How do I know if I need a new chassis or not?

09/14/2016 0 1,196 Total Views

fea_credit-solidworksThere are varying views on whether a chassis should be regularly replaced or simply ‘tweaked’ back to its original position. I hope to give some insight on what to check when setting up a new chassis, and to also offer some tips to those readers who are competing on an older chassis and are looking to maximize performance against competitors on newer equipment, because it can be done!

I often travel to kart meetings around the UK and frequently hear the age old excuse of “I can’t compete with these guys on brand new chassis” or “they’ve got three spare chassis in the back of their truck, it’s not fair”. But, what advantage does changing to a new chassis really bring, particularly mid-season or even part way through a race meeting? Well, I can only offer my experience and opinion and I’m sure if you talk to ten different people, you’ll get ten different answers but here goes…

The Theory The basic principles of kart chassis have remained fundamentally the same since Art Ingels first had his Eureka moment and introduced karts to us in the 1950’s! Despite the availability of stronger, lighter materials and CAD/CAM manufacturing techniques, karts are still constructed with a solid rear axle, no suspension and rely on chassis flex as a means to lift the inside rear wheel, allowing the kart to corner.

The amount of flexibility in a chassis, or torsional stiffness, is principally related to tube material, size and frame design. Often described as torque per unit deflection, torsional stiffness is a numerical method of describing a chassis’ ability to resist twisting, known as torsional deflection, and can be expressed in several different units but the most common is Nm/rad (Newton metres / radians).

Calculating the exact torsional stiffness of a kart chassis can be difficult given the complex geometries that are often used to build a new frame. There are several computer methods now available using stress analysis software, however a ‘simple’ formula has been developed to give an approximate value for the chassis as outlined below.

T = (JG φ) / L = KT φ

T = Applied Torque
J = Polar moment of inertia
G = Material shear modulus of elasticity
L= Characteristic length of cross section
KT φ = Torsional Stiffness Nm/Rad

Typically, a higher torsional stiffness will result in a ‘stiffer’ chassis which is normally desirable for dry racing conditions. In the wet, a lower torsional stiffness is usually desirable to allow the chassis to flex more, hence the reason why so many drivers will choose to remove torsion bars during a wet race meeting. A racing car chassis designer will almost certainly want to increase chassis stiffness, whilst reducing the amount of material used (weight), whereas a kart manufacturer may choose to slightly reduce torsional stiffness to find that optimum performance window.

It is generally accepted that as a chassis is used more the torsional stiffness will vary over time. According to chassis manufacturer, Phil Featherstone of Raider karts, the amount and rate of this variance is largely dependent on the material selection and tolerance of the original chassis construction. “Some chassis might perform at their optimum for more than three years, but in year four the driver might begin to experience handling issues as the chassis begins to deflect from its original position… this might be the result of, for example, consistently driving over the same curb which moves part of the frame by say 1mm. This can have a profound knock-on effect on the balance of the kart” says Featherstone.

So, given that the torsional stiffness can vary over time, and since we know that this has a significant impact on a kart’s ability to handle, surely it is important for a serious competitor to minimise the risk of this occurring through regularly updating their chassis?

Balancing a kart chassis –some influencing factors

There are more factors to consider than just the torsional stiffness of the frame alone before making a quick decision to move on to a new chassis. This is because the handling of a kart is influenced by a large number of other factors including center of gravity height, driver seat position, wheelbase and even the engine fitted to the kart.

Let’s look at a few of these key factors that can influence kart chassis behavior…

Driver position The heaviest part of a moving race kart is usually the driver, the driver’s seat position is therefore crucial to the weight distribution across the chassis. If not positioned correctly, the weight of the driver can cause an uneven distribution of weight to each corner of the kart which will ultimately impact on handling through corners and under braking. The use of seat stays also heavily impact on the torsional stiffness of the chassis.

Engine type and position The second heaviest part of a moving race kart is usually the engine. Using the same principle as above, a seemingly small shift in engine mounting position can impact on weight distribution to each corner of the kart. Typically a chassis is also designed to accommodate a specific engine (Rotax Max, Honda Cadet etc). Therefore don’t be surprised that if you change the type of engine used that the kart handles differently… this is likely due to the difference in weight distribution across the chassis.

Wheel alignment It is important to note that altering the wheel alignment can impact on the way a kart handles. What might seem like a small set-up change to the front track or castor might have a larger impact on weight distribution and resulting torsional stiffness of the chassis.

You might have picked up on a common theme here… weight distribution. In my view, this is the key to optimizing the performance of a chassis, new or old!

Once a chassis begins to deflect, there are varying views on whether a chassis should be regularly replaced or simply ‘tweaked’ back to its original position. It seems the majority of teams and competitors would opt for a new chassis at this stage, seeing it as the most risk adverse and logical resolution, but is it necessary?
My conclusion: it’s all about corner weights
Whether you take delivery of a brand new chassis, or want to optimize the performance of your existing chassis, I cannot stress enough the importance of frequently corner-weighting your kart as part of your basic set-up procedure.

As a result of making set-up changes, or building up a new chassis, the kart might, for example, corner well through right hand turns but under steer through left hand corners – an extremely frustrating scenario. It might be that you’ve checked the tracking and cannot find an obvious problem, but it is likely that there is a significant difference in the corner weights from left to right (cross weight) which might be caused through the chassis having gradually deflected over time or even as a result of the original construction tolerances. However, with either scenario, measuring and adjusting the corner weights can be easily achieved using the basic steps below:

Find a completely flat surface to sit your kart on four accurate scales (one under each wheel). Ideally this should be an engineering table or flat patch rather than what you think might be a level garage floor, but I appreciate not everyone has access to these facilities. If required, use spacers to level off the surface and check that all four scales are perfectly level. A small error can have a significant impact on your results and could lead you to make unnecessary alterations.
Ensure the kart is topped up with fluids and fuel as if you were about to start a race, with tyre pressures set as per your race set-up.

Position the kart on the scale pads with each wheel located as centrally as possible on each scale pad. The driver should then be seated in the kart wearing race clothing and holding the steering wheel in a natural driving position.

Measure the static weight in each corner. From my personal experience, I aim for 50:50 weight distribution from left to right on both the front and rear of the kart, with a 40:60 weight distribution from Front:Rear. Each kart manufacturer will have their own specification and tolerances so it is best to contact your chassis supplier for this information.

If the static weight is not balanced as it should be, then try moving physical objects around and note the impact on the static weight. The obvious object to try moving first is any weight ballast, followed by the battery, and then the driver’s seating position.

If moving the seat position to optimize weight distribution, don’t forget to make sure the driver is still comfortable and can properly control the kart otherwise they’ll almost certainly not be quick. Once the static weight is set, try to fine-tune the ‘cross weight’ as much as possible. Cross-weight percentage compares the diagonal weight totals to the kart’s total weight. To calculate cross-weight percentage, add the Right Front weight to the Left Rear weight and divide the total by the total weight of the kart.

Moving ballast around on the kart will not resolve any differences on the cross weight; it will just alter your static weight. To alter any cross-weight results, the kart will need to be raised or lowered in a particular corner, usually by using stacks of shims to alter the ride height. Make small ride height changes and remeasure your static and cross weight until the kart set-up is optimized.

Maybe, through regular measuring of corner weights, you can say goodbye to the days of thinking ‘my chassis has gone off’, but I will leave you to form your own view on how best to optimize your performance at the race track, whether it be on a brand new chassis or a 10 year-old frame.

As long as you are having fun, keep going!

source: Ben Willshire /