FRONT END ADJUSTMENT
By Mike Unger
By now you already know that the front of your kart points you where you want to go, right? You also know that the front of your kart can be quite complicated with all of the adjustments possible right? I mean think about it, caster, camber, Ackerman, toe, track width, steering rate, ride height, static camber, camber gain, torsion bars, can all be used to help get to where you want to go, but that list can be a bit overwhelming to sort out. This article is intended to give you a better understanding on how to go about sorting out the front of your kart.
With that in mind, we need to have an understanding of the terms used to explain the behavior of the front of the kart. Understeer or push is when you turn the steering wheel but the kart does not want to turn. This understeer can be as severe as it causes you to completely miss the apex or as mild as it simply causes you to turn the steering wheel a little more than you like. Oversteer or loose is when you turn the steering wheel and the rear of the kart rotates too much causing you to get to the apex too soon. Generally severe oversteer is best fixed by dealing with the rear and severe understeer is best fixed by tuning the front end. Mild cases of understeer or oversteer can be attacked with either the front or the rear end adjustments depending on what the problem is.
Let’s talk about what each of the adjustments are and why they are important. After we get the definitions out of the way I will share with you a step by step way to navigate through all of them. Let’s start with an easy one, ride height. Front ride height is simply the height of the front-most frame rail right behind the front bumper to the ground. Adjusting the ride height in most karts is a simple matter of moving spacers to the top or bottom of the spindles. Lowering the spindles in the frame yoke, putting more spacers on the top of the spindle, will raise the ride height and raising the spindles, putting more spacers on the bottom of the spindle will lower the ride height. Raising the front ride height will give the front end more grip and lowering the front ride height will reduce front grip. Ride height tends to be a fine tuning type of adjustment in most karts.
Track width is another easy one to explain. Front track width is simply the distance the front wheels are from one another measured from the outside of each tire. Adjusting front track is a simple matter of moving spindle spacers from the inside of the wheel to the outside of the wheel. Front track is one of the adjustments that you can get carried away with and can cause some confusion. (Here’s a great article on track width) Increasing front track will make the front of the kart respond quicker at turn in but will reduce the amount of front grip at mid corner. Decreasing front track will make the kart less responsive on turn in but have more grip at mid corner.
Front track works very closely and is influenced by caster. Caster is defined as the angle of the front kingpin as viewed from the side relative to the ground. Most karts have caster angles in the 10 to 15 degree range. Adjusting caster varies from kart to kart but in most cases involves using eccentric pills that change the angle of the kingpin. Tilting the kingpin rearward increases caster and tilting it forward decreases caster. Increasing caster will increase front grip throughout the corner while decreasing caster will decrease grip throughout the corner. Caster also has a large influence on the rear of the kart so care should be taken to not only pay attention to the front end while you are adjusting caster but also keep an eye on the rear end too
Ackerman is one of those terms that people throw around a lot but many people simply do not understand what it is. Ackerman in simple terms is thedifference in the left wheel angle and the right wheel angle for a given steering wheel input. Zero Ackerman means that when you turn the steering wheel the right and left wheels turn exactly the same amount as the right. Negative Ackerman is when the outside wheel turns more than the inside wheel for a given steering angle. Positive Ackerman is when the inside wheel turns more than the outside wheel for a given steering input. Most all modern day karts have positive Ackerman built into the front spindle and steering shaft geometry. Ackerman is adjusted two different ways in the kart. The first way is by providing two places on the front spindle to attach the tie rod to. Attaching the tie rod end to the inboard most hole will increase Ackerman. Attaching the tie rod end to the outer most hole will decrease Ackerman. The other way to adjust Ackerman is to change where the tie rod end is attached to the steering shaft. Attaching the innermost tie rod ends as close as possible or even in the same hole will decrease Ackerman where attaching them to outermost holes will increase Ackerman. Increasing Ackerman (making the inside wheel turn more than the outside) will make the front of the kart turn in quicker. Decreasing the Ackerman will make the front of the kart turn in a little slower. Ackerman will also have a very small effect at mid corner but in general only is used to deal with turn in problems. Ackerman may not be adjustable on your chassis. If it is not, don’t worry there are many other ways to tune up the front end.
Camber is the angle of the tires relative to the road when viewed from the front. Negative camber is when the top of the tires are tilted inboard and positive camber is when the top of the tires are tilted outward. Camber is usually adjusted in the same way that caster is. Tilting the kingpin outward will add positive camber and tilting in inboard will add negative camber. Changing the kingpin angle is usually done by rotating eccentric pills in the yoke of the kart. Tilting the tires inboard will remove front grip and tilting them outboard will add grip. I usually set camber based on the front tire wear pattern. If the inside of the tire is wearing quicker than the outside, tilt the tire outboard. If the outside is wearing quicker then add more camber.
In newer karts you can change the steering rate and effort. This is done by changing where on the steering shaft the inner tie rods attach to the steering shaft. Moving the tie rod ends closer to the steering shaft slows the steering down and makes the steering effort less. Moving the tie rods farther away from the steering shaft will make the steering quicker and make the steering efforts go up. Setting the steering rate and effort is largely a driver preference. For young kids a slower steering rate not only makes the kart easier for them to steer but it also smoothes out some of the erratic inputs young kids make.
Just like in the rear the front of many karts have removable torsion bars that change the stiffness of the front of the frame. These torsion bars come in all shapes and sizes from tubes that are flattened in the center to tubes with various wall thicknesses. Making the front end more stiff tends to make the front more responsive and making the front less stiff will make the front less responsive.
OK, now that you understand all of the adjustments that are at your disposal lets talk about how to apply them. Let’s assume it is a hot sticky day and there is a lot of rubber on the track. Your kart is understeering and it feels like it is stuck to the track. This is how I would attack the problem given the tools described above.
Increase the front track. This will increase the weight jacking effect at turn in and will help break that inside rear tire from the sticky track. Be watchful that you do not develop mid corner understeer as you increase track.
Add front bar if not already in. This will make the front end stiffer between the left and right kingpins and allow the weight to be transferred more efficiently. Some karts when you add the front bar in sticky conditions will experience a chassis oop. If it does hop, remove the bar and move to the next tool.
Increase front caster. Tilt the kingpins rearward. This will increase the weight jacking effect lifting that inside rear wheel helping to reduce understeer. Increasing the caster will make the steering more heavy. If it becomes too heavy move the innermost tie rods to holes closer to the steering shaft.
Add positive camber, tilt the kingpins outward. This will get more of the tire surface in contact with the track and help add grip to the front end. Be watchful that this change doesn’t make the front end too twitchy. Some chassis do not respond well to a lot of camber.
For a cold day where you are looking for grip you can go through the same list but simply do the opposite.
Other things to consider when it comes to tuning the front end. If you are having problems with the front being too twitchy consider reducing Ackerman or slowing down the steering rate. If it isn’t turning in quick enough, increase Ackerman and steering rate.
Finally in the end it is important to attack a front end problem by making changes on the front. Sacrificing grip at the rear for an ill tuned front end will not get you to the podium. If the front is not working, tune it up.