THE RIGHT RIM TO MOUNT ON THE KART BASED ON THE GRIP OF THE TRACK
MATERIALS AND SHAPES OF RIMS CHANGE THE KART’S SET UP AND AFFECT RUBBER TEMPERATURES
Rims greatly affect the performance of a kart, both in terms of lap time and during an entire race. This is due to the fact that their materials and shapes affect the temperature of the tyres, which in turn immediately and directly affects the performance of the kart. Alberto Viglino, the engineer responsible for research and development for the Tony Kart Racing Team, explains which OTK rims should be used in different track grip conditions.
1 Let's start with a "classic" differentiation: rubber track or track with little grip. What rim should be chosen?
The MXL model with the wheel track is usually used: this regards a magnesium rim, which allows you to keep the temperature of the tyres as homogeneous as possible during a race. With very low temperatures and a track with little grip, the choice falls on the MXJ model: it is a rim always made of magnesium but made with a different casting process compared to the MXL which allows you to raise tyre temperatures. This characteristic increases the speed with which the tread reaches an ideal operating temperature, even with particularly cold asphalt.
2 Another "classic" question: what rims do you recommend on wet tracks? Magnesium or aluminium?
The peculiarity of magnesium is its ability to dissipate heat and, therefore, keep the rubber and air contained inside it cooler
Vice versa, aluminium is a material that tends to increase the temperature of the air between the tyre and the rim, thereby increasing the rubber’s pressure and temperature. What, with cold and rain, helps trigger grip on asphalt. With a wet track the choice therefore falls on the AXP model, the OTK aluminium rim
3 OK. But, in practical terms, when you get to a track, what are the parameters to check to decide which rims you should mount?
Well, if it's raining, as we've just mentioned, choose aluminium. That's an easy one. However, one dry tracks, one of the most difficult choices is guessing which rims to use. The first parameter to be evaluated is the temperature of the asphalt: high values require rims that generate less grip. Basically, asphalt at over 40°C may tend to require magnesium rims.
However, another important thing to consider is the condition of the track: for example, if it has been raining in recent days, the track is usually not in optimal condition even with high asphalt temperatures, and this makes it better to start testing the kart with medium grip rims. The same goes with tracks that are very slippery because few karts have been on the track or, worse, because of wind has brought sand onto the asphalt.
Having evaluated all this and decided, a priori, which rim to mount, the next step is putting the choice to the test on the track. And this often proves ... that we haven't understood anything!
The difference in tyre pressure before and after a track session is a good indicator to understand if the kart slips and, therefore, if the choice of rims was not the right one
4 What factors show you that you have not chosen the right rim?
In addition to feedback from the driver, the most important factors to consider are the visual appearance of the rubber and the difference in pressure before and after the test. Let's start from here: if, back in the pits, the difference in pressure compared to before the test is over 0.25 - 0.30 bar, for magnesium rims, or over 0.30 - 0.35 bar for aluminium rims, this means that the kart is slippery. In fact, tyres slip when they heat up too much. They transmit heat to the air contained between the rim and the tyre which, in turn, generates an increase in pressure. The higher the pressure difference, the more it tends to mean that the kart slips. Even tread temperature is important, but in this case many factors affect it (the axle, type of seat and its position, etc.) and much depends on the type of rubber used.
However, the simplest thing is to look at the surfaces of the tyres: if they are all dry and little "sticky", this basically means that the kart slips and, therefore, it is necessary to mount rims that generate more grip. If, on the other hand, the tyres look "greasy" and have a darker colour, the grip should be correct. The last possibility is that the tyres are ruined only at the front or only at the rear: this means that the kart is unbalanced, but, in this case, the problem, more than the rims, has to be resolved by working on other components of the chassis (axle , hubs, camber and caster settings, etc.).
5 How many rim models are there in the OTK range? And what are the main differences between them?
There are four different types of rims for single-speed and KZ karts:
MXC: it is the oldest model in the product range. They are made of magnesium.
MXL: also made of magnesium, they differ from the MXC model only in relation to the profile, designed to reduce the amount of air between the rim and the rubber.
MXJ: also a magnesium rim. As mentioned before, it is made with a different process, called the "near melted" process. The use of these rims is recommended in case of low temperatures, or with types of hard tyres, since their characteristic is to quickly reach high temperatures, allowing the rubber to quickly reach the tight temperature and give maximum grip.
6 Apart from the materials, the substantial difference concerns the profile of the rims: how does this affect a kart’s performance?
During a race, the tyre temperature is one of the most important variables to keep under control regarding the performance of a kart. This is also influenced by the amount of air inside the rim: the more there is, the more, with the hot rubber and an increase in volume, the difference in pressure compared to the initial pressure will be significant Because of this, the kart setup will change more during the race.
The volume of air between the rubber and the rim is substantially defined by the rim's profile: a low air rim allows greater control of temperatures, and the MXL model has been studied by OTK to have the least possible amount of air between the rubber and rim.
However, in some conditions, the MXC rim may be the right choice when you need to increase the temperature of the tyres.
OTK has been developing low-energy rims for several years, also based on the advice of Michael Schumacher, bringing F1 rim technology to the kart world
7 How long does a rim last on average?
If the rim is not subject to blows, it is an accessory that lasts for a long time, so I would say that an amateur pilot does not have to worry about replacing them very often. Obviously, there are some precautions to keep them in excellent condition, for example, to avoid leaving them fitted with deflated tyres, since in the long run they can become oval.
The life of the rims also depends on the material: aluminium is less fragile, so in case of collisions it deforms but is difficult to break.On the other hand, magnesium often fractures.
8 How do you know if the rim is wrong? And, in case, can it be repaired?
Warming a kart engine on the trolley is a good idea, at least for the rear tyres, to see if the rim vibrates, which can sound an alarm.
Once you have determined that the rim, and not the axle, is at fault, it should be replaced.
9 Rims are connected to the kart through the hubs. Can we also say something about the hubs? How can the set-up be changed using different hubs of different sizes and made of different materials?
OTK hubs, for the OKJ, OK, KZ and KZ2 classes are made of magnesium and there are different sizes: 77, 92, 110 and 148 mm.
The shortest hubs, of 77 mm, are usually used in the OK and OKJ classes under extreme grip conditions: this configuration is the best for freeing up the set-up and helping a kart come out of bends faster.
92 mm hubs are the ideal choice, again in OKJ and OK, in medium grip conditions. We can say that this is the length we start with because, in the relationship between front and rear axle, it is the length that ensures a better set-up.
However, in geared classes, a length of 110 mm is generally preferred, always for the same reasons of better adaptation to medium grip conditions.
The longest hubs, 148 mm long, are ideal in conditions of poor grip and are mounted, in KZ, when there is a need to increase traction.
