Rollout

[raffaelli]

What is the formula for roll out which includes the internal ratio?

[XJBUCK]

By rollout do mean tire growth at speed? If so there is no difference between that and figuring speed without tire growth. The hard part is figuring just what size your tire is at speed. It will not be tha same as when you let it run free while holding it up because you have no load holding the motor/engine back or a chassis load on the tire which affects the laoded radius. It is also difficult to figure out just what RPM your motor/engine actually pulls to when it running down the track. Without that there is not much point in trying to figure the rollout. The easiest method would be to use a time trap to see how different gear ratios and tires affect speed, but do not forget that higher top speed gearing usually means less acceleration.

[Railman]

Spur/pinion x trans reduction =final ratio
(Trans reduct. = diff gear/topshaft)

I'm not sure exactly what rollout is. I would assume that it would be the actual mph at a given rpm, or inches of travel per revolution on the motor. It really depends on what your trying to find. To be of any use you usually need a benchmark to compare to. Once you have the final ratio you can take the circumferance of the wheel to get the distance traveled by X number of motor revolutions.
Example:
87/20x2.40=10.44 final ratio
If you wanted to know how fast that would be at say 30,000 rpm,
you would need the wheel circumferance. 1/10 trucks are 4" in diam. So circ = 3.14 x 4"=12.56"
So for each 10.44 revolutions of the motor you get 12.56" of travel.
At 30,000rpm:
30,000/10.44x12.56=36,091 inches per minute
To get mph you need to convert to ft & hrs, so:
36,091"/12"/5,280x60=34.18mph

Or:
motor rpm/(spur/pinion X trany) x (3.14 x tire diam)/12/5,280 x 60=mph
Whew... that's more than I can remember! That's why I like to break things down to it's simplist form to solve problems. Anyway I hope this helps.
Joe
;)

[raffaelli]

railman you are an addict!!!

Actually I figured out the formula. I am trying to be able to compare ratios from different car types taking into account for tire diameter on touring cars.

The forulma is just as you said! Yes you can develop it futher ti figure out how many laps you can do. The hand up there is ther constant change in RPM with touring car racing makes it a difficult equation.

[highroller]

To find the transmission ratio for any car count the number of teeth on the drive (diff gear) and the number of teeth on the reduction gear. Now multiple transmission ratio by gear ratio and divide by tire diameter will equal roll out. Rollout is calculating the number times the motor turns to the number of revolutions the tire rotates. Motor 1, tire 88 = 1.88 rollout which is faster than a 2.88.

[Ras Thavas]

Roll Out is the amount of forward travel your car will go per revolution of the motor.

You figure it by the following formula:

1) Height of tires (in millimeters) x 3.14159 (pi)

2) (spur gear teeth / pinion gear teeth) x gearbox ratio.

3) Number from step 1 / number from step 2. This is your roll out in mm forward per Motor revolution.

How does that help you? If you use roll out figures you can more accurately select gear ratios
for your motor application. Since different sized tires can make an impact on gear selection.

To keep this simple, lets consider the pro 3 and 64 pitch gears, 100 spur.

64mm tires, 38 pinion, stock motor - (HPI recommends roll out of 30.99 - 34.38 for a stock
motor)

64x3.14159 = 201.06176

(100/38) x 2.25 = 5.92105

201.06176/5.92105 = 33.96 roll out (rounded up)

That same combination with 63 mm tires

63x3.14159 = 197.9201

(100/38) x 2.25 = 5.92105

197.9201/5.92105 = 33.43 roll out (rounded up)

So assuming your motor revolutions are the same, the 63 mm tires will lower your top speed.
Using the same formula, we see that you can now go up to a 39 tooth pinion, and still be within
safe limits for the motor. With a 39 pinion the roll out is 34.31.

I have created several roll out charts for HPI (Pro 3, Rally/2, EMT) cars and the Losi XXX-S.
They require Excel 2000 (may work on 97), and you can choose different tire heights and motors
and see which gear combinations apply. E-mail me if you want a specific chart.

matai_shang@mindspring.com