HIP TO BE SQUARE: Can You Rock Car Tyres on a Motorcycle?

A car tyre and a motorcycle tyre separated by a question mark

Admit it, after being shocked at the replacement price for that almost-new rear tyre which picked up a big fat wood screw puncture with only half a year of riding, we’ve all considered cutting corners on the replacement rubber.

Maybe it’s a cheap-ass new tyre from a loosey goosey, no name brand, or maybe you considered scouring the classifieds for some ‘good as new’ second hand rubber being flogged by someone keen to make a quick buck. Whatever the case, nothing sucks more than having to stump up a fist full of moto cash unexpectedly.

Actually, one thing does suck more than that. Crashing your bike. And if there’s one surefire way to make that happen, it’s cutting corners on the super critical maintenance parts. Yes, you guessed it; tyres are one of those parts.

So, would anyone seriously consider running car tyres on their moto? Well, you just might if you find yourself stuck in a one-horse town with a shredded rear boot and no other way of getting back to civilisation save for the old car tyre the local ‘mechanic’ is now offering you. But should you do it?

BMW R 1200 GS on side in red dirt with car tyre mounted on rear wheel
What caused this crash? I guess we’ll never know. Image via Advrider.com.

Meet the “Dark Siders”

Now I hope you’re sitting down, because what I’m about to tell you will probably see you needing a stiff drink and a rest anyway; some riders insist that buying bike tyres is a scam and always run car tyres on their bikes. Yes, you read that right. They go by the name “Dark Siders” and bugger me sideways if they aren’t actually serious about the whole crazy idea.

Undoubtedly made up of a high proportion of flat-earthers and fake moon landing loons, they seem convinced that “Big Rubber” (I made myself laugh with that gag) is out to scam all motorcyclists by forcing us to buy expensive, fancy grippers that perform no better than cheap-o car tyres in most conditions.

A motorcycle with car tyre mounted on the rear wheel
Added bonus: the bike stands up by itself! Image via Advrider.com.

Digging deeper reveals that many of these rubber-obsessed weirdos also rock luxury touring bikes that probably weigh just a little less than a fully ladened Titanic. I guess if you’d gone through a bunch of rubber thanks to your road boat chewing up tyres whole, you might be desperate enough to try something as nutty as this, too.

And if you are going to try car tyres on your bike, choosing a ride that likes corners about as much as Donald Trump’s hair likes a stiff breeze is probably the way to go.

A motorcycle takes a corner with car tyre mounted on the rear wheel
It makes total sense, if you’re totally nuts. Image via Motorcycle.com.

Breaking News

For the rest of us, there’s a big stinking problem with the whole hairbrain idea. Bikes—unlike cars and two-wheeled luxo-barges—lean. Yes, you heard it here first. BIKES LEAN. And cars, well they don’t lean unless they are in a serious pickle or if you’ve managed to park them sideways on a set of stairs.

Thusly, car tyres are flat to maximise their contact patch with the road. Hell, even a car going sideways at freeway speeds will remain pretty much parallel with the horizon. But for even the gentlest of manoeuvers on a bike, you’ll be changing the part of the tyre that’s touching the road.

 a homemade motorcycle using car tyres instead of motorcycle tyres
Back away slooowly. Image via YouTube.

Allow Me to Illustrate

Hence, bike tyres need a round cross-sectional profile to ensure that a similar amount of rubber is in contact with the earth at all times. If this were not the case, a rider would have to contend with varying amounts of traction—and that, my inquisitive friends, is a very, very bad thing.

Suddenly finding yourself with less traction than you expected is almost always a recipe for disaster on a bike. Throw riding in the rain into the mix and—just for shits and giggles—some potholes, and you’ve just found yourselves a great shortcut. To the hospital.

Diagram showing contact patches of motorcycle and car tyre when vertical

Thanks to my sweet-ass Photoshop skills, you’ll see above an illustration of the contact patches of both a motorcycle tyre (the left one, dummy) and an average car tyre. Although the car’s patch is segregated, you’ll still see that once added together, they both offer very decent levels of contact—and therefore traction—between the tyre and the road.

 diagram showing contact patches of motorcycle and car tyres when leaned over at 45 degrees

Now I’ve leaned both tyres over at a 45 degree angle, and while the moto tyre still has a proper amount of contact between itself and the road, the car tyre is clearly suffering from a major loss of traction.

Yes, the car tyre would probably deform under the weight of the vehicle it’s supporting and increase the patch size somewhat, but the point remains that not only have you lost a bunch of traction, but you’re also now running on a part of the tyre that’s not designed to come in contact with the road.

Diagram showing various contact patches of motorcycle tyre at varying lean amounts
Surprisingly, motorcycle tyres are designed specifically to maximise contact with the road at all lean angles. I’m kidding about the ‘surprisingly’ bit. Image via ridermagazine.com.

Just Say No

Three quotes to finish up. The first and second are sage-like and the third one is even more so.

The first comes straight from the horse’s mouth. I asked Marcel Bode (from Metzeler Australia) and Simon Pool (the Technical Manager at Pirelli Tyres Australia) for their thoughts.

Their responses were fairly cut and dried. “No you cannot!” they exclaimed. But why?

“The bead profiles of a motorbike tyre and rim are different to car profiles, so you can’t put car tyres on moto rims. The profile of a car tyre and bike tyre are very different, so this would make cornering very difficult; your bike would need to basically stay upright and could not lean into a corner.

Also, everything is completely different from pattern design, carcass construction, rubber compounds, and profiles. Lastly, is it even legal to do, and would the bike be insured when fitted with car tyres?”

Nice pint, that last one. Some insurance companies will take any chance they get to avoid a payout.

Then there’s this gem from the Motorcycle Industry Council of America. “Never mount a passenger car tire on a motorcycle rim; the flat profile of a car tire is incompatible with the dynamics of a vehicle that leans as it corners, and the section of the tire in contact with the rim (the ‘bead’) is incompatible with motorcycle rims.” Pretty clear, yeah?

Finally, there’s Chris Rock, who very clearly encapsulates my thoughts. “Yeah, you could do it… but that don’t mean it’s to be done! Shit, you can drive a car with your feet if you want to, that don’t make it a good fucking idea!”