The specification affecting the safety of a 15 inch car tire being fitted to a motorcycle rim is the diameter difference between a car and a motorcycle rim. The rim has a slightly smaller diameter where the tire seats. Thus the tire is under stress just to get seated.
That's without rim shapes etc
Do so at your own risk.
Apparently tires have been known to fail/blow-out because of this.
People also are known to do this without failures.
I am running a 16 inch (low profile) car tire on my Guzzi.
Mainly to adjust the overall drive ratio.
This is an article by a smarter guy than I.
https://www.goldwingfacts.com/threads/design-differences-between-car-and-motorcycle-rim-tire.400426/
Running a car tire on a two wheel bike IMO is a risk. Some may disagree.
On a sidecar Rig different world. Car tire on rear of bike done right works great and has many advantages.
This was just installed today.
Stock is a 180/55 18
Car tire is a 195/65R 16
The change comes out same height and 6 tenths wider. I keep a factory wheel and tire ready if I take sidecar off. Side by side.
The diameter of a 15" car rim at the bottom of the bead seat is 14.968 inches. Same measurement on a moto wheel is 15.08 inches. This means then that the circumference of the bead seat on a moto tire is approximately 47.375 inches while the car wheel bead seat is approx 47.02" circumference. The high tensile steel cords that form the tire's bead bundle do not stretch. They simply don't. It's inherent in their design and purpose. The only way to find the extra 3/8" you need to seat the car tire on the moto rim is by deforming the bead chafer not to mention the body plies themselves which of course are wrapped around the bead bundle.
As you say, it CAN be done but that doesn't make it advisable. What's an acceptable risk level? One failure for every million miles? Every ten million miles? I don't know, but I will never recommend to anybody that they shoehorn a 15" car tire on a 15" moto rim.
There is a discussion with pics on this Forum from some 8-12 years ago regarding attempting to mount 15" car tires on 15" motorcycle wheels, but I haven't found it just yet. However, I did stumble upon the following information posted** by Claude back in 2011:
YEP it has been done with 15" motorcycle wheels but is not recommended. There have been injuries. Also as hard as it can be to seat the tire one must wonder if any damage was done to the tire during the process which may cause a failure at a later date when on the road. We will not do it WITH A 15" CYCLE WHEEL. See pics below.
**From: https://www.advrider.com/f/threads/15-car-motorbike-tyres.736795/ where you will find additional discussion, including the following comments from a fellow in the UK:
...Nearly killed myself trying to fit a 165/15 vredestien snow plus onto a Bmw r1200c 15 inch rim.Just about managed to get the tyre onto the rim, then just kept pumping air in trying to get it to seat when it exploded. The tyre split on the side away from me which launched the wheel up into my chest throwing me against the garage door. The mass of air escaping from the split blew a shelf off the wall on the other side of the garage and the loud bang left me deaf for 3 hours. But what really pissed me off was I'd just paid 75 uk quid for the tyre.....
Lee
MB5+TW200+CRF250L+GTV300+INT650
XL883R w/Texas Ranger Sidecar
Zuma 50F + Burgman w/Texas Sidecar<Mrs. SwampFox
I have read an article where the 15 inch motorcycle rim was machined down to 15 inch car size.
Another dubious compromise.
It may well be fine but to me to run car tires that diameter on a sidecar?
Fit a car wheel somehow.
