Rollout formula needed

[godale03]

Hey guys,

Do any of you have the formula for rollout?

Thanks

Tom

 

[sweetdiesel]

Rollout = Tire circumference / final gear ratio

final gear ratio = (spur / pinion) x transmission ratio

Example for your T4 with 2.2 bowties

(4" x 3.14) / ((87/20) x 2.6) = 1.1105

 

[R/C Newbie]

Nice Info Thanks

 

[Got Nitro?]

My question is, The rollout formula is used for what?

 

[Diver6127]

it's used mostly by electric guys trying to get optimum gearing for specific tracks. Predominantly, carpet racers since they use foams and foams deteriorate with each passing lap.

 

[sweetdiesel]

Another reason to figure out rollout is when changing to different size wheels. Let's say you have a well geared set-up on your car/truck. We'll use godale's T4 as an example. Let's imagine that he is running a Novak 7.5R setup and is geared 20/87 with proline 2.2 bowties. Let's say for sake of discussion this gearing is absolutely perfect. Good combination of torque and top speed and his electronics are running warm with nothing overheating. Now he decides instead of bowties, he wants to run road rages. Since these are smaller tires he will lose some top speed and his brushless motor might overheat from being undergeared. Using the rollout calculation he can get himself back into the ballpark so that his top speed does not change and his electronics stay happy. We'll run through this with the calculation.

tire circumference of bowties=4 x 3.14 = 12.560 inches
tire circumference of road rages=3.6 x 3.14 = 11.304 inches

Rollout with bowties = 1.1105

Use the formula to calculate proper pinion X with road rages to get same rollout as running with bowties

(3.6 x 3.14) / ((87/X) x 2.6) = 1.1105

X=22.22 so using a 22 tooth pinion will get him close to the same rollout as using a 20 tooth pinion with bowties. He can go one step further and substitute 22 tooth pinion into the formula and calculate the new spur gear to get him even closer to the same rollout.

Edit: if we plug the 22T pinion back in othe formula and solve for the spur we get 86.13, or 86T spur.

Plug the new gear ratio in and we get 1.1122 as the new rollout with road rages, very close to the rollout of 1.1105 with the bowties.

 

[Got Nitro?]

Thank's dom, that's help's. Now too learn to use it correctlly will be my next step.

 

[godale03]

Wow Dom.... I guess I need to go brush up on my Algebra! Thank you for the formula. I am like got Nitro... now I just have to figure out how to do use this formula correctly.

Am I correct in the fact that you can use the roll out number to calculate top speed with your current drive ratio? Not that I am concerned about top speed numbers but I was told that you can use the rollout info to figure it out.

Thank you for your help in figuring out what the formula was and how to apply it. Now I just have to go and figure out how to solve for X.

This all started after I our discussions about proper pinion selections and I started to read about this thing called rollout and so I needed some more answers so I figured I would ask the question here. Thanks so much for your help Dom! I take back all the stuff Monkey Wrench said about you and your astroturf front porch!

Tom

 

[sweetdiesel]

To get the theoretical top speed we will use your brushless motor Kv rating and the battery pack you are using with the rollout number. Just remember that this is a theoretical top speed because there are other factors that will influence your top speed such as drag, weight of the vehicle, and voltage drop of the cells under load. Also Novak does not rate their motors under load, so the Kv rating will be a little lower than what is published on their site.

Let's work with the numbers we have and we’ll probably be close to the actual top speed.

SMC 4200 pack = 7.2 volts
7.5 motor = 5800 Kv

7.2 volts x 5800 Kv = 41,760 max RPMs

Rollout is defined as the distance traveled by the vehicle per revolution of the motor, so

41,760 RPMs x Rollout of 1.11 = 46,353.6 inches per minute. Now convert that into miles per hour and you get your top speed. You have 5280 feet in a mile so

46,353.6 inches / 12 inches per foot / 5280 feet in a mile x 60 minutes in an hour =

43.9 miles per hour

There are also speed calculators on the web that will give you the same answer without having to do all the math. This one does not use rollout, just plug in the appropriate values.

http://www.scriptasylum.com/rc_speed/_top_speed.html