School me on Caster angle.

[crimsonride]

I've done a search and not really found the advice I need.

I've got a SOA YJ with a retarded full time front D44. Currently I have a significant angle on the lower u-joint and I would like to rotate the pinion upward to resolve this... (I will be replacing the axle eventually but until then I would like to eliminate some driveline vibes).

My question is, if I rotate the pinion upward it will change my castor angle to a possibly negative caster angle, what will this do to the handling and feel of the jeep? What options do I have? Also what would be an ideal caster angle?

I do have hydro assist and high steer with a relatively flat drag link.

 

[xjtony]

When I built my Cherokee I set mine at 6 degrees. Too little and it will wander driving down the road and return to center will be affected.

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[xjtony]

With leafs you could try a few different shims to find a good balance

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[BlueWolfFab]

Negative caster angle is not good.

We need to use caster to get the contact patch of the tire, behind the 'pivot point' of the steering system. Think about how that's done on like a shopping cart, where the wheel is not mounted directly under the steering pivot point (the vertical stud in this case) that is what helps the wheel track true when driving forwards. The wheel is wanting to chase the forces of the steering to remain neutral (straight). That's natural and that's what we want.

On a vehicle, a steering system like a shopping cart is impractical, so instead, we rotate our pivot axis (if you were to draw an imaginary line right down the center of both of the ball joints, and extend that line all the way down to the ground, that's our steering pivot point) to where that imaginary line would hit the ground at a point in front of the tires contact patch.

By having negative caster, your pivot point is now behind the contact patch. That would be like trying to steer a shopping cart where the caster wheels are stuck 'backwards' where the tire would be in front of the vertical stud. The shopping cart wheel wants to spin around to its natural state where the wheel is behind the stud again. Same physics on a car, but because of the design, the steering components hold it in place. So with negative caster you're fighting the natural physics of a wheel freely tracking straight.

When you rotate your pinion up, the correct way to fix caster is to cut the weld and rotate the C's. Without doing that, you either sacrifice pinion angle or caster angle.

 

[crimsonride]

Negative caster angle is not good.

We need to use caster to get the contact patch of the tire, behind the 'pivot point' of the steering system. Think about how that's done on like a shopping cart, where the wheel is not mounted directly under the steering pivot point (the vertical stud in this case) that is what helps the wheel track true when driving forwards. The wheel is wanting to chase the forces of the steering to remain neutral (straight). That's natural and that's what we want.

On a vehicle, a steering system like a shopping cart is impractical, so instead, we rotate our pivot axis (if you were to draw an imaginary line right down the center of both of the ball joints, and extend that line all the way down to the ground, that's our steering pivot point) to where that imaginary line would hit the ground at a point in front of the tires contact patch.

By having negative caster, your pivot point is now behind the contact patch. That would be like trying to steer a shopping cart where the caster wheels are stuck 'backwards' where the tire would be in front of the vertical stud. The shopping cart wheel wants to spin around to its natural state where the wheel is behind the stud again. Same physics on a car, but because of the design, the steering components hold it in place. So with negative caster you're fighting the natural physics of a wheel freely tracking straight.

When you rotate your pinion up, the correct way to fix caster is to cut the weld and rotate the C's. Without doing that, you either sacrifice pinion angle or caster angle.

View attachment 110610

Great info! That's what I needed to know. Guess I'll be cutting and re welding my C's.....that's going to suck but whatever.

 

[Herzog]

I've messed with Caster stuff this fall on my buggy. I was at a whopping 15° positive caster and needed to bring my pinion up. I ended up at close to 4° and it feels great. I think 5° to 7° might be the sweet spot for on-road.

 

[Greg]

^^^^ Yep, I run my JK at 4* and it drives well.

 

[Pile of parts]

I agree with previous opinions. I've been able to set mine at about 6* on both a SOA CJ and YJ. One with a 44 and one with a 60. Not sure where it put the pinion angle but it did tilt it up slightly towards the transfer case. Both drove straight as could be but I was also able to unlock the hubs for on road. Many times I would drive back to town from the trail with the hubs still in and didn't notice any vibrations. However, top speed would have been about 60 MPH. Remember, if not using a CV/Double Cardan, the front output on the t-case and pinion should be the same angle and I don't remember what recommended operating angle should be. Fortunately, I have been able to get away with not having to cut and weld the C's.

 

[Gravy]

Cutting and welding C's is not the world's worst deal. It's a pain, but the right way to do it.

Caster is important, but what really effects on road performance is KPI King pin inclination angle.
And it takes into consideration tire size. I could draw a diagram and spend a bunch of time explaining it, but honestly there are better diagrams that already exist. Do a quick search, but basically it's the relationship between caster and center point of tire contact on the ground.