Effects on spring rate from a shackle flip

Not necessarily true. I would imagine the thinner leaves are used for the intended purpose of more movement hence a more progressive engagement of the leaves. I think the only real way is what you suggested of limiting uptravel.

 

Please note that I used stock length shackles in my experiment. Your results of course will vary.

 

Hi Dave and welcome to the show!
First thing that need to be done is understand the components.
The only job a shackle does in a leaf spring suspension is to allow the spring eye relationship to change as the suspension cycles. Without a shackle the leaves would do very weird things as weight is applied.
There are several shackle designs used, including, but not limited to, tension, compression, and slipper. Ironically, slipper eyes are typically used only on very light duty utility trailers and very heavy duty leaf sprung trucks(I have seen them mostly on tandems back in the day).
Compression shackles are typically used on light vehicles. My CJ-5 had them.
Tension shackles are used mostly on pick ups from our Tacos all the way up to my Ford dually.
My friend and mechanical engineer @Sub-Zero did a stress analysis a few years ago showing the difference in stress between a compression and tension shackle, and holy hell what a difference. Simply put, steel is much stronger in tension than in compression. Designing with a tension shackle allows thinner material to be used compared to a compression shackle, saving the manufacturer money, allowing a higher safety margin, etc.
Is a compression shackle unsafe or unsuitable for use on our trucks? I say no. Would I put them on my dually? No way. Disclaimer: I do not run stock shackles with my flip.
Hopefully you are with me so far in that the type of shackle used is strictly dependent on the vehicle, purpose, weight capacity, etc.
On to leaf springs. Take a single top leaf. Lets assume that it has a spring rate of 80# per inch. For the purposes of this discussion lets assume a leaf like on our trucks. Single thickness throughout, not a tapered leaf.
It will take 80# to compress it one inch. Another 80# to compress it another inch, and so on. VERY linear.
Lets look at our second leaf. Same material. But it is shorter, so more force is required to bend it. Lets say its rate is 90#. First inch takes 90#. Second is another 90# and so on. Again linear.
Third leaf we will say is 100# rate. Fourth how about we put at 110# rate? Keep this a nice easy transition pack.
Ok. To keep this simple we are not going to talk about the weight of the truck. We have an assembled pack on our bench. Lets add 80# on it. Assuming the second leaf does not come into play until after one inch of movement of the main leaf, the pack will compress one inch because the spring rate of the only leaf being moved is 80#.
Assuming the third leaf does not come into play until the second leaf moves one inch, to move the pack another inch will require 170#. 80# for top leaf and 90# for second leaf.
Lets say the fourth leaf does not move until the third leaf moves one inch, to move the pack another inch would require 270#.
Since we have no fifth leaf in this pack, we know that to compress the pack another inch would require 380#. and each inch beyond that would be another 380#.
So to compress this pack four inches requires 900#!! Now that is progressive!
As weight removed, the pack will go back to its static shape in the exact same manner.



Tune in tomorrow for a discussion on shackle length as it is determined by the suspension system it is designed to work with.

 

I'm with ya, you won't be talking over my head.

 

Subbed. Keep it coming. I'm in need of a new leaf pack and trying to figure out the best option for my particular uses.

 

Depends on what you want to do with your truck and how much work you are willing to put in.
My advice is to weigh your trucks unsprung weight at each side of rear and have Deaver, Alcan, or another reputable spring company make springs for your exact load out .
In two years of having my shackle flip and Dakars, I ended up breaking one leaf on each side. I wheel the shit out of my truck and use every inch of my 12" of rear travel on every outing. I am still not sure if I am going to move my spring mounts to install longer springs, i.e Chevy 63 type deal, or just link it and add in coilovers. But I need to do research in that are and see how to go about running COs with changing rear loads as I do tow my camper often during the warm season, and my camping gear and trailer tongue weight easily add another 400# to the rear end.

 

That's a tough one. What about coils and bypass shocks? That might not help with the weight though.

You should just get a smaller camper. @HolyHandGrenade

 

Maybe dual rate coil overs would work. Don't really know much about them other than they have two could on them with different rate springs. Was just talking with Tim earlier and mentioned might try quarter elliptical springs for fun. That was a fad 20+ years ago that the desert racers were developing.

 

When you go with multiple rates you have choices, progressive, dual rate, etc. I think it's going to end up with you losing wheel travel when you aren't towing the trailer. You might be able to find a medium point that ride well normally without squatting much under heavy load, though.

You'd probably get the best feedback reading about what works for 4Runners, 80/100/200 series Cruisers and unfortunately Jeeps when they need to balance ride/travel/towing. Or maybe there's something to be learned from the Ram pickups that have coils.

 

Dual rate might work. A lot of high end quad shocks have them. The idea there is you can have a plush ride with the lighter spring and the heavier springs help with bottoming out on big jumps.

I think you just need to ask yourself, How high am I going to jump my truck? And go from there.

They make triple rates also.

 

The newer Rams use air bags. My 3500 dually has leaves and bags 2500s have bags and coils I think.

I don't know how great bags work off road though.

 

I had air bags on my dually also. Lead to broken leaves ...lol. I ended up working with a spring shop and we build a set of springs that worked for my use.
Bags would work well though. Inflated when towing, deflate for trail use and daily driving.

 

I should have been clearer, the 1500 Rams with coils are what I was thinking. I couldn't tell you if they use single rate or progressive springs but I'd have to think they would have tested for empty bed ride vs. loaded to GVWR or with max towed tongue weight.



Dual rate springs are common in a lot of high end applications, but don't think of it like one spring then the second. Your effective spring rate is a combination of the two, at least until the stop is hit. The third spring is I think usually necessary for really long stacks that may completely unload at max droop, so you put in a minder spring that keeps them in place.

 

Most dual rate coilovers that I have seen are pretty long. As I said, I need to do research. If the smallest dual rate I can get is 16" travel, that is no biggie.
Various vehicles over the years have used progressive coils. Essentially the same thing as a dual rate. The issue with pick ups in particular is a wildly varying load, and most coils cannot compare to leafs as they are way more progressive than any coil ever could be.
Even with a triple rate coil, and extra 400# could significantly lower ride height, unless heavier springs are used, which would adversely effect unladen ride quality.

Planning a suspension for a known load is relatively easy. Doing it for a varied load is much more complex.

 

True.

Just throw an extra 400# in when your not towing.


But seriously, if you are going to link it have you thought about shorting the wheelbase.

 

Of course I have thought of that. I do like the challenge of the longer wheelbase though.
Time will tell.