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  • Jay Neal

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.


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