Bikes & Tech

Technical FAQ: Ensuring that your front and rear wheels are parallel

In this week's column, Lennard discusses how to make sure that your front and rear wheels are aligned in the same plane

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Dear Lennard,
Your road bike book is great; thank you for that.

I am curious if there is a standardized, objective way of ensuring that the front wheel is parallel with the back wheel. Having done that, then is there a similarly objective way of determining that the handlebars are perpendicular to the front wheel.
— James

Dear James,
There is certainly a standardized, objective way of doing both, and it involves tools that a consumer is highly unlikely to have.

A framebuilder could assure that both wheels are in the same plane by checking the frame and fork alignment. The frame would be checked on a precision alignment table (aka surface plate) that has a heavy, machined steel vertical standard that clamps through the bottom bracket. The heights of the centers of all of the tubes are accurately measured relative to the machined-flat table surface; if the measurement is the same at both ends of the head tube and both ends of the seat tube, then both of those tubes are coplanar, and the bottom bracket is normal to that plane (i.e., the head tube and seat tube are in the same plane, and the bottom bracket is perpendicular to them).

The heights of the inner faces of the rear dropouts relative to the table surface are measured to ensure that they are equidistant from the plane of the front triangle. Finally, a squared-up rod the diameter of a rear axle is inserted into the dropouts. The rod is normal (perpendicular) to a large, machined base attached to it; if the base sits flat on the table while the rod sits fully into the dropout slots (or holes in the case of a through axle), then the rear hub will be parallel to the bottom bracket and normal (perpendicular) to the plane of the head tube and seat tube. This ensures that a rear wheel that is true (no wobble) and properly-dished (the rim is centered relative to the ends of the hub axle) will be also in the same plane as the front triangle.

The fork is checked similarly on an alignment table. The steering tube is clamped into a long, machined V-block that is parallel to the table’s surface. The fork crown is squared up vertically (can be done with a carpenter’s square) relative to the table, and the height of the inner face of the low dropout (aka fork end) is measured relative to the table surface. Whether one dropout is forward of the other can be checked with a carpenter’s square.

The fork’s steering tube is rotated 180 degrees, and the fork crown is squared up vertically relative to the table with the other dropout down. The height of the inner face of that dropout is measured relative to the table surface, and if it is the same height off of the table as the other dropout was when it was down, then the center of the hub is in the same plane as the frame.

In the same way as with the rear dropouts, a squared-up rod the diameter of a front axle is then inserted into the dropouts. If its base sits flat on the table while the rod sits fully into the dropout slots (or holes in the case of a through axle), then the front hub will be parallel to the bottom bracket and normal (perpendicular) to the plane of the head tube and seat tube. This ensures that a true and properly-dished front wheel will be also in the same plane as the rear wheel.

As a consumer is not going to have these big, expensive, precision tools and is also not likely to have a nearby framebuilder with these tools willing to perform these checks, you can do similar checks with simple tools and materials. I detail the steps along with and illustrations in the frame chapter and fork chapter in both Zinn & the Art of Road Bike Maintenance and Zinn and the Art of Mountain Bike Maintenance.

The ultimate accuracy of eyeballing and using true and dished wheels without tires on them and a string around the frame is less than with a machined alignment table and accurate dial or digital surface gauges. That said, with experience, commitment and care, you can determine with sufficient accuracy in these checks for good bike performance and avoidance of excessive tire wear (if the rear wheel is being dragged slightly sideways, the tread wears faster).

In the event that you’re willing to really take this on, I’m attaching these sections. Here is the frame-alignment-check section from Zinn & the Art of Road Bike Maintenance; in the 5th edition, it is in chapter 17:

CHECK FRAME ALIGNMENT
Exacting alignment checks require a precision surface plate, an uncommon tool in the home workshop. Thus, the following methods for determining frame alignment are inexact, but sufficient for determining gross alignment woes.

If you find problems more severe than moderately bent dropouts or a misaligned derailleur hanger, do not attempt to correct them. Adjusting frame alignment, if it can be done at all, should only be performed with an accurate frame-alignment table by someone who is practiced in its use.

1. With the frame clamped in a bike stand, tie the end of a string to one rear dropout. Stretch it tightly around the head tube, and tie it symmetrically to the other dropout (See Fig. 17.6).

2. Measure from the string to the seat tube on each side (Fig. 17.6). The measurement should be the same to within 1mm.
< Figure 17.6 Checking frame alignment with a string>

3. Put a true and properly dished rear wheel in the frame and check that it lines up in the same plane as the front triangle. Make certain that the wheel is centered between the seatstays and chainstays. If you have an old steel frame with dropouts with tip screws that thread in from the back of the dropout, turn one or the other of them so that the true wheel lines up straight behind the seat tube (centered between the chainstays).

Without the tire on, you can look through the valve hole and see if the opposite side of the rim is centered behind the base of the seat tube and centered under the center of the brake bridge.

The hub should slide easily into the dropouts without requiring you to pull outward or push inward on the dropouts. Tightening the hub quick-release should not result in bowing or twisting of frame members.

4. Remove the wheel and measure the spacing between the dropouts (Fig. 17.7). For 8-, 9-, 10-, or 11-speed rear hubs, this spacing should be 130mm; it should be 126mm for 5-, 6-, or 7-speed rear hubs, and it should be 135mm for a disc-brake hub with a quick-release skewer and 142mm with a through axle. If the spacing on the frame is 1mm less or 1.5mm more than nominal, it is acceptable. For instance, if you have a frame whose rear spacing should be 130mm, acceptable spacing is 129–131.5mm.

5. If you have dropout-alignment tools, put them in the dropouts so that their shafts are fully seated into the dropouts (Fig. 17.8).

If you have an old steel frame with dropouts with tip screws that thread in from the back of the dropout, you can remove them and seat the dropout-alignment tools all the way to the rear of the dropout slots. Alternatively, you can leave the tip screws installed, and before installing the dropout-alignment tools, turn one or the other of them until a true wheel lines up straight behind the seat tube.

Arrange the tool spacers (and the cups, if they are adjustable) so that the faces of the cups are within a millimeter of each other. Tighten the handles on the tools. The tool cups should line up straight across from each other, with their faces exactly parallel. If the tools do not align, one or both dropouts are bent. If the frame has replaceable dropouts, go ahead and replace one or both of them. If the frame has a composite (i.e., carbon) or bonded rear triangle of any kind, there is nothing you can do about the problem.

If the bike has a steel, aluminum, or titanium rear triangle, you can align the dropouts by bending them carefully with the dropout-alignment tools. Hold the cup of the tool with one hand and push or pull on the handle with the other. Titanium is hard to bend because it keeps springing back, steel is strong yet still bendable, and you run a great risk of breaking aluminum by bending.

CAUTION: Never heat the dropouts (or any part of the frame) for alignment purposes. Doing so could irreparably change the strength, temper, or hardness of the part and lead to failure.

Here is the fork-alignment-check section (without the illustrations) from Zinn & the Art of Road Bike Maintenance; in the 5th edition, it is in chapter 16:

CHECK FORK ALIGNMENT
You will need a ruler, a true front wheel, and dropout-alignment tools. If you have an aluminum, titanium, or carbon-fiber fork, this procedure is diagnostic only, because you should not try to realign any of these forks. Checking the alignment may help explain bike-handling problems and may indicate that a different fork could reduce or solve them.

If you find the alignment to be off more than a couple of millimeters in any direction with any fork other than a steel unsuspended one, you will need a new fork. If the fork is new, misalignment should be covered by the warranty.

If a steel fork is more than 8mm off in any direction, you ought to get a new fork. If the dropouts of a steel fork are slightly bent, you can realign them. You can also take a moderately bent (less than 8mm off) steel fork to a frame builder for realignment. Make sure that whomever you take it to is properly equipped with a fork jig or alignment table and is well versed in the art of “cold setting” (a fancy term for bending) steel forks.

1. Remove the fork from the bike.

2. With the front wheel out, measure the spacing between the faces of the dropouts. Adult bikes should have a spacing of 100mm between the inner surfaces of the dropouts. (Some low-end kids’ bikes have narrower spacing—about 90mm or so.) Remember that you are measuring the distance between the flat surfaces that meet the hub-axle faces (and not between wheel-retaining nubs that protrude inward from the dropouts on some forks). Dropout spacing as wide as 102mm and as narrow as 99mm is acceptable. Beyond that in either direction means a new fork. If you have a steel fork, you can take it to a frame builder for alignment.

3. Clamp the steering tube of the fork. Use a bike stand or two V-blocks in a vise.

4. Install the dropout-alignment tools. They can be used on either the fork or the rear triangle of the bike, and thus they have two axle diameters and spacers for use in the wider rear dropouts. Move all of the spacers to the outside of the dropouts so that only the cups of the tools are placed inboard of the dropouts. Install the tools so that the shafts are seated up against the tops of the dropout slots. Tighten the handles.

5. Check the positions of the alignment tool cups. Ideally, the ends of the cups on the dropout-alignment tools should be parallel and lined up with each other. The cups of Campagnolo dropout-alignment tools are nonadjustable and are nominally 50mm in length; the ideal space between their ends is 0.1–0.5mm. The cups on Park dropout-alignment tools  are adjustable in length, so that you can bring the faces up close to each other no matter what the dropout spacing. If they are lined up with each other and the dropouts are spaced between 99mm and 102mm apart, continue to step 5. If a steel rigid fork’s dropouts are not lined up straight across with each other and the dropouts are within the 99–102mm spacing range, skip to section 16-3b to align them before returning to this point for the next steps.

NOTE: The dropout faces must be parallel before you continue with step 6, or the rest of the alignment procedures will be a waste of time. Clamping the hub into misaligned dropouts will force the fork legs to twist. If the dropouts are misaligned, any measurement of the side-to-side and fore-and-aft alignment of the fork legs will not be accurate.

6. Remove the tire from the front wheel. Make sure the wheel is true and properly dished.

7. Install the wheel in the fork. Make sure the axle is seated against the top of the dropout slot on either side, and make sure the quick-release skewer is tight. Lightly push the rim from side to side to make certain that there is no play in the front hub. If there is play, you first must adjust the hub.

8. If the bottom of the fork crown is open (i.e., it’s not a carbon fork), look down the steering tube and through the valve hole to the bottom side of the rim. The steering tube should be lined up with this line of sight through the wheel.

NOTE ON CARBON FORKS: Carbon forks generally are closed under the fork crown, so you cannot sight down through them. In that case, the best you can do is compare the rim’s position (flip the wheel around and install it the other way as well) with the brake hole and with the fork legs on either side of the rim to determine if it is centered in the fork. This will tell you if the fork legs are symmetrical but not if the steering tube is in alignment with them.

a. When you are sighting through the steering tube and the valve hole, you should see the same amount of space between either side of the rim and the sides of the steering tube. You should also see the center of the bottom side of the rim through the valve hole.

b. Turn the wheel around and install it again so that what was the right end of the axle is now the left and vice versa. Sight through the steering tube and the wheel valve hole again.

c. Placing the wheel in the fork both ways corrects for deformation in the axle or any wobble in the wheel. If the wheel is true and dished properly, and the axle is in good shape, the wheel should line up exactly as it did before. If it does not line up but is off by the same amount to one side as it is to the opposite side, when the wheel is turned around, the wheel is off and the fork is fine side to side.

d. If this test indicates that the fork is as much as 2–3mm off to the side, that is close enough; continue on. If it is off by more than 3mm, get a new fork or, if it is steel, have it aligned by a frame builder (do not try to realign suspension, carbon-fiber, or aluminum forks).

NOTE: If you are sighting through the wheel in this way and you cannot see the bottom side of the rim through the valve hole because the hub is in the way, the fork has big problems. In order for the bike to handle properly, the fork must have some forward offset of the front hub from the steering axis. This offset, or “rake,” is usually around 4–5cm. If you sight through the steering tube and see the front hub, the fork is bent backward so much that it has little or no offset! If this is the case, you need a new fork.

9. With the wheel still in the fork, place a ruler on edge across the fork blades with its flat side resting on the rim and its length perpendicular to the steering tube.

10. Holding the ruler in place, lift the fork toward a light source so that you are sighting across the ruler and the front hub toward the light. The ruler’s edge should line up parallel with the fronts of the dropouts. This test will tell you whether one fork leg is bent back relative to the other one. If the two line up parallel or very close to that, the fork alignment has checked out completely, and you can put the fork back in the bike. If one fork leg is considerably behind the other, you need to get a new fork or have this one aligned (only possible if it is a steel fork). If the dropout-alignment tools also indicated that one dropout was ahead of the other, then the fork legs alone could be bent, and not the dropouts.

Even though it is not simple to check that both wheels are in the same plane, whether you have the special tools to do so or not, it is doable even with minimal tools. Of course, with today’s carbon frames, there is nothing you can do about it if the alignment is off.

As for the handlebar being perpendicular to the front wheel, the only remaining thing to check is that the two clamps of the stem are perpendicular to each other. On an alignment table, this can be done with the steerer clamp of the stem tightened onto a 1-1/8” (28.6mm) diameter gauge rod that is clamped into a long V-block parallel to the surface plate. Tighten another gauge rod the diameter of the handlebar into the bar clamp of the stem and rotate the stem to be parallel to the surface plate (establish this by measuring up from the surface plate at each end).

The rod representing the handlebar can then be checked relative to the surface plate with a carpenter’s square. The carpenter’s square should sit flat against the handlebar rod when pushed up against it from any side.
― Lennard


Lennard Zinn, our longtime technical writer, joined VeloNews in 1987. He is also a custom frame builder (www.zinncycles.com) and purveyor of non-custom huge bikes (bikeclydesdale.com), a former U.S. national team rider, co-author of “The Haywire Heart,” and author of many bicycle books including “DVD, and “Zinn and the Art of Road Bike Maintenance,” as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.”
He holds a bachelor’s in physics from Colorado College. Readers can send brief technical questions to Ask LZ