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bubba

How to chop the frame under the pillion?

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bubba
Can it be done safely? Does the tubular frame under the pillion only serve to support the pillion, splash guard, and license plate assembly, etc? Or does it provide some kind of structural integrity?  Thanks!

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hobbs
There's a lot of methods. Hacksaw, plasma cutter, angle grinder, hand file if you're reallllly patient.
 
On the 07, it looks like it supports the passenger, and to some extent the rider, depending on how much choppin' you're doing.

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gregjet
THIS IS OPINION ONLY>>>
I just had a look. I have chopped the subframes ( even removed completely and replaced them with others) on RACING bikes. Never underestimate how important subframes CAN be on some bikes to the stiffness and handling of the total bike. The MT is not a subframe in the ordinary sense of the word. It is part of the frame total structure so you have to be careful.
I presume you are doing this for the track or to cafe racer mod it?
The MT looks a little complicated when it comes to the loads at the swingarm pivot, but I think you could safely chop it where the bent lower tubes almost meets the top tubes just behind the seat mount plate. You would HAVE to join then some how AND make a cross brace to keep transverse strength. In fact , done right , it would INCREASE the strength of the rear of the frame and stiffness of the swingarm mount.
After looking at it you could cut the 4 tubes straight through so there was a plane formed by the open ends of the 4 tubes and weld a plate to their ends across the frame. ( stainless 3mm perhaps). It would mean removing the whole frame to weld it properly though I would think. If you did it at an angle ( ie sloping plate with the top forwards or backwards relative to the bottom) it would give you more weld length and stiffen the crosspiece even more.
Removing and replacing the whole subframe on this bike would require som serious engineering to ensure you maintained structural integrity.
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bubba
THIS IS OPINION ONLY>>> I just had a look. I have chopped the subframes ( even removed completely and replaced them with others) on RACING bikes. Never underestimate how important subframes CAN be on some bikes to the stiffness and handling of the total bike. The MT is not a subframe in the ordinary sense of the word. It is part of the frame total structure so you have to be careful.
I presume you are doing this for the track or to cafe racer mod it?
The MT looks a little complicated when it comes to the loads at the swingarm pivot, but I think you could safely chop it where the bent lower tubes almost meets the top tubes just behind the seat mount plate. You would HAVE to join then some how AND make a cross brace to keep transverse strength. In fact , done right , it would INCREASE the strength of the rear of the frame and stiffness of the swingarm mount.
After looking at it you could cut the 4 tubes straight through so there was a plane formed by the open ends of the 4 tubes and weld a plate to their ends across the frame. ( stainless 3mm perhaps). It would mean removing the whole frame to weld it properly though I would think. If you did it at an angle ( ie sloping plate with the top forwards or backwards relative to the bottom) it would give you more weld length and stiffen the crosspiece even more.
Removing and replacing the whole subframe on this bike would require som serious engineering to ensure you maintained structural integrity.
Thanks for the info, very helpful indeed! I found this pic and added some arrows and have a few more questions. cut.jpg
 
Yellow is the end point of where the lower tube is welded to the upper one. I think I can safely cut here, no? The upper tubes end shortly after the yellow arrow and are connected together by that big metal plate.
 
My preference would be to cut where the red arrow is but that seems like more work as I would be removing half of the length of the welding on the lower bar and as you said would need to address it somehow.
 
The green arrow points to a threaded hole for use with oem luggage rack. There's an identical hole on the other tube. Instead of welding on a new cross brace, could I bolt a piece of metal to these holes to connect the tubes together and for added strength?
 

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gregjet
Me, I'd go furthur forward than that. Have as little of the parallel tubes as would allow each side to have a gusset and then a back cross piece. But your's will work if you ensure the stiffness of the whole structure afterwards. Remember this structure is subject to loads from much more than one direction AND both parallel and torsional loads. Still, done right, it could be much stronger and stiffer, because the tubes will be shorter.
You still haven't said WHY you are doing it
Yes you could bolt a crosspiece but the load would all be taken on the 2 thin and structurally insecure open box arches. Not real stiff nor strong and likely to fatigue at the lower welds IMHO. Get it properly welded with as much 3d load bearing as possible,
 

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