The String Thing
Motorcycle rear wheel alignment
(Pictures at the bottom)
This applies to chain driven bikes the have two sided swing arms
The most common cause of a lot of the shakes and wobbles that afflict single track vehicles is, they are not on a single track. This set of instruction will fix it.
I will elaborate in the main of the String thing to wit:
A Motorcycle gains it's stability from the spinning wheels, which are very heavy gyros just like your toy Gyroscope top, Except many thousand time more powerful.
A gyro has a bunch of physical principals involved when it is spinning. To save typing I limit my explanation to the effect of these.
When a gyro is spinning it wants to stay put that is not turn or tilt This is a very strong force strong enough to hold you 500 lb MC upright.
The back one (rear wheel) is heaver than the front, therefore is has a greater force.
Now to the MC both of these gyros are bolted to the same frame so both must be pointed the same way or they will fight each other for control of the frame.
The stronger one wins in the absence of other forces.
the Caster effect. THe setback of the tire contact point from the geometric center line of the steering axis is call trail It is mostly caused by the fork angle but some bikes have offset triple trees or axele set behind the steering axis.
The hands on the bars adding steering inputs IE: tilting the front Gyro.
Tire friction, and tire carcase self aligning torque. CF of rubber is easy to understand, The self aligning torque is the tire trying to spring back to neutral after is it distorted by cornering or any other side force.
WHy you bike wobbles at certain speeds when your hands are off:
Any miss alignment of the gyros from a common path results in a argument between them. The stronger one (rear) will always win. so the front says we go this way and tries to, but the rear says not so fast bub, where going this way, with a force greater than the front. but the front has steering force the caster effect and tire torque. This cancles out the rears greats force until it build up in the tire flex then the torque overcome the rubbers CF and all the forces are canceled out .for a moment.
This act takes place several times a second. that you sense in a given speed.
Simply adding the weight (force) of your hands overcome all the above and you simply scrub the tires sideways instead.
Thats it folks Get those Gyros in line Do the "String Thing"
Mount the bike upright on the center stand or a race stand or lacking those, a pair of cinder blocks and 2X4s.
Wrap a long string around the back tire as high up as it will go without fouling some other part of the bike. A piece of tape will hold it.
Run the string forward on both sides and tie them to a set of moveable objects.
Check to be sure the string is just touching the same part of the rear tire on all four points and not touching any other part of the motorcycle. Move the front stands so that the string just lightly touches the front edge of the rear tire and is not bent over it.
Now the strings show the path the rear tire is taking on its way to the point where the front tire was. I said that strange sentence to illustrate the point of what were doing here.
Now turn the handle bars to make the front tire parallel to the strings.
Now measure the space from the string to the front wheel rim on both sides. Both measurements should be the same w/in 1/16”
If they are you’re done.
If they are not: adjust the rear wheel chain adjusters to correct the error.
Re- set the strings and measure again. Repeat till OK.
Rocking the Forks
While the bike is still rigged for the horizontal work, do this.
Before we go into the vertical plane it’s important to be sure your front forks are parallel to each other. Forks are often not. This can be easily corrected.
Obtain a flat piece of metal or glass about 3” wide and long enough to span the forks.
Rock this piece across the bare part of the fork tubes just below the bottom triple clamp.
By rock I mean the plate should touch both tubes evenly from top to bottom. If it rocks like a chair with a short leg, that means your tubes are twisted.
To fix this loosen all the pinch bolts on the triple clamps the top center triple clamp nut and the front axel.
Now with you a knee holding the front wheel twist the bars till the plate doesn’t rock.
Then gradually tighten all the bolts while constantly checking the “rocker” till all is tight and the plate doesn’t rock.
Re check the center alignment of the front tire.
Not a necessity but if you want to be really really good about this alignment thing.
Your wheels are really big Gyroscopes and as such the angle or Tilt they are on will determine which direction they want to roll in.
Example: Roll a bike (pedal) wheel first straight up and down the with a slight tilt to the right, then left. See what I mean?
Now back to those two Gyros: The back one is much larger, Guess who rules?
While the bike is still rigged for Horizontal alignment.
Rig a plumb line near the rear tire as close to the wheel center as possible.
Measure from that line to the top and bottom of the rear wheel rim, and shim the stands till the rear wheel is square to the earth. That is parallel to the plumb line.
Now rig another plumb line near the front wheel.
Measure from the line to the top and bottom of the front wheel rim. Both of those measurements must be the same.
If so you’re done.
Depending on how bad, you may be able to adjust this condition.
If worse than ¼” you have a bent frame
If less than ¼” try this:
If the wheel tilts away from the line at the top extent that fork tube in the triple clamps 1/8” conversely shorten the opposite side the same amount.
Re check. Go another 1/8” if necessary.
Re check the horizontal alignment, because you just moved the wheel to the side.
Suplemental instructions: Adjusting the alignment and chain slack
Loosen the big castelated nut, Release the 2 12 mm lock nuts from each other. With the big nut snug but not tight use the 1st 12mm nut against the end cap to adjust the wheel position, mind the chain slack. when a satiafactory alignment is achieved tighten the big nut to 80 lbs. FT. Be sure you have not affecxted the alignment. the back off the 12 mm 1st nut and retighten it to the frame Only finger tight. While holding the 1st nut with a wrench tighten the 2nd nut against it to preserve the settings and secure the end plates. There shoul be no tension on the end plates except the finger tight.
That’s it. No alterations or changes to these instructions will work.
Thanks to retsmah for these images:
Some pictures of my setup
Not a whole lot of clearance here...
This was about as high as I could get the string without hitting the center stand on the right side.
Added Video 8/7/10