Conversion of leaf spring sprung weight to 'capacity' specification

Question

Conversion of leaf spring sprung weight to 'capacity' specification

KPendergast (Aerospace)

(OP)

1 Sep 12 11:28

Hello -

I'm trying to determine which leaf springs to purchase for replacement of worn out springs on a boat trailer. The springs are specified in terms of 'capacity' and dimension. My problem is that I do not know how to convert the amount of sprung (suspended) weight into a capacity value. Is there a well-understood rule to do this conversion that accounts for necessary margin, but doesn't add so much margin that the springs are overly stiff?

Any assistance appreciated. A design reference book would also be helpful so that I could learn quantitatively what is meant by 'capacity'.

Thanks,
Karl P.

GregLocock (Automotive) · Answer 1 · 2012-09-01 13:09:36

No. I have designed or worked with something like 6 different leaf springs for trucks and never come across the term. They probably mean the usual load on the spring, ie (laden axle weight-unsprung weight)/2 for a single axle trailer, but who knows? Definitive reference for spring design is http://books.sae.org/book-hs-788

Cheers

Greg Locock

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patprimmer (Publican) · Answer 2 · 2012-09-01 18:25:25

Having built many trailers, my experience is the same as Gregs. When you buy the springs they are rated by axle load for a single axle.

With boggie axles it depends. If you have a full load sharing arrangement, the axle loading remains the same per axle.

If it is not load sharing, the nominal rating per axle is reduced to 70% of the original loading. In reality, the front axle gets the harder work and the closer the spacing, the less the difference in the load seen by the axles.

Regards
Pat
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KPendergast (Aerospace) · Answer 3 · 2012-09-01 22:58:18

Thanks for the comments, gentlemen. The SAE publication looks perfect, but I'm not sure I want to spend as much on the book as the leaf springs will cost... Will keep looking. Cheers, Karl

JayMaechtlen (Industrial) · Answer 4 · 2012-09-03 23:38:20

You are over-thinking this.

You have the old springs, right?
IF you haven't taken it apart yet, you can get the thing weighed.
Take the old springs to a place that makes leaf springs and tell them you need new ones.
Determine how much higher the trailer needs to ride, and provide the shop with that info.
If the springs aren't terribly rusted or torn up, you might just get them re-arched and heat treated.

cheers
Jay

Jay Maechtlen
http://www.laserpubs.com/techcomm

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Conversion of leaf spring sprung weight to 'capacity' specification