Gears and Acceleration

[ashwright]

Hi,

I have a question, how do you shift gears to get max acceleration?

My bike's motor drives at the pedals and though the gears (BB drive?).
I know an electric motor delivers max torque at 0 rpm, and delivers max efficiency at about 80% of max rpm. (please correct me if this is wrong.). However there is also a current limit (by the controller & battery), so you can't just shift into the highest gear possible and fly away.

So I am wondering what would be the optimal gear to be sitting in, to maximize acceleration? I suspect you will want the motors RPMs sitting at a certain point (say 50% of max rpm), and then gear to keep the motor running at that speed.

Ash.

 

[Whiplash]

That is exactly what I do, basically keep it in the highest gear it will tolerate but be careful, you can overload the controller if you push it too hard...

 

[Rassy]

I agree, as long as you aren't lugging the motor down, you will get the best overall acceleration in the highest gear possible.

On my trike, the 8 speed Nexus will slip a cog if too much power is applied to it in a high gear. Even in low gear if the grade is 20% or greater.

Also, I am more interested in battery performance than acceleration, so I usually take off in low gear, but often upshift two or three gears at a time if not going uphill.

 

[Hillhater]

Acceleration is directly proportional to torque (at the wheel) ...hence more torque = more acceleration.
The lowest gear (highest numerical ratio) will give you the maximum torque at the wheel, and hence maximum acceleration from standstill. ( unless you have enough power to lose traction )

 

[Rassy]

You are also right, Hillhater, and if I could make "instant" shifts at just the right points I think going through the gears would be fastest. But low gear tops out at 7 MPH, so it's pretty quick up to 5 or 6 MPH, but then the upshifting has to be done every few MPH, or more if some gears are skipped. While i'm shifting the one speed setup is leaving me in the dust.

Maybe with a Nuvinci the shifting is seamless and you can upshift with no pause, which might enable you to roll through the gearing while at WOT for maximum acceleration. LI-ghtcycle has probably tried this with his build.

 

[Whiplash]

Theoretically yes, but with my setup at least, if I don't shift up quickly there is not enough load to get the motor to draw enough amps to really rip!

 

[ashwright]

Thanks for all the replies. So in summary, staying in a lower gear gives you more torque at the wheel, but you have to shift up quickly.
Most people just start in a high gear, so they don't have to shift as much, which in practice accelerates more quickly.

Is this right?

 

[AussieJester]

Most people just start in a high gear, so they don't have to shift as much, which in practice accelerates more quickly. Is this right?

Not necessarily, no.. If you start in a high gear its going to heat a lecky motor up as it tries to move the weight from standstill, you get less heat more mojo starting in a lower gear, tiz how it works on my electric two speed bike. Start low and work your way up to high as speed increases :wink:

KiM