I'm not sure Eddie. You may be right, but I feel that if I am in the right gear and assistance level at the lights there isn't much in it between mxus and GSM. When I was running the GSM unrestricted there were very many puzzled car drivers trying to understand why they had to try harder to keep up...

Six e bikes been in this house and none with throttle, although I do have one for the BBS just never fitted.

And I did live in Telford but left so must be fitter now.

Both of mine also have throttles.

Both of mine also have throttles.

 

What about your eBikes?

Two thirds of my e-bikes have had independently acting throttles.

.

Both of mine also have throttles.

 

I didn't realise that you live in Telford as well.

 

 

 

 

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Edited December 29, 20169 yr by EddiePJ

Lets estimate:

The average torque of a 250w motor below 15 mph would be about 24NM. You can get that from the Ebike.ca simulator.

 

The radius of a 26" wheel is about 30cm, so the force at the tyre would be 24/.3 , which is 80N.

 

Acceleration is force divided by mass. Lets say a 75kg rider on a 25kg bike, so 100kg total. That means that the acceleration is 80/100 = 0.8 M/S/S

 

The first equation of motion says that final velocity = initial velocity +acceleration x time or V=U +AT. Turning that around, you get V-U=AT or T=(V-U)/A

 

U is 0, V is 15mph, which is 15x1500/ 60/60 M/S = 6.25 M/S

 

Therefore we have T = (6.25-0)/ 0.8. which is 7.8 seconds.

QED.

Alternative method

Mass of bike and person 100kg

Final speed 25km/ HR or 6.25 m/s. V ( not sure exactly how you got this number my calculation for 25km/HR is 6.9m/s but to maintain comparison I have used it)

Kinetic energy of bike 0.5mass x speed squared. 1952 joules

Energy input 250 watt or joules /s

Time 1952/250. 7.8seconds

Assuming 100% in motor output goes into forward motion no slipping , no wind, level ground

At first, I was surprised that the two calculations gave the same result, but then I thought about it. My calculation was based on the average torque that an average 250w bike makes, using measured data. It seems to correlate very well with an average output power of 250w. That means that, although your bike will probably take as much as 600w or more from the battery under some conditions, it's average power consumption would be much less. That's when going for it with maximum power.

 

FYI I only estimated the conversion between MPH and M/S, which is why we have a difference.

In the first second, the bike is constrained by the maximum torque that can be achieved by the bike's specific motor at very low speed. The power that the system produces can be deduced from P = F * V.

F is equal to torque divided by the wheel radius.

P will rise with V until P becomes limited by the controller, then Danidl's method is the easiest to deduce the time it takes to reach the desired speed.

In the first second, the bike is constrained by the maximum torque that can be achieved by the bike's specific motor at very low speed. The power that the system produces can be deduced from P = F * V.

F is equal to torque divided by the wheel radius.

P will rise with V until P becomes limited by the controller, then Danidl's method is the easiest to deduce the time it takes to reach the desired speed.

 

Of course all of these methods are approximations. Neither of us have taken into account the energy held in the wheels. Assuming that the wheels including tyres are 3kg each this is an additional 6kg (above the static mass included in the previous calculation) of rotational kinetic energy . This is. 0.5 *6kg*6.25^2. 117.joules .

This is one case where the crank drive motor scores as it has less rotational mass compared with hub motors.

if you do that, you should also take into account air resistance, worth about 100W to 110W at 15mph.

How long does it take a 250W electric bicycle to accelerate from 0-15mph without pedal assistance?

 

How fast do they go uphill?

 

I'm not good at equations. Click, click.

This is one case where the crank drive motor scores as it has less rotational mass compared with hub motors.

 

The crank motor has to keep speeding up and slowing down as you go through the gears, so you have to keep destroying its kinetic energy, then re-energising it. I can't see that as any advantage. All the enery going in to the hub-motor is conserved.

 

The crank-drive takes an immediate 5% to10% hit on its efficiency too because of the losses in the drive train.

[mention=14616]Tabs[/mention]

 

Where is the offensive language in this post? http://www.pedelecs.co.uk/forum/threads/0-15mph-acceleration-of-250w-electric-bicycle.26442/page-2#post-347591

[mention=14616]Tabs[/mention]

 

Where is the offensive language in this post? http://www.pedelecs.co.uk/forum/threads/0-15mph-acceleration-of-250w-electric-bicycle.26442/page-2#post-347591

It's that cryptic " get hen for me" in the first paragraph. We know about you country types and what you do with the poor cickens.

It could have more to do with me saying "Pro Connect"

It could have more to do with me saying "Pro Connect"

That's it. Shame on you!

Go wash your mouth out with soap and water.

The crank motor has to keep speeding up and slowing down as you go through the gears, so you have to keep destroying its kinetic energy, then re-energising it. I can't see that as any advantage. All the enery going in to the hub-motor is conserved.

 

The crank-drive takes an immediate 5% to10% hit on its efficiency too because of the losses in the drive train.

 

But from 0 to 25 km/h I am only in one gear - usually 3rd or 4th it is when the motor cuts out that I change up a gear. That is because I am pedalling and adding about another 120 W to the mix.

 

I find I can easily keep up with most cars being driven normally away from the lights up till about 25-27 km/ when they start to lose me. Some are even slow enough to make me brake to not run into the back bumper.

The crank motor has to keep speeding up and slowing down as you go through the gears, so you have to keep destroying its kinetic energy, then re-energising it. I can't see that as any advantage. All the enery going in to the hub-motor is conserved.

 

The crank-drive takes an immediate 5% to10% hit on its efficiency too because of the losses in the drive train.[/QUOT

 

The effect I am referring to is the rotational kinetic energy within the wheel. The moment of inertia of a hub wheel will be greater because there will be a larger rotating mass . It does not matter when in the steady state but does matter when accelerating or decelerating. The crank motors are of small diameter so the embedded energy is less., Even if they are rotating much faster. Also the speed of the crank motor seems to me to be more constant ... At least the low whine seems to be of nearly constant pitch.

from 0-15 mph legal bikes.I think the bosch cx is the fastest.I have tried most ebikes out there.Neo cross is also fast

from 0-15 mph legal bikes.I think the bosch cx is the fastest.I have tried most ebikes out there.Neo cross is also fast

Not "without pedal assistance" asked by OP.

from 0-15 mph legal bikes.I think the bosch cx is the fastest.I have tried most ebikes out there.Neo cross is also fast

that honour should go to the Woosh Big Bear. The only one in the Woosh range that still has a square wave 20A controller. I calculated that the theoretical time to take the Bear to 15mph on throttle alone should be 5.9 seconds. I can't test it at the moment but will try after the holidays.

Not "without pedal assistance" asked by OP.

 

 

I still believe that pedal assist or throttle assist that smoothness equals speed, and crank drives unless linked to a single speed drive train, just don't have the smoothness of gear change that a hub drive set up has.

 

Any advantage that a crank drive might have or indeed if it even does have any which I don't know, would be lost when changing gear.

When Atmosphere held the various world championships, the hub-motored bikes with throttles always got away quicker. That's how the Oxygens did so well. In some cases, they pulled out a 10 meter lead before the CDs could get in the right gear. The CD rider (semi-pro) would just about catch up the Oxygen bike towards the top of the hill, but knackered himself in the process, so the Oxygens pulled away again at the finish. And no, the Oxygens weren't "adjusted" for extra speed or power- just normal 15 amps 36v restricted to 15 mph. The Bosch and other CD bikes were probably running higher currents too, but they all got knocked out by hub-motored bikes in the first round.

 

In the three years it ran, no CD bike ever got in the final, which seems to defeat the idea that CD bikes are better for hills - at least for racing, anyway.

When Atmosphere held the various world championships, the hub-motored bikes with throttles always got away quicker. That's how the Oxygens did so well. In some cases, they pulled out a 10 meter lead before the CDs could get in the right gear. The CD rider (semi-pro) would just about catch up the Oxygen bike towards the top of the hill, but knackered himself in the process, so the Oxygens pulled away again at the finish. And no, the Oxygens weren't "adjusted" for extra speed or power- just normal 15 amps 36v restricted to 15 mph. The Bosch and other CD bikes were probably running higher currents too, but they all got knocked out by hub-motored bikes in the first round.

 

In the three years it ran, no CD bike ever got in the final, which seems to defeat the idea that CD bikes are better for hills - at least for racing, anyway.

How did the Neo cross do?

From my own testing with bafabng bpm 250w abd bafang 250w cst.They are nothing special at 15A.The neo cross has much more low end torque and faster accleration.The neo has a 20 amp controller.But it is stronger than the bpm and cst at 20a also in my experience.The bosch cx is much quiker than the other bosch motors