FAQ: Climbing Physics - Climbing Forces - Anchor Forces
It will be assumed the climber can place solid protection be it
chocks, nuts, hexes, tri-cams, or camming devices. It will also
be assumed the climber knows to evaluate any bolts or fixed anchors
used. What will be explored in this section are the forces generated
within an anchor relative to the locations of the anchor points
and the angles of force between them.
Forces In A 2 Anchor Triangle
of the most common situations climbers encounter are fixed belay
anchors, typically bolts. It is customary to clip into each of the
bolts with a carabiner and sling, then join the slings with a carabiner
at the point of attachment. The climber / belayer hang from a sling
or slings attached to the bolts, attaching to the carabiner at the
midpoint. As the diagram illustrates, this forms a triangle. If
the bolts are at the same height, and the slings are the same length,
the load is shared equally between them.
angle formed between the slings affects the force placed on the
individual anchors. As the chart indicates, the greater the angle
between the slings, the greater the load placed on the bolts. At
angles great than 120 degrees, the forces on the bolts can be greater
than the forces placed on the load carabiner. The diagram indicates
we should strive to keep the angle as narrow as possible to reduce
the load on the anchors.
The diagram shows the loads on each bolt with a 4.5 kN (1000 lb.)
force applied. At low angles, 20 degrees or less, the bolts split
the load. As the angle between the slings grows, the forces grow
rapidly. If the bolts are spaced 2 feet apart, and we build our
anchor using 2 foot slings, the angle is about 60 degrees. This
raises the force on each bolt to 666 lbs.
It should be clear the distance between the anchors is an important
factor. Consider the case of building an anchor using either two
2 foot slings, or a single 4 foot sling doubled between the anchors.
The angle created by the slings varies from 30 degrees if the anchors
are placed 1 foot apart, to about 45 degrees if the anchors are
18 inches apart, and 60 degrees if the anchors are two feet apart.
If the anchors are 3 feet apart, the angle approaches 100 degrees.
A good way to determine if your slings are long enough to keep
forces small is to compare them to the distance between the anchors.
As a rule, the slings should be longer than the distance between
the anchors, preferably twice as long. For example if the anchors
are 1 foot apart, 2 foot slings are adequate. If the anchors are
2 feet apart, four foot slings would be more appropriate.
While 2 good bolts may often be used as a anchor, the situation
changes when the anchor is built with gear placed by the climber.
Whereas bolts are rated to hold loads starting at 4500 lbs. (20
kN), typical climbing gear is rated for loads roughly half that
of bolts. In this case, at least three solid pieces of protection
should be placed, more if there is any question of the quality of
any single piece of gear, the rock is suspect, or if small gear
The same concerns of angles need to be considered with multiple
anchors. With gear anchors instead of bolts, it is even more important
to pay attention to the loads on each piece related to their location
and sling length. Place the anchors close together when possible,
and extend with slings or a cordalette as needed to insure the load
is minimized and shared equally.
Some good recommendations and photographs of equalizing multiple
anchors can be found at the Guiding
Newsletter - November 2000 - Anchor Equalization Tech Tip page.
Analysis (a 13 page DETAILED discussion, complete with physics):
The Physics of
Climbing (for the truly
mathematically minded, A very technical explanation of how
physics applies to rock climbing)
& Ropes (the discussion is about caving, but the principles are
Newsletter - November 2000 - Anchor Equalization Tech Tip
Analysis of Active Anchors
and Climbing: Impact force calculator
Forces in Leader Falls (.PDF file)
on the falling climber depending on different belaying techniques
Climbing Association - Education about bolts
Climbing Training - Advanced Belay Techniques
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