38-39mph will take, on flat well-paved roads with no wind, about 2500W out of the battery. Basic quick simulation using a 72v battery and the GMAC in a 26" wheel and a 40A controller:
Our ebike motor simulator allows you to easily simulate the different performance characteristics of different ebike setups - with a wide selection of hub motors modeled, and the ability to add custom batteries and controllers and set a wide variety of vehicle parameters you'll be able to see...
ebikes.ca
It will overheat the motor doing this in about 12 minutes.
If you have headwinds, hills, or non-flat/paved roads, then it will take more power than that to do the work; you can use that linked simulation to guesstimate different scenarios.
| Graph | Syst A |
| Wheel Torq | 43.0Nm |
| Mtr Power | 2352W |
| Load | 2353W |
| Efficiency | 85.3% |
| RPM | 522.6 rpm |
| Electrical | Syst A |
| Mtr Amps | 49.1A |
| Batt Power | 2756W |
| Batt Amps | 39.1A |
| Batt Volts | 70.5V |
| Performance | Syst A |
| Acceleration | -0.00 mph/s |
| Consumption | 67.8 Wh/mi |
| Range | 23 mi |
| Overheat In | 12 minutes |
| Final Temp | 194 °C |
Is this the bike you have?
If not, please link to the specific one you have so we can see what you've got to work with so we can make specific suggestions.
For now, going with the specs on that page (quoted at the end of the post), it says there's a 350w bafang geared hubmotor in a 20" wheel, with a 48v/14Ah / 672Wh battery.
So to do your ~40mph it will take about 7 times the amount of power that motor is "rated" to take. Even though it could take more (maybe 500-600w+) for short perioids, at seven times that it will probably fail in a few minutes or less from overheating. So you'll need to replace it with something capable of handling at least 2500w; none of the geared motors do this so it will be a heavier and larger DD hubmotor, perhaps the QSmotors' (not the clones, but the actual one) QS205-50H as it is better quality than most of the ones I've seen the insides of.
Since it's nominal speed is around 20mph (even without a limiter it probably can't go very much faster with the equipment on the bike), then to go twice tthat speed you'll need to replace the battery with one about twice the voltage you already have, about 96v, to spin that motor fast enough in that small wheel.
The controller you have can't take that, so you'll need a new one capable of at least 2500w and 96v and 26A+ battery current (possibly with higher peaks during acceleration)..
The battery you have doesn't have enough voltage, so it has to be replaced for that with a 96v or higher voltage battery, and it will have to be able to supply 26A continuously (possibly with higher peaks during acceleration).
I'm not sure that the bike itself and it's brakes and such can safely handle the extra speed either, but I don't know the bike and it's components well enough to say for sure, and it also depends on your riding conditions style / terrain / etc.
Technical Specs
| Best Use | Recreational Cycling
Urban Cycling |
|---|
| E-Bike Classification | Class 1: pedal assist up to 20 mph |
|---|
| Motor | Bafang 350W with Bafang 48V/14AH removable battery |
|---|
| Motor Type | Geared Hub |
|---|
| Motor Torque (Nm) | 80 Newton meters |
|---|
| Battery Type | Lithium Ion |
|---|
| Battery Capacity (Wh) | 672 watt hours |
|---|
| Battery Removable | Yes |
|---|
| Charge Time (hrs) | 8 hours |
|---|
| Pedal Assist Range | 40-50 miles |
|---|
| Frame | 6061 Aluminum |
|---|
| Step-Through | Yes |
|---|
| Bike Suspension | No Suspension |
|---|
| Fork Travel | Not Applicable |
|---|
| Crankset | Alloy with 48T steel chainring |
|---|
| Bottom Bracket | Bafang |
|---|
| Shifters | Shimano Altus, EZ-Fire, 7-speed |
|---|
| Rear Derailleur | Shimano Altus, 7-speed |
|---|
| Rear Cogs | 11-34T 7-speed freewheel |
|---|
| Number of Gears | 7 gear(s) |
|---|
| Brake Type | Hydraulic Disc Brake |
|---|
| Brakes | Tektro Hydraulic with 180 mm rotors |
|---|
| Brake Levers | Tektro |
|---|
| Rims | Co-op Cycles double-wall, 36h |
|---|
| Front Hub | 15 mm thru-axle 36 Hole alloy |
|---|
| Rear Hub | 36 hole |
|---|
| Wheel Size | 20 inches |
|---|
| Tires | Schwalbe Super-Moto-X with Double Defense GreenGuard puncture protection |
|---|
| Tire Width | 2.4 inches |
|---|
| Handlebar Shape | Riser Bar |
|---|
| Handlebar | Co-op Cycles 6061 aluminum |
|---|
| Stem | Promax HQ adjustable |
|---|
| Seat Post | Co-op Cycles aluminum |
|---|
| Saddle | Co-op Cycles Comfort Plus with memory foam |
|---|
| Pedals | FPD alloy |
|---|
| Headset | 1-1/8 x 1-1/2 in. threadless |
|---|
| Chain | KMC for e-bike |
|---|
| Weight | 54 pounds |
|---|
| Bike Weight | Bike weight is based on median size, as sold, or the average of two median sizes. |
|---|
| Gender | Unisex |
|---|
| Sustainability | From a Climate Neutral Certified brand |
|---|
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