TrafficSafe Investigates Riding on the Dark Side

By Jonathan Newell

With National Tire Safety week beginning soon in the USA, TrafficSafe specialists urge riders of large motorcyclists not to use a car tire on the rear wheel, colloquially referred to as the Darkside.

A major online tire retailer in the USA recently published a recommendation to riders of Goldwing and Valkyrie motorcycles to use a Michelin car tire on the rear wheel , quoting advantages such as improved comfort and traction with less effect from road gravel on handling.

The website reassures “darkside” riders that the practice is safe and that the 195/55R16 Michelin Primacy Alpin PA3 ZP is the ideal tyre of choice for the owners of such motorcycles.

The choice of tyre for motorcyclists is particularly critical to ensure safety and there are many factors to consider. Sidecar outfits and custom three-wheelers have special considerations and there may be circumstances when the choice of a car tyre is feasible but for solo motorcycles, the weight of technical opinion from experts is overwhelmingly against the practice of riding on the darkside.

TrafficSafe’s vehicle construction and use expert said, “Motorcycle tyres are constructed differently from car tyres to cope with the kind of side loads that are normal for a motorcycle but would be absolutely abnormal on a four-wheeled vehicle. Car tyres are simply not constructed to take sustained lateral loading to the extent experienced on a motorcycle during cornering. My advice to anyone contemplating using a car tyre on a motorcycle is simple…. Don’t do it!”

We spoke to TyreSafe, a UK non-profit organisation dedicated to raising tyre safety awareness and campaigning for the safe use of tyres. Their spokesman told us:

“A car tyre is not designed for the conditions of use of a two wheel motorcycle. A car tyre develops its cornering force by having a slip angle applied to it, and is not designed to run at the sort of camber angles encountered on a motorcycle. A motorcycle tyre is designed to develop camber thrust as the bike corners at significant camber angles, and it has a profile adapted to this.

The use of a car tyre on a two wheel motorcycle would in our opinion lead to unpredictable handling and cause rapid deterioration in the shoulder / sidewall area leading to possible tyre failure with serious consequences. Motorcycle wheels and car wheels differ in profile, and it is essential that the tyre and the wheel are compatible. Car wheels are not compatible with two wheel motorcycles.

In conclusion, we feel that the use car tyres on two wheel motorcycles should be strongly discouraged.”

TrafficSafe examined the following video footage of a car tyre being used on a solo motorcycle and shared it with other experts in tyre safety. We noted the following points:

1 The tread pattern is designed for car use with the maximum volume of water being displaced when the tyre has flat contact with the road surface. Angling the tyre into corners has an enormous impact on the tyre’s ability to displace water on a wet road surface.

2 As the motorcycle leans into the corner, the tyre presents less surface area to the road, reducing the contact patch at moments when firm contact is most needed.

3 The tyre is over-inflated to 40psi, presumably in order to change the tyre profile to make it less square and thus improve cornering performance. Sudden loss of pressure from 40psi would have a devastating effect on handling.