Level of Difficulty: Advanced
Useful Tools and Supplies 
- Repair Stand. Holds bike for measuring. Use bench vise for actual
- Frame Alignment Gauge
- Frame and Fork Straightener (as necessary for leverage)
- Bench vise and steel bottom bracket cups for shell protection
- Measuring caliper and angle finder
This article will discuss frame alignment and the use of the Frame Alignment
Gauge FAG-2. Frame
alignment issues and repairs are best addressed by professional mechanics
and frame builders. If your frame does not ride quite "right", it is worth
having the alignment checked by these professionals.
Frame alignment is also related to wheel dropout alignment. The use of
dropout alignment tools FFG-1's
is discussed at Dropout
Alignment Procedures. Generally, dropout alignment should be inspected
after checking the frame. Common frame alignment issues arise from the
rear triangle being misaligned relative to the front triangle. Procedures
to check front triangle alignment are discussed at the end of this article.
Frame alignment is important to the performance of the bicycle. Frame
misalignment may result in the following problems:
- Handling and tracking problems, as a result of the wheels not being
aligned to the bike's mid-line.
- Chainline and shifting problems from the rear cogsets being pooorly
aligned with the front chainrings.
- Difficulty in removing and installing wheels. This can result in slow
However, it is important to keep in mind that a bike or frame need not
be "perfectly" aligned to perform well. All manufacturered components
and frames are made to certain tolerances. Frame alignment should be checked
in order to address specific issues and symptoms. It is not typically
measured and addressed as an issue in its own right. If a bike is not
showing alignment symptoms, it probably does not need "fixing".
Park Tool FAG-2
Frame Alignment Gauge acts much in the same way as a wheel dishing
tool. One side of the bike is check for symmetry against the other side.
If the rear part of the bike, the rear triangle, is off either left or
right, it will show by using the Frame Alignment Gauge. In the image below,
the left side is being referenced and then compared to the right. The
rear triangle on this bike is off toward the left side of the bike's mid-line.
It is possible to cold-set or re-bend only certain frames when correcting
alignment problems. Some frame material is either too rigid or too fragile
to bend. When a frame is bent for alignment, you must exceed the "yield"
point of the material. This is the point were the material will bend and
then stay permanently deformed. In some materials, the point at which
it will yield is very close to the point where it will simply fail and
break. Thin aluminum tubing, as an example, should generally not be bent.
Carbon fiber frames, such as the one seen below, will not take a cold-set.
This material tends to simply flex, and then at some point, break. If
in doubt, check with the frame manufacturer.
There are often other methods to correct problems and issues arising
from a misaligned frame. For example, hub spacers can be added or removed
for wheel fit into frame. The wheels may be purposely "misdished" for
better centering to the bike's mid-plane. Different bottom bracket spindles
will re-posiiton chainrings for better alignment to rear cogs.
Aligning stays and dropouts
frame alignment by measuring the width of the hub over the locknuts. Measure
from locknut face to locknut face, where the nuts would contact the frame
dropouts. Write this number down for reference.
inside the width of frame dropouts and compare this to hub. If the frame
is too wide or too narrow as compared to the hub, it may be awkward removing
and installing the wheel. Generally, the frame and hub should width within
Also note the left
and right dropout thickness. If either side is different, record the difference
and account for this difference when measuring the frame. For example,
the replaceable hanger seen below makes the right dropout effectively
2mm thicker than the left side. When this example frame is centered, the
FAG-2 will show a 2mm gap between the pointer and left side dropout.
Place the long straight portion of the FAG-2 along side the left side
of the head tube and the seat tube. Make certain the gauge rests on the
tubes themselves, not head lugs, welds, bottle cages, etc. Slide the adjustable
pointer as necessary to adjust for variations in chain stay length. Turn
the pointer knob until the pointer contacts the dropout face.
NOTE: Large down tubes on small frames may make it difficult to contact
head tube. In these cases it is possible to extend the width of the head
tube by holding a shim, such as a hex key wrench, held flat against head
tube. This allows an accurate measurement from the head tube. Additionally,
some bikes have no seat tube. In these cases, it may be possible to lower
the seat post and use the seat post as a representation of seat tube.
After referencing the left side of the bike, compare this setting to the
right side. Set the FAG-2 to contact the same three points on right side,
mirroring the tool placement at the head tube, seat tube, and rear dropout.
There are three possible results of this comparison:
Result #1: Rear droputs are centered. There is the same three point
contact as on the right side as the left side.
This rear triangle would be centered to the head tube and seat tube.
If dropouts are wide or narrow compared to hub, bend each stay out or
in only one half of the amount of the error. Double check centering
again with FAG-2 after bending both stays.
Result #2: Dropouts are off to left of mid-plane. There is a
gap between right dropout and pointer on FAG-2.
This rear triangle is off centered to the left of the mid-line. The
amount of centering error is one half of gap size. For example, a
1mm gap means a wheel is off only 0.5mm to the bike's mid-plane. If
the frame is too narrow compared to the hub, bend right dropout outward.
If frame is too wide compared to hub, bend left dropout inward. If
frame is the same width as hub, bend both sides toward the right a
slight amount, rechecking both sides with FAG-2.
Result #3: Dropouts are off to right of mid-plane: There is a
gap between seat tube and FAG-2 gauge when pointer contacts headtube
and right dropout.
this case, if FAG-2 is made to rest on seat tube, pointer will sit inside
face of dropout.
The rear stays in Result #3 are off toward the right side of the mid-line.
In this situation, reset the FAG-2 to reference three-point contact
at head tube, seat tube, and dropout on the bike's right side. Move
FAG-2 to check the left side of frame. A gap will now be seen between
the pointer and left dropout. Seeing the error at the dropout rather
than the frame.
Cold setting to align frame
cold set the frame, it is best to hold bike securely by the bottom bracket.
A simple method to do this is to remove the bottom bracket, and install
steel adjustable-type bottom bracket cups. Both cups should extend past
shell. Mount bottom bracket in the hard jaws of a large vise. The cups
will protect the frame.
bending the dropout and stays, it is best to begin with mild hand pressure.
Check progress by re-checking frame width with a caliper and centering
with the FAG-2. Increase pressure as necessary.
If necessary, use FFS-1
Frame and Fork Straightener. This tool provides a lot of leverage, so
use with caution.
When bending a frame, bend one stay at a time as necessary. Always consider
hub width and frame width when aligning rear triangle. Use the FAG-2 and
a caliper together to help minimize the amount of bending necessary. It
is common for the one stay to move slightly when the other stay is pulled.
This is because the stays are joined with a bridge near the bottom bracket
and on the seat stay. Centering tolerance for FAG-2 pointer-to-dropout
gap is generally considered 1-2mm. Adequate tolerance for frame width-to-hub
difference is also generally considered 1-2mm. However, bikes can certainly
perform adequately with even greater error than these numbers. Check with
the frame manufacturer for specific tolerances.
Main frame alignment
The head tube, top tube, down tube and seat tube comprise the "main frame",
or front triangle. Significnat alignment issues are uncommon from this
part of the bike. If this part of the frame has become bent, repair is
especially difficult, and is not generally recommended.
axis of the head tube should be parallel to the axis of the seat tube.
One method to determine this is a frame table, typically used by frame
builders. This is a "surface plate", on which the bike frame is mounted.
A series of measurements are taken to determine alignment. It is the most
accurate method for checking alignment.
An alternative and practical method uses an angle finder. Simply mount
the bike in a repair stand, or even lean bike against a wall. Measure
and note the head tube angle. Compare this to the seat tube angle. If
the two are within a degree, the bike should be adequately aligned. For
example, if the head tube reads 89-degrees, the seat tube should read
between 88 and 90-degrees.
Repair of a misaligned front end is difficult, and is likely to involve
extensive force and possibly more damage. It is generally not recommended.
Typically, a large mandrel is placed in the head tube. The bottom bracket
is held secure in a vise. The head tube is twisted in a direction to counter
the alignment problem. This repair is rarely sucessful, and the bending
places a torsional or twisting load on the top tube and down tube, especially
at the joints. Inspect this bike often for failure of joints.
Bicycle frames are best inspected during cleaning.
Most types of paint tend to be somewhat brittle and will crack if the
material has moved under it. The left image below shows a crack at the
bottom bracket shell. The first indication was paint showing the failure.
An inspection inside confirmed the crack. The second image is a crack
in the left chain stay.
frames are also susceptible to failure. In the frame left, a metal bottom
bracket sleeve was bonded into the frame. This sleeve has loosened from
the frame and is creaking.
downtube of the frame left shows signs of a front impact. The tube has
a wrinkle under the lower side. The properties of the metal in this damaged
area have changed. This tube will likely crack at this point and eventaully
fail. The damage in this bike is not repairable other than by tube replacement.
stress of flexing fork legs is transmitted to the fork crown. This crown
has cracked at the narrow material above the brake caliper mounting hole.
Fork failure is especially dangerous.