Caster Correction Physics and Engineering (1 Viewer)

[categorytheory]

Ok, so a few questions here. I recently lifted my 80 with the OME heavy duty 3.5”: ARB-2863J and ARB-2850J and L series shocks and dampener. I have the 5 degree Ironman 4x4 caster correction plate as well. My question is this.... driving back from having my 35s put on I notice the steering is “squirrelly” and not as “tight” as I accept. I have yet to install the caster plates BUT is this the only reason it would be driving “lose”? And if not what’s the general physics / engineering behind correcting the caster angle? I’m a mathematician by trade so lay it on me thick! I want to get an idea of the issue. I should also mention that I replaced the panhard bars as well.

 

[cruisermatt]

What is your current caster angle? If you have a 3.5" lift without any caster modification then you are going to need more caster. It's 8th grade level geometry

 

[categorytheory]

What is your current caster angle? If you have a 3.5" lift without any caster modification then you are going to need more caster. It's 8th grade level geometry

From what Ive read the 5 degree is needed for a 3-6 in lift..... I mean if it’s simple trig that’s one thing but why does it matter, what’s the reason for the caster correction? As in what the physics of the stability in relation to the caster angle, is more better? Is there such a thing as too much....?

 

[categorytheory]

From what Ive read the 5 degree is needed for a 3-6 in lift..... I mean if it’s simple trig that’s one thing but why does it matter, what’s the reason for the caster correction? As in what the physics of the stability in relation to the caster angle, is more better? Is there such a thing as too much....?

For reference these are the plates: Caster Plate Kit Suited For Toyota 80 Series Land Cruiser/Lexus LX450

 

[cruisermatt]

Yes. It's mechanical trail. No, more is not always better, but you need more. go measure what you are currently at. Any free smartphone angle finder works great.

 

[categorytheory]

Yes. It's mechanical trail. No, more is not always better, but you need more. go measure what you are currently at. Any free smartphone angle finder works great.

this still doesn’t answer my question as to why, or how. all you state is I need more. I want to understand the logic and physics so I can make my one judgement. Thanks.

 

[-Spike-]

Understanding the importance of the caster | TireBuyer.com

Of all vehicle alignment adjustments, caster is probably the least understood. Is caster important to your driving experience?

www.tirebuyer.com

.

 

[categorytheory]

Understanding the importance of the caster | TireBuyer.com

Of all vehicle alignment adjustments, caster is probably the least understood. Is caster important to your driving experience?

www.tirebuyer.com

.

Thank you!

 

[Grandlooser]

Do you like your front wheels to act like those on a beat up shopping cart? This is all the science you need here.

 

[Squeegee]

Is the steering light, as in twitchy or light as in disconnected feeling? Caster is the relation between your upper and lower trunion bearings. When you lift the 80 your kind of rolled the axle forward and now the trunion bearing is “infront” of the lower trunion bearing. Kind of like this \ . This is negative caster, the caster correction plates just roll the axle back into a stockish location where the upper trunion bearing is lounging behind the lower - positive caster - like this /. The steering feels twitchy because you’re steering on the front side of the tire and it wants to flop over side to side when you’re going down the road. I hope this made sense, it’s kinda late

 

[categorytheory]

this is what I was looking for thank you. And it’s light as in twitch but only at around 40mph or so. And in the sense that the steering doesn’t feel very responsive / sloppy.

Is the steering light, as in twitchy or light as in disconnected feeling? Caster is the relation between your upper and lower trunion bearings. When you lift the 80 your kind of rolled the axle forward and now the trunion bearing is “infront” of the lower trunion bearing. Kind of like this \ . This is negative caster, the caster correction plates just roll the axle back into a stockish location where the upper trunion bearing is lounging behind the lower - positive caster - like this /. The steering feels twitchy because you’re steering on the front side of the tire and it wants to flop over side to side when you’re going down the road. I hope this made sense, it’s kinda late

 

[jonheld]

Ok, so a few questions here. I recently lifted my 80 with the OME heavy duty 3.5”: ARB-2863J and ARB-2850J and L series shocks and dampener. I have the 5 degree Ironman 4x4 caster correction plate as well. My question is this.... driving back from having my 35s put on I notice the steering is “squirrelly” and not as “tight” as I accept. I have yet to install the caster plates BUT is this the only reason it would be driving “lose”? And if not what’s the general physics / engineering behind correcting the caster angle? I’m a mathematician by trade so lay it on me thick! I want to get an idea of the issue. I should also mention that I replaced the panhard bars as well.

This has been discussed hundreds of times on this forum in countless threads.
Here's the geometry:

caster correction geometry?

was there a thread in here with a diagram showing the caster correction geometry? stock i think is positive 2 degrees? which means the axle is canter /backward/ slightly? them when you do a lift it goes forward by a degree an inch basically when the bushing mount in the radius arm at the back...

forum.ih8mud.com

 

[Heckraiser]

Positive caster, I think of as the steering axis "leaning back," where the upper pivot point (upper trunion bearing in an 80, or upper ball joint in an IFS/wishbone suspension) is behind the lower trunion bearing (or lower ball joint in an IFS suspension) makes the tire more apt to rest in a straight position relative to road imperfections. It also increases the natural tendency for the steering wheel to return to center after a turn.

Side benefit, with a high degree of positive caster, when the wheels are turned, you'll notice the whole tire angle back. Some older European cars have really high caster, so this effect is visibly quite obvious. Well when the wheel is turned like that, the effect is actually the same as negative camber to the outside tire in the turn (even if your camber is 0*, which is spec for a solid axle truck). Negative camber (where the bottom of the tire sticks out farther than the top of the tire, picture a "stanced" out rice rocket or VW) improves steering turn-in response and roadholding as it counters the effects of body roll, keeping the tire flat to the road surface even when the body is leaned over.

Positive caster is a good thing. And to an extent, more is better, but there is a limit. First off, in a solid axle truck like the 80, the more you increase caster, the more you point your front pinion down. This creates driveline trouble because the front driveshaft u-joints are no longer in alignment with the transfer case output. This is an 80-specific problem. Or a problem specific to full-time 4wd solid-axle trucks, I guess would be more accurate.

But if you didn't have to worry about pinion angle, a nice high caster angle like 6* is usually ideal for road manners and handling. Just this guy's opinion. As the caster angle increases, so does steering effort, so there is a happy medium in there somewhere, but it will be subjective. Super-high caster can also contribute to funky tire wear.

 

[categorytheory]

Positive caster, I think of as the steering axis "leaning back," where the upper pivot point (upper trunion bearing in an 80, or upper ball joint in an IFS/wishbone suspension) is behind the lower trunion bearing (or lower ball joint in an IFS suspension) makes the tire more apt to rest in a straight position relative to road imperfections. It also increases the natural tendency for the steering wheel to return to center after a turn.

Side benefit, with a high degree of positive caster, when the wheels are turned, you'll notice the whole tire angle back. Some older European cars have really high caster, so this effect is visibly quite obvious. Well when the wheel is turned like that, the effect is actually the same as negative camber to the outside tire in the turn (even if your camber is 0*, which is spec for a solid axle truck). Negative camber (where the bottom of the tire sticks out farther than the top of the tire, picture a "stanced" out rice rocket or VW) improves steering turn-in response and roadholding as it counters the effects of body roll, keeping the tire flat to the road surface even when the body is leaned over.

Positive caster is a good thing. And to an extent, more is better, but there is a limit. First off, in a solid axle truck like the 80, the more you increase caster, the more you point your front pinion down. This creates driveline trouble because the front driveshaft u-joints are no longer in alignment with the transfer case output. This is an 80-specific problem. Or a problem specific to full-time 4wd solid-axle trucks, I guess would be more accurate.

But if you didn't have to worry about pinion angle, a nice high caster angle like 6* is usually ideal for road manners and handling. Just this guy's opinion. As the caster angle increases, so does steering effort, so there is a happy medium in there somewhere, but it will be subjective. Super-high caster can also contribute to funky tire wear.

Brilliant thank you!

 

[categorytheory]

So here is a more general question and one that’s been bugging me. I know that caster angle is essentially the orientation of the axle in relation to the top of the shock or upper trunion bearing... but if this is a set point attaching to the frame, then in theory the only way to adjust the caster angle is by lengthening the lower radius arm and drive train. A correction of the angle of attach of the axle itself wouldn’t (at least in my mind) reorient the lower part of the axle to be more in front... unless I’m missing something here....

 

[Heckraiser]

Caster angle is defined by the steering axis. Most of the illustrations show a strut for visualization purposes, but the suspension is actually irrelevant in the most literal sense. I think you're imagining the upper suspension locations as having some affect on caster... they do not. The frame attachment points, the location of the upper spring perch or shock mount, none of them matter. The illustrations are a little misleading in that sense--or most of the illustrations describe a McPherson Strut suspension system, where the upper strut mount does actually constitute the upper steering pivot point. This is not the case with a radius arm suspension (as in an 80) or with a double wishbone suspension (as in 4Runners and such).

Imagine drawing a line through the steering pivot points (in the case of an 80, the upper and lower trunion bearings). This is your steering axis. This imaginary line is the only thing you need to determine caster angle.

 

[categorytheory]

Gotcha! And yeah the depictions of the strut was throwing me! Makes a lot more sense that it’s about the orientation with the steering.

 

[categorytheory]

Caster Correction 80 Series... This video did it for me. Thanks all!