BMW frames, BMW
subframes, and installing sidecar subframes
© Copyright, 2011,
R. Fleischer
airhdframe.htm
sidecar section article 4
Here is an edited exchange, from the Yahoo Groups Sidecar LIST:
> > I am not a BMW owner, but as I understand the problem, the situation
is
> that BMW quit making bikes that would withstand the tremendous lateral or
> side forces that are developed from the addition of a sidecar way back in
> the 1970s or maybe earlier. The frames were thereafter developed
strictly
> for solo bikes as easily seen by the use of the ellipsoidal frame which has
> excellent strength in the vertical plane and very little in the lateral
> plane.
> >
> > xxxx replied: The use of the subframe therefore is not so much as to be able to hang
a
> sidecar onto the BMW frame easier, but to prevent the BMW frame from
> overloading and breaking.
Snowbum replied:
xxxx is quite correct here,
although the SHAPE of the over-all
> frame is not really an issue.
>
> One thing BMW DID carry onwards, at least until the end of 1984
(when they
> changed to ball bearings at the wheels), was the tapered wheel bearings,
> which were way over-designed, and are perfect for sidecar use.
>
> Some BMW models have a single-sided rear end; that is,
they are not a
> twin-shock, twin-boom rear swing arm. On those mono-rear end models
(includes Monolever and Paralever models),
the
> ""wheel bearing"" is the large left side INternal bearing used
inside the rear drive,
> it does double duty, and while fairly robust, needs to be watched for any
> developing extra wheel play. The
> Paralever, is of, generally, zero help with a sidecar, and adds some potential
> maintenance problems. As always, there are tradeoffs. The Mono
and Paralever mono
rear ends
> have very EASY tire/wheel removal. The Paralever models
...especially the
> GS models....have high driveshaft angles, which should be corrected during
a
> sidecar installation, so as to have a LESSER angle than when as a solo
> bike.....BMW had, and continues to have, lots of driveshaft failures, from
> the high angles of stock GS models. A permanent fix is in the
works,
> probably a constant-velocity universal joint modification. Luckily,
on a
> sidecar installation, there is hardly any reason for a road-going machine
to
> have such a high angle.
>
> BMW frames are beautifully made and properly used, are quite adequate.
The
> early frames (prior to the December 1969 introduction of the /5 series)
were
> specifically of heavy walled construction, designed for sidecars, and came
> with two welded-on sidecar ball lower mounts. From 12/1969,
with the
> introduction of that /5, BMW made their frames of different material
> cross-section, and eliminated those ball mounts, and chopped off the main
> frame near the battery under the seat, and had a separate boltable
> structure, which is called a Rear SubFrame...which is NOT, in case anyone
is
> confused here, the same as a "subframe" ADDED for sidecar use.
>
> The later BMW's are, then, more of a PIA to add a sidecar to, requiring an
> added subframe, the purpose of which is to properly distribute forces of
> sidecaring. But, that is pretty much the same for most
motorcycles, you
> need a subframe. It can get somewhat complicated on some bikes with
NO
> frames!
>
> The sidecaring subframes generally attach at both BMW motor mount studs,
and
> at several places to the rear, and upward.
> A solid metal rod is usually added, at existing BMW unused frame tabs, by
> means of just two bolts, above the battery. This ensures that sidecar
> forces from the rear upper sidecar strut is transferred to both sides of
the
> BMW frame. Later BMW frames had a beefed up top frame tube, but this
added
> metal rod takes care of that for earlier models, except, perhaps, for very
> serious off-roading (jumping, etc).
>
> At the front, upper strut mounting area, either a special clamp, or
welding,
> is usually done, and usually is done such that it directs forces right at
> the small round short BMW crosspiece just under the steering head. THAT
> takes care of transferring forces to both sides of the frame structure at
> the steering head area. In the /5+ some later bikes, they did not
have the
> beefed-up steering head the later ones did, and that crosspiece is a must
if
> not there.
>
> None of this is rocket-science, nor hardly complicated, similar to the
> things needed with most sidecar attachings, and actually is pretty simple,
> and will result in a LASTING BMW frame.
>
> When designing and building an added subframe for a sidecar, one of the
> considerations is normal motorcycle servicing, so keep that in mind.....you
> don't want to have to remove a subframe to change an oil filter, as an
> example. You need to consider the spacing between the sidecar
body/frame,
> and the motorcycle parts too. The old rule of thumb is:
Measure three
> times, cut once. I might add a prefix: THINK many times first!
>
photographs of snowbum's subframes, etc., are on this website at: sidecartech.htm
© Copyright, 2011, R. Fleischer
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