Is this a good rule of thumb for selecting springs?

[misterW]

Read this on some forum, can't remember where:

"Choose springs that have a total lb rating equal to 125% of your trailer's fully loaded weight."

What do you think? Does that sound about right?

[Trackstriper]

That's a good question for discussion, maybe I can open a can of worms. I did talk to Dexter Axle several weeks ago and they seemed to indicate that the smoothest ride would be with the springs rated for the load placed on the axle. That's the load that there designed to operate at, not the load where they'll break. The trick is to figure what the load is.

Let's say the trailer weighs 1000# for example. Figure 10-15% for tongue weight placed on the hitch ball. Let's call it 125#. The unsprung weight of the axle is the weight of the wheels, hubs, the axle beam, and probably a portion of the weight of the springs, to be way too technical. Maybe the trailer has 14" wheels....figure 70# for the 2 wheels, another 10# for the hubs, maybe 20# for the axle (wide weight range here)...that's 100# of unsprung weight. Add that to the tongue weight and you have about 225#, leaving the springs supporting 775#.

I like the idea of leaving a little "headroom" against miscalculating the weight of the trailer and all the miscellaneous cargo it will wind up carrying...the stuff that wasn't planned for. So I would think that springing for 100% of the weight of the total loaded trailer would be about right. In effect that's about 125% of the weight the springs are carrying in the example above.

In an ideal world we could build the trailer then weigh the axle and purchase an appropriate set of springs to closely match the load on the springs...not to practical for most folk. But I recently de-rated the factory springs on my cargo trailer somewhat according to Rob's excellent post. I eliminated the short bottom leaf and trimmed the middle leaf in the photo 3/4" at each end. No point in having 3500# springs on a trailer that will have maybe 1700# on the axle. That's a bit overkill. If this mod doesn't work out well when the trailer is finished my fall back position is to buy some 2000# springs from Northern Tool rather than operate with "reasonable guess" rated springs. I just hated to pitch out the heavier springs...seemed like a waste.