Post by Garrett on Apr 25, 2005 14:57:01 GMT -5
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.
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.