Boxerworks.com

For anyone who doesn't know what a Magni frame BMW looks like (tit's the red thing) ..and a little better detail:

Ooh - I'd like to ride that!

As mine is a 1989 mono shock frame - is the subframe inconsequential?
The cross bracing in the tool tray area- is that behind or above the battery? as I'm not sure what it is supposed to do.

The box section at the battery is important in that it often gets bent up in a side collision. On the older bikes the rear frame section stiffens up the part of the frame at the swing arm area.

I never had a way to measure the amount of stiffening that each part supplies. I don't know anybody who did.

Duane- are you ready to rumble?
(Hello old friend)

Good evening, Major!

Nourish,
My thought is that welding the subframe/mainframe connections creates a structure which serves to restrict frame distortion caused by forces acting upon the wheel/swing arm.
The substantial SA of the mono frame addresses these forces.
Still, the rear "down tubes" of the mainframe
are vulnerable.
The diagonal would be welded across the top, from 1 upper sub frame mounting to the opposite side existing cross brace/seat mount.
Gusseting at the 4 corners would also work.
Either serves to complete a structure designed to oppose differential (fore/aft - lateral) twisting of the rear tubes.
I should add that this strategy is easier to "package" as compared to side braces, provides similar rigidity to the rear of the frame, but does nothing to maintain alignment of the steering head and SA.
Close tolerance engine mountings and a structural connection at the upper engine/trans bolts to the mainframe spine tube would do well in that regard.
I would also consider (as HPN) reinforcing the flat plates at the steering head.
My brief (past life) foray into computer analysis of a boxer frame revealed this area ( rear tubes & SA pivot/axis) to be the weakest link of the mainframe.

Thanks Vanzen - That makes sense to me as I couldn't understand why the subframes cross brace had cracked when it wasn't subject to the suspension loads.
I have some K75S forks to fit so will look closer at the steering head stiffening.

vanzen wrote: The diagonal would be welded across the top, from 1 upper sub frame mounting to the opposite side existing cross brace/seat mount.
Gusseting at the 4 corners would also work.

Can anyone show me a photo or illustration of these two braces? I can't quite picture them from the description.

Is this the bracing you were thinking of?

-http://i186.photobucket.com/albums/x97/ ... FRM2-1.jpg

This appears to to be a fabricated box section that is tying the head stock tube to the inner sides of the down tubes to the spine as well bracing the upper rear of the head tube to the spine without having to weld to the top side of the spine.
They also seem to double up on the long gusset plates down the sides of the down tubes.
Would all of this be necessary as it's not going off road - well not intentionally anyway!
If so I was thinking instead of doubleing up on the gussets I could weld in a plate across to the two to make a box section.