RPM

[MikeN]

I'm planning on building a 1:8 scale tank from scratch.
Approximately what sort of rpm for the drive sprocket should I be aiming for?
Thanks.
Also,
BT series tanks don't actually have a toothed drive sprocket. What's the correct term. A drive wheel?

[HERMAN BIX]

The BT 7 had a top speed off road of 50 Kph so that divided by 16 gives you a scale max marker of 3.125kph
If you count the rotations of your drive wheel (mark it with a suitable visual line like white paint pen) over 15 seconds then multiply that by 4 to get the rpm in real time/per minute, then calculate that into the 3.125 kph you can adjust your gear ratio as required.
I reckon a bloke that can use a dividing head to great effect can easily work the maths on that
You may need to use an idler gear to slow down or speed up , really depends on what your gearbox is at the moment

[STHV]

BT series tanks don't actually have a toothed drive sprocket. What's the correct term. A drive wheel?

Dead on!
Drive wheel is the best general term as there are some vehicles out there that have both a toothed drive wheel and a toothed idler wheel, Churchill for example.

[MikeN]

Ok thanks,
I didn't know that about working out scale speed.
So. To achieve an actual speed of 3.125 kph ,
I will have a drive wheel 80mm diameter, that will be laying down track at a rate of 0.251 mtr per revolution
So to cover 3.125 km it needs to revolve 12450 times (in one hour).
Divide by 60 mins gives 207.5 rpm.
I think.

[HERMAN BIX]

If your math's is right , and Im sure it is, thats for the maximum top speed off road .
Depending on your gearbox set up, you may need to allow a different ratio so you get the best drivability at realistic control levels with the overall weight of your creation taken into account.
You can always drive it slower if required!!............

[Herr Dr. Professor]

"ome vehicles...have both a toothed drive wheel and a toothed idler wheel, Churchill for example." Why the toothed idler? Does it help keep tracks on more securely?