Topic: Where to cut the frame?

[mazingerzeca]

Hello all.
I'm planning to cut the frame of my CB750 K7 to install the seat, which will be shorter.
I would like to cut it around the 23 cm mark, but I'm afraid that if I cut that triangle that reinforces the rear shock attachment, it will weaken.
So, the next pooint to cut would be around the 17 cm mark, but I would cut that "bridge" over the two rail frames. I don't know if that "bridge" is critical, if I can cut it or if I should replace it for another one if I cut the frame rails, or maybe it just serves as a base for the standard seat.
If none of these places is safe to cut, I will have to cut the frame around 10 cm.
Where can I cut to be on the safe side:23, 17 or 10?

[seanbarney41]

Just don't

[DaveBarbier]

These frames are weak compared to modern frames so you could cut at the 23cm mark but you’ll need to weld in support. If you don’t weld then go on Craigslist or call up a local tech school to help.

You’ll get a more positive response on DoTheTon because some guys here are less reckless and can’t stand the bike modifications done to these bikes. Totally understandable. But there are many of us here that will cut a frame, there’s just a lower percentage than DTT. Just warning you!

And forgive me but I don’t know how much thought you have into this but for god sakes this is the time to think about rear wheel travel. So many people cut the frame, add a hoop throw a seat on they found on eBay and then realize that the wheel hits the seat when they go over a small bump. That big hump you’re about to cut of is designed for the wheel to raise up into it from the suspension travel. Don’t do another hack job cafe racer.

[Pielz]

+1

[slikwilli420]

As a frame-cutter in my early CB750 years, don't do it. There are a lot of examples on here of people reshaping the stock seat foam and coming up with something really nice, while still keeping the seat lock and functionality there. Too many skateboard seats out there. That said, if you MUST cut it, do it once and do it clean, then make sure when you sell the bike, the hoop comes with it so the next owner can fix your "modifications".

[Erwin83]

I cut my (350f) frame at the 15cm mark and welded in an upswept hoop. As mentioned above, wheel clearance is not to be forgotten, so a flat hoop will not work without other mods (like longer shocks, which will change the stance and ride stability). Cutting half way the factory brace, gives nice extra possibility to weld the hoop to the factory brace, creating a stronger subframe than original, afterall, the hoop itself is also a brace.

[754]

What kind of seat where you thinking of running, that make you think of frame cutting., ?

[mazingerzeca]

Hello. Thanks for the replies. I was not hoping for reckless answers. In fact, I'm conservative too, not only regarding bike's integrity, but safety. This is the Rickman seat that I would like to use. I just would like these rails not to be seen.

[754]

Does the front fit OK ? Ie shock notch is good ?
 How about cut the part showing , 7/8 of the way thru, where it does not show, then bend them  up, would that work? May need to then plug the hole.

[scunny]

Cutting at "10" would be fine.
Personally, I would leave it and mount the turn signals to the ends.

[rotortiller]

I was not hoping for reckless answers.

Cutting the frame behind the shock within reason will not weaken the frame forward of the shock as it is still fully triangulated where it needs it.  I would tie the two sides together after the cut with a hoop etc.to maintain OEM like stiffness. Basically the bike structure will be fine with 9 inches removed as plenty of structure remains behind the shock to support the cafe type seat and your ass. The shock absorber loads are low compared to say swingarm frame twisting moments. I have seen many bikes survive such a cut. Swing arm and wheel loads do not take that load path directly and for the most part travel elsewhere more forward.


I would think that cutting the frame under the gas tank and bolting it back together to be a more risky venture and everyone and their dog that tried it has done it successfully.

The only bad thing for some is that you are reconfiguring the bike from OEM. Me, I say fill your boots as lots of bikes around to destroy and make reborn LOL.