The science behind a droop limiter


At first glance, a droop limiter and the imagined benefit of using one seem pretty cut and dry; in a corner or quick swerve, the body of a car’s left rear wants to raise up and shift weight from forcing down to forcing out and a droop limiter opposes that shift. But one doesn’t have to look much further than the first glance to realize there’s a lot more than that going on here. Right rear travel, top bar angle and total left rear drop, while together are the lion’s share of the math, are just parts of the whole picture. So in fact, there is a complex soup of physical forces mixing and opposing under there as the car moves and conditions change.

droop limiter

First things first. If we’re going to understand this thing, let’s start with the basics. Let’s look at what the forces that we are dealing with are and what are the physical confinements of the forces? Then we will be able to see the ‘wheres’ and ‘whys.’

cent forces

We all know centrifugal and centripetal force. These are the opposite forces at work when we stir paint in a can. You know how, when you stir it really really fast, how it has that spiraling cone down the center that draws paint from the outside, up over the top and down the spiral where it is finally slung to the outside again. The force that is acting on the body of the car is centrifugal force, as it tries to sling the body away from the pit, like a ball on a string. The reason it tries to flip the left up, rather than just slide the whole thing, is because of the sloped angle of the track… the track’s not flat. 

upforce diag

Now, between all the different possible combinations of forces and pressures, there can be as much as a ½ ton of upforce on your top four bar and therefore on your chain limiter, during the course of a race. That being said, at some point your chain limiter will be locked up, stretched as far as it will go. *Note: A droop limiter is not to keep your chain limiter from maxing out. It is to keep it from maxing out all at once. If your limiter maxes out too fast it can be hard on your equipment and may even break your left rear traction loose from the track.

lever diagram

There’s one more thing we need to consider before we start choosing the limiting element for our droop limiter housing, that’s the broader dynamics of our rear end and how its movement affects the behavior of the whole system. When the left lifts up on the rear end, it makes the rear end as a whole, act like a lever to the right wheel. And since the driveshaft is anchored (somewhat) to center, the differential acts like a fulcrum point for the leverage. This motion would take the traction that the left wheel has and add it to the traction the right wheel already has. Obviously, that’s too much and if the left breaks free and is spinning, you’ve lost all your torque anyway.

So, since energy is neither created nor destroyed and merely changes forms, we can say in a nut shell: in a turn or swerve, a car looses traction in the left from centrifugal force. A droop limiter would reclaim prevent the loss of, this energy by offsetting the motion with the weight of the entire rear end.

When choosing a limiting element, there are some obvious reasons to pick one thing or another and there are some NOT so obvious ones. We do not recommend having only one element that you always run. For example, you’ll want a softer spring or puck for a slick track. (Yes, softer.) You may want a softer spring or puck for a more aggressive driver. If your driver is very aggressive in his attack of the corners, a softer spring or puck will allow him to take advantage of ‘side bite.’ If you don’t know what that is, you should learn about it before you try to use the principle. You’ll want a harder element for a super than you will a crate because there is just more energy and higher numbers in all your variables than there are with a crate or in a sportsman. A softer element feels better and stretches further but also loses more than a hard one unless your driver can get some side bite. All that being said, a droop spring element is a single part of your car’s overall ‘set-up.’ And your set up should be a couple of note pages long at least, with changing variables for track, track condition and driver. As a general rule of thumb, we recommend 700lb set for crate, 900lb for super. For example, for a super, if you put a 500lb per inch spring in there: 900 ÷ 500 = 1.8″ drop. Then, just vary that for the other conditions.

droop limiter too

So there ya have it, friends. Now you know how one works and how to puck the right pick! And it just so happens we have a dandy new droop limiter to show you. It’s an in-house design and we made it from scratch, so you know its got that good PPM Quality. After all, our stuff is ‘Just Built Better!’

Look at Droop Limiter