A century ago, 3-wheeled cars were quite popular, like the Morgan below.

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The sale of these cars has been banned later (until recently) in many places due to the associated safety concerns. There are many stories of these cars rolling over such as when turning around corners (for example, the first 10 seconds here and throughout). One source claims the 3-wheel system is actually beneficial as "narrow tires and a low center of gravity ensure grip runs out before lateral forces become a problem".

So the question is, why are 3-wheeled cars considered not effective or safe to drive while 4-wheeled cars are fine? What is the physics that makes this difference with 1 wheel? To me, it looks like lateral forces and banking have to do with this.

  • $\begingroup$ The Reliant Robin sold well... And what is the Vehicle sold and so well used in India? $\endgroup$ – user207455 Jul 18 '19 at 16:47
  • $\begingroup$ FWIW, Where I live, any motor vehicle that is designed to operate with fewer than four wheels in contact with the ground is legally classified as "motorcycle." (Except when it's a "motor-driven pedicycle," a.k.a., "moped".) $\endgroup$ – Solomon Slow Jul 18 '19 at 16:56
  • $\begingroup$ There was an episode of Top Gear (a UK TV program) where they had a Robin Reliant. They tipped it over so many times it became comical. $\endgroup$ – puppetsock Jul 18 '19 at 17:27
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    $\begingroup$ @puppetsock that's the video I linked! $\endgroup$ – KingMongolian Jul 18 '19 at 17:54
  • $\begingroup$ Please stop making trivial edits to the question to get it to the top of the home page $\endgroup$ – BioPhysicist Jul 19 '19 at 13:29

It indeed has to do with their rollover dynamics in sharp turns, which sensitively depends on the wheel track width. A trike has no track width on the (solo wheel) end of the vehicle which means it can never be as roll-resistant as a 4-wheeled vehicle of similar proportions.

This instability in roll is made worse in situations where the weight balance of the vehicle shifts to the solo end in a turn, which occurs when a trike is braking as it enters a turn. Then the inside rear wheel lifts off and the trike readily rolls over.

This tendency can be reduced by putting the solo wheel at the rear of the vehicle and steering with the two front wheels, but it cannot be completely eliminated this way.

Current designs for three-wheeled vehicles call for the wheels to tilt into the turn like a motorcycle, but as the optimum tilt angle depends on both the forward speed and the sharpness of the turn, computer control is required, which adds complexity and furnishes the opportunity for catastrophic failure if the system glitches during operation.

  • $\begingroup$ Have you considered the motorcycle with sidecar combination as that has yet different characteristics? $\endgroup$ – user207455 Jul 18 '19 at 17:27
  • $\begingroup$ Yes: asymmetric left-versus-right turn dynamics, acceleration-versus-braking response asymmetry, and center-of-mass shifts with movement of the passenger. I would never ride one of those! $\endgroup$ – niels nielsen Jul 18 '19 at 17:31
  • $\begingroup$ The passenger has more guts than the rider looking at some of the races :)... $\endgroup$ – user207455 Jul 18 '19 at 17:34
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    $\begingroup$ @nielsnielsen Not crazy at all. We have a couple of engineers in the office who race sidecars. They are probably the most cool headed, straight-thinking, and safety conscious guys who work there. They just take the view that so long as you, and everybody around you, knows what they are doing, and you are using the technology within its design limits, travelling at 100mph with only two inches of air and a layer of neoprene rubber between parts of your anatomy and the road is a safe enough. They have been doing it most weekends for at least 10 years and they haven't had any injuries yet... $\endgroup$ – alephzero Jul 18 '19 at 19:11
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    $\begingroup$ well I get it, I have friends who free-climb sheer rock faces and skydive. Just not for me! $\endgroup$ – niels nielsen Jul 19 '19 at 3:45

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