The UK remains one of the last regions in the world to legalise e-scooters, but why? The Department for Transport are particularly concerned over their safety as there have been studies in the US that have revealed somewhat alarming results. But what is the cause of these injuries? I scoot all the time on my Swifty and it feels safe and fun. The difference is that these statistics are taken from public e-scooter sharing providers who use different hardware, a more compact design with an 8” wheel, whereas all Swifty adult scooters have 16” wheels.
Can scooter safety be improved by design?
With this in mind we decided to measure the geometry of the most popular 8 inch wheeled e-scooter and compared against our 16 inch wheeled scooters.
We also decided to perform a simple simulation by riding both scooters over a pothole. This report explores which design features define a safe and road worthy scooter.
We are conducting this simulation because we believe adults riding small wheel scooters that are poorly designed, could cause a hazard when ridden on the UK’s roads.
In a recent report (US 2018) by the Public Health and Transportation Department in Austin Texas, in association with the CDC it was reported that 20 individuals per 100,000 trips were injured. Cars or other motorised vehicles factored into 16% injuries. The remaining 84% of injuries were caused by the rider losing control for one reason or another. We believe that the geometry and design of these small wheel scooters are a large part of the cause of these accidents, and that safety can be improved through design.
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Choosing the size of pothole for our test
A pothole is a depression or wear in the road, and are common hazards for cyclists in the UK. After a quick internet search, we discovered that most potholes are repaired (within 7-28 days) if they are deeper than 40-50mm (depending on the council). However, most shallower potholes can remain unrepaired for a long period of time.
Therefore we decided to design our test using a pothole 50mm deep and 300mm wide as our control. We have ignored some factors, for example, rider ability, the weather and third-party influences like other vehicles.
Wheel Size - Is bigger better?
In this study we are comparing a 16” big wheeled scooter and an 8” wheeled e-scooter. The 8” wheeled e-scooter is the most common form factor currently on the market.
Figures 1.0, 1.1, 2.0 and 2.1 demonstrate the critical angles and dimensions of each scooter and also a simulation and suggested trajectory after impacting a pothole.
This simulation does not take into account weather conditions, rider ability or other external factors that might affect the outcome.
So for wheel size, the bigger the better! But it’s more complex than that, the larger wheels allow for other design elements that collectively make the 16” scooter is far more stable than an 8” scooter. Let’s examine those critical design elements.
Handling and Control - Head angle, rake and trail
Bicycle geometry and engineering is a very well researched and studied topic. Principles such as trail, wheel offset, headtube angle, rake, wheelbase, wheel size etc are all taken into account when a stable and comfortable ride is desirable. It is common practice that a bicycle trail is no less than 50mm. It is the fork rake and headtube angle that creates trail. The 16” scooter shown in Fig. 1.0 has a trail of 53mm, this is within the recommended threshold of 50-70mm recognised by bicycle engineers. The larger trail provides a geometry that delivers stable handling and straight line of travel. The compact design of the 8” scooter only provides 22mm trail [Fig. 2.0] which will feel very twitchy to steer, and therefore harder to control. This trail is a result of small wheel size, poor headtube angle and negligible rake. When riding over the pothole [fig 1.1 and 2.1] the 8” scooter will be difficult to control, even by an experienced rider.
Stability - Wheelbase
The dimensions for the wheelbase, centre of footplate and headtube-to-footplate are all greater on the 16” SwiftyAIR scooter [Fig 1.0 and 2.0]. This results in the rider’s weight being further back from the front wheel. When the rider experiences the front wheel impact of the pothole [Fig 1.1], the energy transferred to the rider is less sudden and greatly reduced. The shorter wheelbase of the 8” scooter means the rider’s weight is further forwards and close to the front wheel at the point of impact. This results in a greater sudden impact and higher energy transfer to the rider. This makes the 8” scooter harder to control. Again the smaller wheel and the geometry of the more compact design will be harder to control.
Steady Manoeuvres - Deck Drop
Deck drop, is the measurement between height of the wheel axles and the height of the deck. The 16” scooter has a positive deck drop of +88mm [Fig 1.0] whereas the 8” scooter has a negative deck drop of -28mm [Fig 2.0]). A positive deck drop (i.e below the centre line between front and back axel) is desirable as it lowers the rider’s centre of gravity. This is a factor that is important when a rider is manoeuvring or trying to avoid an obstacle. A negative deck drop or high centre of gravity drives less force into the tyres on turning than a low centre of gravity. A well designed scooter maintains suitable deck drop, and also ground clearance – a critical measurement in scooter design, and another advantage of the 16” wheel. This is particularly important when it comes to avoiding a pothole or other obstacles in the road.
Product Longevity - Crumple zones and tyres
The 16” scooter in Fig.1.0 has pneumatic tyres and bicycle spec wheels (hub, spoke and rim). The 8” scooter in Fig 2.0 has solid rubber tyres and solid wheels.
The hub, spoke, rim and tyre design of a bicycle wheel is easily serviced, maintained and replaced. It’s also designed to absorb energy under impact and crumple in extreme cases. Pneumatic tyres also help to absorb energy and provide the most suitable way to create traction and grip. A tyre always creates a flat spot when it comes into contact with the ground, this is varying dependent on wheel size, tyre design and tyre pressure. This flat area effectively creates the grip needed to securely drive forwards, bank, turn and brake with confidence. Small wheels that are solid with no rim or spokes, fitted with a solid tyres have the opposite effect, they produce poor contact with the ground and reduced performance when driving forwards, banking, turning and braking.
Trajectory after impact
The simulated trajectory after impact as shown in figure 1.1 and 2.1 demonstrates a catastrophic result for the 8” scooter and substantially lower risk for the 16 inch scooter.
The combination of poor geometry, short wheelbase, negative deck drop and solid small wheels would suggest that under impact, the front wheel of the 8 inch scooter would stop suddenly. It is unlikely that the front wheel would roll out of the pothole.
One must also consider the 8” scooter grip-angle-to-vertical dimension (7 degrees) shown in figure 2.1. This is half that of the 16” scooter figure 1.1 (14 degrees). This increases the likelihood of the 8” scooter and rider being propelled forward over the handlebars when colliding with the pothole.
When you consider the combination of poor braking (mechanical and electric) that 8 inch scooters most commonly use, and the high and forward centre of gravity shown in our simulation, we believe that driving over an obstacle like a pothole would present a high-risk hazard in an 8” scooter.
By comparison, when the 16” scooter shown in fig 1.1 strikes the pothole edge it is probable that the larger pneumatic tyre would deform under impact, cushioning the blow, and promoting the wheel to roll up and over the edge of the pothole. The lower centre of gravity, larger headtube angle, greater trail and larger bicycle spec wheel and pneumatic tyres are all factors that would help support this result and conclude that this would be a low-risk event to the rider.
Conclusion
It is our opinion that wheel size is the critical factor when designing a scooter that provides a safe and comfortable ride and that is roadworthy. It is also worth mentioning as a private company we are not the only manufacturer who makes scooters with larger wheels, and we have no claim to this wheel size. However, we are in a situation where scooter market is dominated by small-wheeled scooters because the price and compact design is appealing to consumers.
In 2020 Swifty Scooters will celebrate its 10-year anniversary, so when it comes to scooters we know a thing or two about scooter design engineering but most importantly safety. Many of the new e-scooter sharing operators generally speaking excel when it comes to their connectivity, mobile sharing platforms and logistical operations. However, the choice of hardware (scooter supplier) must evolve to be safer and more suitable for road conditions and for novice riders. Historically, the majority of scooter-share operators have bought OEM products from the largest scooter manufacturer in the world, Ninebot.
Ninebot is a Chinese conglomerate producing over a million scooters a year and supplying a large proportion of the scooter sharing platforms. Many of the early scooter sharing operators were in such a rush to deploy and scale that they simply didn’t consider the predicament of road suitability that we now are facing. There are some who are investing in improving their hardware, but they all seem to pursue similar geometry, design and wheel size. This is driven only by price and plentiful supply.
The public perception of the small-wheeled scooter form factor
Scooter-share operators have scaled so fast that the compact form factor has become the common public perception of what a scooter should look like. Many of our customers have expressed that they wished they had known about larger wheeled scooters before purchasing a small-wheeled scooter. Many of these customers have unfortunately been injured as a result of losing control of their small-wheeled scooter (both electric and kick-scooter) and have been suddenly thrown to the ground.
At Swifty Scooters we take rider safety and rider experience very seriously. We design, engineer and manufacture all our products to our own specification. This has taken years of research and development and is the reason why we believe our vehicles are the winning form factor. For a safe and stable adult scooter, our testing has proven that you must have 16 inch wheels as standard, whether it be electric or kick-powered.
When it comes to regulation, we would like to propose changes that support a larger wheel size in order to suit variable road conditions.
It’s also important to note that as a company we welcome other manufacturers to take heed of this study. Our mission at Swifty Scooters is to encourage more people to choose lightweight, active and electric transportation. We are experiencing a transport revolution, where a variety of small, light and electric new vehicle types are emerging. Scooters are just one type among many that will need space on our roads in the future.
There is clear public demand for scooters and we believe the UK has a great opportunity to lead on this front when it comes to scooter legislation. The UK’s legacy for transport innovation could extend into the future by setting a new gold standard in safety and legislation to support the adoption of light energy-efficient vehicles.
Swapping car miles for scooter miles – the evidence
We are very proud of our customers for putting in lots of scooter miles to date! According to a recent questionnaire, we estimate that our users have clocked up 6,900,000 miles since we started our company. That’s 277 times around the world on a scooter! Our study revealed that 40% of these scooter miles were replacing car journeys, others were used to make public transport more accessible by speeding up the ‘last-mile’. Find out more about how swapping car miles for scooter miles saves CO2 in our recent blog:
Great write up. I agree, the small wheels on the rental scooters make them rather dangerous to ride. Although I am a huge fan of their service, the scooters themselves give me anxiety. One bad crack and I’ll go flying. That’s why I like the swifty scooter, because they have a much more stable and safer ride. I watched your podcast about the engineering behind the design of the swifty scooter. Your observations are right, in which the rental scooters are fidgety because of their high center of gravity and vertical stem.
Andres on