Chassis softness / hardness

The only real difference in OTK tuning to many brands is the axle stiffness they use. Though BirelART’s axle theory is somewhat similar now.

The rest of tuning is similar to most brands. In simple terms, some karts do their work on turn-in and some do their work at apex and exit. It just depends on tubing stiffness, geometry, components and frame design.

The same theories on how to get a kart to corner apply to most brands universally. But different karts have different quirks that require different adjustments to get to that same goal.

This thought is new to me. Can you expand a little? Can you give me an example of each? Teach me, please!!

Also, can you give an example of a couple chassis?

In VERY general terms, I’ll give my opinion on it based on my experience.

If you go up and down a big grid at a big national event, you will see a variety of kart makes using a front bar, and a variety of karts running no front bar. Some karts are designed so that you always have to run the front bar to make them work (OTK) where other karts are designed so that you rarely run a front bar (CRG sometimes), and then there are karts that can work either way (Merlin). But it seems like there is usually a full design philosophy that leads to a baseline setup of bar or no bar. This probably has to do with tubing materials, specifically the front hoop and it’s stiffness, the waist width, and the location of the bends in the waist, and the mounting location of the torsion bar which all will affect how the the kart transfers weight and twists at load. The Merlin seems to be able to work with or without a front bar, which maybe indicates that the bar location in that kart is less effective than it is in other karts so it isn’t as big of a swing, but that’s a complete guess.

I think this is related to where the kart is working best in the corner. For example, if you take the front bar out of an OTK kart, it turns into a noodle-y mess and will not turn. This is because the OTK is very front positive. It has snappy turn-in, hikes the inside rear wheel high, and feels springy. When I raced the ART GP kart (this was the last brand I ran besides OTK), it felt completely different. We never ran the front bar in this kart, in fact the team told us to throw it away so we wouldn’t be tempted to try it. So this kart had no front bar, but it was still able to turn-in competently. However, it was clearly working differently than the OTK kart was. The front felt very soft and light, and it took a fair bit more wheel input to get it to react. But it was unbelievably good from apex to exit. Once the inside rear wheel came off the ground, the kart held it up and rotated really nicely and shot off the apex like a cannon because it lacked a bunch of the scrub in the front end the OTK has built in.

Two different models, accomplishing the corner in different ways. ART GP did all it’s work in the second phase of the corner, OTK did all it’s work at the turn-in. I think if you understand how your specific chassis is working, you can tune to it’s strengths a bit more. This could be why some people swear that old 32mm shifter chassis work great in 206, but others think that 28-30mm chassis is the only way. Maybe one kart is doing its work somewhere else in the corner, and the driver is driving to that strength and the tuner is tuning to that strength.

This might be more evident and more valid in a faster, softer tire class like X30 too, where you have engine power and tire grip to overcome some of the trade-offs in how the kart is working.

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On its own it doesn’t indicate much, because I didn’t lay out the materials or geometries or loads. The scale is in mm though.

What you could take from it is in the corner entry the most displacement is at the stub C’s. But, i can see more (obviously) and that’s not due to the front bar bending. It’s from twist in the central part of the frame. And this is why adding a bar makes a difference, if I put in that part, the displacement decrease by 40%.

So with reference to what TJ was talking about with front bars, there is an article from Dino Chiesa (KR designer/owner) in which he describes the reasons for using different bars. He effectively says what TJ said. Some chassis are softer so need the bar all the time, some chassis are stiffer and don’t need a bar or use a nylon bar to act as a dampener. I say this is all about the middle of the kart, and not the material, but the width of the waist and the length that it is straight (where the bends are).

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