thanks both!
So it would just be for everyday commuting I’ll list some of the criteria I’d ideally want the bike to fit into (please let me know if anything I say in unrealistic/not worth the price)
- my weight is 76kg
- It would be for daily commuting, max distance of an hour. So a battery life of about an hour would be great (not sure what the average battery life is?)
- Id like to go 20mph as a top speed
- I live in London and it’s mostly flat - some hills here and there. (Not sure what normally happens if an e-bike that isn’t purpose built for hills was to go up one?)
- in terms of pedalling, not sure the best way to measure how hard I pedal. But I use gears 3 and 6 so maybe a little bit less than that.
- and I’m not too handy with bikes but willing to learn where I need to!
That means in principle you can use any kit. The normal 36v 15 amp ones would give you enough power. The 20 mph complicates things a little since most kits are designed for 15.5 mph, which is the legal limit.
Some kits fix the max speed to 15.5 mph so that you can't change it. You must check before buying whether that's the case.
Next, you need to understand how motors work. There's a max speed any motor spins to at any voltage. If you increase the voltage, it will spin faster. A motor that can spin 200 rpm at 36v will spin to 260 rpm at 48v. A motor makes max power at 75% of its max speed, after which the power ramps down to zero, so no power at max rpm.
15 mph is about 200 rpm with a 26" wheel, so if you want a modal speed of 15 mph, you need a 260 rpm motor, and if you want a modal speed of 20 mph, 330 rpm. You can get the 330 rpm by running a 36v 260 rpm one at 48v.
Now bear in mind that a 36v battery is 42v when full and 32v when empty. That also affects the motor speed, so if you only do short journeys and your battery is nearly always full, you can get about 10% extra speed.
What it all comes down to is how important that 20 mph is. Let's say you got an ordinary 36v kit. Normally the motor is 260 rpm for 15 mph cruising. If unrestricted, it has some power above 15 mph. With a full battery, 20mph would be achievable with medium pedalling, but not with an empty battery.
The problem is that many sellers of kits don't tell you the rpm of their motors.
When you buy a mid drive kit, none of this matters because you use the gears to get whatever speed you have power for. Instead, the speed of the motor needs to match your pedalling speed. If you pedal too fast or too slow compared with the motor's speed, you can't get the power you want, and it can affect the motor's efficiency when you pedal at the wrong speed, so choosing the right one is important unless you can pedal at any speed comfortably. The downside to a mid drive is that you have to do a lot more gear changing to keep the motor turning at a suitable speed, which is not so relaxing as a hub-motor, and it can be downright annoying in winter with frozen fingers. Also, reliability is not so good. You nearly always need to open up the motor from time to time to service or repair it, especially if you use it in all weathers. Lastly, the power goes through the chain, which causes extra wear on the drive train, so a bit more expense and maintenance. None of those things are a big deal, but the next is something to think about. In heavy traffic with derailleur type gears like yours, when you forced to stop suddenly in the wrong gear, not only is it more difficult to start up again, but you can do some serious damage to the gears. That's not a problem for country riding, but it's going to be annoying in heavy London traffic. When you have a hub-motor, it'll bail you out until you get enough speed to start pedalling an shift gears comfortably.
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