find my rollout then I need to know my pinion size which I'm not sure which to use. If I want to find my pinion size I need my rollout which I don't know either. Do guys know the rollout automatically by the size of the track or what.
Try this simple program. just input your tire size and spur gear and it will show you what the rollout will be for each pinion 16-65
 

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t4mania said:
I'm a little confused about this rollout thing. Say I wanted to find my rollout then I need to know my pinion size which I'm not sure which to use. If I want to find my pinion size I need my rollout which I don't know either. Do guys know the rollout automatically by the size of the track or what.
Hit or Miss... and or experience and knowledge of what a motor can do..

I can tell you I was a LOT BETTER with it in the old days...because I knew MORE of what a BRUSHED MOTOR did - (winds & voltage)

The BRUSHLESS ESC and MOTOR deal changed a lot of that - because now every Motor / Esc combo will be different...sometimes HUGELY different in rollout.

Things I personally look for

Pretty precise TRACK SIZE
Estimated Lap Time
Tire Diameter

so say a 165 ft track
est. lap 4.80 sec
= 34.375 ft. per second (this would be 23.43 MPH)

so using a 2.12" (diam.) tire (which would be .555 ft ) the tire would spin approx 61.94 times per second (or 3716.4 RPM's at the axle)

If you know your motor will spin a certain RPM under load at a given voltage - you can set your ratio from there.

say a 17.5 = 2200 Kv (RPM Per Volt) Unloaded, but loses 40% of it's RPM under load....

at say 3.7 volts you could be looking at about 4900 loaded motor RPMs

4900 (motor RPM's) / 3700 (axle RPM's) would be approx a 1.32 RPM Ratio

Say I'm running a 88 tooth Spur Gear...I'd be looking at a 66 - 67 tooth pinion just using basic crude numbers. *88/66 = 1.333 88/67 = 1.313

I believe if you run the ROLL OUT formula that comes to a 4.995 roll out.

Pinion (66) / Spur (88) * Tire Diam (2.12) * Pi (3.1416)
 
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