| Understanding your Chassis - by Mike McCarty |
I've read it many times on 4cycle.com about what the optimum setting is for cross. You don't hear it as much with nose and leftside, but the question will pop up. The answer is that there are no perfect set-up numbers. What must be realized is that each kart and each driver do not perform the same. Therefore each will require different numbers to perform to both the kart and the driver’s ability. These numbers may even go outside what the manufacturer has suggested. The thing that separates the really good drivers form the mediocre ones is experimentation. These racers use the set-up sheets as a baseline to start from and tweak the numbers from there. Now the question is how do you find your perfect numbers? Let me explain.
First you need to understand some things. Setting up a kart is nothing more than a balancing act. What we are looking for is the perfect balance between forward traction, turning traction and sideways traction (sidebite). If you have too much forward traction, the kart does not want to turn into the corners. The front of the kart just slides. This is an understeering (pushing) kart. If the kart has too much turning traction then the rear end of the kart slides up the track. This is an oversteering (loose) kart. If the kart has too much sidebite it will 'feel' great in the corners but loses speed. This is a binding kart. What is important to remember right now is that turning traction hurts forward traction and vise versa. Then if you have too much turning and forward traction then you have a 'binding' kart as well.
What you are looking for is corner speed. Corner speed equals greater straightaway speeds. This means that you want the maximum amount of forward traction with the least amount of turning traction that you can get away with in the corners. Too much or too little of one or the other or even both will slow you down. Let’s look at some things in basic terms.
Nose weight: Nose weight affects how a kart turns. More nose weight equals more turning traction. Less nose weight is the opposite. Too much nose will be loose on entry while too little will cause a push on entry.
Leftside: Leftside controls sidebite and how much weight is transferred to the rightside tires. Too much LS will cause a 4-wheeled drift and too little will be tight on entry.
Cross: Cross affects how a kart turns and the distribution of weight transfer to the rightside tires. You have varied opinions on cross. Some say increasing cross loosens the kart while others swear it tightens the kart. My advice is to experiment with cross and find out how it affects your kart. I will tell you that the closer cross comes to equaling leftside, the tighter the kart will be. High cross is usually run when there is a lot of grip. Low or lower cross is usually run when there is little grip. On coke syrup, cross is usually negative, meaning that you run under 50%, usually 42 to 48%.
Rear Stagger: Rear stagger affects how well a kart rolls in the corners. More rear stagger means that the kart will turn in a tighter circle and less rear stagger will make the kart turn in a larger circle. Careful because rear stagger affects straightaway speed.
Front Stagger: Front stagger affects how the kart redistributes weight as the front wheels are turned. Usually high cross or small tube chassis require less front stagger while low cross or big tube chassis require more front stagger. More front stagger will reduce cross while less front stagger will increase cross. Other factors are with camber, especially camber gains on the RF with steering input. All I'm going to say is try 1 1/4" front stagger then 1 3/4" front stagger (resetting your numbers) and add steering input. Look at your numbers (nose, LS and cross) and camber changes compared to both. You will be surprised. Front stagger also changes your scrub radius and caster trail. It will also set the rake of the chassis. The rake of the chassis is the tilting of the front of the chassis relative to the ground. This adds a certain amount of “twist” to the chassis much like preloading a spring. It also affects the natural camber that the kart “sees”.
Caster: Generally, more caster equals more front bite, less caster equals less front bite. There is a lot more to the caster puzzle than this though. I hear this statement a lot: "Caster is a driver preference adjustment". To me there is way more to the caster game than most will admit to. Caster sets into motion the rate of weight transfer. Either caster setting will 'unload' the left rear. One does it mechanically (left front) while the other does it by nature (right front). Most will leave the caster settings 'as set by the factory' and few will play with caster. Caster is a great tool to use to tune the kart. You can generally add bite to the kart by decreasing the split or by adding caster to both sides. If you increase the split you can take bite out of the kart. These are very general statements. There is a lot more related to caster than what is obvious. Generally, as a track gains bite you can reduce caster. When the track is green or has little bite you have to add caster. Caster gives the front tires more leverage over the rear tires.
Camber: This adjustment has several things tied to it. They are: tire grip and the timing for weight transfer. This is why changes in camber can drastically change the handling of the kart. The amount of camber that you need depends on such things as track surface and configuration, tires, and the amount of cross that you have. Generally high cross set-ups need -3.0 camber or more. Low cross set-ups need -2.5 or less. The more nose weight that you run the more sensitive the kart is to camber adjustments.
Basically kart setup is a balancing act between turning traction (nose weight), forward traction (rear weight) and sidebite (leftside). As discussed above, too much forward traction (rear weight) will cause the kart to push and not enough will cause a kart to be loose. Too much sidebite (low leftside) will cause the kart to bind while too little (more leftside) will cause the kart to 4-wheel drift. Too much turning traction (more nose weight) will make the kart loose while too little (less nose weight) will cause the kart to push. Then we have probably the most confusing setup number … cross. I don’t know why but people really confuse themselves with cross. Let’s take a better look at cross.
Cross Weight: You can ask ten different questions on cross and get ten different answers. I'm going to explain it like this: Cross is NOT a primary adjustment tool. Cross should be used to fine tune the kart. If your kart is close on setup here is how to adjust cross. I adjust by air pressure (AP) build up and using tire temps. If in all cases my right front builds more AP than the right rear then I reduce cross. This will relieve the right front of some of its dynamic loading. If the right rear is building the AP then I increase cross to relieve it of some of its loading. A lot of what you do will depend on your front-end settings. Generally adding cross decreases the amount of bite in the kart making it more loose if you are moving away from leftside. Say leftside is 56%, then going from 59% cross to 62% cross will free up the kart. On the other hand decreasing cross adds bite to the kart making it want to push. This would be the same leftside as above but moving the cross to 57%. Like I have said before cross can affect your kart differently depending on your balance (nose vs. leftside). I would explain how to use tire temps to indicate that you have too much cross; it would just take up too much space. Basically, you take the average of the left rear and the right front minus the average of the right rear and left front. This number should be zero. If it is a plus then you have too much cross. If you have a negative you have too little cross.
I know many race both dirt and asphalt. So what changes do you need to make? Do anything to take bite out of the kart. That would be to increase leftside, cross and right front camber and reduce caster and nose weight.
Weight Transfer: The amount of weight that transfers is proportional to the height of the center of gravity (CG), and inversely proportional to the track width of the kart. The following formula will always hold true: Total Transfer = Total Kart Weight * CG height * Cornering G’s/ Track Width. You want to minimize weight transfer as much as possible but not to totally eliminate it; this in turn prevents a decrease in overall traction. Remember that the more loaded a tire is the more it grips. Most will confuse car tire loading with that of a kart. When a car tire is loaded more it will generally lose grip. The amount of weight transferred is also dependent on other factors like the karts speed, the radius of the turn and the kart’s path through the turn. The fact that the amount of weight transferred is proportional to the radius of the turn is one of the reasons why a large, smooth radius is the fastest line when cornering: it minimizes weight transfer, so it maximizes overall grip and cornering power.
Obviously, weight transfer has an overall effect on handling: more weight on a tire means more grip. So, if the CG is located further towards the rear, the kart will have a lot of rear traction, which is nice to have if acceleration is important. If the CG is located further towards the front, the kart will have a lot of steering, but it might lack rear traction, which increases the risk of spinning out. This is the same for lateral weight distribution in LTO karts except it works left to right. So the CG is very important because it determines the karts roll characteristics and weight transfer.
The reason you run so much leftside weight in LTO karts is to better distribute the weight when you corner. Asphalt is a little different but not much. The reason you run even more leftside on asphalt than you do on dirt is because when you corner you create more G-force. Because of this, more leftside is needed to counteract the greater side-to-side weight transfer. Same thing for dirt. When the track is wet/slick you need less leftside because the weight is not going to transfer like it will do when the track is hard/fast. So the more G's you create the more leftside that you need and vise versa. On coke syrup you need a ton of leftside, usually more than 60%. This is to keep the kart from flipping over due to excessive G-force.
To a certain extent cross plays a role in dynamic load transfer. Chassis design plays a very important role in how a kart distributes dynamic loads. Karts are geometrically and mechanically softer in the front than in the rear. Part of this has to do with CG ‘roll’. Something to consider is that dynamic weight transfer will always seek the path of least resistance. Here is an example: due to low lateral acceleration the CG must be able to roll easily (low front roll stiffness). Low front roll stiffness will make the transition stage between turn in and keeping the left rear unloaded from the effect of horizontal acceleration easier. All I’m saying is that weight will stay off the left rear as the ‘jacking effect’ is coming off, rather than having the left rear ‘reload’ due to excessive front roll stiffness. This is speaking in terms of a track with low grip. It's opposite for a high grip track. The waist of the kart is usually the softest part of the chassis.
As a very general rule, entry handling problems can be adjusted at the front-end while exit problems can be adjusted at the rear end. Usually front-end settings affect how a kart enters the corner and the rear end affects how a kart exits.
As for tires and air pressure well… there are so many combinations to that puzzle that it is nearly impossible to cover them all. Not only do you have different combinations of air pressure, tire compounds and tire manufactures but each person’s setup has an affect on these combinations as well. The best thing to do is experiment.
This is a very brief explanation of understanding your chassis. I have a manual out that goes into more detail about how all this stuff affects your kart. Most call it the “bible” on understanding you chassis. Go to www.kartcalc.com for ordering information. |
| Weight Transfer - by Pat Dotson |
INTRO
In any discussion of kart handling and setup, "weight transfer" is a term that will always come up. It is usually used in a general sense, and is almost meaningless without some additional information. Here is why; on a kart, there are three different ways that weight shifts from one tire to another.
The purpose of this article is to define each of the three types of weight transfer. With a better understanding of weight transfer, discussions on kart setup can be more detailed, and a greater understanding of kart dynamics will be possible.
THREE TYPES OF WEIGHT TRANSFER DEFINED
The three types of weight transfer are:
1) Mechanical Weight Jacking - This type of weight transfer occurs when the steering wheel is moved, causing the front wheels to turn. It can be seen and measured on scales.
2) Dynamic Weight Transfer - This type of weight transfer occurs in corners. It is caused by lateral G-force shifting weight off of the left side (LS) tires and onto right side (RS) tires of the kart.
3) Dynamic Weight Jacking - This type of weight transfer is really just a part of dynamic weight transfer. When weight transfers from the LS to the RS of the kart in a corner, most of the weight will tend to go toward either the right front (RF) tire or the right rear (RR) tire. It is determined by chassis design and setup.
MECHANICAL WEIGHT JACKING
In some ways, mechanical weight jacking is the easiest mode of weight transfer to understand. It occurs when the steering wheel is moved, turning the front wheels. With steering input, the LF tire will move downward, while the RF tire will move upward. This is easily seen and measured on scales. The amount and timing of the weight transfer is affected by all of our front-end setup parameters; camber, caster, KPI, scrub radius, etc.
Despite how easy it is to see and measure, controlling this type of weight transfer is really the "black magic" area of kart setup. The individual setup parameters that control how mechanical weight jacking occurs are simple to understand. Understanding how they relate to each other, and the ability to use them to your advantage, is something very few people have mastered.
I'm not going to go any further into the details of this mode of weight transfer here. Look to Mike McCarty's "Understanding Your Chassis" article in the 4-Cycle Chassis Tech Section (http://www.4cycle.com/chassis/art.asp?art_id=34&art_content_id=210&FormArticle_Page=1&) for a more complete description. For the purposes of this discussion, it is only important to recognize that this type of weight transfer differs from the other types.
DYNAMIC WEIGHT TRANSFER
The concept of this type of weight transfer is very simple; when a kart is turning, weight transfers from the LS of the kart to the RS. Dynamic weight transfer refers to the total amount of weight that transfers from left to right. It can be easily predicted.
There are only a few items that affect dynamic weight transfer:
1) VCG - the height above the ground of the center of gravity of the kart and driver.
2) Kart Track Width - the width of the kart. The distance between the outside edge of the rear tire tread can be used.
3) Lateral G-Force - the sideways force generated in a turn.
4) Total weight of the kart
In the "Understanding Your Chassis" article in the 4-Cycle Chassis Tech Section, Mike McCarty mentions the formula for calculating dynamic weight transfer:
"Weight Transfer: The amount of weight that transfers is proportional to the height of the CG, and inversely proportional to the track width of the kart. The following formula will always hold true: Total Transfer = Total Kart Weight * CG height * Cornering G's/ Track Width."
Think about it this way; the length of the VCG is a lever. The total weight of the kart and driver is distributed between the LS and RS of the kart. As G-force builds in a turn, it pushes to the right on the VCG lever, attempting to lift the LS of the kart. As the LS of the kart lifts, weight is taken off the LS tires, and moved to the RS tires. As more G-force pushes on the VCG, more weight will be transferred.
It is important to understand that the amount of dynamic weight transfer does not depend on tire pressure, chassis stiffness, or any other characteristic of kart design or setup. Also, on a kart, this type of weight transfer happens almost instantaneously. There isn't much that can be done to speed dynamic weight transfer up or slow it down, but it is easy to control the total amount that occurs. Adjusting the VCG height or the track width of the kart will change the total amount of weight transfer that occurs. Adjusting the amount of static LS weight will determine how much total weight ends up on the RS of the kart in a turn, which controls "side bite".
DYNAMIC WEIGHT JACKING
As mentioned previously, dynamic weight jacking is really just a part of dynamic weight transfer. It is simply the distribution of the dynamically transferred weight between the RS tires. Some percentage of the transferred weight will go to the RF tire, and the remaining percentage will go to the RR tire. These percentages are determined by setup parameters and, more importantly, chassis design.
The goal of chassis design and setup is to reach a neutral balance in handling during turn-in, at the apex, and during the exit of a turn. The key to this balance is controlling the loading on the RS tires throughout the turn. In controlling this balance, the unloading and re-loading of the left rear (LR) tire is critical.
Statically, the LR tire on a kart carries the most weight. In a corner, most of the weight that will transfer is going to come off of the LR tire. It will not transfer directly from left to right. Much of the weight statically loaded on the LR tire will transfer to the RF tire.
From a chassis design perspective, the distribution of dynamic weight transfer is determined by things like tubing diameter and wall thickness, and the stiffness or softness of the waist of the kart. Another factor is the stiffness of the rear axle. Some chassis are equipped with split tubes that can be clamped. This allows some adjustment of the dynamic weight jacking characteristics of the chassis.
From a setup perspective the main thing that affects distribution of weight is the relative stiffness of the RF and the RR of the kart. It helps to think of each corner of the kart individually. Anything that changes the relative stiffness of the RF corner of the kart compared to the RR corner will affect transferred weight distribution.
The key point to remember is that a stiffer corner will be more resistant to weight transfer. Anything that softens a corner of the kart will cause more weight to transfer to it.
Let's go back for a minute to the first type of weight transfer discussed; mechanical weight jacking. When the steering wheel is turned, it is obvious that weight is transferred from the RF/LR tires onto the LF/RR tires. What is less obvious is that the RF is being made softer compared to the RR. With the steering wheel turned, more of the transferred weight will be distributed to the RF tire. More caster, scrub radius, etc. (anything that causes more mechanical weight jacking), will cause more weight to transfer dynamically to the RF, creating more grip on the RF tire and less on the RR tire.
Cross weight will also affect the distribution of transferred weight. With low-cross setups (generally 54% or less) lowering cross weight will further soften the RF, increasing bite via more weight transfer.
High-cross setups work differently. A high-cross setup creates a relatively stiffer RF compared to the RR, and so less weight will transfer dynamically to the RF compared to a low-cross setup. This type of setup works because of the grip provided by weight that is statically pre-loaded onto the RF tire.
CONCLUSION
These are the basic issues surrounding weight transfer. Mike McCarty goes into a much more detailed discussion of weight jacking axes, and chassis design variables in his manual "Understanding Chassis Theory and Dynamics".
Next time the subject of weight transfer comes up, think about which type of weight transfer is really being discussed. When you have questions about a setup issue, try to use these terms to describe your thoughts. |
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