FRONT TRACK WIDTH
By Chris Livengood
Front track width is a function of at least four different factors. For new karters, front width is adjusted by moving the wheels in or out on the front stub axles. Changing the width alters at least four separate factors. These four aspects are vertical center of gravity across the front of the chassis, scrub radius, the leverage on the chassis exerted by each wheel, and finally the dynamic change in wheelbase across the inside and outside of the kart when the steering is turned.
For all intensive purposes I am going to flat out ignore the dynamic change in wheelbase. To be honest, I have not fully explored this factor and cannot venture even a guess on how it changes chassis characteristic. For now I will focus on just the first three factors.
By decreasing or increasing your scrub radius through adjustment of overall front width you are also changing the amount of jacking effect created by the front end geometry of your kart. Scrub radius is the distance from the center of the tire to the kingpin bolt. Combining scrub radius with caster angle is what allows the inside front wheel to drop and the outside front wheel to rise when the steering is turned. A chassis with zero scrub radius will have a zero net gain in wheel height change even with excessive amounts of castor. Additionally, a chassis with zero degrees of castor and an infinitely large scrub radius will again experience zero net gain in wheel height change when steering input is changed.
It has been shown those scrub radius and castor angles are both directly related to jacking. As a result, it can be concluded that across a certain amount of steering input the inside front wheel will drop a given amount and that the outside front wheel will raise a certain amount. Through observation of the kart on the stand this jacking effect can be further confirmed.
The amount that each wheel changes height is thus a function of castor angle, scrub radius, and finally kingpin inclination. Kingpin Inclination (KPI) is best described as the angle at which the kingpin either leans towards the center of the chassis or away from the center of the chassis. Other than the previous short blurb I will largely ignore KPI in this article. Suffice it to say that KPI is also very important in controlling the rate at which the wheels raise or drop.
You are probably now wondering how does changing front track width or scrub radiuses alter chassis handling and what might the driver feel when making these changes? As was already demonstrated, wider front track width creates a greater amount of jacking effect across the front of the chassis and narrowing does the opposite. I have found that the majority of the time this creates the feeling of quicker turn-in and conditionally greater mid-corner grip. Theory regarding turn-in revolves around the speed at which the inside front tire drops.
Given the amount of steering input (steering angle) and the speed at which the steering wheel is turned remain constant the inside front wheel will literally drop quicker than an inside front wheel that is set to a shorter scrub radius. Remember that because the inside front wheel drops and the outside front wheel raises it can be deduced that karts must at a minimum create initial turn-in force using the inside front tire.
Practically speaking this should result in a kart that tends to turn-in faster. Moreover, this outcome can to a degree be associated with a slight compounding effect. Not only should increasing scrub radius help the kart turn-in more positively it should also require less steering angle from the driver.
Though, this is where we must consider the increase in leverage exerted across the front of the chassis. Imagine taking a very long lever and lifting on the inside front of the chassis. If the lever is extremely long it will be easy to not only lift that side of the chassis up, but it will also be easy to deflect the frame.
The paradox that karters experience with front width is that as you increase front width you also increase wheel leverage on thefront of the chassis. Leverage on the frame effectively decreases the frames ability to translate jacking effect efficiently. At an unknown and completely chassis dependent point the creation of a larger scrub radius to increase jacking will produce quickly diminishing returns. Simply, the point at which any chassis experiences this is dependent upon the almost infinite variables built into each brand. Theoretically, this phenomenon can happen to every chassis though the point at which the phenomenon can occur may be outside of the karts built in adjustability.
Furthermore, narrow front widths tend to translate jacking effect more efficiently into the frame. Keep in mind that wider front widths effectively soften the chassis. To make this situation more easily understood here is a theoretical explanation. Imagine a chassis that is 100% stiff. This chassis no matter how much load you place on it will never deflect. Turning the steering will then translate 100% of the wheel height change through the chassis. Now if the chassis is only 5% stiff and the steering is turned the result will be that only 5% of the total jacking effect will be transferred across the chassis. This means that the soft chassis deflects and in doing so absorbs 95% of the jacking effect.
Drivers with experience driving soft chassis, often comprised of 28mm tubing, call the point at which increasing front track width no longer produces greater turn-in “going soft.” This phenomenon can also occur with other stiffer chassis too, though, they are less likely to do so unless you are running very extreme front width settings.
If you have established that your chassis goes soft you may then experience that the kart actually experiences better turn-in characteristics by going narrower. I stress that this in most cases is experienced with very soft frames or when you are running extremely wide front widths. If you find yourself in this dilemma and you feel that the kart needs to turn-in more positively you may need to approach the entry understeer with different devices (hint: narrow the front and add castor).
Finally, we must discuss the effects of center of gravity across the front of the chassis. When we look at a kart and imagine the center of gravity we try to view the point at which the mass is centered around. This point exists in three dimensions. By widening or narrowing the front width we primarily change how the vertical dimension or the height of the center of gravity interacts with the two front wheels. A wider front track in effect lessens the amount of leverage that the vertical center has over the two front wheels.
There are two different theories that I have explored regarding the vertical center of gravity and front width. I believe each theory to be true but to be conditional depending on the characteristics of the chassis at any given time.
The first theory says that karts require weight transfer onto the outside tires to generate grip from the tire. If this holds true then narrowing the front width should gain mid-corner and exit grip. In this scenario the narrow width allows the weight to transfer with greater force to the outside tires.
The other theory says that a wider track lowers the center of gravity allowing the mass of the kart to be carried more evenly across the two front tires. The wider width does not literally move the center of gravity to a lower position but rather lessens the amount of leverage the existing center of gravity has over the chassis. Effectively, this is the same as lowering the center of gravity. Dispersing the mass of the kart across two tires should better than one and once again this theory should increase mid-corner to exit grip. However, this relationship is a bit more complicated. Remember that wider scrub radius also increases the rate at which the inside and outside wheels change height. As a result you get an effectively lower center of gravity and a mechanical situation (weight jacking) that also places weight on the inner front tire at a greater rate. Further, if you increase track width very far and as a result increase jacking to an extreme you may experience a paradoxical effect where the mechanical jacking actually takes paramount over the effective reduction in center of gravity. Thus you begin to overload the inside front and under load the outside front tire. It is a tricky situation indeed.
Before I review take a second or two to digest the previous paragraph. Put more simply, the wider track width tends to resist lateral weight transfer to a greater degree than a narrower one. Then, due to greater wheel height change a greater amount of weight is put on the inside front tire. These factors may interact with each other differently depending on which end of the spectrum you are on.
How these two factors compliment each other is extremely dynamic. Regarding on track characteristics I boil both of these theories down to two factors. These being tire loading and tire temperature. If you are running in conditions where tire temperatures are on the low side a good strategy for increasing grip may be to generate greater load on just one of the tires. In this scenario you may be looking to get weight on the outside front or the inside front and this may help to generate better tire temperatures and the feeling of increasing front grip. Conversely, if you are understeering on a day where front tire temperatures are normal to hot you may find that attempting to spread load across the two front tires will produce the feel of greater front-end grip. This philosophy however is largely qualitative and before you live or die on these words I suggest you engage in extensive testing.
I contend that both theories regarding front track width hold water, however, it is important to remember the complexity and conditionality of kart tuning. For example, if the track at the moment is yielding high levels of grip. Weight transfer may then occur in greater quantities than opposite track conditions. As a result it may be possible to actually overload the outside front tire and under-load the inside front tire. It may then be true that reducing weight transfer either via front track width or by other means may increase the sensation of front grip.
As always with karting it is important to remember that adjustments are almost always conditional. Moreover, in condition-Z making track width change-B will produce effect-1. Conversely, in condition-Y making track width change-B will produce effect-2. This applies not only to track condition but also what phase of the corner you are attempting to effect. In summary, karts are fickle instruments to tune. Making one chassis change often results in several changes in handling characteristics. Always make sure to evaluate the environment in which you are tuning before making a chassis adjustment. Finally, test chassis adjustments regularly and evaluate the results of the adjustment being sure to consider the condition that any given result was found.