How much does leaf spring length matter? (2 Viewers)

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Fjtdi

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I've been going back and forth on what leaf springs to use for my fj40 build. Its currently set up with a SOA and I'm split with what springs to use. I have a set of Rubicon Express re1445 that are around 45.5 inches long with a centered pin, brand new. I originally wanted to go with fj60 rear springs (47.5 inches in length) before I got a great deal on the express springs( which based on my research of leaf springs, have a lot of pluses out side being jeep springs).

Let's assume the shackle angles are perfect, is a few more inches in leaf spring length going to make a huge difference in ride quaility and flex?

Thanks.
 
I believe that the factory springs were somewhere between 42-44” depending on how flat/used they are. The longer the spring the more droop you will get. Ride is really dependent on a number of factors, length being one of them but a stiff spring will counter any benefits from length.

I would go as long as you have room. My springs were custom made and they were 54” in length and I had well over 12” of flex.
 
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Longer leafs make a huge difference both in ride and suspension travel/usable articulation. Longer is generally better, short springs go through more arc change for any amount of travel. I’m not going to describe it in super correct engineering terms, but longer will be less harsh, generally smoother, and give more travel. But too long in a shorter rig makes for packaging and overhang issues. Spring center pin position and shackle angle are also major factors.

I ran multiple combos on my old ‘84 minitruck and liked it best with minitruck rears up front, and Marlin Crawler rears, I think they were 56” or so. Both were longer than stock and I mixed leafs to make custom packs that were not flat but had lots of flex and low lift. Combined with a nice 2.0 smooth body reservoir shock, the rig at 108”-110” could haul fairly fast through the desert and forest trails, and was very stable as a crawler.

My current FJ40 has stock FJ60 springs front and rear with a leaf removed and 102” wheelbase. It seems to be good, but I have less quality shocks on it and not as much tuning time with the combo. Trying to get it out more this year, better shocks and rear shock position are probably what’s next.

Reading your situation, I’d be less concerned with the roughly 2” difference in length as I would with where the spring pin is and making sure you get your axle and wheelbase where you want for the size tire you are gonna run. If you’re fabricating your own mounts, you can put them where you want, and could run the 60 springs with the short side forward for good length and approach angle.
 
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I ran a variety of springs underneath what is now my dedicated trail '40. Factory springs, a set of the old aftermarket 3 leaf "kidney busters", a set of Pro Comps and a set of OME all in SUA configuration. Once I went SOA, I tried OEM FJ40 springs and FJ45 springs and eventually settled on 56 inch long 8 leaf "one ton" springs from the rear end of mid '80s Chevy duallys. When installed under a '40 with the Cruiser style shackles above the spring rather than the chevy truck style shackles beneath the springs, they are not as strong or a s stiff as they would be in their original configuration.

I could go on more about the various modifications, considerations and factors of the different installations, but the short story is, the longer chevy springs outclassed any other setup. They drooped, compressed, articulated, flexed, carried load and did everything a spring is supposed to do far better than any of the shorter springs.
Crawling over rocks or blasting over rough trails, longer is better.

Mark...
 
It also depends on if you are staying with stock hanger locations ? I would use the 60 springs , I recall something about the end wraps dont fit the 40 hangers ?
 
peesalot said:
It also depends on if you are staying with stock hanger locations ? I would use the 60 springs , I recall something about the end wraps dont fit the 40 hangers ?
Click to expand...

If you're using stock hanger locations you're severely limited. You're right, but if the OP has enough skill to pull off a proper soa, they can move spring hangers, which means they can run any springs they want.

40 rears iirc the 54" offset pin Dakota springs work well. Chevy 63"s for wagons.
 
Did it say the op had done a SOA ? Your right, you can do whatever you want, anytime.
 
This is all very helpful and gives me some ideas. What's the best center pin placement? Centered vs offset. I would think a centered pin would make the most of the leaf spring.
 
Fjtdi said:
What's the best center pin placement? Centered vs offset.
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I think what is best for your sheet metal, aesthetic, or wheelbase desires. One of my 40's sits on FJ60 rear leaf springs front and rear. The rear springs are flipped to gain wheelbase with the military wrap leaf re-drilled to line up with the flipped leafs. I wasn't worried about sheet metal and wanted the most wheelbase I could without some ridiculous rear hanging shackle hanger. I ended up around 102" wheel base.

Here is a decent leaf spring option chart that has been floating around for a couple of decades:

 
I have been flipping springs for a bit of extended wheelbase for the last 30 years or so. Never really noticed an difference in how the spring rode based on the center pin location. However, these days, the '40 I mentioned that I have tried all sorts of springs under is sitting at about 106 inches of wheelbase and graduated a while back from the 40 inch boggers to 18.4x16.1 tractor tires. So I haven't really paid attention to road manners in a while I guess.

Mark...
 
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