They vary in cross-section shape, construction material, and installation position.
Here is how optional bars condition kart set-up
Removable bars increase frame rigidity in the section where they are installed (the front end, the left side in correspondence to the seat, or the rear end). When removed, instead, the chassis becomes more flexible.
The most common kind in circulation today are made of steel and teflon. Carbon bars – which are lighter, stiffer (because of the higher elastic modulation of carbon) and more expensive – are basically forgotten, in part due to the fact that they are banned in international competitions.
In terms of installation, front bars may be attached in different points for slightly different effects on overall kart set-up. In terms of cross-section, both front and rear bars, steel or teflon, can be circular or flattened at the center. In this last case, technically
speaking, rigidty increases the more the flat section rotates from horizontal to vertical. In practice, though, today flat-section bars are either postitioned horizontally (minimum rigidity) or vertically (maximum rigidity).
Finally, “additional” bars haven’t always been removable. In the past, some karts had the front bar welded to the frame or cut in the middle. In the first case, you couldn’t adjust stiffness, while in the second you did so by central lock-down of the two sections. Bars used to be fixed to compensate for the structural weakness of the joints between bar and chassis, which causes non-uniform flexing overall. Today, manufacturing technicques are so sophisticated as to make this issue non-influential. In fact, almost all chassis on the market come with a removable front bar.
Any time we make the chassis stiffer in a specific section, we add bite to the correspondent tires. This is because the main forces at play, the combined weight of kart plus driver and centrifugal force (through bends), tend to discharge on those wheels that are linked to a more rigid structure.
So, the front bar makes the front end of the kart stiffer and gives the front tires more grip. And, since greater stiffness also means more-responsive vehicle reaction, the front bar will also make the kart faster at corner entry and changes of direction. Of course, the same goes for the rear bar: the rear end will get stiffer and rear tires will gain bite and become more reactive, especially at corner exit.
The reason for increased reactivity is this: when one end of the kart becomes less flexible, the forces that act on the corresponding tires reach their maximum intenstity more quickly. Here’s an example: when trying to move a heavy object by leveraging with a tube, the object will move faster if the tube is metal than if it’s plastic.
This is because the tube made of softer material will have to flex much more before discharging its force onto the object and thus making it move.
Coming back to chassis bars, we yet have to speak of the lateral kind. This goes on the left side of the frame, at seat height, and adds stiffness to the central section of the chassis, causing an overall increase of frame rigidity, rather than having a localized effect on front or rear end torsion. This means that all four tires gain grip, but also that the kart becomes more “edgy”.
Now, a counter-inutitive “paradox”: an a wet track you want to remove all bars, to make the kart softer and so more “malleable” in direction changes. This helps you remain in control of the vehicle, especially coming in and out of corners (due to prolonged understeering in the first case and oversteering in the other in low grip conditions). The reason why adding stiffening bars in wet conditions won’t produce the desired effect of greater grip is that traction is just too low in absolute terms.
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