Kart Weld Quality / Kart Frame Metallurgy

Maintenance, Repair, Equipment & Tools
102

Yes. Emerson and Wilson Fittipaldi know a LOT about racecars. Remember, they built and raced their own machinery long before Emmo got to F1. Look up their twin-engined Beetle that they used to outqualify the Porsches at a sportscar race. It would not surprise me at all if they’d found a really skilled metallurgist and worked with them for decades including on their kart project.

You could absolutely induction-harden an axle under and next to the bearing carriers so that it would be a lot harder to damage at the stress concentrations yet still be able to yield away from there.

103

Yes. They really do operate in the plastic deformation region. Putting a chassis on the table after a single race on a rubbered-in track will show that it is bent. For a shocking demonstration, cone off an “oval” layout using turns from a sprint track the Monday after a race and you’ll see notable permanent set after enough laps to get the tires hot.

104

AMV started rating axles by yield strength a couple of years ago and others have started to catch on. Factory medium axles are 420 MPa.

105

I can 100% confirm that the source of change for different chassis hardness and flex properties (and ultimately performance) is down to the treatment that is done to the tubing prior to welding. 99% of the steel aloy tipology used for the construction of frames is the same amongst everyone, with different prefered suppliers.

The tubing diamater must be mantained to conform to homologation specifications, but the variance in wall thickness along with the treatment the tube receives to yeild the sought properties for a given chassis is not regulated.

I am not a mechanical engineer by trade but my understanding is that there is an inverse relationship between wall thickness and the hardening process that is conducted. Ergo: The thicker the wall is, the softer you can treat the material. Inversly, the thinner the wall is, the harder the treatment must be to mantain performance. As of today, the standard wall thickness for most comercially availble chassis is 2.0 mm, with race chassis varying from 1.4 to 2.0mm depending on the manufacturer.

Keep in mind, the mixing of wall thicknesses and differently heat treated tubes on a frame is common and very experimented with, to vary the end behavioral result depending on what you are efter.

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106

I was under the impression that wallthichness had to be 2mm according to the FIA.

107

We do not specify wall thickness for chassis frames. The only thickness that’s specified within the regs is the steering column’s one :smiling_face:

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108

#1 Any metal specimen that undergoes plastic deformation permanently changes shape. Chassis do bend, but they are not designed to bend. It is a unfortunate side effect of usage. At the top levels, when a chassis bends (changes shape) it goes on the scrap heap (ie…gets sold off…). The whole idea that a chassis is designed to operate in the plastic region is nonsense. If that were true, the mfgs would be selling us all sorts of jigs and fixtures to continuously re-bend our chassis…

#2 Hardness and flex (modulus) are not the same. Hardness (ie the stress level that causes yield) does change with heat treatment. Flex (the modulus of elasticity) does not. If you can prove otherwise, you should be in for all sorts of academic prizes as your discovery will be counter to 100+ years of Engineering and Industry data and experience. Anyone who convinces themselves about stiffness as a function of heat treatment is falling victim to a placebo effect.

#3 The only reason you would heat treat a chassis is to stress relieve the HAZ (heat effected zone) near a weld.

#4 Wall thickness and heat treatment are completely independent of each other.

#5 Proper heat treatment that yields controllable results is quite EXPENSIVE. You have to heat the part with a specified time gradient. Hold for a certain period of time. Quench (cool much more rapidly), which “locks in” the microstructure. This will create a “dead hard”, or max hard condition. How hard the dead hard condition is depends on how much carbon you have in the steel. You then perform ANOTHER heat treatment process called tempering, which takes the edge off the dead hard condition, and makes things less brittle / more ductile and less prone to fatigue. The higher the tempering temperature, the more you increase ductility and toughness while lowering hardness/strength. When you look at the data sheet for a alloy steel suitable for heat treatment, they will include a tempering curve, which shows what tempering temperatures give what hardness. Where you run into issues with 4130 is rapid localized cooling in the HAZ that can create hardness gradients. This can lead to cracks. Based on the cost of a kart chassis, I would conclude that NOT A SINGLE mfg is heat treating their chassis in a manner that gives controllable results. If they were, chassis would cost a LOT more. At most, they are doing a localized temper / anneal (heat up with a OA torch to just under critical temperature, and slow cool) to temper the weld area and reduce hard spots, etc.

#6 Most 30mm chassis tubes are 2.0mm wall. I personally think this is too large and thin, which makes it prone to bending. This reality is probably what leads CK to make his statements about chassis bending. You can actually build a frame from different diameter tube, with a different wall…and achieve THE SAME bending stiffness, but there is a compromise. The basic beam constant is “EI” where “E” is the modulus of elasticity (200-215 Gigapascals for Steel), which we can not change…no matter what anyone claims…and the “I”, which is the Area Moment of Inertia, which we can change. Any and all beam equations contain this “EI” term. If you go to the Engineer’s Edge website, they have a calculator for the Area Moment of Inertia Tube/Pipe Calculator. What you will find, if you plug in some numbers, is that commercially available (in the US) 1.125" x .095" wall tubing has an “I” that calculates to 97.5% of that of 30mm x 2.0 mm ( 1.181" x .078" wall). In other words, you can trade diameter for wall thickness, and get about the same bending stiffness. But there is that compromise I noted previously. That is weight. The smaller thicker tube will weigh more per foot. But here is where it gets interesting. If you actually do a stress analysis (Shigleys Formulas for Stress and Strain), or FEA analysis of the two different sizes, you will find the smaller, thicker, heavier tube can take more load, and be displaced further, for a given bending moment. In other words, if you can afford the weight, going to a smaller diameter but thicker wall will give you a chassis that is more like to hold up over time (not bend) while exhibiting the same flex. So, what you get for your extra weight is “toughness” in a chassis.

#7 If I were mfg kart chassis…I would specify a hotwire TIG process, such as EWMs “TigSpeed”
using the oscillating wire feed. This allows you to put a lot less heat into the weld. It is also very easy to adapt to robotic control. The oscillating hotwire process was created by a Swiss Company called TipTig about 20 years ago, but they never made anything of it because they refused to license the technology and wanted to “do it themselves”. Typical Swiss. The patents appear to have run out, and other mfgs are now incorporating into their high end tig wire feeders.

That is enough Engineering and Mfg tech for today…

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109

So if you chopped up a frame from every manufacturer, and tested each 30mm tube with 2mm wall thickness, they would all flex the exact same?

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110

Yes. Steel is steel.
Simple quasi-static elastic deformation will be similar regardless of heat treatment or metallurgy.

As would the results for ‘soft’ and ‘hard’ axles of the same wall thickness.

Permanant deformation will however ocurr at sub-yield loading levels after adequate load cycling. ie fatigue

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111

Well that’s what my materials science book was saying. Heat treatment has little effect on elasticity. Maybe it’s in the noise for selecting materials for an “engineering” application, but there is enough of a difference to effect a kart frame.

112

Okay, I’m just trying to simplify this because I am not an engineer so equations and laws go over my head.

So essentially heat treating moves the yield of the steel but doesn’t change how it actually flexes given a specific force? But doesn’t that effectively change how the chassis works? If you have a kart that is treated to have a higher yield, it can flex further and still remain elastic? Does that not affect how a kart will handle? You’re changing the kart’s “limits” before it enters the plastic region, no? And doesn’t heating a material affect the arrangement of the molecules? Which would change how a piece of material reacts to physical stimulus?

I’m looking at a hacksaw blade in my shop. The saw blade is springy, it can flex a lot and not deform. But the teeth cannot flex or they wouldn’t cut. So they heat treat the blade’s edge to keep it hard so it can cut. You can physically see where the steel is discolored on the edge from the heat treat. Or an actual metal spring? The steel is heat treated there too to make it springy, or it would immediately deform when it was flexed.

I push back on this because I know people who are designing and building chassis and they tell me the opposite of what’s being said in this thread. And like I said, I have tested a few chassis this year that are the same exact dimensions of tubing but different “heat treat” according to the manufacturer and they feel different. Drastically and noticeably. And I know you’re gonna tell me it’s placebo, but after doing this for over 22 years and having a fair bit of technical experience in tuning and driving, I like to think I’m pretty immune to placebo in this case. It seems like it would be a pretty big waste of time and resources to test these different chassis if the reality is they are actually all the same and the manufacturers are lying to us.

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113

Some points I’d like to make:

Pushing on the saw blade vs. teeth is not a good example. If you push the blade an inch, each part of the blade is undergoing very little deformation - it just adds up incrementally over a long distance. Heat treatment is generally regarded to not change how much force it takes to flex something within the elastic region.

Here’s an excerpt from a mil-spec document. I’ve circled the F_tu (when it cracks), F_ty (when permanently deforms), and E (the ‘flex’). You’ll notice that there’s a huge variation in the “strengths” (F_tu and F_ty), but they’ve slapped one single modulus on here. This NOT the same modulus for all steels, but all steels are close. Different alloys DO have different flex (E), and karts are extremely sensitive to small changes.

Manufacturers are also notoriously dishonest. This is worth keeping in mind.

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114

What about the damping properties of steel with heat treatments? Does that change?

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I have never worked on a project that considered damping for steel in the elastic region, nor do I even know of any material models that consider. There’s a strong possibility that those using that word for kart frames are not using it in a technically correct way. There are other properties of metals that can appear to be damping and do in fact reduce forces, but they are not damping. Torque bars for an impact gun are a good example of this. They reduce impact torque, and I am sure there are people on the internet say they do use through damping, but they don’t.

For background, I am aerospace/mechanical engineer doing high rate structural analysis for a decade. In the plastic (permanent deformation) region, there are strong damping effects for metals, but kart loading rates are relatively low in the grand scheme. All loads going into the chassis are passing through rubber tires and a flabby human, ignoring impacts will solid objects for obvious reasons.

“High rates” for steels are on the order of 1000 inches of deformation in one second for a 1 inch thick part. That’s stuff like bullet impacts, missile hits, etc. Karts would be quasistatic as far as modeling the steel itself.

For metals, the elastic and plastic regions are massively different. Think of a bucket of sand. If you push on it gently enough, none of the grains of sand move around and the sand returns to the original shape. Push hard enough for the grains to shift and your hand will leave an imprint. This is analogous to the grains in steel, and heat treatments change the sizes and compositions of the grains which changes what happens if and when they start shifting.

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116

Damping is so low in steels, that doubling or tripling the damping still results in “low” damping. Steel damping is in the range where it would only be felt over many many cycles (hundreds), such as in oscillating motion, resonance, etc. In other words, it takes many cycles for the energy loss due to damping to add up to where you can observe the effects. Research damped spring systems to understand the most basic form of damped oscillating motion.

That is NOT how a kart chassis loads and unloads. Unless you get a hoppy bouncy thing going…you are talking about ONE cycle. Load…unload. In a low cycle case like this, you will not feel damping till it approaches critical damping. That amount of damping is orders of magnitude greater than what steel has.

This is why CAST IRON is used for machine tools where damping is critical. It’s damping is about 15-20 TIMES that of steel. This is due to the graphite deposits in the microstructure (which is also why CI is so messy to machine).

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117

I’ll expand on this and say that, even over large cycles, external damping is far more dominant than molecular damping in real-world applications. Things like air drag (in the case of the tuning fork) and ancillary components (like the squishy human and sliding tires in a kart situation) wash out any damping you’d see inside the steel itself. Even in a hopping problem, your frequency is orders of magnitude lower than the resonance of the steel-only structures. But it is close to that of a human being bounced around.

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118

Not quite accurate. A quick glance of the technical regulations still has minimum wall thickness for axles and for group 3 karts.

I’m sure there used be some for group 1 and 2 but I guess no longer.

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Yep… 100% agree. My background is in machinery dynamics, vibration analysis, etc. I deal with natural frequency issues in steel rotating machine frames, and the amount of damping in steel is close enough to zero to be considered zero.

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@dodo and @Thomas_Williams, watching you two nerd out on mechanical systems discussion is a hoot. :upside_down_face:
Spent most of my technical focus in combustion and heat transfer before landing in management focused roles. I remember enough to follow your conversation and agree, but that is about all. No way I could articulate some of that information to others. Just haven’t used it much depth since college. :man_shrugging:

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121

My bad Nik, oversight.

Indeed, minimum thickness for Grp 1/2 axles is 1.9mm at 50mm diameter.