bent /2 frame

/2 frame alignment tests

by Duane Ausherman

I will only address the BMW’s from the early 50’s till the late 70’s, the ones that I know.  We repaired hundreds of wrecks over the years, from minor to “totals.” This is a collection of what we learned and I hope that it may be useful for others.  BMW frames are not symmetrical, so don’t judge it by comparing right to left.  They are not even the same for the same year and model.  The only thing that is important is that the correct parts must line up.

One of the easiest ways to check for a bent frame is to ride it.  The bike must have accessories removed that may affect it.  Empty the bags, better yet, remove them.  Any type of fairing may make it “lean” too.  A single mirror will affect it.  Two identical mirrors that are mounted identically should “balance” out each other.

At anything above 30 mph, with no wind, it should go in a straight line with hands off of the bars.  It sounds too easy, so here are a few more things to try.  You might think that the camber of the road will make it drift.  Go to the center of the road and try it.  In a place where there is no traffic, try it in several places on the road.

As you are going straight, try twisting your body slightly to change the air flow.  The bike will drift off to one side or the other.  As you twist your right shoulder forwards of the left one, the bike will drift to the right.  You can steer it by twisting your body around, meanwhile staying straight upright.  If the bike wants to drift off to one side, try leaning the opposite way to get it back straight.  How much must you lean to get it going straight?  We measured the amount of lean as the distance from the centerline of the riders head to the centerline of the bike.  Brand new bikes would vary by an inch or two.  I remember a new bike that was bent and we had to lean about 3-4″ to go straight.  In our shop, we decided that a “lean” in the 4-6″ range was “bent.”  One can ride it, but over long distances, it will make your shoulders and/or back sore.  Some bikes are so bent that you would have to climb over on one side to go straight.  An experienced person can actually “feel” that it is bent while just pushing them around on a smooth surface.

It is possible for a bike to track straight and still be bent.  The “box section” just behind the battery is commonly bent.  The real way to check a frame is with a frame jig or a very fancy setup.  BMW never offered a frame jig for the /2 and I am not aware of any such aftermarket product.  The frame jig for the /5 allows one to test a bare frame or a partially disassembled bike.  One could easily be made with the use of a known good frame that is stripped down.  Without the availability of a jig it could be sent off to a frame straightener for checking.

Do not be concerned about the swing arm adjustment, or wheel offset by the factory.

The /2 frame

If the accident was head-on, then that is relatively easy to check.  There are several indicators to look for and they are not very hard.  The frame head angle is usually bent less than the down tubes of the Earles forks, depending upon the year.  The later frames have gussets added for strength.  Look for less distance between the curved cross tube on the forks.  Check your log book to see what it once was.  You didn’t write it down? You didn’t even measure it?  This is one of many things that should have been recorded.  Depending upon the year, the measurement changed.  If you checked 10 new bikes of the same year on the showroom floor you would find differences of up to 1/4.”

Did the front wheel get smashed? If not then the chance is very small that the frame is bent.  Did the forks hit the front engine cover?  If it did, then the frame is most likely bent.  If the forks broke a hole in the front cover, then I would bet that the frame is bent.  I never saw one with the hole that wasn’t bent.

If the bike is ridable then it is easy to check.  It should go straight with hands off and be neutral in corners with stock tires.  Neutral means that one can remove hands in a corner and it will stay on the same curve.  If it tries to stand up or fall down it isn’t neutral.  If the tires are of a different profile than those that are recommended, then the wonderful trait of neutral steering may have been lost, even before an accident.

If it had been hit sideways, one could string line the wheels.  I have seen one case of Earles forks that were visibly bent off to one side.  The bike tracked very crookedly.  With straight forks, it was better, but not perfect.  The frame was also bent.  Usually, bent forks don’t cause tracking troubles.

How to “string line” a motorcycle frame

As stated above, one of the best tests is to ride a bike.  The most common error is to have the frame track crookedly.  That is, it won’t go in a straight line with hands off.  The bike could track straight and still have a bent frame.  A more experienced rider is needed to “feel” the error.   The older BMWs were neutral steering too.

It is often the case that one may want to get an idea of a frame on a bike that can’t be ridden.  This simple test will do just that.  With experience, one can visually see that the front wheel doesn’t align with the rear wheel.  Without as much experience with the visual test, one can use a string to test alignment.

The idea is that one wants to take the plane of the rear tire and project it forwards to the front tire.  A string can be used for great accuracy.  The string should represent the plane of the sides of the rear tire.  The string will run back and around the rear tire and forwards alongside the front tire on each side.  I like to use a string that has no fuzz and is smooth.  Fishline works great.  This allows for it to just touch the sides of the front of the rear tire accurately.

Depending on the bike, it may have a center stand that interferes with the running of the string.  In this case, the center stand had been removed and I had little choice.  You can see that it is held up by milk crates.  Usually, the stand only makes it slightly harder to “thread” the string through and around the rear tire.

frame2.jpg (39135 bytes)

This is a view from the front of the bike.   A clear path exists for the string.  With the bike very stable, align the front wheel as well as can be done visually.  With experience, one can see that the side of the front tire is parallel to the side of the rear tire.  Now go over to the other side and see that it is parallel there too.  The rear tire is usually wider than the front tire.  Look very carefully to see if the same amount of rear tire “overlaps” the front tire on each side.  If the front appears to be centered, that is great.

frame3.jpg (40072 bytes)     frame4.jpg (40484 bytes)

I can’t tell from the photos if it is aligned or not.  Many people can’t read a frame this way, so don’t worry if it all looks confusing.

A weight will be needed on each end of the string.  Anything heavy enough to hold the tension is OK.  A brick works well.  Start at the front and run the string back and around the rear tire and back up front.  Make sure that the string doesn’t drop down into a groove at the rear of the tire.  If it does, then that makes it impossible to get it to align along the rear tire.  It would be getting wider at the front end and the strings wouldn’t be parallel.  Carefully move the weights back and forth to get the string to just touch equally on both sides of the front of the rear tire.  Carefully check to see that the string doesn’t “bend” as it goes past either side of the rear tire.

frame5.jpg (40462 bytes)

Measure the distance between the strings just in front of the rear tire.  This is the tire width.  Now measure the strings just in front of the front tire.  The measurement must be the same.  If they aren’t, then the string is bent at one side of the tire because the string is touching a different amount.  Keep moving the bricks until both measurements show the same, and neither string looks bent at the place where it goes past the front of the rear tire.

frame7.jpg (40533 bytes)     frame8.jpg (41236 bytes)

Carefully adjust the forks so that the strings are parallel to the tire on each side.  Then measure the two spaces.  In this case, the left side (looking back) is about 7/8″, and the right side is only 3/16″.  This shows that the frame is bent.  If in doubt, repeat the test.

Riding this bike with “hands off” would require a “body lean” of at least 6″-10″ and that is enough to make cross-country riding very hard on one’s shoulder muscles.

In doing this for the camera, I “saw” less error by eye than the string showed.  I had to do both again to double check my results.  My eye/brain is no longer able to do this as accurately as 30 years ago.  I trust the string more than my eyes.

I suggest that you try this test on your bike that tracks exactly straight.  Get some practice.

How not to do the test

A search with Google (motorcycle alignment string test) brought up a site with fancy directions on how to do this test.  The writer ran the string around the front tire and extended the string back to the rear.  Then he moved the string until it just touched the rear tire at the front.  Next, he tried to measure the distance between the rear of the rear tire to the string on each side.  Any difference shows the misalignment of wheels.  The mechanical advantage is lost and the amount of difference at the rear will be very slight.  Testing in this way has a low degree of accuracy.

Updated 3 Nov. 2019