The deal with droop

Jun 23, 2011 4 Comments by

Suspension tuning is not a simple thing.  A typical RC car has more adjustments than you can shake a hex driver at, and choosing the right thing to adjust in the right way in the right situation takes years to learn.

One of the more subtle tuning attributes on a car is droop.  A lot of people think they understand droop (and some of them actually do), but plenty of others have never even heard of it.  In both cases there is a surprising amount of confusion around the topic, which is why I’m writing about it.

The first thing to understand is that droop is not a setting; it’s an attribute. It is affected by other settings, each of which have their own unique names, but at the end of the day droop is a result of the way your car’s suspension has been adjusted.  Droop allows (or limits) weight transfer from one side of your car to another – a factor that can drastically alter handling.  Adding droop on one end of your car will generally increase grip on the other end, although this is not always the case.  Running a lot of rear droop, for example, allows more weight to transfer to the front wheels during braking, which may or may not be desirable depending on what type of driving you’re doing.  I won’t go into the nuances of tuning with droop, largely because the effects differ greatly depending on vehicle, surface type, driving style, and more.  Just know it is one of many options open to you, and adjusting it can have profound effects.

Ok, let’s get down to exactly what droop is.  Put your car down on a flat surface and let the suspension settle; this is your car’s ride height.  Now, slowly lift up on your car’s chassis until the wheels just begin to leave the ground; the distance your suspension is able to extend downwards before the wheels leave the ground is the droop value.  Put more simply, droop is the total amount your chassis is able to rise above its natural ride height.

 

This rally car is running some serious droop - the wheels are extended almost completely out of the fenders.

Many cars feature screws in the chassis or suspension arms that limit suspension travel. Although often referred to as “droop screws”, these are more accurately described as downtravel, downstop, or drop stop screws.  Although downtravel adjustments affect droop, they don’t define it.

Confused?  Ok, here’s an example.  Let’s say that your sedan’s chassis is 5mm off the ground at rest.  You lift up on the chassis, and at a height of 10mm the wheels leave the ground.  This means you have a 5mm ride height and 5mm of droop.  Easy.

You want to reduce the amount of droop your car has, so you make an adjustment to limit the downtravel of your suspension by 1mm.  The ride height of your car remains 5mm, but now as you lift up on your car the tires leave the ground when the chassis is at 9mm, which gives you 4mm of droop.  So far so good.

You now decide that you want to raise the ride height of your car, so you add 3mm of preload to the shock springs.  Your car now has an 8mm ride height.  So far so good, right?  You lift up on the chassis, and because of the downtravel adjustment you just made, the wheels again leave the ground at a height of 9mm.  But wait… That means you now only have 1mm of droop.  Clearly then, both preload and downtravel are unique and independent settings, but both affect droop.

For the most part, any time ride height is changed droop changes as well.  This includes spring changes, weight distribution adjustments, shock angle adjustments, and so on.  It is worth noting, however, that unsprung ride height changes (e.g., tire and rim changes) do not affect droop.  Additionally, any adjustment that limits suspension travel will also change the droop value.  Along with downstop screw adjustments, travel limiters can be added to shock absorbers to reduce the amount they are able to extend.  Before suspension-mounted downtravel screws became common on RC vehicles, shock travel limiters were pretty much the only way to adjust droop.

This concept can sometimes be a bit confusing, but that’s ok – nobody was born knowing this stuff.  Just give it some time and practice and you’ll be a pro before you know it.

 

 

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About the author

I got my first RC car way back in 1985 - a Tamiya Wild One - and have been involved in the hobby ever since. I've made every mistake in the book and loved every minute of it... Well, except for that one nitro engine I could never get to run properly. I've bought and sold more vehicles than I care to count, from cars and trucks to planes, helicopters, boats, and more. I'm a dedicated basher, certified bench racer, and collector of random tools. My very favorite part of the hobby is fixing things I've broken.

4 Responses to “The deal with droop”

  1. Nivlac says:

    so what does lowering or raising the droop level do for the handling?

    • Tom Ross says:

      I knew someone was going to ask that. :)

      There are many, many variables that play into the ultimate handling impact for droop changes, but I’ll give you some general guidelines. Know that droop is all about weight transfer, so it all depends on if you’re accelerating, decelerating, cornering, or maintaining a steady velocity (both laterally and longitudinally). Generally:

      More front droop:
      -nose lifts on acceleration
      -more on-power steering
      -less high-speed steering stability

      More rear droop:
      -rear lifts on braking
      -better bump handling
      -less stability under braking

      Less front droop:
      -nose stays flat on acceleration
      -less on-power steering
      -improved high-speed steering stability

      Less rear droop:
      -rear stays flat on braking
      -less stability over bumps
      -improved stability under braking

  2. Jesse says:

    Sounds ummm… A little confusing

    • Tom Ross says:

      It is, but don’t sweat the details. The most important thing to remember is not to mess with it until you feel more comfortable with the concept.

      If your car or truck isn’t handling the way you like it, the best place to start is by changing the tires. Droop is way down the list of other adjustments to make.

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