Topic: CB750 K8 Build

[calj737]

Reconstructing 40 year old manufacturing techniques has little practicality. If this approach were still prudent, it would still be in use. These frames are notoriously flimsy and the metal weld used were bulk manufacturing techniques. Simple as that.

s for 1/4" or greater plate, as I said, bevel it and then you get full penetration of a 2x edge against a thin tube. The purpose of using thicker than original plate is to provide stronger load bearing support against the remainder of stock thin tubing.

As I said, your bike, your neck. In my opinion these types of choices are exactly what future owners point out when abasing POs and their hack jobs. I guess I would rather have something very strong than maybe strong enough. And I build to "better than" stock specifications. Guess I'm funny that way.

[firebane]

Reconstructing 40 year old manufacturing techniques has little practicality. If this approach were still prudent, it would still be in use. These frames are notoriously flimsy and the metal weld used were bulk manufacturing techniques. Simple as that.

s for 1/4" or greater plate, as I said, bevel it and then you get full penetration of a 2x edge against a thin tube. The purpose of using thicker than original plate is to provide stronger load bearing support against the remainder of stock thin tubing.

As I said, your bike, your neck. In my opinion these types of choices are exactly what future owners point out when abasing POs and their hack jobs. I guess I would rather have something very strong than maybe strong enough. And I build to "better than" stock specifications. Guess I'm funny that way.

I'm not here to create an argument or have one and I am listening to your valid points. But if a 40 year old bike with 40 year welds on 14-16g sheet metal is used for shock mounts and it was that much of a safety concern we would see a lot more 40 year old bikes with broken shock mount plates or shock mount issues. When generally the only shock mount issues I've seen is the thread bolt part gets broken off due to rust or over tightening.

Like I said earlier adding 1/2" of steel to support the shocks when there isn't a shred of structural support on the bike made from 1/4" just seems silly. You would be adding 1" of extra material onto the back of the bike.

If you can show me any instances where stock setup has failed in the shock area I would be glad to see it but I would say that it was engineered well and as long as its done the same way again it should be right back to where it is.

I will heed your warnings and your feedback and thoughts and take it into consideration.

[calj737]

I never told you to use 1/2", I said 1/4"-3/8". And the reason is simple, it's about the ratio of metal to metal when welding thin tubing to thicker metal. I'll give you an example, then I'll give you my "rationale" for my position.

Your tubing is 0.060. If you use 1/4" plate, bevel 1/16" from each edge. That leaves 1/8" landing area. That is 2x the thickness of the tubing. When you weld something that is 0.060 thick, you can not get any penetration without burning through. Using 1/4" (or more) allows heavy penetration against the plate, with a tie-in to the tubing. This is very strong and allows the plate to support the tubing.

Welding 0.060 wall to 0.060 sheet gives you nothing to tie to. That weld will be very weak. In fact, depending upon how it's done, it will either be cold, or crack adjacent, and likely fail any structural stress test. The original was a stamped gusset welded on an assembly line and tied into the rails where you have cut it.

I literally just did this same modification 3 weeks ago for a friend of my son, on his KZ400. When I showed him the sample pieces (welding my approach versus the stock gussets) after saw cuts and stress tests, he was shocked (pardon the pun) at the weakness in the gussets. Had I known this topic would surface, I would have taken pictures as they would now be quite informative to the point.

Now, why do I harp on this? A few years back I was fortunate enough to be introduced to a local guy who for 28+ years has been building spec race car tube chassis's for NASCAR, SCCA, etc. The guy is a prominent builder. He was kind enough to make a subframe for me because a mutual friend took me to him. It was he, that explained the importance of suspension gussets and the flaw in the stamped gussets. So I follow his professional advice on frame mods, bracing, and material thicknesses without question as he is the expert.

I'll say it one more time; it's your bike, do as you wish and as you are comfortable. When I see something that I strongly believe is not prudent or sound, I'm going to speak up. Good luck with your project.

[754]

I could be wrong here but generally welding a 4x thicker piece to another can create a place for stresses to concentrate, and that can lead to breakage.
 That is why they use doublers and sometimes layers.
 Look at the way the sidestand mount is welded to a 750 frame...do they just weld a heavy piece on ? No they double it up and weld the thick piece to one or both of the reinforcement pieces.
 I have not read thru all of this, but would like to see these 1/4 mounts compared to the stock boxed in section.
 What I should mention if I were to relocate shock mounts, if possible I would machine the two mounts out of one piece of bar, and part the material most of the way thru..where they would get cut off . Then install the whole assemble if possible, keeping everything in plane.. Then tack or finish weld..then cut the centre part out.

[firebane]

Someone decided to want my bike as is so I sold the 750 and that saga ends but a new saga begins as I now own a 76 cb550f