scars_of_carma
11-02-2009, 09:59 PM
So I happened to read this interesting snippet in Popular Hot Rodding today. Quote:
DRAG SPRINGS
Regardless of how fancy a rear suspension setup may be, without the correct front spring rate it will never put the power down. Bill Buck says the ideal front spring rate for a drag car is just enough to hold the front end up in the air. From there, it's up to the shocks to fine-tune the rate of weight transfer. Furthermore, choosing the correct front rate is far more important than choosing the correct rear rate.
-Stephen Kim
"The lighter the front spring, the more weight it will transfer rearward when you hit the gas. Since springs with lower rates squish down more when loaded than stiff springs, they store more potential energy," he explains. "You can use stiffer springs to prevent the suspension from bottoming out, but that can hurt weight transfer, so I prefer using softer springs and travel limiters. If a car transfers too much weight and does violent wheel-stands, then it might be necessary to use stiffer springs. If Project Fox was a track-only car it could get away with 170lb/in springs, but since it will see some street time a 200lb/in spring is a better compromise. AJE's coilover conversion relocates the spring right on top of the ball joints. That means that the spring rates and wheel rates are very similar, which allows running a relatively soft spring." -Bill Buck
Weight Transfer and Tires
Although Buck doesn't mention the affect of weight-transfer on lateral grip it seems to me that the biggest difference is how the forces act on the tires. Tires are designed to roll that is their mode of least resistance. Braking and accelleration g's press down on the tires yes but they aren't forcing the tires out of their element. They simply make the tires roll slower or faster.
Generally speaking a cars suspension is designed to focus sprung-weight at the center of the contact patch. Pressing more weight on the tire will flatten the bottom of it out a bit but the shape and size of that contact patch isn't going to change drastically under accelleration and braking. Under lateral g's however the contact patch deforms and shrinks. Weight transfer stresses the sidewall as it always would but under lateral g's it's totally concentrated on the outside sidewall of the outside tires. Conversely braking and accelleration g's are spread evenly on both sidewalls of both tires.
Even as the contact patch deforms and shrinks Lateral g's want to drag the tire across the ground. Remember if the car is in motion the tires are constantly rolling. Under throttle or braking in a straight line they are not being told to roll in a direction other then the direction they are pointed in. In other words, the vector of the car is in-line with the tires so the tires are still in the mode of least resistance.
When you attempt to steer the car the vector of the car changes in relation to the tires. The vector of weight pressing on the tires is no longer in-line with the tires mode of least resistance... see where I'm going with this? Obviously there are ways to help the tires cope with cornerning forces. Negative-Camber, toe, tire design, tire composition/compound etc. etc. There are ways to help the suspension cope with cornering forces such as bushings and there are also ways to help the chassis cope with cornering forces such as swaybars...
Now Buck was saying that softer springs let you transfer more energy. Pressing more weight on the tire will flatten it increasing the contact patch in addition to creating more friction. I think it is true that soft-springs may achieve a higher peak grip then stiff-springs for this reason. However, it is also very easy to overload a tire in this fashion and the g-load must be constant or its unsustainable.
The real benefit of stiff springs is that it's easier to feel and maintain the tires grip threshold under lateral g's. In other words the usable grip is nearly equal to peak grip. With soft springs the peak grip is hardly usable under lateral g's.
DRAG SPRINGS
Regardless of how fancy a rear suspension setup may be, without the correct front spring rate it will never put the power down. Bill Buck says the ideal front spring rate for a drag car is just enough to hold the front end up in the air. From there, it's up to the shocks to fine-tune the rate of weight transfer. Furthermore, choosing the correct front rate is far more important than choosing the correct rear rate.
-Stephen Kim
"The lighter the front spring, the more weight it will transfer rearward when you hit the gas. Since springs with lower rates squish down more when loaded than stiff springs, they store more potential energy," he explains. "You can use stiffer springs to prevent the suspension from bottoming out, but that can hurt weight transfer, so I prefer using softer springs and travel limiters. If a car transfers too much weight and does violent wheel-stands, then it might be necessary to use stiffer springs. If Project Fox was a track-only car it could get away with 170lb/in springs, but since it will see some street time a 200lb/in spring is a better compromise. AJE's coilover conversion relocates the spring right on top of the ball joints. That means that the spring rates and wheel rates are very similar, which allows running a relatively soft spring." -Bill Buck
Weight Transfer and Tires
Although Buck doesn't mention the affect of weight-transfer on lateral grip it seems to me that the biggest difference is how the forces act on the tires. Tires are designed to roll that is their mode of least resistance. Braking and accelleration g's press down on the tires yes but they aren't forcing the tires out of their element. They simply make the tires roll slower or faster.
Generally speaking a cars suspension is designed to focus sprung-weight at the center of the contact patch. Pressing more weight on the tire will flatten the bottom of it out a bit but the shape and size of that contact patch isn't going to change drastically under accelleration and braking. Under lateral g's however the contact patch deforms and shrinks. Weight transfer stresses the sidewall as it always would but under lateral g's it's totally concentrated on the outside sidewall of the outside tires. Conversely braking and accelleration g's are spread evenly on both sidewalls of both tires.
Even as the contact patch deforms and shrinks Lateral g's want to drag the tire across the ground. Remember if the car is in motion the tires are constantly rolling. Under throttle or braking in a straight line they are not being told to roll in a direction other then the direction they are pointed in. In other words, the vector of the car is in-line with the tires so the tires are still in the mode of least resistance.
When you attempt to steer the car the vector of the car changes in relation to the tires. The vector of weight pressing on the tires is no longer in-line with the tires mode of least resistance... see where I'm going with this? Obviously there are ways to help the tires cope with cornerning forces. Negative-Camber, toe, tire design, tire composition/compound etc. etc. There are ways to help the suspension cope with cornering forces such as bushings and there are also ways to help the chassis cope with cornering forces such as swaybars...
Now Buck was saying that softer springs let you transfer more energy. Pressing more weight on the tire will flatten it increasing the contact patch in addition to creating more friction. I think it is true that soft-springs may achieve a higher peak grip then stiff-springs for this reason. However, it is also very easy to overload a tire in this fashion and the g-load must be constant or its unsustainable.
The real benefit of stiff springs is that it's easier to feel and maintain the tires grip threshold under lateral g's. In other words the usable grip is nearly equal to peak grip. With soft springs the peak grip is hardly usable under lateral g's.