spring rate effect on handling
07-19-2001, 06:16 PM
#1
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spring rate effect on handling
I'm a little confused on the effect of a softer spring rate on the cars handling. Everthing being equal, why does a softer spring rate create (lets use the front on a 911 as an example) oversteer? Wouldn,t the softer rate make the car push? I always thought a stiffer rate made that particular end dig in more.
confused.
j.p.
confused.
j.p.
07-19-2001, 10:05 PM
#2
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Courtesy of "Going Faster" by Skip Barber School: "All other things equal, a stiffer spring will increase the resistance to roll and absorb more of the load transfer at the end of the car on which it is installed. At the front, going to a stiffer spring (higher spring rate) will increase understeer. Going to a softer spring (lower spring rate) will decrease the roll resistance in the front, moving the cornering balance away from understeer." They go on to say "while a softer spring should increase the relative grip of that end of the car, you have to keep in mind how the additional roll allowed by the softer springs affects the grip of the tire".
07-20-2001, 03:30 AM
#3
Slip angle (SA) is the difference between the direction the wheel is pointed, and the direction the wheel is traveling. In a turn these are not the same. This is because tires are elastic and lateral forces deform the tire.
SA is caused by tire deformation up to the limit of adhesion (loss of traction). In the real world this might be 15 degrees or more. Beyond that point, SA is caused by slippage - that is traction has been lost = and the tire is sliding causing extreme slip angle.
Understear is the condition when the SA of the front tires is greater than the SA. Overstear is the reverse condition.
All else being equal, increasing the load (weight) on a tire increases the SA. Hence a stiffer spring places more load on that tire in a turn and increases the SA at that end of the car.
However, all else begin equal, increasing the load (weight) on a tire increases the limit of adhesion. That means that a stiffer spring (while incresing the slip angle before limit of adhesion) will increase the limit of adhesion enabling that tire to generate a greater cornering force without breaking into a slide.
Note that this increase in the limit of adhesion in a turn will be at the expense of the limit of adhesion at the other end of the car.
Ideally the car should be balanced such that both tires will reach their limit of adhesion at the same time. Anything else will cause a spin before the maximum cornering force is acheived.
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Chuck Moreland '86 Cab - "Sparky", '77 Targa - "Sweet Pea"
SA is caused by tire deformation up to the limit of adhesion (loss of traction). In the real world this might be 15 degrees or more. Beyond that point, SA is caused by slippage - that is traction has been lost = and the tire is sliding causing extreme slip angle.
Understear is the condition when the SA of the front tires is greater than the SA. Overstear is the reverse condition.
All else being equal, increasing the load (weight) on a tire increases the SA. Hence a stiffer spring places more load on that tire in a turn and increases the SA at that end of the car.
However, all else begin equal, increasing the load (weight) on a tire increases the limit of adhesion. That means that a stiffer spring (while incresing the slip angle before limit of adhesion) will increase the limit of adhesion enabling that tire to generate a greater cornering force without breaking into a slide.
Note that this increase in the limit of adhesion in a turn will be at the expense of the limit of adhesion at the other end of the car.
Ideally the car should be balanced such that both tires will reach their limit of adhesion at the same time. Anything else will cause a spin before the maximum cornering force is acheived.
------------------
Chuck Moreland '86 Cab - "Sparky", '77 Targa - "Sweet Pea"
