Got my CR500AF running since a couple of weeks and I discover a little hairline crack on the front of the frame at the upper Y weld. So we grooved the crack, rewelded it and put a 1/4 plate over it to reinforced.
Looks like it did the job but on the next ride I did at a local MX track the 2 craddle tube had snapped just over the front engine mount, just over the weld. I have read multiple time that welding 6061 can weaken it by as much a 2/3 of its original strenght. But I also know that most of you don't have your frame heat threaten after the welding is done. I have read alot on this forum and the other one and don't remember this beeing a common failure.
What do you think? How would you fix it? I have a couple of idea but let-s hear about yours.
Just looking at it you can see the engine mount bolt has been pulling together...the gap on the inside of the crack is alot narrower then the outsite...supprising when you see how faraway the mounts are from the up rights....alot of filling weld went into that....makes alot of heat Greg, what you recon
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it looks to have a crease on the outside of the tube like a bender mark it does look like a big weave on the weld but i would like to see a bigger photo
KE 336 wrote:Maybe from bending tube in T6 condition or head stay not providing enough support???
I did bent the tube in T6 condition, but the tube cracked in the straight part, not where it was bent so I don't think this is the issue for now. The head stay may not be the best design though.
if I remember, there was little to no tension when I bolted the engine but maybe there was some tension in the frame tube when we welded them.
Greg, I can take other pic, what would you want to see exactly?
craigf40 wrote:Is your motor maybe vibrating alot?
This is the first 500 I have so I can't really compare, it does vibrates but I don't know if this is "a lot" for a 500. All bearing are new in the engine, piston is a Vertex 90mm. Crank was within specs so I didn't mind rebuilding it.
if the headstay is not built right, the motor can vibrate too much in the cradle and fracture the frame above the top frt motor mounts as yours has.
I bought a bike with problem and I am having a new headstay design fabbed up.
I would guess thats due to too much engine movement.
we just finished a crf 450 conversion long story short i had a cr250 frame that i cut the head stay off and welded to the 450 and fabbed head stay plates like a stocker
i would like to see build pic if i could i am no expert and i am not a engineer but im a qualified boilermaker so what i tell you is only my opinion
Good grief, it’s not just the British bikes that snap then.
Just how hard do you ride, are you at pro level doing massive jumps??
Looking at how the metal has fatigued and looks like it crystallised in the break area, it looks like continuous back and forth stress from the engine moving around.
How would you say the engine was aligned in the frame with regards to the swinging arm and chain run?
Did you have to fight the frame at the front to line up the front mounts for welding. By this I mean did you set the alignment at the swingarm and weld up the front where it sat, or did you go for a symmetrical look at the front assuming that to be right thing to do but possibly skewing the engine out of alignment. Just curious as to how you approached the framework and made sure the engine was in the right place.
98cr500rider wrote:
By this I mean did you set the alignment at the swingarm and weld up the front where it sat
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This is what I did. I first put the engine with the swingarm bolt tighten and we tacked the Y, the tube and the 2 lower mount in place. I removed the engine so the welder can finish the welds. Then I put back the engine and fabbed the 2 front mounts and head stay. I bolted the front mount to the engine and he tacked them where they sat. Same with the headstay. I removed the engine another time so he can finish thes weld too. Engine is aligned perfectly I would say. When there is only the swingarm bolt thighten, the engine is "free" (but theres no real gap) in the other mounts.
My headstay might be a problem. It's ugly and it was made quickly with what was available. I took the 2 500 headstay and the 2 450 ones and weld a lenght of sqare tube between each pair. Result is something like shaggy's headstay but thinner with welds instead of billet.
the factory head stay is designed this way so why change it the head stay is a real problem with the 450 conversion and i dont believe that the halo style head stay is really sufficient to hold the engine and stop vibration that is my opinion others will disagree im sure. i have 3d renderings of what i have done if you are interested from what i see you are starting your conversion again. pm me if i can help you
After looking at Gregbro's headstay, I would still be slightly concerned about the length of the mounts. Seems like there would be considerable leverage side to side. I quess the headstay is mainly for the fore to aft movement. The 125 and 250f chassis we did have much shorter headstays. Still the most stock 500 mount on a 450 to date. Good work.
cmotodad wrote:After looking at Gregbro's headstay, I would still be slightly concerned about the length of the mounts. Seems like there would be considerable leverage side to side.
Well, maybe I could do a combination of what I have and what Greg has. Or just something like Greg, I thought about it at first but I was concerned about the lenght of the mount too. What thickness are they? I have some 1/4 flat bar here but maybe I should go 3/8 ?
I will have to move my coil, this is exactly where it is
I believe that head stay is the answer the motor looks like it was moving up and down and it makes sens to me that the head stay even if its long will stop that movement and i don't think there is any need to stop side movement coz there shouldn't be any
Do I get a prize for being the first to suggest a head stay problem?
I considered doing mine the way Gregrobo did also BUT I myself do not care for just a butt weld of aluminum holding something having a pulling force. Not saying that way is wrong, I just don't put a lot of faith in it.
I know honda does it that way but they also have the benefit of knowing exact alloy type and probably have their own grade of filler rod and post weld heat treat. If you think about it, not matter how beefy the support block is, the only thing holding it to the frame is the 1/4" or so of fillet which is a marrige of the two alloys welded together blended with filler.
My which is probably all it is worth.
I'll put my 2 cents in again on this subject- my first frame had a neat hoop design that used the 450 mounts and went around the back of the head.
From the start the bike vibrated too much and the head stay continually cracked. The nipples on the radiators cracked after only a few months.
The next frame I used 125 brackets and cut a block to make everything fit together- less vibration and no problems.
I don't care if you don't trust butt welds, the other way will screw up your bike. Do it the right way.
nmdesertrider wrote:I'll put my 2 cents in again on this subject- my first frame had a neat hoop design that used the 450 mounts and went around the back of the head.
From the start the bike vibrated too much and the head stay continually cracked. The nipples on the radiators cracked after only a few months.
The next frame I used 125 brackets and cut a block to make everything fit together- less vibration and no problems.
I don't care if you don't trust butt welds, the other way will screw up your bike. Do it the right way.
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So those are 125 brackets? Do you know what year? Thay look to have almost the good lenght...
which is probably all it is worth.
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