Kart Chassis Tubing Diameter\Durability Discussion (From Pro Tour Discussion)

I’ve considered running a 32mm chassis for TAG just due to my build. While I can get down to 360lbs for KA/X30/Rotax I’m just broad up top and tend to transfer alot of weight around.

Might be why I’m always popping seat struts and baseline OTK settings never work for me.

Splitting this one off since I think it deserves it’s own topic.

I ran two seasons with a roadrebel (32/32 ) - the frame felt stiff. I was always trying to make the rear grip. I had most success by fitting another sniper kit at the front and taking all the caster out. Hard rear axle with widest rear hubs also helped. Fitting a nylon front bar made the front feel like I could apply more pressure to the fronts than running without one. Fitting a round steel tube made the front very hard - made the kart unstable to cut any curbs

Now my current ride is 2022 kt5 crg (30/32). Very different kart to rr. Softer ride for sure. I like driving it. Tons and tons of rear grip. I havent had to reduce the rear width once. I always did that with the roadrebel to maintain rear grip level as the rear tyres wore. With the kt5 I have tried various ways to reduce the rear grip. Kt5 feels like it bends more in corners and stores/releases energy more easily than rr. Front end doesnt feel that much different with or without stabilizer bar. Running over the curbs with a round steel stabilizer doesnt cause much issues. Feels weird as rr was so sensitive about it

What’s your height/weight? Light build I assume?

So it appears that yield (plastic deformation) is the largest problem with current kart chassis. I agree with the report then that using a different tubing material such as Docol R8 over 4130 makes sense for long-life kart frames. Not only would it last much longer, the welds do not over-harden the tubing making it crack at seams.

When I order a custom kart from Margay I’ll have it made with a better material such as this. For now I’ll just modify the old CRG 32mm one I have with extra bracing for torque.

I’ve ran a Docol kart chasiss. No bueno. They don’t flex right.

About 80kg, 176cm. My weight is in my upper torso. I have no leg muscles - I used to be a gym rat who always skipped leg days. This summer started to run and bicycle more to have some stamina to drive a shifter. Didnt do much cardio exercising during the last winter - big mistake. I was so out of shape when the season started. Ran out of steam fast at the track

Thanks for the feedback.

I plan to do that as I’m not concerned about flex as I only plan to use it for road racing, not sprint.

Of course possibly they used too thick wall tubing. I thought this quote was good:

“Yeah, the NHRA went Docol, probably 8-9 years ago with all the top-fuel cars. Because all the chromoly cars were breaking in half. In a top fuel car, the frame is the suspension. The frame is the suspension. There is no suspension on those cars. They were flexing so much to make traction, they were breaking in half.”

“They stopped it. Docol will not break. I mean, it will not break. But, it is expensive.”

It’s just stiffer?

“Well, to make something flex, you gotta thin it down. When you thin it down, then it flexes and it cracks and it breaks, then you wreck.”

“This Docol, you can get it pretty darn thin. It flexes, it’s got memory, it comes back, it doesn’t break. It’s badass.”

My Coyote Zenith is docol. I switched from a birel and was instantly faster.

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The important part I can’t stress enough is that material is just one of the variables. You have diameter, section, material, how it’s bent (e.g. what type of bending machine is used by the manufacturer), welding tech (mig vs tig), machine vs handwelding, who is the welder in question, heat treatment, design, setup etc…saying Docol is better than 4130 is the same as saying TIG is better than MIG or 30 is better than 32…I’m sure you all experienced a combination that worked great for you in that particular set of circumstances, that’s absolutely great so try to stick with it as you’ve hit jackpot, but doesn’t mean the same combo will work for everybody else, all the time

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High strength steels that won’t plastic deform for the same weight and are less likely to crack at welds are better than lower strength steels like 4130 that do - unless you are a frame manufacturer.

There is nothing in their interest to improve the product by using better materials when it means this will decrease sales. However, it is very much in the interest of the consumer to be able to buy a product that will last much longer. The only way to force this change is probably down to regulation. But I suspect that like with anything that involves money, manufacturers will pad the pockets of regulators rather than spend R&D money to innovate.

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Wouldn’t disagree with any of that but the report seems to be open to a lot of criticism.
In effect we get “people say frames go off after half a season” but this isn’t supported by any testing. Surely the new OTK frame should have been driven for the equivalent of half a season’s use to demonstrate this fall off?
The report then debunks aĺl the usual theories as to why the chassis goes off and comes to the conclusion that it is due to the chassis getting bent in use.
As I understand it at any level frames are on and off straightening rigs every few minutes which according to the conclusions of the report should restore the frame to good as new condition and this 'straightening 'can be repeated indefinitely?
Further the 'replica ’ frame built out of the sponsors material manages, in the hands of one driver at one track, to complete ‘the equivalent of a full season’s racing’? without fall off in perfornance .Without comparative testing there is no proof that the OTK chassis would not have done the same. Perhaps I need to read it again?

If the frame has been bent (tubes have had plastic deformation), it will never be the same. Straightening a bent frame makes it weaker, and it will bend easier the next time.
What it is important is how much you can stress the material before plastic deformation occurs, so that it springs back to shape after the load is removed. A stronger material will take more load and not deform permanently, allowing it to remain “like new”.

My only comment would be to please not get hung up on chassis outer tubing diameter per se as a fundamental metric on flex characteristics. I understand how it is logical to conclude 32 > 30 so = stiffer, but it is not always the case

There are very stiff 30mm tubes, and incredibly soft 32mm tubes. It is possible (and it actually already happens) to have a full 32mm chassis that deforms/flexes a lot more than a full 30mm counterpart.

Tube quality, along with chassis geometry and application purposes define the final torsional characteristics for chassis tube choices.

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Nice and brave report on a difficult subject but…

…It is hard to believe that the replicant Docol kart was as fast as the TonyKart. There is more to chassis dynamics than just making the static stiffness the same. There is the dynamic response (damping properties) that needs to be considered. I’d compare kart chassis response theory to be similar to axle response theory. Hence, much as an axle, I’d expect the response of the chassis to be affected by the hardness of the material.

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In the linear range below yielding of the steel, what’ll influence the torsional stiffness of a 2-rail steel frame is:

  1. Lateral spacing between the tubes - narrower is more flexible, and this is a property of distance SQUARED.
  2. Tube diameter
  3. Tube wall thickness

The parallel-axis-theorem contribution dominates, all that really matters from 2 and 3 is the total cross-sectional area of the tube. It is very easy to build a 30 mm x 2mm chassis as stiff as a 32 mm x 2 mm chassis, all you need is to make the waist 5% wider.

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I’ve been waiting for you to confirm my thinking :slight_smile:

There’s a fine line between “useful advertising for Factory Karts” and “giving away Factory Karts engineering information on a public forum before the karts that use it even go on sale” and I’m not sure how much more Willy would want me to say.

There are other areas that experience flex, it’s just more prominent in the chassis “waist”. Still, a tube can be made stiffer just by increasing its diameter, & relative to a smaller diameter tube (& same material properties), a larger one can have comparable rigidity with thinner walls.

No one will believe you anyway so I wouldn’t worry about it.