Alignment Question - rear spring change
- Thread starter JKD
- Start date
Register to hide this ad
DaMonkey
New member
Figure wheelbase is 105"
So, for every 1" height change in back, it impacts caster by 0.55°
Check my math? I think I did that right.
What are your alignment specs now before you drop the springs an inch?
I don't think you have much to worry about. Symmetrical change of that amount prob not noticeable and won't effect tire wear like camber or toe would.
Lower springs will give you additional caster which in general is what people want with lifted 3rd gens because suspension lifts push adjustment limits toward loss of caster.
So, for every 1" height change in back, it impacts caster by 0.55°
Check my math? I think I did that right.
What are your alignment specs now before you drop the springs an inch?
I don't think you have much to worry about. Symmetrical change of that amount prob not noticeable and won't effect tire wear like camber or toe would.
Lower springs will give you additional caster which in general is what people want with lifted 3rd gens because suspension lifts push adjustment limits toward loss of caster.
IBallEngineer
New member
I have to say I don't really understand the inverse cosine, I have only a foggy memory of geometry 101 that I took about 40 years ago.
But I do have years of teaching wheel alignment under my belt, and I've done all kinds of experimenting on an alignment system. I can tell you that raising or lowering the rear ride height by 1" will not change your caster by half a degree. I'd say you might get a reading change of .01, maybe. Caster is measured, or changed, by the relationship of the upper and lower ball joints in relationship to the contact point of the tire. What I think you are calculating is the number of degrees of movement at 105" radius. Which if you figure the circumference of a circle 2piR, you come up with 659.7". Then divide 360 degrees of a circle by that number and you have .545. That is at degrees of movement at the rear axle, not at the pivot point, or center of the circle. .5 degrees is much more movement than you would get up front from a 1" drop at the rear.
Next time I demo on my 4runner, I'll let air out of the rear and check this both ways to confirm with actual readings.
DaMonkey
New member
Exactly. Unless they set it to more than 3.0 degrees with the rake (which is a lot - both for what is possible in the range, and for how a tech would set it)... your shorter springs are nothing to worry about on alignment.
But see how it feels. If your steering feels heavy when stopped or in parking lot situations, that’s what too much caster would do. Too much caster: when you cycle steering all the way, it swings the whole front end side to side slightly and up on the ends like a ‘U’. That’s the heavy part. Imagine a chopper motorcycle raked out.
With what your talking about, no meaningful impact to feel or tire wear, actually prob an adjustment in the right direction.
DaMonkey
New member
I think that would be great to see real example, and good example for classroom environment.
I just used intuition and an online triangle calculator. I go over it in theory and I think your real example will show it change by 0.55 degrees.
See this triangle, as the whole chassis rotates by 0.55 degrees, so will the UBJ relative to the LBJ?
DaMonkey
New member
I know I don’t need to show you this pic lol, but for the benefit of pedanticism (OCD’s proper cousin), I’ll share it.
If the chassis rotates 0.55 degrees, so will caster?
I think you nailed it, yes the inv. cosine would be degrees of movement at a radius of 105....but am thinking that's the same angle moved by erm, triangles and geometry and stuff..
I should draw it out...I see DaMonkey said same thing now...Other thought though, the contact point of the tire will change minutely too?
Last edited:
DaMonkey
New member
it's relative to vertical. so shorter springs will roll front tire back minutely, but so will hub center.
the caster angle between UBJ and LBJ is fixed against the chassis. so a change in chassis is a change in caster.
i think the question is how much does the chassis angle change. i believe 1" change in springs is 0.55° chassis angle change (and caster), but i'm open to the idea something else is happening.
IBallEngineer
New member
That is chassis angle measured at the rear axle, not at the front axle. The front axle is the pivot point for the movement at the rear.
DaMonkey
New member
That's the whole point. Drop the springs by 1" on 105" wheel base will pivot that caster line (UBJ / LBJ intersection) by 0.55° at the front axle. That's where it rotates.
I don't mean to be stubborn, I may be totally wrong but I don't see it yet. I'm usually pretty good with this stuff, but I'm a weekender, not a pro. I guess I just need it spoon fed to me. Or show how to calculate the actual change to caster?
2016 4Runner SR5
Senior Member
Don't know about the 3rd gen but I can tell you 4th and 5th gens sweet spot is right around 4* positive caster. If you're really worried, have the shop check it again.
IBallEngineer
New member
If you tie a string to a fixed point (the front axle) and extend it out to 105" then move it up and down 1", how much does it move at the fixed point? Nothing right? Now extend the string forward to the front bumper and pivot the string at the (former) fixed point and the string will move some, but not .5 degrees.
DaMonkey
New member
Still not following....
My brain is stuck. You don't have to keep trying to pull me out of stubborness, but I'm stuck.
Your string from axle to rear axle is a good example. If you string it to the rear hub center, and drop it 1 inch, the angle difference at the front hub center is 0.55°. The caster line rotates with it, but vertical stays the same. Or maybe it does not, maybe the fog will lift.
Let's say that frame rails are perfectly horizontal to ground. But you flatten the tires so that frame rails now angle upward by 0.55°. Would you agree that caster just changed by 0.55°? I think you would agree to that, because the ball joints are fixed angle against the frame (with steering wheel straight forward).
Assuming you agree with that, then what angle do you think the frame rails change if you drop 1" in the back? Seems to me that it is 0.55°.
IBallEngineer
New member
OK, I found all kinds of problems with the post I was trying to make my point with. I just read an old book quoting frame angle change and Caster to be 1 to 1. So I'm going go with your diagnosis for now. This of course means I'm going to have to put my 3rd gen on the rack and check it with 1" of change in rear ride height to see what the Caster change could be. I'll get with you within a week or so for sure. I'll take pictures!
Last edited:
DaMonkey
New member
An A / B test for alignment... and you don't have to touch an adjustment bolt, just let air out of the tires
Just remember to measure the 1 inch after both sides are already down. If you measure 1" on the first side, it will rise slightly after the other side is lowered (depending how far outboard your measurement point is from the fulcrum point when you lower the next side). <--- joke
no but really though.
:lol:
IBallEngineer
New member
An A / B test for alignment... and you don't have to touch an adjustment bolt, just let air out of the tires
Just remember to measure the 1 inch after both sides are already down. If you measure 1" on the first side, it will rise slightly after the other side is lowered (depending how far outboard your measurement point is from the fulcrum point when you lower the next side). <--- joke
no but really though.
:lol:
I have air assist bags
IBallEngineer
New member
An A / B test for alignment... and you don't have to touch an adjustment bolt, just let air out of the tires
Just remember to measure the 1 inch after both sides are already down. If you measure 1" on the first side, it will rise slightly after the other side is lowered (depending how far outboard your measurement point is from the fulcrum point when you lower the next side). <--- joke
no but really though.
:lol:
OK so I had some time yesterday and I had a student perform two alignment checks.
First I added air to the bags, alot, about 80psi, so it would go up as much as possible. He did the roll back, performed the Caster Sweep and the caster readings were as follows.
Left side 1.9 +
Right side 2.2 +
Then I put a measuring tape in the center of the rear wheel opening and let enough air out to drop it 1" exactly. Did a second roll back, and another caster sweep, the readings were as follows.
Left side 2.2+
Right side 2.5+
So there was definitely a change, but it was not the .5 change that it pencils out at. I stand corrected, but I've seen hundreds of alignments and never seen .5 degree caster change with such a small ride height change to the rear. So I'm only half corrected, or 60% to be accurate.
:hungry:
