I’m not an engineer and I don’t play one on TV. But this is what I’ve culled from years of reading about the subject of uneven tire wear. And if I’m full of bull, one of our resident smarter people (of which we’re blessed with many) will tell me so. Gently, please.

All of the advice above is excellent. However, no one thing is responsible for uneven tire wear. Instead, it is a combination of the above and how it takes place as you ride and corner.

As Greiffster points out, a simple 1.5% road crown will not create the problems you’ve experienced. However, when you add the two sides together, you get 3% (we’ll get to how that math works in a minute), and 3% can be a substantial contributor. For the sake of this discussion, let’s assume the measurement of 1.5% so we have a constant to work with. And since it’s probably better to work in degrees, let’s just round it up to 2 degrees of road crown, or a summed difference of 4 degrees between the two sides.

First, there’s almost imperceptible differences in side-to-side tire wear when riding on a crowned road in a straight line. A bit more when braking, because the front tire deforms (compresses) some and thus exposes a larger portion of its side tread to the 2 degrees of road crown that exists (plus there's a microscopic amount of side slip due to the road crown), but we’re splitting hairs. Uneven front tread wear comes from cornering and, if you’re a bit of a trail braker, from that, too. But for this explanation, let’s work on the cornering.

Let’s assume a cornering angle of 30 degrees. Let’s make this our cornering constant. As has been explained, in countries where we drive on the RH side of the road, the road falls away from us to the right (for water drainage). Also, on any road, a left curve takes the outer of the two radii, the right, or opposing lane, represents the inner radius (going the other way). On a right curve, the inverse is true as the right lane is the tighter and (in terms of speed) the slower of the two. As anyone who’s watched a wheel go around knows, the parts closer to the axle (axis) spin slower than those farthest away from it. So if you look at any corner, left or right, as a part of a circle, i.e. an arc which has an axis, you can understand more easily how one side is the faster side depending on which way the corner goes.

Now let’s take that 30-degree motorcycle lean angle and carry it through a left-hand curve. The 30 degrees is measured from vertical. This assumes a road surface perpendicular to that vertical line. But it’s not perpendicular and thus not really “just” a 30-degree cornering angle because if you add in the 2 degrees of road crown, you’ve got an effective cornering angle of 32 degrees when arcing left. Not only that, but two of those 32 degrees are being used by gravity to pull you away from your intended line, because of the negative grade represented by the road crown. Thus, in order to hold your line, your tire scrubs, but so subtly that you usually don’t feel it. Still, it’s wearing by sacrificing its surface to the heat and shredding that it takes to change the direction of a loaded 700-lb sport-touring bike.

Now let’s take that same curve and make it a right hander. Suddenly the 30 degree lean angle is reduced to 28 degrees because of the 2 degrees of camber (road crown) that works to create a positive road angle into which to arc your bike. Thus the differences between a left curve at 30 degrees of lean and a similar right curve at 30 degrees of lean, total 4 degrees. That’s enough of a difference to create some different wear characteristics. More scrub and wear going left, and less going right (all else being equal).

Beyond this, we can calculate in a number of other factors if we want.
• Take away our constant of a 30-degree lean and in the real world, road speeds would be different (greater) through a left hander vs. a right hander. More speed, more scrub (tire wear).
• Plus there’s the fact that riders are generally more comfortable leaning to the left (most of us are right handed and are more protective of our natural side). Thus we’re more comfortable cornering harder to the left.
• Plus turning left leans our bodies away from the throttle and straightens out the right wrist which makes for more comfortable and controlled throttle application leading to earlier application (vs. leaning our bodies right which cramps the wrist’s ability to rotate smoothly and often delays confident throttle application), all of which contribute to making the front tire work harder and sooner when exiting a left curve, and on an already 4-degree surface differential,
• etc. etc. etc.

The above, along with many other factors (many of which are touched on above) is what I’ve been led to believe are the major contributors to uneven front tire wear. Ten, twelve, fifteen curves per mile on your favorite set of twisties, mile after mile, through hundred and eventually thousands of miles, and the inequities build up until the tire wear differences are noticeable.

Cary, Gleno, Rodger, Paul,
Godspeed, my brothers.
We will never forget you.

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