Motorcycle.com
“The man was in such deep distress,” said Tom, “that I could do no less than give him good advice.” Said Jim: “If less could have been done for him I know you well enough, my son, to know that’s what you would have done.”
—Jebel Jocordy (Ambrose Bierce)

If there was one piece of advice that you’d think was universal to all motorcyclists, whether they’re riding dirtbikes, sportbikes, cruisers, tourers or ice racing in Wisconsin in January (as the MO crew once did), it’s this: don’t put a car tire on a motorcycle. This advice is so natural-seeming, so obvious, that I don’t think I’ve ever had to actually type that phrase before, so I’m going to do it again, because it’s so much fun:

Don’t put a car tire on a motorcycle.

You know where this is going, don’t you?

I know it may be old news, but maybe you’re like me a week ago and didn’t know that this is, as they say, a Thing. There are numerous discussion sites and motorcycle shops that are fully supportive of slapping a car tire on your motorcycle’s unsuspecting rear (or even front) rim. But why, Lord, why? Just because you can do something doesn’t mean you should do it. So why do thousands of riders roll over to Pep Boys to do it?

One obvious answer to ‘why’ is – and I’ll bet you could guess – it saves you skrilla. How much? A high-mileage motorcycle tire is what … $150? $200? $250? And you’ll be lucky to get 20,000 miles out of it if you’re really anal-retentive about tire pressure and whatever else it is that conscientious motorcycle owners do. An Arizonian Silver Edition car tire can be had at America’s Tire for a mere $73 (with free shipping) and comes with a 50,000-mile warranty, plus the waiting rooms at car tire shops have vending machines and a barely audible TV from 1998 playing the Steve Harvey Show. Finding a shop with the stones to mount it to your motorcycle rim can be challenging, but they’re out there.

Tom’s rear tire leaning into a turn. Yes, it’s square, but it still leans. You just have to push a little more. If you are old, you may remember motorcycle tires having a much more car-like, squared-off profile than they have today.

I found out about this from the latest issue of Iron Butt magazine, the official organ of the Iron Butt Association. If you don’t know, the IBA is “dedicated to safe, long-distance motorcycle riding and sponsors the Iron Butt long-distance rally every other year as well as other smaller events. The IBA has over 50,000 members, and these people ride – a lot.

It’s a surprisingly interesting and readable magazine, with great photography and the sort of articles you really aren’t going to find in other publications unless the editors there are profoundly off their meds, like “Confessions of a Rally Spouse or “Priorities – How to Make Long-Distance Relationships Work along with thrilling shoot-out comparisons between Ibuprofen and Naproxen.

In “Examining the Dark Side (Iron Butt Magazine, Fall/Winter 2015), IBA Chief Technical Adviser Tom Austin not only writes in great detail about mounting car tires (the hipsters call them “CT’s for short), he mounts up a Kumho 195/55-16 Run Flat on his own Gold Wing, with resulting benefits I hadn’t thought of.

For one, the car tire, with its far stiffer sidewall and greater load rating, ran cooler than the moto-specific tire he was using before. Austin points out that anecdotally, there have been no reported heat-related catastrophic failures of a CT on a motorcycle, but such failures are well-known on heavyweight bikes riding at high speeds at high temperatures. Another benefit is (according to posters on Dark Side fora) the availability of stickier-compound rubber in oddball touring and cruiser sizes. Another advantage is long tread life – up to four times what you’d get out of a motorcycle skin. Oh, and you can also use a run flat, which can, in an emergency, let you ride on a flat tire for a few dozen miles.

Did you know there’s an actual Tire and Rim Association that sets standard bead sizes for car and motorcycle tires? Maybe you should listen to them before you try to mount a car tire on a motorcycle or vice versa. Or not. What do they know, anyway? Diagram: Tom Austin.

Of course, there are some disadvantages, too. The main one is strange handling characteristics from the CT. No, really! Yes, it seems that the squared-off profile of the car tire results in heavier steering feel and abrupt transitions from lean to vertical, which unless you live somewhere that’s extremely flat and straight, you probably figured out on your own. The car tire can make the bike hard to hold upright on a sloped road, a problem for those with short inseams and heavy motorcycles.

Also, there’s an issue (according to Austin) with the design of the motorcycle rim vs. that of a car. The car wheel’s “bead hump” – which is behind the tire bead and keeps it from unseating – is a few millimeters farther back from a motorcycle’s, which means the bead will not seat into the motorcycle rim like it’s supposed to. The risks created by this are enough to make a products-liability attorney wet his or her business suit (or slather with greedy anticipation depending on the side of the courtroom he or she is on). “An incredibly bad idea,” writes Austin.

And yet, you don’t hear about motorcycles crashing because of running CT’s. Lo and behold, it turns out that motorcyclists are the kind of people who are willing to take risks and continue doing something that sounds insane to outsiders. And kudos to the IBR for allowing the use of car tires at its sanctioned events, even though a recent Iron Butt Rally finisher set a record of 14,185 miles on Metzeler ME880 motorcycle tires. In the IBA’s view, there’s no data supporting the notion that car tires affect tire safety or motorcycle handling, and there is data supporting the notion CT’s run cooler – so it’s kosher.

How’s this for freaky: to mount a car tire on a motorcycle rim, not only do you have to be okay with this, you also have to mount a size smaller – a 16-inch car tire fits a 17-inch moto rim. What could possibly go wrong? According to Dark-Side riders, nothing. Diagram: Tom Austin.

Austin does point out the disadvantages probably outweigh the benefits of running a CT, and I’m with him. In fact, I’m going to state for the record, on behalf of myself, Motorcycle.com, the USDOT, the MIC, UN, and every other organization you could think of, including the Organization of Satanic Temples (which may, in fact, not exist), that you should under no circumstances, ever put a car tire on your motorcycle.

But if you do, I unofficially think you’re a badass. Ride on.


Gabe Ets-Hokin is a product development specialist for SuperKid, a Hong Kong-based toy company. His latest products, Broken Glass n’ Rusty Scrap Iron Discovery Center and Baby’s First Meth Lab, are available at your local toy retailers.