In the world of electric bicycles, one type of motor drive reigns supreme: the hubmotor. Sure, mid-drive ebikes are slowly gaining momentum, but hubmotors are king right now. And if hubmotors are king, then that makes friction drive electric bicycles the lowly cousin, still part of the royal family but never destined for the thrown.
Perhaps I’m being hard on friction drive electric bicycles. Let’s take a step back and look at the evolution of the friction drive to get a more fair assessment. First, some mechanics.
Friction drives, like their name would imply, work by powering the bike through a friction connection. This usually manifests itself in a roller on the rear wheel of the bike. The roller is connected to the output shaft of the motor, or even occasionally is the motor shell itself. Some type of abrasive coating is applied to the roller to help it grip the tire, and some method of tensioning is applied to help the roller stay connected to the tire. When the throttle is engaged, the roller spins the tire which in turn powers the ebike.
Friction drives were popular in the early days of the modern electric bicycle. A great example is the EV Warrior ebike, which was a California designed, Mexican produced dual motor friction drive ebike designed to help automakers in California adhere to a law requiring a certain number of sales of electric vehicles in all car dealerships in the state.
The EV Warrior had two opposing motors powering a friction drive roller over the rear wheel, a long frame designed for cruising at high speed and an all around reliable setup. Alas, under pressure from the oil companies, the California Air Resources Board backpedaled on their tough electric vehicle legislation and the rest is history. Friction drives fell back into obscurity as mid drives and hubmotors became the new kids on the block, winning praise and finding an audience all over Asia and then after a few more years in Europe and the Americas too.
Friction drive electric bicycles fall into obscurity
So why did friction drive electric bicycles lose out to hubmotors? Friction drive ebikes have a number of problems inherent to the friction drive that hubmotors and mid-drives simply don’t have.
The first major problem is efficiency. Powering a wheel by rubbing on the tire simply isn’t a highly efficient way to get around. A lot of energy is lost as heat and scraping off bits of rubber, which brings us to our next point: tire wear.
Friction drives are tough on tires. A normal bicycle tire only makes contact once every revolution. A point on the tire touches the ground, spins around, and repeats. On a friction drive electric bicycle, the tire now has two points of contact for each revolution, and one of those points of contact is a screaming, 3,000 RPM rough and jagged steel roller. Tires tend to wear out pretty quickly under those conditions.
Another downfall of friction drive electric bicycles is that the system isn’t an all weather solution. Friction drives only work when there is sufficient friction between the roller and the tire. Get a little water on the tire and suddenly the coefficient of friction drops considerably. All it takes to sideline a friction drive can be riding through a puddle of water.
Rainy day? Forget about it.
Snow? Good luck with that.
If you live in sunny southern California, you might have been able to get away with a friction drive electric bicycle. But anywhere else in the country that gets even sporadic rainfall and you’ll find yourself leaving your ebike in the garage more often than you’d like.
Tire choice is also pretty limited. Many people like to use mountain bike tires that have knobby tread patterns and give better grip for riding on dirt or gravel paths. There are many hybrid tires that have a good combination of smooth tread for roads and knobs for trails. Most of these tires won’t work as well with friction drives because you need a fairly flat and regular tread to interface with the friction roller and still get any reasonable degree of efficiency.
Friction drive electric bicycles are making a comeback
Friction drives certainly have their downfalls, but that’s not to say that newer versions of friction drives haven’t been improving. Two members of Endless Sphere, Kepler and adrian_sm, have been developing a new generation of friction drives based on RC airplane motors which promise higher efficiencies and less tire wear. These new designs weigh in at only a few pounds, self regulate the pressure between the roller and the tire, and use motors that operate more efficiently. The jury is still out on how much better these designs work when the tire gets wet.
A new commercial, easy to install friction drive kit called the Rubbee is also taking a shot at the market. Its slick design incorporates the motor, battery and controller all into one unit to make installation easy and painless. Well, almost painless, if you don’t count the $1,200 sticker shock. For the time being though, unless you’re looking to build something custom and already have a bunch of appropriate parts laying around or want to fork over the cash for a retail friction drive electric bicycle kit, consider a nice hubmotor or mid-drive electric bicycle kit instead.