How much caster do I need?

[tbisaacs]

I tried to search, didn't find much. I'm running SPC adjustable control arms and they are set to max caster, eg closest to rear of wheel well:

I'm getting a ton of rubbing on the fender liner when fully stuffed with my 35x12.50s & 25mm offset.

For those of you running SPCs, what is your caster?

Thanks!

 

[Polecat]

You've run into probably the main issue with adjustable UCAs - they actually move the wheel back to increase caster, usually resulting in the rubbing you're experiencing. The resolution (the specifics of what I think you're asking about) is to decrease caster with the UCA and compensate with a higher-than-normal caster setting on the LCA.

This feels a little like an "it depends" question, but I think the easiest answer is to find a shop both (1) that is willing to mess with aftermarket UCAs and (2) that understands how both UCA and LCA adjustments impact not only the caster angle but also the placement of the tire in the wheel well.

That said, mine are set to D in the diagram below (should be the same as yours, but it's too small for me to confirm definitively).

(LC200, 295/65r18 (33"), ET28)

Driver (front of vehicle to the left):

And here are the readings from my last alignment. I specifically asked the guys to utilize both upper and lower adjustments to center everything in the wheel well, not just get the total caster reading within spec with the LCA only.

 

[tbisaacs]

You've run into probably the main issue with adjustable UCAs - they actually move the wheel back to increase caster, usually resulting in the rubbing you're experiencing. The resolution (the specifics of what I think you're asking about) is to decrease caster with the UCA and compensate with a higher-than-normal caster setting on the LCA.

This feels a little like an "it depends" question, but I think the easiest answer is to find a shop both (1) that is willing to mess with aftermarket UCAs and (2) that understands how both UCA and LCA adjustments impact not only the caster angle but also the placement of the tire in the wheel well.

That said, mine are set to D in the diagram below (should be the same as yours, but it's too small for me to confirm definitively).

(LC200, 295/65r18 (33"), ET28)

View attachment 3910563

Driver (front of vehicle to the left):
View attachment 3910569


And here are the readings from my last alignment. I specifically asked the guys to utilize both upper and lower adjustments to center everything in the wheel well, not just get the total caster reading within spec with the LCA only.

View attachment 3910533

View attachment 3910564

View attachment 3910566

Thanks, I think mine is set to "A" which is +3° which seems way too high

 

[skrypj]

"I'm getting a ton of rubbing on the fender liner when fully stuffed with my 35x12.50s & 25mm offset."

Um, yeah, I bet you are. At +3.0 I almost wonder if they had to bring the LCA backwards.

Do you even need more positive caster? Or could it have been aligned by adding caster using the LCA, which would have brought the tire forward.

 

[tbisaacs]

Do you even need more positive caster?

That's what I'm asking. Is there a standard amount I should be specifying, or just take it back to my alignment guy and say make it not rub.

 

[Polecat]

That's what I'm asking. Is there a standard amount I should be specifying, or just take it back to my alignment guy and say make it not rub.

You can ask for both, because "standard" is a range.

 

[skrypj]

That's what I'm asking. Is there a standard amount I should be specifying, or just take it back to my alignment guy and say make it not rub.

He should try to get it into the stock caster range.

If the LCA's have enough adjustment range, you could try to set the UCA's to F or G and see if you can pull the tires forward a ways.

 

[TeCKis300]

To answer your question about how much caster you need, it depends. Short answer is you won't need as much as stock spec because of larger overall diameter tires.

What you're really dialing in is mechanical trail. There needs to be a certain amount of mechanical trail to provide the caster effect of wheel directional stability and feel. From the graphic, imagine the blue circle as factory 31s and black circle as the 35s you're running. You can see with the same caster angle, that the larger diameter 35s will have more mechanical trail.

Caster can also be used to compensate for other parameters. Rigs with more aggressive offsets may lose some directional stability which additional caster can help with. Too much positive camber can also decrease directional stability.

Long story short is you probably only need +3 or just over that for caster.

IIRC, you're at about 2.5" of suspension lift?

I would just about max the LCA caster adjustment while keeping any positive camber in check (ideally camber close to 0), and use the upper UCA to dial in a hint more caster. Pretty much what @Polecat is recommending.

As an aside, this is part of AHCs advantage of maximizing clearance for larger tires because of lower static lifts that don't require UCA caster correction which pulls the tire towards the firewall. While having lift on demand that can exceed any static lifts.