Best wheel size

[swbluto]

What do you guys think the best wheel size for the "driven wheel" is? I'd guess a smaller wheel!

The reasoning being is that a gear ratio at 26" would be equivalent to half that gear ratio on a 13" wheel. In real life, the next practical size is 16" but you get the idea: You can substantially reduce your gear ratio by designing with a small wheel in mind. This can mean the difference between a one stage and two stage drive which then means cheaper, increased drive efficiency and, most importantly, more POWER!

I wondered why bicycles have 26" wheels to begin with, though. I thought maybe it was to help handle pot holes or something like that, so I went to a motorcycle wheel seller and it seemed 18" was the most common size! I think motorcycles have to worry more about potholes than bicycles but yet there common size seems a tad fair bit smaller suggesting that a smaller wheel won't hurt.

Ok, an example... let's say a gear ratio of 12.0 seemed about right on a 135 kV motor on a 26" wheel. On equivalent gear ratio on 16" is about 7 which seems to be well within the single drive system.

I have questions about how the brakes would be put on the bike, though. Some gearing setups interfere with disc brakes but it seems calipers wouldn't be ideal on a 16" wheel on a 26" mountain bike, so it seems back brakes would be eliminated on mountain bikes unless one has a clever circumvention or uses a right side drive which seems to introduce other complications.

 

[etard]

a 16 inch wheel is awful small. D, had problems with crank arm clearance from the ground when he tried to put 24" rims on his bike. So I would think that would be the biggest drawback. Also, motorcycle rims might be 18 inches, but after adding in the tire you probably end up with a 22-24" diameter. Plus, bicycles tires tend to be getting even bigger with the addition of 29ers and bigger yet on the horizon. It's true though that smaller diameter wheels would make it alot easier to deal with reduction, but aesthetically they look like little toy bikes, or folders which aren't known for great handling. To solve the problem of rear brakes, you could use a coaster brake if you had to.

 

[swbluto]

To solve the problem of rear brakes, you could use a coaster brake if you had to.

Do pedals free wheel with coaster brakes? I wouldn't immediately think so, but I can imagine it's physically possible for the pedals to "not rotate" but yet the brake wouldn't engage until you actually started applying pressure backwards.

 

[EVTodd]

To solve the problem of rear brakes, you could use a coaster brake if you had to.

Do pedals free wheel with coaster brakes? I wouldn't immediately think so, but I can imagine it's physically possible for the pedals to "not rotate" but yet the brake wouldn't engage until you actually started applying pressure backwards.

The pedals do indeed freewheel with coaster brakes. Well, in the sense that they don't move when you don't pedal. Man... It would be a bear to stop it if they didn't freewheel!

 

[liveforphysics]

Pedals do freewheel with coaster brakes. They use a slick double sprague clutch mechanism to allow this. However, coaster brakes are pretty bad brakes. I would go with disk, and find a work-around to let it happen with your drive sprocket.

As far as wheel size effecting performance, it looses stability at the square radius. It also has a reduced contact patch (which likely wont matter for these applications). If a 16" OD tire hits something over 8" high, it has no reason to go over that obsticle, it's just as likely to try to direct momentium towards the ground, or to try to stop the bike instantly.

For obsticles under half the radius of the tire, as the tire size gets larger, the time interval for the required upward displacement increases, which decreases the intensity of the impact. This also improves energy efficiency.

For an exagerated example of the effect, imagine trying to go over a rough cobble-stone street on a skateboard or roller blades. Very tough to roll, and the bumps are sharp and jolting. Now imagine going over the same street on a mountian bike. Rolls pretty free, and the bumps aren't so bad (of course pnumatic tires play a part in this).

So, i guess the best answer for your question is for smooth paved roads, it's doesn't make much of a difference what tire size, and smaller makes the drive more mechanically simple.

 

[Chalo]

What do you guys think the best wheel size for the "driven wheel" is? I'd guess a smaller wheel!

I wondered why bicycles have 26" wheels to begin with, though. I thought maybe it was to help handle pot holes or something like that, so I went to a motorcycle wheel seller and it seemed 18" was the most common size! I think motorcycles have to worry more about potholes than bicycles but yet there common size seems a tad fair bit smaller suggesting that a smaller wheel won't hurt.

Large diameter wheels have a smoother, more comfortable ride, better traction, and lower rolling resistance than smaller wheels, all else equal. This is why mountain bikes have 26" wheels when 20" wheels would be much stronger and lighter, and it's also why the new 29" wheel size has arisen recently in the mountain bike world.

Motorcycle wheel sizes are designated by the diameter of the rim, while bicycle wheel sizes are designated by the approximate size of the inflated tire. A so-called 26" bicycle wheel (which is technically referred to as ISO 559) has a rim 22" in diameter. Keep in mind that a fat motorcycle tire can be much taller than a bicycle tire too, and thus add more to the wheel's outside diameter. Also note that motorcycles benefit from relatively plush long-travel suspension, and this helps offset the ride quality shortcomings of a small diameter wheel.

I have questions about how the brakes would be put on the bike, though. Some gearing setups interfere with disc brakes but it seems calipers wouldn't be ideal on a 16" wheel on a 26" mountain bike, so it seems back brakes would be eliminated on mountain bikes unless one has a clever circumvention or uses a right side drive which seems to introduce other complications.

it would be relatively easy to braze another set of cantilever brake studs onto a steel frame at the correct position for a small wheel; the insurmountable problem is that the bike's cranks will be dropped closer to the ground by an amount equal to most of the reduction in the wheel's radius, and this may cause the pedals or chainring to hit the ground in a turn or when ridden over bumps.

For instance, switching 26" wheels out for a 16" wheels will drop the bottom bracket (crank axle) about 5". Since an average mountain bike has a bottom bracket height of about 11-1/2", substituting 16" wheels at both ends would drop it down to about 6-1/2". Well... normal cranks are over 7" long from the bottom bracket spindle to the outer end, which means your prospective 16"-wheeled MTB could not be pedaled in a straight line! If you used a 16" wheel only on the rear, you might be able to pedal it in a straight line, but you'd face intractable problems cornering or rolling up/down/over obstacles.

If using a small wheel solves a major motor reduction issue, then it might be worth dealing with the other complications it creates. But changing an existing bike's wheel size more than a couple of inches usually makes such a mess of things that you're much better off finding a bike that comes equipped with your desired wheel size.

Chalo

 

[swbluto]

Hmmm... yes, I'm seeing it.

Get a bmx frame with the desired back wheel size. Replace the handlebars/headset/forks/front-wheel-assembly with a mountain bike's(or whatever preferred - personally, I think I'm more cozy to a "banana bar" riding style that doesn't seem particularly common among mtbs or roadies).

Hmm, now this makes me wonder if a dual suspension bmx bike exists. :lol: Or, perhaps, a back suspension bmx. Or, perhaps, a 16" dual suspension that you can fit discs onto.

 

[swbluto]

http://www.amazon.co.uk/Raleigh-Boys-Invader-Dual-Suspension/dp/B000W49H76

Here's a 16" dual suspension! Now, just need to replace the front with a 26 incher.

(Down side: the spring constant was probably not chosen for someone of my weight)

Oh, another one! http://www.etoys.com/product/index.jsp?productId=3156733

 

[Lucky_Hoodlum]

on and off road motorcycles have smaller than 24 and 26in wheels on them, something closer to 20in i would think
so i would guess smaller would be better option, benefit of smaller wheels size i see would be good torque for the hub motor uses downside
would be slower top speed. This could be good way for those in not so flat terrain to get a non geared hub motor to perform on hills
maybe? For non hub motor users i think it might not matter so much as the ability to alter their gearing ratios
is possible with either swapping the fix sprockets out or simply 'changing gear'

 

[zukster]

If you put a motor designed for a 26 inch wheel on a 16" wheel, you will indeed
increase your torque (perceived power), but you will loose top speed I think. That's
why companies like Bafang, offer the motors for so many wheel sizes. So the
16 inch motor on a bike with 16 inch wheel will go about the same speed as a
26 inch motor on a bike with 26 inch wheels.

We have lots of hill here, so I put a 26 inch motor in my son's 24 inch wheel
bike. It'll give him more torque for the hills ( a bit) and I don't want him going
fast anyway.

I tried to get a 28 inch motor but Keywin does not sell that one, altough Bafang
makes one. I would not want to go more than a few sizes different anyway.

 

[swbluto]

Oh, so a gear ratio of 12 is within the province of practical? I thought Matt was saying something about "Keep it under 6 per stage" or some such like that.

And to those who mention "You'll lose top speed", you won't lose anymore speed than the gear-ratio-equivalent setup at 26" as the output power of the motor will be the same at the same forward speed. Essentially, at a given forward speed, your thrust at the wheel would be the same - the torque is lower, but the thrust would be the same since the decrease in wheel size makes up for it. For the exact same motor and exact same gear ratios, typically a smaller wheel will result in a lower top speed and greater thrust at a given speed though that's not universally true. If you have a sufficiently tall wheel, the thrust wouldn't be great enough to overcome any kind of resistance so it could be really slow, so there's an "optimum value for top speed" somewhere and it's not always higher than a size that you're considering.

 

[zukster]

Lots to think about there. Its prob different for diff kinds of motors too.
I think the motor windings are different in some for different wheel sizes. For
geared motors, maybe the universal gear sizes are modified too. Then you have
to consider what the controller is doing with the info from the hall sensors
if its a hall motor. I don't know enough yet to muck around too much. Are
you going to go ahead and try it on the 16 inchers?

 

[Miles]

I decided on 20" (406) wheels with suspension as the best compromise, for my use.

Another plus for smaller diameter wheels is proportionately less torque through hub gears.

Obviously, if you're going to put the motor drive through the cranks (and pedal with the motor), there is no advantage, for the motor, from having smaller wheels....

Luke,
Everything else being equal, the shape (aspect ratio) of the contact patch will vary with wheel diameter but the area of it will stay pretty much constant, yes? The size of the contact patch is determined by the load and inflation pressure.

sw,
While motor speeds need gearing down, crank speeds need to be geared up. This is why (aside from the other advantages) bicycles tend to have large wheels .

 

[johnrobholmes]

I have some wheels on the way with 23" inflated tires. Rims are 17" diameter. I will tell you how it rides compared to a mountain bike 8)


Small wheels make a rough ride, that is for sure. Much easier to get reduction with them however.

 

[Chalo]

Oh, so a gear ratio of 12 is within the province of practical? I thought Matt was saying something about "Keep it under 6 per stage" or some such like that.

My first e-bike had 12:1 reduction in one stage (#25 chain with 144t and 12t sprockets). There were issues that made me move on to the next version, but they were not directly related to the amount of reduction.

And to those who mention "You'll lose top speed", you won't lose anymore speed than the gear-ratio-equivalent setup at 26" as the output power of the motor will be the same at the same forward speed.

Geared to the same overall ratio of RPM to road speed, you are basically correct. In the bicycle world, the term we use is "gear inches". A 3:1 gear on a 20" wheel would yield 60 gear inches, and thus would travel the same distance per crank rotation as a high wheeler or unicycle with a 60" wheel. There are more convenient ways to express this ratio when the power input is a motor turning thousands of RPM, but the concept is the same.

However, since a small wheel has more rolling resistance and uses up more of your kinetic energy jarring you and your bike, the top speed really will be slightly lower with a small wheel.

Chalo

 

[spinningmagnets]

16" wheels are normally found on childrens bikes, so they are cheap and weak. Due to the popularity of BMX bikes, its possible to find 20" wheels that are heavy duty (keeping in mind that you can still find cheap and weak 20" wheels.

Here's a 20" BMX wheel with 7 gears

 

[Miles]

16" wheels are normally found on childrens bikes, so they are cheap and weak. Due to the popularity of BMX bikes, its possible to find 20" wheels that are heavy duty (keeping in mind that you can still find cheap and weak 20" wheels.

Rubbish

Strength is rarely a problem with small wheels.......

 

[Mathurin]

700X32-38 seems to be optimal for urban use in my neck of the woods.

 

[spinningmagnets]

Dear Miles, I apologize if it sounded like I was disparaging all small wheels. After your comment, I Googled Moulton and found this:

http://www.bikeaccess.net/bikeaccess/Articles_db.cfm?Search=Folding%2CFriday%2CMontague%2CMoulton&Article=Moulton

I like your bike. I was merely pointing out that if the builder was living in the US, "most" of the 16" wheels that are easily found have very inexpensive bearings, spokes, etc.

I'm sure with access to the internet and being able to ship worldwide, small-wheel components can be found and purchased that are robust and well-made. There is so much creativity in bicycle and E-bicycle building, I'm certain there are many important uses for smaller wheels...your Moulton, and the front wheels of tadpole/reverse trikes comes to mind.

 

[Miles]

spinningmagnets,

No apology needed - I was only jesting - I'm not easily offended...... I'm sorry I used the "R" word

16" (349) and 20" (406) wheels are used on folding bikes, recumbents, trials bikes and BMX (as you said). Alexa and Sun do rims in these sizes, using the same section as for their larger wheels. 20" trials rims are incredibly tough.

The thing to note is that, for a given rim section, wheels get stronger as they get smaller...

 

[TylerDurden]

Looks like Moulton has a new 2-speed planetary chainwheel...

 

[Miles]

Looks like Moulton has a new 2-speed planetary chainwheel...

Duomatic hub gear and Schlumpf Speed-Drive.