Since I am decidedly not a motorcycle rider, I actually didn’t know people sometimes swap car tires onto their motorcycles. I have wondered about it, though. And if you’re thinking about trying it out, here are some things to think about.
Dark Siders (they call themselves that) are part of one of the most evangelical groups of motorcyclists I’ve encountered. They are deeply passionate about their practice of using car tires in place of prescribed motorcycle tires on their bikes. And they are more than anxious to profess those beliefs in the hope others might also see the light. Or the dark, as it were. It borders on a religious movement. Needless to say, it was not difficult to get their side of the story. In fact, after I broke the news to the prospective student, I was soon flooded with emails from other Dark Siders, each enthusiastically sharing his personal testimony, telling of his conversion to a life of riding on the dark side, never to return to conventional practices again. “Remember, the earth was once flat too,” one Dark Sider offered.
The biggest difference you’ll observe between a car tire and a bike tire are their profiles. A car tire is pretty flat, whereas a bike tire is more U-shaped. This is because a motorcycle leans far more than a car does and the tires need to keep a uniform contact patch with the pavement during those leans.
First, there’s the load capacity. Car tires are designed to withstand far more weight. The bike tire FortNine uses as an example has a load capacity of 716 pounds; the car tire has a load capacity of 1,433 pounds. This might not matter if you’re riding something like a little Honda Grom, but if you’re sitting on a behemoth Honda Gold Wing with a passenger and cargo, you start thinking about stuff like that.
Then there’s the tire bead (which is the edge of a tire that stays seated on a wheel or rim). See, both the spinning action and the turning action want to fling the tire off the wheel.
“Each of these problem forces [need to be] be counteracted by two opposing forces for redundancy,” the video explains. “We have air pressure squeezing the tire against the bead flange to stop it curling inward under corner force. We also have the bead hump, squeezing the tire against the bead flange to stop it curling inward from corner force.”
“Meanwhile, the circumference of the rim being slightly larger than the tire’s stretches the rubber taut against the bead seat to stop it throwing outward under centrifugal force. And the bead lock also stops the tire throwing out from centrifugal force.”
The big problem here is a car tire doesn’t really fit as ideally as a motorcycle tire. But it seems to work for dragsters and off-road riders, where a big, flat contact patch gives the best grip.
Then there’s the handling. A dark sider rider has to keep applying countersteering pressure throughout a corner because the bike just won’t lean and stay leaned over. The car tire will always try and right itself.
But it makes sense why someone would try and put a car tire on a bike. Because it does work. They can get more miles out of the tire, it can withstand lower temperatures and carry heavier loads. And they’re generally cheaper to buy. But the trade-off is a poor bead fit and sub-optimal handling.
This great video details the rest.
h/t to Dave!