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Improving the set-up by changing the axle, advice from Albino Parolin
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With karts, the total absence of suspension means that the rear axle acts as a shock absorber (find out more about its function by reading the article dedicated to this element of a kart in the "Technique" section). The different hardnesses and thicknesses available change the set-up of the kart and the behaviour of the vehicle on the track. The axle is a component that can generate smoothness and traction according to the needs of the different classes. With Albino Parolin, owner of Parolin Racing Kart, we discover how to decide which axle to choose and how the hubs and bearing shells affect its bending.
N.B. Although the indications and technical advice provided by our interviewee may be useful for deciding how to manage the setup of any kart, they are particularly specific to Parolin Racing Kart chassis.
There are different axle hardnesses. With our racing team we usually work with four: hard, medium, soft and very soft. The medium hard axle is fitted as standard when starting to run at a race weekend and the track is not rubberized. The choice of the medium axle is the correct solution, for all classes and all types of tyres used, in the first rounds of tests. After a day of practice sessions, when the track begins to rubberize, you can think about whether or not to move to a softer or a harder axle to improve the kart's set-up. It’s fair to say that there is no universal rule for all chassis when choosing the hardness of the axle; it depends on the structure of the body, in other words the bending capacity of the same. A “soft” chassis, which bends easily, is often paired with a harder axle; vice versa on a more “rigid” chassis, with less flex, a softer axle is mounted to balance the structure.
To define the correct axle, divide the bend into three parts: entry, middle and exit. This will allow you to evaluate where to intervene to improve the setup.
A hard axle can give better grip when entering bends, while in the middle of a bend its stiffness benefits the kart's smoothness. Finally, when exiting bends, traction increases.
A soft axle gives less grip in the first part of the bend, in the middle of a bend it “binds” more - that is, it increases the grip provided by the rubber - while it reduces traction when coming out of bends.
If a kart with the medium axle understeers, that is, when entering a bend, there is a loss of grip at the front compared to the rear with a consequent lack of directionality of the kart; the advice is to mount a soft axle. The effect you get is a minor push from the rear in the first part of the bend (entry); this will allow the correct front grip and the optimal setting of the trajectory. Conversely, if we have too much front entry, thereby oversteering when entering bends, it means that you have a rear with little grip and you can use a harder axle that will help you generate more grip from the rear when entering bends. Changing the grip at the rear by changing the axle is an activity that must always be carried out after changing the geometry at the front. In the event of understeering when entering bends, the caster angle must be loaded by at least 2 degrees to increase the grip of the front axle. If you have oversteering when entering, decreasing the caster angle will cause a loss of grip at the front, giving a more balanced set-up.
In the single-speed classes, a kart set-up is sought that rewards the smoothness of the kart in the middle of a bend and coming out of bends. The driver will have to set an entry trajectory that avoids "getting stuck" in the middle of the bend with the consequent decrease in engine rpm. Basically, you need to have a bend that allows you to exploit the torque of the engine in the exiting phase; to do this you can mount a more rigid axle. A rigid axle, compared to the soft one, "binds" less in the middle of the bend and allows smoothness in the middle of the bend. In the Junior classes, where the minimum weight is 140 kg and the axle is less stressed, you can avoid fastening the central bearing shell to the chassis for greater bending. This will allow the mounting a hard axle (more smoothness), but at the same time it will not cause the rear to lose grip when the kart is in the middle of a bend. This solution should be used on tracks which have a series of fast bends and a balance is sought between smoothness and grip in the middle of a bend.
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