tests for a bent frame

BMW motorcycle 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.  You can actually “feel” that they are 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.  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 the 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 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 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.

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 headstock of the frame and has arms that go back to the swingarm pivot holes.  A special bolt that is sort of a “bull’s 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 visual test 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.

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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 the backbone 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 a photo of this damage?  This damage isn’t visible as it is under the rubber boot.  It is also on the inside part of the bike 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 during manufacture, 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 owner died a few years ago and the business is gone.

The second shop, G.T. Enterprises, is up north in Montague, Ca.  Try calling 530-842-6511.  He is slower and cost less.  Customers 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 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.  Usually, the stand only makes it slightly harder to “thread” the string through and around the rear tire.

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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 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.

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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 great accuracy.  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.  The string should represent the plane of the side of the rear tire.

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 tire.

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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 rear tire.

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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 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 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.  This method is not accurate.