Cycle Sense
Why front forks
bend
You wouldn’t want it any
other way
By John Schubert
M
any, if not most, of us have dented a rim at
some point. You’re riding along, blissfully
inattentive, you don’t see the pothole in time,
and you dent the rim. Damage: one rim, plus
the labor to replace it.
That experience would seem to be at odds with another
fairly common experience: you run into a fixed object and
bend your fork, but the wheel isn’t harmed. How can that
Area where stress
is concentrated
REARWARD FORCE
FORWARD FORCE
(the rider’s
moving mass,
pushing the
bicycle)
FIGURE ONE:
Here’s what
happens
when you
ride into a
brick wall.
The wheel
transfers the
force straight
rearward on
the dropout.
Tremendous
stress is concentrated at
the top of the
fork, and
bending is
likely.
be? How can a whisper-thin rim and spokes be undamaged
when a stout fork, made from one-inch-diameter tubing, is
destroyed?
Related to this conundrum is another question that I’m
often asked: “Doesn’t the bend show that the bike failed,
causing the accident?” The answer is virtually always “no.”
The fork failed (or, in other cases, the wheel failed) after the
accident was already in progress. Forks are strong structures. But by design, they’re not terribly stronger than they
need to be, nor should they be. And any structure is stronger
in some ways than others.
I’ll start this discussion by reviewing the definition of
strength, because people often confuse strength with stiff-
ness. Strength is the measure of a material’s resistance to
permanent deformation, whereas stiffness is the measure of
a material’s resistance to temporary deformation.
In other words, if you push on an object and it springs
back to its original dimensions, you’ve experienced its stiffness. If you push and it bends permanently or breaks,
you’ve exceeded its strength. The difference is key, because
a fork needs to be designed with both of them in mind.
Most people don’t realize how much a bicycle fork flexes in routine use. When you sit on the bike, your body
weight causes the bike to squat approximately 1/8 inch —
because the front forks splay forward under your weight.
Similarly, as you ride the bike the forks flex in response to
bumps in the road. This flex is less than the shock absorption offered by your pneumatic tires, but it is still significant, and it makes the bike more comfortable to ride.
The more stiff the fork is, the less it flexes in response to
bumps. In general, a fork gets stiffer when you make it
stronger. So lots of strength would make a fork too stiff.
(This doesn’t apply to suspension forks.)
Forks are stronger against some forces than others. Since
a fork is made from tubing, it’s very strong against compressive forces. That’s why a rim-denting pothole so seldom
damages the fork. The sudden upward kee-runch of the pothole’s far rim is aligned almost perfectly with the column of
the fork, and so the fork takes this load primarily in compression.
“Most of your normal loads are vertical bump forces,”
said Jim Papadopoulos, a well-known bicycling engineer
who is currently co-authoring the next edition of Bicycling
Science. “The fork is a lot stronger in that near-vertical orientation. With those forces, you flatten the rim before you
destroy the fork.”
On the other hand, when you run the bike into a brick
wall, the fork bends rather easily. Confounding matters, the
rim is much less likely to get damaged in such an accident.
Why’s that? Several factors team up against the fork and in
favor of the rim. The fork is no longer loaded in compression. It’s loaded almost purely in bending.
“It’s like a big long diving board, and you put a load on
the end of it and bend it,” Papadopoulos said. (Papadopoulos estimates that the fork is about 2 1/2 times stronger in
resisting vertical loads than horizontal loads.)
Meanwhile, the wheel actually sees less of a load than it
did when you ran over the brick. So it isn’t damaged.
Thanks to the testing department at Cannondale Corp, we
have some numbers to put on this.
A well-built 700C bicycle wheel can support about 2000
pounds in its own plane. (But at that weight, the slightest
whiff of a side load will, naturally, crumple it quickly.)
Front forks withstand a bending force, applied at the
dropouts, anywhere from 400 pounds (for ordinary steel
forks) to 800 pounds (for the most robust of the mountain
bike suspension forks) or 1000 pounds (for carbon fiber
forks). These numbers are considerably higher than the 200pound requirement of the U.S. Consumer Product Safety
Commission (CPSC).
Frames vary a lot in strength. Cannondale mountain bike
frames can withstand a bending force of around 1200
pounds. Thus, the numbers explain a common bent-bike situation: You ride your bike into a parked car. The wheel is
Adventure Cyclist • July 2000
37
Area where stress
is concentrated
FORWARD FORCE
(the rider’s
moving mass,
pushing the
bicycle)
Wheel acts as a
long lever arm,
applying even more
force to the fork
REARWARD FORCE
intact; the fork is bent. None of the bike’s
parts were defective.
Does a stronger fork make the bike
safer? No. The fork doesn’t bend until
extraordinary forces are acting on it, so it
can’t bend when you’re controlling the bike
normally. Also, when a bike part bends during an accident, it does not perform the
energy-absorbing function of crumple zones
in a car. A bike can’t do that because the
bike doesn’t contain its rider.
Next question: why is almost every
pitchover accident accompanied by a bent
fork? In those accidents, you didn’t ram a
brick wall or a parked car. You only slowed
the bike down with friction from the tire
contact patch. But that friction can cause
dramatic forces in an accident situation. The
reason why is easy to visualize if you imagine yourself pushing on the dropouts. If the
fork blade were twice as long, you could
generate the same force by pushing half as
hard. That’s called leverage. It so happens
that you do have twice as much leverage to
bend the fork blade when your force is
applied at the tire contact patch (compared
with force applied at the dropouts, as it is in
a brick-wall accident). So if your fork takes
400 pounds to bend in Cannondale’s testing
machine, it will take 200 pounds of force
applied at the contact patch.
How do you get the contact patch to
apply 200 or more pounds of rearward
force? Slamming on the front brake works
real well. So does getting a stick caught in
the spokes. That’s why pitchover accidents
often result in bent forks.
My Cannondale contact says the
pitchover accidents he sees don’t result in
damage to either frame or fork.
“That’s because you work for Cannondale,” I told him. Remember, oversize aluminum is much stronger than regular steel.
(A common cause of foreign object
entanglement is a poorly maintained front
fender. If you have fenders, inspect the
FIGURE TWO: This is the
factor that surprises people:
A rearward force applied at
the tire contact patch can be
as high as several hundred
pounds. When multiplied
through the lever arm of the
entire wheel and fork blade,
this can easily bend the fork.
What does it take to create
such a rearward force? Ordinary tire traction will do it,
when combined with a
strong reaction to force. A
panic grab of the brakes is
sufficient retarding force (if
your brakes are in good condition). So is a foreign
object in the spokes.
mount at the brake bridge often and use
blue Loctite on those threads. And think
long and hard about buying Esge fenders,
whose Secu-Clip mount is reported to
reduce the likelihood of the fender fouling
the front wheel.)
There is one good thing about bending
your fork: You may avoid bending your
frame. If the frame is sufficiently stronger
than the fork, one of these little accidents
will cost you the price of a new fork, but
not the price of a new frame. In fact, most
bikes are made so that the frame is stronger
than the fork. The frame is well triangulated, and the fork is cantilevered, so this
strength difference is a natural outcome. But
if you have a beefy front suspension fork, a
tandem fork fortified for a front disc brake,
or other newfangled contraption, all bets are
off.
One of the seldom-appreciated aspects
of the bicycle’s elegant design is the way a
front fork bends in an accident.
“A taper gauge fork is designed so it will
have equal resistance to bending over a
large portion of its length,” said cycling
writer/engineer John S. Allen. “In a crash, a
Reynolds tubing fork would bend evenly
from top to bottom.”
Stop and think about it: if the fork is
bent as John describes, no part is stronger
than any other part. But if most of the fork
remains unbent, and the bending is concentrated in a small crumple zone, that tells you
that the rest of the fork was stronger than it
needed to be, and, therefore, heavier than it
needed to be. It would be stiffer too, and
transmit a tad more shock to the rider.
In addition, Allen added, “If you make a
fork just strong enough, it will absorb vibration more than a vastly stronger fork. The
fork will flex in concert with the vibrational
frequency from the road surface to keep the
energy of that vibration from reaching the
handlebars, as long as that frequency is
above the resonant frequency of the fork
Adventure Cyclist • July 2000
38
and wheel system.”
But if you make the fork stiffer and
stronger, he said, you raise that resonant frequency, which passes more of that vibration
on to the rider.
“The fatter and softer the tire, the less
you will notice this difference,” Allen said.
Finally, Allen added, “Cheap bike forks
are often too strong.”
Now there’s a criticism those manufacturers probably weren’t expecting! (It’s
cheaper to make it strong. You use a heavier
gauge of a cheaper steel.)
So, the next time someone complains
about his fork bending in a little accident,
you can tell him, “Tough break, buddy. But
the universe would be a worse place if your
fork hadn’t bent. Everyone’s bike would
ride a little rougher and vibrate people’s
hands more. And you might be looking at a
totaled frame instead of a replacement fork
now.”
Like many of you, I was saddened to
learn that Executive Director Gary MacFadden plans to leave Adventure Cycling. I’d
like to give y’all my personal take on this.
Bottom line: I’m gonna miss Gary.
I’ve always been befuddled by the
breadth of Gary’s talents. He’s the only person I know who can (a) write the code for a
database program that helps run Adventure
Cycling’s business side, (b) walk into a studio with a 4x5 view camera; from there
walk into the darkroom; and from there produce really classy photos; (c) execute a
passable chandelle in his Cessna 175; (d)
spend hours talking about bicycle advocacy
and make sense the entire time.
He’s also a conscientious and realistic
manager, steadily increasing the reach of
Adventure Cycling’s programs. For example, the Great Divide Trail stands in my
mind as a supreme accomplishment against
tough odds.
I owe Gary a personal debt. Back in
1988, when the Vulture Capitalists fired me
from Bicycle Guide, the magazine I had
founded, I was in a deep funk. I thought my
career in bike magazines was over. Then
Gary called, and in that phone call, Cycle
Sense was born. Over the years, I’ve been
privileged to be a part of what Gary and
Dan D’Ambrosio have made, in my opinion, the best cycling magazine the nation
has to offer. And it’s become that way with
little fanfare and remarkably efficient use of
resources.
This organization cares about cycling,
it’s mature enough to put judgment ahead of
ego, and it lets its accomplishments speak
for themselves and serve the members.
That’s Gary’s style. ●
Technical Editor John Schubert
answers one E-mail out of every
ten at [email protected].