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Rolling Resistance

Road tire size and other contributors to rolling resistance

Thirty years ago in Illinois, we raced many individual, 2-man, and 4-man time trials. Not only did we get techy about our training, we were “very scientific” about our gear. And now, more than thirty years later, I realize that what we believed about tire width, wheel diameter, inflation, and rolling resistance was all wrong. For example, on my 4-man TT team, we all rode funny bikes with 650 mm front wheels. On fairly rough rural roads, we inflated our skinny 19 mm tubular tires to 160 PSI. In short; we did almost everything wrong. Unless we were riding on smooth asphalt, every road surface felt like washboard. So, as it turned out, not only was I wrong about rolling resistance; the opposite of much of what I believed was actually true.

If you want your eyes to glaze over, there are highly-technical articles on the contributing factors to rolling resistance. This ain’t one of them-there articles. We’ll point out the variables that contribute to rolling resistance, some of which make a small difference; and some of which make a tiny difference. A good, efficient position on the bicycle contributes more to going fast than how wide or narrow your tires are, but decreasing your rolling resistance can make a noticeable difference.

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TIRE WIDTH. A 19 mm tire and a 28 mm tire, inflated to the same PSI, deflect to the same amount of contact area. The 19 mm tire, however will have a flatter footprint because it compresses more longitudinally, and it will have higher rolling resistance. This is a generalization, though, and subject to many other variables.

RIDER/BIKE WEIGHT. A heavier rider on a heavier bike will deflect more and have higher rolling resistance. If you and your bike weigh a total of 250 lbs., you will have to inflate your tires maybe 30 PSI more than someone weighing 70 lbs. less. Think of a heavier car having higher spring and shock rates due to the extra weight. A relatively light car that has very stiff shocks will ride like a buckboard and actually handle worse by momentarily losing road contact, especially in bumpy corners. The term in racing is “bump steer.” We’ll talk about compliance some more, and how it reduces rolling resistance.

ROAD SURFACE. Here in Southern Utah we have varied road surfaces, ranging from about 2% sublimely smooth asphalt to the more common bumpy, worn slurry coat, and the Dreaded Chip Seal Nightmare. (That will be the name of my next band.) If you inflate your tires higher and ride on a rough surface, you increase rolling resistance because the bike tends to micro-bounce over every minute imperfection. You lose contact with the road, and every time the bike lands, rolling resistance is increased by friction. The idea is to inflate the tires high enough so they’re not squishy, but soft enough that they don’t feel like they’re made of concrete. I’ll adjust my tire pressure depending upon where my ride is taking me. Most of the time, on 25 mm tires, the sweet spot is between 95-110 PSI. Just because the info on your sidewall says you can pump the tire to 160 lbs. doesn’t mean you should. Not only will you be slightly slower, you’ll be uncomfortable. FRAME/WHEEL VERTICAL STIFFNESS. Most newer road bike frames are intentionally somewhat vertically-compliant. They are supple enough to help you track bumps better and they also provide a more comfortable ride; and lower rolling resistance. If you can recall frames that had thin, vertical-bladed top tubes years ago, you’ll remember those frames had bone-crushing rides. Brands that come to mind are Cervelo, Cannondale, and Kestrel. Not only were they fatiguing, the vertical stiffness increased rolling resistance. A less-compliant bike will not track road irregularities as well, and it will bounce.

Michelin Tire Pressure Chart

The same is true of wheels that are not vertically compliant. Wheels should have high lateral stiffness, but be somewhat supple in the vertical plane. I notice this on my two nearly-identical S-Works Tarmacs, which share all the same components except for the wheels. The wheelsets both weigh around 1,250 grams and they’re from the same unnamed Italian manufacturer. One wheelset has rims that are 21 mm deep, and the other wheelset’s rims are 50 mm deep. The deep-rimmed wheels have a harsher ride, and chip seal and bumps are more obvious. Even though these wheels “look faster,” and they’re certainly more aerodynamic, they don’t maintain road contact as uniformly as the lower-profile rim, and I suspect rolling resistance is a bit higher. I prefer the ride of the less-flashy low-profile wheels.

TIRE CONSTRUCTION. There are many variables here. An aggressive tread pattern may have higher rolling resistance. That can be mitigated by riding a road tire with a raised, smooth center. That way, you are essentially on a treadless tire until you corner, at which point traction becomes far more important than rolling resistance. Some sidewalls are suppler than others. Tubular tires (the only tires I’ve ridden on since 1971) have lower rolling resistance, categorically, and that, along with generally lower tire and wheel weight, explains why most pro teams use them. You can run a tubular at 10-20 PSI higher, and have less deflection and more comfort than you might have with clinchers. That said, clinchers have gotten really good, and some rival the ride and rolling resistance of tubulars.

There are other bike-weeny considerations, such as thread count (higher is better), slightly increased rolling resistance in tubulars due to the mounting glue or glue strips, and other characteristics that may rob you of three precious watts.

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FINDING A HAPPY MEDIUM. Just because lower tire pressure “can” roll better and wider tires “can” have lower rolling resistance, don’t go overboard. There is a definite sweet spot for road bikes. Lighter riders can ride 23 mm or 25 mm tires, and heavier riders seem best served by 25 mm to 28 mm tires. Tires wider than 28 mm start to have a weight and aerodynamic penalty, and that may negate any advantages of going that big. And at some point, you can ride with too little tire pressure, and feel like you’re pedaling through sand, even though you’re on Snow Canyon Parkway. 60 PSI is too little for a road bike, even if you weigh 100 lbs. And remember that the heavier you are, the more pressure you need to run or you’ll deflect more and have higher rolling resistance. Your tire and pressure choices depend upon your weight, your desired comfort level, the road surfaces you ride, and your frame and wheel compliance. To optimize your setup for the least rolling resistance, you must consider the variables, and not just tire width and tire pressure.

Writtten by Paul Scarpelli

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2 Responses to “Rolling Resistance”

  1. Michael McGrath on 17 Feb 2018 at 7:21 pm

    I have a question comparing speeds of a mountain bike with narrow Road bike tires
    With the speed of a Road bike.

    How much difference on speed will you see if the same Rider rides both bikes on a flat road?

  2. Paul Scarpelli on 19 Feb 2018 at 1:08 pm

    I can’t answer that question, but I’ll wager the reason a mountain bike will be slower is non-aerodynamic rider positioning, and, secondly, weight. You would be dealing with increased wind resistance. If the tires were the same size and inflated to the same PSI, rolling resistance will be close. One variable would be suspension, though. A suspension tends to even out the ride, reducing rolling resistance. Bottom line…Dunno!

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