JFab frame rails breaking

[jholt]

Any one having any issues with lower frame rails breaking on their 500af. Mine broke this weekend while racing motocross, they broke about two inches down from the "Y" section.

[KevinTwoStk]

Do you A/F guys heat treat your frames after welding?

Did the frame rails break at a weld, or close to a bend?

Do you have photos?

I don't have an A/F bike, but I'm curious anyway.

Thanks!

[kxpegger]

Mine was built by Service Honda. Don't know what their process is but I've beat the snot out of mine in the Nevada desert and no issues yet (4 years). I know welding aluminum is a tricky business. The metal has to be cleaned spotless and from what I have read it needs to be pre-heated to avoid bittleness and hardness while welding. There's also the type of aluminum, rod filler and amperage while welding to consider. A lot of welders can make it look pretty without really knowing what they are doing.

[stevea100m]

I bent my own rails out of 6061 if i remember right and had them welded about 6 months ago now. Now Im no A motocrosser but I did have my cousin who is race it a couple of weekends ago at one of the roughest track here in michigan and no problems and Ive rode a few enduros and harescrambles with the same results and I did not heat treat the frame

[jholt]

Did not heat treat they broke right above front motor mount and right below the upper bend I have motocrossed this bike for about 3-4 months

[sandblaster]

Just thinking out loud here.... I'm wondering if when the motor mounts were positioned if perhaps too much of the weight was positioned on the carriage and not enough on the head.
It seems to me that a balance should be maintained.
I'm getting ready to build my engine mounts for my AF and had considered this a possible problem.

[jBernard]

^ i agree with the above.
looks like the weight of the motor tried to torque back and basically pulled the bottom rail section away from the top/yoke.
do you have an upper headstay mount to help distribute some of the weight?

[don46]

I don't know about weight, the motor is positioned through the swingarm bolt, and two sets on the cases, I also wouldn't want the head to be supporting a significant amount of weight as it may cause some distortion to the head causing it to leak. I believe jholt said they broke right above the upper motor mount, so the frame should have been supported at the Y and at the upper mount, to me it would seem that the failure was more of a structural issue, ie vibration, break at the end of a weld or perhaps wrong material. The first one I built I had a crack, re-engineered and no further problems, the second one was like the first redesign and no problems.

[kxpegger]

It looks like your frame rails curve inward and were directly welded to the main spar but it is hard to tell from the pictures. If this is the case I think you lose some structural integrity because it's a longer distance for the frame rails to try and support. More leverage torsion, compression, lateral and strain forces work hardening the metal and causing fatigue stress on the rails. Just my guess? Here's a pic of how SH did it on mine.

[jholt]

here are a couple of pics of the head stay and how the "Y" section was welded together. I did it like the original 08 kx 450 frame was the only difference is its 3 inches longer than stock.

[trim]

On my bike I used the same style headstay mount as yours accept added a extra tab linking the round cross shaft to the lower engine mount bolt holes.The reason for this was looking at it made me think the motor would pull down in the frame with nothing stopping it from rolling forward. Just my .02
If you look on the last page of my build it shows a pic

[kxpegger]

After looking at your pictures I'm convinced that it is your headstay design that put to much stress on the frame rails. It is not a straight tight pull. It is a scissor that will allow forward and down movement of the engine. I would post a pic of how they did my SH but to get a good view I would have to pull the tank. Anyway they welded a very beefy mount to the cross spar of the frame not the perimeter frame tabs that the 450 engine used. The way SH did it there is absolutely no way the engine can rock forward and down unless the head stay of the cylinder head breaks or the frame cross spar or weld breaks. This area was very well over engineered by SH. I'll try to get you a pic tomorrow as it is about 120F degrees in the garage right now and I'm not in the mood to set up the portable swamp cooler. I live in Vegas!

[suicyde]

My brother in law broke his service honda frame as well, he is a pro class mx'er. that af got manhandled, and it didn't hold up. It got patched up, and sold off.

[jBernard]

^ do you know exactly where it broke?
i cant say ive seen a stock frame rail ever brake. obviously they are forming them in a less stressful way and probably heat treating after welding. something to consider since i'm surprised no one has gotten injured yet. pretty scary.

[KXDINO]

The tubing looks kind of thin does it not? I like what ktm does,4130 and lighter  than any aluminum jap frame out there;plus there more room to work on where carby goes etc.