Bent backbone?

Guido · Feb 19, 2012

I started a post about me not being able to get my swing arm spindle to slide into the bushings, that they were misaligned.
reaming-swing-arm-bushes-t12369.html
It showed the swing arm was bent, twisted.
The guy at the frame shop said the frame may also be bent. I am stripping it down now.
I noticed that the back bone has a slight hump to it. I checked out my other bike and it has the same slight hump in it's back bone.

If someone who has their bike torn down would check theirs with a straight edge to see if they also have the same hump in the back bone I would really appreciate it.
I just want to see if the back bone is suppose to be perfectly straight or if the hump is how it was designed.
The bend starts at where the rear frame tubes are welded to the middle of the back bone.

concours · Feb 19, 2012

Pics please...

bwolfie · Feb 19, 2012

I remember reading many years ago that the slight bend is intentional on all commando's. I'll double check the 3 frames I have and report back.

L.A.B. · Feb 19, 2012

Guido said:
I noticed that the back bone has a slight hump to it. I checked out my other bike and it has the same slight hump in it's back bone.

All perfectly normal and nothing to worry about.

Guido · Feb 19, 2012

That takes a load off. My swing arm was noticeably twisted.
It wore out a new tire in a matter of a few months plus the axle bolt was real hard to start in the dummy axle, the rear sprocket was worn badly on one side of the radius.
If there is a way to check the frame for straightness at home, I'd like to know how to do that.

Guido · Feb 19, 2012

bwolfie said:
I remember reading many years ago that the slight bend is intentional on all commando's. I'll double check the 3 frames I have and report back.

While your checking, see if you have any exhaust nuts lying about as I crushed mine trying to get them off.
Never use a plumbing pipe wrench to remove these nuts. Idiot!

rvich · Feb 19, 2012

In the workshop manual, section F, figure F4 gives frame dimension data. It is hard to read online but easier in the actual manual. The Haynes manual has the 850 data on page 92. Short of that you can take careful measurements from points like the front isolastic mount to a point on the opposite side like were the frame loop meets the tail loop. Then compare it to the same points on the opposite side. In theory these should all match. The more places you can find where you can measure left to right at an angle and the right to left between the same (but opposite) points the better. I have no clue what kind of tolerance there was in the frame jig when these were built but if you start finding measurements that don't match you could research further. This is not unlike squaring up a framed structure before you put on sheeting. Hope it makes sense.

Russ

gjr · Feb 19, 2012

You might block the bare frame up so it sits level (side to side), put a 3/8" rod through the rear isolastic mounting holes, and drop a plumb bob down through the steering stem to see if the steering head axis is perpendicular to the isolastic bolt. I taped a small square to the bolt and lined up the string by eye.

rvich said:
The more places you can find where you can measure left to right at an angle and the right to left between the same (but opposite) points the better. ... This is not unlike squaring up a framed structure before you put on sheeting.
Russ

That is a good idea.

Greg

rpatton · Feb 19, 2012

Guido said:
That takes a load off. My swing arm was noticeably twisted.
It wore out a new tire in a matter of a few months plus the axle bolt was real hard to start in the dummy axle, the rear sprocket was worn badly on one side of the radius.
If there is a way to check the frame for straightness at home, I'd like to know how to do that.

The guy that checked my frame told me that every Commando frame he'd seen shows the hump. He said it has to do with the stresses built up from when they weld the frame together.

Which side of the sprocket showed the wear? When you get the swingarm back make sure to ask him about the offset on the swing arm. The axle pads are offset 1/8" to the right to make up for the 1/8" offset on the cradle. You can check the chain alignment with just the cradle/trans/swingarm/rearwheel. The frame doesn't enter into it.

I'm told that a lot of BMW's have had a small offset to the their wheels that doesn't cause any problem with handling or tire wear, but the wheels are parallel.

http://www.oldbritts.com/19_063968.html

Guido · Feb 19, 2012

rvich said:
In the workshop manual, section F, figure F4 gives frame dimension data. It is hard to read online but easier in the actual manual. The Haynes manual has the 850 data on page 92. Short of that you can take careful measurements from points like the front isolastic mount to a point on the opposite side like were the frame loop meets the tail loop. Then compare it to the same points on the opposite side. In theory these should all match. The more places you can find where you can measure left to right at an angle and the right to left between the same (but opposite) points the better. I have no clue what kind of tolerance there was in the frame jig when these were built but if you start finding measurements that don't match you could research further. This is not unlike squaring up a framed structure before you put on sheeting. Hope it makes sense.

Russ

I've got 2 manuals and one, the Haynes, shows all the dimensions.
I get what you are saying about left to right and right to left. I'm a contractor and use that to measure for square too.
Why didn't I think of that?
Gotta start thinking out of the box. Square box that is.

Hobot · Feb 19, 2012

As stated all Cdo have camel hump spines. Only need frame square enough to assemble fairly easy. Only alignment that matters is the drive chain sprockets. A number of other well respected cycles beside BMW run a bit out of line tire centers w/o notice. What may be an issue i've run into is the under spine tube knocked
off to one side putting a bind on the head steady unless space over to accommodate. Don't be shy to bend and bash, drill and grind the head steady once rest is assembled so its in most neutral unloaded state at rest.

Bent backbone?

Guido

concours

bwolfie

L.A.B.

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Guido

Guido

rvich

gjr

rpatton

Guido

Hobot

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