I am planning on converting my road bike into an electrical one and I can see that I can get average speeds of 35-40kmh. So I am planning on biking long distances at this average speed. Is it safe to do it? What can fail at such high speeds in long run, considering different types of asphalt quality. I know that it depends on the bike manufacturer, but can a generic failure be anticipated?

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    40kmh is 25mph, a speed which many cyclists can maintain for an hour or longer on reasonably smooth, flat roads. Commented Sep 7, 2017 at 12:42
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    Depending on your country, an e-bike which can go 40kph with the assist on can be legally considered to be an electric motorcycle, and then it's forbidden to go on bike lanes, you must wear a motorcycle helmet (!), need a license plate and special insurance... So make sure you check before ordering any parts!
    – bobflux
    Commented Sep 7, 2017 at 15:26
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    Except for the steepest of climbs, I'd imagine that almost all of the Tour de France is ridden at a pace above 25MPH. Granted, they ride professionally maintained bikes, but they're not rebuilt every night checking for "speed-induced wear".
    – FreeMan
    Commented Sep 7, 2017 at 15:53
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    I'd think that the torque is still something to be concerned with -- a typical hub motor will put a lot of torque into the seat stays / chain stays that may not be designed for it. On a normal bike, the force is mostly from the chain being pulled forward, putting the chain stay under compression (mostly), but a strong hub motor is going to put a twisting force on the rear fork ends. It probably won't matter much on a cheap, heavy steel framed bike, but it could cause problems on a lightweight racing bike.
    – Johnny
    Commented Sep 7, 2017 at 16:23

3 Answers 3


Stopping distance is definitely something to be aware of. As speed increases, the distance needed to stop increases non-linearly. For illustration, in cars, with 1 sec reaction time:

  • 20 km/h = 8m
  • 30 km/h = 13m (50% increase in speed, 63% increase in distance)

(Source: http://www.nzci.co.nz/tools-calculators/stopping-distances.html)

In other words, it may feel safer than it really is.

Also make sure your conversion has a brake sensor cutoff, you wouldn't want your braking distance increasing because the motor is pushing while you're frantically trying to stop.


Yes - the bike can stand higher speeds fine.

Generally. wear is caused by distance travelled, not the speed it travels.

You might suffer increased wear through increased vibration but this would be very hard to measure and is likely quite small.

On the other hand you might suffer increased damage through accident or sudden pothole, which would have been more avoidable at a lower speed.

Additionally, your bike's braking distance will be increased as velocity increases. So your electric bike requires more defensive riding, anticipation and prediction of what will or could happen. Consider replacing brake pads with really good ones (like kool stop) and consider your tyre tread too.

I rarely mention head protection on SE, but do consider that your momentum is increased by riding an electric assisted bike. That energy has to go somewhere if things go badly, best not through your head.

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    Personal story - I have an old 90s rigid MTB which I electrified with a hilltopper kit, a front wheel hub motor. It worked stunningly well, but I was doing peaks of 40-43 km/h on flat roads, and came close to being doored at least twice. So I started riding a lot further into the lane. If you got doored at 40 km/h you're in for AT BEST a world of pain. Example youtube.com/watch?v=VOV9dl2ms4w I saw it coming.
    – Criggie
    Commented Sep 7, 2017 at 9:19

As you may know kinetic energy is ½mv² where m is mass, v is velocity. You have more mass on the bike with the addition of motor and batteries. A bigger concern is that energy increases with velocity squared.

For 100 Kgs of bike, motor, batteries and rider:

  • 10 Km/h: 0.4 Kj
  • 20 Km/h: 1.5 Kj
  • 30 Km/h: 3.5 Kj
  • 40 Km/h: 6.2 Kj

Note that going from 30 to 40 Km/h almost doubles the kinetic energy.

The two things I would be concerned about that have to absorb energy are brakes and wheels.

Brakes obviously have to absorb all the kinetic energy when you need to stop. There are concerns about stopping distance as noted by others. The wear on pads rotors or rims will also be increased. You will also have think about the heat capacity of the brakes. Is repeated stopping from your max speed going to overheat the brakes?

You don't say if you have disc or rim brakes. I personally think I would not have enough confidence in rim brakes for this application. I'd want to fit the beefiest disc brakes possible, preferably a set intended for a downhill mountain bike.

The wheels are going to take more punishment as they roll over bumps at higher speeds. If you have a set of skinny rims with a low spoke count, I'd be looking to upgrade to something strong with a high spoke count (which you would actually need to support strong brakes also).

  • So your answer is, basically, "No, a standard bike can't do that", even though the pros have been riding at closer to 50km/h with rim brakes for decades and huge numbers of amateurs zip around at 40km/h, too, without needing extra-strong brakes or extra-strong wheels. 40km/h is what road bikes are designed to do. Commented Sep 7, 2017 at 19:52
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    @David Richerby My thinking is that the difference is with a motor the bike will be seeing sustained and constant use accelerating up to then braking down from 40 Km/h. Note I didn't say "no this is not possible", I brought up two issues I personally would be concerned about. Commented Sep 7, 2017 at 19:58
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    "[S]ustained and constant use accelerating up to then braking down from 40 Km/h." Yes. That's normal for a road bike. It's what they're designed to do. Commented Sep 7, 2017 at 20:14

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