# What makes scooters unstable at high speeds?

Scooters and skateboards are obviously meant for slower speeds, below 5 MPH. Typically if you go faster than that, it becomes harder and harder to control the scooter, and you will fall off. An example of this is the speed wobble, examples easily found on youtube.

What's the deal? Why is it that comparatively, bicycles can handle higher speeds with more stability and comfort?

• Do you think it may be related to wheel diameter and gyroscopic effect... – Solar Mike Jun 10 '18 at 15:02
• I was suspecting distance between wheels, but that may be it. – user16388 Jun 10 '18 at 16:47
• From your grossly erroneous speed estimate, it would appear you've never actually seen someone riding a skateboard with determination. Similarly, it's widely reported that the "share" electric scooters go about 15 mph, not because they could not go faster but because their electronics are capped at that as a matter of policy. – Chris Stratton Jun 13 '18 at 5:48

As has been mentioned above the size of tires plays a major role. Let's assume the manufacturer standards for a wheel after improving the shaft allows 1/100 mm of play. This much play would encourage a small wheel to resonated while shimmying much more than a big wheel which would need much mor time to complete one turn.

A 10 cm diameter wheel at 20 km per hour rotates roughly 10 turn/ second which is much closer to its natural frequency of vibration around the shaft versus a large wheel which takes ~1 seconds at the same speed. Also, the small tire has a very small I, moment of inertia WRT the shaft axis so it will bounce around easier. $$\omega_p = \frac {mgr}{ Iw}$$

Precession is inversely proportional to the spin of the wheel speed meaning the small wheel will precess much faster and because it needs much less play at the shaft to wobble, it will resonate and shimmy.

• A 10 cm wheel at 20Km/hr rotates at 55 turns per second (20,000m/hr x 10 turns/meter /3600 sec/hr =55.5rpm). – BobT Jun 10 '18 at 21:21
• @BobT, Thank you for checking the arithmatic. I used 10cm diameter which is approximately 63cm circumference. Still my calculations were wrong and I will edit my answer. But the point is when you increase the wheel's diameter its precession increases nonlinearly and with the same degree of play it won't resonate, hence no shimmy. – kamran Jun 10 '18 at 23:40
• You're right of course... 10cm diameter is 31.4 cm circumference. My mistake... – BobT Jun 11 '18 at 2:50

There is less gyroscopic effect but this is not as much a factor. It is camber of two wheeled vehicle that is self correcting. If you push a bicycle backwards it will not stay up.

With the smaller radius the rpm is higher. If the tire is out of round it will wobble. They often put cheaper tires on a scooter and / or do not balance them.

Smaller wheel base makes them less stable.

Racing scooter attain high speeds.

The smaller the wheel the less gyroscopic effect is created. Death wobbles can occur at slower speeds with smaller tires. Taller tires is what gives motorcycles and bicycles their stability compared to the smaller tires of a scooter as the speed increases.

• Wrong -- it's an urban myth that wheels provide stability via gyroscopic effect. If that were true we'd never be able to steer a bicycle. – Carl Witthoft Jun 11 '18 at 18:56
• @CarlWitthoft do you have anything to back that claim? You don't turn your wheel but lean after a certain speed. – user4139 Jun 11 '18 at 19:07
• yes, and that "lean" would make you fall if you didn't turn the front wheel such that the resultant centrifugal force "pushed" you upright. You can find the analyses at any number of "bad science" web sites which discuss the physics of bicycles in great detail. The offset front fork mount is a critical design element. – Carl Witthoft Jun 11 '18 at 19:13

I agree that wobble magnitude vs. wheel size is a significant source of instability.

My suggestion is that another major instability source is the relative lateral acceleration in a turn compared with your ability to compensate. The center of mass is very high on a skateboard, compared with the wheel axes, so a little centripetal force leads to the CM going "outboard" and causing a crash. By comparison, not only is the CM of a bicycle much closer to the wheel axes (in height), but you can force a far greater out-of-plane tilt on the wheels to correct against that centripetal force.

To enlarge a bit upon Carl Witthoft's answer, which I upvoted, remember that the swivel mechanism in the skateboard's wheel trucks is designed to provide a certain (fixed) amount of steering action for a given amount of board tilt. At ordinary board speeds (a brisk run) the board tilt provides the rider with approximately the right amount of lean angle to make the turn coordinated (as it would be for instance on a bicycle in a turn).

The rider has no way to decouple his lean angle from the board tilt angle to produce a coordinated turn at speeds different from the board's "design speed"- meaning that at slower speeds, the rider will tend to fall towards the inside of the turn and at greater speeds he will tend to fall towards the outside of the turn.

At high speeds, fighting this tendency puts the board into a speed wobble in which the upper part of the rider's body does not track the movements of the board while the lower part does. Mayhem ensues.