rear spring rates...why?

I have noticed that many setup suggestions on this site seem to suggest LR softer than RR. We used mastersbilt setup sheets for our 02 Boss, which recommended 250LR, 225RR for starting point..the Steve Smith book from '04 also lists that LR should be stiffer than RR..

when we got our car it had 200LR, 250RR...we changed it based on neutral baseline setup sheets..

Just curious why the suggested setups have changed...

I know that car setup is based on what the driver feels the car needs..and we are working towards that (season over now though..gotta wait til next year)..

opinions on the change? I guess in essence I would like to know what to expect if we switched spring rates...

a softer spring stores more energy,for instance you say you have a 250 lr spring,now you change it to a 200 if you dont put rounds in it the car will loose bite and ride hieghts will be off (thats if all you do is change the lr spring)so you reset everything and you had to put rounds in and sometimes even preload it a little to get the bite you want and the ride hieghts right,so what did you do??? first a 200 lbs spring will spring up more and quicker than a 250,second chances are the lr will never completely come off the spring because its preloaded a little,hope this helps just a quick idea of whats going on but like anything else on a race car there are alot of other factors in there to

Tighter entry and tighter "on the bars exit". Exit will be looser if you are off the bars when you pick up the gas. What you are seeing is the fact that people are after a different attitude in the chassis now. It takes a different approach to make that work.

As our chassis dealer always tells us, "It's all part of the puzzle" meaning it's a component of a certain set-up. You can use a stiffer LR spring than the RR, but the rest of the set-up (RF SPRING, and BAR ANGLES, ETC.)must compliment that. It all depends on what the chassis is telling you along with driver feedback.

If a guy did have a stiffer lr then right rear would he need add more bite and rf spring to add drive.

The Steve Smith book you are referring to is great but it has some information that doesn't exactly conform with today's setups and this is probably the most glaring. The book says that a stiffer LR will make the car tighter on exit. This is based on old school understanding that as weight transfers to the rear and you apply the "heavy spring gets the weight" rule of thumb, the LR is going to get more load and hence drive harder causing the car to be tighter under acceleration. The problem with applying that logic to today's setups is that the LR spring is usually completely unseated once the car is at full hike and the car is only being held up by the bars. The spring isn't doing anything once it comes unseated. So the question now is is what the spring doing WHILE the car is hiking?
When you drive the car into the corner, a certain amount of weight is going to transfer from LR to RR and that number is completely independent of springs and only really dependent on roll center and vertical center of gravity (assuming you enter the car into the corner the exact same every time which I'm sure we all do ;).
Let's us a hypothetical example for the sake of simple math. Let's say on our car the LR has 500 pounds of static weight and the RR has 500 pounds of static weight and that when we go into the corner 250 pounds transfers from LR to RR. If we had 250 pound springs across the back of the car, this would make the LR spring unload 1 inch and the RR spring compress 1 inch.
Take the same car and now put a 200 pound spring on the LR and reset your ride heights. For the same amount of weight transfer that LR spring now has to unload 1.25 inches. That extra unloading means extra hike which means more bar angle which means more LR thrust angle which means tighter on-throttle race car.
This is a crude example for the sake of simple math and the reality is even more extreme. There are plenty of fast 4-bar cars out there with LR spring rates down under 150 pounds.

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Originally Posted by Matt49

The Steve Smith book you are referring to is great but it has some information that doesn't exactly conform with today's setups and this is probably the most glaring. The book says that a stiffer LR will make the car tighter on exit. This is based on old school understanding that as weight transfers to the rear and you apply the "heavy spring gets the weight" rule of thumb, the LR is going to get more load and hence drive harder causing the car to be tighter under acceleration. The problem with applying that logic to today's setups is that the LR spring is usually completely unseated once the car is at full hike and the car is only being held up by the bars. The spring isn't doing anything once it comes unseated. So the question now is is what the spring doing WHILE the car is hiking?
When you drive the car into the corner, a certain amount of weight is going to transfer from LR to RR and that number is completely independent of springs and only really dependent on roll center and vertical center of gravity (assuming you enter the car into the corner the exact same every time which I'm sure we all do ;).
Let's us a hypothetical example for the sake of simple math. Let's say on our car the LR has 500 pounds of static weight and the RR has 500 pounds of static weight and that when we go into the corner 250 pounds transfers from LR to RR. If we had 250 pound springs across the back of the car, this would make the LR spring unload 1 inch and the RR spring compress 1 inch.
Take the same car and now put a 200 pound spring on the LR and reset your ride heights. For the same amount of weight transfer that LR spring now has to unload 1.25 inches. That extra unloading means extra hike which means more bar angle which means more LR thrust angle which means tighter on-throttle race car.
This is a crude example for the sake of simple math and the reality is even more extreme. There are plenty of fast 4-bar cars out there with LR spring rates down under 150 pounds.

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Crude but effective. It was explained to me as a "jack in the box" effect. To achieve the correct ride heights with a softer spring, you need to cranks some rounds in it. This squishing of the spring has a jack in the box effect when the chassis starts to roll over. Helping pop the LR up in the air. It also helps to think of things in extremes. If you had a 1000 lbs spring on the LR, how much popping up would it do with a spring that stiff.? The car is essentially sitting on a block of steel.

There is to many variables in today set ups i think it's mostly what the driver wants. I also run the soft RR and Heavy LR but i also only run 30 lbs of LRW. But there is alot more that goes along with that LS %,R%,Shock angles,roll centers,J bar lenght and height,lead placement track size and lets not for get the everchanging shock package. Start with a baseline set up make changes till the driver is comfortable with the car then run it a few weeks if thats what the driver likes now you have a base line that works for you then you can start changing looking for more out of your car.