Bronco coilover spring specs?

[Silver-Bolt]

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

Awesome! Thank you!

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[87-Z28]

Here are some spring rates from @Snacktime bilstein 5100s thread. Some fairly soft springs.

Here are the specs comparing the Bilsteins/OME. The Bilstein 6112 fronts sit between the OME medium and heavy, and the rears match the medium OME springs.

Ok

 

[87-Z28]

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

I think that one of the differences with the Bronco may be the motion ratio of the LCA. Bronco has a small wheel amplification from shock travel. Probably only about 1.5. My earlier calculations in this thread did not account for that. Load at the wheel should be multiplied by this factor to get spring load in the shock for the front end. So I believe my earlier spring rate estimates are a little low.

the FJ cruiser may have a larger motion ratio. Maybe closer to 2.0. Given similar vehicle weights it might then need larger spring rates. By a factor of 2/1.5 or 1.33. So a 500 fj cruiser spring might be equivalent to a 375 bronco spring. Maybe that is one reason the front spring rates seem lower. Just a thought. Any idea what the motion ratio for FJ cruiser is?

 

[87-Z28]

Some lessons learned. Thanks @Silver-Bolt for input. So I think it is a challenge to measure displacements on truck and estimate spring forces accurately. Weight distribution and CG are not really known. So load on a wheel is difficult to get. Gravity and an incline also have an effect. In the IFS, motion ratio can not be ignored in the front. Too many sources of error.

Perhaps the best way is to use the linear spring theory. K= G * d^4 / (8 * N * D^3) for a gut check, using this on OME data, d=15.75 mm=0.62”, N=10.5, G=11x10^6 psi, K=332, so coil diameter calculates as D=3.9. Table doesn’t list coil diameter so can’t calculate K directly. Table does list spring weight (5.2) kg, so coil diameter can also be calculated from weight as 4.0”. Very similar to 3.9” from spring theory. Fox coil diameter was reported as 3.6” for front. Ford springs have a fairly large diameter at about 5.5” and don’t sit as snuggly on the shock as OME or fox.

Seems that the spring theory works fairly well for the OME springs. So from earlier calculations, the Ford spring rates from my 4dr 2.3 OBX hitachis calculated as:

• Front 350 lbs/in
• Rear 225 for initial linear range

I now believe those are pretty good numbers. SAS springs are likely heavier to achieve additional lift and not affect either preload or usable extension travel.

IFS motion ratios. PIA… For bronco this seems to be close to 1.5 for the front. Close to 1:1 in the back. So if rear spring rate is 225 then a similar rate spring on front would need to be 225*1.5 or 340. The Bronco spring rates calculated above then seem fairly balanced on all four corners.

 

[Silver-Bolt]

Some lessons learned. Thanks @Silver-Bolt for input. So I think it is a challenge to measure displacements on truck and estimate spring forces accurately. Weight distribution and CG are not really known. So load on a wheel is difficult to get. Gravity and an incline also have an effect. In the IFS, motion ratio can not be ignored in the front. Too many sources of error.

Perhaps the best way is to use the linear spring theory. K= G * d^4 / (8 * N * D^3) for a gut check, using this on OME data, d=15.75 mm=0.62”, N=10.5, G=11x10^6 psi, K=332, so coil diameter calculates as D=3.9. Table doesn’t list coil diameter so can’t calculate K directly. Table does list spring weight (5.2) kg, so coil diameter can also be calculated from weight as 4.0”. Very similar to 3.9” from spring theory. Fox coil diameter was reported as 3.6” for front. Ford springs have a fairly large diameter at about 5.5” and don’t sit as snuggly on the shock as OME or fox.

Seems that the spring theory works fairly well for the OME springs. So from earlier calculations, the Ford spring rates from my 4dr 2.3 OBX hitachis calculated as:

• Front 350 lbs/in
• Rear 225 for initial linear range

I now believe those are pretty good numbers. SAS springs are likely heavier to achieve additional lift and not affect either preload or usable extension travel.

IFS motion ratios. PIA… For bronco this seems to be close to 1.5 for the front. Close to 1:1 in the back. So if rear spring rate is 225 then a similar rate spring on front would need to be 225*1.5 or 340. The Bronco spring rates calculated above then seem fairly balanced on all four corners.

It would be super simple if I had factory springs to put in our spring rate machine. No guessing/speculating required. I will be adding a fair amount of weight to my Wildtrak when it arrives. ARB Summit bumper, winch, Metal-Tech rock rails, skid plates, and rear bumper with swingout tire carrier. I don't want a lift, I just want to handle the extra weight at the factory ride height.

 

[Bikeric]

It would be super simple if I had factory springs to put in our spring rate machine.

The unfortunate thing would be you need to test 11 different front springs and 6 different rear springs.
All springs lengths should be the same for front, and again for rears. The heaviest spring rate would have to be from a 4-door First Edition as it had EVERY OPTION. Find someone with a 4-door FE and ask for their VIN so you can determine what spring P/N they have.
(I grabbed this VIN from BaT: 1FMEE5EP2MLA40056 )

 

[87-Z28]

Guessing and speculation is part of the fun.

agreed on the testing. The simplest would be to test the springs from heaviest and lightest trims. Then weight ratios could be used to get all the rest.

I would send mine but $60 in shipping for ground ups and I want them back. Surely someone close to you has replaced their coilovers.

 

[Silver-Bolt]

Guessing and speculation is part of the fun.

agreed on the testing. The simplest would be to test the springs from heaviest and lightest trims. Then weight ratios could be used to get all the rest.

I would send mine but $60 in shipping for ground ups and I want them back. Surely someone close to you has replaced their coilovers.

Local would be ideal. No problem to return the springs after testing. Testing is completely non-destructive. The machine just compresses the spring and measure the rate at specified compression lengths.

 

[87-Z28]

Understood. I have tested concrete and metal samples in axial compression and tension. Both non destructively and to failure. The testing would also provide the nonlinear response of the rear spring as displacement increases. If I was within 500 miles of you I would bring mine and watch. Awesome stuff.

I am moving my daughter to Seattle in January. If we don’t have numbers by then I may stop by.

 

[87-Z28]

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

can you also share the preload in inches that fox is using on the front out of box at lowest setting. Or whatever setting they use when advertising 3” of lift over stock SAS.

 

[AccuTune Offroad]

I don't have those specs to share at the moment. We will be putting together a very detailed tech article, it's just going to take some time.

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