Calculating the potential top speed of a motor/car?
 

This is all theoretical, but am I on the right lines?

My lazer zx-5 takes 2.5 turns of the spur to move the car a distance of 27cm.
Based on gearing of 78/24, the motor would need to turn 3.25 times to turn the spur gear once (to travel 27cm).

If a motor has a rating of 5200Kv, and is running a 7.2 volt battery, then potentially it's RPM is 5200 x 7.2 x 0.9(90%) = 33696 rpm

At 33696rpm, the motor can turn the spur gear 10368 times in a minute (33696 / 3.25 = 10368).

If one turn of the pinion makes the car travel 27cm, then 27 x 10368 / 100 = 2799 metres. (THIS IS THE BIT I HAD WRONG)

So, at full throttle, the motor can make the car travel 2799 metres per minute, if we divide that by the number of metres in a mile (approx 1609), that's 1.74 miles per minute.

If the car travelled for a full hour, it would cover 104 miles per hour!

Surely that can't be right, where am I going wrong in my calculations?

EDIT: I know where I was going wrong.

It takes 2.5 turns of the spur to make the car travel 27cm, but I've not factored that into my calculations.

So, the motor can turn the spur gear 10368 times per minute, it takes 2.5 turns to do 27cm, so 10368 / 2.5 = 4147. So in a minute the wheels will turn 4147 times, 4147 x 27cm / 100 = 1119 metres per minute. Divide that by the number of metres in a mile, 1119 / 1609 = 0.7 miles per minute, over an hour = 42 mph...that's better

or strap a iphone/garmin sat nav to it and it'll log the max speed reached...

I've just created a little spreadsheet that does it for me, bit crap, and pointless, but I was interested in figuring it out.

Don't think your calcs take account of the motor being under load.

No they don't, they're simply theoretical.

...and b0llocks! :thumbsup:

A car in the first heat of 2wd at our regional was clocked at 35mph on the straight using a police radar gun.:thumbsup:

Back to the drawing board then!:D

You forgot the transmission loss factor of 0.843452178 approximately in a Losi xx4 :)

Also forgot about uneven surfaces and loss of traction

This is genius:

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

42 mph is easily achievable so your calculations seem realistic, that is if its based on carpet

Aerodynamic drag squares with speed, so you very quickly reach a point where most of the motor power is being used to push the air out of the way.

I'm sure these calculations will be very useful for anyone racing in a frictionless vacuum...