Bronco coilover spring specs?

[Direwild]

Tried looking but cant seem to find dimensions and spec on the coilover springs for the 6th gen broncos. Im looking for the ones on the 2dr base manual but having a list for all the trims and everything would be nice too. I just want to possibly replace the springs and not the whole coilover and save some money, i would just like to know the default springs before I even think about touching it.

Sponsored

 

[Tricky Dick]

I haven't seen specs, but I have put stock springs from a heavier trim on my shocks and it firmed up the ride and gave lift. I highly recommend it.

 

[userdude]

You might contact some of the aftermarket companies, like Eibach.

 

[Direwild]

well decided to lift it up. The length of the front is 14inch length and 18 in the back. i think have the rest for the front and back springs but i don't want to sit here and give out false info. But the front spring rating is 600/in and the back is 250. i could be very wrong but i could be in the ball park. i still want to find confirmed spec for the stock springs.

 

[87-Z28]

well decided to lift it up. The length of the front is 14inch length and 18 in the back. i think have the rest for the front and back springs but i don't want to sit here and give out false info. But the front spring rating is 600/in and the back is 250. i could be very wrong but i could be in the ball park. i still want to find confirmed spec for the stock springs.

Estimated spring rates.


Weight total 4577, front 2543, rear 2034. 4dr OBX with 2.3L and no extra weight mods. SAS springs with bilstein 6100s.
Rear 19” free length and about 13” compressed at static load (ride height). Effective rear spring stiffness: (2034/2) / (19-13)= 169 lbs/in. Call it 175..

Front 16” free length and about 11.5” compressed at static load. Effective front spring stiffness: (2543/2) / (16-11.5)= 282 lbs/in. Call it 300.

It was a little difficult to measure displaced length on truck but probably accurate to 1”. So above estimates seem reasonable to me. Very soft.

 

[87-Z28]

Alternative method to estimate spring stiffness.



Theoretical spring stiffness for linear spring with constant diameter. Front spring is slightly non-linear with major diameter of 5.5” but drops to 5.0” for last coil on each end. Rear spring is more significantly non-linear with major reduction on diameter at ends. However, linear stiffness might be estimated as: K= G * d^4 / (8 * N * D^3)



G= is elastic shear modulus and mostly independent of strength for steel, about 11 x 10^6 psi. D is major diameter (5.5). d is diameter of wire (3/4). N is number of coils (7.5)



Front spring stiffness estimate from theory is 350 lbs/in.



Rear spring stiffness estimate is 225 lbs/in, if assuming 11.5 coils and all other variables same as front. This is probably not a good way to do this for rear spring since it is more non-linear than the front.



Nonetheless, numbers are similar to measurements of displacements at ride height from my OBX. I doubt the springs in the front are much over 350 or 200 in the rear. Very soft.



Since both springs are non-linear to some degree I would tend to believe the stiffness numbers from the ride height displacements. This should provide a more real word effective spring stiffness. The non-linearity should not be significantly engaged until the spring is nearly full compressed and the stiffer ends (smaller major diameter) begin to do the work.

 

[87-Z28]

Spring rate estimates for other trims.

Badlands 2.7 4dr seems to be heaviest trim (steel bumper and bash plates probably). 2907 lbs in front. So if Ford wanted to keep similar SAS ride height as my OBX 2.3 (probably lightest 4 Dr trim), then same spring displacement is required in front or about 4-4.5”. The spring stiffness can be estimated from the increased weight ratio (2907/2543) or 1.14 (15%). The “heaviest” stock spring is probably 350 if my OBX is 300. These ratios can probably also be made for the 2dr variants. In the rear there is probably not much significant difference.

It would be interesting to see if the non SAS springs are lighter. If they have same unloaded length as SAS and have lower ride height with all other geometry identical, then they should be lighter. One extra inch of displacement on my OBX for a non SAS ride height would result in a spring stiffness of (2543/2) / (16-10.5)= 231 lbs/in. Maybe 250.

It seems ford made small changes in the spring stiffness across the various trims in order to achieve consistent ride height and the dynamic performance or displacements under loading that they intended. Seems to be a very fine tuned system. For better or worse, depending on your use case, but clearly fine tuned with intent.

 

[Silver-Bolt]

Spring rate estimates for other trims.

Badlands 2.7 4dr seems to be heaviest trim (steel bumper and bash plates probably). 2907 lbs in front. So if Ford wanted to keep similar SAS ride height as my OBX 2.3 (probably lightest 4 Dr trim), then same spring displacement is required in front or about 4-4.5”. The spring stiffness can be estimated from the increased weight ratio (2907/2543) or 1.14 (15%). The “heaviest” stock spring is probably 350 if my OBX is 300. These ratios can probably also be made for the 2dr variants. In the rear there is probably not much significant difference.

It would be interesting to see if the non SAS springs are lighter. If they have same unloaded length as SAS and have lower ride height with all other geometry identical, then they should be lighter. One extra inch of displacement on my OBX for a non SAS ride height would result in a spring stiffness of (2543/2) / (16-10.5)= 231 lbs/in. Maybe 250.

It seems ford made small changes in the spring stiffness across the various trims in order to achieve consistent ride height and the dynamic performance or displacements under loading that they intended. Seems to be a very fine tuned system. For better or worse, depending on your use case, but clearly fine tuned with intent.

Are you assuming the spring rates based on static weights? I guarantee that the front springs are not 350lb springs. More likely in the 500-550 range. I have tried to get someone to send me the stock springs to test but so far no luck. I can test and return the springs.

 

[87-Z28]

Yes. Static weight displacing spring. And very very crude measurements. Could easily be off by 25%. That would bring 350 up to 450. They are also not exactly linear. I wouldn’t be surprised if these estimates are low. But not sure they would go up to 600. Maybe. I could see closer to 500. Initial estimates are usually poor. It would be very good to get real numbers from a compression test.
I did think 350 is kinda low so that is why I tried estimating with the linear spring theory. And those numbers are also kinda low. The springs are mildly nonlinear though.
Just trying to add some initial data for others to respond so we can get better data. I have non SAS springs lying around that I switched out.

 

[Silver-Bolt]

Yes. Static weight displacing spring. And very very crude measurements. Could easily be off by 25%. That would bring 350 up to 450. They are also not exactly linear. I wouldn’t be surprised if these estimates are low. But not sure they would go up to 600. Maybe. I could see closer to 500. Initial estimates are usually poor. It would be very good to get real numbers from a compression test.
I did think 350 is kinda low so that is why I tried estimating with the linear spring theory. And those numbers are also kinda low. The springs are mildly nonlinear though.
Just trying to add some initial data for others to respond so we can get better data. I have non SAS springs lying around that I switched out.

Not the same application but My old FJ Cruiser factory springs were 500lb. Went up to 650's for a heavy bumper and winch.

 

[87-Z28]

Agreed. 500 seems more reasonable. Maybe my numbers are bogus. I am real curious however where the design thoughts of ford were. If the springs are 25% softer. Closer to 400. Would be great to know.

Another major assumption is the static weight on the spring. I just grabbed generic data from ford. How weights are actually distributed. Probably only 10% error tho and nothing better to use.

Just a thought, if springs are 500 then 1000 total for front end. For front end weight on my OBX (2543) then only 2.5” static displacement. I am pretty sure I have quite a bit more than that at ride height. Just tried to measure again. Hard to get in there, but sure looks closer to 4”. I am having a hard time getting to 500. My springs are probably the lightest tho, since my weight is lower, 2.3 with plastic bumper and skids.

a side note, I have the bilstein 6100s installed on the lowest preload setting. They only come in one flavor for 4dr. So that setting should get you close to level as advertised. It was probably designed for 2.7 heavier front end so folks wouldn’t end up with squat. So mine is not level with some small rake, slightly lighter springs.

the range in front end weights from ford is 2907 to 2338. That is about 25% increase at max weight. If spring rate is nominally more like 500 for heaviest trim, then to maintain ride height rate could drop by 25% for lightest trim or to 375.

I think Ford has some pretty light springs out there for some of us. Also clearly by intent. if anyone has some better data please add and/or correct.

 

[87-Z28]

Hi Bronco people. I can provide a few answers to some of the questions out there for damper travel and spring rate.

Damper travels for 2 door and 4 door are the same. Damper travel is not equal to wheel travel...I do not recall the motion ratio for Bronco but it wouldn't be that hard to back into.

FA - full stroke (metal to metal) 161mm or 6.3"
RA - full stroke (metal to metal) 239mm or 9.4"

FA spring rate is around 415in/lb
RA spring rate is little tougher...the spring is pretty complicated...the first 4" of travel are approx 165in/lb, the remaining travel is approx 290in/lb.

ESCV - I can't really add more detail than the Bilstein cross sections and media have provided. It is an amazing piece of tech that is first to market on monotube dampers. It was a fun ride from concept to first test parts...6 very fast weeks.

Just an observation from me on some of the coilovers coming out, be very aware of the damper travel. One of the kits (a very expensive one) has an inch less travel than the stock Bilsteins for the rear damper. I don't like giving up travel

I might be able to answer more questions...toss them my way. If I can answer, I will.

Silent Panda - former Head of Damper Magic at Bilstein

Found this in an early thread from 2020. Not sure if these numbers are design data or actual as built. According to this the front spring is 415. I believe there are various different front springs. I know I have a softer one. So maybe range in front is 350-450.

rear is definitely nonlinear and early in compression range seems to be close to 175. This would include static load at ride height. It will stiffen up significantly near end of compression travel. Not exactly clear at what stroke but seems to stiffen to near 300. You can see major diameter decrease in the spring near the ends

@Silent Panda. Any additional data?

@Evolkidbell any thoughts or insight. Maybe you have some data since you were on original thread.

@AccuTune Offroad have you guys taken apart any oem springs and measured them. I think most of the tunes you guys do are fox and king but thought maybe I would ask to see if you have played with the oem springs.

I had the day off today and kinda got wrapped up in this, sorry.

 

[AccuTune Offroad]

I don't have too much stock info to share, we are more focused on adapting Fox/King shocks to upgraded Broncos looking for more performance.

 

[Silver-Bolt]

I don't have too much stock info to share, we are more focused on adapting Fox/King shocks to upgraded Broncos looking for more performance.

Can you share what the spring rate is for the Fox/Kings?

 

[AccuTune Offroad]

Can you share what the spring rate is for the Fox/Kings?

Out of the box:

Fox
Front: 16.55 X 3.630, 365lb
Rear: 15.5 X 3.57, 185 LB

King
Front: 16.00” x 3.00x 550lb
Rear: 4.00” x 3.00 500lb / 14.00” x 3.00x 350lb

Sponsored