Frame Alignment Level of Difficulty: Advanced Useful Tools and Supplies [1] Repair Stand. Holds bike for measuring. Use bench vise for actual bending. 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 wheel changes. 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".

The 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.

IMPORTANT NOTE 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 Begin 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.

Measure 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 1-2mm.

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.

In 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 To 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.

When 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. The 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.

Composite 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.

The 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.

The 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.

Suggested Park Tools PCS-9, PCS-4 or PCS-1 Repair Stand FAG-2 Frame Alignment Gauge FFS-1 Frame and Fork Straightener (as necessary for leverage)