Tech Talk | The axle

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THE AXLE

Carlo Forni

17 February 2016

This is the rod that connects the rear tires – plain in appearance, but of paramount importance for chassis set-up and proper functioning conditions

WHAT IS IT?

HOW DOES IT WORK?

IN PRACTICE

The defining feature of kart chassis is that they don’t have shock absorbers. This means that wheels and tires are rigidly connected to the frame. On the rear end of a kart, it’s precisely the axle and the hub assembly that provide a minimum of suspension. Today the axle of KF and KZ karts has to meet a mandatory diameter of 50 mm and can only be a few

millemeters thick, guaranteeing the margin of flexibility required to withstand the weight of kart plus driver and the effect of centrifugal forces.

However, axles can be more or less stiff, depending on the type of steel alloy they are made of and exactly how thick they are.

SINCE KARTS HAVE NEITHER SHOCK ABSORBERS NOR DIFFERENTIAL, THE AXLE PLAYS A KEY ROLE IN KART SET-UP AND TRACK BEHAVIOR

WHAT IS IT?

HOW DOES IT WORK?

IN PRACTICE

AXLE IN GENERAL STIFF SOFT

AXLE IN GENERAL

STIFF

SOFT

The axle connects the wheels to the rear end of the frame and to the engine through the transmission assembly, consisting of gear wheel, chain, and sprocket. It is the axle that provides traction, making the tires spin.
On straight stretches, the axle sits more or less parallel to the ground; while through bends, centrifugal forces and steering angles cause it to tilt, lifting the rear inside tire from the ground (as in the above illustration). It is thanks to this movement that the axle makes up for the absence of the rear differential.

WHAT IS IT?

HOW DOES IT WORK?

IN PRACTICE

Since chassis behavior and set-up vary depending on axle flexibility, it’s interesting to note what happens with axles that have different mechanical properties. Of course, to guarantee top performance, any axle has to have good elasticity, which means the ability to return to a straight shape after flexing. This said, a stiff axle will condition kart set-up needs and rear tire behavior differently than a soft one.

Stiff
All axles generally flex more in the section between the lean-in tire and the outside bearing. A stiff axle will tend to do so in a less pronounced way. As we said, through bends, the axle tilts relative to the ground, so the rear inside lifts off the track, while the outside lean-in tire gets pressed into the ground, under the combined effect of weight and

centrifugal force. Since a stiff axle has limited flexibility, the tire footprint will only be in partial contact with the pavement and will do more “work” on the outside. In addition, since a stiff axle resists the effect of applied vertical forces, these tend do unload moslty on the lean-in tire, releaving the other two tires that touch the ground (the ones up front).
Soft
A softer axle flexes more, so through bends it will tend to keep more parallel to the ground compared to a stiff one, with the resulting effect that the rear inside tire will lift less. Instead, the lean-in tire will work more level to the ground, using the inside portion of the footprint. Finally, since a soft axle “feels” the effect of vertical forces, it distributes them predominantly to the two front tires.