PeT said:
Actually, I do want to understand how a 29er wheel can have a bigger footprint, better traction, and less rolling resistance -- as has been claimed more than once in this forum.
Guess it's time to trot out some old links:
http://www.precisiontandems.com/artbillwheelsize.htm
http://www.gtgtandems.com/tech/700-26.html
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Pasted from the precisiontandems site:
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
Bill
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Pasted from the gtgtandems site:
700c vs. 26": Testing Reveals Best Choice for You
by Bill McCready, President of Santana Cycles
Santana sells similar numbers of tandems with both wheel sizes and has no ax to grind.
Four years ago a respected bike designer confidently predicted the impending demise of 700c tandems. While this was not particularly frightening to Santana (we had designed and promoted 26" road tandems as far back as 1983), it also didn't ring true. I've watched as hundreds of customers tested both sizes before making a decision --- was it possible the majority who chose 700c tandems were mistaken?
Many people who advocate one size over the other insist on comparing fat 26-inch tires and skinny 700c tires. Some make recommendations based on the availability of a particular tread pattern. Others confuse the issue by comparing 700c tandems designed for pavement with 26" tandems designed for dirt. Santana's question was simple: if you were to eliminate the differences in tread width, tread pattern, inflation pressure and frame geometry, is a 26" wheel superior to a 700c wheel strictly on the basis of its diameter? If yes, why? In an attempt to discover the truth, we prepared some test tandems and asked a number of teams to evaluate them.
To reduce extraneous perceptions our test bikes used identical tubing and direct-lateral frame style, 26" and 700c rims produced from the same extrusion, and tires with the same width and tread pattern inflated to the same pressure. The honest attempt was to discover the best size --- after all, life here at Santana would be a whole lot simpler if we could standardize on 26" wheels.
But first, some background. The argument over wheel size did not start with tandem riders. Alex Moulton of England produced pro racing bikes with 14-inch sew-up wheels in the late-'60s. In the mid-'70s, Tarn Cycles of Chicago built a series of Campy-equipped full-race singles (and at least one tandem) with 20-inch wheels. In the early-'80s California's first production mountain bikes, built by Victor Vincente, were equipped with 20" wheels. All of these builders argued that bikes with smaller wheels would be superior due to lower weight, stronger wheels, quicker acceleration, and less wind resistance.
Critics of these designs claimed bikes with smaller wheels were slower and less stable. While slower was difficult to prove, some organizers banned small-wheeled bikes from racing (where they might have disproved the "slower" argument) fearing "diminished gyroscopic effect" would inevitably lead to crashes in pack racing events.
Fred de Long, Technical Editor of Bicycling, disproved the "gyro" theory in the late '60s when he assembled a unique bike with side by side front wheels --- a normal front wheel plus an identical counter-rotating wheel slightly above and to one side. The second wheel (which rotated at the same speed but never touched the ground) offset the gyro effect of the first. His finding: a bicycle's gyro-stability is a myth. He postulated (and I agree) that all us cyclists remain upright by continually steering through/across the path of our imminent fall. (You can quickly prove this to yourself by riding a bike with an over-tightened headset --- the results are extremely convincing).
Three years ago there was yet another resurgence of interest in road-racing bikes with smaller-than-700c wheels. For a time you could buy road racing bikes with 26-inch wheels from many serious builders including Serotta and Paramount.
While a few large-frame time trial and triathlete bikes are still built around a pair of 26" wheels, the designers of these bikes are admittedly chasing tiny aerodynamic and weight advantages that will be lost on a tandem (where doubled power reduces the significance of these advantages by 50%).
So what did we learn during Santana's testing? Our panel of testers uniformly found 700c tandems were more stable at higher speeds. Most testers also believed the tandems with 700c wheels were faster. The difference in "feel" was substantial enough so that an envisioned follow-up "blind" test with carefully shielded-from-view wheels was deemed unnecessary.
Why were 700c tandems clearly more stable? At the time of the testing, none of us had a clue. I later developed a theory, first published three years ago, that the answer was due to the shape of the tires' contact patch (footprint). If the same mass is supported on tires inflated to the same pr essure, the area of the contact patch must also be the same --- this is, after all, the meaning of p.s.i. or "pounds per square inch." The difference in wheel diameter causes the footprint of the bigger wheel's tire to be more elongated than the footprint of the tire on the smaller wheel. I reasoned a longer footprint would provide greater directional stability at high speeds (as is the case with longer skis, surfboards, and skates). Until someone comes up with an alternative explanation, this theory not only explains the increase in high speed stability, it also explains why off-road riders might reasonably prefer 26-inch wheels --- the rounder footprint provides less steering resistance and easier maneuvering at low speeds.
While my original "footprint" theory explained stability, it didn't explain the perceived difference in speed. I originally thought it probable our testers were mistaken about a speed advantage for 700c wheels. If they actually rode faster with 700c wheels, I felt certain it was an ephemeral result of enhanced rider confidence. Put simply, if riders on 700c test tandems felt more confident at higher speeds (because of stability resulting from the shape of the footprint), this confidence might allow a temporary increase in performance. If there was an enduring speed difference, it seemed likely to me the lighter and smaller 26" wheels would have the advantage.
Some of you might think the difference in diameter between 26" and 700c is too small to matter. Actually, even though we all know 700c rim is slightly smaller than 27" rim, a 700c rim is a full 2-1/2 inches larger than 26" rim.
Two-and-one-half inches?! How can difference between 26" and 27" exceed 2-1/2"?! Answer: a ridiculous tradition dictates that sizes of bicycle wheels --- unlike car and motorcycle wheels --- indicate the nominal outside diameter of the TIRE, and not the actual diameter of the rim. While the out side diameter of a traditional 26-inch "balloon" tire is about an inch smaller than the original 27-inch "racing" tire, the rim is nearly 3 inches smaller. The same tradition exists in Europe where there are no fewer than 4 diameters of rims that accept "650" tires (labeled 650-A through 650-D). To compare the "real" size of a rim or tire you must know the "bead seat diameter." Fortunately, this number is found molded into the sidewall of most tires. The real size of a 27" rim is 630mm (about 24.8"), a 700c rim has a bead seat diameter of 622mm, and the "26-inch" rim found on tandems a nd mountain bikes is only 559mm (a mere 22"). If matching width tires are installed, the outside diameter of a 622 (700c) tire is 63mm (2.5") larger than the outside diameter of a 559 (26") tire.
I've since realized the testers who reported faster speeds on a tandem with 700c wheels were correct --- and here's why:
Remember that the area of a tire's contact patch (or footprint), because it is purely a function of weight and inflation, owes nothing to the diameter or width of a tire. It follows that our test tandems with 11% smaller wheels produced footprints that were exactly 11% shorter and, therefore exactly 11% wider. Shorter explains the stability difference and wider explains the speed difference.
Why is wider slower? To apply the extra width against the pavement, the tread and sidewall of the smaller yet equally-wide tire is forced to undergo a great deal of additional contortion --- and tread and sidewall squirm are the primary causes of rolling resistance.
Is the difference in rolling resistance enough to produce a significant difference in speed? Because rolling resistance is a much smaller factor than wind resistance, until a few months ago I would have guessed no. Today I'm convinced otherwise --- whereas aerodynamic and weight differences are probably only half as significant for tandems (because of doubled power), internal tire friction is probably twice as critical (because of doubled mass). Even if 26" someday proves itself the superior size for road racing singles (it hasn't yet), the optimal wheel size for a racing tandem will always be larger.
While determining an exact difference in rolling resistance would be fairly easy, the effect on speed is difficult to ascertain. My best current estimate is a 26" tandem with equivalent rims, tread width, tread pattern and inflation will be 2-4% slower than a 700c tandem. While this will be a small difference for those who want the flexibility of using their tandem off-road, those interested in ultra-fast pavement rides might expect a cruising speed difference of up to one mile per hour (or a century finishing time difference of 5-10 minutes).
A couple of final thoughts about ultra-fast road rides on a 26" tandem. To achieve the same gearing as a 700c road tandem with a 54 tooth chainring, a 26" racing tandem will need a 60 tooth ring --- which is incompatible with the curvature of modern front derailleurs. And when you want to stop, because braking power is a squared function of effective brake radius, a rim brake on a 26" tandem is 19% less effective than the same brake on a 700c tandem.
Does this mean 26" tandems are stupid? Hardly. If you want to conquer the toughest terrain, 700c wheels simply aren't strong enough. And if we built a 700c frame with sufficient clearance for as-yet nonexistent 2.5" knobbies (700x63), captains shorter than six feet would have a hard time straddling the top tube.
If you want a tandem for tackling rugged trails or rutted fire-roads, a 26" tandem with clearance for wide knobbies is the only choice. If you can limit your off-road excursions to graded dirt, a good 700c tandem is adequately strong and will always be faster on pavement.
