Need some info on Castor angle and Currie arms

Mtb275

Mtb275

Member
This last weekend, I installed my lift. It is a JKS 3.5". I opted to install 8 adjustable Currie arms. I got everything finished up except for castor. I read Eddie's Write up on alignment, centering wheel and Castor, it was a huge help. He mentioned the Castor should be around 92 or 88 depending on which way you are holding your angle finder.

My diff on the stock rubi 44 has a slight angle down towards ground.
IMG_1478655019.013067.jpg
I orientated my phone angle finder so it was the same as the one in Write up, It read 87 degrees.
IMG_1478653685.221161.jpg
When I hold the angle finder on the ground, it reads 89.
IMG_1478653766.410242.jpg
So, this suggests I have 2 degree slope down towards ground, Correct? Is that a negative castor?

Do I need to rotate the diff up 4 degrees, so it is more in line wth the drive shaft?

Depending on answers, I have a few more questions.



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Speedy_RCW

Speedy_RCW

Hooked
Yes it needs to be rotated up. How much depends on you. I'm personally at about 3 degrees caster.


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Guess we need to know what you mean by diff though before I can actually say that. Are you measuring at the pinion or on the machine surfaces on the front of the pumpkin next to the cover?
 
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Speedy_RCW

Speedy_RCW

Hooked
cozdude said:
This is wrong. You want -4.2* caster to be back at stock specs. Since his garage floor has 2* in relation to his diff he needs to subtract 2* to get closer to stock specs.


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Assuming his diff is pointing down 2 degrees that would put him pretty close to stock. Hard to tell from the pictures where he's measuring which is why I edited my previous post.


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Mtb275

Mtb275

Member
Speedy_RCW said:
Yes it needs to be rotated up. How much depends on you. I'm personally at about 3 degrees caster.


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Guess we need to know what you mean by diff though before I can actually say that. Are you measuring at the pinion or on the machine surfaces on the front of the pumpkin next to the cover?
Click to expand...

Measuring at machined surface next to cover.


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Mtb275

Mtb275

Member
cozdude said:
This is wrong. You want -4.2* caster to be back at stock specs. Since his garage floor has 2* in relation to his diff he needs to subtract 2* to get closer to stock specs.


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I'm measuring at the round machined surfaces next to the diff cover. It definitely points down some, you can see the angle at the drive shaft. So I need to SHORTEN my upper arms '2 degrees'. Correct?

Is there any concern with the angle of the drive line at the yoke? It is a JE Reel 1350 driveline.


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cozdude

cozdude

Guy with a Red 2-Door
Mtb275 said:
I'm measuring at the round machined surfaces next to the diff cover. It definitely points down some, you can see the angle at the drive shaft. So I need to SHORTEN my upper arms '2 degrees'. Correct?

Is there any concern with the angle of the drive line at the yoke? It is a JE Reel 1350 driveline.


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Correct you need to shorten your upper arms ever so slightly to get your caster as close to 4.2* as you can


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Speedy_RCW

Speedy_RCW

Hooked
Mtb275 said:
I'm measuring at the round machined surfaces next to the diff cover. It definitely points down some, you can see the angle at the drive shaft. So I need to SHORTEN my upper arms '2 degrees'. Correct?

Is there any concern with the angle of the drive line at the yoke? It is a JE Reel 1350 driveline.


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Shortening your upper arms will point the pinion down more. It's going to depend on where you axle is located which ones you'll want to adjust. Maybe both of them will need to be adjusted. Set your pinion pointing up two degrees and go from there. Ideally you want the pinion to point up an equal amount as your driveshaft angle so there is no angle difference at the pinion side u-joint but that's just not feasible. You have to compromise on caster and pinion angle. Clear as mud? The higher you rotate the pinion, the shittier your jeep will handle.


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Speedy_RCW

Speedy_RCW

Hooked
cozdude said:
Correct you need to shorten your upper arms ever so slightly to get your caster as close to 4.2* as you can


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Am I just brain farting here but are you saying he needs to rotate the pinion down more? In stock configuration isn't the pinion pointing up 2 degrees? If he's already down 2 he needs to lengthen uppers or shorten lowers right?


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cozdude

cozdude

Guy with a Red 2-Door
Speedy_RCW said:
Hope I'm not fucking you up man. I think the confusion lies at where the pinion is actually at right now. If it is indeed pointing down already, you don't want to shorten your uppers.


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You are getting it backwards. Your rear shaft you want to be pointing in line with the t case. The front you need to tilt the top of the ball joint back towards the driver which in turn points the pinion down. This is negative caster.


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Mtb275

Mtb275

Member
Just trying to figure it out. No worries. Went through the calculus in college and the positive negative caters is f'ing me up. Feeling dumb......

I drove it last night, seemed to drive fine. Didn't drive it a bunch or get it to freeway speeds. I didn't get any drive shaft vibration.

I just don't want to F up my driveline.

And if I need to lengthen my upper arms, I'm not sure what I'm going to do because the Currie arms can't be lengthen any more. I would have to shorten my lowers, which would move my axle slightly off center, but would in turn change my caster.

I'm going to find a stock JK to measure tomorrow.


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Speedy_RCW

Speedy_RCW

Hooked
cozdude said:
You are getting it backwards. Your rear shaft you want to be pointing in line with the t case. The front you need to tilt the top of the ball joint back towards the driver which in turn points the pinion down. This is negative caster.


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Isn't that positive caster? Regardless of negative or positive, (I can never keep it straight) I agree the top ballpoint should be tilted back. But if his pinion is pointed down 2 degrees already that would give him 8 degrees caster correct? If it was at 0 degrees caster would be 6. And pointed up 2 degrees would put caster at 4. The way I see it or read it, he needs to point the pinion up.


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Speedy_RCW

Speedy_RCW

Hooked
Mtb275 said:
Just trying to figure it out. No worries. Went through the calculus in college and the positive negative caters is f'ing me up. Feeling dumb......

I drove it last night, seemed to drive fine. Didn't drive it a bunch or get it to freeway speeds. I didn't get any drive shaft vibration.

I just don't want to F up my driveline.

And if I need to lengthen my upper arms, I'm not sure what I'm going to do because the Currie arms can't be lengthen any more. I would have to shorten my lowers, which would move my axle slightly off center, but would in turn change my caster.

I'm going to find a stock JK to measure tomorrow.


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I guess we just need to know for sure if the pinion is actually pointing down as it is now. Shortening your lowers won't change the axle center but it will move it back a hair. Might just need to lengthen uppers a little and shorten lowers a little. That's assuming you pinion is actually pointing down now.


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Speedy_RCW

Speedy_RCW

Hooked
Mtb275 said:
This where I'm measuring angle at.
View attachment 230176

And Eddie's gauge shows 92 and mine is at 87, with my floor at 89.



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Yea, looks to me like that's 4 degrees caster on Eddies jeep so if yours is rotated 4 degrees from that picture (clockwise as you look at the picture) that would be 8 degrees caster. Which means you'd need to lengthen uppers and/or shorten lowers to get back to stock.


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