Chassis size vs axle choice

Introduction

In a previous section entitled ‘Kart setup’, the effect of axle stiffness was discussed and certain advice was provided.  Rather than having to refer to that, it is repeated below.

The axle has a very strong influence on rear grip characteristics and in almost all cases, you should use a mid-range axle.  The hard or stiff axle doesn’t flex like a soft one and forces energy into the tyres, thus making them work harder and provide more grip.  It is used when the weather is cold, or in slippery track conditions.  The softer axle is used if conditions are extremely ‘grippy’ or where excess rubber has built up on the track.  In general, high power karts need stiffer axles for more traction.  Lower power karts have a greater need to ‘free up’ the chassis and will run softer axles.  The rear axle acts like a spring and the softer the spring, the more/longer the inside rear wheel will stay up.  Tall drivers generally will need softer axles to help reduce grip.

Virtually all of the literature on kart setup will provide the same advice, and it is generally true when one is racing with a 1050mm chassis used in the senior classes regardless of the manufacturer.  However, if you are running a 950mm chassis and making axle changes, you may be battling with the above advice because it doesn’t always hold true.  Let’s therefore unpack this in a bit more detail.

Chassis construction

A chassis is constructed by welding a number of bent-up tubes together, thereby making a frame which has longitudinal rigidity i.e. resistance to vertical twisting around the Z-axis – this manifests itself as bending between the front and rear axles.  You see the effect of this when the driver jumps into the kart and the chassis ‘springs’ downwards, but for this discussion we are not really concerned about it.  The chassis also has torsional rigidity i.e. a reluctance to twisting around the X-axis along the centreline of the kart.

So, what gives the chassis its torsional rigidity.  Referring to the diagram on the RHS, the wider one makes the element being twisted viz. the chassis width across its waist, the more resistance it will have to the twisting action.  That said, it is also closely related to the outside diameter of the tubes being used.  It is the caster angle which forces the front inner wheel downwards when the steering wheel is turned and provides a twisting force into the chassis.  The torsional rigidity or stiffness of the chassis then determines how easily the kart will lift its inner rear wheel when negotiating a corner.

Putting the Bambino chassis which has a 770mm wheelbase to one side for a moment, the other two chassis sizes available use either a 950mm or a 1050mm wheelbase.  Apart from the additional 100mm in length between the front and rear axles, another major difference between them is the size of the tubes used in their construction.

Chassis tubes

Regardless of the manufacturer, all of the currently available 950mm wheelbase chassis are constructed of 28mm diameter tubes for the main members.  The 1050mm chassis on the other hand use 30mm or 32mm diameter.  Birel is a bit of an anomaly however, because their 1050mm RY29 chassis uses 28mm tubes whilst the RY30 uses 30mm, and the RY32 uses 32mm tubes.  One can therefore expect these chassis to react slightly differently to each other.

Unless you have access to someone inside a manufacturer’s factory or have managed to lay your hands on some of their ‘classified’ material, you are really left in the dark about the actual material composition of these tubes, or whether they have been hardened or not.

Chassis shape

So, if we don’t have access to that information, then apart from the tube size that one can easily measure with a Vernier, what other differences are there that can one readily observe that will assist in understanding how a chassis reacts to changes in setup.  The most obvious of these is the overall chassis shape as shown in the diagram below.

Essentially, there are two basic frame shapes that are used viz. the CRG shape which has a short but fairly wide waist, and the OTK shape that has a longer but narrower waist.

The chassis made by CRG for their own use employ this short wide waist.  Despite some Zanardi chassis having been made by CRG, those chassis shapes use the OTK layout.  The OTK shape shown in the diagram viz. a longer and narrower waist, has been adopted by Birel, Energy, Intrepid, Parolin, Ricciardo, Tony, and Zanardi.  Therefore, the conclusion is that they should all react in a similar manner when changes are made to their setup.

Torsional rigidity

Referring to the diagram above, then assuming that the tube material and hardness used in both shapes is identical, one can draw the conclusion that a chassis conforming to the CRG shape will be torsionally stiffer than one that uses an OTK shape.  Apart from using smaller tubes than its 1050mm counterpart, a 950mm chassis also has a significantly narrower waist.  What this boils down to is that it is torsionally a lot softer than its bigger brother and this has a marked effect on the overall setup.

If the chassis is torsionally stiff, then the rear end of the kart reacts readily to the steering input when going around a corner and the inner rear wheel lifts fairly easily.  This is because you can liken the chassis to a steel rod – twist one end and the other end must follow.  If the chassis is torsionally softer, it performs much like twisting a rubber rod – twist one end and the other end is reluctant to follow.  So on a torsionally softer chassis, the inner rear wheel wants to ‘stay’ on the track and needs a bit more ‘coaxing’ to lift itself up.

Effect on axle choice on 950mm

What we see from the above is that a 950mm chassis is a lot softer torsionally compared to its bigger brother, and that’s where the softer axle helps.  By using a softer axle combined with a softer chassis, you are effectively forcing the outer tyre to work more thus creating additional side bite i.e. grip.  This of course is contrary to all the literature out there and flies in the face of what is true for the bigger karts viz. harder axle equals more grip.

Regardless of chassis size or shape, more grip is useful in situations when the track is ‘green’, there isn’t much rubber laid down yet, the temperature drops quite a bit, it starts to rain, or any combinations of these conditions.  Obviously the converse is true.  A word of caution for whatever chassis you are running.  You need to be able to distinguish between the rear end sliding i.e. you want to add grip, as opposed to the rear end hopping where you need to reduce grip.

Axle charts

Axle charts for both CRG and OTK are presented below.  The one for CRG provides the actual hardness of the axle on the Brinell scale, referred to HB.

Note that you are allowed to use an axle from any manufacturer i.e. it doesn’t need to be the one supplied with your kart, as long as it complies with certain dimensions.  These apply to the outside diameter and wall thickness specified in the regulations pertaining to your class.  Lastly, the length of the axle on any 30mm axle class must be 960 ± 10mm.  You may well need to do some trimming to comply with that specification but don’t end up being shorter than the laid down specification or you’ll be excluded.

Summary

To alter the grip level, a change in axle isn’t the only item one needs to consider.  Factors such as driver height, seat position, seat stays, rear track width, tyre pressures, hub width, amount of Ackermann, etc. all play a big role.

For these reasons it is always judicious to only make one change at a time so you at least know what effect that has made to your times on the stopwatch.  This experience doesn’t come overnight and there’s still nothing to beat track time and a lot of effort if you want to become successful.  To sum up, when making changes it’s very much a case of try what the literature says, and if that doesn’t work for you, then do exactly the opposite!!

Emile McGregor - MSA Technical Consultant