By Chris Livengood
One of the most common problems I encounter at the track while driving my equipment, watching others drive, and while helping others tune their chassis is anoversteer off of the exit of the corner. It often makes applying throttle at the apex a difficult task and zaps valuable forward thrust that is needed to propel your kart down the following straightaway. In this article I will discuss theories pertaining to the causes of an exit oversteer as well as several other aspects of the condition.
As a driver coach at BeaveRun in Wampum, Pennsylvania I work with many young and developing drivers. One of the biggest hurdles developing drivers face is how to establish what are good chassis handling characteristics and what are not. Confusion often results from what might be a very comfortable setup and thus easy to drive for a driver who is not 100% confident. From my experience comfortable almost invariably means a kart that is too “tight.” I like to define “tight” as a chassis that gives the perception of having a greater proportion of rear grip than front grip and as a result is not balanced.
One common outcome of a tight chassis is an understeer. Understeering karts tend to be more predictable under braking and may inspire more confidence through high-speed corners. However, understeer often results in a snap oversteer off the exit of tighter corners which can destroy speed entering straightaways.
In this scenario the cause is the entry understeer and the effect is the exit oversteer. In karting circles this is often known “push-kick” or “push-whip.” The most common theory, and the one that I personally subscribe to, says that the entry understeer prompts the driver to increase the amount of steering angle to compensate for a chassis that is not approaching the apex at a quick enough rate. At some point in time after the steering angle is increased the front tires achieve enough load or slip angle to turn the kart. However, the wheels are now at a greater steering angle than should be required for the turn. The instance that grip is achieved happens very quickly. The final result of this quick increase in steering traction is a snapping or whipping of the rear end of the kart or what I am calling an exit oversteer.
It has been established that the effect is an oversteer and cause is the understeer. So in theory if we correct the cause we will also correct the effect. Effectively, we should be able to kill two birds with one stone. I must also note that fixing an entry understeer is done differently than correcting an exit understeer. Keep in mind that an exit oversteer can exist on its own with different causations than we are discussing here.
Before going straight to chassis adjustments I will also say that my own tuning strategy is based on what I call “123 ABC theory.” Practically speaking, if you get the kart right in the first part of the corner then it is more likely to perform correctly at mid corner and apex. If the kart performs correctly in the first two parts of the corner then it is again more likely to perform correctly in the third and final part of the corner.
One final thing, before I make changes to the chassis setup I like to evaluate overall chassis neutrality. For this I evaluate the karts current setup and decide if the kart is currently set to any adjustment extremes. An extreme for me includes low rear ride height, really high or low castor settings, and extremely stiff or extremely soft axles. This list will be different for just about everybody and it is important to note that some chassis will be setup towards extremes a lot of the time. Many Yamaha Can class karts are good examples of this. Use my list only as a consideration of potential areas of chassis setup that might represent extreme outliers and as a result could be obvious causations of an entry understeer or any other handling ailments you may come across.
At this rate you probably thought I was never actually going to get to chassis setup. An entry understeer is 99% of the time is the result of a kart that is not able to transfer weight quickly enough. Take this literally. All the evidence I have collected says that a kart begins the initial turn-in sequence on the inside front tire. This is a direct result of the castor built into the front-end geometry of the chassis. Simply put, if weight is not applied in great enough quantity or instantaneous enough to the inside front tire an entry understeer will likely exist.
For the “newbies” out there I will give a quick explanation of castor and its application to kart dynamics. Castor angle as well as scrub radius causes the inside front tire to drop and the outside front tire rise when the steering wheel is turned. This is due to the castor angle or the leaning of the top of the kingpin towards the rear of the kart. This effect is often called “jacking.”
Ultimately this means that at the instance the steering wheel is turned a percentage of weight is placed on the inside front tire and a large percentage of weight is removed off of the outside front tire. Furthermore, if the steering wheel is turned and weight is not being added to the inside front tire at a fast enough rate or in great enough proportion the chassis will understeer. For practical purposes I will ignore factors like weight distribution in this discussion as that factor would add thousands of words to the article. Suffice it to say however, more often than not more front weight increases the rate at which the chassis will turn initially. Ignoring weight bias there are several other options to increase the karts ability to turn-in and reduce entry understeer.
The biggest of all adjustments would be the addition of castor. This means leaning the top of king pin further back. This increases the rate at which the inside front wheel drops as well as the rate at which the outside tire rises. The results being a quicker weight transfer onto the inside front tire and in increase in the karts ability to turn-in. Increases in positive camber can also help increase turn-in. Positive camber involves the movement of the tops of the front wheels away from each other. Increasing positive camber means that at turn-in the inside front tire, the one turning the kart in, has a greater lean towards the apex of the corner. This is the same principle as leaning a motorcycle. Leaning the tire towards the apex produces what is known as camber thrust and actually causes the tire to drive towards apex without additional steering angle.
Front torsion bars can also help. Putting the front torsion bar in stiffens the frame across the front of the chassis and creates a more positive or efficient transfer of “weight jacking.” Another adjustment to look at is front width. Increasing front width increases scrub radius. Scrub radius is the distance from the center of the wheel to the center of the kingpin bolt. Speaking simply, the greater the scrub radius the greater the jacking effect. This works very much the same as increasing castor angle. However, on some softer chassis increasing front chassis width might actually have the opposite effect. This can be attributed to the relation between increase in scrub radius and the resulting increase in leverage that the wider width puts on the chassis. Greater leverage can create the symptom of what boils down to the softening of the chassis and thus may negate the added jacking effect gained by greater scrub radius. I mean this in a mechanical sense not in the terms of actually softening the materials the chassis is comprised of. As far as front width is concerned my experience says the majority of 30mm and 32mm chassis will turn-in quicker with greater front width while some 28mm chassis might actually turn-in better with a narrower scrub radius. Testing will establish this for your given chassis but generally the rule above appears to hold true.
For finer adjustment I may attack the rear width. Narrowing rear width can have the effect of making weight jacking slightly easier. This is due to several factors. A narrower rear track has less leverage over the front of the chassis and thus making weight transfer easier. Moreover, a narrower rear track effectively raises overall center of gravity, which can help unload the inside rear tire and allow the kart to turn. Rear track adjustment is a small tuning tool when considering turn-in. I recommend using it as a fine-tuning tool.
There is a lot of information in that paragraph above so I would like to boil all of it down to a few main points. Castor adjustment may create the biggest change to the karts turn-in characteristics. Front width is a finer adjustment but is where I recommend starting if castor is seen as too big of an adjustment. Camber is a good tool also and I have found it to be highly effective in “green” track conditions but be aware that camber adjustments have many side effects in other parts of the corner. The front bar is always a solid choice but again be aware of the side effects. Rear width is effective for small adjustments to turn-in characteristics but narrowing the rear can be detrimental to the braking stability of the chassis.
We got through the most common cause of an exit oversteer but there is at least one other scenario that has not been discussed. This scenario is far less common but I do come across it from time to time and find that it is especially frequent in rain conditions. One of the goals to going fast in a kart is unloading the inside rear tire. This inherently means that you are turning on one rear tire. Sometimes the rear tire remains unloaded for too long. Yes, it actually can happen. If it does occur it can result in a kart that has a tendency to exit oversteer. Usually a good characteristic of this scenario is that the kart turns-in correctly but once you apply the power the kart rotates excessively. The bottom line here is that you are attempting to turn and accelerate forward on one tire. The basic strategy in this situation is much the opposite of the strategies I presented for the “push-kick” scenario. In this situation you are looking to set the inside rear tire down a bit sooner. Once you get both tires on the ground you will find that you no longer have oversteer. This however is a fine line to walk. You need to set the tires down just enough so that you eliminate the exit oversteer and gain forward traction but not so much as to “bind” the chassis up.
The theory being applied is the reduction of weight transfer. Remember however that we are attempting to go in small adjustments because an exit oversteer is leagues better than an exit understeer. Slight decreases in castor through either direct adjustment of castor or through reduction in scrub radius (reduction in front width) are obvious adjustments to try. Slightly lowering weight transfer through a slightly wider rear width (effectively lowering the center of gravity) can help get your inside rear wheel back on the tarmac a bit sooner. If the exit oversteer is extreme then lowering the rear ride height may also be a step in the right direction. Very extreme over transfer conditions can be customary with taller and bigger drivers who may have relatively higher mass.
Again I will review the adjustments and try to sum them up in terms of usability. Castor can be adjusted in large or fine adjustments depending on the adjustability of your specific chassis and can help fix the problem immediately if adjusted correctly. Front width is a smaller adjustment and as a result is less risky. If the exit oversteer is small then start by narrowing the front width by a spacer. Rear width is a fine adjustment and by going a touch wider that last little bit of exit stability might be found. Rear ride height is a big adjustment in my experience. If you are feeling like the kart has more than enough mid-corner grip but is exit oversteering badly this is a good adjustment to try. Before using any of these adjustments one must always consider possible side effects of each chassis change and pick the compromise that will maximize benefit and minimize negative side effects.
In this article I explained the theory behind two different types of exit oversteer conditions and discussed several potential chassis adjustments. While I find this theory to be reliable and repeatable under most circumstances there are always conditions in which it may be proven wrong. Additionally, there are adjustments that I ignored so as always I recommend that you keep on testing.