# A 3 wheel 'motorcycle' from an automobile conversion?

A Buick has been modified to replace the two rear wheels with a single wheel centered in-line where the originals wheels were located. A 3
wheel 'motorcycle' was the result.
http://tinyurl.com/7tnyq
Assume that if instead of where it was placed, the rear wheel was positioned to the rear of the back bumper, what effect, if any, would occur on the loading of the two front wheels and the resultant handling?
And why?
BoyntonStu
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Dear stu:

Been done long ago by R. Buckminster Fuller (but not with a Buick).

Free body diagram, about the "front axle". All other things being equal, the loading on the front wheels will increase.
The stability will likely increase by moving the single rear wheel further back (all other things being equal). The turning radius will be unaffected, but this likely will wear out the single rear tire through "scuffing".

Because it is not nice to fool mother nature! ;>)
David A. Smith
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David,
Please expand some on this: "Free body diagram, about the "front axle". All other things being equal, the loading on the front wheels will increase."
BTW His Buick rear wheel has had very little wear in the origiinal wheelbase position.
Thanks,
stU
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Dear stu:

The sum of the torques about the axle. The "weight" of the car at its center of mass, times the perpendicular (shortest) distance to its line of action. Next the supporting force on the single rear wheel, times the perpendicular distance from the front axle to the line of action of this wheel. So that these two terms say that the *car* does not spin about it axis.
"weight torque" = "rear wheel support torque"
M*g * x_cm = f_rw * x_rw M = mass of car g = acceleration of gravity x_cm = perpendicular distance from from axle to line-of-action of the center of mass x_rw = perpendicular distance from from axle to line-of-action of the rear wheel support (this whole line-of-action stuff is really simple if the tires are all the same diameter)
Note that M*g = f_rw + f_fw
You don't really care about the force on the rear wheels, so... f_rw = M*g * x_cm / x_rw
Now substitute and solve for f_fw (front wheel supporting force) M*g * (1 - x_cm / x_rw) = f_fw
As x_rw goes up, so does (1 - x_cm / x_rw).

I would expect that the tread would tend to "scallop", as you moved it back. Just depends on the steering geometry.
David A. Smith
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I can't really describe why, but my "An Introduction to Mechanical Vibrations" (3rd ed.) by Steidel has a paragraph that shows why it might not be a good idea: "good vehicle design places the center of percussion about one axle with the centre of oscillation about the other". (This is in the section about compound pendulums.) I'll let you read up on the topic yourself...
-Paul
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That link does not work for me, can you post either the full link, or else make sure that your TINYURL link works?

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http://www.geocities.com/doolenthreewheelers/DoolenHome.html
I have spoken with the inventor and he says that after 3 years of daily use, he is convinced that the idea is very worthwhile.
Again, all things being the same in the car with the exception of moving the 3rd wheel aft 5 feet, I cannot see a major problem developing.
The Dymaxion was a 's t r e t c h e d' 3 wheeler and Bucky was no fool.
Common sense also tell me that limos are OK and they are much longer than what I am considering.
stU
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While I agree with a the physics of what has been posted regarding the three wheeler car, there are other aspects that should be considered.
First, if you remove a wheel, then you remove some safety for the car. What I mean is that if one of the three remaining tires is damaged, then the car would go handle worse than a 4 wheel car.
Second, if you convert a car to 3 wheels, how are you going to change the wheel that is located in the center of the car? When designing any consumer product, safety is always a large factor. By putting the third wheel in the center of the car, in a location that is not very accessable you put a person in danger while changing the tire.
Also, you may want to look at the type of weather in your area. If you are in an area where it snows a lot during the winter, conformity may be the way to go. Since all of the other traffic will wear down two tracks, your third wheel may end up sliding around a lot more.
I didn't see anything mentioned about how the 3 wheel design fits on an automotive lift.
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Good points.
My idea is to mount the 3rd wheel aft the rear bumper. Tire changing is not a problem.
Going flat on a rear centered wheel is not as dangerous as a flat on the rear corner of a 4 wheeler.
Getting on a lift is not a problem with chassis lifts which lift from under the passenger area.
Changing the centered tire is safer than being on the side of your vehicle next to traffic.
3 wheelers in snow areas have not reported any major problems.
Have you visited the Doolen link? His family loves them.
stU
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snipped-for-privacy@aaronj.com wrote in

Not so. If you lose one wheel on a 4 wheeler the remaining wheel on that axle will provide some lateral resistance.
If, on a three wheeler you lose the single wheeler then you have very little yaw stability. Have you ever had a front wheel go flat on a motorbike at high speed?
A three wheeler is not a crazy idea, dynamically, none the less you HAVE lost a margin of safety.
Oh, and since you'll probably ask, yes I have, and it won the race it was designed to win.
Cheers
Greg Locock
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Greg,
The one wheel design in front of a motorbike (2 wheeler) is obviously dangerous in a blowout. Ditto for a delta 1F2R trike.
One wheel behind a tadpole trike (2F1R) is less problematic. Your front wheels will steer and your rear hub and flattened rubber tire should act like feathers on an arrow and keep the vehicle stable as you brake.
What say you?
stU
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Dear stu:

Move the flat to the driver's side front. Note that you have placed most of the weight on the flat and one tire that will not prevent a "flat spin". By moving the lone tire further out, you may be losing the "laterial stability" necessary to prevent your vehicle going into a "flat spin".
My intended meaning of "flat spin" is pushing a pin down the center of gyration for the final vehicle, down into the Earth, then flick the vehicle parallel to the surface of the Earth. That one tire pushed way out may be less effective.
Do a hovercaft... ;>)
David A. Smith
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Ice skater.
Arms out, slow spin.
Arms in, fast spin.
The longer the wheelbase the straighter she want to go without spinning.
stU
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Dear stu:

"All thing being equal", *this*skater is out of control. The skater's arms are relatively massless, unless you are going to move the engine backwards with the rear wheel.
Also, I don't think this can be registered as a motorcycle, since two wheels steer...
David A. Smith
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snipped-for-privacy@aaronj.com wrote in

I agree it is less frightening, it is still unstable. It will tend to oversteer uncontrollably if you get a flat, and spin.
Let's get this straight, there is nothing seriously wrong with a properly designed trike. They do have the problem I mention above, but that may be an acceptable compromise in the light of other advantages.
But as a conversion from a 4 wheeler, well, I have doubts about all sorts of things.
Cheers
Greg Locock
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Hmmmm.... Doolen says "If you got to this site you are probably aware of the advantages of three wheel transportation. Also the drawbacks."
For those of us who are more uninformed on the subject, what are the advantages?
snip.....

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1> A little better gas miileage.2> Surprisingly little wear on the rear tire.3> One less tire to buy.4> All the advanrtages of registering as a motorcycle. Tags, insurance, inspections, emission.
5> Meet curious gawkers.6> Relatively easy to power rear wheel and make car into a hybrid.
Also see: http://designmassif.com/trihawk/articles/index.htm
Read the Car & driver reviews, etc.
stU
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snipped-for-privacy@aaronj.com wrote in

I suggest you start by reading the Beckman 3 wheel cars article on the Trihawk site. It says (paraphrasing) that if you leave the CG where it was originally for a 4 wheeler then you are building a roly poly deathtrap. In fact you will probably raise the CG, making things worse.
In the UK they used to have a 3 wheeler, the Reliant Robin aka the Plastic Pig. An enthusiastic driver could roll one going round a skidpan. You cannot do that in a typical conventional 4 wheeled sedan.
Cheers
Greg Locock
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Trihawk - (304 - Two seater -Roadster)
Rating: 10/10
Review by Warren Behler. URL: N/A
Manufactured: Jan 1985. Engine: 1,299cc
102,000 miles and still going.
The 1983 / 84 Car Driver Yearbook when writing about going around corners said of the Trihawk It's the Best Handling vehicle you can buy ....Period. It is. It handles so well because of its front-wheel-drive with 75% of its weight up there. The local Porsche drivers around here don't wave to me anymore. Trihawks run best when they have a Porsche or two for breakfast. The vehicle is dead reliable. I've been thru 17-western states as far east as Des Moines, Iowa. My wife and I went 100-mi in a snowstorm in Nebraska, with the top doors and the heater on we were nice and cozy. Its the most fun you can have on three-wheels. I think Ill keep it around for another 102,000-miles......
Placing most the weight (75%) up front is the key. Whether 3 or 4 wheels, the ~ 70/30 ratio will almost never oversteer.
If a 4 wheel car handles OK, it is not unreasonable to assume that a rear wheel placed where the Ackerman points to, should not perform exactly the same?
If I am incorrect, reasons please.
Beckman URL?
stU
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http://designmassif.com/trihawk/articles/3wc/gallery/html/3wc-1.htm
In the article, Beckman basically says that 3 wheelers can be as good handling as any 4 wheeler and that the yaw reaction time is much quicker.