That ^2 is a load-bearing exponent which changed my entire perception of safety.
With the oil crisis making it a good time to buy an ebike, I see the same problem popping up on bike forums that I faced when making my original purchase. At least here there’s the option between a Class 2 (32kmh max, usable on paved trails) or a Class 3 (45kmh max, limited to roads). When I bought my first bike I got a Class 3 because I’m getting 40%~ more speed for the same price. I could arrive at my destination 40% faster than I do on my cargo bike.
Assuming a rider+bike weight of 100kg and converting the two to m/s: 8.89m/s, 12.5m/s-
Class 2- 0.5(100)(8.89^2)= 3950 Joules of energy. That’s what you experience when crashing at max speed, the equivalent of falling off the roof of a building with one floor.
Class 3- 0.5(100)(12.5^2)=7,810 J.
Despite the speed only increasing by 40%~, the kinetic energy you experience in that crash almost doubles. The energy your brakes have to absorb to stop, and with that the stopping distance and wear, almost doubles. Your risk of fatality jumps significantly. You are now jumping off the tallest house in your neighbourhood and that is the level of injury you can expect from the slightest mistake.
If you or anyone you know is tempted to get a hyperbike because it goes much faster than a cargo bike, especially beyond the Class 3 speeds that I don’t even want to think about the injuries from, that impact force needs to be a primary consideration. I am confident that with proper safety gear I have a 90% chance to survive a crash with injuries that won’t last a lifetime. If my cargo bike was Class 3 instead of Class 2, I could not maintain that same level of confidence. It’s such a minor difference when you are looking at generic speed numbers but you will crash and that is the standard you need to prepare for.
Buy a Class 2 and maximise your weight-bearing utility over speed. Slow is smooth and smooth is fast.
The energy your brakes have to absorb to stop, and with that the stopping distance and wear, almost doubles.
The stopping distance is mostly a given but otherwise this is mostly a non issue despite getting brought up a lot. Halfway competently maintained V-Brakes which are easy as hell to service will block a tyre which is much more the limiting factor on a bicycle. As in: It’s not that hard to do 45kmh on an acoustic bike and v-brakes are fine for that, even with the bit of weight all the electronics add.
When I bought my first bike I got a Class 3 because I’m getting 40%~ more speed for the same price. I could arrive at my destination 40% faster than I do on my cargo bike.
I see the idea but I think that generally never works out in urban areas fairly regardless of mode of transportation bar jogging instead of walking and from my experience in the EU, being able to use paved trails, footpaths and such generally gets you to places much faster compared to more max speed both due to being able to cut across certain infrastructure instead of taking the long way around and the fact that things like bicycle + pedestrian infrastructure doesn’t usually have any sort of meaningful traffic or traffic lights in the way that car infrastructure does
Motorcycle speeds require motorcycle gear, plain and simple. The law hasn’t caught up yet but eventually it will and every bike capable of true road speeds will require a full face helmet, and only be able to be sold if it has motorcycle-tier disc brakes. And that’s if we get lucky and they don’t decide that they also require license plates and road taxes!
this equation is the physical reality of why SUVs are so deadly to pedestrians and other cars and it has to do with their mass
Bought a sub-compact car because I’d rather absorb the risk than transfer it onto something innocent, was able to stop just in time to touch the fur of the only animal I’ve almost hit in a decadeDefinitely one of those moments where I wish I was more broadly literate in maths. There are probably plenty of equations as horrifying as this one but it’s one of the few I have to consciously interact with daily.
mass and grille height. a lower front end could sweep you onto the bonnet, which is much safer than being knocked to the ground or pushed along, eventually under the wheels.
idk, you aren’t obligated to red-line the throttle the entire time. we don’t drive cars over 160kph much or at all, even if they’re capable
Originally I made this as a comment on /r/ebikes. The part I didn’t copy-paste here is that I normally ride at around 24kmh (6.67m/s), decreasing to a 10kmh or slower crawl around anyone I have to pass. On my 15 minute commute that might cost me up to an additional 5 minutes. But I spend those moments smelling flowers, watching birds, and doing other things that make that time enriching instead of stressful.
(0.5)(100)6.67^2= 2224.4 J. If 3950 J is a 100kg weight falling from a 4m one-floor building, now I’m falling from a 2.2m doorway. My padded jacket is enough to avoid a bruise instead of a broken arm.
edit: Not to say that 32kmh is without its merits. I think that’s the perfect balance between urban speed and safety as the equivalent to what I’d drive on the adjacent streets. I can react and stop fast, crash relatively safely, and save a couple minutes over driving+parking. It feels as fun as driving 70kmh on nearby winding canyon roads.
I agree with your post, but I think the power is the class 3 is very useful when used in the proper situations and the proper respect for the speed (as you described).
There are times I wish I had a Class 3. If I’m riding on a road or a perfectly flat and empty trail, it would get me from A to B faster. That’s ultimately the goal of my commute and I think Class 3 should be a legal category for those purposes, with everything beyond it being legally considered a motorcycle.
That equation isn’t just the stress I face when crashing though. It’s the stress the bike’s components face when riding normally. My Aventon Abound has a 40kmh mode I could unlock but I avoid it because the vibrational stress from hitting slightly uneven sidewalk panels at 32kmh has shaken bolts loose. You’re trading off short-term gain for long-term pain when you apply motorcycle speed stresses to bicycle components.
It’s true I have had to dump a ton of maintenance into my bike
the surface of a pedestrian sidewalk certainly isn’t designed or engineered for those speeds. i’ve gone that fast on a push bike but it was bombing a hill.
Sidewalk panels was bad word choice. Most of the urban trails here use a larger version of sidewalk panels that seem to have the same material. Generally even the ones that are nearly half a century old are in good condition with a legal speed limit of 32kmh and a courtesy speed limit of 24kmh. The main hazard on those trails comes from tree roots shifting the edges of the panels, but their size means there are few places where it isn’t a gradual enough transition to cushion the shock.
In my case I almost never rife in the sidewalk (which is why keeping up in a 25mph zone is handy)
Is this the reason all powerful e-bikes look so over built and heavy?
Built for torque. Electric motors can really fatigue a bike frame over time.
I’m a multi-modal kinda guy, so I’d like a lighter ebike especially so I can put it on bus racks or bring on the train.
I have 5800km on my e-bike, bought it in October 2023, and is running out of steam. I will need to bought a new battery any day soon. I have to charge it at the office and then back in home again.
Safety is a big factor but one that is pretty easy to plan for for those who want the speed. Like if you wanna go 60km/h plus on what is basically an electric dirtbike you can just wear a full face dirtbike helmet and gear. It’s still a very good point you make since I don’t wanna put on all that gear for a quick trip to get groceries.
I think for me one of the other huge factors in taking it slow on a bike or with personal transport in general is energy. The energy required to overcome wind resistance also scales with velocity squared. So going 1.4x as fast should require about double the battery capacity. Actually more than double, since the slower you go, the more significant your own pedal power is. I don’t have an e-bike yet, but even without the battery, I can ride so much farther if I take it easy and average like… 15km/h instead of trying for 20.
force = change in momentum / change in time
where momentum = m * v
is another very relevant equation here when looking at the mechanics of collisions. reduce the momentum change or increase the collision time and you reduce the literal (peak) force of the impact.
interestingly, kinetic energy is the time integral of momentum.
Good post, I would never consider such a huge difference in fatality chances between a 13 km/h max speed difference when trying to choose a bike, it seems so insignificant when you only compare the speeds
