damon_rinard wrote:
It's a little old now, but you might be thinking of Bob Bundy's FAQ article measurements.
From
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Subject: 8e.2 Frame Stiffness
From: Bob Bundy <
bobb@ico.isc.com>
As many of you rec.bicycles readers are aware, there have been occasional, sometimes acrimonious, discussions about how some frames are so much stiffer than others. Cannondale frames seem to take most of the abuse. The litany of complaints about some bike frames is long and includes excessive wheel hop, numb hands, unpleasant ride, broken spokes, pitted headsets, etc. I was complaining to a friend of mine about how there was so much ranting and raving but so little empirical data - to which he replied, "Why don't you stop complaining and do the measurements yourself?". To that, I emitted the fateful words, "Why not, after all, how hard can it be?". Following some consultation with Jobst and a few other friends, I ran the following tests:
The following data were collected by measuring the vertical deflection at the seat (ST), bottom bracket (BB) and head tube (HT) as a result of applying 80lb of vertical force. The relative contributions of the tires, wheels, fork, and frame (the diamond portion) were measured using a set of jigs and a dial indicator which was read to the nearest .001 inch. For some of the measures, I applied pressures from 20 to 270 lbs to check for any significant nonlinearity. None was observed. The same set of tires (Continentals) and wheels were used for all measurements.
Note that these were measures of in-plane stiffness, which should be related to ride comfort, and not torsional stiffness which is something else entirely.
Bikes:
TA - 1987 Trek Aluminum 1200, this model has a Vitus front fork, most reviews describe this as being an exceptionally smooth riding bike
SS - 1988 Specialized Sirus, steel CrMo frame, described by one review as being stiff, hard riding and responsive
DR - 1987 DeRosa, SP/SL tubing, classic Italian road bike
RM - 1988 Cannondale aluminum frame with a CrMo fork, some reviewers could not tolerate the rough ride of this bike
TA SS DR RM
---------- ---------- ---------- ----------
ST BB HT ST BB HT ST BB HS ST BB HT
diamond 1 1 0 2 2 0 2 2 0 1 1 0
fork 3 11 45 3 9 36 4 13 55 3 10 40
wheels 2 2 2 2 2 2 2 2 2 2 2 2
tires 68 52 66 68 52 66 68 52 66 68 52 66
total 74 66 113 75 65 104 76 69 123 74 65 108
What is going on here? I read the bike mags and this net enough to know that people have strong impressions about the things that affect ride comfort. For example, it is common to hear people talk about rim types (aero vs. non-aero), spoke size, butting and spoke patterns and how they affect ride. Yet the data presented here indicate, just a Jobst predicted, that any variation in these factors will essentially be undetectable to the rider.
Similarly, one hears the same kind of talk about frames, namely, that frame material X gives a better ride than frame material Y, that butted tubing gives a better ride that non-butted, etc. (I may have even made such statements myself at some time.) Yet, again, the data suggest that these differences are small and, perhaps, even undetectable. I offer two explanations for this variation between the data and subjective reports of ride quality.
Engineering:
These data are all static measurements and perhaps only applicable at the end of the frequency spectrum. Factors such as frequency response, and damping might be significant factors in rider comfort.
Psychology:
There is no doubt that these bikes all look very different, especially the Cannondale. They even sound different while riding over rough roads. These factors, along with the impressions of friends and reviews in bike magazines may lead us to perceive differences where they, in fact, do not exist.
Being a psychologist, I am naturally inclined toward the psychological explanation. I just can't see how the diamond part of the frame contributes in any significant way to the comfort of a bike. The damping of the frame should be irrelevant since it doesn't flex enough that there is any motion to actually dampen. That the frame would become flexible at some important range of the frequency spectrum doesn't seem likely either.
On the other hand, there is plenty of evidence that people are often very poor judges of their physical environment. They often see relationships where they don't exist and mis-attribute other relationships. For example, peoples' judgement of ride quality in automobiles is more related to the sounds inside the automobile than the ride itself. The only way to get a good correlation between accelerometers attached to the car seat and the rider's estimates of ride quality is to blindfold and deafen the rider (not permanently!). This is only one of many examples of mis-attribution. The role of expectation is even more powerful. (Some even claim that whole areas of medicine are built around it - but that is another story entirely.) People hear that Cannondales are stiff and, let's face it, they certainly *look* stiff. Add to that the fact that Cannondales sound different while going over rough roads and perhaps the rider has an auditory confirmation of what is already believed to be true.
Unless anyone can come up with a better explanation, I will remain convinced that differences in ride quality among frames are more a matter of perception than of actual physical differences.
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