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.

Since the US online tire retailer had quoted a Michelin car tire as an “ideal example” of what to use on a motorcycle, we spoke to Michelin directly and showed them the YouTube video. Michelin had this to say:

“Using a car tyre in the way illustrated in the video could result in rapid deterioration in the shoulder and sidewall area due to the abnormal flexing which the tyre will undergo, and so in our opinion running car tyres on two wheel motorcycles should be strongly discouraged.”

Finally, we spoke to a UK insurance company on whether fitting a car tyre on a motorcycle would affect any claims. The spokesman said “Such a change to the standard specification of the motorcycle would require prior approval from the insurance company otherwise the policy would be invalidated. In the event of an accident, the rider would lose the right to claim on the insurance. Under current UK law, the insurance company would be obliged to pay third party claims if the motorcycle rider was at fault but the insurance company would rigorously pursue subsequent compensation from the rider at fault”.

We also spoke to the largest insurance company in the USA which specializes in motorcycle insurance who declined our invitation to comment. TrafficSafe would therefore recommend that US motorcyclists check their policies and enquire with their insurance provider regarding the validity of their policy before fitting non-standard equipment to their vehicles.

Given the overwhelming weight of opinion against the use of such tyres for two-wheeled motorcycles, TrafficSafe urges motorcyclists not to be deluded into believing that riding on the darkside is safe. Websites and forums promoting such a riding style have a duty of responsibility to put the record straight. TrafficSafe particularly urges the US online tire retailer to retract its public statement encouraging darkside riding and to fulfil its duty as a responsible tire supplier by not marketing car tires to solo motorcyclists.

Jonathan Newell

Studied Engineering at Loughborough University and now involved in broadcast and technical journalism. Jonathan is based in London and Almaty.

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TrafficSafe Road Safety Blog

12 Responses to TrafficSafe Investigates Riding on the Dark Side

  1. jnewell says:

    TrafficSafe is extremely pleased that the US tyre supplier referred to in this article has fulfilled its responsibility to the motorcycling community by issuing a statement withdrawing its claim that the Michelin Primacy Alpin tyre is suitable for use on a motorcycle rear wheel.

    After reading the article, watching the video and making its own consultations, the online tyre supplier does not recommend the use of car tyres for use on any two-wheeled motorcycle and doesn’t believe that riding on the Darkside is a safe practice.

  2. sumncguy says:

    Well …

    The 2012 goldwing is just about 1k pounds. The best motorcycle tire as far as load is the Dunlop e3 which is load rated at 991 pounds.

    This is a REAL issue.

    Is it better to overload the motorcycle tire or ride the Dark side ?

    What do you have to say about this ?

    What studies have been done to back up what you are saying ?

    • Steel Horse says:

      Motorcycles have two wheels, with the weight of the motorcycle, rider, pillion, and luggage divided between them. The load on the rear tyre will be a portion of this total load, not the whole, with the front tyre also taking its share.

      Unless you only use your 2012 Goldwing for wheelies, or you habitually overload the bike beyond the manufacturer’s recommendations (which will be in the manual, as will be recommended tyre specifications) you should be fine.

  3. sumncguy says:

    and you should also have your facts straight. The Max pressure for the Michelin Primacy Alpin PA3 ZP is 51 psi ! 40 lbs isnt over inflating the tire !!

    • Steel Horse says:

      It’s true: you can run this tyre at up to 50psi. On a standard car though tyres tend to be inflated to 28-32psi, with some variation outside that. In that context, at 40psi the tyre is overinflated relative to its normal use. Perhaps the article could be more explicit about that, but the point still stands.

  4. jnewell says:

    Thanks for the link to the article, it was an interesting anecdotal treatise of one guy’s experience of riding on the dark side. However, my point doesn’t change.

    The writer’s experience of 100k miles with a car tyre on the back doesn’t tell me it’s safe. There are people who’ve smoked for 60 years and died happily at 95, it doesn’t mean smoking’s good for you.

    The debate on dark side riding is very controversial and is dominated by safety advocates on one side and passionate enthusiasts on the other. There is very little middle ground and so the divisions are wide but the debate should nonetheless be rational… from both sides.

    TrafficSafe’s take on this is that in the absence of objective test data, motorcyclists shouldn’t take the risk.

    If car tyres can be used on motorcycles then there should be type approval for them (there isn’t). Without type approval, how does the average consumer (by which I mean people without the level of experience of the article’s writer) know what tyres match what rim sizes, what inflation pressures to use, which tread patterns work the best, whether it can be used on a front wheel and whether it can be used on sports motorcycles with higher cornering angles.

    In short, the limits should be defined but they’re not.

    Dark siders are one very small subset of the vast motorcycling community and it’s that wider group of people that we’re trying to reach with the message that without extensive testing and type approval, don’t put car tyres on your bike.

  5. Mike Woodley says:

    over here in the UK its normal practice to fit a car tyre on the rear of a motorcycle and sidecar. I have been running a GTR1000 Kawasaki outfit for 18 months and scrub out a standard 150 80 16 in around 3000 miles. I am about to change to a 165 70 16 car tyre. I have made this choice because I discovered reducing rear tyre pressure to 30psi seems to improve the low speed stability, in addition the 16 inch car tyre seems a good match for the motorcycle rim. I already run the standard front tyre at 22psi and the next mod, assuming car tyre fitments at least as good as the rear bike tyre, will be to increase the width of the front tyre to 120 with as low a profile as I can get. Any tips or genuine comments direct to my e-mail would really be appreciated.


  6. Juan Arturo says:

    It will be helpful for us to know how many MC accidents has been caused by the use of a CT on the rear of a bike. If someone has access to that statistic, please share with us.
    One thing is for sure (I did test it), with a CT on the rear of a bike, one could apply the rear brakes to all force without having any tailing on the bike or loose of control of it.
    When riding a MC danger is everywhere around one. Most accidents are caused by “cagers” hitting bikers than from tire failure. IMHO

  7. GaryODS says:

    An interesting article.

    “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.”

    I’m assuming that the blogger does understand that the tire he is referring to in the US is a ZP, capable of running 50 miles @ 50 MPH with no air in it?

    In my experience it’s not the shoulders that wear out first, and I’m running mine at 32 PSI with approximately 25K miles on it and I’d guess that it’s got an additional 10K – 15K left to go.

    This isn’t for everyone, there are handling characteristics that are different, but for me they just became the new normal.

    I really don’t care what he, or anyone else chooses to ride on – whatever trips their trigger. For me I actually feel safer on the CT, and on long touring rides in remote area’s I certainly like the option to go to the closest car tire dealer instead and get back on the road instead of waiting for days or more for a tire to arrive.

    Ride Safe

  8. Bill says:

    This reminds me of the old line “you can’t fix stupid”.

    The biggest and most valid reason not to put a car tire on a motorcycle is the rim bead on a motorcycle and a car are different and not compatible. Then you have the handling issue, a tire is not designed for motorcycle and even in this comments section it has once again been admitted that it changes the handling and it is never for the better. Everyone who uses a car tire says that yet they still do it all to save a few dollars on the cost of a tire.

    This is what it all boils down to is owners who cheap out. Maybe a better path for those owners would be to sell their motorcycle if they can’t afford to put the proper tires on it.

    If car tires were so great don’t you think all the motorcycle manufactures would put them on from the factory?

    There was a time when some motorcycle tires had a square tread profile. Bike back then were not as powerful and did not handle as well as bike today do. Tires were have evolved to the design they are today because that is what works best and is safest. The tire engineers figure out that a square tread design was just not the best solution for a motorcycle.

    I will happily but motorcycle tires and enjoy the handling that was engineered into my touring bike rather than to degrade the handling.

    I think the only thing that has saved those who have switched to a car tire is street motorcycles are designed to only lean up to 45 degree with many not even able to reach that amount of lean angle. Racing motorcycles are designed for lean angles up to 50 degrees before hard parts start to scrape.

    In my opinion it is completely stupid to buy a 20,000 to 30,000 dollar motorcycle and then put a car tire on it and alter the handling in a negative way.

    Goes back to “you can’t fix stupid”.

    As for the comments about crash reports documenting crashes of bikes with car tires, in the United States police crash reports are not designed to capture that data so there could have been a few hundred crashes involving motorcycles using a car tire and we would never know because the data is just no captured on the crash report.

  9. Hack says:

    So, it’s not safe to run a car tire on a 1,00 lbs or less machine that has 2 wheels because they can’t take the side loads. Explain how the car tires can withstand this.

    • Steel Horse says:

      Oh boy… Two points:

      (1) I might be wrong but I suspect the rigours of stunt driving require more frequent tyre changes than standard road driving due to excess tyre wear – something to which driving on two wheels would contribute.

      (2) Those guys are driving on two wheels without incident, and it is impressive, but they’re not doing it at highway speeds. Not even close. If I were to run a car tyre on my ZZR 1400 and repeatedly throw it through bends leaned over at high speeds the results would I’m sure be quite different.

      Seriously, tyres are what keep you on the road: don’t skimp.

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