Wrenching with Rob - Chassis Alignment Basics, Part II

By Dr. Rob Tuluie, PhD

Editor's Note -- This story is part two of the "Wrenching with Rob" series, in which Vintage Editor and Technical Writer Dr. Robin Tuluie discusses, in depth, technical and theoretical topics that make motorcycles function. This time the focus is on motorcycle chassis basics, in particular, how to do your own wheel and chassis alignment. Today, Rob covers parts (3) Determining if the sprockets align and (4) Checking whether the swing arm, forks and frame are straight. Before reading on, make sure you've read part one of the series.

Part 3: Checking the sprocket alignment.

Similar to the method we used previously to align the rear wheel relative to the front wheel, we will now align the sprockets. It should be clear that if the wheels are aligned, the swing arm and frame straight and the sprocket offset and rear wheel spacers correct, then the sprocket alignment will be true. However, for many custom built bikes using different wheels and spacers, relaced spoke wheels or different sprockets this may no longer be the case. I've found that especially on older Vintage machinery often the rear axle adjustment at which the wheels align does not coincide with the adjustment at which the sprockets align. Older Vintage machinery has often been subjected to several previous owners and one is never quite sure what is original and what has been replaced. Even newer machines with cast wheels can suffer from sprocket misalignment if the wheels are not OEM-spec, aftermarket sprockets or wheel spacers are used or a bent swing arm is suspect.

Remove the chain from the motorcycle and chain guard and front sprocket case cover if necessary. Referring to figure 1b below, simply string a line from the front sprocket to the rear one. Align the line at the rear sprocket as shown. If the sprockets are aligned, then this line should just graze the edge of the front sprocket. Make sure that the line is straight.

If this alignment cannot be achieved, then assuming you have previously aligned the wheels as in Part 1, the logical conclusion is that the rear axle adjustment which yields proper sprocket alignment is not the same as the rear axle adjustment which yields true wheel alignment. Hence something is wrong. First check that the rear sprocket offset is correct. Do this by comparing to the correct OEM wheel and sprocket assembly.

Next check that the rear wheel spacers have the correct length. Also check to make sure the front wheel is installed with the proper spacers, otherwise the wheel alignment would not be correct. If you have OEM wheels, spacers and sprockets and still the sprocket alignment cannot be achieved without disturbing the wheel alignment, the fault lies in the wheel alignment itself or in a bent chassis or bent chassis component (such as the forks or swing arm).

The next step is to look at the chassis itself an determine if the frame, forks or swing arm are straight. This step is also the next step if a proper wheel alignment results in the wheels being out-of-plane as in Fig 2 of part 1 of our article. Just because the wheels and sprockets align doesn't mean that the chassis is straight - the wheels must be in a single plane as well (as described in part 1). Referring to figure 3 of Part 1, this means that the angle

	theta = 57*(a_r -a_f)/s_f

should be close to zero (plus/minus 1 degree). If it is not within this spec, then the chassis needs fixing.

Part 4: Checking The Chassis

So let's suppose that somehow your chassis alignment and sprocket alignment don't agree and/or the front and rear wheels are out of plane. After all, if they do agree (to within the limits of accuracy stated above), then you're home free, your wheels and sprockets aligned and both wheels in the same plane. At that point, there is little reason to suspect a bent chassis unless several parts are bent and conspire to give aligned (which also means "in plane") wheels and sprockets - a somewhat unlikely scenario!

The first check is to see if any of the axles are bent: either roll them on a flat surface or hold a straight edge along them, turning the axle in the process to at least two different spots. The next check is to see if the swing arm is bent. The most common way for the swing arm to be bent is a twist in the cross tube - imagine standing behind the bike, taking hold of the swing arm legs and pushing one down and the other up. This would result in a twist of the cross tube. An easy way to discern this without dismantling anything is as follows: Referring to figure 2, and with the sprockets and wheels aligned (or as close to it as possible with the suspect bent chassis), get the rear wheel vertical to ground, using a carpenters level or the string method. Next see if the swing arm axle is horizontal - you can do this by either feeding a string or straight rod through the swing arm axle (if hollow) and holding a level along the string, or by measuring the distance d1 and d2 of figure 2 from the axle center to either a flat and level ground or to a carpenter's level that is set up on the ground in a horizontal position, perpendicular to a line connecting the front and rear tires.

Your swing arm is straight enough if the twist angle is less than 1 degree, that is if the difference in the distance d1 and d2 of figure 2 from the axle center to horizontal is less than

	(d1 - d2) < w * sin(1 degree), or

        (d1 - d2) < (w / 57.0)

If it turns out that (d1 - d2) exceeds the above limit, I recommend removing the swing arm and checking it again on a flat work bench, using the same principles as above.

It is also possible for the swing arm to be bent sideways, such as would occur if the rear wheel were impacted from the side. Any sideways bend would manifest itself in an offset of front sprocket to rear wheel sprocket and would be easily detectable during a sprocket alignment.

If the swing arm is indeed bent it can usually be straightened and braced by a decent frame shop, or you could do it yourself if familiar with such operations.

CHECKING THE FRONT END

The first thing is a quick preliminary check which consists of sighting along the plane in which the forks lie. Gauge one fork tube against the other by making sure they are parallel. If not, try loosening the bottom triple clamp bolts and front axle and, if applicable, any fork brace, and twisting the front wheel back and forth slightly. Bounce the front end up and down a few times (make sure the top triple clamp bolts are tight!) to center the wheel, then snug the bottom triple clamp bolts and and then the front axle. If the fork tubes are still not parallel, then either they are bent or the triple clamps are bent. To find out which one is at fault, disassemble the forks first and rotate the fork tube in either a lathe or on V-blocks using a dial indicator, or roll it on a flat surface. If the forks are straight, put them back into the triple clamps and, if the triple clamps are indeed bent, lying a flat plate across the fork tubes will immediately show this. Both fork tubes and triple clamps can be straighten if not bent too severely. However, as soon as any crease or chipping of the chrome is evident on the fork tubes they are history.

CHECKING THE FRAME

Checking any frame is not a conceptually difficult part in so far as there are only two important dimensions involved (apart from making sure that none of the motor mounts, seat mounts, etc. are bent). A frame's function is basically to connect the steering head and swing arm pivot points in as rigid and economical a manner as possible and also to provide mounting points for various ancillaries. For our purposes, the question of whether the chassis is aligned or not condenses now to whether the frame's steering head is perpendicular to the swing arm axle. The inclination of the steering head with respect to the ground (the steering head angle) is also important for the handling of the machine but will not affect anything we've discussed above as far as wheel alignments and such are concerned. We will return to the question of measuring the steering head angle in part 3 of this series of articles. We therefore represent the frame as in figure 3.

Figure 3 shows that the steering head must lie in a plane normal -- that is perpendicular -- to the swing arm axle. It is not generally required that this plane intersect the swing arm axle in the center of the axle, 1/2 way between the frame spars. Some machines have asymmetrical frames with respect to the center of the swing arm axle. However, in all cases, the steering head MUST lie in a plane normal to the swing arm axle. In addition, for a perfectly aligned chassis, the steering head must lie directly vertically above a line connecting the center of the tire contact patches when the wheels are aligned and vertical.

Now that we have an understanding of the basic function and necessities that make a "straight" frame and chassis, we can set about checking how close your frame comes to that ideal geometry. Checking the frame for straightness, measuring the steering head angle, wheel base, center of gravity and other features one would like to know will be discussed in the next issue of "Chassis Alignment Basics".

<< Back To Part one

Originally from http://www.motorcycle.com/mo/mcnuts/chassis2.html