Most roads, especially narrow, mountainous twisty roads are higher in the middle than on edges. The roads which clearly show this characteristic are called high-crowned roads; however, to lesser degrees, most roads, even city streets, have the same characteristic, mainly for drainage purposes. This means in the US, the left side of motorcycle tires wear off more than the right side, since on average, due to the crowning effect of roads, the left side of tires contacts the road surface more than the right side.

You can understands the effect by looking at the following diagram with a cross-section of an imaginary, exaggerated, high-crowned road and an upright motorcycle:

Diagram: A Motorcycle and a Crowned Road in the US

2002 California Motorcycle Handbook specifically mentions how to handle high-crowned roads:

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SLOPING SURFACES

A road surface that slopes from one side to the other is not difficult to handle when you ride straight ahead. However, in a curve, a slope can make the turn harder.

A high-crowned road is higher in the middle than at the sides. A turn to the left on a high-crowned road is like a turn on a curve that is banked the wrong way. The crown makes the turn more difficult because it:

• Cuts down on the clearance between the left footpeg and the surface.
• Adds the force of the downslope to the outward force of the turn, which increases the chance of a skid.
• Makes it necessary to turn uphill.

The only way to handle the "wrong way banking" is to slow down. This will straighten the motorcycle and reduce the outward force.

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I started speculating last year that in the US, more crashes occur in left-handers than in right-handers. To date, I haven't found the US statistics which clearly indicate that motorcycle riders have more crashes negotiating left-handers than right-handers. However, I found a little interesting statistics at a UK website:

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However, an in-depth analysis of accidents in Cheshire between 1 April 1998 and 31 October 1998 reported in Street Biker (Feb-Mar 2000), the MAG newspaper. John Moss MBE, Chief Road Safety Officer for Cheshire (and MAG member) revealed that fully 67% of all the crashes studied were due to rider error and that the traditional view that most motorcycle accidents are down to blind Volvo drivers is badly flawed.

Let's look at the bald statistics:

lost control on right hand bend - 16.7%

lost control on left hand bend - 13.7%

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Obviously, if my hypothesis is right, in the UK, riders should have more crashes in right-handers since the direction of traffic in the UK is the opposite of that of the US.

Unfortunately, the number of samples (470) in the above analysis was too small to make the difference noted statistically significant. It is interesting to note, however, that in this motorcycle accident analysis, the distinction was made between right-handers and left-handers.

In any case, since most roads are crowned to some degree even if not clearly detectable to our naked eyes, in the US, I believe it pays to be extra-cautious when we approach all left-handers.