Topic: Best places to brace a frame?

[KB02]

So, I did a search and found a little information, but am still looking for more.

CB750k8.

Looking to brace the frame for less flex and better handling. I have found the the down tubes at the neck could use better welding, and the area around the swing arm could use improvment (Although I couldn't find out the best way to improve it.)

Any other suggestions?

[ColinMc]

My plan with my frame now that I have it stripped down is to do the same thing. Just minor reinforcements here and there and to improve some of the welds...some sections have gaps in the welds...by the steering head and whatnot where the flat metal folds over if that makes sense. I want to complete the weld and put some minor cross bracing in...i'll draw up a diagram of my plans this weekend and post it. I've done this to newer steel and aluminum frames with great success, not sure if doing it to an older frame will have other adverse effects?

[Short Round]

When I get around to my frame after my engine is back together, I plan on taking it down to bare metal and going over all the welds on it to smooth them up.  I am going to try to make them as seamless as pollible before it goes to powdercoating.  Not sure if Im going to brace the frame anywhere but I have not thought of doing that until now. HMMMMMMMMMMMMMMMMMMMMMM............................Chris

[Roach Carver]

is there any concern of warpage with no jig?

[mlinder]

Be careful what you weld up.
Remember that certain places SHOULDN'T be welded.
The corners of gussets, for example.

I was considering doing the same thing, but somebody mentioned that new bracing and gusseting would likely crack and bend our old, fatigued, soft-steeled frames and welds.
Make one area too stiff, and other parts have to flex even more to compensate.

After careful consideration, I realised that only 1 stiffening mod to teh fram could be made without causing too much stress on various joints and frame member...
You'd have to run directly from headstock to where the swingarm bolts up.
Would have to go around the engine. Would make a kickstart unusable. Would be a pain in the ass. But it would really, really stiffen things up, without overstressing any old frame part.
Could make them bolt-on, for removal.
BMW did this on their 1976 r90s superbike racer.
I'm not going to bother. Too much work.

[RRRToolSolutions]

Here is a factory drawing of the stock frame. I personally think the engineers did a good job with it's design. Remember back in 1969 - this was the biggest, highest powered machine made and was over-built (except for the chains). I also know that it is the lightest frame I've ever worked on. With over 20 different restorations behind me, the Honda CB750 frame shocked me with it's light weight. I'm currently doing a 1967 Honda CB450 Bomber and it's frame is 20% heavier than the 750's. The infamous Kawasaki Mach III's and IV's all have much heavier frames and lighter engines, but rumors still surround those old triples - again, I think Honda did a great job.

I do feel they should have made the engine accessable. As is, no work can be done to the valvetrain, cyliderhead, or cylinders without complete removal of the engine. Honda left us 1/2" of space to remove that - as many know - not nearly enough.

Regards,
Gordon

[bwaller]

You can weld the joints on the two front downtubes.(where they're spot welded) Maybe add a brace near the swingarm pivot, but what made the most sense to me was from Gordon Jennings and the Gentlemans Express article where they used reamed fit, oversize engine mounting bolts to allow the engine to act a structural member.

Don't go overboard re-inventing the wheel here. I'm fairly certain if it's used on the street you won't experience enough flex to even notice. If you do all the other right things, bearings, bushings, wheels, tires, forks, shocks you'll have a decent handling ride.

Although it can be a fun project just tweaking a little!

[754]

I got a question re valve cover clearance.

If you bent the tubing and carved out a weldable joint to go on the front part of the tube in question; how high would you have to go up with the tube?? And how close to the weld where the tube meets the frame would you have to start travelling up. I am thinking 60 deg or greater angle.

I am thinking a weld in piece milled out of solid that permits a bend sharp enough to allow this at the front portion, ie the tube would be dificult to bend at a sharp enough radius using conventional bending equipment. The welded in piece or fitting would actually slip into the tubing on both ends, then be welded around and plug welded.

 These fitting would be doable on a CNC mill or maybe manual (way more finishing)

[Crusty_Butt]

is there any concern of warpage with no jig?

Warpage is a major concern, even with a jig. An experienced welder can minimize warpage by skipping around to different spots and welding a little at a time, like following the torque sequence when you are tightening down a cylinder head. A jig can help but if you weld it wrong the frame will just distort when you take it out of the jig.

Bret

[RRRToolSolutions]

Here is a repair kit that let's you fix those missing frame tubes - Gordon

[KB02]

Be careful what you weld up.
Remember that certain places SHOULDN'T be welded.
The corners of gussets, for example.

I was considering doing the same thing, but somebody mentioned that new bracing and gusseting would likely crack and bend our old, fatigued, soft-steeled frames and welds.
Make one area too stiff, and other parts have to flex even more to compensate.

After careful consideration, I realised that only 1 stiffening mod to teh fram could be made without causing too much stress on various joints and frame member...
You'd have to run directly from headstock to where the swingarm bolts up.
Would have to go around the engine. Would make a kickstart unusable. Would be a pain in the ass. But it would really, really stiffen things up, without overstressing any old frame part.
Could make them bolt-on, for removal.
BMW did this on their 1976 r90s superbike racer.
I'm not going to bother. Too much work.

First Part - Good Point
Second Part - I guess I'm not really sure what you mean. So you're suggesting an additional tube running from head to swing arm pivot. Aside from the obvious PITA that would be to create, how would that make the Kickstart unusable?

You're right, it's way too much of a pain to do, so I won't even be thinking of attempting it. I like the engine bolt idea, though. Anyone have dementions on that? Or is there a like to the article?

I got a question re valve cover clearance.

If you bent the tubing and carved out a weldable joint to go on the front part of the tube in question; how high would you have to go up with the tube?? And how close to the weld where the tube meets the frame would you have to start travelling up. I am thinking 60 deg or greater angle.

There are kits available to make these tube removeable. Do a search and you should be able to find them.

[754]

I was thinking going to a little extra effort to weld in a permanant solution to the problem and never have to think about it again.

As far as the engine bolts go that was general knowledge years ago on racers and hot street bikes. ( I was surprised when they talked of straightening the footpeg engine bolt and said using ready rod would be OK!) If you want stiff use rods that fit tighter not looser.

Frames are built to production tolerances and are not neccessarily straight, it would not hurt to check any frame (moreso on a modified one) if you want better handling..

[mlinder]

Be careful what you weld up.
Remember that certain places SHOULDN'T be welded.
The corners of gussets, for example.

I was considering doing the same thing, but somebody mentioned that new bracing and gusseting would likely crack and bend our old, fatigued, soft-steeled frames and welds.
Make one area too stiff, and other parts have to flex even more to compensate.

After careful consideration, I realised that only 1 stiffening mod to teh fram could be made without causing too much stress on various joints and frame member...
You'd have to run directly from headstock to where the swingarm bolts up.
Would have to go around the engine. Would make a kickstart unusable. Would be a pain in the ass. But it would really, really stiffen things up, without overstressing any old frame part.
Could make them bolt-on, for removal.
BMW did this on their 1976 r90s superbike racer.
I'm not going to bother. Too much work.

First Part - Good Point
Second Part - I guess I'm not really sure what you mean. So you're suggesting an additional tube running from head to swing arm pivot. Aside from the obvious PITA that would be to create, how would that make the Kickstart unusable?

You're right, it's way too much of a pain to do, so I won't even be thinking of attempting it. I like the engine bolt idea, though. Anyone have dementions on that? Or is there a like to the article?

I got a question re valve cover clearance.

If you bent the tubing and carved out a weldable joint to go on the front part of the tube in question; how high would you have to go up with the tube?? And how close to the weld where the tube meets the frame would you have to start travelling up. I am thinking 60 deg or greater angle.

There are kits available to make these tube removeable. Do a search and you should be able to find them.

Exactly, from steering head to swingarm pivot, both sides. Go look at your bike. The tube on the right would cross over where the kickstart is. You'd not be able to use it.
The actual fabrication would be pretty simple. It's just the other headaches it would create.

[crazypj]

If you check just about any new sport bike you'll find they use engine as stressed member.
There is no 'top tube' just the side pieces holding steering head and cylinder head holding the sides, preventing them opening like a bow (and a more or less straight line from swing arm pivot to steering head. The rear shock loads are fed into same area
The frame mod will work but wont actually have much effect with the bends in it until you tie the sides together with cross pieces (looks like one above exhaust pipes and another under carbs) When thats done, you have to fix the rear subframe and swing arm as they will still move independent of one another (the 'hinge in the middle' feeling of a 750 pushed into a bend)
PJ

[HondaMan]

If you check just about any new sport bike you'll find they use engine as stressed member.
There is no 'top tube' just the side pieces holding steering head and cylinder head holding the sides, preventing them opening like a bow (and a more or less straight line from swing arm pivot to steering head. The rear shock loads are fed into same area

Yup, CL72/77, Honda 90, etc. Even the 450. The 350 was the first to use the frame by itself.

The frame mod will work but wont actually have much effect with the bends in it until you tie the sides together with cross pieces (looks like one above exhaust pipes and another under carbs) When thats done, you have to fix the rear subframe and swing arm as they will still move independent of one another (the 'hinge in the middle' feeling of a 750 pushed into a bend)
PJ

You're right about that, too. Bracing the forks, improving the wheel and steering head bearings, tightening the (usually worn) swingarm bushings go a long, long way toward removing that "hinge". Also, spoke torque (evenly, high), evenly-seated tires, balanced tires, evenly-damped shocks and forks (air forks, too)...now, it starts to feel like the 130 MPH bike it is! 

[754]

I'll buy you a 6pack or two if you show us a pic of the 450 with engine as stressed member.................................

[turboguzzi]

joining in late but here are my thoughts.....

first have a read here:
http://www.tonyfoale.com/Articles/Frame.mod/KawaMods.htm

then, how hard are you riding? using the whole tire? no chicken strips on the edges? are you grinding footpegs in corners? essentially bracing adds weight so you would want to do it only if frame flex is holding you back in corners. That's assuming that you have very good shocks and sorted out supsension (worn units can cause weave) , you'd have to go pretty fast on a stock SOHC for it to be a problem.

have braced my frame extensively, racing Avon's give extremely high grip but that's for the track and after removing a bunch of other brackets to cut back weight .... 

On the other hand, with these frames being made out of very mild steel, all welding is pretty easy, took all my brackets and frame to a pro TIG guy who did it all for a tenner or two.
TG

[754]

Pretty good simple read.. shows how a rider in an average workshop can actually stiffen a frame themselves..

One think I did not see mentioned yet;

Try to be careful with thickness of gussets and tabs, if you deviate from the thickness of what is on the frame it can cause trouble.. thicker is not better. Can lead to higher concentractions of stress than is desired... and sometimes lead to cracking..

[KB02]

So, this has been a very interesting post so far. That was a great article to read. Makes a lot of sense. Although I can't really understand what that Pyramid thing over the swing arm does on that kawi frame... Oh, well...

[HondaMan]

I'll buy you a 6pack or two if you show us a pic of the 450 with engine as stressed member.................................

I'll try to find one: it was one of the early "black bomber" versions, in a "scrambler" chassis. The forward downtube was just one, bolted to a crosspiece at the lower front of the engine with a skidplate going off of it and underneath. They were seen on dirt tracks in the Midwest in the late 1960s. The lower crankcase (not the cylinders) was the stressed member.

[754]

Ahh..like a Ducati single.. i just could not picture the frame bolting to the head with no front tube.. you are taling factory frame??