Padraig’s eBike Explainer – Regenerative Braking

As a result of my day job, I see very few ads that are for anything other than e-bikes. I miss the days of seeing ads for crazy shoes. Recently, there has been some heavy advertising for e-bikes that claim they charge going downhill or while braking. I’ve had three different friends inquire about them. 

If my friends are asking me, I’m betting our listeners are wondering about them as well. 

So let’s cover the basics about recharging an e-bike by going downhill or braking. Yes, this is a thing. Some e-bikes can do it. The question is, do you want it? Before you say yes out loud, let me explain just what this entails. 

First, it requires a very specific kind of motor, called direct drive. This is a hub motor that is usually quite large. Unlike most hub motors, it doesn’t have gears inside. Direct drive motors generally weigh on the order of an extra kilogram compared to geared motors, sometimes more. Also, direct drive motors don’t produce much torque until the RPMs are high, which means that when starting and at low speeds, these motors tend to be sluggish. If the hill is steep enough, it might just slow to a stop. 

E-bikes with mid-drive motors, which are generally the best motors out there, can’t do this. 

Some e-bikes with direct-drive motors can recharge the battery with braking. That said, in my limited experience, this braking is good for scrubbing speed for turns, but not for precisely controlled stops. 

Also, you’ll want to give some thought to how much time you spend braking. For most of us, it might only add up to a minute in an hour-long ride. That’s not much time for charging. Also, you have to think about efficiency. Braking this way really doesn’t recapture much energy. 

But there’s also recharging on downhills. The first rule of generating electricity is that you either need a chemical reaction, like with a battery, or you need mechanical energy to be converted into electricity. Going downhill is a terrific opportunity to take that energy—momentum—and turn it into electricity. How fast we go downhill depends on how much resistance there is, though, right?

If you sit up high you go slower. If your wheels have rotten bearings, you go slower. If your tires are inflated to 8 psi, you go slower. And if there is a mechanism taking that energy and converting it into electricity, that is going to create drag at the rear wheel. And drag means slower. 

For riders who don’t like the zoom, don’t like going fast, regeneration is a terrific way to keep an e-bike’s downhill speed under control. But. 

I keep doing that, don’t I?

The recapture isn’t all that efficient, as with braking. You could descend for a couple of miles and maybe only claw back 1% of the battery’s charge. 

The reason why the recapture is so low owes to another factor. Cyclists don’t weigh much. There’s not much energy tied up in a 180-lb. Cyclist going downhill. Even less if you only weigh 130 lbs. 

These e-bikes aren’t bad. I’m not against them and I’m not going to tell anyone not to buy one, but I do want to make it clear that the marketing claims I’m seeing make them seem almost perpetual-motion machines. They aren’t. 

For me, the real deal-killer is the fact that it would sap the whee from every descent. Put another way, you have to trade dopamine for electricity. I’d be willing to trade electricity for dopamine, but not the other way around.