Matching Springs to Load Carried

[Terrytruett]

Hi Kevin @KevinP,

I think you have the gist of the discussion/debate from way-back-when . Some summary notes . . .

LCI was firmly against different strength springs on opposite ends of an axle.
They never got the F=kX basic engineering of matching the spring strength to the load being carried.
They tried (and still try) to use "spring natural frequency" as the excuse . . . showing that they know even less about how weight affects natural frequency.
Several of us now have thousands of miles on suspensions with spring strength matched to load carried . . . with no problems.

The 337 guys seemed to have enough spring strength so they went with different spacer blocks on each side to get level (Paul's situation)
The 303 guys did not have enough spring strength on the left side so we went with stronger springs on that side only (Pat, myself and others)

It is important not to over-strengthen your springs.
This means a rougher ride for the trailer and the tire sidewalls will be doing more than their fair share of cushioning the bumps.

I would strongly suggest that you measure your current loaded arch height for each spring. See https://www.mygrandrv.com/fo...ng-Arch-Height This is the only way that you will know for sure that your springs are overloaded (and by how much) or how much your future changes will help. Just "looking at" the springs can be misleading.

Lots of experience with this, on this forum . . . don't hesitate to ask, if you have any questions.

Rob

Hi Rob, thank you for your work on the spring info. Just swapped my kitchen side to 3000# on my 303 per your info.

Question. You had mentioned that the OEM on axles were concerned about natural frequency and using an unmatched pair of springs (3000# one side and 2700# on the other) would cause issue. Do you have any more info in this area of study? Just curious.

TIA

TT

[Cate&Rob]

Hi Rob, thank you for your work on the spring info. Just swapped my kitchen side to 3000# on my 303 per your info.

Question. You had mentioned that the OEM on axles were concerned about natural frequency and using an unmatched pair of springs (3000# one side and 2700# on the other) would cause issue. Do you have any more info in this area of study? Just curious.

TIA

TT

Hi TT,

Quoting from an earlier discussion . . . see this thread https://www.mygrandrv.com/fo...525#post103525 . . . particularly around post 54.

"LCI will claim (at least they did to me at the Rally tech session) that they can’t have different springs because this will create an imbalance of spring natural frequencies side-to-side. They seem to be forgetting that natural frequency is affected by the weight carried by the spring. Not to get too deep into the math, but natural frequency varies directly as the spring constant and inversely as the mass, so increasing the strength of the spring will actually compensate somewhat for the increase in weight, keeping the natural frequencies closer together. Whether the natural frequencies should or should not be matched in a four spring undamped system is way more math than I am up for "

Rob

[Terrytruett]

��. Thank you!

[Gyro Gearloose]

Hi TT,

Quoting from an earlier discussion . . . see this thread https://www.mygrandrv.com/fo...525#post103525 . . . particularly around post 54.

"LCI will claim (at least they did to me at the Rally tech session) that they can’t have different springs because this will create an imbalance of spring natural frequencies side-to-side. They seem to be forgetting that natural frequency is affected by the weight carried by the spring. Not to get too deep into the math, but natural frequency varies directly as the spring constant and inversely as the mass, so increasing the strength of the spring will actually compensate somewhat for the increase in weight, keeping the natural frequencies closer together. Whether the natural frequencies should or should not be matched in a four spring undamped system is way more math than I am up for "

Rob

Rob,

I am in full agreement with you (previous discussions). I found this calculator for a coil spring but I think the concept of different weight on a spring changing its natural frequency rings true. With that said, putting more weight on a heaver spring will give it the same Natural Frequency as a lighter spring with less weight on it. So with us putting different springs on 303's to compensate for the weight difference side to side solves the Natural Frequency problem that Lippert created by not using different springs side to side.

So to kind of prove the point I used this calculator for a 2600 lb./in spring and a 3000 lb/in spring. https://www.amesweb.info/Vibration/N...of-Spring.aspx

Values
2600 lb spring
Spring constant [k] = 2600 (lbf/in)
Mass [M ] = 2600 (in) Used 2600 to simulate 1" of spring compression
Resulting First natural frequency [f1] = 3.127 HZ


3000 lb. spring
Spring constant [k] = 3000 (lbf/in)
Mass [M ] = 3000 (in) Used 3000 to simulate 1" of spring compression. My 303 has about 400 lbs of extra weight on the drivers side.
Resulting First natural frequency [f1] = 3.127 HZ


Hmm, same natural frequency. So if I'm thinking correctly, putting different size springs side to side actually helps correct natural Frequency issues. Just as we think.

Try putting different weight on the 2600 lb spring in the calculator and watch the Natural Frequency change.

Another Gearloose moment,
Pat

[Cate&Rob]

Another Gearloose moment, Pat

Hi Pat,

I should have known that you would come along with the math support for my foggy memory of an engineering course a long time ago .

Thanks!
Rob

[MidwestCamper]

Looking this up was easier than cracking my Vibs book.

Natural frequency of spring mass system
A spring does not have a natural frequency of its own. The natural frequency of a mass spring system depends both on the stiffness of the spring and the mass of the body supported by the spring and is f=1/(2pi)*square root (k/m) where k is the stiffness of the spring and m is the mass.