[JP (OP)]

So I just finished teaching an intro physics summer course, and one of my students emailed me to ask about some real-life physics.

She saw a 3-wheeled car driving along, with 2 wheels in front and 1 wheel in back.  Her question was whether this would be more stable or less stable than the other way around (1 wheel in front, 2 in back).  She figured the 2 wheels up front would be more stable: the engine and most of the mass is probably up front, and so the center of mass would be closer to the wider wheel base (making it less likely to tip over).  Off the top of my head, I tend to agree, but these things tend to be trickier than they at first seem. 

Any thoughts?

[LeeE]

Consider the area defined by the wheels - if the CoG falls outside that area it will tip over.  There currently seems to be a fad for three-wheeled pushchairs for young children and infants, and they're advertised as being more stable, implying that they're less likely to tip over - they're not.  They are just less likely to wobble on an uneven surface - stability comes down to the CoG staying within the plane defined by the wheel layout, and a three wheel layout, within the same overall dimensions as a fourwheel layout will be more likely to tip over because a three wheel layout, within the same overall dimensions will have a smaller area.  If three-wheeled vehicles were such a good idea they'd be used in Formula 1 and rallying.

This doesn't however, explain why so many young mothers will walk up to the side of a road and wait to cross it, with the rear wheels of the pushchair on the kerb and the front of the pushchair - the bit where their child is - overhanging the road.

[DoctorBeaver]

A friend of mine had a Reliant Robin and I drove it a few times. It was a lot more stable than you would imagine. However, the Bond Bug was notorious for tipping over in corners. Both are 1 wheel at the front and 2 at the back.

The only 3-wheelers I can think of that had 2 front wheels were the Isetta Bubblecar and the Messerschmitt. I believe both had a rear engine.

http://www.3wheelers.com/archives/archive1/body_gal43.html [Links inactive - To make links active and clickable, login or click here to register]

for instance...

[RD]

She saw a 3-wheeled car driving along, with 2 wheels in front and 1 wheel in back.  Her question was whether this would be more stable or less stable than the other way around (1 wheel in front, 2 in back).  She figured the 2 wheels up front would be more stable: the engine and most of the mass is probably up front, and so the center of mass would be closer to the wider wheel base (making it less likely to tip over).  Off the top of my head, I tend to agree, but these things tend to be trickier than they at first seem. 

Any thoughts?

Remember to include the weight and position of the occupants of the vehicle in your COG calculations.
The occupants will weigh more than the engine. 

[Design for new three wheeler with two wheels at the front..
http://www.theregister.co.uk/2008/02/06/electric_3_wheel_sports_car_lotus_reliant/ [Links inactive - To make links active and clickable, login or click here to register]]

[LeeE]

Which ever way you look at a three-wheeled design, the mass at the third wheel end of the vehicle i.e. the suspension (and any steering) gubbins must be mounted to the sides the wheel and therefore fall outside the plane of stability defined by the wheel plan.  Depending on whether the third wheel is at the front or back of the vehicle, there are also weight transferrance problems, during braking or acceleration, respectively.  This is especially important while cornering, and in each respective case, the transfer of weight is towards the apex defined by the third wheel.

The only real advantage of three-wheeled motor vehicles is that, as the Reg article points out, they don't need a full driving license and can be driven on a motorcyle license, at least here in the U.K.  If the vehicle is kept to a low weight there can be manufacturing and maintenance savings but if the vehicle gets heavier the third wheel suspension stuff needs to be substantially stronger than the paired wheel stuff, which pushes the costs back up.

[Alan McDougall]

Hello,

I three wheel vehicle must be less stable than a four wheel vehicle. In fact any odd number wheel vehicle must be more instable than even number wheeled vehicles.

A three wheel vehicle with two wheels in front will not affect the stabilty

I would prefer my three wheel vehicle to be the reverse as it would be easier to steer.

Alan   

[Bored chemist]

This statemnet" In fact any odd number wheel vehicle must be more instable than even number wheeled vehicles." is made without any grounds to believe it.
I invite the author to consider the fact that young chlidren learn to ride tricyles before they progress to bicycles.

[Madidus_Scientia]

And why would it be easier to steer

[turnipsock]

Speed Bumps are a nightmare. It's hard to avoid pot holes in the road as well.

Has anybody else noticed there seems to be less road kill on the roads since the decline of the three wheeled car?

[ukmicky]

In fact any odd number wheel vehicle must be more instable than even number wheeled vehicles

I could see where you were going but unfortuanatly you took the wrong turning []

[JP (OP)]

Good points, everyone.  Thanks for the insight.

[LeeE]

I saw a two-seater (side-by-side) three-wheeled pushchair today and it really made me wonder how such a dangerous thing could ever have been made - I can't help thinking that it's a law-suit waiting to happen.

It was no longer than a normal single-seat three-wheeled pushchair, but much wider.  Now at first thought, this might seem like a good idea - making it wider between the paired wheels should make it even more stable, shouldn't it?  Well, if it was for just one child then yes it would, but as it had been done so that two children were seated beside each other, just ahead of the paired-wheel axle, it meant that their CoGs were located only a little inside the plane of stability and their legs were entirely outside it.  The thought of what could happen, with just a single child seated in one of these is very worrying, especially if one were to run away and collide with something.

[Alan McDougall]

BC,

I invite the author to consider the fact that young children learn to ride tricycles before they progress to bicycles.

Did you ever own or ride a tricycle as a kid. If you did you will remember the falls and bruises.

A Little four wheel kiddies car never has this problem because it is more stable than a trike.

[Madidus_Scientia]

Did you ever own or ride a bicycle as a kid?

Have we been teaching children wrong all this time? should we start them on bicycles and work them up to tricycles?

[LeeE]

I think everyone realises that an in-line two-wheeled vehicle is inherently unstable whereas a tricycle has some inherent stability - it's just not as great as a four-wheeled vehicle.

[Bored chemist]

Please let me know how you can have two wheels that are not in a line.
It's quite easy with 3.
Incidentally I never even tried to ride a unicycle.
Perhaps there's a better hypothesis than "even number of wheels good; odd number of wheels bad" involving the number of wheels.
My recolection of riding a trycyle is a bit hazy but I remember that you can stop without falling over; that's tricky on a bike.

[LeeE]

Are you being picky about my hyphenation of 'inline'?

A non-inline two-wheeled vehicle will be stable if it's CoG is both between and below the line joining each of the two wheel's axis.

...Or if the stability is artificially supplied i.e. Segways.

Stability, on both land and water depends upon keeping the CoG either below or within, if not below, the plane or axis of stability.  The number of wheels, when on land, only comes in to it because within the same overall dimensions i.e. length and width, the plane of stability defined by a three-point system is smaller than for a four-point system.  Adding further wheels, within the same overall dimensions does not increase stability because it does not increase the size of the plane of stability.

[Alan McDougall]

MS,

Did you ever own or ride a bicycle as a kid?

Have we been teaching children wrong all this time? should we start them on bicycles and work them up to tricycles?

I do not concur with your logic, When I was a kid there were no kiddie 4 whee cars , if there were I am sure my dad would have started me on one, then progressed to a tricycle and finally a bicycle on which I had some awful falls

[Madidus_Scientia]

It wasn't logic, it was sarcasm.

[Bored chemist]

"Are you being picky about my hyphenation of 'inline'?"
No. I'm pointing out that with 2 wheels they must touch the ground at (as an idealsied model) 2 points. Thos points will define a straight line.
You need a third point before you can say whether it is on that line or not.
2 points can't help being in a line.
"A non-inline two-wheeled vehicle will be stable if it's CoG is both between and below the line joining each of the two wheel's axis."
Similarly, if you hang a unicyle upside down from a wire and hang from the saddle, it will be stable with just one wheel.
It seems that neither the number of wheels nor the parity of that number is sufficient to establish stability.

However this still shows the inadequacy of the original postulate "In fact any odd number wheel vehicle must be more instable than even number wheeled vehicles"


Incidentaly I think zero is even, so does anyone have anything to say about the stability of vehicles with no wheels?
For example I can point out that driving a hovercraft is an absolute pig of a problem; on the other hand I have no problem walking and I have no wheels.