fj80 caster questions (1 Viewer)

[lacresta]

I am wondering if there is a formula for calculating how much caster is effected in relation to front to rear body lift hieght.
example question, what would the caster angle change in deg if the rear was 2" higher then the front?

 

[leonard_nemoy]

I wouldn't imagine that changing the height at the rear of the vehicle only would affect caster.

For example if you lift the rear of an 80 series 2" but the front stays the exact same height than your caster would not change. Only a height change in the front would change your caster.

 

[lacresta]

I wouldn't imagine that changing the height at the rear of the vehicle only would affect caster.

For example if you lift the rear of an 80 series 2" but the front stays the exact same height than your caster would not change. Only a height change in the front would change your caste

I wouldn't imagine that changing the height at the rear of the vehicle only would affect caster.

For example if you lift the rear of an 80 series 2" but the front stays the exact same height than your caster would not change. Only a height change in the front would change your caster.

pretty sure it will. the installed lift did not end up level without the wieght of a bumper I have on order whiich like everything else is 12 weeks out and the arms I want to order to correct the caster are also 8-10 weeks out. becasue springs dont always give you advertised lift hight I got the caster checked after lift installed as a base line rather then just saying I have a 3" lift when ordering arms. I was not able to add wieght that simulated the bumper when I got the caster numbers. I know it will be level when I get the bumper and would like to order arms without waiting for the bumper to be installed and re check caster. as it sits now the frame is angling down toward the front which must have some effect on caster angle right?

 

[BILT4ME]

pretty sure it will. the installed lift did not end up level without the wieght of a bumper I have on order whiich like everything else is 12 weeks out and the arms I want to order to correct the caster are also 8-10 weeks out. becasue springs dont always give you advertised lift hight I got the caster checked after lift installed as a base line rather then just saying I have a 3" lift when ordering arms. I was not able to add wieght that simulated the bumper when I got the caster numbers. I know it will be level when I get the bumper and would like to order arms without waiting for the bumper to be installed and re check caster. as it sits now the frame is angling down toward the front which must have some effect on caster angle right?

It most definitely affects caster by having the rear at a different height than the front.

I have no caster correction on my truck (I need it, just have not done it) and the truck steers MUCH better when it is low in the rear. When I pump up the rear airbags and remain unloaded, it is squirrelly as hell. It will grab every line and edge, dart all over the road.

If I intentionally load a 500 lb wheelchair on the rear and don't pump up the bags, it steers great but you can feel the tail wag the dog.

 

[leonard_nemoy]

If you change the angle of the steering axis than you are changing caster. It's that simple.

I personally do not understand how changing the rear ride height only can change the angle of the steering axis.

Changing rear ride height will certainly affect the rear panhard which will definitely change the way the rig handles.

 

[LandLocked93]

pretty sure it will.

~.25deg/in @ 112.2wb
Ever so technically yet near as makes no difference, yes. A decades worth of non-maintenance would render the same performance with no lift.
Otherwise, it's roughly -1deg/in lift on the same axle. All other things being equal probably.

 

[lacresta]

~.25deg/in @ 112.2wb
Ever so technically yet near as makes no difference, yes. A decades worth of non-maintenance would render the same performance with no lift.
Otherwise, it's roughly -1deg/in lift on the same axle. All other things being equal probably.

1st I always appreciate help from all mud members there is a wealth of knowlage here.. panhard bars locate axles side to side. front and rear go in opisite direction when lifting the way the fj80 is set up and of course will effect tracking or what I call crabing . caster also has a very specific effects on steering. when lifted the caster changes because the radius arm angle changes and pulls the axle back shortening wheel base and decreases caster on a radius arm set up. . improprer caster can cause a few different problems but the one that most feel when lifting creating neg caster is the darty feeling with steering input when at speeds over 40mph. I understand suspension and how all the components play their part in how the truck handles. for sure when the rear is higher then the front it effects caster.. what i was hoping to find was someone that knew a formula for calculating the effect on caster when the rear is higher then the front.. ie the rear is higher then the front by 2" it will yeild 1 deg of - caster or if the frame is pointing 4 deg down toward the front it will cause a -1 deg of caster.. maybe this formula does not exist.. dont mean to sound like a smart ass but i am not looking for suspension advice or what to do to fix it just a way to calculate effect on caster when the truck is not level. again thanks to all that try to help its so cool to have a site like this with so much experience..

 

[LandLocked93]

panhard bars locate axles side to side.

You asked about front axle caster delta with +2in height delta 112" away.

 

[leonard_nemoy]

what i was hoping to find was someone that knew a formula for calculating the effect on caster when the rear is higher then the front.

I think @LandLocked93 gave you the equation

~.25deg/in @ 112.2wb

If I am understanding his equation correctly than lifting the rear 2" two inches would change the caster by .125 degrees or in other words it would be a negligible change.

.25/2 = .125 degrees of change for a wheelbase of 112.2"

 

[-Spike-]

I think @LandLocked93 gave you the equation




If I am understanding his equation correctly than lifting the rear 2" two inches would change the caster by .125 degrees or in other words it would be a negligible change.

.25/2 = .125 degrees of change for a wheelbase of 112.2"

I think it's inches x .25 degrees. 2 inches = .5 degrees.

 

[lacresta]

I think it's inches x .25 degrees. 2 inches = .5 degrees.

landlocks formula looks good I must have skiped over his.. .5 would make it easy. thanks for catching me not seeing it.. it was short and sweat lol..

 

[leonard_nemoy]

I think it's inches x .25 degrees. 2 inches = .5 degrees.

landlocks formula looks good I must have skiped over his.. .5 would make it easy. thanks for catching me not seeing it.. it was short and sweat lol..

But @LandLocked93 clearly used the division symbol, not the multiplication symbol.....

 

[-Spike-]

But @LandLocked93 clearly used the division symbol, not the multiplication symbol.....

He used the 'per' symbol. Which is also the division symbol, but logic says it was intended to be .25 degrees per inch of lift in the rear.

 

[leonard_nemoy]

He used the 'per' symbol. Which is also the division symbol, but logic says it was intended to be .25 degrees per inch of lift in the rear.

What logic would make divide mean multiply...

Either way it is probably better for the OP to err on the side of caution and run with .5 degrees because it is nice to get a little more caster than factory specs for tires bigger than factory size.

 

[lacresta]

What logic would make divide mean multiply...

Either way it is probably better for the OP to err on the side of caution and run with .5 degrees because it is nice to get a little more caster than factory specs for tires bigger than factory size.

ya foward slash is the division symbol. need to put my glasses on so can I see.. I have seen the for every inch of lift you get 1 deg change for the same axle just never knew the .25 change front to rear for every inch.. or that you then divde it by the length in inches.. trig is for sure not my strong point.. I am going to see if my son in law the engineer math master can check us haha..

 

[-Spike-]

What logic would make divide mean multiply...

ya foward slash is the division symbol.

I don't know how you've gone through life without seeing a forward slash as indicating 'per', but here ya go:

https://www.allacronyms.com/per/abbreviated

 

[leonard_nemoy]

Ha, I am still learning something new everyday.

 

[lacresta]

Ha, I am still learning something new everyday.

my math expert son inlaw instantly called the foward slash that

means over.. or in old guy non math terms divide.. I had to fact check him haha. if you belive everything on the internet.

 

[baldilocks]

I wouldn't imagine that changing the height at the rear of the vehicle only would affect caster.

For example if you lift the rear of an 80 series 2" but the front stays the exact same height than your caster would not change. Only a height change in the front would change your caster.

So what it you lifted the rear 12”? Lifting the rear will affect castor angle but to a lesser degree than front lift.