RC motor KV & Actual speed

[Larby]

Morning all,

I’m getting ready to build a RC based mid-drive and I’m trying to figure the reduction ratio needed.

The motor I’m looking at is going to be 130-140KV, running a 8S pack for example, so theoretical no-load speed around 4000rpm.

This is the motor I’m looking at using, unless people have a better suggestion?

https://hobbyking.com/en_us/turnigy-sk8-6374-130kv-sensored-brushless-motor-14p.html?

I’m aiming for a max cadence of 100rpm, I don’t care for it being able to assist faster than I can pedal. It’s intended to be a lightweight assistance system, not standalone propulsion.

Thing is, I’m assuming there’s no point basing my reduction ratio on the theoretical 4000rpm no-load as a maximum.

Any suggestions on how I’d figure a good speed to base it on? I considered just halving it, but that’s a simple guess. I’m looking to use a Neugart planetary gearbox or similar, so would prefer to get ratio right from the offset!

As always, any advice etc is welcome. Every day’s a school day!

Cheers!

 

[Larby]

Literally just after posting this, I finally found what I think is the answer on this website - about 70%!

I guess I’ll leave this thread here for anyone else searching for the same answer in the future!

http://www.recumbents.com/wisil/e-bent/rc_drive/tutorial.htm

 

[larsb]

70% seems too low as loaded rpm. I usually use 85% as top speed assumption. Has worked so far.

Some cons:
That motor is an outrunner and combined with the reduction will make a LOT of noise at higher rpm. Some like to hear the motor scream when going flat out but i find it gets tiring.
Neugarts have trouble with the high rpm/high torque in these kinds of use cases. I’m afraid the lifetime of the drive might be limited due to this.

Pros:
Cheap (not really dirt cheap since a cheap neugart is hard to find)
High torque for the weight and size due to the outrunner

An inrunner motor with belt reduction would be my first choice, especially if you like more ’zen’ riding.