Over running a brushless hub motor

[Bluefang]

I have been thinking about doing a motorbike build with a hub motor mounted normally with a 2nd motor, driving a chain powering the hub as well. Due to voltage limitations the top speed is borderline to what i want. My question is can a hub motor be over run by 20% even with the controller at max. So under full acceleration the hub would reach max at 100km/h and the 2nd motor would keep driving the bike up to 120km/h, would the over driven hub cause a major resistance to the 2nd motor? Would it blow up the controller( i doubt this)? Would the hub cause a small resistance to the 2nd motor?

I am thinking and hoping that all i will have to contend with is the cogging effect in the hub motor and also the hub been unable to use regen until its below the controllers max voltage setting. But would like some reassurance

 

[mlt34]

One thing you'll need to be careful of is not accidentally regen-ing from the hubmotor being overdriven. That would happen to me a lot when I'd go down hills on a 6x10, and somewhat less on a 8x8. If I was traveling faster than the hub's max speed, I'd see negative amps on my cycle analyst screen indicating regen. If you aren't prepared for this, and if your second motor is really working hard, you could end up blowing the BMS in your battery if you aren't careful...

why not just put a second hub up front, or even simpler, increase your battery voltage if you are just looking for 20% more speed?

 

[John in CR]

Due to the wind resistance at 120 you're going to need high power up there, so gear it so both motors are pulling all the way up to 120kph. A pair of HubMonsters with the in-wheel motor in the larger tire available that ends up a 21" OD and the second Hubmonster geared at 1:1 would be able to do what you want on 23s if you keep the total load under 330kg and your bike has reasonable aerodynamics. That would be with the kind of acceleration performance in my videos with my using one motor and 180kg with a 19.2" OD wheel.

Personally I'd shoot for 250kg all up with half of the increased weight being motor, controllers, stronger frame and wheels and half of the added weight dedicated to more battery. Double the torque and 115% more power (15% higher voltage) on tap pushing, but only 40% more weight and 10% steeper gearing, sounds like a winning combination to me.

An alternative is to go for a lighter build and use a single Hubmonster as a mid-drive, and go to 32s vs the 20s I ran for 8 months. At that voltage you could easily gear it down to achieve 120kph with some reasonable aero, since I get to 105kph on 20s. Then you'd still have extra power to push more load and squeeze out more performance at an all up weight of say 220kg.

Which way to go is a close call, so it depends how much you want the bike to weigh. Dual motors on 23s can be done on 4 cheap controllers. 32s on one motor requires a pair of higher priced controllers.

You might want to see how my single motor in wheel works out on 32s as I am preparing now. The motor is vented, so I'll be able to go up on current as well as the 60% increase in voltage, so as I turn the controllers higher I'll be able to more than double the peak power input with only about 5kg more weight.

John

 

[John in CR]

Finding a hubbie that will push to 120kph would be hard enough, so why handicap it by making it push another motor past its no-load speed? Besides the fact that hubbies act a little weird when pushed above no-load, there's no benefit. At your 120kph one motor would need to produce the full power required (13kw at the pavement with the aero of an MTB) plus the extra amount the other motor is dragging. A motor capable of doing that well doesn't need the other motor.

OTOH, 2 high hubbies capable of the rpm required and splitting the load so the output at the road of each is only 6.5kw, sounds like a piece of cake with the right motors, and it would give you killer performance from 0 all the way up.

John