How good can a full-size solid axle suspension be?

mk216v

Der Chef der Fahrzeuge
At some point I found a chart which showed the front axle weights and possibly some spring rates for the solid axle F series front springs. I will see if I can find it. I agree though, most aftermarket spring kits are way to stiff. They are targeting those who want more lift, and don't want to get complaints about body roll or loose handling.
All you had to do was ask! ;)
Pg 49 for '19 Ford SuperDuty pickup; https://madocumentupload.marketingassociates.com/api/Document/GetFile?v1=4448972&v2=082218092022&v3=60&v4=3a356440177318e6661ce4291a1c606943fd66e39b5e8d79ceb4e67b&v5=False

Pg 61 for '19 Ford SuperDuty chassis-cab; https://madocumentupload.marketingassociates.com/api/Document/GetFile?v1=4448983&v2=082218092443&v3=60&v4=38f8030153407e24633f589da7df71bb3eaa21ea896c45ec7d49ee10&v5=False
 

Metcalf

Expedition Leader
Even with 4,500-5,000lb front axle weight, I think 400lb/in is too high. 350lb/in would be reasonable I think. If you are adding travel, you could go with even less, 325lb/in or maybe a bit lower. You would still be able to carry additional weight without riding on the stops, and your back won't be breaking on every bump. You would need a bit of preload depending on your desired ride height, but that should be doable.
I posted the scale weights from my truck a few pages back.

My total front axle weight is 3720lbs right now. I suspect about 750lbs of that is the front axe assembly. Another ~100lbs for each tire/wheel. That gives me roughly 1400lbs sprung weight per corner.

The truck may only be running a few inches of droop, but I need to measure the stock shocks to confirm. It has about 4.75" on the front from ride height to metal on metal contact. I am not sure that the stock bumpstops are designed to go all the way into the cup however?
 

Metcalf

Expedition Leader
The lack of transparency is frustrating for sure. Everyone wants to hold onto their secret sauce, which is understandable, but still frustrating to the engineers/enthusiasts not working at said company.
Yup, I miss the 'tech' aspect of it all....the WHY it works like it does. I know that companies make money on that, but dang it makes it difficult to learn much.
 

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luthj

Engineer In Residence
Assuming you don't intent to add significant extra weight. About 250-300lb/in is what I would suggest with 5" compression from ride height, 1400lb sprung per corner, and zero preload. That would yield a suspension frequency of 1.3-1.5hz Which is within the typical comfort zone of 1-1.5hz.
 

nitro_rat

On a Suburban Excursion
If you are planning to keep all the available up travel then your final ride height has nothing to do with tire fitment. You need to pull the springs and make sure whatever tire you plan to run clears at full lock in both directions at full stuff. The amount of trimming you have to do will be the same with no lift or 6".
 

Metcalf

Expedition Leader
If you are planning to keep all the available up travel then your final ride height has nothing to do with tire fitment. You need to pull the springs and make sure whatever tire you plan to run clears at full lock in both directions at full stuff. The amount of trimming you have to do will be the same with no lift or 6".
Yes, Agree.
 

Metcalf

Expedition Leader
Assuming you don't intent to add significant extra weight. About 250-300lb/in is what I would suggest with 5" compression from ride height, 1400lb sprung per corner, and zero preload. That would yield a suspension frequency of 1.3-1.5hz Which is within the typical comfort zone of 1-1.5hz.
Spring frequency is a great topic in itself for this thread. There is much debate if it should apply to long travel 4wd stuff.....
 

luthj

Engineer In Residence
For actual desert racing, my take is that matching front/rear frequencies can be important for stability. On longer wheelbase vehicles its less important though. A natural frequency in the comfort range is a good starting point. Higher frequencies cause a ride to feel harsh, especially with lots of unsprung weight. Which then necessitates high damping rates. Which finally increases suspension response time, transferring more energy to the chassis. Obviously there are control issues with too low of a frequency, but thats typically handled best with sway bars, and shock tuning.

Really all frequency is telling you is the ratio of mass to spring rate. Its a convenient way to set a starting spring rate. To maximize usable travel, lower is better. Go too low and your suspension can skip over a very rough surface. Then use bump stops, shock valving, or progressive springs to control bottom/topping effects. For track racing etc, they often choose a much higher spring rate. Which has a lot more to do with suspension response time and weight transfer than comfort.

My take is to choose a rate which lets you have good usable travel and ride height with minimal preload. Then have the shock tuned to control bottoming, and prevent stacking or slow response. A position sensitive shock can help with really frequency/rate, as you get high rates at the top/bottom, but faster response (less damping) in the middle range (typical travel).
 

Metcalf

Expedition Leader
For actual desert racing, my take is that matching front/rear frequencies can be important for stability. On longer wheelbase vehicles its less important though. A natural frequency in the comfort range is a good starting point. Higher frequencies cause a ride to feel harsh, especially with lots of unsprung weight. Which then necessitates high damping rates. Which finally increases suspension response time, transferring more energy to the chassis. Obviously there are control issues with too low of a frequency, but thats typically handled best with sway bars, and shock tuning.

Really all frequency is telling you is the ratio of mass to spring rate. Its a convenient way to set a starting spring rate. To maximize usable travel, lower is better. Go too low and your suspension can skip over a very rough surface. Then use bump stops, shock valving, or progressive springs to control bottom/topping effects. For track racing etc, they often choose a much higher spring rate. Which has a lot more to do with suspension response time and weight transfer than comfort.

My take is to choose a rate which lets you have good usable travel and ride height with minimal preload. Then have the shock tuned to control bottoming, and prevent stacking or slow response. A position sensitive shock can help with really frequency/rate, as you get high rates at the top/bottom, but faster response (less damping) in the middle range (typical travel).
I generally set maximum spring rate as what provides zero preload at full droop. In my opinion that is as high as you should go on a long travel off-road suspension. I don't think you want the coil repeatedly dropping out ( or going negative with spring retainers ) at high speeds. There is a pretty big following in the off-road world, with companies like Accutune, that adding preload isn't a bad thing. That seems to directly conflict with frequency tuning theory, especially on vehicles that are droop biased. The maximum amount of preload you can have is dictated by the risk of blocking the coil at full bump.
 

jwpnrp

New member
Attached should be screenshots of stock super duty coils ( with tags still on) Screenshot_20200129-171540.png

and 80-96 bronco coils made by deaver sold by bronco graveyard (softest I could find that when researching for my truck) Screenshot_20200129-171405.png mounting looks close enough that they may interchange fairly easy, should be able to confirm at a junkyard. Those early coils are lift coils but designed for a lighter weight front end so they may sag more in a heavier application. With your skill raising the top of the coil bucket to compensate for too tall of a coil may help you get the lighter spring rate you want in a semi production part ( a little time consuming though :)
 

Metcalf

Expedition Leader
Attached should be screenshots of stock super duty coils ( with tags still on)

and 80-96 bronco coils made by deaver sold by bronco graveyard (softest I could find that when researching for my truck) mounting looks close enough that they may interchange fairly easy, should be able to confirm at a junkyard. Those early coils are lift coils but designed for a lighter weight front end so they may sag more in a heavier application. With your skill raising the top of the coil bucket to compensate for too tall of a coil may help you get the lighter spring rate you want in a semi production part ( a little time consuming though :)
Any idea what the ID and spring rate is? Looks like on the website they say they are about 22" long unloaded. I am surprised to see them be a dual rate spring. I would rather avoid that at this time.
 

Metcalf

Expedition Leader
Assuming you don't intent to add significant extra weight. About 250-300lb/in is what I would suggest with 5" compression from ride height, 1400lb sprung per corner, and zero preload. That would yield a suspension frequency of 1.3-1.5hz Which is within the typical comfort zone of 1-1.5hz.
So gathering a little more data...

The shock on the truck is sitting at 19" from eye to the stud top mounting surface at ride height. Most replacement shocks are listed as 13.36" compressed and 20.8" which is 7.44" travel. I think you typically have to add about 1/2" to the mounting length with stem top measurements. That seems to jive with the roughly 4.75" of available uptravel to metal on metal contact.

So if the truck only has about 7.5" of travel.....and 4.75" up from ride height.....that only leaves about 2.75" of droop from ride height. I guess those high factory spring rates start to seem a little more reasonable. I don't really see the stock bumpstop compressing all the way into the cup, but who knows. I don't love the idea of re-designing things based on a moving target.

It seems like with something like the Carli coil/spring tower I can package a shock with a 16" compressed length and still maintain all the stock up-travel. It seems like there are a few options in 2.5 shocks with that compressed length that squeak over the 9" travel mark.



Rough numbers, the front coil is about 16.5" from the steel upper spring mount to where the coil starts to be full diameter on the bottom.

At least this starts to form the box I am working in....
 

jwpnrp

New member
Deavers are somewhere around 300 in/lb if I remember right, I'll see if I can get ID this weekend ( coils in storage at different location)
 

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toddz69

Explorer
Deavers are somewhere around 300 in/lb if I remember right, I'll see if I can get ID this weekend ( coils in storage at different location)
That sounds about right. I'm not as familiar with the TTB Bronco spring rates as the solid axle stuff, which is generally a little less unless you used the old red Ranchos which were a teeth-jarring 485 lb/in.

Regarding custom coils, I've had good luck with Custom Coil Specialties in Kansas. They did a run of coils for us (for early Broncos) in the late '90s and also did a set for me a few years ago through Deaver.

Todd Z.
 
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