Acceleration - Top Speed - Torque

Hello,

Optional intro
I'm building an electric kick bike (have build three ebikes before). The controller I want to use (and already have) is limited to 48V and 40A. For motor I'm thinking a DD hub motor in a 20" rear wheel. To be road legal I'm limited to a top speed of just 17-18MPH. To make it fun I would like it to accelerate quickly up to the allowed max speed and also reach top speed uphill. The motor should not overheat even when accelerating and riding at top speed for longer times.

I have used this controller before with a Crystalyte H 4065 motor in a 28" wheel with a 36V battery and it had a top speed of around 20MPH and acceleration was 'OK'. Since I will use a 48V battery, a much smaller 20" wheel and a lower top speed, I expect a decent increase in acceleration.

Question
Which DD Hub motor in a 20" wheel, running on 48V, with a requested top speed of 17MPH/27KMPH will give me the best, reliable, acceleration and torque?

Options
- Crystalyte HT 3525 Rear motor
http://shop.crystalyte-europe.com/product.php?productid=16445&mode=download&file_id=66
-- 250 - 2000 Watt, 24 - 72 Volt; 40 - 90 Nm Torque; 45 Amp Max; 6,5 RPM/V; 6,5 Kg weight
-- 20" Wheel @ 48V = Top speed exactly 17MPH/27KMPH

- Crystalyte H 4065 Rear motor
http://shop.crystalyte-europe.com/product.php?productid=16566&mode=download&file_id=113
-- 250 - 3000 Watt, 24 - 72 Volt; 55 - 120 Nm Torque; 50 Amp Max; 8 RPM/V; 7,5 KG weight
-- 20" Wheel @ 48V = 21MPH/34KMPH

Since the lighter, cheaper, HT 3525 has the exact top speed I require, I would assume it will use the applied power more efficiently in the speed-range I require and so it should accelerate quicker?

Thanks

For your specific needs, a lower rpm motor will work best for you if you run 48v. Otherwise, you'd still end up with a 35 mph bike running 2000w. I would not get too hung up on top speed though, if it goes 20 mph, close enough. Chances are, if you pedal any, your top speed will be limited to 17 mph just by gearing.

I built a similar bike, but my aim was more for lower wh/mi, yet able to tow a heavy trailer up a mountain on only 1000w. I used a lower rpm (10 turn 9 continent type) motor and 20" wheel. It ran 18 mph on 13s 48v. To pedal at that speed, I ran 56t front ring, with the typical 14t freewheel on the motor. Slow up a hill though. Only more watts can get you faster up hills. basic physics.

A more powerful motor, and same voltage, and same motor rpm @48v, would have meant a very snappy take off, but still close to the same top speed. Just a bit faster, because of course more copper and magnet would lug less under load on the flat. I will guess 20-22 mph would have been possible. and it would have been fast up hills of course. This is closer to what you want.


You have 2000w with that controller, so the motor does not need to be huge, but get one with wider than 28 mm magnets if you want that better take off. Torque takes copper, and big magnets, unless you volt up to 40 mph.


If you needed to do this with a typical rpm hubmotor, then run huge wattage 36v. Get a 36-48v controller, with 60 amps. 36v will do 20 mph, in about any motor, if you give it 2000w or more of it. The wheel size won't affect your top speed near as much as you think. That is why I went with the very slow motor in my bike. it was too fast to be economical with the regular motor.

But the smaller wheel will really improve torque, so that is what you want. BTW, a low rpm motor will be sold by many as a higher torque motor. Its not. Its actually lower, because back emf builds at lower rpm.


So 36v and normal rpm is your best bet for actual torque, but to get it, more than 2000w.

Thank you for that explenation.
It is for a kickbike, so no pedaling, no chainrings, just throttle and pushing it manually

18 mph will suck even on that thing. Don't worry about limiting the top speed to comply with the law. Nobody is EVER going to check, and if you don't whiz by cops at 40 mph nobody will ever know either.

I checked on my cell phone the speed at which I tend to ride, seems to me that 40km/h is super fast for my liking, though I do ride that speed when its safe to do so, its good to get that out of the system but I am never in a rush. 35kph is more to my liking for a fast cruise, while 30kph is a good pathway speed but I tend to hang back behind Lycra's/Others who pedal at a good pace. 10mph is 16kph, so I'd say 20mph/32kph is a good cruiser speed, but its good to have that bit extra.

Lets say you got a 70V 50A setup, thats 3500W. Now lets say you want to go slower but still have power, then 35V would require a 100A for that same 3500W. Its a teeter totter, or a balance game, same goes with Tops Speed and Torque. One way is to go with a smaller wheel, but you lose top speed unless you get more voltage. Also realize if you go with a slower motor, the strands are thinner meaning you cant dump as many amps into the motor.

Its a matter of what you want, what you will be riding most of the time.
--------------------------------------------------------------------------------------------> Balancing Game

My aim with my cargo bike was similar-- good load carrying and competent climbing while remaining legally compliant, but taking advantage of Texas not having a specified power limit.

I use a Crystalyte 5305 in a 20" wheel, 35A controller, and 48V (13S) battery. Fresh off the charger, it just about reaches 22mph on flat pavement. Once the initial highest voltage portion of the charge is used up, it still accelerates expediently while remaining under the 20mph limit.

I recommend a Leaf Bike motor with whichever winding gives a no load speed of about 35 kph with your wheel diameter and voltage. (365 rpm if it's a true 20" wheel.) Use a generous controller current for strong acceleration and climbing; this will taper off at cruise speed to the same power level as if you used a low current controller.

My cargo carrying capacity is limited to six 2L bottles hanging off the left side of the handlebar. I would joke about how much alcohol is could carry but its never seen it, just Club Soda.

I still think the slower motor will do him fine, and can handle enough power to make him take off snappy.

If he chooses the bigger motor, yes, more torque even on the same watts. Why? enough wire to use the watts, vs making it hot. But the faster motor will not only have a faster top speed, which could be limited, but it will also have a more hard to control throttle at lower speeds.

Might not be touch it and you wheelie off, but I think he can put plenty of power through the smaller, lower rpm motor, and still have a throttle that is easy to use.

Screw being over the top speed, but he can still be less than 30 mph I think, on 48v, and simply give it less throttle when in crowded riding conditions.

^^That is why I like the mass and material (stator) of the mxus 3kw even though I hardly use it to its maximum, the Leaf 1500W is a solid choice as well, which is what I would probably be fine with, with my weight and some of the hills I do.

YES, if he wanted to ride 30 mph all the time, perfect. Don't know if you can get that motor in a slower wind though.

Again,, the ONLY reason to choose the slower wind, is to build the slow bike. Otherwise, pick a voltage for your faster wind motor to get the speed you want, with tons of copper, for the win. Just putting a fast motor in 20" wheel does not slow it down all that much. Give it 48v and 60 amps, and it will take off like mad. But it would only go 29 mph vs 31.


Don't really like the idea of running a big fast motor on 24v, but it could be done to slow it down.


His bike is going to be faster, at least potentially, than 17 mph. But with a slower wind, and 20 inch wheel, he might get it to top out under 25 mph. Then ride it, only 17.


At ebikekit.com they do sell a slower rear motor, the 10 turn wind. Its only a 28 mm motor though, so it will only stand about 2000w. On 13s, 22 amps, and 20 inch wheel, I got a bike that topped out at 18 mph. Likely go 20 if you gave it 40 amps though.

Many controllers have a programmable speed limit. You could just program the limit you want and it would accelerate like mad until it reaches the limit then not go any faster.

I ordered a Crystalyte H 4065, I will put a resistor inline with the throttle to limit the top speed. With a switch I could bypass the resistor and open up the top speed.

thanks

michielk said:

I ordered a Crystalyte H 4065, I will put a resistor inline with the throttle to limit the top speed. With a switch I could bypass the resistor and open up the top speed.

Click to expand...

Doing it that way will limit not only speed, but torque and power. That's great for increasing range in most situations, but bad for climbing steep hills and accelerating from a stop.

If you want to keep your speed limit but also have your maximum power and torque available, use a programmable controller with RPM limiting, or a Cycle Analyst.

Hello,

Thanks for your feedback. I'm aware of the impact that it also limits power, torque and this is fine for me.