Tech FAQ: E-Bikes for rehab while riding with fast friends

This article originally appeared on Velo News

Have a question for Lennard? Please email veloqna@comcast.net to be included in Technical FAQ.

Dear Lennard,

From an accident I was in at 17, I had to have a knee replacement six months ago. Recovery is going well. However, I wanted to be able to get out and start riding with my friends again. Especially now that the race season is about over. The group I ride with is a fast group, pros to cat IIIs. I thought what if I get an e-bike to do the group rides. I was initially a little concerned because most of the e-bikes I see people riding are city bikes aimed at commuters and recreational riders.

I did a search, and at first I was really excited. Race looking/race geometry - check, full carbon fiber - check, 105 to Dura Ace components - check. But then I read the fine print. They're class 1 eBikes and assist turns off at 20mph! This left me dumbfounded. They're sheep in wolf's clothing!

Therefore, it turns out on any flat section with my group I'd have zero assist and be pushing around a bike that weighs 20ish pounds more than my regular bike! On a flat ride I'd get almost no assist the entire ride. Here I was thinking I can set the assist to start kicking in at certain programable power level. Then as I get stronger, I would raise the kick-in power level and/or lower the amount of assist.

Am I missing something? The hacks I've seen on my searches don't appear to work with these race (looking) bikes. I was assuming bikes like these would be targeted to people like me, but it seems we're not there yet.

– Mike

Dear Mike,

This is a great question, and it's important for people to understand this distinction you're pointing to if they plan on using an e-bike to ride with fast friends. Yes, using a Class 1 e-bike is an absolute non-starter if you want to go on group road rides. There are other options, however.

The basics: Class 1 e-bikes are defined as being pedal-assist only (i.e., no throttle) and are limited to only assisting up to 20mph. Class 2 e-bikes have a throttle (i.e., the rider need not pedal; the motor will drive the bike with or without the rider pedaling), and, like Class 1, they also offer no motor assist beyond 20mph. Finally, Class 3 e-bikes are, like Class 1, pedal-assist only, and they will assist up to 28mph.

After the motor-assist speed limit has been surpassed, whether it is 20mph (Class 1&2) or 28mph (Class 3), an e-bike just becomes a heavy bike and is also (a bit or a lot) harder to pedal than an "analog bike" of comparable weight. There are a number of reasons for this. On mid-motor e-bikes, the chainring is not fixed to the crankarm; when you turn the cranks backwards, the chainring does not turn. When pedaling, there are more parts turning with the cranks than just the spindle to turn the chainring, so there will be more resistance than spinning a crankset spindle inside of a pair of bottom-bracket bearings. Furthermore, on e-bikes with a tiny front chainring, the chainring rotates faster than the crank, making it effectively a larger chainring, and those planetary gears inside of the motor housing that gear up the chainring to turn faster than the legs create noticeable resistance when pedaling.

For example, Bosch Gen. 2 Performance motors turn the chainring 2.5 times the rate of the cranks, so an 18-tooth front sprocket is equivalent to a 45-tooth chainring, and a 20-tooth front sprocket is equivalent to a 50-tooth chainring. Because of the planetary gears turning inside to make that happen, pedaling feels very heavy on an e-bike with its Bosch Gen. 2 Performance motor turned off. By contrast, a Bosch Gen. 4 Performance motor's chainring is a standard size and turns at the same speed as the crank, so the resistance the rider feels when pedaling it without assist is far less.

Even with a relatively slow group, whenever the road turns downward gently, the group's pace will exceed 20mph, and a fast group will constantly be above 20mph on flat ground, as you said. A rider trying to keep up on a Class 1 e-bike will have not only the lack of motor assist but also the greater pedaling resistance and weight of the e-bike making keeping up harder. If the rider is on an e-bike because he or she otherwise couldn't comfortably keep up with this group, then a Class 1 e-bike will make it even more difficult to do so.

A Class 3 e-bike, however, which is what I ride, is a joy to pedal in a group or with individual fast riders. I can keep up with anybody on an analog bike and have fun while doing it. My wife, however, won her e-bike in a raffle--a 65-pound, mid-motor, flat-bar, 26" balloon-tire, fender, rack, and chainguard-equipped practical machine. Even after I took 23 pounds out of it by switching to carbon wheels, fork, seatpost, and drop bar, losing the fenders and chainguard, and switching to 11-speed SRAM Red hydraulic-disc-brake drivetrain and fast 700C tires, group rides were still not doable due to the 20mph limit. More on this later.

America's winningest couple, Davis Phinney and Connie Carpenter, are e-bike riders, and it was Davis who first convinced me to get an e-bike, after my riding had become short, solitary, slow and boring due to developing a heart arrhythmia. At the time, Davis had a Trek and Connie had a Cannondale, both Class 3, and we have done many rides together on them, including a number of Davis Phinney Foundation fundraising rides.

Davis also has a carbon Pinarello e-bike, and, incredibly, as you found in your search, despite it being a very high-end, pricey bike, it is a Class 1. He can ride it with others who understand its limitations and adjust their riding accordingly, but, for the reasons described above, it is not the tool of choice for riding his own charity rides and other group rides.

Which brings me to hacks to increase the motor-assisted speed. I don't know which ones you alluded to that would not work with the e-bikes you found. There is a very simple hack that I'm convinced will work with any e-bike, and that is to lie to the bike's computer about the wheel size. For instance, I converted my wife's e-bike from Class 1 to Class 3 simply by inputting in her computer controller the diameter (or circumference--I can't remember which it asked for) of a 20" wheel, even though she (now) has 700C wheels. I had to put a separate bike computer on her handlebar that would display her actual speed, since the speedometer on the e-bike computer display is now useless. When her e-bike computer now says she is going 20mph, she is actually going 28mph--voila, a Class 3 e-bike emerges from its Class 1 beginnings (and it still has the Class 1 sticker).

All e-bikes, at least mid-motor ones, generally measure wheel speed with a magnet on the spoke of the rear wheel, and the speedometer of course needs to know the wheel size to do so, meaning that it can be manipulated by adjusting the wheel-size input. Some e-bikes only allow a dealer to access the inputs for the on-board computer, which of course may preclude this. But low-end ones are as simple to set up this way as any cheap bike computer. And many high-end ones use a smart-phone app as a display and a controller, putting the inputs in the users' control.

While that speedometer hack can give you precision for the assist-limit speed, there are other hacks that can remove limits entirely, although they may preclude the updating of a high-end e-bike's firmware. For instance, moving the speed sensor from the inside of the chainstay to the outside of the chainstay and moving the magnet from the wheel to the crankarm fools the e-bike's on-board computer into thinking that the rider's cadence is the rotational frequency of the wheel. Since a rider can never spin their feet as fast as a 700C wheel going 20mph, much less 28mph, this effectively removes any limitation on speed at which the motor will still assist. A high-end, dealer-serviced system like Bosch would recognize that it had been hacked, and if it were to ever be plugged into a Bosch dealer's or service center's computer, it would be flagged as such and would be rendered unserviceable.

Addressing your original contention that "we're not there yet" with e-bikes on which you can ride with fast friends, au contraire! There are plenty of Class 3 drop-bar e-bikes that would do what you want without any hacking. I mentioned that Trek and Cannondale make them. The Specialized Turbo Creo SL is also Class 3, and there are other examples, including my stock e-bikes as well as my custom ones. (Given my height as well as my mentality, when I acted on Davis Phinney's e-bike suggestion, I of course was going to build one for myself rather than buy one.)

I love your labeling of high-end, drop-bar, lightweight "race (looking)" Class 1 e-bikes as "sheep in wolf's clothing." As you say, there are high-end ones like this Look that, seemingly inexplicably, assist only up to 20mph. One reason could be state laws limiting e-bikes in state parks and on bike paths to only Class 1 or 2. Since it is impossible to tell the class of an e-bike except by close inspection, I think it would make more sense to instead legislate an enforceable standard, like speed limits on bike paths, but I don't make the laws.

-- Lennard

Lennard Zinn, our longtime technical writer, joined VeloNews in 1987. He is also a custom frame builder and purveyor of non-custom huge bikes , a former U.S. national team rider, co-author of “The Haywire Heart,” and author of many bicycle books including “Zinn and the Art of Road Bike Maintenance,” “DVD, as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.”

He holds a bachelor's in physics from Colorado College.

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