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TKART magazine Special | Guide to purchasing OTK Kart Group rims (for Tony Kart, Kosmic kart, LN Racing kart, Exprit…)
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GUIDE TO PURCHASING OTK KART GROUP RIMS (FOR TONY KART, KOSMIC KART, LN RACING KART, EXPRIT…)

Jacopo Colombo
21 November 2023 • 23 min. read
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A range of 7 products designed for Junior/Senior Racing karts (both Shifter and Direct-Drive), MINIs and also the Rental scene. We explain how the Italian company’s rims are produced, the construction decisions made by its R&D department and which model to choose based on the weather conditions and the grip out on the track.

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THE COMPANY

OTK Kart Group – which includes the brands Tony Kart, Kosmic Kart, Exprit Kart, LN Racing Kart, Gillard Kart, EOS Kart, Redspeed Kart - is the leading manufacturer of chassis and components for Racing Karts (but also produces Rental karts, Viper Kart). As regards the production of rims, OTK Kart Group manages the design and manufacturing phases of its products entirely "in-house". The only parts delegated to external suppliers are the casting of the rough components and everything concerning the final treatment processes. However, both processes are carried out according to the design rules and production standards established by OTK Kart Group itself.

Design

The R&D department of OTK Kart Group designs every part of the karts it sells and, therefore, all rim models found in the price lists of the seven brands it possesses, led by Tony Kart. It always starts with finalizing the various projects (technical drawings) created using CAD (Computer-Aided Design) and "Creo" software in 2D and 3D, in order to evaluate different options that meet the needs of rim behavior and performance out on the kart track that the company has set out to achieve. Once the most convincing alternatives have been selected, there’s the production of prototypes using 3D printers. During this phase, both complete and (for magnesium models, which, unlike aluminum ones, have variable thicknesses) sectional prototypes are produced, in order to have an all-around vision of the various parts of the rim and be able to evaluate any modifications to improve the original projects. Once this preliminary development phase has been performed, there’s the production of the mold for the new rim model and the manufacture of a small batch of samples. This is followed by a test phase out on the track, which is crucial for monitoring the performance of the new products and checking that there are no problems related to their use that were not envisaged at the design stage. If the track tests show positive results, then there’s mass production of the new rim models, which are consequently officially added to the product catalog for the OTK Kart Parts line.

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The first step in the production of each rim model is the definition of a technical drawing. The image shows the section and front perspective of the MXJ model rear rim.
Once the 2D drawing has been produced, there’s the creation of a 3D representation which allows a first view and analysis of the new product.
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The design phase involves the production of specific prototypes, created using 3D printers.

Casting

Based on the characteristics it wants the rim to have, but regardless of the material (magnesium or aluminum), the Italian company uses two distinct production processes:

  1. Low pressure casting: is applied to magnesium rims as well as to the aluminum model used for rental karts and has replaced the gravity shell casting, guaranteeing superior production quality. In fact, it’s a process which - precisely due to the low pressure involved in the phase of putting the material into the mold – improves the filling of the mold itself, thereby reducing the forming of porosity, which can be harmful to the structure of the end product, given that it can generate cracks and the consequent breaking of the rim. 
  2. Die casting: for aluminum rims and certain magnesium models. It’s a faster casting process compared to low pressure casting, in fact the material is literally injected into the mold. However, this rapid action increases the risk of creating turbulence and blowholes within the structure of the rim itself. That said, the strict quality controls implemented by OTK Kart Group guarantee the structural integrity of each product.


The type of casting used directly influences the behavior of the rim out on the track. In fact, the rims produced by low pressure casting guarantee that the kart slides better in track conditions with hot asphalt and high grip. Those produced by die-casting, on the other hand, are optimal in conditions featuring cold tracks or ones that are not yet perfectly rubberized, as they guarantee the wheel assembly better road holding.

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A magnesium rim made by low pressure casting: as you can see, the amount of excess material that must then to be removed by CNC machining is significant.
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A rim made by die casting, once removed from the mold, has a minimal amount of "burrs" that need to be removed so that the product is of the desired final shape and size.

CNC machining

To guarantee the highest quality of its products, the Italian company uses technologically advanced machinery and has automated most of its production processes. Rims produced by low pressure casting require more CNC machining, as they have a lot of excess material. They are easily recognizable because they are processed both internally (meaning the area which is then covered by the tire) and externally (i.e. the part of the flange, meaning the part that’s visible when the tire is mounted). The last step for models produced using the low-pressure casting method is painting. In fact, this production methodology results in raw products that require more processing by robotic machinery. Every single rim made by die casting is checked by an operator and then processed externally, by means of robotic machinery, on the bead part of the tire. In terms of processing, die-cast rims are finished only on the edge since this production technique produces minimal excess material. Once this phase is completed, the rim is painted with a powder coating and is then placed back into the machine to carry out the work necessary to complete the product (such as, for example, centering the hub and making the holes for the anti-loosening screws). 

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The machinery used by OTK Kart Group processes the rims, eliminating excess material and creating the structural details that characterize them.
The robotic machinery that processes the rough pieces that arrive at OTK Kart Group from the foundry.
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The robotic machinery that processes the rough pieces that arrive at OTK Kart Group from the foundry.
The robotic arm moves the rim from the CNC machine to the other stations in the production process, such as the one that washes the rim and the one that performs the leak tests.

Final treatments

The last production phase to which OTK Kart Group rims are subjected involves the treatments, which differ based on the casting method used and the construction material of the model.

  1. Magnesium rims produced by low pressure casting are subjected to a passivation process, which consists of immersing the rims in a solution. This process cleans the surface of the impurities and iridescence that are typical of magnesium, resulting in the product having better aesthetic qualities. Furthermore, it creates a very thin protective layer on the surface capable of protecting the rim from corrosion. Passivation, however, doesn’t have a sealing function. That’s why each individual rim is subjected to "impregnation" by immersion in special liquids and is then placed in a vacuum. By doing so, the material absorbs the impregnating substances which make the rim watertight. Subsequently, there’s the liquid painting phase, which creates a very fine layer of color ranging from 8 to 12 µm, unlike powder coating, which can reach up to 120 µm. Thinner paint layers guarantee greater heat exchange, meaning the rim can guarantee lower and more consistent operating temperatures. 
  2. Die-cast aluminum rims are subjected to a single final treatment called powder coating. In fact, aluminum doesn’t require protective treatment against oxidation, given that, from a chemical point of view, unlike magnesium, it’s less subject to this phenomenon. Furthermore, taking into account that the aluminum models are recommended by OTK Kart Group as being optimal in conditions where there’s "rain/low temperatures/poor levels of grip out on the track", powder coating gives this type of rim low heat exchange and, therefore, a greater ability of the rim to accumulate heat, an aspect that helps tires to reach the correct operating temperature range sooner.

This kind of treatment is not exclusive to aluminum rim models, however. Some magnesium rims, made using die casting, are also powder coated. The reason is also linked to maintaining temperature levels: these specific models are meant for use in cold track conditions, which therefore implies the need for more heat at a tire level.

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This is what die-cast rims look like in the three main production phases: [1] after casting, [2] after CNC machining and [3] after final treatment. In this case, we can see the AXJ model, made of aluminum.
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In this photo, we can see the three processing phases of the AXJ model from the external side perspective. Since it’s a die-cast model, we can see how it is structurally "ready" immediately after casting. Indeed, the lines of the spokes extended up to the edge of the rim are visible, as well as all the holes for connection to the hub already being present.
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This image also shows the AXJ model, seen from a third perspective: the hub side.
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This is what low pressure casting rims look like after the three macro production phases: [1] after casting, [2] after CNC machining and [3] after the final treatment. In this case, we can see the MXC model, made of magnesium.
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In this image, we can see that the low pressure casting process requires additional subsequent machine processing. In fact, unlike die casting, in this case the amount of excess material present on the rim is significant. Furthermore, the holes for the hub screws, as well as those for the inflation valve, are not present after casting, but are made subsequently by CNC machining.

Quality controls

Quality control, in the OTK Kart Group production cycle, doesn’t represent an actual phase that’s positioned before or after a specific process. It could be said that this task accompanies all the various steps in the construction of a rim, starting from the "initial" phase, i.e. the rigid checks and related quality tests that are carried out on the raw materials (aluminum and magnesium) purchased. Once a rim has completed all the steps in the production process, it is once again examined by the OTK Kart Group quality control department. Here, the engineers verify that each model conforms with certain safety and structural standards. The first step, to which all products are subjected, is the leak test: by immersing the rim in water, "putting it under pressure", it checks that no bubbles escape from the structure. In fact, the appearance of air bubbles indicates that there are cracks or damaged parts. The second step regards the analysis of eddy currents. This phase is carried out on a sample basis and represents a more in-depth test, which checks whether the rim has any micro-fractures or structural imperfections.

The eddy current test is carried out using a special instrument equipped with a "pen" probe that the engineer passes over the entire surface of the product, detecting any defects. If any deformity should result, a variation of the graph will appear on the monitor to which the probe is connected every time it passes over the crack.

MEASUREMENTS TO KEEP AN EYE ON

The structure of the rim plays a very important role in track performance, which is why it’s important to pay attention to these measurements.

Width

By width, we mean the distance between the two profiles of the rim, internal and external. This measurement has maximum limits in CIK-FIA regulations: 135 mm for front dry rims and 215 mm for rear ones. However, as for wet rims the maximum measurements are 130 mm and 180 mm respectively (designed narrower, since a narrower tire "cuts through" the water more efficiently out on the track). These values are the same for all Junior and Senior chassis, corresponding to the following FIA Karting categories: OK, OKJ (Direct-Drive) and KZ (Shifter). As regards MINIs, the limits set are 120 mm at the front and 150 mm at the rear, valid for both dry and wet models.

For its part, the OTK Kart Group applies the following widths to its models: 

  • the magnesium MXC, MXJ and MXQ, designed for Junior/Senior karts and for use in dry conditions, measure 130 mm at the front and 212 mm at the rear;
  • the aluminum AXP - for Junior/Senior karts in wet conditions - measure 130-180 mm (front-rear);
  • the magnesium MXC - for MINI karts in dry conditions - measure 115-147 mm (front-rear);
  • the aluminum AXJ - for MINI karts in wet conditions - measure 110-145 mm (front-rear). For this last model, the single 130 mm size is also available, designed for certain championships in which competitors race with four identical rims;
  • the aluminum AXP - for Rental karts in both dry and wet conditions - measure 130 mm at the front and 180 mm at the rear.
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Offset

In the automotive sector, offset commonly means the distance (measured in millimeters) between the central plane of the rim and the plane where it is attached to the hub. In karting, however, it’s customary to define offset as the distance between the hub stop and the plane passing through the internal edge of the rim. This is a measure free from regulatory constraints and which is defined at the discretion of each individual manufacturer. This data, in fact, directly affects the torsional capacity of the wheel. To be clear, increasing the offset increases the rigidity of the wheel which, in turn, increases the level of grip that the kart will have on the track, especially when exiting bends. This is why it is customary to combine wheels with greater offsets to shifter karts (where the search for grip is generally preferred to smoothness, unlike direct drive classes) and to track situations with fairly low levels of grip. Despite this, OTK Kart Group has opted to standardize all its rims (Junior/Senior, MINI and Rental) with a front offset of 48 mm and a rear offset of 58 mm. That’s because these sizes adapt nest to the chassis of the brands that the Italian company owns (Tony Kart, Kosmic Kart, LN Racing Kart, Exprit Kart, Gillard Kart, Redspeed Kart and EOS Kart). In addition to that, the choice to standardize the offset measurement is based on the fact that the variation of this parameter can occur in terms of mm, measures which don’t guarantee significant changes in performance. Results can be achieved by working on many other aspects of correcting the flexibility and geometry of a chassis. 

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Thickness

This is the measurement, in millimetres, which expresses the thickness of the rim structure. Like the offset, FIA Karting regulations do not set any standards for this parameter, so each manufacturer can "play" on this aspect in order to give the rim a more or less rigid structure. In fact, when it comes to this part, OTK Kart Group rims have higher or lower values depending on the resistance or flexibility desired. Nevertheless, the measurement is always around 3 mm. 

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PCD

This means the diameter, measured in millimetres, of the hypothetical circumference created by joining the anchoring points of the rim to the hub. There are no regulatory restrictions here, either, but OTK standardizes the value for all models at 58 mm, the measurement that has always been used by the Italian company for its rims.

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