Kinematic model of OEM SAS front suspension

[RobBob22]

@87-Z28 this is really cool data. You proved some of my thoughts wrong because while I do believe that some suspension clearly do increase droop I didn’t that the angle change was so dramatic, this shows otherwise.

Now I do believe any sort of spacer/puck lift on the front beyond that ~24.5in length is a terrible idea. I wonder what the “guaranteed break point is”

What is the reduction in CV angle for some of those shorter designs like the Fox?

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[87-Z28]

What is the reduction in CV angle for some of those shorter designs like the Fox?

Fox has same extended length as many of the other coilovers (24.3”) just like 4WP example shown. So the droop angles will be identical, just the jounce limits will be different since they have different max travel numbers.

 

[RobBob22]

Fox has same extended length as many of the other coilovers (24.3”) just like 4WP example shown. So the droop angles will be identical, just the jounce limits will be different since they have different max travel numbers.

Fair, that’s the @AccuTune Offroad length, which Ive debated changing at least that Fox length in my table to his number because we have verifiable evidence from him that Fox is probably measuring it differently.

With the shorter bump stop I’ve started debating is it really that different then most the of brands with ~3” bumps stops.

 

[87-Z28]

Adding this here.

A diff drop doesn’t really maintain oem geometry. It only helps improve CV half shaft angles at full shock extended length. I would keep the extended shock length under 24.3” and run the ufdu. Better solution for 37-38s if you are pushing things and has a much improved intermediate shaft.

I know I am in the minority here but not a diff drop fan. It is really just a band aid fix that is only minimally effective. Need to really drop diff (2+ inches in many cases) to get back to oem CV angles and then plunge becomes a real concern. Not worth the effort.

first figure shows CV angles as a function of shock displacement during a suspension cycle. Zero shock displacement is at full extension. Oem CV geometry is shown (angles snd plunge). Also shown is a typical aftermarket shock with the extended length of 24.3” (4WP coilover but many fall into this range including fox and king). Also shown is the 4WP shock with a 1” spacer increasing effective extended length to 25.3”.

notice that CV angles significantly increase with increasing shock extended length. This is why the diff drop is considered since it can decrease those angles. Also notice the CV plunge remains fairly tight and less than +/- 3 mm. This is because of the OEM location for inner CV pivot point on half shafts. This point has been located along the line of action between LCA and UCA pivots. Thereby necessarily minimizing CV plunge.

adding a diff drop will move the inner CV pivot point downward and thus begin to effect CV plunge. The question is how much can a diff drop really help CV angles and how bad does plunge get?

second plot shows 24.3” extended length shock with a 1” diff drop. Also shows a 25.3” EL shock with a 2” diff drop. Notice CV plunge begins to increase as diff drop distance increases. CV limits for plunge are in the +/- 10 mm range. So a 1” diff drop is still within limits but 2” is not. Starting to get serious plunge >20 mm. Also notice that it really takes near 2” of diff drop to get back to stock CV angles.

 

[87-Z28]

For reference this was first posted in the following thread. Added here to be thorough.

https://www.bronco6g.com/forum/thre...u-had-any-problems-with-your-cv-axles.136698/

For OEM style CVs, the axles are free to move (not restrained) along their axis (longitudinally). This allows for some misalignment along CV axis as the wheel moves up and down during suspension cycling.

For example, consider the imaginary distance from wheel hub center to the half shaft insertion into FDU. Call this “plunge”. If this distance decreases during a suspension cycle then there is positive plunge. If it increases then negative plunge. If the plunge were rigidly fixed or constrained, then CV would pull apart during negative plunge.

The oem CVs have a maximum allowable angle that is related to plunge. If they operate near zero plunge (+/- <10 mm) then they have the greatest allowable angle. As plunge increases, either pos or neg, their allowable angle decreases. They are fairly symmetric wrt pos or neg plunge. From oem CV allowable angles vs plunge, 10 mm of plunge is a reasonable limit state and will get you close to 30 deg of angle.

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