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July 05, 2013 7 Comments
Avid cyclists lust after frames. Saddles provide a styling touch as compelling as fashionable shoes (notice all the Brooks-type seats). Drive trains and wheels arouse great aspirations. But tires? How many feel deeply partial or in awe of tires?
Tires are too small to arouse the viewer. They get dirty, look much alike, and wear out. We pay a lot for these rubbery belts that can be wasted in a single incident. Maybe they too-much resemble garden hoses. Or maybe their magic is too hard to comprehend, like wheels. What could be more transparent than brakes and drive trains? Frame and cockpit function is as visually obvious as furniture. Tires and wheels are subtle and obscure.
The closer you look at tires, the more they are positively dazzling. These deceptive little bladders keep us suspended on air and work by principles that are unique among all the bicycle's many systems. If the wheel was a prime catalyst to civilization as we know it, the pneumatic tire was the wheel's most important advance. The pneumatic tire super charged wheels and bicycle racing was instantly transformed.
In less than one year, the entire world bicycle industry was converted to pneumatic tires. Everything else was instantly obsolete. Nothing in our time was so transformative. Not jet engines, not semiconductors, not LED's. Why?
Yikes, it's Alive
The pneumatic tire is an organic device that mimics living tissue. How do sherpas carry 40kg packs over granite and ice, barefoot? How do arteries maintain strength, flexibility, and a controlled measure of size change? Synthetic rubber (thanks to Charles Goodyear) is only a short modification of rubber tree sap. Alone, it makes balloons. No control.
Combine rubber with fibers and you have a system with promise. The arteries and intestines of organisms evolved such technology over millions of years. Elastin is a fibrous protein that gives strength and a limit to size change. Collagen is a flexible protein that moves like rubber. Together they are building blocks for living tissue and extremely similar to tires.
Tires are the living tissue of our bikes. So how do they support our weight on pillows of air? A tire has a contact patch with the ground. This is the deformation of the tire that matches the usually-flat contour of the earth. Wider the tire, the wider the contact patch relative to its length. Small tires have long, narrow contact patches.
The arithmetic is elementary. If a tire carries a load of 100lbs, then its contact patch will respond accordingly. If the air pressure of the tire is 100psi, then the load will create a contact patch of one square inch. If the pressure is 50psi, then the contact patch will be two square inches. No calculus here.
Your contact patch will vary in shape according to the tire size. But the contact patch area is independent of tire size. Notice that the larger tire has a wider, shorter patch than a smaller tire. The shorter the patch the lower the rolling resistance. Why? Because rolling resistance is a function of tire sidewall deformation. The longer the patch, the greater the sidewall deformation. As the sidewall deforms and reforms, there is work occurring in the fabric. Heat is the outcome and watts are consumed.
Small vs. Large Tires
If larger tires always have shorter patches and lower rolling resistance (at equal pressures) then why ever use small tires? Several important reasons:
(1) Smaller tires are much lighter and the rims that best support them are too.
(2) Smaller tires have much lower wind resistance. Much lower. This reality is being oddly distorted in much of today's aerodynamic discussion.
If smaller tires are so good, why use large ones? Remember:
(1) Small tires must use higher pressure because they offer less volume to protect against road hazards. Low pressure is often preferable for comfort and control.
(2) Air doesn't weigh much. Larger tires weigh more but most of their size is air. The weight penalty of larger tires is smaller than it looks.
One more important issue with tires is traction, not rolling resistance but the ability of a tire to connect with the ground. By traction, I mean friction:
Ft ≤ μ Fn
Simplified, friction is a function of tire stickiness and load. Friction is NOT a function of the size of the tire patch. How can that be?
Basically, lower tire pressures only give greater traction under conditions where the riding surface is unstable. Unstable surface means that greater area offers more chances for some positive interaction between tire tread and road (or trail). If the road is uniform, lower pressure does not increase traction.
Why does lower pressure seem to increase traction on uniform road surfaces? This is a suspension effect. Lower pressure does not increase friction but a compliant tire casing maintains more continuous contact with the road so traction is better. It's not friction, it's suspension.
I'm reminding you of the magic we enjoy on every ride. Generalizations found in simple-minded explanations and marketing can be really confusing if you miss the underlying principles.
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