Bob Bundy's Vertical Deflection Measurements

"BikeRacer" <[email hidden]> wrote in message
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Quoted message said:

I looked at the FAQ to find Bob Bundy's deflection measurements, and have a quick question - there
is a chart with numbers in it but no explanation about what the numbers represent. 1 what? 2 what?
Can the FAQ be updated with a better explanation?

In the preamble, he mentions that these are vertical deflections measured at three points under a
static vertical load of 80lb. Although it's not explicitly stated, I assume, since he writes that
the measurements were made to the nearest thousandth of an inch, that the unit used is one
thousandth of an inch.

Here's the entire FAQ entry from: draco.acs.uci.edu8e.2.htmlOpen ↗

<quote>

Subject: 8e.2 Frame Stiffness From: Bob Bundy <[email hidden]>

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 tortional 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.

<unquote>

James Thomson