Teach Me Please

[Mithion]

My answer to the question you will read shortly is this- Because the battery is rated at a higher kWh.

Here is the question-

An electric bicycle weighing about 46lbs with a 2.3Kw/3hp motor running off a .53kWh battery has a top speed of about 31 mph.

Another electric bicycle weighing about 43lbs with a 1.2Kw/1.6hp motor running off a .81kWh battery has a top speed of about 62 mph.

Why does the bike with the less powerful motor have a faster top speed?

Is my answer correct, if so please explain. Thanks so much for the knowledge.

[protomech]

1. All too often published specifications are just another form of marketing, particularly when they're published via press release. Which is to say they're fiction, or at least heavily qualified facts.
2. It's possible one bike is more aerodynamic. The human-powered land speed record is 82 mph on flat ground; a top cyclist can usually put out about 500-600 watts of mechanical effort for a minute or so. The LSR was done on a very specialized aerodynamic recumbent bike.

The capacity of the battery is only a factor if the total battery power output limits the motor's power output. For example, the 2004-2009 Toyota Prius has a 1.3 kWh NiMH battery rated for a 20C discharge (20 * 1.3 kW = 26 kW). The MG2 motor is rated at 50 kW, but because of the small battery size in combination with the 20C discharge limitation, the motor's actual output is limited to about 25 kW.

The C-rating for a battery is a measure of how quickly the battery can charge or discharge. A 1C discharge is the discharge rate (power) at which the battery will be completely depleted in 1.0 hours; a 10C discharge is the power at which the battery will be completely discharged in 0.1 hours (ex 10 times the capacity of the battery).

Ex for 1.3 kWh battery:
1C discharge is 1.3 kW, the battery can supply 1.3 kW for 1.0 hours
10C discharge is 13 kW, the battery can supply 13 kW for 0.1 hours
20C discharge is 26 kW, the battery can supply 26 kW for 0.05 hours (3 minutes)

This is a pretty simplified explanation, and that's good enough for hand-waving : P The Audi e-bike battery can fully power the motor if it can discharge at > 4C, which seems pretty reasonable. That leaves either the less powerful bike's top speed claim being a fabrication or else it's extremely aerodynamic.

[Mithion]

The first bike is indeed the Audi E-Bike, the second is the PG Bikes Blacktrail which is very similar being all carbon fiber. The Blacktrial, Audi, KTM and a few other bikes all claim to use the same Clean Mobile drive system which is strange since all of these bikes use the same drive system but all have differing top speeds, ranges etc.

[Mithion]

Would the 3 pound weight difference have any effect, somehow I doubt it.

[Deadly Silent Ninja]

That leaves either the less powerful bike's top speed claim being a fabrication or else it's extremely aerodynamic.

Could it be a "perfect conditions" claim? I mean, if there was no drag or friction and they didn't factor the weight of the rider, would it be possible? A computer-simulated situation. It would still qualify as "fabrication," but they wouldn't be "lying." Or as protomech said: "just another form of marketing." 

[Mithion]

Could gearing have anything to do with it. You could look up both bikes for more specifics- Audi Wothersee E-Bike and PG Bikes Blacktrail.

There has to be a reason for a 30 something mph difference between the two.

[protomech]

http://www.jaylenosgarage.com/at-the-garage/electric/pg-bikes-blacktrail-electric-bicycle/

Brief coverage of the bike. The CEO of PG-Bikes says it's 3.6 kW motor in the video.

One other thing about motor ratings - there's a continuous rating, which is usually the power output over 1 hour.. then there's a peak rating, which is power output over a shorter period of time. It's possible the 1.2 kW rating is a continuous rating, and the 3.6 kW is a peak rating.

http://www.gizmag.com/blacktrail-worlds-fastest-ebike/16290/

Claimed max range 25-37 miles at max speed (25-35 Wh/mile w/ an 810 Wh battery). That sounds more like a 40-45 mph type top speed than a 62 mph top speed .. I don't care how light the bike is.

[Mithion]

I think you may be right with the over exaggeration of the top speed of these bikes. It's funny because there is a somewhat common practice for auto makers to put out lower power numbers for sports cars so people will be plesantly surprised when they see for themselves.

[Brammofan]

The top speed could also be something that the manufacturer set up in the motor controller software in order to conserve energy, increase range, decrease litigation, control liability insurance costs, make the bike more/less fun, or any number of other reasons.

[Mithion]

Did some research and discovered that-

The PG Bikes Blacktrail apparently uses a Clean Mobile Gear Reduction system with a Plattenberg Predator RC Motor.

Maybe this is how it could achieve the advertised 62mph top speed, the Audi E-Bike uses a motor with no gear reduction system.

[Richard230]

One thing that I have noticed is that every "production" electric motorcycle I have owned has had an optimistic speedometer, ranging from 20% off on my second GPR-S to about 8% off on my Zero.  Both of my BMW's also have speedometers that read about 8% fast. It seems to be an industry standard.  It is a lot cheaper to calibrate the speedometer to read faster than you are actually going than it is to make a more powerful motor. My guess is that most riders can't really tell the difference at high speeds and prefer the higher indicated speed for bragging and "bench racing" purposes.   

[Mithion]

Found this video, it's in German but right around the 2 minute mark they begin to show all the motor components. Cool video that shows some behind the scenes stuff.

[Richard230]

That is a neat e-bike, but it looks kind of expensive.   

Here is Honda's version:  http://www.visordown.com/snippets/hondas-latest-cub/20677.html

[Deadly Silent Ninja]

Instead the UNI-CUB is currently just a showcase for what Honda's robot engineers are capable of, so don't expect to see one in your local dealer anytime soon. If this is the future, perhaps a stronger version for stout users is in need as the pounds will pile on some users when they no longer have to walk anywhere. Perhaps Honda could upgrade the seat for a porcelain throne and get it to run on gas? Just a thought.

Read more: http://www.visordown.com/snippets/hondas-latest-cub/20677.html#ixzz1vGY8kcH5

Gas burning moving toilet??? Even if it was electric, I think that's where I draw the line...

[protomech]

It's electric. It'll probably spend all of its time indoors, anyhow..