20" vs 26" bike wheel, what's best?

cwah

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Hello,

I've heard a lot of time here that the smaller the wheel, the better as motor spins faster and is more efficient... and that there are also better acceleration.

But still, most of the bike I see are 26" wheel... or sometime 24" wheel.

I don't really see any smaller wheel, and I wonder why?

Most of the ebike frame kit are for 26" too:
https://endless-sphere.com/forums/viewtopic.php?f=3&t=68280

So if smaller wheel are so much better for motor performance, why no one is having 20" wheels???
 
cwah said:
So if smaller wheel are so much better for motor performance, why no one is having 20" wheels???

The reasons are similar to why recumbent bikes are so rare despite being more efficient through the air than a more typical bicycle. Add the myth that persists about DD motor winding turn count having a benefit similar to a change in gearing that allows a larger wheel to be used, and you end up with a perfect combination for logic and science to be ignored.
 
A hub will max out at a specified RPM. If there is enough power, the same motor on a 20 moved to a 26 will go faster. Also with some of the bigger hubs it can be tricky to attach the fitting on an air pump.
 
a lot of the foldable bikes use 20 inch wheels and 16 inch wheels . The small wheels make a small package and lighter weight , this is a desired effect when trying to fit a bike into the trunk of a car or making it small enough for placing in the overhead luggage rack on a commuter train. I should point out that there are many 16 inch wheels in use, two common are the 16 inch 305mm used on bob type trailers and the 305mm is also the front wheel of the out of production recumbent bikeE. The 16 inch 349mm wheel is used on front and rear of the brompton folding bike and three wheels of the greenspeed gt3 tadpole trike.
the different wheels do not fit into different frames without modifications and the tires are usually not interchangeable.
The 20 inch wheel also comes in many different sizes
The only e-bike kit seller that i have seen post the ISO metric number is ebikes canada -grin tech. This ISO number is need to insure the wheel will fit in your frame.
Back to small wheels, the pro are they make a smaller package, accelerate faster, and can be geared low to climb hills easier due to the gear inch formula.
big con is when hitting potholes,
bigger wheels have a lot of pluses including ability to get into higher gear inches using a derailleur system and maintain a high speed easier, not spin out
i am sure others can add many more
mixing wheels sizes as in done in many recumbents also aids in aerodynamic design. Goldrush uses a 622mm rear and either a 406mm OR 451mm front wheel
 
I suppose the question may be about the comfort of use vs efficiency of the motor?

For example, recumbent are obviously more efficient but as a commuter it's not as convenient as a stand up bike. Thinking about how I'd climb stairs, manage potholes, car not seeing a lower bike or dealing between road and pavements are all factor that made me keep the stand up bike...

Maybe it's the same for 26" vs 20"?

The question would be if the added acceleration and efficiency are enough to compensate on the practicality of a full size 26" or even 29er that make the road much smoother, manage stairs and pavement jump way easier?
 
ebent said:
A hub will max out at a specified RPM. If there is enough power, the same motor on a 20 moved to a 26 will go faster. Also with some of the bigger hubs it can be tricky to attach the fitting on an air pump.

Simply increase the voltage.

FWIW going with a larger wheel does not always result in higher speed. That's because typical DD hubmotors are too steeply geared when used in large wheel.
 
Well, bear in mind so few bikes are made for a motor. But it's true, most ready made ebikes keep the 26" wheel. It's not because it's better, its so they can still sell them. People want to buy "normal". Only if you pedaled without power a lot, would a 26" wheel for street use really make the most sense. For rolling resistance. And once you get into cargo bikes, 20" would still be better, for better torque.

Once you hub motor, then the only good reason for 26" rear wheels would be off road, like rock staircases.

BTW, finished my 20" er. Really rides nice. Re the speed, at the very most, I see only 2 mph less speed when running the exact same motor in 20" vs 26".

One thing though, a 20" hubmotor needs to be in the right frame. I'm NOT impressed with a full suspension MTB with 20" wheels, and pedals cut down to shorties, for example. Nor do I like riding a bmx bike. Nor do I want folder. But a full size bike who's frame was designed for, or modified to work right with 20" rear wheel can be great.

This is why I set out to learn to weld better.
 
Like for like, 20 inch wheels have a much harsher ride, more rolling resistance, less traction, and accelerated wear characteristics compared to 26 inch wheels. The rubber and casing have to distort a lot more to establish a contact patch of a given size.

If you use a fatter tire on the small wheel, you can improve the ride and traction up to a point, at the cost of even more rolling resistance.

It's not coincidental that normal bikes for adults have used 26 inch or larger wheels since the invention of the pneumatic tire. If you check out car and motorcycle wheels, all of those are larger in outside diameter than 20", and they have suspension too. Bigger wheels ride better and work better.
 
Exactly right, for pedaling. We're not talking about a bike without a motor.

Definitely, the harsher ride needs to be compensated for, by lengthening the rear end. That's why no folders, or bmx bikes for me.
 
I still suggest answering for yourself why you never see cars with such small wheels.
 
Yes, look at them. Their tires' outside diameters are much closer to 26" than to 20".
 
Chalo said:
Yes, look at them. Their tires' outside diameters are much closer to 26" than to 20".

There are no electric motorcycles with hubmotors, but powerful electric scooters with hubmotors in wheels with an OD of 21" or less are made literally by the millions. Why do you think that is?

Those who continue to insist on putting big wheels on DD hubmotors and run them at high power or under strenuous low speed conditions will continue to have heat problems. A smaller wheel means you can run a smaller lighter motor at the same power, or run the same motor at higher power.

Excuses to avoid running a DD with the proper gearing are lame. My 19.25" OD wheel rides like on a cloud due to the suspension, so that excuse doesn't fly. Motorcycles commonly run smaller wheels on the rear for better off road traction, so that one doesn't hold water either. While riding on stairs may be kinda ok now, it's only because almost no one is doing it. Imagine the problems if it became common, so that's not a valid excuse to gear your motor too steeply either.

If your ebike is really a bicycle with electric assist, then it makes perfect sense to stick with what holds true for bicycles.
 
When we see legitimate mass-market electric cars and motorcycles with hub motors, they will have wheels substantially larger than 20 inches OD.

People adapting hokey scooter motors to build even hokier small wheeled homemade bikes is not proof of merit. That's like deciding that shopping carts are suitable street vehicles just because you can find people pushing them in the streets of every city.

Scooters of all kinds are acknowledged to be much less capable than motorcycles with full sized wheels.
 
Chalo,

The part you're missing is that running a larger wheel requires more torque for the same thrust. That cannot be accomplished by any change in the windings, only a larger heavier motor can make more torque unless the evasive high temp superconductor becomes available. That means until a better motor is designed we're stuck with smaller wheels ruling the roost for direct drive hubmotors. What's required is the same magnetic forces at a greater radius without gaining weight and without creating greater iron cores losses or losing structural integrity.

I have little doubt that the big motorcycle manufacturers are exploring hubmotors, since getting the motor out of the way is such a space saving advantage, but I expect their solution to be high power geared hubmotors where the internal gear reduction can make any size wheel possible. They already have the metallurgy knowledge needed for long life, low noise gears, and simply need to solve the cooling issue of geared hubs. Such geared hubbies will also enjoy the weight advantage of smaller higher rpm motors delivering the same power.
 
A larger diameter motor can do the job; in the same way that Nine Continent or Magic Pie motors bear little resemblance to the things we were getting from Wilderness Energy in the '90s, hub motors will continue to evolve towards a better match with wheels big enough to work on their own merits, not just the motors'.

I reckon in time we'll see motors that are wheels and not simply hubs. Or at least hubs large enough that they can attach directly to rims without the use of spokes. Along the way, we'll have to lose the stupid flatted-axle-as-torque-anchor shortcut that the Chinese have been palming off up to now.

Whether motors must become coreless to make that transition is outside my area of expertise, but seeing as hub motors have become larger in diameter and more efficient over the last twenty years, I expect they'll continue to do the same going forward.
 
Actually, I owned a car for about 6 years, that had tires with an OD smaller than my 20" bike tire wheel. And yeah, the ride sucked badly. A good size pothole was a disaster in that car.

My new 20" rear wheel bike rides fine, but only because my seat is not positioned right over the rear tire. I'd want a 26" wheel, if my ass was close to the rear axle.

Much depends on the routes you will typically be riding, if I was still riding that crappy bike trail to work every day, I'd still stuck on a bike with rear shocks. Where I street ride now, I need no suspension, nor a big wheel. Just a bit longer bike is all I need now. That bike trail I used to commute on had crazy bad bumps for the water channels crossing it that would launch you into a jump at 20 mph. With shocks and cross laced 26" wheels, you stayed on the ground and didn't break spokes.

Now I ride entirely in the "new" burbs, and the streets and bike paths on the east mesa are much better than in the old city center. 20" rear wheel is going to be fine for that. In the city center, the best routes are old dirt road irrigation canals, with huge washboards and bumps in them. Full suspension territory.
 
It's simple and it's not. What's best in one scenario may not be the same in another. Think about road conditions and how you want to use the bike. If you want to pedal along with the motor or not, etc.etc.etc. Considering only the electric system alone and the smaller wheel is better, but that's not and shouldn't be the only consideration. What's best for you may not be for another person.
 
wesnewell said:
It's simple and it's not. What's best in one scenario may not be the same in another. Think about road conditions and how you want to use the bike. If you want to pedal along with the motor or not, etc.etc.etc. Considering only the electric system alone and the smaller wheel is better, but that's not and shouldn't be the only consideration. What's best for you may not be for another person.

If it's different for everyone, why most people choose 26"? Clearly it is not. I can't find a single high power (more than 2 kw) ebike frame for 20" wheel?
 
What is common on the market is one thing, what we build is another. I build with large wheels to ride the mountain, smaller wheels to ride the streets. I'd go even smaller if the streets were nicer and I wouldn't ride the curbs, parks and stairs.

What we build by ourselves, we can optimize for the usage that we plan. What is best is always a compromise, and you decide what you are willing to lose somewhere to gain elsewhere.
 
Chalo said:
When we see legitimate mass-market electric cars and motorcycles with hub motors, they will have wheels substantially larger than 20 inches OD.

People adapting hokey scooter motors to build even hokier small wheeled homemade bikes is not proof of merit. That's like deciding that shopping carts are suitable street vehicles just because you can find people pushing them in the streets of every city.

Scooters of all kinds are acknowledged to be much less capable than motorcycles with full sized wheels.


Chalo, I know you have your opinions and all that.... but as somebody with a "hokey" scooter motor conversion, I know you're dead wrong. After running like a champ for over 9000 miles, what I thought would be a silly thing to get running for fun turned out to be an amazing performer. It climbs and accelerates great, runs fine off a cheap generic controller, the casting helps act as a heat sink, never a worry about a broken spoke or a broken axle (oversized with 14mm flats), the built in drum brake is powerful and well modulated, whats not to like? Coupled with a decent frame, its been an unbeatable cheap performer with solid 40 mph top speed. I ride a LOT of high end electrics, and a lifetime of normal pedal bikes so have some basis for comparison, and your ideas about harsh ride or unsuitability for general use are just not based in reality. Every person I've ever let drive it comes back raving about the handling, smooth ride and unstoppable power band. That's my "proof of merit".

I recently went for a high speed 30ish mile ride with an out of town guy with a Cromotor and Sabvoton with regen braking, and he was blown away by the cornering, power, braking and climbing of my dirt cheap, low tech setup. I ran into another guy at random at the top of a mountain road, who turned out to a retired aerospace guy who built his own tilting trike, a dual motor mini open wheel race car and an electric commuter van, and he was again blown away by how cool the motor was after the extended climb to his house, and wondered where I got such a "special" motor... His face when I said "off a broken down scooter I got for a hundred bucks" was priceless...lol. So unless you have ridden somebodies "hokey" conversion... you're just spouting nonsense on this particular topic. And for anybody else who might be swayed by that off base opinion, if you like power, cool running, great braking, low cost, unbreakability, and hands off 15 to 40 mph stability, don't sell a cheap scooter motor short.

[youtube]nD3s0gEmiLs[/youtube]

And that was how it handled with my old 60lb lead acid pack strapped to the top.... with the new frame and bttery combo it;s even dreamier.

[youtube]eVc7e-yb0p8[/youtube]

I'd like to see any other sub $200 motor/controller combo match that...
 
Cwah,

I saw you state it here and in the cargo bike group buy thread, and I've seen others use similar wording on numerous occasions, so I want to try to clear up a common misconception. Spinning a hubmotor faster doesn't necessarily mean greater efficiency. In fact, the only thing directly rpm related regarding efficiency is the iron core losses, which increase with rpm. This loss is generally small in comparison to copper losses, which is the bulk of where the heat originates.

Where the gain in overall efficiency comes from for a hubmotor when using a smaller wheel is that less current is required for the same thrust. Copper losses are current squared times resistance, so the 23% reduction in current required for the same acceleration with a 20" wheel vs a 26" wheel results in over 40% less heat. If instead you keep current levels the same, then you'll accelerate more quickly and make heat for less time. Personally, I shoot somewhere in the middle, so I get more performance and acceleration using less current.

That brings up the potential where a larger wheel can be more efficient. With a large wheel if you are running for long periods at steady-state speed, and it's a low enough wind and friction load that the motor approaches peak efficiency at cruise, then the smaller wheel would put you past peak efficiency and the greater core losses of higher rpm (assuming same speed) would make efficiency lower at cruise with the small wheel. This would be rare but possible for someone with a long flat commute with few stops or someone touring long distances without headwinds or hills.
 
We're just saying it in a way that does not make it completely clear. Because of what you just said, there is a butter zone rpm where a motor runs efficient as it can. Trying to over volt to increase the rpm more than the butter zone may not be better at all.

And then there is the slower rpm as you leave a stop sign, which is definitely too slow for the motor to run efficient under a heavy load. (weight of rider and bike)

And then there is the amount of time you spend at that lower rpm, making heat. Those big wheel hub motor cargo bikes can spend a long time in that heat making rpm, especially if only powered at 1000w, it's even slightly uphill, and the load out is 400 pounds.

I think the same situation with the 20" wheel will prove to waste less energy, because it should get up to speed a bit quicker. But I simply never got your point previously, that the smaller wheel would do the same thrust with less current. I had the idea that it was more torque, but the same current. (current always being whatever the max is, leaving a stop sign)

As always, I just have a damn hard time understanding this correctly. Will I have more torque? Or will I just stop pulling max current lots quicker? Both?

The key thing here is going to be if it can get out of that poor rpm quicker when the bike is loaded down heavy. In theory it will, but it might be that in fact, what is really needed is 20" rim AND a more powerful motor, such as what you run. Unfortunately, it will be a long time before I can give my new bike my definitive test. Ride to the top of Augustine pass on a hot day, loaded up to 400 pounds. Then compare heat at the top with the same motor,controler, battery, but 26" rim. It might get to the top and be just as hot as the 26" bike. We'll see next June.

I may never really understand why one setup gets to the top of the pass cooler. But I can at least say, this works, that works, the other really did not.

It's crystal clear what realllly works. Your wheel size, big magnets, plenty of power to do the job. It's also clear what doesn't work for me. My body riding one of those scooters built for a person 5' tall. :lol: Uncomfortable as hell in 10 min. Otherwise I'd just buy a scooter.
 
John CR and Dogman others are right on I am now going from a 26 to a 20 or maybe a 24 but I know I need to go smaller , Chalo is so wrong its funny he should read the history of the motorcycle
 
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