Can someone post the formula for calculating rollout again? I had it but lost it.

first multiply the tire diameter by pi (3.14), we'll call this number "A". Then, divide spur by pinion, we'll call this number "B". Finally divide "A" by "B".

example
2.25 (tires) x 3.14 =7.065
120 (spur) / 23 (pinion)= 5.217

7.065 / 5.217 =1.35

so your rollout is 1.35

if you have your tire/pi ratio and have a rollout you want (we'll say 2.40 in this case) do this:
2.25 (tires) x 3.14=7.065
7.065/2.4= 2.944 (this is the gear ratio you need)
now take your spur gear already on the car (116 in this case) and divide it by your ratio above:
116/2.944= 39.402 (this is the pinion you'll need)

so you need a 116/39 to get a rollout close to that. As for the .402 you can round either way just understand that there is a difference in rollout +/- depending on which way you go. I like to round down by default by you can do either just be consistant in which way you go.

Tire dia multiplied by 3.14 multiplied by pinion size divided by spur size

HEY Q BALL
ITS SIMPLE TIRE DIAMETER x 3.14= ? divided by gear ratio

example ( 2.25 x 3.14=7.065 / 3.85=1.8350649 which is 1.83 or close to 1.84 on rollout if you round off to the high side.


thanks RACERX1

pinion divided by spur x 3.14 x tire size

all correct ways of saying the same thing :thumbsup:

Desired rollout/tire diameter/3.14*spur=pinion

So why go backwards? this gives you the pinion you need rather than you guessing until you get it right

Tire dia multiplied by 3.14 multiplied by pinion size divided by spur sizeAlso my method...Very simple, like me :thumbsup:

I just thought I'd throw this in ...

Rollout = Distance (inches) the car travels for each motor revolution.
Steve

DELETE

I have a Excel Spreadsheet made up with rollouts calculated for different spur/pinion combos with tire sizes from 2.0 to 2.35 on the charts.
email me if interested @ wowracer@charter.net would be nice for a small paypal donation if you felt my charts were helpful to you.
paypal addy wowracer@hotmail.com

http://www.hobbytalk.com/bbs1/showthread.php?t=101738

Try the above link to an example rollout chart for a TC3 w/96T Spur

what is the best rollout # for a flat track w/ 200' runline

OK, so a guy is NOT a mathematician...

He knows HOW to calculate ROLLOUT...but now tell us WHY or WHAT..or HOW to use it...

It is just the way to determine the final ratio and keep it the same with changing diameter foam tires. Just one more thing to help to stay fast

but now tell us WHY or WHAT..or HOW to use it...

Shorter track you want the motor to wind out quicker = lower rollout

Longer track you want the motor to take longer to wind out = higher rollout.

Also Motor RPM and Torque come into play.
The more RPM a motor and less torque = a lower rollout
The less RPM a motor has and more torque = a higher rollout

Say you have a 200' track and you have a Brand Z motor that has 23000 RPM and the rollout everybody runs is a 2.60

Then you switch to Brand X that has RPM of 20000, you would probably need to gear up about 3-5 teeth to a 2.78-2.90 range to run similar lap times.

If you are running stock motors, use a temp gun as soon as you come off the track, typically if you can get the motor to come off at 85-100 degrees you are in the right ballpark on gearing. Also most tracks a 3 to 4 tenths drop from start to finish of a race in lap times is about right. 2 tenths you might not have enough gear, 6 tenths and you might be overgeared.

I use this formula:

Pinion divided by spur X 3.14 (Pi) X tire size = rollout

whichever you choose, stick with it and it will become second nature, dont forget to throw that calculator in your box.

Rollout is a better method of trying to stay consistent as the tire size changes, so does the rollout and you will have to adjust your gearing accordingly to maintain your rollout.