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Accurate Rear Wheel Alignment Using Mason Line

Although sometimes overlooked, rear wheel alignment is quite important. The result of a poorly-aligned rear wheel is excessive wear to rear tires, sprockets, and chains.

All tire alignment methods I'm aware of rely on a straight line, be it a straight edge, a laser, or a mason line. For me, mason line has proven to be an extremely accurate and reliable method as long as three conditions are satisfied:

  1. Use mason line and not string and pull it very taut
    Discounting for the Coriolis effect and gravity (both negligent over six feet) a taut mason line is about as straight as it gets.
  2. The line is shimmed off the tires
    Anything that touches a line between its two attached ends will push the line and the line will not be perfectly straight.
  3. Result is double-checked after preliminary alignment
    For explanation, see below.



Materials Required


  1. With bike on center stand, run a taut line up one side and down the other around front and rear tires

    1. Begin by fixing the line to a rear tire spoke on the drive side and wrapping it across the back of the tire about seven inches off the ground. A picture-hanging hook tied to the line works great.

    2. On the timing side, run the line through the center stand and around the front tire tread, keeping the line parallel to the ground. If necessary, adjust the height of the line so that it is as high as possible without fouling the center stand.

    3. As you go around the front tire, pull the line on the timing side good and tight before going down the drive side and passing it through the center stand.

      Tip: A finger-sized loop knotted into the line just ahead of the center stand on the drive side makes it easy to keep the line taut while you thread the loose end through the center stand.

      Diagram of rear wheel alignment using mason line.

    4. Back at the rear tire, pull the line taut, cross it over the tire, and tie it off using the TS shock absorber bolt and the folded passenger foot peg.

      Tip: Wrap the line around the foot peg five or six time, crossing it over itself each time, and then secure it with a half-hitch.

You now have a taut line all the way around the outside of both tires. If it's not taut, go back and teach it!

Shimming string from tires

  1. Next, use the shims to space the line away from both front sides of the front tire, and both rear sides of the rear tire. The shims must all be equal.
  1. Carefully adjust the front wheel side-to-side until the distances between the line and both sides of the front tire are equidistant. You can measure with a 6-inch machinist's rule, or slide a slightly thicker wooden shim between the string and tire on both sides and eyeball the small gap.

    Measuring gap, front tire

    Once the front wheel is centered between the lines, immobilize it with either the steering damper if fitted, or with something like a couple of cement blocks. Recheck gaps to be equal.

  2. Now measure the distances between the line and both sides of the rear tire. If they are not equal use the axle adjusters to move the rear wheel side-to-side until it is centered between the lines.

    After making the initial alignment, go back and check to see that the front wheel is still perfectly centered between the lines. This step is crucial, because if the rear tire was badly mis-aligned to begin with, it introduced a slight error when centering the front wheel in Step 3.

    Re-adjust the front wheel if necessary and re-align the rear wheel using the adjustors. Once both tires are perfectly centered between the lines, the rear tire is perfectly aligned.

Measuring gap, rear tire
  1. Alignment completed, tighten the axle nuts incrementally to factory-recommended torque: 100 pounds for 1969 T120R.

  2. It's a good idea to make one final check after tightening the axle nuts.

  3. Lastly, remember to check the tension of the drive chain before you go for a ride on your perfectly aligned Triumph motor bike.
650 Triumph