FORK PERFORMANCE: HOW DO WE KNOW WHAT WE KNOW?

FORK PERFORMANCE: HOW DO WE KNOW WHAT WE KNOW?

Post by Bobb » Sun, 27 Apr 1997 04:00:00

My enthusiasm for this subject has been quelled by the sickening feeling
that a fellow cyclist, a brother, died and the "authorities" are again
shirking their m***obligations.  I'm sorry.  It happened here a couple
years ago.  The officer on the scene remarked, "It's that time of year -
skunk, raccoon, bicyclists - all in the wrong place at the wrong time."

So anyway, I have sensed a wee flicker of disagreement here in this
newsgroup, regarding certain characteristics of forks.  The interested
participant [are there any?] can address this issue in a number of ways:

1) GO WITH THE EXPERTS.  

This is one way to go through life, probably quite appropriate in many
instances.  The problem here is that the experts appear to disagree [I
say "appear", because I don't believe there is really much disagreement
among the experts].  On one hand, we have Mr. Ernesto Colnago, who
believes his straight fork provides more efficient power transfer.  If it
does this, it does so by the incursion of less flex in response to
vertical stress - i.e. his rider stomping on a pedal. Mr. Colnago has
built bikes for several decades, puts his name on them, and has watched
them cross the line first all over the world.  We have Tony Oliver,
physicist and metallurgist, who writes, "We have concluded that the
amount of offset for a given head angle will determine the steering
characteristics.  However, there is more.  The shape of the rake will
determine the degree of comfort.  Sadly, modern trends are going in
favour of large radius banana-shaped fork blades.  These may be fine for
the time-trial rider who cycles for only a short period of time but most
inappropriate for anyone who sits on the bike for more than a few hours.
 A blade with a small radius bend low-down near the fork-end will
minimise the transmission of road vibrations through to the bars.  To
overcome modern, pneumatic-drill banana blades, cyclists are expected to
ride with thick foam, shock-absorbing pads either on the bars or on
special cycling mitts - a strange solution to bad frame design... Fashion
unfortunately causes some dire compromises and many younger riders prefer
to have numb hands rather than to be seen riding a fork that looks a
little old-fashioned."  [Oliver, Tony  TOURING BIKES: A PRACTICAL GUIDE.
 Crowood Press, Wiltshire, UK, 1990; p. 18]  The fork design with the
largest radius of curvature is a straight-blade one [radius of curvature
= infinity].  We have Richard Talbot, who is a professional engineer and
author of DESIGNING AND BUILDING YOUR OWN FRAMESET. He writes, "Fork
length may be defined in two ways.  The conventional method is simply to
state it as the straight-line distance between the front wheel axle and
the front brake hole.  A second way is to measure the distance between
these points along the blade center line, taking blade curvature into
account.  Of the two methods, the second is a more accurate indicator of
performance characteristics, because a given fork rake and crown may use
any number of blade curvatures.  For example, the blade may be made with
a large radius bend that starts immediately below the fork crown, or it
may be straight for most of its length and then curve abruptly at the
tip.  Each shape has a different center line length and each will give a
different feel to the bike."

On the other hand we have Mr. Brandt, whose authority and reputation in
these matters is unsurpassed.  Mr. Brandt feels that curved forks and
straight forks behave identically, given the same degree of offset.  
There are apparently data to support this view.

So in this case, the decision to "Go with the experts" requires another
decision:  which experts?

2) THINK IT THROUGH FOR YOURSELF

I ask you to consider the following examples.  Suppose we have two
bicycles which are identical, except that one has a straight blade fork
with a large offset while the other has a straight blade fork with a
small offset.  The one with the large offset will flex more in response
to vertical stress than the small offset fork.  This will occur because
of two attributes of the large offset fork:  longer blades and shallower
angle of approach to the road surface [thus vertical force would be more
perpendicular to the fork blade - imparting more torque on the blade].

Now consider two bicycles that are identical except that one has a
curved-blade fork and the other has a straight blade fork.  The offset is
the same for both.  The curved-blade fork will flex more in response to
vertical stress than will the straight-blade one.  This is true for the
same reasons as in the first example:  longer blades and shallower angle
of approach to the road surface.

3) DO AN EXPERIMENT

Ride 'em.  The guy that did the little experiment with the string taped
to the fork [posted on this newsgroup "Re: Why Straight Fork?"] takes the
prize.  Simply brilliant!

The data which demonstrate that fork performance is determined solely by
the amount of offset have been referred to here and elsewhere quite
often.  I have not seen them all.  But I believe that they largely deal
with two facets of fork performance:  steering response and "shimmy".  
Racers apparently want "oversteer" so they should use small offset forks
and steep head angles.  Tourers and tandem riders apparently want
"neutral" steering so they're not dancing all over the road under load.  
The "experts" all seem to agree that fork shape DOES NOT affect steering
response - given the same amount of offset.  The second factor is
"shimmy" which is high-frequency oscillation of the front wheel in the
left-right directions.  This causes oscillations in the handlebars which
are clockwise-counterclockwise in orientation.  The "experts" seem to
agree here to that tendency to shimmy is determined by the fork offset,
and NOT by fork shape.  So a racer "divin' inta da corner" won't get any
better steering response nor any less shimmy from a straight-blade fork
than he would with a curved one.  

So I think [but don't know for sure, since I haven't seen ALL the data]
that the oft-repeated refrain "fork performance is dictated solely by
offset and not by shape" refers to those two very important aspects, and
is correct.  But I don't think the refrain applies to dampening of road
vibration or to dissipation of other vertically-applied stress.

[Everybody knows this:]  Fork performance depends on many factors:  
materials, cross-sectional geometry [circular, elliptical, true blade,
..], crown design and composition, workmanship, and many others.  These
factors probably overshadow the influence of blade curvature. If not, we
can always get some of those "special cycling mitts".

That's how I see it.

Bobby