Friday, December 4, 2020

Motorcycle Wheel Alignment


I just aligned the wheels on a new motorcycle I just acquired. This is one of those activities that is an arcane procedure.
Back in the 1990's I made a post to the Ducati mailing list describing this procedure. And then I posted it on my blog back in 2012. But I keep seeing confusion about how to do this properly, so this is a refresh of that description, now with pictures!

A couple notes to begin:

  • For this to work, your motorcycle needs to be in good condition and undamaged (no warped wheels, bent swing arms or loose bearings that allow the wheels or steering head to wobble).
  • This method ignores how the chain and sprockets are aligned, and focuses on how the wheels and tires are aligned. I can imagine that having the sprockets and wheels be perfectly aligned simultaneously may not be possible, and I'd rather have the wheels lined up.

The "Thread Method" or 'String Method":
I've always aligned my wheels the way I saw Rich Oliver do it on his TZ250 years ago. I think this way is standard "racer" practice. 

You really should use a thread to do this. It is possible to use a string or something else thicker than a thread, but it doesn't work as well. The thicker the string, the harder it is to see if the string is bending around the wheel instead of just touching it. This will make more sense when you understand the procedure.

The theory of this procedure is that you run two perfectly straight parallel lines forward along each side of the bike. These lines are arranged to touch the edges of the rear tire. Then if the front tire is narrower than the rear tire, the front tire should be perfectly centered between these lines. Like this:

(image credit: Scott DiRoma - see the end of this post for more great info)

Put bike on service stand. Take about 15 feet of thread or string (elastic thread works best!). Tape center of thread to back of the rear wheel about 4" off the ground.

In the picture above, I have wrapped the thread around the wheel, taped it, and then run the end forward the front of the bike. Note how the lower thread that runs forward runs over the top of the tread block: it is NOT in the notch of the tread. It is important that the thread runs over the outer edge of the tread.

Bring the ends of the thread to the front of the bike. Tie the ends of the thread to some movable objects (I use a pair of jack stands), at the same height (about 4"). 

Make sure the thread is as high as possible without hitting anything under the bike. It must not hit anything like the exhaust or side stand.

The idea is to set the threads so they are parallel and "just" in contact to the front edge rear tire by moving the jack stands. 

This is where using a thin thread really helps. Above you can see the thread is just barely in contact with the front edge of the rear wheel. If it more than just touches, the thread will bend around the wheel and will no longer be running straight forward. Check this very carefully!

With the threads taped to the back of the rear tire, and "just" touching the front edge of the rear tire, they should make two parallel lines that run forward, passing along either side of the front tire. 

 It is hard to see the threads in this picture, but they are there. Make sure the front wheel is pointed perfectly forward by adjusting it carefully at the handlebar. The thread should be the same distance from the wheel at both the front and back edges of the tire.

Now the alignment of the front and rear wheels can be easily observed by examining the clearance of the front wheel and the thread on either side. 

 Above, you can see the right side of the tire (on the left in this photo) is is 23mm from the thread.

Here, above, you can see the left side of the bike (on the right in this photo) is about 15mm from the thread. So this front wheel is not perfectly aligned!

Again, be sure the front wheel is pointed directly forward with these measurements the same at the front edge and rear edge of the front tire. Also, be sure the thread hasn't fallen into the tread on the rear tire and is just barely touching at the front edge of the rear tire.

Adjust the chain adjuster appropriately if necessary, and then recheck. After you move the chain adjusters, you will have to recheck how the thread runs and touches the rear wheel and you WILL have to move the jack stands because you have moved the rear wheel!

 It's a pain to do the first time, but then it gets easy. Use the thinnest, elastic thread you can find. Also, try rotating the rear wheel to several points to make sure you aren't just adjusting for uneven tire manufacture. And make sure the thread isn't on some bump of "mold seam" rubber or in a tread at the edge of the tire. This would screw up everything.  

When using this procedure on my '92 750SS, a quarter turn of one of the chain adjusters is quite significant. When complete, the adjusters appear to be at even marks at either side of the bike, indicating adequate swing arm manufacturing. However, a quarter turn seems to effect the alignment significantly, but is just about invisible in relation to the adjuster marks. Doing this procedure properly will result in much better alignment than just counting on the adjuster marks.

There is ONE video on youtube that I have found that explains this well. Be sure to check it out here, by Scott DiRoma: