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.

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.

Hands on:

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.


Vertical alignment
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.

If not:
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.
FOG

Thanks to retsmah for these images:[/color]


*MODERATOR NOTE*
Added Video 8/7/10
KS

http://www.youtube.com/watch?v=RBxK-OQ--fg

Sent to the top and stickified.

thanks FOG!

FOG - last night I did the string thing with a laser as well as a string.

The laser that I have paints a vertical line.

I just had the laser knick the front and back edges of the rear tire, and it would paint a line forward, all the way past the front of the bike.

No real advantage, just another option.

Yep, good Idea, I got me one O them laser things too. Never used it for a plumb line though, but it has that option.

On another list I used to dole out Smart pellets for a good Idea or correct answers. Like Pavlov.

That'en worth a food pellet.

FOG

I for one normally simply trust the manufacturer's increment marks on the swingarm. I also normally trust the triple tree clamps are a flat plane so if the forks are adjusted to the same height on each side it is correct. However, I could see this as useful in making a minor adjustment to a bike that has been wrecked.

Sorry Bud, but your wrong on both counts. Your Trust is misplaced. The Swing arm marks are often "Off" and even if they are correct it's much harder to be accurate with them. A tiny error that would be imposable to see can be magnified at the wheel. And the forks can be, and almost allways are, twisted. Even if the tube are level. The top tree can be misaligned Radially, with the bottom one so that the forks form a kind of Spiral. in this condition as well as binding the forks and aggravating the bushing wear you induce a kind of Bump steer that turns the wheel as the forks compress.
In almost every case I have checked, to achieve vertical alignment I've had to slip one fork tube up or down in the triples to get the front wheel parallel to the rear in the vertical plane.

Pleas read the instruction and do it correctly

Is it possible to post a digital pic with your bike doing the string thing...

So I was doing a chain tensioning and decided to do a string alignment this time rather than using the factory marks. I am finding that the center stand is right in the path of the string on the right side of the bike.

Anyone else have this problem? Mine's an 06 if that matters. I will work on it some more tomorrow, it's possible the wheel is pointed way off to the right.

I've been trying to cure a front wobble issue in my bike ('03) for a while now, and have gotten pretty adept at setting up "the string thing" that FOG described above. I know what you mean when you say some of the centerstand parts get in the way. You'll need to play around with the rear to front placement of the string, but the strings will definitely go from the rear to the front without touching anything else, including the center stand.

I ended up using fishing line (don't know the test poundage ) since its thin, round and can be "cinched up" really taught. I wrapped the string around the back of the rear tire (you'll need to experiment with the height on the tire, but go as high as possible without touching anything but the front edge of the rear tire), fished the line through the centerstand, then tied the line to two mini-sledge hammers which I placed about to feet in front of the front wheel. I fiddled with the placement of the hammers to make sure the line from the rear tire was as straight as possible without any deflection due to touching the front of the rear tire. You can follow the rest of FOG's instructions at this point and get the system to work.

If this setup is not correct, I'm 100% positive someone will jump in to let us both know. Keep working with the setup, it takes a little practice at first, but you'll get it.

Close enough, but I wonder if you wouldn't be better able to gage the fron wheel if the front of the strings were a bit higher. Parallel to the floor at the same level as the rear would be Ideal.

Did you also do the vertical check too?

FOG

Well, I found that if the rear wheel is aligned correctly the string on my bike is about 1/8" inch away from the right side of the center stand. So you have to start out either correct, or with the back wheel pointing to the left

I think I've got it aligned pretty well, the left / right side measurements on the front wheel were off by a little less than 1/16 after torquing the axle... well, at least as well as I can measure with a ruler and string Still need to clean the chain and put the plastic bits back on, then I can take it out for a test ride.

Some pictures of my setup, feel free to criticize: (These pictures might be down tonight, flickr is up for some scheduled maintenance here soon)



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.

Looks exactly right to me in terms of the rough setup. FOG may be able to suggest additional minor tweaks to get the most accuracy out of the method, e.g. making sure your bike is exactly vertical in relation to the ground prior to setting up the horizontal string thing measurement.

Went out for a brief ride to see how the alignment feels, didn't notice anything out of the ordinary although I only got up to 50 or so.

The factory alignment marks seem to be pretty good on mine, I used them to do the initial alignment and when I checked it with the strings it looked to be right on.

Hey Slugger those Pictures Retsmah posted are perfect illustration of the string thing set up. If you could add them to my discription that would be super. Of coruse Give Retsma the Byline.

FOG

I gotta say I've had the exact same experience in my 23 years of riding. The factory alignment marks have always been spot on when I've checked them with several different alignment methods on various bikes. So, for the last 10 years or so I've never used anything but the factory marks.

Think about it.... In our litigiuos society, I can not believe the cycle manufacturers would allow the (very important) rear wheel alignment marks to be inaccurate. The liability would be far too great to allow it.

Methinks it wouldn't much matter from the liability standpoint. It really only rears it head when you take your hands off the bars and it's out of alignment but matched to the marks. When you start your description of the accident with, "I took my hands off the bar for just a second," it's a pretty sure bet that their liability stops there.

Nice catch-22 there. You think you notice it so you take your hands off the bars and confirm it, but without taking your hands off the bars it's impossible to confirm.

The point of the string thing is to see actually what those Factory marks are supposed to do. It's like a Magnifying glass to help you see better. If you have read my little stories about gyros and wobbles, and you do the string thing you WILL better understand what make your motorcycle work.

If you don't CARE Shut up

Note to administration. Once a Item has been made a sticky it should be closed to further comment except through the Admin.
I understand them to be information not to be corrupted by inane useless commentary

FOG

Then if you are going to do the string thing 100% properly to check the factory alignment marks...

You need to remove the rubber and align to the rims only. Did you ever notice how much runout there is on the tire sidewalls?

Try it. Put the bike on the center stand and rotate the rear wheel. Watch the side walls of the tires. They wobble don't they?

Seems to me way too much runout on the rubber to accurately measure alignment within a few millimeters at the ends of the string.

Just a few hundredths runout of the rear tire will be much greater at the front tire due to the string angle and distance.

Yes this is true, But easy to check, just spin the tire and watch it. If it bad or your really anal. just adjust the strings bu measuring to the wheel rim.
You can also fit four small equal sized blocks of wood to the nice flat surface on the rims to space the strings out off the tire.

Good point thanks for ading it.

FOG

Now that's a good idea! Never thought of that. This way you get rid of the runout from the tire and get much more accurate measurements up front. They should measure to the front rim as well and not the front tire.

That's the problem.... I'm too anal!

Just a quick calculation:
assuming approx 17" across the rear tire where the string hits and approx. 4 feet to the front tire for one measurement...

With only 1mm (.04") of runout on the rubber the string will be off approx. .113" (almost 1/8 of an inch).

This dimension will only get larger as the 4 feet (assumed distance above) gets longer.

I know you already know this FOG, just pointing it out to the younger guys so thay can take this into consideration...

so then when someone has a question, they have to post a new thread about it?

its always better to post in an existing thread than to start a new thread IMO.

I feel the Stickies in the FYI section, are supposed to be instructional and thoroughly proven or tested and should not be subjected to Dilution by a lot of "what if" type comments.

FOG

Just did this and the rear tire is straight as an arrow, however I am still getting some headshake
I also "rocked" the forks and they are perfect.

Check your steering head bearrings and change the front tire. Finally straight may not be where the rear wheel (gyro) wants to be. Yes turn it a bit . Warning this will be a trial and error and hardly worth it.

FOG