Bike Cop:
Be aware that not all 'dark-siders' are evil. I will qualify this by saying that going 'dark-side' for the right reason, and the safe way, should not be an issue - legally, or liability-wise
, period, full-stop. That said, i do agree that automotive tires, with the soft sidewall, mounted upon an unmodified motorcycle rim and being used as a solo motorcycle, is a huge-gamble in the legal and liability department that is indeed potentially worthy of a ticket or three. Doing this to a single track motorcycle in the interests of tire life is a false economy. Doing it with a multi-track vehicle, like a sidecar or forecar/trike configuration - entirely safe and suitable!
There are the ones that have
properly done it... and the ones that have done it
properly, have done so at generally great expense, for equal reasons of safety and tire longevity.
There is a fundamental difference in motorcycle rim sizes, and car tire sizes .. as such, most are generally incompatible. The tire size that is most likely to work, and least likely to blow out either while installing, or on the road, is the 17" and 18" tire sizes. Even given that, those that i know that have used the 17" stock motorcycle rim and a Smart-Car spec 175 or 185/55/R17 tire, have also taken the added steps to machine the proper automotive bead seat into the motorcycle rim.
MC Rims:
15": 15.080"; 16": 15.978"; 17: 17.080"; 18: 18.080"
Passenger Car Rims:
15" 14.968"; 16": 15.968"; 17: 17.188; 18: 18.188"
Lower than 17 inch the car rim sizes are 0.032" smaller than the nominal size,
Thus, a 15" rim has a rim size of 14.968"
and a 16" rim has a rim size of 15.968"
But bigger than 17" the rim sizes are 0.188" bigger than nominal size
a 17" rim size has a rim size of 17.188"
and a 18" rim size has a rim size of 18.188"
Now, i researched this about 10 years ago, as part of my own decision-making process into the 'dark-sider' controversy, and how it applied to my own project at the time, this comes from a text file i saved from back then. In the end, although it was considered acceptable to mount a 17" car tire onto a machined 17" motorcycle rim in the sidecar community, i took the added steps to do it
right from a safety perspective.
My sidecar rig is the result of a 3 year fabrication exercise. It is a heavyweight at 1200lbs, with car rubber on all three rims. Not much beyond the frame, gas-tank, and motor is stock.
Sidecar wheel - Asuna alloy 14" with 185/55R14, mounted on suspension of own design and fabrication. $80.00 junk yard.
Rear wheel - Honda Civic alloy 15" with 185/55R15 mounted on custom machined hub. $1000 for hub, $200.00 to import a pair of US spec Honda rims.
Front wheel - 3 piece composite custom imported from the sidecar capital of the world - Holland. $1800. Mounted on custom LL front end of own design and fabrication.
The project was on the road from 2004-2010 before i was side-lined due to health issues. 55k km's on the project, with no issues related to automotive tires being used out of spec.
My point of the above, beyond being likely one massively annoying brag to others, is that you cannot judge the use of automotive tires in a motorcycle application the same way, across the board. Circumstances and application should and must be assessed.
It's been my unfortunate experience to be inherently distrustful of what the law decrees, especially in setting legal precident or even 'cracking down' - a blanket tends to be thrown over all, using the phrase "summary-offense"... in the hands of an ignorant, non-riding cop not familiar with the safe application of car tires on a vehicle other than a car, assigning of a notice of an offense for using car tires on another (non-automotive) vehicle with similar road-handling characteristics, would be a mis-application of the intent and spirit of that law - to keep them off of solo, single-track motorcycles where they definitely (IMHO) DON'T belong.
I just don't trust your average traffic cop to make an accurate assessment of all that, road-side. Not when it comes to my disposable income and the potential of traffic fines taking it away, all due to ignorance by a cop throwing the ticket and mumbling "explain it to the judge".
