Front coil spring puzzle

[Bustedbroc]

Hi Folks. First a recap, I purchased a 1974 U15 302 bone stock far as I can tell 10/2023. Working my way thru it. New front and rear springs (not installed) were provided with the sale. Rear leafs are and now working on replacing the front coils. All the springs appear to have been purchased from Tom's. The leafs had a Tom's "stock" label on them and I have to believe the fronts were purchased at the same time. When I removed the old front spring and compared it to the new side by side it measures 1 inch taller/longer. 16" vs 17" and a tad beefier. On the Tom's website they have these:
https://tomsoffroad.com/parts/66-77-ford-bronco-coil-springs They are they only ones they offer for a "stock" ride height.
My question is, does anyone know how high/long factory coils are? I called Tom's and she said stock Bronco springs are 16", the same as my new springs. She also stated that if someone installed F150 springs they would be 17" or 1" longer which is what I pulled out. I intend to put these new springs in but it would be nice to know if the 16" statement is true. I can see someone has been messing with them at one point because there was only 1 bolt in each bottom retainer cup and 1 snapped off. I find it curious that Ford did not have different springs for 6 cyl and V8? I guess they weighed about the same? Unless Tom's only offers 1 size fits all for stock ride height.

 

[Broncobowsher]

Free height of coil springs is a really bad comparison. You need to know the spring rate as well. And the sprung weight that they are holding up. Divide the weight (per side) by the spring rate and that will tell you how much they will compress at ride height. If one of the spring designs has a softer spring rate it can be taller free height and lower installed height.

Now the Tom's springs are even harder to figure out. They are a progressive rate. Instead of easy math of weight and spring rate for compressed height, the spring rate changes during compression. So while these may be stock height, they are not the stock spring rate of stock. The stock springs were single rate, not progressive. The free height of the Tom's springs will not compare to a true "stock" spring. The Tom's springs are an upgraded spring over stock with a different design.

Now what you took out, it is used and good chance sagged since it was installed. And you have evidence that it may not even be stock. There have been so many different springs out over the years as well. Total guess on what you got with the Bronco.

 

[Bustedbroc]

Thanks for the reply. There is a lot of "unknown" and "mystery history" with this truck. Part of the adventure, LOL!

 

[Bustedbroc]

Free height of coil springs is a really bad comparison. You need to know the spring rate as well. And the sprung weight that they are holding up. Divide the weight (per side) by the spring rate and that will tell you how much they will compress at ride height. If one of the spring designs has a softer spring rate it can be taller free height and lower installed height.

Now the Tom's springs are even harder to figure out. They are a progressive rate. Instead of easy math of weight and spring rate for compressed height, the spring rate changes during compression. So while these may be stock height, they are not the stock spring rate of stock. The stock springs were single rate, not progressive. The free height of the Tom's springs will not compare to a true "stock" spring. The Tom's springs are an upgraded spring over stock with a different design.

Now what you took out, it is used and good chance sagged since it was installed. And you have evidence that it may not even be stock. There have been so many different springs out over the years as well. Total guess on what you got with the Bronco.

Hello, I think I may have figured out the "mystery". My truck is finally off the jackstands and on the ground with new wheels/tires. I was shocked that the ass end of the truck, (bumper height), was approx 3 1/2 - 4" higher that the front. It had quite a rake to it and the front appeared too low with the "stock" springs from Tom's. After sleeping on it and poking around on the internet to get replacement front springs with more height, I realized that the stock heights springs are not appropriate on a truck that has a dealer installed Koenig winch setup that probably adds 200 lbs. That would explain the difference between the springs that came off the truck and the stock springs that someone else purchased. I see that Jeff's BG offers springs to compensate for the added weight.

1966-77 Ford Bronco Coil Springs, 1 1/2 - 2 1/2 Lift-Broncograveyard.com

This is a pair of variable rate 1½ - 2 ½ coil springs.br Generally, these moderate lift coils are used to level-up your vehicle, and / or to offset the weight of a winch / bumper or snow plow.br Unloaded spring height is approx. 18br

shop.broncograveyard.com

 

[jamesroney]

I find it curious that Ford did not have different springs for 6 cyl and V8? I guess they weighed about the same? Unless Tom's only offers 1 size fits all for stock ride height.

Ford offered a variety of coil springs on the early Bronco. The suspension capacity and spring rate was independent of engine displacement.

I've owned 6 cyls with 1000# coils, and V8's with 850# coils.

Your assumptions are incorrect, but your conclusion is correct.

 

[bmbm]

Ford offered a variety of coil springs on the early Bronco. The suspension capacity and spring rate was independent of engine displacement.

I've owned 6 cyls with 1000# coils, and V8's with 850# coils.

Your assumptions are incorrect, but your conclusion is correct.

Are those ratings stamped on the springs?

 

[Yeller]

Are those ratings stamped on the springs?

Might have been 50 years ago but I’ve always had to do the math. Coil spring calculator

 

[jamesroney]

Are those ratings stamped on the springs?

No, they are published in the build literature. My Marti Report lists them in the options.

It should be measurable in the wire diameter, but I don't know.