Kinematic model of OEM SAS front suspension

[87-Z28]

I have been working on trying to accurately model the OEM front suspension for a while (way too long). There is a thread with lots of disconnected thoughts.

https://www.bronco6g.com/forum/threads/some-stock-suspension-calculations-and-measurements.73917/

which may be difficult to read. So I have started this thread with a more refined model and results. I know this may be way to deep for many but it is interesting to me and was very helpful in my understanding of the OEM front suspension details. Hope some find this interesting.

The inspiration needs to be credited to my favorite B6G troll, @Snacktime. His post and analysis of the 5100 bilsteins is fantastic and very worth a read for many wanting to learn more about the details.

https://www.bronco6g.com/forum/threads/snacktimes-5100-bilstein-shocks-review-takeaways.56435/

This was the first thread I read that made me stop and really think.

Sponsored

 

[87-Z28]

After lots of thought, perhaps a good way to model vertical wheel motion is as follows; track 3D coordinate locations of LCA frame mounts, lower knuckle ball joint, and both upper and lower shock mounts. These parameters fully constrain the kinematics for vertical wheel motion, a quantity we all care about greatly.

The UCA helps to keep a reasonable king pin axis during LCA movement by constraining knuckle upper ball joint. But does not affect kinematics of vertical wheel motion, only wheel orientation (camber, caster, and toe).

Model is 2D in frontal plane only. However, out of plane motion in for-aft direction has very little effect on vertical wheel motion. This can be seen from side view and front view pics attached. <1 deg shock angle for-aft and the LCA for/aft frame mounts have only 12 mm vertical difference.

Getting accurate 3D coordinates has been a challenge. Just taking measurements from front end is a PITA and not very accurate. Accurate results were obtained from good 3D coordinates. The overall model results are now easily +/- 5%. This kinematic model only requires 4 point locations for the link pivot points.

 

[87-Z28]

Kinematic motions from model can be plotted as functions of the LCA angle (positive downward). The primary input variable is the shock length, which drives the motion of the LCA angle. Vertical wheel motion can then be plotted along with motion ratio (vertical wheel displacement divided by shock displacement).

The attached plot shows the results for the OEM SAS configuration. Static equilibrium (ride height) is shown for a shock length of 20” and LCA angle of 2 degrees. Also shown are the max compression and extension lengths for the SAS bilsteins (17.06 and 23.31 inches).

Note. Primary input parameter for model is shock length. Kinematics dictates 20” shock length achieves 2 degree LCA angle. Exact shock spring rate/preload combination determines length for ride height, but is not relevant to quantify kinematic motion of LCA. This is an important realization.

Some observations wrt OEM design

• LCA swings through 36 degrees, from -15 to 21 degrees
• 6.25” of shock travel yields 9.3” of wheel travel
• Shock travel wrt ride height is fairly balanced, 3.3” in extension and 3” in compression
• Motion ratio is nonlinear and varies from 1.5 to 1.7
• Wheel moves 4.5” upwards (compression), so top of tire to inner fender clearance needs to be at least 5” for OEM SAS ride height.

 

[87-Z28]

Any shock configuration can be put into kinematic model, since shock length is primary input variable. Some common aftermarket shock design characteristics have become evident, such as maximizing extension to 24.3” (observed CV binding limit) and maintaining travel in the 6” range.

As an example, Attached plot shows results for hypothetical aftermarket shocks with max extension at 24.3” and 6” of travel (so 18.3” max compression). Assuming a balanced shock motion at ride height (3” jounce and rebound), then the ride height shock length becomes 21.3” and LCA angle is 8 degrees. The lift at wheel then becomes 2” above SAS. A spring rate/preload combo needs to be designed to achieve this.

Some observations wrt aftermarket shocks.

• LCA still swings through near 36 degrees, but now from -7 to 28 degrees
• Only 6.0” of shock travel now yields 9.7” of wheel travel (better than SAS), since motion ratio increases with increasing LCA angle
• Shock travel wrt ride height remains balanced, 3” in extension and 3” in compression
• Motion ratio on average is greater than SAS and varies from 1.55 to 1.75
• Wheel now moves <3” upwards (compression), this allows for 37-38s and prioritizes clearance at full compression. Some jounce shock travel is likely left on the table. Fox and ride shocks even have sub 6” travel to further prioritize jounce clearance.

 

[87-Z28]

Attached is a figure showing the king pin axis at ride height for OEM SAS configuration. A side view is included to show caster angle (4 degrees in this configuration). The following equation can be used to calculate scrub radius. This should be accurate for all wheel/tire setups that use OEM hub mounting surface (not the portal guys). This assumes king pin axis is close to OEM alignment, so all lift heights that accomplish this are applicable. Equation also includes variable for wheel spacers.

Rscrub = ( 147 - Woffset + Wspacer ) * 0.233 Rtire - sin (Acamber) Rtire.

Rscrub is scrub radius, Woffset is wheel offset, Wspacer is wheel spacer thickness, Rtire is true tire radius, and Acamber is camber angle. All units in mm.

Some observations:

• Interestingly SAS scrub radius is very near zero, only a positive 5 mm.
• A common wheel/tire combo of 0 offset and 37s (36.5” true) gives a scrub radius of 29 mm.
• 74 weld portals (assuming 4” increased track width per side and 4” vertical drop to new hub mounting surface) using +30 offset SAS wheels and 40” tires has a 63 mm scrub radius. Note this was calculated from a different equation due to portal drop height.

 

[userdude]

Yeah! What he said.

 

[zoober]

I would love to be able to play with those models in Creo

 

[Brian_B]

Wow, Impressive!

 

[PWillette]

Kinematic motions from model can be plotted as functions of the LCA angle (positive downward). The primary input variable is the shock length, which drives the motion of the LCA angle. Vertical wheel motion can then be plotted along with motion ratio (vertical wheel displacement divided by shock displacement).

The attached plot shows the results for the OEM SAS configuration. Static equilibrium (ride height) is shown for a shock length of 20” and LCA angle of 2 degrees. Also shown are the max compression and extension lengths for the SAS bilsteins (17.06 and 23.31 inches).

Note. Primary input parameter for model is shock length. Kinematics dictates 20” shock length achieves 2 degree LCA angle. Exact shock spring rate/preload combination determines length for ride height, but is not relevant to quantify kinematic motion of LCA. This is an important realization.

Some observations wrt OEM design

• LCA swings through 36 degrees, from -15 to 21 degrees
• 6.25” of shock travel yields 9.3” of wheel travel
• Shock travel wrt ride height is fairly balanced, 3.3” in extension and 3” in compression
• Motion ratio is nonlinear and varies from 1.5 to 1.7
• Wheel moves 4.5” upwards (compression), so top of tire to inner fender clearance needs to be at least 5” for OEM SAS ride height.

This is pretty cool stuff!

Just out of curiosity, could you plug in my Eibach 2.0s (24.2" extended, 17.77" compressed)?

 

[Snacktime]

Now calculate the difference in bump steer and how much toe changes. I got wear issues on my tires!

 

[SierraBronco]

So frickin awesome man. We need to get some measurements on the other portal manufacturers to see what their scrub radius is. I think the Tibus portals are wiiiiiiiiiiide with the knuckle not being incorporated into the box

 

[Jtbob3]

Now calculate the difference in bump steer and how much toe changes. I got wear issues on my tires!

I suspect toe angle changes a lot more at the extremes of travel. That would lead to more bump steer which could result in some vehicle wander and would definitely wear the tires a bit more.
The other thing to consider is wheel camber. It will also have bigger changes with more travel and camber leads to tire wear.

 

[murphtron]

I have been working on trying to accurately model the OEM front suspension for a while (way too long). There is a thread with lots of disconnected thoughts.

https://www.bronco6g.com/forum/threads/some-stock-suspension-calculations-and-measurements.73917/

which may be difficult to read. So I have started this thread with a more refined model and results. I know this may be way to deep for many but it is interesting to me and was very helpful in my understanding of the OEM front suspension details. Hope some find this interesting.

The inspiration needs to be credited to my favorite B6G troll, @Snacktime. His post and analysis of the 5100 bilsteins is fantastic and very worth a read for many wanting to learn more about the details.

https://www.bronco6g.com/forum/threads/snacktimes-5100-bilstein-shocks-review-takeaways.56435/

This was the first thread I read that made me stop and really think.

This is amazing and I will study. But I want to humble brag that I finally met the legendary @Snacktime in the flesh snowheeling a few weeks ago and got his extensive suspension download. Great guy!

 

[87-Z28]

I suspect toe angle changes a lot more at the extremes of travel. That would lead to more bump steer which could result in some vehicle wander and would definitely wear the tires a bit more.
The other thing to consider is wheel camber. It will also have bigger changes with more travel and camber leads to tire wear.

yeah. The LCA swings through large angles. It is a challenge for the UCA to keep up at the extremes. So the knuckle orientation changes a bit. None of that out of plane motion is included in the model. It won’t affect wheel up and down motion at all, just wheel orientation.

At some point I do want to add the UCA motion into this and get the full 3D kinematics. Snacktime can worry about his own toe issues, seen to many pictures of his feet on his posts.

 

[87-Z28]

Just out of curiosity, could you plug in my Eibach 2.0s (24.2" extended, 17.77" compressed)?

yeah. That is why I did the aftermarket comparison using max length of 24.3”. Your shocks are going to start at the same LCA angle at full extension, right side of curves. You will just end up with slightly more compression since yours travel 6.4” and not the 6.0” I used in the plots. So left side changes some. Also not sure where your ride height is. But I assumed 2” of generic lift for the plots.

Sponsored