Springs, Tires & Weight-Transfer

[scars_of_carma]

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.

[Sh4d0w]

While I do not dispute the points you mentioned, I wanted to point out that the focus of those drag cars is RWD - they WANT more weight on the back because that's where the power goes - thru the back tires to the road. Our 3g's (except that one guy with a RWD) are all FWD, so we want to prevent as much of that rear weight-shift as we can, to maximize our force going to the powered wheels up front.

Am I wrong?

[scars_of_carma]

While I do not dispute the points you mentioned, I wanted to point out that the focus of those drag cars is RWD - they WANT more weight on the back because that's where the power goes - thru the back tires to the road. Our 3g's (except that one guy with a RWD) are all FWD, so we want to prevent as much of that rear weight-shift as we can, to maximize our force going to the powered wheels up front.

Am I wrong?

That makes sense you're not wrong.

[markmdz89hatch]

While I do not dispute the points you mentioned, I wanted to point out that the focus of those drag cars is RWD - they WANT more weight on the back because that's where the power goes - thru the back tires to the road. Our 3g's (except that one guy with a RWD) are all FWD, so we want to prevent as much of that rear weight-shift as we can, to maximize our force going to the powered wheels up front.

Am I wrong?

I completely agree with scars and you shadow on all points mentioned. Just to add to this, in a FWD car it's imperative to reduce the 'squat' of the rear so as to avoid a front-rear weight transfer and take all weight off the front wheels. The best method for this is a stiff stiff ass end. Not only will it reduce squat, but under some load, the sprung weight will want to release and will then do it's best to transfer the weight back to the wheels that need traction most (the front). Now that's on g's associated with acceleration.

For lateral g grip, scars, I completely agree that you don't want to be too soft up front because you don't want to start rolling all over your tires sidewall and essentially killing the tire's effectiveness to maintain traction and grip. The fact that FWD wants power on the wheels that have more of a tendency to transfer weight, as compared to the rear wheels, it's a delicate balance to play with. On this, what I've read up on is that a semi-stiff front spring, combined with a stupid stiff rear is a decent place to start. From there, it's a question of limiting the lateral weight transfer of the rear by giving it a seriously stiff anti-sway bar. This will still allow the rear suspension to operate independently on minor undulations in the road, but operate as more of a straight axle rear on full weight transfers. Up front, those semi-stiff springs will keep the weight planted and will still allow the tires to maintain a decent contact patch to the road surface, especially when combined with a mild / moderately stiff ant-sway bar. This will allow weight transfer, but not complete inside wheel lift on hard turns, which will help maintain traction on the powered wheels, and should reduce the push of understeer. Oversteer can be corrected in many cases with more throttle and/or corrective steering, but understeer usually forces you to let off the throttle in order to regain front wheel traction.

(On a side note, and no mean for thread jacking, but I just played with my tire pressure and rotated my 'slippery~er' tires to the back on my Protege in an effort to prepare it for Auto-x yesterday. Then I took it for a test run in a local empty parking-lot, and was completely disgusted with how little traction under power the front had, yet the rear was damn near impossible to break free. I felt like the front tires were on ice and the rear was just pushing it straight. Useless steering sucks. Dive into a turn quick and try to pik the apex nice and late, and the 'turn-in' just turns into 'eat your sidewalls while the car just toboggans in the direction it was going before you cut the wheel. Sucks balls. I tried to play with tire pressure up front and out back, to no avail. Truly annoying. So much so in fact that I pulled the plug on my auto-x fun for yesterday. I'm now shopping for a rear anti-sway bar as a place to start.)

[scars_of_carma]

...what I've read up on is that a semi-stiff front spring, combined with a stupid stiff rear is a decent place to start.

I agree thats how my car is setup right now and it feels good.

(On a side note, and no mean for thread jacking, but I just played with my tire pressure and rotated my 'slippery~er' tires to the back on my Protege in an effort to prepare it for Auto-x yesterday. Then I took it for a test run in a local empty parking-lot, and was completely disgusted with how little traction under power the front had, yet the rear was damn near impossible to break free. I felt like the front tires were on ice and the rear was just pushing it straight. Useless steering sucks. Dive into a turn quick and try to pik the apex nice and late, and the 'turn-in' just turns into 'eat your sidewalls while the car just toboggans in the direction it was going before you cut the wheel. Sucks balls. I tried to play with tire pressure up front and out back, to no avail. Truly annoying. So much so in fact that I pulled the plug on my auto-x fun for yesterday. I'm now shopping for a rear anti-sway bar as a place to start.)

Bummer, "plowing" cars are no fun. My ZX-R acted like that more then I would have liked... Refer to the last section of my last post in the using brakes thread perhaps that will give you more ideas.

[Importordomestic]

you have said that you are always trying new setups on your car. What is your method for changeing setups? do you make very subtle changes or drastic to try and figure out a real quick median? also how long do you run a setup before changeing?
I see setups working in 2 directions. One is a setup that will allow the care to be physics wise the best that it can be and the other being the setup that allows you to drive the car the best. I tune to what gives me a better feel. I like to think that by going this way i will eventually find the best setup for me and my car.

[lostforawhile]

we need Johnny O to chime in on this, good thread.