Does Axle Hardness/Stiffness Affect Go-Kart Performance?

Go-karts are simple machines and they have very few moving parts compared to cars. The axle is one of the most important parts as it connects the two rear wheels, but it also plays a vital role in kart setup. So, you might be wondering if the axle’s stiffness will affect the kart’s performance.

Axle hardness or stiffness can affect go-kart performance by altering the rate of lift at the rear when cornering, but it’s unlikely to have a huge effect. Other setup changes, like altering tire pressures or even changing track width, are normally more effective and easier to make. 

There are varying levels of stiffness that a driver can choose from in the rear axle. Before you choose one for your own kart, it’s important to understand what it does, and which axle might work best for you. Keep reading to find out more about axle hardness and how it affects your kart.

What Do We Mean By Go-Kart Axle Hardness/Stiffness?

The axle of a kart is the rod that connects the two rear wheels. The axle plays an important role in a kart because it not only drives the kart forward, but it can also affect its handling. There are various types of axles that you can install onto your kart, and this is where some confusion comes in because there has been a lot of debate on how this affects the handling of the kart.

There are two main ways that you can identify the stiffness or hardness of an axle that is used on a kart. The first is by looking at the color markings on the axles. Some manufacturers use this method to identify an axle. Other manufacturers will simply use a letter marking on the axle to identify where it lies on the stiffness scale. 

Different Kart Axle Stiffnesses 

Stiffness	Color	Letter
Super Soft	Copper/Orange	U
Soft	Red	Q
Medium/Neutral	Green	N
Medium Hard	Yellow	H
Hard	White	HD
Super Hard	Blue	HH

Stiffness varies for a given axle diameter and material, so you’re not sacrificing the strength of your axle when choosing between the different types as they will be the same diameter and made from the same material. The difference is in the material hardness of the axles, which is determined by the way the axle is made.

All of the axles go through the same manufacturing process in terms of being built, but in order to get different levels of stiffness in the axles, there are different processes applied to each one. The way that the axle is made and processed (using processes like heat treatment) will determine its flexibility and stiffness. 

How Does Changing Go-Kart Axle Stiffness Affect Performance?

Kart axle stiffness will affect the rate of lift of the rear wheels during cornering. This can influence the jacking effect, and how quickly the kart rotates around a corner. Changing the stiffness of the axles is not necessarily a performance factor, but rather driver down to preference.

There has been a lot of debate on how changing the stiffness of the axle will affect the performance of the kart. Many drivers have tested different axles but found there to be no difference between the varying levels of flexibility in the axle. Overall, the axle can affect the handling of the kart, rather than the performance.

Some drivers claim to feel a difference in their kart and see a difference in their lap times when they switch out their axles, but others state that the difference is simply psychological rather than giving the kart a true performance advantage. But, before we get into what an axle would theoretically change on your kart, let’s first understand how a kart’s “suspension” works.

Grip

Before we get into the suspension and the axles, we need to first understand that none of this will affect the amount of grip you get from your kart. Grip is directly related to the tires, as they are the point of contact between the kart and the tarmac. So, if you’re struggling for grip, then the answer might not be in the axles.

Changing the axles will affect the way that the kart handles, rather than the amount of grip that it has. Those are two different elements, and this is something that is important to keep in mind as you explore how a kart’s chassis and axle work together to determine how it handles around corners.

Understanding Go-Kart Cornering

Because a kart doesn’t have a suspension system, the rear axle (at the inside rear wheel) needs to lift off the ground for the kart to have optimal rotation. This can either be described as “rate of lift” or “jacking.” This jacking effect is essential for karts to get around medium and slower speed corners because it allows the kart to rotate faster. 

The rear axle, in combination with the flexing of the chassis during cornering, is what will determine the rate of lift of the inside rear wheel. In general, the higher you get that inside wheel to lift, the more your kart is going to rotate (more oversteer). If the wheel stays flat on the ground while cornering, you’re going to have more understeer and will need a slower speed to get the kart rotated.

The more your axle and chassis flex, the harder it’s going to be to get the inside rear tire to lift off the track. This means that you’re going to have more understeer. But if your inside rear tire lifts too much, you’ll likely be fighting the kart or experience hopping through the corners, which will slow you down even more.

What’s important here is that this will not affect the performance of the kart. Instead, it affects how the driver can control the kart. Every driver has a unique driving style and would prefer a kart that is either more balanced, has more understeer, or has more oversteer. Some drivers can’t go faster with a kart that rotates quickly and vice versa.

The Axle Theory

So, in order to get that inside wheel to lift more, you need less flex in the axle and the chassis. A stiffer chassis and axle will cause the wheel to lift easier, which will give you more oversteer and “bite” through the corners.

Based on this theory, a harder axle will give your kart more oversteer, and a softer axle will give your kart more understeer. The more the axle flexes, the more the inside tire will be touching the tarmac during cornering. You might have more rubber touching the road, but that’s not optimal for rotation in a kart.

What you will notice though, is that the kart will have better traction with a softer axle because the rubber is touching the tarmac more often. So, if you need to get a better push out of slower corners, then switching to a softer axle could do the trick. 

NOTE: A lot of the discussions here are based on what could happen, and there are so many variables at play that mean what does happen is often different

In Practice

While the difference in flexibility between axles makes sense in theory, it’s often very different in practice. Many drivers have tried and tested different types of axles without seeing different results. Some drivers might set the same lap times and achieve the same results using two completely different axles, with the same feeling in the handling of the kart.

In reality, the materials used to make different kart axles are very similar, and the types of steel used normally have very similar Young’s Moduli, which is a property related to the elasticity of a material. This property essentially tells you how easily the material can stretch and deform, and for various types of steel, the differences are too small for there to be a notable difference on a kart.

Plus, the wall thickness of a kart axle is usually constant for a given range of axle stiffnesses, such as 2 mm. So, the real difference in kart stiffness comes from the way the metal is processed and heat treated, as this can affect the physical properties of the material, namely its damping capabilities, which could be perceived by the driver. 

The Placebo Effect

There may be a placebo effect when changing the axles of the kart. The driver is often told what the difference will be, and so they expect a change in the handling of the kart. If they then set a faster lap time, they’ll associate that with the change they made to the axle, even if the real reason for the faster speed lay elsewhere. 

If you have tried different types of axles and you find that you are going faster with one axle as opposed to the other, then it’s likely that the specific axle is suiting your driving style better than others. Remember, everyone has a unique driving style, so changing an axle might not have a big impact on one driver, but for another driver it could help them gain half a second per lap.

Different Circuits

Just like every driver is unique, every circuit is as well. So, you may find that changing between different types of axles will allow the kart to handle better, or even allow the driver to set faster lap times around different circuits. While a hard axle might work on one track, a soft axle could work better on another.

Axles also act as shock absorbers and dampers on a kart, so if you’re on a bumpy circuit or a track with high kerbs, you may need to swap out to a softer axle to try and settle the kart down when it goes over the bumps.

This could allow you to ride the kerbs more aggressively or brake later if you manage to keep the kart stable over bumps in the entry of a corner. But you always need to keep in mind that you want to control the rate of lift of the inside rear wheel, as that’s what allows your kart to rotate.

Different Weather Conditions

When the rain starts to come down then it’s likely that your entire kart set up will change. In wet weather conditions the setup of the kart becomes crucial, otherwise it will be undriveable. In the majority of cases, karts tend to struggle in the rain because the chassis can’t flex enough when the tires don’t have enough grip.

When the tires can’t grip onto the tarmac, the chassis will often be too rigid to flex during corners. This is where it might help some drivers to put a softer axle on their kart to help the chassis to flex some more during low grip cornering. But again, this is not giving the kart more grip, it’s simply changing the way that the kart behaves under load.

KEY POINTS

• Axle stiffness can affect your kart’s handling

• Different axle stiffnesses may suit different driving styles

• There are lots of other factors at play that can affect how your kart handles

Is It Worth Changing Your Axle To Boost Kart Performance?

When it comes to the setup of your kart, there are several other elements that you can check and change before you start changing your axles around. In the majority of cases, changing the axle will be a last resort, and even professional teams won’t change the axles on their karts too often.

Most teams will start their practices or testing with the medium axle that has the letter ‘N’ or the color green stamped on it. This is the most neutral axle and allows the team to move up or down the range of axles if they need to. But in the majority of cases, they don’t. Before changing the axles, they’ll first look at the tires, suspension geometry, seat height, and the track width.

These are the elements that will make the biggest differences in the drivability of the kart and will have the biggest effect on the lap times that the driver can achieve. Before getting into different types of axles, these are the setup elements that you need to try out first.

Tires

As we mentioned earlier, the tires are directly responsible for the grip that the kart has. If you’re struggling to get your kart to bite when going around corners, the tires should be the first thing you check. Changing tire settings is something that most drivers are cautious of, but in reality, it’s much simpler than you might think!

All you need to do is make some adjustments to your tire pressures. There’s nothing too complicated about it, you just need to know what changes you can make based on how you want the kart to behave while you’re on track. Everyone will have a baseline in terms of tire pressures, but you can alter them in a way that suits your driving style better.

If your kart is oversteering and you need more rear grip, then you can raise your rear tire pressures by one PSI or increase your front tire pressure by one PSI. If your kart is understeering, you can do the opposite to try to improve your kart’s balance.

If your kart has too little grip and you find that the kart is not biting through the corners at all, then you can simply raise the air pressure in all four tires by just one PSI. Having too much grip on the other hand can cause the kart to bounce through the corners, or even lift both the inside wheels. You can counter this by lowering the air pressure in all four tires by one PSI.

Suspension Geometry

Suspension geometry is another aspect of the kart that you can work on to improve its overall handling. This one is much trickier, and it may require you to use some measurements and tools to get the balance right. But if you’re feeling confident and you have the right equipment, this is one of the best ways to change the way the kart behaves.

First up is the toe. This is the angle of the tires pointing inwards or outwards. Adjusting this will change the amount of grip you get from the front of the kart. Reducing toe-out (tires pointing more centrally) will reduce your front grip. The neutral setting for the toe on a kart is 1 millimeter toe out. Increasing the toe out will give your kart more grip and bite going into a corner.

You can also alter the amount of grip at the front of the kart by adding, removing, or even loosening or tightening the torsion bar. This is a setting that many drivers tend to overlook, but adding or tightening the torsion bar will increase the amount of grip the front of the kart has while it’s out on track.

Seat Height

Many drivers who are new to the sport will change their seat settings based on what is most comfortable for them. But that means that you are sacrificing performance for comfort. The driver is the heaviest part of the kart, so changing the seating position will alter the way the kart behaves, much like moving ballast in a car.

Moving the weight around your kart can significantly affect the way that your kart handles. For example, if it starts to rain, moving your seat higher up will be one of the first things you should do. Moving the seat upwards or backwards (or both) will shift the weight further onto the rear axle which will give you more stable braking and traction, as well as increasing your rear grip.

Moving your seat forward will increase the amount of front grip that your kart has. Lowering the seat will help to reduce excessive bouncing. You can also add or remove seat stays to get the same effect, or change the position of the ballast (if you have any) on your seat.

Even changing to a stiffer or more flexible seat should be considered before changing the type of axle that you have on your kart. The seat plays a vital role, and altering the way that it reacts to the chassis and how it shifts the weight will impact the handling of the kart.

Track Width

Finally, adjusting your track width is another option you should consider before changing the type of axle on your kart. Changing the track width is much easier and quicker to do. Changing the front is often enough because it’s about how wide the front is compared to the rear. 

TOP TIP: There’s no need to widen the front track width and narrow the rear, as this will have double the effect – only change one thing at a time!

If you find that the kart is jacking too much and the inside rear wheel is lifting too much, then increasing the front track width will help. Increasing the front track width will also reduce front grip and it will cause the kart to understeer more going into the corners. Shortening the rear track width will have the same effect.

You can increase the rate of lift by increasing the rear track width, and you can get more oversteer from your kart by reducing front track width. It’s often best to move half a spacer at a time when it comes to the front track width, because the effect of changing this element can be dramatic. 

Final Thoughts

Axle hardness/stiffness can affect go-kart performance, but there are several other setup changes that you should try first before you start looking into different types of axles. In reality, the stiffness of the axle usually has a minimal effect on the performance of the kart.