How does one decide what size wheel to put on a given conveyance?

Factual Questions
1

I saw loads of vehicles, trailers and whatnot on a long drive this weekend, and as I was a passenger I had lots of time and attention to devote to pointless mental meanderings.

So how is it we, the civilized world, decide what size tire to use on sundry rolling obects? For instance, all the horse trailers I can recall have wheels more or less car-sized, but I’ve seen other trailers that carry the same weight or more using a wheel closer in size to what you’d see on a wheelbarrow.

I’m sure ecomics plays an overwhelming role in most design decisions, but if you removed cost as an issue and said to your designer: "Design the perfect wheel, sized for:[list=a]
[li]a bicycle[/li][li]a 500 pound ATV [/li][li]a trailer for two 500 pound ATVs[/li][/list=a]

Not that I want a specific answer for these cases, but more generally:

Can an “ideal” wheel size be designed for a given task? If you limitations on materials costs, could your designer explain why a 15.3" wheel was a better choice for a given task than a 15.7" wheel, or vice versa.

2

Well, at the very least a wheel has to be greater than twice as large as any obstacles it has to roll over. In general, the larger the wheel diameter, the smoother the ride. Beyond that, I don’t think there are any hard-and-fast rules.

3

There are several considerations. The weight per surface area is one. More weight requires more area so you need wider and/or bigger diameter. Also, rotation speed and balancing. Also the type of terrain: smoother means you can use smaller.

An example. I had a forklift do a job for me. It had wheels of a very small diameter which were fine for the concrete floor of a warehouse but it got continually stuck in the gravel outside as they just dug themselves in. A bigger diameter would have been better. think of agricultural tractors.

4

Sometimes it’s to keep the trailer from being too tall. For example, I’ve noticed that the semi-trailers used by moving companies often have wheels that look way too small, like the owners went to a junkyard and got the wheels off an old Geo Tracker. The trailers that have these wheels are much taller than ordinary semi-trailers, and if they had normal wheels they probably would hit bridges on the freeway.

5

I remember reading Tech magazines (Popular Science, Popular Mechanics, Mechanix Illustrated, Science and Mechanics!) from the early 1970s mocking the small wheels on the compact cars coming out then. There was questions of handling, traction, wear and reliability (i.e. no high speed blow-outs) with these smaller wheels (13", I think), and one editor said “If you want a compact with 15” wheels, get a VW Beetle" - so clearly, even though the compacts didn’t weigh as much as the full-sized cars of the time, and so the manufacturers thought they could get away with smaller tires, there was much second guessing on everyone’s part as to if this was a good thing or not…
A quck googling brought up this quote from ‘Cooper Tire’ : "With 12-inch and 13-inch tires completely phased out of new cars in the US market, and 14-inch tires dwindling quickly, tires are getting bigger and better " - so it seems the second guessers from the 70s were right after all about the smaller tires (I know my mini-SUV takes 15 inch - no puny tires for me).
Now, at work, we have a TopKick (large GMC truck) with what looks like ridiculously small tires - however, since this TopKick pulls comparatively lightweight trailers over paved roads and well compacted ground, those tires are perfectly adequate - if we wanted to plow snow with this TopKick though, we would need larger tires to get good traction and handling in difficult conditions.
This doesn’t really answer your question except to validate that old reliable answer - ‘It depends on what the usage is’

6

** originaly posted by Q.E.D **

So that’s one in favour of bigger being better. Smaller ones having problems with obstacles.

** originaly posted by sailor **

smaller wheels can work on smooth surfaces, but bigger wheels can also handle smooth surfaces, and are much better on rough terrain.

So far it’s all bigger is better

** originaly posted by Diceman **

so sometimes smaller wheels are used where larger ones would perhaps be better, to allow the vehicle they are attatched to pass under obstacles imposed by the medium along which they are travelling.

** originaly posted by SirRay **

suggesting that larger wheels are preferable, but that smaller wheels can be used if they satisfy the requirements for the job.
The way i’m thinking is that bigger is better, with smaller wheels being used where either:
a) They allow the vehicle they are mounted on to pass under obstacles.
b) The smaller wheels are adequate for the purpose they are being used for, and larger ones would be wastefull ( hammer to kill a fly )

Having said that, what affect would having huge wheels on cars have on: Wind- resistance, steering, and power transmission?

What diameter should these wheels be?

  • Images of using 20 foot wheels ona little car to trundle around in fields *

what’s wrong with a car with an axel really high up ( on the roof even ) adn huge wheels that keep the vehicle teh sme height off the road as curretn wheels mounted the car do? ( with the exceptions fo the following: doors not opening ( large spokes in the wheels you can get out through perhaps) steering, and height restrictions)?

-Qis

7

Qis, I’m glad you’re getting into the spirit of the question!

There has to be some relationship between the wheel size and engine power available to deliver an expected range of speed.

I have a feeling that there’s a correlation between size and safety; i.e., “larger’s safer”, as vehicles/trailers that carry humans or livestock tend to have larger wheels than those that carry ATBs, snowmobiles, etc.

But when you think about it, wheels are one of those products which time hasn’t reduced the variety of. I’m sure if you look hard enough, you can find wheels of just about any size from 1/2 an inch to 50 feet. Which leads me to believe there are plenty of engineers who said something like, “Well, the Goodyear 15” tire isn’t good enough for reason x, and their 16" tire is too big because of reason y. What we need is a 15-3/4" one".

8

Now that I think about it, this question comes up all the time on one of my favorite shows - Junkyard wars.

They just repeated the Mega-Wars show where 3 teams had to create vehicles which could a) Race fast on a flat desert, b.) off road scramble through a boulder-filled course, and c.) operate amphibiously through a water course.
The teams were allowed to modify their vehicles between race segments, and so one of the teams originally planned (but in the event did not do) to change their vehicle’s tires - small, narrow, hard (high psi inflation) tires for fast racing over the hard-surfaced flat desert course, and large, soft (low psi inflation) tires for the rocky off road course. This makes some sense since on a staight run for speed you don’t want the tires gripping the surface that much, since that would slow you down - conversely for rocky, uneven terrain you want every bit of traction you can get…

9

I’ll take just one of your examples and show real world conditions - a bicycle.

First, you have road touring bikes. These are made for flat surfaces with little to moderate roughness, aka roads. They use tires that are small and high pressure, so they will roll easily and not grip the road as much.

Then you have mountain or trail bikes. These use similar sized rims, but the tires are big and balloony, and have big treads for gripping.

Both are very similar vehicles with different requirements. The wheel diameter (rim size) is determined more by the size of the rider, i.e. the frame of the bike big enough to allow legs to pedal it, etc. The tires are changed.

This points out a question: do you just mean tire size (thin vs. thick, narrow vs. wide), or hub size (diameter of the rim), or both?

For rim/hub size on a bicycle, you have constraints from functionality. As I said, they have to be large enough to allow the pedals to revolve without scraping the ground, and give some clearance in the process. Wheel size could be smaller than conventional and still use the same frame size. Other factors include ability to climb over obstacles (i.e. curbs, rocks, etc), and rolling energy. Note how most bikes use gearing to allow different ratios of turning between the pedal cycling and tire rotations. This allows you to adjust for the same amount of effort in pedaling to match different terrain requirements that take different levels of energy: slope, wind resistance, roughness, etc. Different wheel sizes would require different gearing.

Think about the physics of rotation. Velocity at the rim is proportional to the radius. A larger wheel size has a faster rim velocity for the same angular rate (i.e. rotation speed). Think about it - you turn the same angle, say 180 deg. If r= 6 in, the corresponding circumference (i.e. distance traveled) is less than if the radius is 12 in. The distance traveled is bigger, but the time to travel is the same. v = d/t = wr/t

w = omega, angular rate
d = distance
t = time
v = velocity
r = radius

So if you have smaller wheels on your bicycle, you will travel at a slower speed for the same gearing and pedal speed.

To relate this to trailer wheels, which are not geared. The smaller your wheel diameter, the faster the wheel will rotate at a given vehicle speed. The wheel has to go around more for the same travel distance. Same vehicle speed or linear speed means same distance traveled in the same amount of time, ergo the wheel goes around more.

10

Trial and error. Wheeled vehicles have been around for hundreds of years now and they’ve learned what works and what doesn’t. Each new application furthers the knowledge base.

11

Don’t smaller wheels also get you better torque than larger wheels? Something about the distance between the center of the wheel and the road surface? The term “moment arm” should be used somewhere in here, I think.

12

One other thing that’s been ignored - ground clearance. Obviously the amount of ground clearance any vehicle has is at most the height of the axle above the ground. The way to increase this height is to use a larger wheel diameter. Of course, this raises the center of gravity of the vehicle, and can lead to instability during turns.

13

Usually this isn’t an important concern. Almost all vehicles have gears or some other adjustable transmission. If a vehicle is too slow for a given drive axle RPM, the solution is not to increase wheel diameter but to adjust the gearing. The only exceptions I can think of are earlier examples of steam locomotives and Ordinary (high-wheeler) bicycles - these didn’t have gears, and their wheels got bigger and bigger as their speeds improved. It’s true that if you use a very high gearing and very small wheels, the drive axle will rotate at a very high speed; but with properly designed bearings this doesn’t cause any problems.

In the recumbent bicycle community the question of wheel size is always a big topic of debate. Ignoring practical concerns (cost and availability of parts), advanteges of big wheels are:
[ul][li]Smaller rolling resistance[/li][li]Easier to go over obstacles (road irregularities and potholes)[/ul][/li]Disadvantages are:
[ul][li]Heavier weight[/li][li]Weaker against lateral force[/li][li]Less aerodynamic[/li][li]Makes the vehicle large and tall, which increases air resistance and affect handling[/ul][/li]So there are two schools of thought when it comes to racing recumbent bike design - a “lowracer” uses small wheels (or at least a small front wheel) to minimize air resistance, while a “highracer” uses large wheels to minimize road resistance. Lowracers tend to do much better on ideal road surfaces (racing tracks) while highracers are better suited for real roads. On recumbent trikes, the wheels don’t tilt so lateral strength becomes a big issue. And since you’ve got wheels sticking out on both sides of the vehicle, air resistance of the wheels becomes a bigger issue. So all modern racing trikes (e.g. the Greenspeed GLR) have small (16") wheels.

14

No, that can be compensated by adjusting the gearing. A 16" bicycle will have exactly the same torque as a 26" bicycle if you give the 16" bicycle a 63% larger chainring.