Some stock suspension calculations and measurements

[87-Z28]

Well I went from a 1" differential drop to ruffly a 5/8" drop. Only reason was better fitment for the front differential. I still think a mild differential drop is worth it to minimize the additional wear of a lift.

Absolutely agree. <1” a no brainer. Just some kinematics on large diff drops >2” and how that affects CV plunge. Nothing in IFS occurs in a vacuum.

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[87-Z28]

Estimated scrub radius and king pin axis geometry for OEM design envelope.

The axis about which the tie rods turn the wheel (king pin axis) is three dimensional. Considering only the wheel camber alignment plane, the angle of this axis can be calculated as tan ^-1 (8/20) or about 20 degrees. Measured to the nearest inch for stock oem UCA/LCA combo. This ignores wheel caster plane angle, which is very small anyway (a few degrees).

As the vehicle static ride height is lifted, the king pin angle decreases (due to pivoting UCA and LCA) which pulls ground surface pivot point Inward. This also affects wheel camber. If you have lifted your rig a few inches you can easily see the disturbing new camber angles.

LCAs can be pulled outward to readjust camber, and the king pin axis should also then readjust. When LCAs are at their outward limit of adjustment, UCA ball joint may need to be pulled inward (with new UCA) to adjust camber and king pin axis for additional lift.

the king pin axis at ride height for a lifted vehicle can be set back to stock if proper alignment is achieved. Scrub radius is then only a function of wheel centerline on the ground surface; or wheel offset and radius.

a 20 degree king pin axis angle will move the location of the ground pivot point (8/20) or 0.4” or 10 mm for every inch of wheel height achieved (radius). Plus or minus 1-2 mm??

can’t accurately measure where the king pin axis pivot point is on the ground surface. But likely there is some small positive scrub radius in the oem design. The base 30” tires are on +55 offset. Moving to SAS 34.4” tires would be an increased wheel radius of 2.2” and thus require 22 mm of decreased offset or 33mm in order to maintain oem scrub radius. SAS wheels are +30 mm offset so numbers make sense.

so going from SAS 34.4” to a 36.5” (37s) tire would require about 10mm of decreased offset for a 20mm offset wheel to maintain SAS scrub radius. Using a 0 offset wheel on SAS 35s results in significant additional positive scrub radius that can add increased force couple to wheel when braking, accelerating, turning,… and thereby increasing wear.

very rough numbers but I think they are reasonable. Perhaps a bit over simplified.

 

[87-Z28]

Analysis of global axle housing stiffness/deflection and strength.

attached is a plot of axle housing stiffness/strength normalized to the OEM M220. Wall thickness shown in 1/32” increments. This assumes equal axle lengths and material (elastic modulus and yield). Bending and torsional stiffness are identical when normalized.

Numbers used for oem SAS m220 are 2.75” with 9/32” wall. Got them off internet from Jeep axle. If anyone has actual numbers please include? I can update to be more accurate. For reference the extreme duty Spicer D60 (3.5” with 0.4” wall).

Diameter has a stronger affect than wall thickness. That is likely why the camberg and spidertrax only have a 0.25” wall with 3.5” diameter. Interesting…

Wheel/tire load factors were generated based on weights and normalized to OEM SAS (34.4” and 90 lbs). 37s assume 110 lbs, 38s are 120 lbs, and 40s at 130 lbs. wheel/tire weight drives ultimate load factors. Load factors included are linear acceleration (f=m a), torsional equivalent (T= I a), and moment arm (leverage) increase. Torsional inertia calculated as I= m r^2 (strongest contributor).

Assuming linear superposition with load factors normalized to OEM. This is extreme worst case but may serve as a reasonable design threshold. Final load factors were 37s at 1.8 times oem, 38s at 2.0, and 40s at 2.5. These wheel/tire thresholds were included on plot.

normalizing everything to OEM allows for scaled comparison and doesn’t require actual loading quantification. It however assumes OEM m220 was designed for 35s at limit state.

This only compares axle housing capabilities. Housing is the most straightforward and simplest to compare from an engineering design point of view. But if the rest of axle is designed around housing strength (flanges, pumpkin, bearings, axle shaft) then all is good. Hopefully aftermarket manufacturers will do this. But doubtful. There seems to a bunch of variations in just housing designs.

Just strengthening/stiffening oem m220 housing doesn’t address other components and seems to become a rather expensive band aid ultimately resulting in alternative weak spots. Poor design philosophy from a systems point of view. Always chasing the next failure point.

the spicer extreme D60 axle tube housing is a beast. Almost 3x stronger than oem. If the shafts, bearings, pumpkin, etc was designed to match then seems like a great bolt-on choice for 40s.

 

[87-Z28]

Analysis of local stiffness/strength of axle tube housing to impact loading.

Attached is a plot of local axle housing stiffness with respect to a point impact load (Rock impact on housing tube). normalized to the extreme spicer D60 tube (3.5” with 0.4” wall). This plot assumes a flat plate response and is only qualitatively correct, but is very representative of the local response (displacement and stiffness). As long as comparing only tubes with tubes and not plates with tubes.

unlike for the global axle bending or torsion response, thickness is king here. 1/4” tube thickness is likely 25% more vulnerable to impact deformation than the 0.4” tube from spicer.

the extreme spicer tube looks pretty stout in terms of both global and local stiffness.

 

[87-Z28]

Estimated rear drive shaft angles

My 4 door 4a transfer case and sitting near SAS ride height. Maybe 1” over at most. Flange to flange my rear shaft is just over 50” long. My ride height shaft angle is close to 7 degrees. With plus or minus 4” of rear pinion up and down motion (based on oem suspension travel of 7-8”). I won’t likely see more than 12 degrees.

Attached are some shaft angles. For near SAS geometry. 2 door in yellow and 4 door in red. Assumes 2 door shaft is 35” long (don’t have one to measure just scaled wheelbase). Dashed lines for increased angles from a 2” spacer lift or effective shock length increase. Re-drilling lower mount up 2” does same lift. Springs and preload will just move equilibrium ride height angle. 0 point for pinion motion.

the 4 door caps out at full droop from 12-14 degrees. 2 door at 17-20 degrees. Plus or minus 1 degree or so. 17 degrees on unmodified SAS 2 door doesn’t seem that high. It’s hard to get the 4 door angles excessive.

 

[Snacktime]

Can you run the driveshaft angle graph out to 12 in of droop?

King pin angle and added steering force is a fantastic talking point. Enjoying the topic over in the other thread. Would love to see you run a chart of offset vs force increase just to show negative effect on the steering.

 

[mpeugeot]

Let me see if I am understanding things correctly, as I am relatively suspension stupid when it comes to off-roading.

If I have a set of Eibach pros with threaded bodies, how does that differ from a perch collar lift on a set of stock Hitachi shocks (ignoring for a moment the actual ability to dampen movement). By raising the lower perch you are essentially lifting the the spring in the same manner as a perch collar?

Along this line of thinking, I assume that the perch collar height is the difference between the Bilstein Sasquatch and the Bilstein Badlands non-Sasquatch; is that correct?

Or I am I missing something?

I am considering installing a set of Eibach Pros along with the the upper control arms from a Zone off-road 4" lift kit (no desire for a hybrid spacer/perch collar lift). This seems like it would give me a large amount of adjustability in ride height, possibly enough to run 37s (would that be a possibility with a small body lift?).

 

[87-Z28]

If you just look at the red lines on the plot you see full compression, full extension, and ride height. When shock/spring system is installed onto bronco on Jack stands they are at full extension. When Jack stands are removed spring will compress and settle to an equilibrium position due to gravity, ie, ride height. This is near the center of shock travel for “SAS” depicted in plot.

Ride height can be altered by changing spring forces.

1) increasing spring stiffness with new spring. A stiffer spring will result in less spring compression when loaded by gravity (for same vehicle weight) and new ride height will increase because shock is now riding closer to full extension. effective shock length at ride height is longer so distance between frame and wheel hub increases, adding lift. A softer spring will cause ride height to move closer to full compression and decrease ride height.

2) increasing spring preload with same spring. The spring preload, usually referenced as spring compression in inches, adds compressive force to the spring. For me it is easier to think wrt forces. So when loaded by gravity of the vehicle the spring compresses less since less force is required to reach equilibrium due to the additional preload force. Then ride height increases and the new equilibrium position is closer to full extension. Excessive preload force will cause ride height to be near full extension. Plot shows ride height for a 3” preload (an additional 1200 lbs for a 400 lbs/in spring stiffness).

Both a perch collar and the threaded spring preload adjustment do the same thing. Add preload force by compressing spring. Adjustable threaded version has benefits. I have the 6100 bilsteins that allow for preload adjustments but are not threaded. Circlip and groove moves spring to discrete locations. I wish mine were threaded. Pita.

 

[mpeugeot]

If you just look at the red lines on the plot you see full compression, full extension, and ride height. When shock/spring system is installed onto bronco on Jack stands they are at full extension. When Jack stands are removed spring will compress and settle to an equilibrium position due to gravity, ie, ride height. This is near the center of shock travel for “SAS” depicted in plot.

Ride height can be altered by changing spring forces.

1) increasing spring stiffness with new spring. A stiffer spring will result in less spring compression when loaded by gravity (for same vehicle weight) and new ride height will increase because shock is now riding closer to full extension. effective shock length at ride height is longer so distance between frame and wheel hub increases, adding lift. A softer spring will cause ride height to move closer to full compression and decrease ride height.

2) increasing spring preload with same spring. The spring preload, usually referenced as spring compression in inches, adds compressive force to the spring. For me it is easier to think wrt forces. So when loaded by gravity of the vehicle the spring compresses less since less force is required to reach equilibrium due to the additional preload force. Then ride height increases and the new equilibrium position is closer to full extension. Excessive preload force will cause ride height to be near full extension. Plot shows ride height for a 3” preload (an additional 1200 lbs for a 400 lbs/in spring stiffness).

Both a perch collar and the threaded spring preload adjustment do the same thing. Add preload force by compressing spring. Adjustable threaded version has benefits. I have the 6100 bilsteins that allow for preload adjustments but are not threaded. Circlip and groove moves spring to discrete locations. I wish mine were threaded. Pita.

Thank you, that helps and it's how I generally thought it worked, usually I am going the other way (lower).

I managed to get a set of the Eibach Pro Truck 2.0 for $900, and while not the most Guichi suspension, they seem adequate for the money. I was also able to get a Zone 4" lift kit for $300 and will use the UCAs from the kit. Seems like it will get me a 3" lift with little drama (but I will still need an adjustable track bar to get everything straight).

 

[zuke]

I am considering installing a set of Eibach Pros along with the the upper control arms from a Zone off-road 4" lift kit (no desire for a hybrid spacer/perch collar lift). This seems like it would give me a large amount of adjustability in ride height, possibly enough to run 37s (would that be a possibility with a small body lift?).

This is exactly what I'm running now after starting with a Zone spacer lift..

No body lift, Zone Upper Arms, stock Trackbar, JKS Max Tire Clearance Kit.

With the Eibach's set to give about 2.5" of lift, I'm running Maxxis Razr A/T 37x12.5r17's (True diameter 36.5) on ProComp 69 Series wheels (9" rim with -6 Offset) with no rub issues.

The rear axle is moved about 3/16" to the right from stock at ride height, and it's imperceptible when looking at it or driving, I don't see the value in an adjustable Trackbar or lifted trackbar mount at that difference.

You're two door is likely way lighter than my 4d Badlands loaded up with gear, So you'd probably end up with less pre-load to achieve the same height and ride, but it's pretty easy to adjust the Eibach's (I'd look for a better spanner then the one that comes with them though, I broke the nub off doing the third shock I adjusted with it).

The ride is way nicer than it was with the spacer lift on the road, and I haven't seen any loss of articulation or issues off-road. From a value perspective, I'm very happy with the Eibach's.

 

[Fordified1]

The rear axle is moved about 3/16" to the right from stock at ride height, and it's imperceptible when looking at it or driving, I don't see the value in an adjustable Trackbar or lifted trackbar mount at that difference.

There is some value to getting the track bar level. As the more level it is, the less side to side movement you will get when flexing the rear suspension. (especially extension) relocation kits are pretty cheap.

 

[zuke]

There is some value to getting the track bar level. As the more level it is, the less side to side movement you will get when flexing the rear suspension. (especially extension) relocation kits are pretty cheap.

That's absolutely true, and yes, the Rockjock bracket is available from lots of places for around $120.

As soon as we start talking "Value" it becomes a little subjective though

For low speed off road, (Rock Crawling like I do) unless you're getting binding in the driveshaft or control arms, or rubbing of components (Brake Lines, Shocks/Springs against the frame), the axle swinging further right isn't a big deal.

For any off-roading where you cycle the suspension a lot, (Desert, Dunes) especially fast, Like whoops and moguls, keeping the axle more centered is a lot more important, and the relocation bracket has a lot more value.

 

[87-Z28]

ProComp 69 Series wheels (9" rim with -6 Offset) with no rub issues

do you think running something like a +18 offset wheel in your setup would have any rub issues? I like the minimal lift (~2”) to get 37s on with no rub at full articulation. But would like to get the scrub radius closer to stock. Probably a small 1” body lift is needed?? I guess it’s just really hard to know until you try it.

 

[mpeugeot]

do you think running something like a +18 offset wheel in your setup would have any rub issues? I like the minimal lift (~2”) to get 37s on with no rub at full articulation. But would like to get the scrub radius closer to stock. Probably a small 1” body lift is needed?? I guess it’s just really hard to know until you try it.

That is the exact reason why I have a 2" body lift going on. I may go with less body lift, but I figure that with a 2-3" lift from the Eibachs and a 2" body lift, 37s should not be a problem. I do think that it will require some body lift to get 37s fully in there. I know that some will disagree with the body lift, but it seems like an easy way to ensure that the fender clearance is there.

 

[zuke]

do you think running something like a +18 offset wheel in your setup would have any rub issues? I like the minimal lift (~2”) to get 37s on with no rub at full articulation. But would like to get the scrub radius closer to stock. Probably a small 1” body lift is needed??

I "think" it would work, the main rub point is usually the inside-back of the fenders when turning at or close to lock, with less scrub radius I think it ends up further from it not closer... But, as you said;

I guess it’s just really hard to know until you try it.

You really need to find someone already running the setup you propose, and that's always a challenge.

As long as you still have your old tires/wheels, you can always put the lift on and test the new combo, Then get a body lift if you need one. and run the old tires while you wait...

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