Some stock suspension calculations and measurements

[87-Z28]

Yeah that comes straight from the workshop manual. I am pretty sure it actually states 18 degrees. I will dig out the text and post it here when I have some time. I am traveling this week.

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[87-Z28]

From the workshop manual.
Halfshaft
Special Tool(s) / General Equipment

Tire Lever

Removal

NOTICE: Do not hold the halfshaft by the CV joint.

NOTICE: The inner CV joint must not be bent more than 18°.

@Aonarch

Of course this is just recommendation from the shop manual. It is not clear what the increased wear and longevity is at higher angles (>20) or the catastrophic failure limit at severe angles (>30).

The SAS OEM geometric design seems to limit angles to less than 20 degrees. Severe use at more than 30 is likely a challenge for the stock CVS.

 

[climb2descend]

Seems pretty clear at this point that someone needs to make a limit strap kit for these extended length shocks. Seems like we would need a 90deg tab on the top of one of the bar pin bolts.

 

[builtMIbronco]

Seems pretty clear at this point that someone needs to make a limit strap kit for these extended length shocks. Seems like we would need a 90deg tab on the top of one of the bar pin bolts.

I think limit straps are definitely an option, but maybe not the option leaving the most performance on the table. A diff drop that drops all 3 mounts even just an inch would reduce angles pretty significantly, and still allow for the full travel range we see out of pretty much every coilover setup available.

I ordered a T6 Ranger diff drop from across the pond to see how well everything fits and what I'll need to fab up to make it work.

 

[climb2descend]

I think limit straps are definitely an option, but maybe not the option leaving the most performance on the table. A diff drop that drops all 3 mounts even just an inch would reduce angles pretty significantly, and still allow for the full travel range we see out of pretty much every coilover setup available.

I ordered a T6 Ranger diff drop from across the pond to see how well everything fits and what I'll need to fab up to make it work.

Fair point, we need all the droop we can get. Will be interesting to see your results, and how much it actually helps and how much u need to lower the skid plate. Did you just get a spacer one?

 

[builtMIbronco]

Fair point, we need all the droop we can get. Will be interesting to see your results, and how much it actually helps and how much u need to lower the skid plate. Did you just get a spacer one?

Yea I'll definitely have to notch the diff skid at a minimum even more than it is already for the rc diff drop. I ordered the full t6 ranger "bolt-on" kit but I really only wanted their upper mount spacer and the driveshaft spacer. The overseas ranger kit drops all 3 mounts unlike the diff drops we have here that only drop the front of the diff half inch or so. I'll have to either just space out the passenger side with washers or come up with a new bracket, and then I plan on taking the front mount as it goes. If I can use the one they send great, if not I'll build or modify what I have between theirs and the RC bracket. I believe the kit I bought said it moves the diff down a full inch and forward 20mm or so. From what I can tell this will barely clear the cross member.

All will be experimental and I'm not expecting it for a couple weeks still, but it was fairly cheap just comes from Australia lol

 

[87-Z28]

Yea I'll definitely have to notch the diff skid at a minimum even more than it is already for the rc diff drop. I ordered the full t6 ranger "bolt-on" kit but I really only wanted their upper mount spacer and the driveshaft spacer. The overseas ranger kit drops all 3 mounts unlike the diff drops we have here that only drop the front of the diff half inch or so. I'll have to either just space out the passenger side with washers or come up with a new bracket, and then I plan on taking the front mount as it goes. If I can use the one they send great, if not I'll build or modify what I have between theirs and the RC bracket. I believe the kit I bought said it moves the diff down a full inch and forward 20mm or so. From what I can tell this will barely clear the cross member.

All will be experimental and I'm not expecting it for a couple weeks still, but it was fairly cheap just comes from Australia lol

Yeah, a decent well thought out diff drop is definitely needed. All the ones out there seem like a quick fix afterthought. Dropping only the front two mounts. Post your results when you finish. I would be curious how the interface with the drive shaft works out.

 

[huck2080]

do you happen to know what the lift height was set to out of the box on those ?

 

[TeocaliMG]

Yeah, a decent well thought out diff drop is definitely needed. All the ones out there seem like a quick fix afterthought. Dropping only the front two mounts. Post your results when you finish. I would be curious how the interface with the drive shaft works out.

I called RC but it seems they are catching on and refuse to sell their diff drop separately from their lift kits. I decided to build my own drop bracket, I will post a thread when I get it all installed.

It is not as intuitive as it might seem, as much as i'd like to drop everything equally straight down at all 3 points of contact, the rear contact is very difficult to drop since it has a weld nut and stud in place that need to be removed. Big Meats has a good thread on how he did that, but it's a chore. Mine will mostly drop the front, and right side, and add an extension plate to the back to help compensate for the slight rotation downward.

The second important thing that must be considered is CV plunge. If you are using a diff drop, and not dropping the lower control arms and using taller knuckles, you will be introducing extra plunge in the half shafts as they are misaligned to the geometry of the suspension. By my calculations 20mm of diff drop is the limit (with stock suspension hardpoints, no cradle drop) where you use up all the half shaft plunge. Dropping lower than that will compress or pull the CV's further than they are designed to travel which is certainly worse than a slightly higher CV angle. Even dropping 20mm will reduce max angle by more than 10%!

 

[Snacktime]

I called RC but it seems they are catching on and refuse to sell their diff drop separately from their lift kits. I decided to build my own drop bracket, I will post a thread when I get it all installed.

It is not as intuitive as it might seem, as much as i'd like to drop everything equally straight down at all 3 points of contact, the rear contact is very difficult to drop since it has a weld nut and stud in place that need to be removed. Big Meats has a good thread on how he got around that, but its a chore. Mine will mostly drop the front, and right side, and add an extension plate to the back to help compensate for the slight rotation downward.

The second important thing that must be considered is CV plunge. If you are using a diff drop, and not dropping the lower control arms and using taller knuckles, you will be introducing extra plunge in the half shafts as they are misaligned to the geometry of the suspension. By my calculations 20mm of diff drop is the limit (with stock suspension hardpoints, no cradle drop) where you use up all the half shaft plunge. Dropping lower than that will compress or pull the CV's further than they are designed to travel which is certainly worse than a slightly higher CV angle. Even dropping 20mm will reduce max angle by more than 10%!

Only an issue is you don't run bump stops. Also the more you lift the more you adjust out your lower control arms which extend the shafts.

 

[TeocaliMG]

Only an issue is you don't run bump stops. Also the more you lift the more you adjust out your lower control arms which extend the shafts.

The CV limit is more in full rebound than jounce when running extended travel coil overs (I am running Icon). With the half shaft travel set in its center at ride height (stock everything), you run into the compression limit at full rebound at 20mm drop. Actually, it's already exceeding by a mil or two but that's fine by me, bushings are compliant.

You can get a bit more drop by pulling the LCA's out like you said, but you will quickly then run into the pull-out limit in jounce, maybe you could run 25mm safely. Regardless, if you don't have adjustable UCA's you will quickly run into camber problems by pulling the LCA's out.

20mm is the sweet spot for happy CV travel and angle IMO. I'd rather beef up the CV's than run more drop at that point. No way 1.5" drop (~35-40mm) on stock half shafts is not hitting travel limits internally.

 

[87-Z28]

The CV limit is more in full rebound than jounce when running extended travel coil overs (I am running Icon). With the half shaft travel set in its center at ride height (stock everything), you run into the compression limit at full rebound at 20mm drop. Actually, it's already exceeding by a mil or two but that's fine by me, bushings are compliant.

You can get a bit more drop by pulling the LCA's out like you said, but you will quickly then run into the pull-out limit in jounce, maybe you could run 25mm safely. Regardless, if you don't have adjustable UCA's you will quickly run into camber problems by pulling the LCA's out.

20mm is the sweet spot for happy CV travel and angle IMO. I'd rather beef up the CV's than run more drop at that point. No way 1.5" drop (~35-40mm) on stock half shafts is not hitting travel limits internally.

do you have numbers for oem internal CV travel limits?

 

[87-Z28]

some estimates of CV axle plunge wrt a 2” diff drop.

considering only the camber plane (no out of plane motion) the wheel motion and CV outer joint location can be modeled wrt shock travel. The attached figure shows this for max jounce and rebound CV angles of 30 degrees. The inner CV joint pivot point was located to have uniformly equal plunge at either max jounce and rebound. Likely how this was designed.

The measurements are for oem UCA/LCA and accurate within 0.5”. Achieving resolution on the order of a few mm for CV plunge is a challenge. Nonetheless, the effects of wheel travel on CV plunge can be seen. Possibly +/- 20 mm at the extremes when a 2” diff drop is considered.

Regardless of the absolute numbers, increasing diff drop can quickly affect kinematics of CV motion and thus affect CV axle plunge. not sure about actual CV limits or out of plane effects, but just a visual example of CV plunge kinematics. Sure, for lifted vehicles, the LCA will be pulled out to help with plunge at max rebound, but then you can quickly run out of extension during jounce.

this analysis assumes rigid links (no compliance for elastic bushings) and no adjustments of LCA pivot point for alignment effects. This is just an example of the kinematics, not sure where the limit states are, but larger than a 2” diff drop seems challenging for oem CV plunge

 

[87-Z28]

For oem geometry, CV angles roughly change by only 4-5 degrees, for every 1” change of inner CV joint pivot location (diff drop). Sure that can make a big difference when you are at or past CV limit states in full droop and most definitely necessary. But a diff drop does not magically fix all of the IFS geometry issues. The oem design envelope is fairly tight.

 

[Snacktime]

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.

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