Originally Posted by kitesurfer
i don't know why a new wheel is required. GOLDWINGS have been running car tires for a few years now. good reports from all that do...BAD reports from those who won't try.
It all depends on what the bike's stock wheel/tire size is as to wether there is a safe car tire substitute without doing a wheel conversion.
The modern BMW's run a 17" rear wheel with a single-sided, double-pivoted swingarm. That swingarm is pretty beefy and doesn't offer much extra clearance for an oversized tire. That is true both in the overall width, as well as the height of alternate tires.
There is a big radius brace molded into the corner of the swingarm that limits the size of a square profile tire.
Add in that there just aren't that many 17" car tires available in smaller widths in the United States, and those few which are made are on the pricey side; and the cost of a conversion quickly becomes practical.
There is one inmate over in the Hacks forum who recently fitted a 17" car tire to the stock wheel of his K-bike sidecar rig (Yokahoma S-Drive 195/40-17 ~$110). The first size he bought didn't fit (rubbing), and the smaller one he bought second required THREE wheel spacer shims before it would clear the swingarm, which altered the centerline of the wheel.
(And he recently reported that he is still getting rubbing at speed due to tire growth when it is spinning at highway speeds.)
On the other hand, there is a relatively wide selection of 15" car tires which easily fit into the space available, and they are MUCH less expensive ($50-70).
And if on a trip these tires are readily available from nearly any tire outlet, service station, etc. to keep you rolling.
(In my researching this conversion I found that most of my local car tire outlets will not mount a car tire to your bike rim for you, but if you have a car rim - no problem!)
So for half the tire cost, you get wider selection, availability, and easy servicing if you have an issue out on the road.
Just from the cost of the tires alone the conversion can end up paying for itself in the long-run.