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″.

Trailer Leaf Spring Length By Dexter Axle
A page from the Dexter Axle informational publication showing spring length and axle positions.

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.

ScreenShot of eTrailer's Selection of Trailer Leaf Springs @ 3500#

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 we say.

The effect of spring length on trailer construction is shown with the 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 so 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 Really 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 better ride.  In general, longer springs give a better ride.  You can see this in the evolution of pick-up trucks.  As the years go, truck and SUV’s are getting longer rear leaf springs.  They support load just as well long or short, but the bump dynamics are better with longer springs.  In general, longer springs have a lower deflection rate which helps to absorb bumps better.  It allows more motion with smaller forces.  For trailers, a longer leaf spring length means the trailer doesn’t react as quickly to bumps, so it doesn’t jar as much.
  2. Longer springs spread forces 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 puts all the force at one-ish point on the frame.  A really long trailer leaf spring length puts two force points, half as much, far apart — and that makes less stress in one area.
  3. Longer springs weigh more and typically cost 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 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.

Spring Length 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.

Slipper Spring Tandem Axles
Slipper Style Tandem Axle Assembly with a Medium Trailer Leaf Spring Length

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 on Leaf Spring Length

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.

11 Comments About “The Value Of Spring Length”

  1. I have built well over a dozen trailers in the past 50 or so years and have used several spring lengths and sizes. I built two trailers using rear light truck springs, one a single axle and the other a tandem. Both trailers were built with smooth ride in mind. The single axle is a 6 x 16 enclosed trailer with outboard wheels and was built for moving equipment for sound re-enforcement. I just kept adding leaves to the springs ’til I got the ride I wanted and with shocks added it has surpassed my expectations for ride. Even empty it is exceptionally smooth. The 8 x 20 tandem was built as a mobile stage to be used at outdoor shows and also in parades where music cds were played. I set the axles quite close together which meant I had to overlap the springs without equalization. I accomplished this by turning the springs slightly on the axle mounts ’til the rear cleared the front springs. The rear axle has heavy duty shocks added to reduce sway and bounce. After 20 years of hauling all kinds of loads it has given absolutely no trouble and the looked for results were achieved. With the length of the springs and the built in clearance to the frame equalization has not been a factor. I’ve often wondered why all light duty trailer springs are built so short. With heavy trailers I can certainly see the advantage of the shorter springs.

    Reply
  2. I am entirely of the same opinion when it comes to spring length. The longer you can get, the smoother the ride and the less stress on the main beams.
    I built a 4×8 trailer back in 1987 using the front drop axle of a 1970 Chevy van (rated at 3500#) combined with a set of 48 inch long leaf springs. The ride is absolutely fabulous compared to every other trailer which I either purchased or built using standard trailer springs.
    The trailer is somewhat over built with an empty weight of 950 pounds but even without any cargo, it hardly bounces at all going over large bumps and potholes in the road.
    The suspension has a long travel but can still accommodate loads of 3000 pounds before the springs flattens out. Truly remarkable!!!
    But most practical of all is when it comes to moving delicate furniture made with glass components, the thing is so smooth that you don’t notice hardly any shocks or vibrations transmitted to the items being transported on normal road conditions with bumps and holes here and there.
    We can’t use any other trailer with shorter springs for this type of use unless we either deflate the tires significantly beforehand or add at least a half ton of weight inside the trailer so that the tiny leaf springs (approx. 25 inches in length) don’t send the trailer bouncing all over the place at the first bump on the road smashing the precious cargo.
    Unfortunately, this makes me the most popular gal in town on July 1st of every year when everyone scrambles on moving day ….. or any other moving day for that matter.
    I should have know better.
    The final word is “the longer the better” when it comes to leaf springs.
    Trailer manufacturers should rethink their strategy and offer much longer springs for single axle trailers.
    Much smoother ride without affecting the payload capacity. True they are heavier (contain more steel) and cost more but the benefits are amazing and well worth it.
    You can look in scrapyards and get long truck springs. Remove or add blades to adjust the springs to match the size of your axle and you’ll have the best trailer possible.
    My trailer is 33 years old and still going strong, enduring the most diverse set of tasks imaginable.
    Hauling 3000 pounds of tiles or a large china doll it does it all.

    Reply
  3. On the nomenclature above regarding the spring lengths, is that length before you put weight on them or after its setting on its feet ? The reason i ask is I have a Stellar toy hauler triple with 6k axles… I have one broken spring and need to just do the rest at this point…. the springs are a 4 stack… under pressure they are 26″ long…. relaxed their 25 1/4.

    all my measurements to the schematic up top says it will fit 26 inch springs…. do I go with a 6 stack 26 or a 4 stack 25 1/8 ?

    I guess what really bothers me when reading everywhere… I cant find what sag measurement is once its on its feet…

    Reply
    • I believe the measurement is static, unloaded. I would ask before you buy, and if possible, take one to the shop when you order them. Yes, I know it’s not as easy as buying online, but it might avert a problem. Other option, look at etrailer.com. They usually have drawings with their products, and a video to explain them.

      Reply
  4. Hi a question… I want to build a dump trailer 2 meter wide and 4 meter long… I’m gonna put a double leaf springs suspension ( so 4 wheels).
    My question is: How do I know where to weld the equalizer? What is the som for that? How do I get the measurement? From the back of the trailer to the tongue where it hook to the ball of the truck, or what? The dump will be 2 meter wide and 4 long but to the tongue will be another 1 meter and a half…. Can you help me out? Or sent me a link that explains that good. Thank you.

    Reply
    • I’m not sure what you mean by equalizer … I’m guessing you mean the equalizing rocker that attaches the front and back axle springs? If so, then that is the “axle position”. When using multiple axles, the “axle position” is the center between the front and rear axles. Please see our article “Where Does The Axle Go?” which gives the equations to figure that out. Good luck with your project.

      Reply
    • I’ve built a few trailers and I use a scale hanging from an engine hoist and a piece of pipe under the trailer to roll the trailer on until I have less than 300 lb of tongue weight. Wherever the pipe ends up under the trailer is the fulcrum where the equalizer for a tandem axle will go or the axle Centerline for a single axle. Its best to build a trailer to carry most of the load well-balanced with very little load transferred to the truck. Assuming you’re pulling it with a single rear axle truck.

      Reply
  5. Looking for a single leaf spring 112 cm long (between eyes) 60 mm in with and 10 mm in thickness. It is for a 500 kg trailer. Used on a rough roads. Could send a photo. What is the closest that you can offer?

    Reply

Leave a Comment

11 Comments About “The Value Of Spring Length”

  1. I have built well over a dozen trailers in the past 50 or so years and have used several spring lengths and sizes. I built two trailers using rear light truck springs, one a single axle and the other a tandem. Both trailers were built with smooth ride in mind. The single axle is a 6 x 16 enclosed trailer with outboard wheels and was built for moving equipment for sound re-enforcement. I just kept adding leaves to the springs ’til I got the ride I wanted and with shocks added it has surpassed my expectations for ride. Even empty it is exceptionally smooth. The 8 x 20 tandem was built as a mobile stage to be used at outdoor shows and also in parades where music cds were played. I set the axles quite close together which meant I had to overlap the springs without equalization. I accomplished this by turning the springs slightly on the axle mounts ’til the rear cleared the front springs. The rear axle has heavy duty shocks added to reduce sway and bounce. After 20 years of hauling all kinds of loads it has given absolutely no trouble and the looked for results were achieved. With the length of the springs and the built in clearance to the frame equalization has not been a factor. I’ve often wondered why all light duty trailer springs are built so short. With heavy trailers I can certainly see the advantage of the shorter springs.

    Reply
  2. I am entirely of the same opinion when it comes to spring length. The longer you can get, the smoother the ride and the less stress on the main beams.
    I built a 4×8 trailer back in 1987 using the front drop axle of a 1970 Chevy van (rated at 3500#) combined with a set of 48 inch long leaf springs. The ride is absolutely fabulous compared to every other trailer which I either purchased or built using standard trailer springs.
    The trailer is somewhat over built with an empty weight of 950 pounds but even without any cargo, it hardly bounces at all going over large bumps and potholes in the road.
    The suspension has a long travel but can still accommodate loads of 3000 pounds before the springs flattens out. Truly remarkable!!!
    But most practical of all is when it comes to moving delicate furniture made with glass components, the thing is so smooth that you don’t notice hardly any shocks or vibrations transmitted to the items being transported on normal road conditions with bumps and holes here and there.
    We can’t use any other trailer with shorter springs for this type of use unless we either deflate the tires significantly beforehand or add at least a half ton of weight inside the trailer so that the tiny leaf springs (approx. 25 inches in length) don’t send the trailer bouncing all over the place at the first bump on the road smashing the precious cargo.
    Unfortunately, this makes me the most popular gal in town on July 1st of every year when everyone scrambles on moving day ….. or any other moving day for that matter.
    I should have know better.
    The final word is “the longer the better” when it comes to leaf springs.
    Trailer manufacturers should rethink their strategy and offer much longer springs for single axle trailers.
    Much smoother ride without affecting the payload capacity. True they are heavier (contain more steel) and cost more but the benefits are amazing and well worth it.
    You can look in scrapyards and get long truck springs. Remove or add blades to adjust the springs to match the size of your axle and you’ll have the best trailer possible.
    My trailer is 33 years old and still going strong, enduring the most diverse set of tasks imaginable.
    Hauling 3000 pounds of tiles or a large china doll it does it all.

    Reply
  3. On the nomenclature above regarding the spring lengths, is that length before you put weight on them or after its setting on its feet ? The reason i ask is I have a Stellar toy hauler triple with 6k axles… I have one broken spring and need to just do the rest at this point…. the springs are a 4 stack… under pressure they are 26″ long…. relaxed their 25 1/4.

    all my measurements to the schematic up top says it will fit 26 inch springs…. do I go with a 6 stack 26 or a 4 stack 25 1/8 ?

    I guess what really bothers me when reading everywhere… I cant find what sag measurement is once its on its feet…

    Reply
    • I believe the measurement is static, unloaded. I would ask before you buy, and if possible, take one to the shop when you order them. Yes, I know it’s not as easy as buying online, but it might avert a problem. Other option, look at etrailer.com. They usually have drawings with their products, and a video to explain them.

      Reply
  4. Hi a question… I want to build a dump trailer 2 meter wide and 4 meter long… I’m gonna put a double leaf springs suspension ( so 4 wheels).
    My question is: How do I know where to weld the equalizer? What is the som for that? How do I get the measurement? From the back of the trailer to the tongue where it hook to the ball of the truck, or what? The dump will be 2 meter wide and 4 long but to the tongue will be another 1 meter and a half…. Can you help me out? Or sent me a link that explains that good. Thank you.

    Reply
    • I’m not sure what you mean by equalizer … I’m guessing you mean the equalizing rocker that attaches the front and back axle springs? If so, then that is the “axle position”. When using multiple axles, the “axle position” is the center between the front and rear axles. Please see our article “Where Does The Axle Go?” which gives the equations to figure that out. Good luck with your project.

      Reply
    • I’ve built a few trailers and I use a scale hanging from an engine hoist and a piece of pipe under the trailer to roll the trailer on until I have less than 300 lb of tongue weight. Wherever the pipe ends up under the trailer is the fulcrum where the equalizer for a tandem axle will go or the axle Centerline for a single axle. Its best to build a trailer to carry most of the load well-balanced with very little load transferred to the truck. Assuming you’re pulling it with a single rear axle truck.

      Reply
  5. Looking for a single leaf spring 112 cm long (between eyes) 60 mm in with and 10 mm in thickness. It is for a 500 kg trailer. Used on a rough roads. Could send a photo. What is the closest that you can offer?

    Reply

Leave a Comment

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