Are your bike tires slowing you down?

Your bike tires may be costing you time, lots of time.

“Come on,” you say, “a bike tire is a bike tire, right?”

Or “I paid $50 per tire for my Continental Force and Force Attack tires. The bike shop said they were the fastest. They are fastest, right?”

Well, not according to Tri-Maine race leader, Kurt Perham, they are not. Perham is nearly obsessed with what he refers to as “free speed.” From fast transitions to perfecting bicycle aerodynamics, Kurt is always on the hunt for an edge. In a recent interview with OA, the race leader and multi-sport coach shared with us the fact that we could replace our Continentals with a set of Michelin Pro Race 2s and save ourselves both time and money. A set of Pro Races 2s can be purchased on line for $30 less than you can buy the Continentals, and they will save you as much as a two minutes over a 40K bike course like Urban Epic or Lobsterman.

How is this so?
Kurt shared with OA a presentation he developed with his colleague, Rick Ashburn, on bicycle tire drag. The discussion is a little technical but well worth the read.

“A pneumatic (air-filled) tire has a flat spot where it touches the ground – the contact patch. About half of a rider’s weight is on each tire, making an oval flat spot a little less than one square inch in size. As the tire rolls forward, the leading edge of the contact patch moves around the tire. Some rubber from the tire that began its life round becomes bent and flattened. As that spot on the tire leaves the back end of the contact patch, it bends back into its favored round shape. That bending and un-bending action doesn’t happen freely – it involves friction within the tire. That little bit of friction, happening constantly and rapidly, adds up and slows the bike down.

Now, before you go thinking that pumping the tire up really hard will reduce this drag, you need to consider bumps in the road. While it is true that high-pressure tires can minimize the bending and unbending friction, they do so at the cost of bouncing over tiny bumps in the road instead of “squishing” over them. Every bumpy bounce causes the bike to divert some of its kinetic energy into an up-and-down direction instead of forward, and that energy is effectively lost. So, a nicely rolling tire needs to strike a balance between being pumped up enough to minimize the contact patch drag, and not so much as to have it losing speed to the tiny constant bumps encountered on normal asphalt.

The answer to this little optimization problem is found by tire testing. Cutting to the chase, the answer is: For racing on normal public roads, you should pump your tires to between 100 and 120 pounds. Just because the tire might have “190psi” printed on the side of it doesn’t mean you should do it! That is a maximum number printed for your protection, and would apply only if you were racing on a track.

There is an equation for calculating the drag that the tires impart to the bike. For our purposes, it is convenient to express that drag force in terms of watts:

Watts to overcome tire drag = Crr*M*g*V, where

Crr = Coefficient of Rolling Resistance (determined by testing the tire)

M = Mass of bike plus rider
g = The gravitational constant, 9.8m/s^2 (remember high school physics!)
V = The ground speed of the bike.

Since M is nearly fixed, and g is most certainly fixed, you can see that tire drag varies directly with Crr and with bike speed. If you go twice as fast, you use up twice as many watts overcoming tire drag. Since you want to go as fast as you can, the sole variable left under your control is the Crr of your tires.

The Crr of a name-brand racing tire will range from 0.004 to 0.007. At a glance, those numbers look like they are close together, and you can buy highly regarded and expensive tires at both ends of that spectrum. However, let’s do a little math

Crr = 0.004 or 0.007
M = 66 kilograms
g = 9.8m/s^2
V = 8.94 m/s (20 mph)

Rolling along at IM bike pace, I am expending either 23.12 watts or 40.48 watts to overcome tire drag. Which would you prefer? This difference of 17.36 watts represents – are you sitting down – about 2.5 minutes in an 40K bike race!

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