Basics of axles, rims & tyres

In most instances where one doesn’t have the budget for a specialist kart mechanic, many decisions and adjustments on the kart are either done by the driver or the parent. They are also often based on information they have heard via other competitors or mechanics without a real understanding of some of the basics involved behind these choices. This chapter attempts to provide some of the basics regarding the axle, rims and tyres used in karting as they all ultimately play a huge part in the overall scheme of things.

Rear axle

The rear axle is a major contributor to the setup and handling of a kart. Essentially there are four different external diameters used for karts, and they are based on the chassis size and engine being used in a particular class. The Bambino class which runs a 77cm chassis (that’s the wheelbase i.e. the measurement between the front and rear axles), uses a 25mm outside diameter (OD) hollow axle. Note that only the Topkart axle (19mm ID) or the EMR Axle (18mm ID) are allowed and both are roughly 950mm in length. The next range of classes that run a 95cm chassis (Kid Rok, Mini Rok, Micro Max, Mini Max) use a 30mm OD axle. For all CIK or Rotax homologated 105cm chassis, a 40mm OD is used on the DD2 gearbox classes, and a 50mm OD axle is used for all the others.

Except fore the very old Bambino chassis that used a solid rear axle, all modern axles are hollow as this not only reduces the overall mass of the axle, but makes them far less stiff i.e. a lot more ‘springier’. Going around a corner requires the outer wheel to travel a longer distance along the intended path compared to the inner tyre, and that is why a car has a differential fitted to the rear axle. However, there is no such luxury on the rear axle of a kart and therefore one has to rely on lifting the inner tyre off the track to prevent it being ‘dragged’ along. This is an important concept to grasp as the stiffness of the axle assembly to a large extent determines how much and how quickly the inner wheel will lift (and also return to the track). It thus controls the behaviour of the rear end of the kart and whether it understeers or oversteers.

Looking at the axle in isolation, it acts similar to a cantilevered beam with its attachment point being the bearing block on the chassis as shown in the diagram below.

Axles are made from one or other grade of steel, and although these have different values of E, they are all fairly similar. Manufacturers thus alter the stiffness of their axles by altering the material and also manipulating the inner diameter of the shaft. The latter has a significant influence on the axle stiffness because its 4th power is used in the equations.

That said, one must not lose sight of the fact that there are other restrictions placed on the axle by the technical regulations. These are threefold - the mass of all 30mm axles must be 2 900grams ± 10grams, and the wall thickness of all sizes of axle must be within certain limits as per the category specific regulations, or if not stated therein, then the SSR’s or CIK rules will apply. Lastly, the overall length of the 30mm axles must be 960mm ± 10mm. If you are not within those limits and a technical check is conducted, it will result in an exclusion. So, you have been warned.

Rear track

It’s not only the stiffness of the rear axle that affects how the kart will react, and the ride height, track width, along with some other factors such as hubs, rims, tyre pressures, etc., all influence the kart handling. The rear track is defined as the width measured from the centre of one tyre or rim, to the centre of the opposite tyre or rim. For this discussion we’ll look at the tyres, and the wider the track, the lower the grip. This is due to the fact that a wider spacing between rear tires i.e. more overhang from the bearing carrier to the wheel, permits greater deformation or bending of rear axle whilst negotiating a corner. In short, its equivalent to having a softer axle which means it’s more difficult to lift the inner wheel and the kart will tend to slide around the corner i.e. less grip resulting in oversteer.

The regulations also place a limit on how wide you can go on the rear width. This is measured between the outside of the rear tyres or rims whichever is the greater, and is not to be confused with the rear wheel track (centre to centre distance between the rear tyres). Exceeding these limits will result in a technical infringement i.e. heat exclusion or possibly an exclusion for the whole of the event. Generally, these limits are as follows:

Bambino, Micro Max, Mini Max, Mini Rok = 1100mm
Jnr. Max, Snr Max, DD2, OKJ, OKN, KZ = 1400mm.

Rims

Rims are attached to the axle, either front or rear, by one of two methods. The simplest style, and that is only used on the front end, is known as direct spindle mount or DSM. The central portion of the rim is machined to house bearings, and is held in place on the stub axle with a single nut. The second style of rim is bolted to a hub using three studs and the hub itself houses the bearings. The two styles are shown below.

For rims that are bolted to hubs, there are two bolt patterns in common use and they are known as Metric, and Rotax front. To be perfectly correct, it’s actually three patterns but we will ignore the one used in the USA because they always like to be different to everyone else, and we don’t use them in South Africa anyway. The differences we need to be aware of are the size of the central hole, and the pitch circle diameter (PCD) of the three bolt holes as shown below. Both rim types use 8mm studs for attachment to the hub itself.

Metric rims have a central hole of 40mm and a PCD of 58mm.
Rotax front rims have a larger central hole of 55mm and a PCD of 67mm.

Rims are available in aluminium and also magnesium with the latter being lighter, softer, and also roughly double the price of the aluminium ones. Aluminium rims are produced either by using a spinning method on a lathe or being cast, whilst magnesium ones are always cast. The magnesium rims also gain/shed heat quicker than their aluminium counterpart. Different shapes are also produced as shown in the picture. As may be appreciated, because the one on the LHS has more ‘meat’ in the central area, the volume of air that is capable of being held in the tyre is reduced accordingly. This style of rim is therefore also known as a ‘low volume’ rim.

Because the tyre heats up due to friction between it and the asphalt surface as the race progresses, the air inside the tyre will also heat up and expand thus increasing the tyre pressure. What happens then is that the tyre’s outer circumference increases to compensate, so having less air to start with means there will be less increase in the tyre’s rolling circumference as the race progresses. The rolling circumference can have a fairly significant effect as discussed later.

Rims are available in a variety of widths varying from around 100mm up to 210mm, this being measured between the insides of the rim flanges. The width one uses is pretty much decided by the tyre size and the technical regulations also have limits per class on the rim widths allowable. As a result, most competitors in a given class will run exactly the same rim width but some may go a bit narrower. That said, using a slightly wider rim decreases the tyre sidewall flex and removes grip from the tyre because the sidewall can’t roll over as easily.

Sometimes rims of the same width are available with a different offset - the offset being defined as the horizontal distance from the inner mounting face of the rim to the inner vertical edge of the rim. So, if one has a rim with an even offset (the mounting face is halfway across the rim width), then to achieve the same track width as a rim with a shallow offset, you need to set the rim further out along the axle. This can be achieved in two ways viz. using a longer hub, or sliding the hub further outboard on the axle. Mounting the assembly further outboard means there is more axle exposed so the rear end of the kart becomes softer and loses grip. Using a longer hub effectively stiffens up that portion of the axle and increases the rear grip on the kart. On the front end, grip can be increased by mounting the hubs further outboard. This is achieved by re-arranging the spacers that come in 5mm or 10mm thicknesses.

Hubs

Similar to rims, hubs are available in aluminium or magnesium. As we saw earlier, the longer the hub, the more grip is produced and this applies to both the front and the rear of a kart. On the rear, the use of a longer hub is akin to using a stiffer axle, but to a lesser degree. When purchasing, it is important to remember that the front spindles can be for 17mm or 25mm bearings, so the hub needs to be specified correctly. Similarly, hubs for the rear are available in differing axle sizes. The hub lengths can vary from about 40mm right up to 105mm, and a selection of these for both front and rear are shown in the pictures below.

Tyres

So, this is where we are once again make use of the good old Imperial system of measurement that we discarded 50 years ago in 1970 because, unfortunately, tyre designations are always in inches. All racing go-kart tyres are designed for use with a 5” diameter rim, and there are two tyre designation systems that are in use. Why that is the case is anyone’s guess, but the following diagrams with examples, show you how to interpret what the numbers on the side of the tyre mean.

The next important thing to consider when mounting a tyre onto its rim is the direction arrow embossed onto the sidewall. Note that on LeVanto tyres there isn’t a directional arrow, and what you think might be the arrow, it’s actually part of the manufacturer’s logo. For Mojo or Bridgestone tyres, once mounted you can’t simply swap, say, a RH front wheel to the LH side because the rotation direction will be incorrect. This is regarded as being a technical infringement and can result in an exclusion. So, you may well ask why that is the case – does it provide an advantage? According to the Mojo factory to whom I posed the question, the simple answer is NO - the directional arrow is simply used for safety reasons. The outer layer is effectively a strip of rubber where the ends are connected by overlapping at a certain angle, and the correct rotational direction prevents the tyre from being ‘peeled apart’ when accelerating or braking hard. For rain tyres, the direction also plays an important role in providing optimum water displacement.

The dry weather racing tyre is known as a slick, and that’s because there are no ‘knobbly bits’ that stick out of the surface like that on an off-road vehicle tyre. On a slick, the more rubber one has available to mould into contact with the track surface, the more grip one has. Secondly, because there are no gaps in the surface of the tyre (no knobbly bits or tread), each molecule of rubber has support from the one next to it - the tyre becomes more stable and is therefore far less likely to squirm or slide across the track surface when large cornering forces are applied to it. This ability then allows the manufacturer to employ the use of relatively soft and stickier tyre compounds which provide even more grip without the downside of the tyre breaking up under load.

In the wet, most of the above doesn’t apply any more. A slick can still work relatively well at low speeds or when there isn’t a lot of water on the track because the tyre still has sufficient time to squeeze the water out from underneath it. However, as the kart speed increases, the water directly ahead of the tyre builds up into a wedge and the tyre starts to ride on the surface like a water skier, and you have little or no control. In the process, the water has also cooled off the tyre, so there is even less grip to play with. So, the solution is to have a tread pattern which effectively breaks up the surface into smaller blocks of rubber that are in contact with the track surface. This has a twofold result viz. there are grooves that allow the water to escape, and because there is less rubber in contact with the road surface, the vertical pressure exerted by the tyre onto the track is increased thus assisting in the action of displacing the water.

The tyre diameters, or more to the point, the rolling circumference, doesn’t matter too much on the front of a kart because they roll totally independently of each other. However, the rear end of the kart has a rigid axle so the rolling circumference of those tyres does matter a lot more. You’d like to think that you have two identical ones, especially if they are brand new, but that is not necessarily the case. According to the tyre manufacturer’s specifications, the Mojo tyres have a manufacturing tolerance of ±5mm on their diameter, and the LeVanto and Bridgestone ones are marginally more at ±6mm and ±7mm respectively.

Let’s say you inflate your tyres to their normal racing pressure and measure the rolling circumference. It’s better to do this using Mom’s floppy sewing tape which will sit flat on the circumference of the tyre, rather than using a steel tape measure which will have lots of kinks because you are trying to bend it the wrong way. As an example let’s say the circumference of the one tyre is 885mm and the other is 895mm.

Along a 100m length of straight track the first tyre will try to rotate through 113 revolutions whilst the larger one wants to do 111.7 revolutions, but it can’t do so because of the rigid axle. This difference of 1.3 revolutions ‘binds up’ the kart but manages to dissipate itself via scrubbing the tyres along the track. This results in more heat build-up in the tyres, tyre degradation, plus a slower speed down the straight because of the effective waste of engine power in overcoming the scrubbing. There is luckily a way to fix this on new tyres. After measuring the circumferences, increase the pressure of the smaller tyre by about 0,5bar for every 2mm difference in circumference being careful not to inflate the tyre past its maximum, and leave the tyre out in the sun for roughly 20minutes. Drop the pressure, let it cool for a few minutes, re-inflate the tyre to the correct pressure, and re-check the measurement for comparison.

Emile McGregor - MSA Technical Consultant