OTK are so prevalent so it appears that they break all the time.
The metallurgy matters. There are other contributors here that could provide more insight.
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I imagine most Euro manufacturers have specific alloys of steel they are developing and using, and they arenāt just using 4130 CrMo like all their websites claim.
Like when Intrepid tried that stainless or whatever it was chassis.
Iāve heard from people who have built their own karts that theyāve tried making a straight up 4130 or mild steel chassis and it handled like garbage. Like it wouldāve needed a big redesign to function well with that material.
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100% 4130 is a-ok. Running on par with Eagles, MGMs, OTKs, and Birels. 
Went through a few rear bar changes and baseline setup changes to figure it out, but weāve stayed 4130 the entire time.
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Iāve heard from some guys in chassis R&D that even within 4130 you can have annealed vs non-annealed and that makes a significant difference in the tubing also. Not sure what youāre using, but as long as it works. I think anymore itās not so much about having outright speed, but having a wide tuning range and not a peaky kart through a run. Iāve also been told that most of the European karts arenāt technically on the scale of chromoly, that they have higher carbon and chromium parts compared to mild steel but not enough to be on the chromoly scale 
. All very secretive on kart composition, but Iāve heard from guys that had tubing analyzed from different karts.
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OTK isnāt higher carbon. Itās lower. Itās definitely a chromoly classified tubing though. Iāll pull the sheet and post it here in a bit when I get a moment.
Annealed tubing isnāt a thing for 4130 that Iām aware of unless you have the resources to do the annealing yourself. I could be wrong but Iāve never seen it commercially available. Annealed and normalized typically pertains to sheet.
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Yeah youāre right, 4130 annealed is definitely not readily available in the sizes required for karts. I have heard of various alloys tried in karts using annealed vs normalized. āDocolā is another one Iāve heard of guys trying in the states.
The chassis welds are Mig from factory, so it canāt be very high Cr content. Base on the Mig, that further reinforces my initial though that it was temperate control combined with high loading that resulted in our cracks.
Today we are at SpeedSportz shaking down the repairs and trying to figure out how the locals get around this place so quick. Based on pace thus far it looks like his streak of podiums will end at 3 in a row. 
Couple pics of the Tig repair. Friend of a friend of my dadā¦yeah, one of those deals. 
Chassis welding background, current aviation welder did the work for us. He worked out the HAZ at the weld/base interface, cleaned up a spot that I screwed up taking off powder coat, and the re-welded. Took lots of time and let stuff cool between repositions or new passes. Pretty pleased with results. Hopefully the lower temperature weld process helps.
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Docol is what Iāve been told the Coyote Zenith is made of. Itās on my list to test early next year.
OTK tube is is on the high side of the Cr range of 4130.
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Fe 97.24
C .22
Mn .68
Cr 1.05
Ni .10
Mo .20
P .011
S .001
SƬ .24
Cu .15
Iām aware thatās not 100% but itās the lab result I got back.
4130 is a range:
Iron, Fe 97.03 ā 98.22
Chromium, Cr 0.80 ā 1.10
Manganese, Mn 0.40 ā 0.60
Carbon, C 0.280 ā 0.330
Silicon, Si 0.15 ā 0.30
Molybdenum, Mo 0.15 ā 0.25
Sulfur, S 0.040
Phosphorous, P 0.035
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I wonder how Big difference there is from the different brands of karts,
Welding steel like this dont that require preheat and slow cooldown to be sure it lasts?
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You donāt need to preheat because our tubing is so thin.
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Curious, is anyone using laser welding with kart tubing?
From what Iāve read about it should be a good fit for tubing and create very high quality joints.
No, it shouldnāt require preheat. Controlling heat and/or PWHT might still be a good idea. The OTK welds were Mig, which is going to be more difficult to control heat on than Tig. I have no idea what the PWHT practice is, if any for OTK.
So with the Tig repair, we effectively took out the HAZ area at the cracks, and the HAZ area on the other side of the bead that didnāt crack. Then made lots of passes with the Tig, being slow and methodical between passes and preventing lots of heat buildup. Spent 45-60 minutes making those two tiny weld repairs, just trying to control the heat a little.
I have Zero Lead on my kart. Just my oversized A$$ in the seat. Mine has broken twice just below where it bolts up to the seat. The first guy I had weld it did it in just a few minutes and was heating the weld after finished. I understood that is a big no no with chrome-moly. Second time a different guy took quite a bit of time and the weld was much prettier. Looked like a layered stack of dimes vs the buggered mess from the first guy.
I know almost nothing about metallurgy or welding other than what I have learned from the interwebs, but I do understand that the temp you are fusing metals makes a big difference in the joint strength. There was a YouTube video I saw a couple of years ago talking about joint failure on ChM tubing on drag cars I found very interesting. Its a bit lengthy, but worth the watch if you are into those kind of things.
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When i have our chassis repaired i always get them tigged, it seems to last compared to the few times i had something mig welded.
I dont think any of the chassis manufactors do much In controlling heat or cool down for that matter.
It has been claimed that Birel uses tubing that has been heat treated if this is true i dont know. Or if any of the other is doing it.
I just had the right side seat mount weld break when testing without seat struts. This was a brand new Redspeed at the beginning of this year. No major wrecks or straightening done. Guess the old fat guys like me are too much weight in the seat up high.
If this point of failure is so universal why arenāt we looking at replaceable secondary seat struts?
Imagine a system wherein one can unthread the broken strut and replace or something.
I donāt mean the clamps that are used in a pinch, something better engineered for more permanence but with understanding it will fail repeatedly.
Has such an approach been tried?
Yes, by Ikart.
On the regulatory side of things, I recall I had some preliminary conversations on homologating similar systems, and hell no was the answer. So there may be some safety or some other reason behind e.g. lateral impacts
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Some of the dirt chassis out there have adjustable seat posts that are not welded to the frame. That is what we rigged on our broken seat at the track. It wasnāt a perfect fit for our kart, but with no other option we made it work!
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That lines up with the results we got from testing out here in California; TonyKarts are made from 4125.
Iām not sure why youād use an alloy steel and not heat-treat it for the extra strength, or why youād use a chrome-moly steel and GMAW rather than GTAW and end up with brittle welds, or why youād go all the way down to 0.22% carbon and lose the whole strength advantage over mild steel, but OTK sells plenty of karts. I can point to some advantages of a GTAW 4130 frame but Iām not going to advertise on your thread.
If I was having tube made at a mill for a special run I have a reasonably inexpensive alloy in mind, but even Billy thinks Iām crazy for wanting 350+ KSI ultimate strength in the joints.