'tis true. The rolling resistance between 20 and 23's are too small to really contemplate.<br /><br /><br />SLICK VERSUS PATTERNED TREADS<br /><br />Practitioners of motorized sports recognized the traction benefits of slick tires a long time ago. Drag racing is the sport in which the most obvious benefits of slick tires are demonstrated, and it only takes a few seconds to see that treaded tires just slow the vehicle down.<br /><br />While that was evident to drag racers early on, it took a generation for grand prix and rally drivers to figure that out, and another generation before motorcycle racers caught the hint. Where I live, in the San Gabriel Mountains, cafe racers test their skill with high-powered motorcycles, which now use slick tires routinely.<br /><br />I find it interesting that when I get my Continental GP3000 tires they've still got a tread pattern. Fortunately, most road bike tire makers have come 'round to the realization that slicks are in and tread is out.<br /><br />Bicycle tires (as opposed to car tires) don't need to have a tread pattern to prevent hydroplaning, as a bike tire's round cross section has the ideal shape for the rain. While it is hard to demonstrate this on the road, I'm told that machines that measure traction show that smooth tires corner better on both wet and dry pavement, and that lean angles can be greater with smooth tires than with treaded.<br /><br />Smooth tread also offers lower rolling resistance, because its rubber does not deform into tread voids. Rubber does not compress, but it is elastic and it deforms. It changes shape, but not volume. For a tire with an internal tread pattern, the rolling resistance is increased because the rubber bulges and deforms into tread voids when pressed against the road. Tread squirm is one term I've heard attached to this phenomenon, and it is mostly absent with smooth tires.<br /><br />SMALLER- VERSUS LARGER-WIDTH TIRES<br /><br />I've heard, though I haven't seen, reports of rolling resistance studies Continental performed that included an analysis of tire width. While I've heard about the conclusions of those studies, it's only third-hand, so I can't report them here with any authority.<br /><br />I've heard, and it seems intuitive to me, that there is not much if any measurable difference in the rolling resistance of a 20mm tire versus a 24mm tire, all other things equal. Thinner tires require more attention, though, in that they're more susceptible to increased rolling resistance if they're not inflated to a sufficient pressure.<br /><br />I'd be more concerned about the rolling resistance of tires that have some profile other than round. Certain tire makers have tread molds that cause the tire to have a flatter aspect, perhaps in the hope that they'll corner more securely. Whether or not that actually serves the intended purpose, the question is, what does such a profle do to the profile of the contact patch when the bike is going in a straight line?<br /><br />This is easy enough to test, though it'll require a bit of clean up. Apply a thin layer of wet paint or ink on your garage floor. Roll the tire over it with your weight on the bike, and see what kind of impression it leaves after a full revolution (that won't mean much to you unless you compare the ink profile to that of another tire).<br /><br />After you've satisfied yourself (or if you're willing to take my word for it) that a 23mm or 24mm tire will roll as efficiently as a 19mm or 20mm tire, you can move to the next two issues, which are aerodynamics and resistance to flats.<br /><br />As to the former, it depends on the wheel you're using. Remembering that what's aero seems to change every six months according to the various "experts," I distinctly remember a series of conversations between both those who make, and those who wind-tunnel test, aerodynamic bicycle wheels, in which one of the significant parameters was the match between the width of the tire and of the rim upon which it was mounted. In most cases and with most carbon race wheels, 23mm and 24mm tires appear to blend more cleanly than do 20mm tires and thinner.<br /><br />Rolling resistance<br />by Dan Empfield September '02<br />
<br /><br />ps: how did this weird rumour start that 23mm tyres are more efficient than 20mm?
