Help me understand: pinion angle vs castor

[centuryhouse]

2015 Rubicon JKU 4" lift, 35s 65k miles

I got this JKU this summer and just noticed the front driveshaft had a lot of slop so replaced it with a double cardan shaft (single u-joint at front differential, double at transfer case).

I installed it yesterday and now have a mild vibration at 55mph that gets more noticeable the faster I go. Everything was tightened to spec, greased, and feels solid.

Reading revealed the pinion must point to the transfer case with this style DS and mine does not.

- Adjustable lower control arms should allow me to point the pinion up - is that correct? Or is it uppers that are needed? (Plenty of room in wheel well for travel either way)

- How does changing pinion angle impact the castor measurement?

- If I get adjustable CAs, do I just eyeball the pinion to point to the TC? Is more precision needed?

 

[FishFam JKU]

I would suggest Adjustable Upper control arms. And possible a match vendor set designed for 4" of lift and able to reset the geometry to correct angles.
Rolling the Caster back to proper numbers should also bring your Camber back into proper ranges. Use a an angle finder to get the Caster close, hell use the iPhone angle indicator. Be sure to compensate for the angle of the floor our on as well, don't be me

Not sure why you would get vibrations with a new shaft when it was not present with the old shaft. First suggestion would be to remove the drive shaft completely and run it. Should tell you quickly if its the new shaft. If you don't get vibes with the drive shaft out, take it back and have them re-balance it.

Scott

 

[JeepJeezy]

You will need both upper and lowers to get closer to stock 4.5°. Your uppers will get your pinion angle while the lowers will get your caster within spec. If you look at your front springs, right now they're bending at the bottom towards the rear. Your need the bottom to move towards the front of the Jeep. When you get that straightened out, no pun intended, then work on your upper control arms to get your pinion angle. What were the measurements of your new driveshaft? Collapsed and extended.

 

[centuryhouse]

I would suggest Adjustable Upper control arms. And possible a match vendor set designed for 4" of lift and able to reset the geometry to correct angles.
Rolling the Caster back to proper numbers should also bring your Camber back into proper ranges. Use a an angle finder to get the Caster close, hell use the iPhone angle indicator. Be sure to compensate for the angle of the floor our on as well, don't be me

Not sure why you would get vibrations with a new shaft when it was not present with the old shaft. First suggestion would be to remove the drive shaft completely and run it. Should tell you quickly if its the new shaft. If you don't get vibes with the drive shaft out, take it back and have them re-balance it.

Scott

Thanks Scott. When I started Googling this I found a lot of references to vibrations on new double cardan DS if the pinion wasn't straight in line with the shaft.

Removing the DS is next to see if it still does it, but it definitely didn't have vibration the day before with the old DS and nothing else has changed. I just had to remove the DS 3rd time. Already had to remove it once right after installing it and realizing I'd missed an inverted grease fitting you can't get to without removing the shaft.

 

[centuryhouse]

You will need both upper and lowers to get closer to stock 4.5°. Your uppers will get your pinion angle while the lowers will get your caster within spec. If you look at your front springs, right now they're bending at the bottom towards the rear. Your need the bottom to move towards the front of the Jeep. When you get that straightened out, no pun intended, then work on your upper control arms to get your pinion angle. What were the measurements of your new driveshaft? Collapsed and extended.

It is 34.5" collapsed and 38" extended.

I am confused about something still - why do people say the lower CAs set pinion angle and the upper CAs control caster?

It seems to me both rotate the axle, and since pinion angle & caster are tied together wouldn't changing the lowers' length change both pinion angle and caster? And same with changing the upper CAs' length?

 

[Dirty Dog]

Thanks Scott. When I started Googling this I found a lot of references to vibrations on new double cardan DS if the pinion wasn't straight in line with the shaft.

Removing the DS is next to see if it still does it, but it definitely didn't have vibration the day before with the old DS and nothing else has changed. I just had to remove the DS 3rd time. Already had to remove it once right after installing it and realizing I'd missed an inverted grease fitting you can't get to without removing the shaft.

See, that doesn't make sense to me. One of the reasons for using the double cardan shaft is to reduce vibrations when you have a steep pinion angle. I built a YJ that had room for a rear drive shaft that was about 3" long (ok, it wasn't that bad, but it was really really short). Double cardans on both ends.

 

[centuryhouse]

See, that doesn't make sense to me. One of the reasons for using the double cardan shaft is to reduce vibrations when you have a steep pinion angle. I built a YJ that had room for a rear drive shaft that was about 3" long (ok, it wasn't that bad, but it was really really short). Double cardans on both ends.

From what I read, it is because these have a single u-joint on the pinion end which can't handle the angle, and a double u-joint set up by the transfer case which can handle it. So it can handle the angles, but only at back.

 

[FishFam JKU]

From what I read, it is because these have a single u-joint on the pinion end which can't handle the angle, and a double u-joint set up by the transfer case which can handle it. So it can handle the angles, but only at back.

Pinion should be pointing towards the T-Case, within a degree or two I believe. No stress on the single joint. The double cardan is to absorb all the remaining angles at the case.

LCA or UCA can be adjustable but yes they serve the same purpose in a stock suspension. If you are not modifying the mount points you only need one set to be adjustable. Typically having both adjustable is due to a long travel or custom suspension needing positioning and rotation. You only need the pitch.