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WHEEL SIZE 26 vs 700C

Testing 26 vs 700

By Bill McCready, president Santana Cycles, Inc.

A Posting from Bill at Santana

Because of an aol software problem we lost a week of postings and only caught the tail end of the most recent 700c vs 26" battle.

Many of the comments were on-target and should be amplified.

First, the physics of the 26vs700 discussion are only meaningful if we standardize the other variables: rim width, tire width, tread pattern and inflation. To perform comparisons here at Santana we used two diameters of otherwise identical Avocet tires mounted to two diameters of otherwise identical Sun rims. We installed these wheels in frames built from identical tubesets. The frame geometries differed only to the degree that the resulting bottom bracket heights were equal. Components other than the wheels were identical. We inflated the tires to 115psi.

Between 700c and 26" there is no difference in the AREA of the contact patch. "Pounds per square inch" means just what it says. If we know the loaded weight of a tandem and the inflation of the tires we can accurately predict the area of the contact patch. Actually, if we know any two of three variables (weight, inflation and the size of the contact patch) we can calculate the third. Tire diameter has no effect.

Tire diameter does, however, effect the SHAPE of the contact patch. Because the 26" wheel has a diameter that is 11% smaller (559mm vs 622mm bead seat diameter) the resulting contact patch or footprint of a 26" tire is both 11% shorter and 11% fatter.

The shape of the footprint affects handling. With all other things equal (especially fork rake and bottom bracket heights) the rounder contact patch of a 26" front tire dramatically improves low speed maneuverability. Conversely, high speed stability is enhanced by the longer and narrower footprint of a 700c front tire. While the two tires will feel different in a hairpin curve--the smaller tire corrects quicker and the larger tire holds a smoother line--because cornering speed is a function of area and grip, maximum speed through a sharp turn is the same.

If they both corner at the same speed, is either wheel size more efficient? Yes. Because of its smaller diameter, the 26" tire is forced to deform more to apply its equal-area-yet-fatter contact patch to the ground. When we put the same weight on both bikes it's easy to observe more "bulge" in the sidewall where the 26" tires meet the ground. Greater tire deformation (sidewall flex and tread squirm) equals greater internal tire friction; the leading cause of rolling resistance.

Why not compensate for the extra rolling resistance by inflating 26" tires to higher pressures? While many of us fear blowouts, the leading justification for lower pressures (and wider tires) is COMFORT. Because the smaller wheels start with a comfort handicap (smaller wheels are less compliant), higher pressures won't be a popular option.

If rolling resistance effects speed, why do leading triathletes use 26" tires?

For certain events (triathlons, track pursuits and time trials) rolling resistance is less important than the frontal area of the tire--in these no-slipstreaming events a solo bike with 26" wheels has an advantage. But for pack cycling events (criteriums, sprints and road races) the aerodynamic advantage of the smaller wheel is not great enough to offset increased rolling resistance.

Is the wheel efficiency equation different for tandems? Yes. Compared to a solo bike, a tandem tire's frontal area is roughly half as important (twice as much power to push each tire through the wind). Further, a tandem's doubled weight can make sidewall deformation and rolling resistance twice as critical. Subsequently, there are no on-road races where a tandem with 26" wheels will be faster.

If a 700c tandem is faster, why does Santana offer nearly twice as many models with 26" wheels?

Even though a 700c wheel is actually slightly heavier than a 26" wheel, the difference in "bash-strength" (the ability to survive impacts) is enough to render a 700c wheel damned near useless for rutted jeep trails and urban curb-hopping.

If you want one tandem that does it all, 26" is the only wheel size that makes sense. While a 26" mountain tandem can easily be converted into a pavement scorcher that will keep you abreast of the fastest roadies on their solo bikes, a tandem with 700c wheels is too fragile for real mountain biking.

And even if you never plan to venture off pavement, the "bigger is faster" argument is limited by the size of the riders--tandems built around 700c wheels are inefficiently tall for captains shorter than about 5'7".

Prove it to yourself section:

Because one or two netizens might (again) find it easier to malign the messenger than to attempt to understand the accuracy of the message, I've included the following quick and simple experiments to allow everyone to test the verity of this posting.

Experiment 1: To confirm identical contact AREA and differing footprint SHAPE use an ink pad, a sheet of paper, a bathroom scale and a pair of different diameter wheels with similar width rims. Install similar-width smooth tires--worn out tires from the discard pile of a local bike shop work great. Inflate both tires to the same pressure before testing.

Experiment 2: To determine that tire deformation effects rolling resistance attach any bike to a wind or magneto trainer. After riding a bike for a couple of minutes, overtighten the adjustable roller (or lower the chainstay support pad) and try it again. Where does all that extra energy go? If you can stand to ride the "tight" setup for a few minutes you'll confirm that a bulging tire converts energy into heat.

Experiment 3: To discover the diameter-dependent differences in stability and handling visit a Santana dealer and ride a Visa & Vision, or Arriva & Fusion, or Sovereign & Encore back to back. These three 700c / 26" model-pairs have the same tubesets, neutral handling characteristics and identical components (except rims and tires). Experienced tandem riders will easily note the differences in crawl-speed maneuverability and high speed stability. If the dealer is willing (most are) bring along your cyclo-computer and tape or tie-strip it to each model before coasting down the same hill to determine the speed difference.

Happy trails


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