Full speed/ coasting and cruising economy.

[handmadematt]

Hi guys, I've introduced myself in the new members area with pictures and data of the new bike that I've built myself.
Here:
http://www.pedelecs.co.uk/forum/threads/ive-just-finished-building-my-first-trail-ebike-pics.18787/

When she hits around 20 mph (or slightly over) I feel that the motor has reached a terminal speed. It's like a wall and all acceleration is lost. Is this the designed built in maximum of the motor or is it the controller limiting the speed? It really doesn't feel like the motor runs out of juice. It feels restricted.
It's a 36v 500w motor, I'm sure if one was to put 48v into the motor it would spin faster, no?
(I don't necessarily have any interest in doing so I am just keen to learn and find out what is going on.)

My economy question is connected, it's about coasting and cruising. My thinking is that the full power output potential of the motor (500 watts) is obviously used for acceleration but I can't imagine that the full power output is used to maintain this "speed ceiling." (The same goes for legally speed restricted machines once they hit 15mph.) This effect would be even more pronounced going downhill with the accelerator turned to maximum. Some downhills I can even coast/ pedal to high 27-28mph at which point opening and closing the accelerator has no noticeable effect on the bike at all. Is the controler clever enough to stop/ slow power going to the motor in these situations or not?

The only way I can think of to observe this in action is to fit a shunt and a little ameter to show real time amps, I could then easily calculate the watts at any given time.
It's a bit elaborate though just to satisfy my curiosity and not actually serve any purpose.
Can anyone here shed any light on how all this works for me? (Or point me towards some reading material.)
Many thanks.

 

[Deleted member 4366]

To check whether you notor is limited by the controller, charge up the battery to full, then spin your motor at full throttle with the wheel off the ground. Check the speed. Then go out for a ride to use up half the battery. With the battery half used, do the spin-up test again. If it goes at the same speed, your controller is limiting the speed. If the speed's about 10% less, then there's no limit.

The motor has a natural top speed, which depends on the voltage. Power falls off rapidly as it reaches maximum RPM.

 

[handmadematt]

Thanks again mate.

The motor has a natural top speed, which depends on the voltage. Power falls off rapidly as it reaches maximum RPM.

This could be what I am experiencing. I have a laser RPM meter so this test will be easy. Thanks.

If anyone has any comments on the economy of cruising and coasting with the throttle open that would be great.
Ta.

 

[Deleted member 4366]

The less you open the throttle, the more efficient your motor becomes. as long as the motor can work without struggling.

 

[handmadematt]

Sure.
If am pedaling faster than the motor speed though and I open the throttle I can't see that it's going to be outputting it's peak power... Where is this energy going? It's not helping me as a peddler, it's not becoming sound. Heat maybe? (Probably not.) I have a feeling that the controller simply shuts off, or the nature of motor technology means that it's not energised or something? I.E it's not using any (or very little) power.
Maybe I need an ammeter to see exactly what's going on.
Thanks again for the great discussion bud.

 

[handmadematt]

UPDATE
With a full battery my rear wheel is spinning at 1,276 RPM.
I'm going for a ride this afternoon after which I will measure it again and report back.
Ta.

 

[Alan Quay]

.....With a full battery my rear wheel is spinning at 1,276 RPM.....
Ta.

That's got to be wrong - in a 26" wheel is about 100mph!

 

[handmadematt]

I'll double check it.

 

[handmadematt]

It was 305. Which sounds more reasonable.
I guess the laser was malfunctioning earlier with the shiny rim or spokes.

 

[Deleted member 4366]

That's 22 mph no-load, which will be about 20 mph on the road.

 

[handmadematt]

I got back from a 12 mile ride on forest trails with some decent hills too. It was great.
The green light had long gone from my battery indicator lights and I was on a steady amber.

Immediately I measured:
35.5v and 278rpm.
After letting the battery settle for around an hour I measured:
36.4v and 285.5rpm.

So it appears that there is no speed limit imposed by the controller but the natural peak performance of the equipment is more abrupt than I am used to. My experience of motorcycles is that they creep to their top speed very slowly, electric feels different in that respect. It's all power and then instantly nothing. Subtle, but noticeable to me.