Sprung vs. Unsprung Weight
First lets define each one and what they mean to us as dirt racers.
Sprung Weight: I have a feeling this is going to get me in trouble with some people because the ideals about this are all over the place. Sprung means that the 4 springs in the car is carrying the weight on them, the amount of total weight being carried is the sprung amount, lets look at that. A car weighing 2450 (race car dirt modified) is carrying about 1640 pound of weight on the springs and 810 pounds off the spring. This means we have 1640 sprung and 810 un-sprung weight in this car. Understand this is not spot on but a general ideal of the numbers. The un-sprung weight is stuff like the tires, wheel, brake Assy, rear axle Assy, springs, and ½ the shocks weight, everything else is sprung weight. The sprung weight is than divided into two categories, in the spring box and outside the spring box. Weight in the box is controlled by the springs (good) and weight outside the box is leverage on the springs and tires (bad). In our standard cars it is hard to get weight inside the spring box or weight from outside the box that is, sometimes we must add weight where we do because there is just not a place for it. Stock cars often must hang a lot of weight around the fuel cell because it is the only place that they can do so.
Unsprung Weight: things like the wheel, tire, brake caliper, rotor, hubs, spring, ½ of the lower control arm, ½ the shock, this all adds up to what is unsprung weigh. (see list of unsprung weight) This is stuff that the shock valving is going to need to control when the car hits stuff on the racetrack. The lighter the unsprung weight is the faster the tire makes it back to the ground and in most cases that means more traction at the tire. But there is a bit of a disconnect in the plan, and that starts with the way we think about our race cars. We were always tough to think that the lighter we got our race car the faster it will be. The old saying was, make your car as light as you can so you can move the weight to the spring box area, and this will make your car faster. We just kind of shorten that down some to, make your car as light as you can, and it will make it faster. The problem is we need that weight pushing down on the tires to help make traction and as our cars get lighter, we keep losing weight from the spring box area and not the unsprung area and this is causing handling problems. To understand what I just said our cars have gotten lighter while the unsprung weight has stayed close to the same, which means the ratio between the two has gotten worse. Increasing unsprung weight by decrease sprung weight is going to create handling problems at the tire.
Controlling the weight is going to be the next step in the puzzle and one in which we will need to do with the shock. This is where technology comes in, it allows the guy with info to get to the answer fast than the guys without. Technology is like putting together a puzzle with a picture on it versus putting together a puzzle that has no picture. (Rant over) Controlling the tire motion and keeping it placed on the ground is done by understanding 3 things. One weight of the unsprung matter, two, the speed of the unsprung matter, and three the effect that the matter has on the tire. We can use these 3 things to help control the tire and reduce the amount of time the tire is off the ground. 1st the unsprung weight, the total weight of the matter, (see list to determine weight) ½ of the unsprung weight is what we have to control with the shock. Now the speed in which that tire is moving up and down, this info will be harder to determine and will take data or trial and error to come to a conclusion.
Well how important is unsprung weight, well it is pretty important to the overall outcome of the car but into days world very few people will read this let along do the work to determine where they are at with unsprung vs sprung weight. After doing this research we will be looking at RR compression and will determine which direction we will be going to improve the right rear tire and its load.
REV: 7-9-21: RR wheel load.
For many years we have had conversations about the RR, most of them have been in the shock and now a lot of it is focus on the spring. So what is the big deal with the right rear that make it so important to the car, FEEL. This is a touchy subject because it affects us all different and different from week to week in a lot of cases. For years we have worked to improve feel in the shock, and I think we have made small improvements in this area, I also think we have learned to put it on the back burner and just go with the car. Either way we need to be looking and understanding how the car responds to the RR.
Today it is somewhat different with the motor package that we are on, where in the past we were all on an open motor we tended to drive closer to the same, but with todays crates we need to drive different. With that being said the RR takes on a new look and job as we enter the corner, it is about holding the car up vs making side bite and dig. In todays racing world we talk about wheel load a lot and it seems to have a lot of different meanings so let us look at a couple of those.
We need to be in a range of 350-425 depending on the car / racetrack / tire / and speed, therefore we need to adjust this setting. Remember this, if your car weight is 2460 with driver than you will need to add lead to the car to move weight to the right rear. The lighter the car the less you can turn jack bolts to move weight, and you can only do it 1 or 2 times at most than you need to add weight to the car. When moving weight in a car using the coil overs or jack bolts you need to turn all 4 corners, if you need to increase wheel load at track add lead to the right rear. Hear is the procedure to move weight.
Why is this number so important to the car, it allows us a good starting point to determine wheel load and rear steer in the car and makes it more consistent cornering for the car. So, what we are looking for and will need to be determine by you for your car is a down number at 2” below ride. We know in a Late Model that the number we are looking for is 900 lbs @ 2” and when we hit that number the car is good.
On other cars unless you have run data and know we are going to have to determine the down number by collecting data over the next 6 weeks of racing. If we are deeper than 2” of travel, we will need to increase our static load number or maybe spring rate. This effects the car entry to center and increasing speed in this part of the corner is highly important to decreasing your overall lap time. Tuning the car on the right rear with load / rear steer / and stager will determine the free roll steer and speed in the corner (entry to center) tuning the car with rebound / bar angle / j-bar will get the car center and off.
Improving the right rear feel in a car is a hard thing to do and can be done in many ways but here are a few things you need to be doing to make it right. Oh yea by the way when you get done adjusting the RR spring load you can add your left rear preloaded spring setup back to the car.