Gear ratio / top speed

[Kizum]

How do you calculate the estimated top speed for a rc car given the gear ratio, spur gear, pinion gear, tire size and motor RPM? What is the formula used?

What does "Rollout" mean? How do you calculate this?[]

[sosidge]

When doing these sums - remember that drag increases greatly as the car goes faster - so the motor will never manage to pull it's full no-load revs - more like 1/2-2/3rds, where the peak power is, when you're pushing for pure top speed runs...

Anyway, to work out Rollout (which is the distance the car travels for every rotation of the motor)...

(spur/pinion)*internal ratio=overall gear ratio

(tyre diameter*3.14)/overall gear ratio=rollout

And to work out top speed...

Motor RPM*rollout=top speed.

There will be some scaling factors involved - if you're using metric, you'll probably end up with the speed as "mm/minute". To make it KPH, multiply by 6, and divide by 100,000. To make it MPH, multiply by 6, and divide by 160,000.

In imperial, it'll probably be inches/minute - to make MPH, multiply by 6, and divide by 6366.

Remember - only 2/3rds that mathematical speed is a "realistic" value.

Hope that adds up!

[Kizum]

Thanks! I suppose you have some degree in mathemetics? [8D]

[mud4fun]

Just a further note: if you are running soft tyres they will compress when the car is laden so the actual rolling radius will be significantly different to the unloaded radius, maybe as much as 10-15%. It is the actual rolling radius you'll need for the calc above. (This will apply more to buggies and trucks than touring cars)

[sosidge]

quote:

Originally posted by Kizum

Thanks! I suppose you have some degree in mathemetics? [8D]

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I have 2 A-levels in it...

The maths is simple once you start using it though - it just seems like a lot when it's written as a word formula!

There are a couple of programs out there that do the work for you.

[theMonster]

You also could just use this. I love this site!

http://www.arroyoc.com/index.cfm?fuseaction=utilities.gear