Legal speed restrictions on electric bikes

[Jdsk]

Wot's this? A confusion over power? Well, we do tend to use the terms for energy use and storage rather loosely at times. In common-speak:

Battery capacity (it's maximum stored energy) is measured in watt-hours - the quantity of power stored.

We do tend to use imprecise terms for the physical quantities. It sometimes isn't a problem. But of course we should switch to precise terms when needed, typically in a logical chain of argument but often in a disagreement.

Battery capacity is measured by the energy stored, not the power. It isn't possible to measure power in watt hours.

It's also better to leave units of measure out of it until the type of quantity being discussed is clear. Energy and power are different types of quantity whatever units are used.

Jonathan

[pete75]

Incidentally, the legal speed limit for electric bikes (that 25kph) is allowed a variance of 10% in practice, or so I read. Presumably this is because the power-limiting software governing the e-motor employs a single assumed wheel size? Putting a fat tyre on a wheel will make it's wheel circumference significantly longer, so your bike will go further per wheel revolution than the assumed circumference-value used by the speed limiting software to cut off motor power. You'll be able to go a bit faster than 25kph before the motor cuts out.

Cugel, now waiting to be corrected.

Yep. I know someone who has built a motor intended for a 20" wheeled bike into a 700C wheel. It thinks it's doing 25kph when it's really going a lot faster.

[rogerzilla]

The battery contains all the power on an ebike. The motor doesn't produce power it, converts battery power into motion.

Power = energy (or work)/time.

The battery contains energy, not power. The motor develops power from the energy stored in the battery.

Is a watt not a unit of power then?

It is. So batteries aren't measured in watts (at least, not by anyone who knows what they're doing). Watt-hours or kilowatt-hours, yes. Confusingly, many batteries are actually measured in ampère-hours or milliampère-hours, which is a measure of electrical charge (like coulombs) and not energy. You have to multiply by the voltage to get watt-hours. There are also assumptions about discharge rate used in the Ah figure.

[simonhill]

I liked the analogy that your battery is your fuel tank.

[andrew_s]

There is, in Europe, a class of ebike called the S-pedelec, which is restricted to 45 kph (28 mph), rather than 25 (15 mph) as in a regular e-bike.

They can be bought in the UK, and the sellers will usually give assistance/advice on getting them properly registered.
Once registered, they are effectively an electric moped, and you'll need a licence, insurance, and a motorbike helmet.

There's some advice here:
https://www.google.com/amp/s/ebiketips. ... 1637%3famp

Unfortunately, I think a DIY conversion will be a fair bit more difficult.