'73 CB250 - Doing it right this time round.
- Thread starter Green199
- Start date
the side supports that you have planned out are a bit "spindly" and in the engineering sense not quite correct. i am sure you can improve on your design, i have a few suggestions, when attaching a structural gusset or plate it is much better to have it land on the tube one side or the other not down the closest centerline.
the webs should probably be thicker(basicall;y not such a large ammount of material gone in the center) and more tapering at the tube so they plant with a wider "foot" at the tube in fact it would be better as a plate with some cool speed holes take a look at the frame below and see what i mean as far as planting gussets and plates over at the edge not dead down centerline of tubes,and welding it just at the inside joint is fine dont try to fill the big v that would form where the plate hits the tube overwelding is gonna possibly create distortion as well
take great care that you dont weld them on with gaps make sure the fit perfect a gap at the joint can cause excess shrinkage after cooling and before you know it the holes dont line up you cant get the pivot bolt in or out and or the wheels end up out of line
the webs should probably be thicker(basicall;y not such a large ammount of material gone in the center) and more tapering at the tube so they plant with a wider "foot" at the tube in fact it would be better as a plate with some cool speed holes take a look at the frame below and see what i mean as far as planting gussets and plates over at the edge not dead down centerline of tubes,and welding it just at the inside joint is fine dont try to fill the big v that would form where the plate hits the tube overwelding is gonna possibly create distortion as well
take great care that you dont weld them on with gaps make sure the fit perfect a gap at the joint can cause excess shrinkage after cooling and before you know it the holes dont line up you cant get the pivot bolt in or out and or the wheels end up out of line
and depending on how loose the pivot bolt fits in the frame center section it may be wise to verify that the front and rear wheels are riding the same centerrline track after the pieces are fitted tacked-up and before you commit to the final welding, you dont want to be riding crooked ,crab like down the road
xb33bsa said:the side supports that you have planned out are a bit "spindly" and in the engineering sense not quite correct. i am sure you can improve on your design, i have a few suggestions, when attaching a structural gusset or plate it is much better to have it land on the tube one side or the other not down the closest centerline.
the webs should probably be thicker(basicall;y not such a large ammount of material gone in the center) and more tapering at the tube so they plant with a wider "foot" at the tube in fact it would be better as a plate with some cool speed holes take a look at the frame below and see what i mean as far as planting gussets and plates over at the edge not dead down centerline of tubes,and welding it just at the inside joint is fine dont try to fill the big v that would form where the plate hits the tube overwelding is gonna possibly create distortion as well
take great care that you dont weld them on with gaps make sure the fit perfect a gap at the joint can cause excess shrinkage after cooling and before you know it the holes dont line up you cant get the pivot bolt in or out and or the wheels end up out of line
I know what you mean about them looking 'spindly'. After putting the pics up and looking they don't look adequate. I was always thinking about adding some ribs for strength, but still the overall lack of material needs to be improved. I am now considering using the original bolt on plate adapted to be welded in place, with speed holes to reduce weight.
Originally I didn't want to 'blend' the rearset mounts in to the tubes as much and risk any distortion from over welding, but I severely messed the first mount up at the top, and didn't take the time to step back and think things through about starting over. Instead I carried on and kept blending, meaning I had to do the same for the other mount. I guess that's the main regret I have so far about welding them myself as opposed to getting someone who knows exactly what they are doing.
I will check wheel alignment once I weld everything up. I think if I do use the original plate then I will dramatically reduce and chance of the wheels being out of line.
Again, thanks for all your input Xb, it really helps beginners like myself get back on the right track, and without the sarcastic tone a lot of other people take. It is much appreciated.
Back to the drawing board over the swingarm pivot supports. My original idea was to trim the original plate (as shown below), and weld this to the frame.
But having just tested this out, the plates really could do with being removable, as it is a right pain in the ass to get the swinger installed with the supports in place. So back to square one at the minute.
So briefly this morning, I cut out the replacement seat pan (steel compared to the original aluminium one so I can weld it in place) which fits much better than the original, and matches the contour of the seat internals on the rear. This should make the seat itself much more secure when it is bolted on to the pan.
It also has the added benefit of being mounted about 1/2 an inch higher on the frame, so will extend the clearance to my tyre.
But having just tested this out, the plates really could do with being removable, as it is a right pain in the ass to get the swinger installed with the supports in place. So back to square one at the minute.
So briefly this morning, I cut out the replacement seat pan (steel compared to the original aluminium one so I can weld it in place) which fits much better than the original, and matches the contour of the seat internals on the rear. This should make the seat itself much more secure when it is bolted on to the pan.
It also has the added benefit of being mounted about 1/2 an inch higher on the frame, so will extend the clearance to my tyre.
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DohcBikes
Fuck You.
No no. See, XB is one of if not THE most sarcastic sonofabitch on this site. You simply happened to encounter him when he is doing his damndest to hide it.Green199 said:
Within 3 posts of my membership here he had told me I "fucked up" and "good luck, that was a nice bike". No sarcasm there, uh uh.
And what is it with people here telling guys not to get their panties bunched up? That's a baited line if I ever saw one. In person it would not go over. Keyboard Commando favorite.
DohcBikes said:
I don't want this thread to turn into another thread bashing members attitudes. Some people may view XB as very sarcastic, but it hasn't come across when he has been dealing with me. Likewise, to you. As i've seen in a few threads recently, people have that opinion of you. But again, I haven't experienced it and therefore don't have that opinion. I just appreciate more members stopping by and giving advice.
I like your new outboard mount for the swing arm much better than the previous one. This is one of the more important parts of your motorcycle. The swing arm places a pretty significant torsional load on the frame where it attaches (both vertically and horizontally), and the wider this fixturing is the the stronger the whole assembly is. Additionally, there is always lost motion (slop) in this assembly, and the wider it is the less result the lost motion will generate. This is a particularly important location to be mindful of XB's recommendation of attaching the plate to the outside face(s) of the frame tube. Often this sort of thing is done with two thin sheets, one on the inside and the other on the outside of each tube and formed so the meet flat together where the pivot bolt goes - or a box section is made instead and a tube welded in for the pivot. The idea is to not weld on radius line to the tube cross section because the tension/compression loads tend to fatigue the tube resulting in failure. Welding the plate on a line tangent to the tube cross section places a shear load on a larger portion of the tube and results in less trouble. Attaching to both sides of the tube eliminates a twisting component on the tube. You will likely be fine with a single plate , but make it fairly thick, and large enough to spread the load over a decent length of frame tube. Weld it to a tangent line to the tube cross section. Keep it flat if you can, but bending a jog in it so it lines up is better than attaching on a radius line. It is a small part and fairly heavily loaded so choose strength over weight. Don't count too greatly on the center portion of the pivot where it passes through the stamped backbone - it will be doing very little compared to the outboard plates.
As a matter of fact I have - if you consider a race track a road. Though fatigue cracked front down tubes I suppose doesn't qualify as broken in half, the rider certainly considered the bike unrideable. I've seen a lot more issues on tube chassis race cars both from fatigue and being outright over stressed. Very often due directly due to ignorance in understanding very well known engineering principles and general good practice for fabrication. Very often problems that could have been completely avoided with a little knowledge and a fairly subtle alteration in design. There is no question that a lot of incredibly hacked together work would seem to function just fine but that does not mean that anything hacked together will any more than not having seen something means it isn't a concern. Since it takes very little if any additional time or material to do a thoughtful job over a guess, why wouldn't you?
After looking at the options mentioned above, and looking at what I have available, I decided to go down the route I originally planned.
The original plate was tacked in place to check fitment with the swingarm in place, and then welded into place. Due to the plate originally fitting around the frame tube, I trimmed it so that it still slightly wrapped around the tube. This way, the plate is not fitting perpendicular to the frame tube, and is running into a slight tangent.
I also shaped and mocked up some additional tubing which ties the frame tubes and the backbone together. I ran out of welding wire so can't carry on with this til I get another roll.
[Edit] - I don't know why all the photos are rotated now, but if you click to see them larger, they are the correct orientation.
The original plate was tacked in place to check fitment with the swingarm in place, and then welded into place. Due to the plate originally fitting around the frame tube, I trimmed it so that it still slightly wrapped around the tube. This way, the plate is not fitting perpendicular to the frame tube, and is running into a slight tangent.
I also shaped and mocked up some additional tubing which ties the frame tubes and the backbone together. I ran out of welding wire so can't carry on with this til I get another roll.
[Edit] - I don't know why all the photos are rotated now, but if you click to see them larger, they are the correct orientation.
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DohcBikes said:
You are making yourself look foolish again. On the motorcycle I mentioned one of the front down tubes was cracked entirely in two, the other half way or so. No the whole bike was not broken in two. No doubt ridden long enough this machine would have developed much more serious problems. FATIGUE cracks invariably get worse and lead to catastrophic failure as in this case, broken in two qualifies as catastrophic.
jpmobius said:
Just trying to stiffen the frame up as much as possible. I'm not entirely confident on the pressed sections. I plan to add an additional tube linking the brace to 'triangulate' it. Although, this may have to be curved or something to ensure I have enough clearance to the rear tyre.
