Mounting Car Tires on Motorcycles - Shop Talk

Tech Matters

I know there are enough riders mounting car tires on their motorcycles to warrant discussion, but as a topic of personal interest it ranks pretty low on my totem pole. For starters, it's not something that has a huge impact on the general population. Loud pipes, for example, attract a lot of unwarranted attention, a fat tire, not so much, so as far as social impact goes the issue is a non-starter. Secondly, I've known guys that have been running car tires on their bikes since the early 60's. Since none of them are running amok leaving havoc and mayhem in their wake, I'd have to think the practice is fairly innocuous, although one of them has been known to say nasty things about his ex-wife.

I mean it's your bike and your hide, so if you feel comfortable mounting a gigantic car tire to the rear of your VTX have at it.

Bottom line, even if mounting a car tire to a motorcycle rim isn't something I condone, nor for that matter do the tire or motorcycle manufacturers, it certainly isn't something I feel compelled to rail about in these pages.

However I recently received e-mail from a fellow that subscribes to the Darkside, () which is forum for car/motorcycle tire conversion enthusiasts. It stated in part that "the number of riders going to a car tire is growing by leaps and bounds and I think it (the use of car tires) needs to be tested by pros and the information published," So I suppose some comment is expected.

At the risk of sounding argumentative, not that it's ever stopped me before, I'd point out that both the tire and the motorcycle manufacturers have already done plenty of testing, and that's exactly why new motorcycles come with the tires they do, It's also why both warn against fitting anything but an approved motorcycle tire to a motorcycle. I'd also suggest that if we mounted a car tire to a test bike, and then wadded the thing up, we'd be left holding a large and very messy legal bag that no one, especially no one employed in the legal department at Motorcycle Cruiser, would be interested in taking off our hands.

Now normally that'd be the end of the discussion, but I like to think I'm an open-minded type of guy, so I clicked on to the Darkside forum to see what it had to say. It was enlightening. In some respects these guys do make a reasonable case for themselves. Particularly since, as one poster noted, "This mod is for a cruiser only because by the time we get leaned close to getting off the tread and onto the sidewalls, we are dragging everything on our bikes and couldn't lean any more if we wanted to."

Their contention is that using a car tire on the rear of their motorcycles, (and let me be clear on that point, nowhere did I see anything suggesting that using a car tire on the front of the bike was a good idea,) offers certain advantages, primarily: car tires are cheaper and last longer than bike tires, and they also offer a smoother ride. While the last raised my eyebrows the first two are reasonable points, so I can certainly understand why the owner of a big inch road burner, particularly one that racks up lots of miles and burns through two or three tires a year would find the idea intriguing.

But here's the problem; Car tires are fundamentally unsuited for use on motorcycles. Moreover, that's not just my opinion but also the opinion of every tire engineer I've ever discussed this with. We all understand that the dynamics of motorcycles and cars are vastly different. As such, they demand different types of tires and at the risk of great over simplification I'd point out that if a car type tire worked well on a motorcycle everything from a moped to a MotoGP bike would be wearing them as standard equipment. The fact that they can even be used on a big cruiser says more about the way those bikes are designed and ridden than it does about the suitability of using a car tire on one.

According to most of what I read on the Darkside (and similar forums) none of that matters. Some dispute the facts provided by the tire manufacturers and at least one guy said he didn't much care what the facts were. He liked his car tire and even if side by side testing showed that the motorcycle tire was superior, he was going to keep on running it. It's hard to argue with logic like that.

Knowing what I do about the dynamics of motorcycles, (and to be fair here I'm not a big fan of overly wide motorcycle tires either) I think I'll put my faith in the engineering staffs at Dunlop, Avon, and Metzler etc, and stick with motorcycle tires on my bikes, at least until someone can prove me wrong. Along those lines, I'll make a standing offer to any Darksider. Since I don't own a motorcycle that'll accept a car tire, and I'm fairly sure Honda would have a hemmorage if I installed one on my long term VTX, I'm going to ask one of you guys to hook me up. If you're game, we'll meet some sunny Sunday and take a ride. I'll show you some of my favorite roads and I'll even spring for lunch. If the car tire works as well as you say it does, I'll be happy to say so in print, you've got my and Motorcycle Cruiser's word on it. Any takers?