The Value Of Spring Length

Why choose a long or short leaf spring for your trailer?  They come in different sizes, not only for strength, but also in trailer leaf spring length.  So, why do I want long ones or short ones?  Does it even matter?

These are great questions, for sure, and it can certainly cause some confusion when looking at the specs.  So, let’s look at some basic trailer leaf spring length geometry and reasoning.  We’ll shed some light, even if we don’t have all the answers.  Since they make different lengths, there must be a reason.

Is This A Thing?

Let’s start by looking at some standards.  Because the most popular axle capacity for DIY is 3500#, we’ll use this as our example.  Other capacities have similar standards and effects.

To get a feel for sizes, we’ll turn to the Dexter Axle publication on 3500# axles.  This graphic below gives a lot of information, but focus for a minute on the red circle items.  There are at least 3 popular-ish spring lengths indicated.  Looking at other sources (other axle suppliers) we also find 27″ is sort of common (was more common), as well as 25″.

The info here is from Dexter, and since they are probably the biggest manufacturer, it makes sense to consider what they think is important to publish.  Another big one to check is eTrailer.com  And a quick search there shows similar available trailer leaf spring length.  Here’s a partial screenshot.

As you can see, there really isn’t a true standard.  However, from experience, the sizes that are close will all work with the same mounting.  And, if you’re building new, it’s best to follow the manufacturer’s suggestions for spacing anyway.  Even with our trailer plans, we specify the spring mount positions — then, defer to the components you want to buy rather than forcing you to do it as designed.

The effect of spring length on trailer construction is shown with the many green circles above.  It does not change the axle position on the trailer, just the spring mount positions relative to the axle.  You can see the differences in the graphic.  Distance between the mounts and the resulting axle spacing (for tandem axles) does make a difference.

Trailer Axle Leaf Spring Length – Why?

So, why do they have the many different spring lengths?  What trailer leaf spring length should I use?  Does spring length even matter?

Let’s start with the last question, and work to the first.  I don’t have definitive answers, but I’ll share some facts and perspective.

Does Spring Length Matter?

In a word, Yes.  How much is debatable, and it depends on what you’re doing with the trailer.  So let’s break it down into a few key points.

1. The biggest advantage to long springs is ride.  In general, longer springs give a better ride.  You can see this in the evolution of pick-up trucks.  As the years go on, truck and SUV manufacturers are finding ways to make rear leaf springs longer.  They support load just as well long or short, but the bump handling dynamics are better with longer springs.  Basically it helps absorb bumps better by allowing more motion with smaller force changes.  For trailers, the longer spring means the trailer can’t react as quickly to a bump, so it doesn’t jar as much.
2. Longer springs spread the load more.  With leaf springs, the load effectively divides from the axle so the front of the spring has half the load, and the rear has half.  If we think about extremes, a super short spring makes all the load apply to the frame at one-ish point.  A really long trailer leaf spring length puts two load points far apart — and that makes less stress in one area.  Less stress concentration in one area of the main beam.
3. Longer springs weigh more and typically cost a little more.  If you want cheap, go short.  That said, the small difference in price — if that’s the only reason — is probably just chincy.
4. More suspension travel with longer leaf springs.  Using the travel feels nice because bumps are not as harsh — which goes back to ride quality (#1 above).  The side effect?  Add a little more space for vertical movement of the wheels and axles.  (Yet, for the differences we’re talking about, the added space is pretty minimal.)
5. Longer springs need more space.  If you have things on the frame conflicting with long leaf springs, then go shorter.  Make the trailer leaf springs as long as practical in the space available.

What Trailer Leaf Spring Length Should I Use?

This second question is more complicated.  Not that choosing needs to be complex, but to give proper advice, we must know the situation.  In general, longer springs are better for most applications.  While ride quality is not amazingly better by adding 2 or 4 inches to the springs, it is better.  That means (slightly) better trailer control, and harshness on other trailer components is less.  As a side note, torsion axles are touted for better ride, yet just like adding a couple inches in trailer leaf spring length, the difference is small.

The main reason to prefer leaf springs over torsion axles is stress distribution at the trailer frame.  That said, longer springs spread the stresses even more, and from an engineer’s perspective, that has value.

So, without other considerations, choose longer leaf springs.  That is especially true for single axles.  That said, if space does not allow, of if you’re simply replacing existing springs, use the longest acceptable alternative.

Why So Many Spring Lengths?

Our third question is even harder to answer.  I suppose the number of lengths has evolved for many reasons, but I don’t know them.  It might have something to do with tire sizes, and it might be something completely unrelated.  It’s a little disconcerting that there are so many minor differences in trailer leaf spring length, but that’s the way it is.

Please, if you have knowledge on this point, we would all appreciate you sharing.

For Tandem Axles

Single axle mounting is not as particular with spring length.  If you’re building new, you can specify the trailer leaf spring length and go with it.  In tandem and triple axles, spring length becomes more important.  For instance, if the springs are too short, the tires might interfere with each other.  Look at the green circles in the graphic above for the letter “C”.  Axle spacing is set, in part, by spring length.  Note the differences in the tables.

For tandems, and triples, we recommend you look first at the tire size you wish to use.  Add at least 3″ (preferably 4″) for space between the tires, then find an option of the next size up for axle spacing.  For example, if you choose a 29.2″ diameter tire, add 4″ space, and it gives you 33.2″ ideal axle spacing.  We can choose 33″ axle spacing (per the chart tables above), and end up with a 3.8″ space between the tires.  That will work.  Select mounting components to fit that decision.

If you are concerned about wanting larger tires in the future, go to the 35″ spacing.

Another generality for tandem suspensions is this.  For shorter trailers, consider closer axle spacing.   While it does not matter when towing straight down the road,  it does make a difference with turning tight.  Especially when man-handling the trailer.

Eye-Eye vs Slipper Style Leaf Springs

Does it matter if the springs are Slipper style or Eye-Eye style?  Not a bit.  Different base lengths are available in each of these categories, but the effect of spring length is the same.

One bit to understand, however.  These two different styles come in different capacities and lengths, so they are not so easy to compare apples to apples.  Additionally, slipper style springs more often appear on the heavier duty trailers, so their tires are usually larger in diameter.  That will usually dictate longer springs.

Final Thoughts

The concepts of trailer leaf spring length are not something to lose sleep over.  While there are differences for performance, choosing one length over another will not make or break the trailer.  We bias toward longer springs because it’s the right direction, but other factors can influence that decision more.  A tandem axle trailer with really small tires, for instance, might want short springs just for maneuverability.  There is nothing wrong with that choice.

The biggest factor in choosing springs should be the availability of correct components to give you the action and function you want from the trailer.  All other things being equal, choose long.

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