On 16 Nov 2004 14:35:50 -0800, [email hidden] (John McGraw)
Quoted message said:Jeff Starr <[email hidden]> wrote in message news:<[email hidden]>...
Quoted message said:Kurd said:He had the Easton carbon road bar. It looked like it was cut right at the
edge of the stem. Our Easton rep said that while they recommend an Easton
stem, a 2 bolt stem is superior to a 4 bolt stem when dealing with their
carbon bars.
Hi, did he explain why?
It would be helpful, if you followed convention, by not top posting.
Life is Good!
Jeff
I'm very skeptical about Easton's 2 bolt comment. Proper TQ & careful
inspection & breaking the edges if necessary seem to me to be the real
issue. It amazes me that none of the major players monitor this site,
such as Easton.
Best, John
I'd be surprised if they didn't.
Here is a link to a Tech Bulletin on Easton's site:
http://www.eastonbike.com/downloadable_files/r&d_files/R&D-06-2Bolts.pdf
==========================
T E C H N O L O G Y R E P O R T
R&D Q&A NUMBER SIX
TIGHTENING COMPONENTS
TWO BOLTS VS. FOUR BOLTS
Should form follow function…or fashion? How many bolts does it take to
hold your handlebar securely without squeezing the life out of it?
Two? Four? Six? Eight?
The other day we were in our favorite bike shop admiring a beautiful
new Easton EA70 road stem. The face plate of the stem attached with
two bolts. “It's a cool stem,” exclaimed the shop owner, “it would be
even cooler if it had four bolts.” “Really, why is that?” we asked.
“Because four-bolt stems are the current fashion and style,” the store
owner offered.
Suddenly, our excitement about the stem made us feel like we had we
left the office wearing one blue sock and one black
sock. But we were walking better than ever, so in our defense we
asked, “If four bolts is cool wouldn't six or eight be even
cooler?”
What does the number of bolts have to do with anything anyway? Well,
here is where we may get ourselves into trouble with the
fashion police. (We’re sure a warrant is being prepared even as we
write this.)
Function vs. fashion
The purpose of stem bolts is to secure the handlebars between the
stem's front cap and rear perch. The bolts should allow the front cap
to be tightened securely enough to keep the bar from rotating or
slipping during use. Does a four-bolt stem offer any advantages over a
two-bolt stem…or vice versa?
Let's start by looking at the bolts themselves. Bolts are available in
a myriad of materials, diameters and number of threads per inch.
Typical stem bolts are made from aluminum, titanium or chromoly steel.
Each of these materials has different properties.
Aluminum has the lowest density and offers the lowest weight. But
nothing comes for free. Aluminum bolts just don't have the necessary
strength. Since durability and strength are important attributes,
aluminum may not be the best choice. How about titanium? While
stronger than aluminum, Ti bolts still don't offer enough strength and
toughness for this application. They are somewhat brittle and subject
to breaking under load.
Steel bolts are heavier than aluminum and titanium, but more than make
up for it in strength, toughness and dependability.
Weight is a factor in any performance product, and two bolts are
definitely lighter than four (assuming the bolts are the same size).
But before we decide that a two-bolt stem is superior to a four-bolt
stem, let's consider how the bolts and the face plate interact and
function.
Hold on a minute
Today's handlebars are getting lighter and lighter. Aluminum road bars
are around 200 grams and composite bars are even lighter. While these
bars are very strong, they got this light by using less material. As a
result, they have thinner walls.
A two-bolt system has definite advantages when it comes to clamping
bars with thinner walls. It does a better job of spreading the
clamping force evenly over the entire area of the face plate. The
center of the bar suffers less trauma when tightened.
Four-bolt stems pose a couple of challenges. First, most four-bolt
stems use smaller diameter bolts. This is done partly to compensate
for the increased weight of the extra bolts. To further offset this
weight issue, stem designers will often use bolts made from titanium.
Either way, they are not as strong as the chromoly bolts used with a
two-bolt stem.
A second drawback is the way that four bolts apply clamping force to
the bar. It's much more of a challenge to apply even clamping pressure
across the center of the bar with a four bolt system. If installed
improperly, four bolt face plates can pinch the bars at the corners
of the face plate, damaging the center area of the bar.
Even with proper assembly, a four-bolt stem places maximum clamping
stress at the edge of the stem/bar interface. This is the location of
the bar's maximum bending stress during use. These combined stresses
can lead to premature fatigue failure or crack initiation.
A two-bolt design is self centering; it allows clamping stresses to
dissipate before reaching the edge of the stem where the bar exits.
Time to talk torque
It seems like everybody is bent (pun intended) on over-tightening
components. Why is this? Lots of reasons: people are afraid that their
components will come loose; people compulsively tighten their bolts
(sometimes before every ride); and finally, people don't use torque
wrenches.
Let's assume that we do use a torque wrench. Our owner's manual or
assembly instructions generally recommend a torque specification. But
is this a suggestion or a requirement? If 60 in/lbs of torque is good,
why not use 90 in/lbs for that added security?
At some point the bar is tight enough to function and not slip. Is the
torque value supplied by the stem manufacturer not enough, just
enough, or too much in relation to what it takes to hold the bar
securely without slipping? How do manufacturers arrive at their
recommended torque values?
The torque values supplied by manufacturers are frequently based on
the forces that *the bolt* can withstand. The bar is probably tight
enough at some point prior to reaching the maximum torque specified.
Does this mean that you should not tighten to the recommended torque
value? The fact is you probably don't need that much force to secure
the bar. Just make sure that you NEVER tighten beyond the recommended
torque value.
This is an important issue because there are so many people who suffer
from Chronic Compulsive Tightening Syndrome (CCTS). (You who suffer
from this affliction know who you are.) The problem occurs when, with
the best of intentions, they install a bar and torque the stem bolts
to the recommended torque value, And then, prior to the week's epic
ride feel the need to…you know, just snug them up a bit. It isn't long
before the maximum torque specification has been left in the dust. The
center of the bar is crushed and the bolts are just one big bump away
from snapping.
Four-bolt stems are much easier to over-tighten than their two-bolt
brethren. That's because when tightened to the same torque value, four
bolts exert twice as much pressure on the bar as do two bolts. It is
very easy to reach dangerous levels of torque by simply
hand-tightening with an allen wrench.
Stem Bolt Torque Guidelines
TWO-BOLT STEM* TORQUE (IN/LBS)- DESCRIPTION
45 - Adequate to safely secure most handlebars.
50 – 60 Torque that can readily be generated by hand with a
screwdriver.
70 - The maximum safe torque for lightweight handlebars.
80 - Torque level that can permanently damage lightweight aluminum or
composite bars.
50 – 110 Torque value that could be indicated as the maximum value by
a stem manufacturer (based on the torque specs of the bolts used).
* Four-bolt stems exert twice the pressure on a handlebar as two-bolt
stems at any given torque value.
Recently, at a major race in Europe, one of the larger stem
manufacturers sent their engineers to the race to offer technical
advice to the various team's mechanics. The engineers provided torque
wrenches and set up each rider's bike with a stem and bar torqued to
the correct specification. A week later, at the end of the race, the
engineers returned and rechecked the torque on the bikes. They
discovered that, on average, every stem bolt was nearly double the
maximum torque allowed. When questioned, the mechanics acknowledged
“snugging up” the bolts after each days stage.
The moral of this story is tight enough…is tight enough. Once you have
reached the maximum torque value there is no need to constantly
snug-up the bolts. If you feel the need to recheck the bolts, break
out the torque wrench to ensure against over-tightening. Your bolts
and bar depend on it.
More twists and turns
The issue of over-tightening also applies to brake levers and bar
ends. While none of us want to have our components slip during a ride,
we often compensate by squeezing the life out of our bars and
seatposts.
Clamp designs vary, but the best designs allow the brake lever, bar
end or seatpost clamp to remain round as pressure is increased. In any
event, there is no reason to tighten the brake lever to the point
that, in a crash, the lever breaks rather than rotates around the bar.
By not over tightening you get to reposition the lever rather than
replace it with a new one.
Lightweight aluminum and carbon seatposts also suffer from over
tightening. The most common symptom is what we call a snake bite — a
vertical crack that is generated by over tightening the seat collar,
forcing the frame to dig into the outer layers of the composite. This
eventually leads to premature failure of the seatpost.
May the force be with you
With proper installation and care, modern, lightweight components are
designed to last for many seasons. Mankind is their only natural
enemy. When installing components, don't just rely on feel — rely on
an accurate torque wrench (or a mechanic that possesses and uses one).
Leave the breaker bar in your [censored] pack. Take out your stress on the
trail, not on your bolts.
