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Most customers loved their new bikes and heaped praise on Holland Cycles. But occasionally a customer would complain that his new bike was more flexible than his old one. The rumor that titanium frames were noodles for stiffness was going around at the time, and as a result we had been bumping up the diameter of the main tubes to get stiffer frames. We thought the frames were pretty stiff. We were wondering how much stiffer we had to make frames to get them stiff enough to satisfy those customers who wanted more.
Because we wanted a scientific answer to the question, I designed a simple non-destructive deflection test to measure the flexibility of frames. That way I could make a numerical comparison of the stiffness of various frame designs.
To measure front triangle deflection, I slipped a solid steel bar one inch in diameter through the head tube to simulate a front fork. I located the bar on the ID of the head tube by means of closely machined steel cones at each end. The cones held onto the bar with set screws. I hung 47.5 pounds (21.6 kg) on the bar 23 cm below the bottom edge of the head tube. This load on the front triangle is a combination of lateral bending and a little torque between the frame's head tube and the bottom bracket shell. I measured front triangle deflection at the point on the bar where I applied the load. Even though the steel bar weighs a bit too, I measured deflection of the frame due only to the weight of the 47.5 pounds, having zeroed the dial indicator after I installed the 1 inch steel bar.
After removing the bar from the head tube, I measured rear triangle deflection. I installed a Dura-Ace axle set and skewer in the rear dropouts, being careful to tighten the skewer the same for each test. I hung the same 47.5 pound load from the dropouts and measured rear deflection at the point of loading.
Limiting this test to measuring just front triangle and rear triangle lateral stiffness means that the few beam style or other non-diamond frames I tested may flex in ways that I did not measure. An example of such flex might be the movement of the top of the rear wheel from side to side on frames without seat stays, or lateral seat movement in the case of beam frames, etc. I ignored any vertical loads on the frame, as I believe a traditional diamond frame resists such loads easily due to its truss design in the vertical plane.
It should also be noted that this test measures deflection directly, not stiffness, but the two are related. Stiffness can be thought of as the mathematical inverse of deflection. For example, to calculate the stiffness-to-weight ratio of a given frame, you would first take the inverse of the frame's total deflection ("one divided by total deflection") to find a number which represents the frame's stiffness, then divide that result by the frame weight. This gives a stiffness-to-weight ratio.
Over a period of about a year, I deflection tested over 100 frames as the opportunity arose. The most flexible one had a total deflection of 0.86 inches (a steel aero-tubed Pinarello). The stiffest one deflected only 0.26 inches (a Cannondale track frame). I believe this represents about the total range. I don't think we missed any that are much stiffer or much more flexible, though I would have liked to have measured a Vitus 979 or a TVT carbon, both of which have a reputation for flex. Nevertheless, the range of deflection that I found is probably representative of virtually the whole spectrum. For this reason, I no longer perform this test.
Is it possible for a frame to be too flexible? I believe it is. Under a hard effort by the rider, a flexible frame actually allows the wheels to come out of plane with respect to each other. You can see it in any sprint if you can get a view from the front or behind. All frames do this to some extent, but large or powerful riders may prefer a stiffer frame to minimize this effect.
And a too-flexible frame can be hard to handle in some riding situations. For instance, on high speed descents and in corners, an overly flexible frame can weave around enough to become a handful to keep under control. In other words, flexible frames can be scary! I once read a quote attributed to Andy Hampsten to the effect that the only thing scarier than descending on a TVT frame was climbing without one! A very flexible frame can be disconcerting enough to cause a rider to back off in some situations where a stiffer frame would allow him to keep up speed.
With frame deflection data and the ability to measure deflection, frame designers can practically match your favorite frame's stiffness, nearly guaranteeing your new frame will neither lack the stiffness you want, nor be over designed.
"Size" is the frame size in centimeters measured from the center of the bottom bracket to the top of the top tube. Note that since there are several popular methods for measuring frames, this number may not agree with the frame size as declared by the manufacturer.
"Front" is the deflection of the frame's front triangle in inches. A larger number is more flexible, and a smaller number is stiffer.
"Rear" is the deflection of the frame's rear triangle in inches. A larger number is more flexible, and a smaller number is stiffer.
"Total" is the sum of front deflection and rear deflection in inches. A larger number is more flexible, and a smaller number is stiffer.
|Eddy Merckx||753R||lugged steel||51.0||0.44||0.17||0.61|
|J. Durso||TIG||AerMet100 (1)||52.0||0.47||0.19||0.66|
|Pinarello||Gavia TSX||lugged steel||52.5||0.37||0.15||0.52|
|Holland||#50||db Ti 31.8/31.8/34.9 (2)||53.0||0.40||0.18||0.58|
|Bob Jackson||531 C||lugged steel||54.0||0.38||0.14||0.53|
|Richard Sachs||lugged steel||54.0||0.38||0.16||0.55|
|Casati||Gold Line||lugged steel||54.0||0.44||0.15||0.59|
|Trek||5200||carbon tubes & lugs ì56 cmî||54.0||0.35||0.24||0.59|
|Holland||#69||Ti 31.8/31.8/34.9 (2)||54.5||0.37||0.18||0.55|
|Specialized||Allez Comp||lugged steel OS||55.5||0.44||0.18||0.62|
|Kestrel||KM40 (650c whls)||carbon||55.5||0.46||0.18||0.64|
|Masi||3-V||lugged True Temper (3)||56.0||0.38||0.12||0.50|
|Holland||SL/SP||lugged steel (4)||56.0||0.38||0.13||0.51|
|Serotta||Colorado Legend||Ti (6)||56.0||0.37||0.21||0.58|
|Kestrel||200 SC||carbon (700c whls)||56.0||0.41||0.18||0.58|
|Holland||#3792||Genius fillet brazed||56.0||0.44||0.18||0.61|
|Holland||#80||db Ti 31.8/31.8/34.9 (2)||56.0||0.45||0.20||0.65|
|Holland||#60||db Ti 31.8/31.8/34.9 (2)(7)||56.0||0.45||0.22||0.67|
|Trek||2300||lugged Al w/3 main carbon||56.5||0.39||0.18||0.57|
|Pinarello||X142||Columbus "Air" (8)||56.5||0.70||0.16||0.86|
|Eddy Merckx||TSX||lugged steel||57.0||0.39||0.14||0.53|
|Hedgehog||EL OS||lugged steel||57.0||0.40||0.17||0.56|
|Litespeed||custom||Ti 31.8/31.8/31.8 (2)||57.0||0.44||0.23||0.67|
|Holland||#1994||Logic >31.8/31.8/34.9 (2)||58.0||0.29||0.12||0.41|
|Holland||"Herrick" track||TIG steel (9)||58.0||0.31||0.10||0.41|
|Holland||#64||db Ti 34.9/34.9/38.1 (2)||58.0||0.32||0.19||0.51|
|Kestrel||200 SC||carbon (700c whls)||58.0||0.41||0.17||0.58|
|Litespeed||custom||Ti 31.8/31.8/31.8 (2)||58.0||0.42||0.22||0.64|
|Tesch||S-22||oversized fillet brazed (6)||58.0||0.31||0.12||0.43|
|Eddy Merckx||653||lugged steel||58.0||0.48||0.20||0.68|
|Schwinn||mid '80s||Tange #2 lugged||58.5||0.41||0.17||0.58|
|Eddy Merckx||lugged steel (10)||59.0||0.40||0.14||0.54|
|Holland||#63||db Ti 34.9/34.9/38.1 (2)||59.0||0.38||0.19||0.57|
|Hedgehog||EL OS||lugged steel||59.0||0.44||0.17||0.60|
|Serotta||Colorado LT||lugged steel (6)||59.5||0.31||0.14||0.45|
|Holland||#4092||Genius fillet brazed||60.0||0.46||0.17||0.63|
|Holland||#3992||Genius fillet brazed||60.0||0.46||0.17||0.63|
|Rigi||(twin seat tubes)||lugged||60.0||0.49||0.17||0.66|
|Holland||#66||db Ti 34.9/34.9/38.1 (2)||60.0||0.48||0.19||0.67|
|Eddy Merckx||(10)||lugged steel||60.5||0.39||0.14||0.53|
|Holland||#62||Ti 34.9/34.9/34.9 (2)||60.5||0.35||0.22||0.56|
|Holland||#17||db Ti 31.8/31.8/34.9 (2)||60.5||0.43||0.20||0.62|
|Holland||#15||Ti 34.9/34.9/38.1 (2)||62.0||0.37||0.22||0.59|
|Centurion||Ironman||Tange #1 lugged||62.0||0.46||0.18||0.64|
|Richard Sachs||lugged steel||62.5||0.49||0.15||0.63|
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