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. I am told that newer BMWs don't track straight by intention.
I have no idea what that is about.
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 center
line of the riders head to the center line 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. You can
actually "feel" that they are bent while just pushing them around on a smooth
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. I
don't think that BMW ever made a jig for the /2 for the dealers to use. 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 year, the measurement changed. If you checked 10 new bikes on the
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, but it could happen I suppose.
If the bike is rideable 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 in the same curve. If it tries to
stand up or fall down it isn't neutral. If the tires are of a different
profile then the wonderful trait of neutral steering may have been lost, even
before any 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 to become
The /5 and later
I have never seen a /5 that tracked crookedly and didn't have a bent frame. Another way of saying it is that slightly bent forks probably won't cause it
track badly. It is possible and I am sure that someone has had it happen,
it is just that I have never seen it.
BMW supplied a frame jig for testing frames. It bolts up into the head stock
of the frame and has arms that go back to the swing arm pivot holes. A special
bolt that is sort of a "bulls eye" and gets inserted into the pivot pin holes. One can easily see how far and which way it is bent. Frame straighteners
have their own system of measuring and it is as accurate, or more so.
Has your bike recently suffered a crash and you want to know more about what
condition it is in? You don't have access to the frame jig or any likely way to
check a frame. There are a few "likely" things that are indicators. If you can
ride it and it goes straight and is still neutral in corners, you are home safe.
If it can't be ridden, I would perform a few quick visual checks.
1. Is the flat sheet metal steering damper plate that attaches to the frame
perfectly flat? It gets bent very easily and is a great indicator. It only gets
bent in a crash. Just because it is straight doesn't mean that the frame isn't
bent. I don't have one to photograph. I hope someone will take a picture of it
and send it to me for this page. The /6 uses a hydraulic damper and so
this one is only for the /5.
2. The frame gusset, on each side, should be perfectly flat. Use a reflection
in the light to show any "warp" that could only result from a crash. If it is
warped, the frame is bent. Just because it is straight doesn't mean that
the frame isn't bent.
The left side gusset on a /6. It is the part with the
warning label on it.
3. Remove the tank and lay a straight edge along the main frame backbone. It
should be absolutely straight. If bent, it will usually be in the first few
inches behind the steering head. On the /6, this area is obscured by the master
brake cylinder, as can be seen in the above photo in the upper right. Just
because it is straight doesn't mean that the frame isn't bent.
4. If the crash had a sideways component, then the traction of the rear wheel
on the ground will sometimes cause the rear frame section to twist. That is the
rectangular part just behind the battery. I know of no visual test for this
"box" section being slightly bent. If moderately bent it can give you trouble
with getting the rear swing arm pins to go in easily. That isn't an easy visual
check however. If severely bent it can cause the output shaft of the
transmission to move sideways enough to break the transmission rear cover.
Would someone send me photo of this damage?
This damage isn't visible as it is under the rubber boot. It is also on the
inside part where it is the hardest to see. Loosen the front boot clamp. Peel
back the boot and check the flange part on the inside (midline of the bike) for
cracks or actual missing aluminum. Not only is the frame bent, but the
transmission is damaged too. We actually found one output shaft so severely hit
that it broke. Nothing was visible without inspecting under the boot.
Motorcycle frame straightening
I must admit that 25 years ago, BMW cautioned against getting a frame
straightened. That was only in the North American market. That was very
hypocritical of them, as it was a common service in Europe. A BMW dealer could
send it back to the factory frame shop for straightening. I was there and saw
it. There must have been at least 10 old used scratched up frames for rework. To
add to it, every frame BMW made was crooked, as a result of the welding process,
and had to be straightened. If it was straightened once, why not twice? We had
many frames successfully straightened over the years. At least two well
qualified shops have been doing this in Northern California for over 15 years. I
am sure that several other places do it as well. It is not magic.
The Frame Man 916-927-9712, 1645 Silica Ave, Sacramento CA 95815. Frame
straightening/repair, forks, wheel straightening, etc. Dealers and racers from
all around send their work here. Will do business via UPS. He is
fast, but costs a bit more.
The second shop G.T. Enterprises
is up north in Yreka, Ca. He is slower and cost less. Owners report
that it may take months.
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 the front wheel doesn't align with the rear wheel.
Without as much experience one can use a simple string to test alignment.
Depending upon 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. Usually the stand only makes it slightly harder to "thread"
the string though and around the rear tire.
This is a view from the front of the bike. You can see that it is held up by
milk crates. 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 with 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.
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.
How to string line a frame on a BMW motorcycle
Now for the string test. 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
fairly great accuracy. The string will run back and around the rear tire and
forwards along side the front tire on each side. I like to use a string that has
no fuzz and is smooth. Fish line works great. This allows for one to get it touching the sides of
the front of the rear tire accurately.
A weight will be needed on each end of the string. Anything heavy enough to
hold the tension is OK. 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
Measure the distance between the strings. This is the tire width. Now measure
the strings just in front of the front tire. The measurement must be the same. If it isn't, then one side of the tire has the string touching a different
amount. Keep moving the strings until both measurements show the same and
neither string looks "bent" at the place where it goes past the front of the
Rotate the forks so that the string is parallel to the tire on each side. Then measure the space. In this case the left side (looking back) is about 7/8"
and the right side is only 3/16". That shows that the frame is bent.
If in doubt, perform the test again by moving the strings while watching them
touch the rear tire.
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
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 eyeballing the tires.
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 of 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 problem is that a
great deal of mechanical advantage is lost and the amount of difference at the
rear will be very slight. That web site is far nicer looking and artful than
this one, but the content isn't what it should be. I emailed the author and we
discussed this issue. He agreed with me and vowed to change it. That was in 2003
and nada. We are all very busy.