Jeep tj control arm length chart

I was certain this info was here somewhere already but can't fargin find it. :banghead:

Intending to swap in RE 3.5" springs this weekend. Is there a list of what the lower CA lengths (front and rear) should be? I have 8 adjustable CAs, just need an idea whereabouts the lowers should be at.

Thanks,
Keith

RE recommends 16 1/4 F&R lowers as a starting point

n4phth4 said:

I was certain this info was here somewhere already but can't fargin find it. :banghead:

Is this what you were looking for?

CHIJPR said:

n4phth4 said:

I was certain this info was here somewhere already but can't fargin find it. :banghead:

Is this what you were looking for?

Yes, exactly that. TYVM!

I'm trying to derive the formula that was used to calculate those lengths. Does anyone know the angle between the control arms and horizontal on a stock Jeep? Pretty sure it isn't 0 degrees (which would make the calculation very simple: sqrt(15.75**2 + lift**2), because the numbers don't match up. Simplifying it to a right triangle like that gets you pretty close (off by .05" at 2" of lift sizes, off by 0.16" at 10" of lift), but from the numbers it looks like a more complicated formula was used. I'd like to take the stock control arm angle into account, and see if that matches up...

wsm04rubi said:

I'm trying to derive the formula that was used to calculate those lengths. Does anyone know the angle between the control arms and horizontal on a stock Jeep? Pretty sure it isn't 0 degrees (which would make the calculation very simple: sqrt(15.75**2 + lift**2), because the numbers don't match up. Simplifying it to a right triangle like that gets you pretty close (off by .05" at 2" of lift sizes, off by 0.16" at 10" of lift), but from the numbers it looks like a more complicated formula was used. I'd like to take the stock control arm angle into account, and see if that matches up...

Bill, while the math is simple enough, the model used to find it isn't.

If the rear LCA's are lengthened with the axle rotated for a DC DS, then there will most certainly be contact between the differential and gas tank skid when stuffed. There will also probably be contact between the rear wheel well body/tub and tire when one wheel is fully stuffed if the tires are large enough. Another factor to consider is when the axle is rotated the spring is compressed unevenly, add armor, and the actual lift height is different than expected.

My rear LCA's are set at nearly stock length to avoid these contact issues while the front LCA's have been lengthened quite a bit to get some of the lost wheelbase back. The UCA's are adjusted to get pinion and/or caster back into specification.

I for one would not take these table lengths as definitive. There are lots of other variables that may or may not make the lengths optimal.

Just my $0.02...

ken

Ken White said:

Bill, while the math is simple enough, the model used to find it isn't.

If the rear LCA's are lengthened with the axle rotated for a DC DS, then there will most certainly be contact between the differential and gas tank skid when stuffed. There will also probably be contact between the rear wheel well body/tub and tire when one wheel is fully stuffed if the tires are large enough. Another factor to consider is when the axle is rotated the spring is compressed unevenly, add armor, and the actual lift height is different than expected.

My rear LCA's are set at nearly stock length to avoid these contact issues while the front LCA's have been lengthened quite a bit to get some of the lost wheelbase back. The UCA's are adjusted to get pinion and/or caster back into specification.

I for one would not take these table lengths as definitive. There are lots of other variables that may or may not make the lengths optimal.

Just my $0.02...

ken

I agree, I was just curious how they were calculated. The numbers are much too precise to be measurements someone took with tape measure, so they must have been calculated based on stock suspension geometry (and with the goal being a stock wheelbase).

I think I figured it out - I assumed that whoever came up with this measured the stock control arm angle, so I reverse calculated that angle using their numbers for stock CA length and 3" lift CA length. Came up with an angle of 1.1 degrees.

I won't go through all of the calculations that got me to here, but this Smalltalk code snippet matches their numbers (a couple of them are off by 0.01, but that is probably rounding differences):

| a b c liftHeight caLength |

liftHeight := 3.0.

a := liftHeight + 0.30427.
b := 15.7471.
c := ((a*a) + (b*b)) sqrt.
caLength := c roundTo: 0.01.

caLength inspect.

Got this via a combination of the law of sines and the pythagorean theorem...

BTW, that formula is for the lowers. I haven't figured out the math on the uppers yet...

wsm04rubi said:

| a b c liftHeight caLength |

liftHeight := 3.0.

a := liftHeight + 0.30427.
b := 15.7471.
c := ((a*a) + (b*b)) sqrt.
caLength := c roundTo: 0.01.

caLength inspect.

Got this via a combination of the law of sines and the pythagorean theorem...

Interesting forumla. Gives a caLength of 16.2 for 3.5" height, which jives w/ RE's "recommended" value of 16.25.

Could you PM more details about how you came up w/ that equation? My geometry is rusty but I'm just curious where those two constants came from. (BTW I'm a programmer by trade but smalltalk isn't one of my fluent languages)

Keith

I pm'ed you as well, but here is my math:

On this diagram, x and b2 are assumed to be constant regardless of lift height.

From the control arm length chart using the numbers for a 3" lift:
a = 3.0
b = 15.75
c = 16.09

Law of Cosines: cos(B) = (a**2 - b**2 + c**2)/(2 * a * c) = 0.20536151

so
B = 78.15 degrees
A2 = 11.85 degrees

Law of Sines: a2 / sin(A2) = c / sin(90)

so
a2 = (sin(11.85) / sin(90)) * 16.09
a2 = 3.3042667

x = a2 - a = 0.3042667

Pythagorean Theorem: b2**2 = c**2 = a2**2 = 16.09**2 - 3.3042667**2

so
b2 = 15.747

so our 2 constants x and b2 are solved:
x = 0.3042667
b2 = 15.747

You can also calculate the stock control arm angle from this - it comes out to 1.1 degrees...

CHIJPR said:

n4phth4 said:

I was certain this info was here somewhere already but can't fargin find it. :banghead:

Is this what you were looking for?

Wow, can you imagine a 10" SA lift -- talk about evil riding!!

At what angle below horizontal would the front lower CA be?

wsm04rubi said:

I pm'ed you as well, but here is my math:

On this diagram, x and b2 are assumed to be constant regardless of lift height.

From the control arm length chart using the numbers for a 3" lift:
a = 3.0
b = 15.75
c = 16.09

Law of Cosines: cos(B) = (a**2 - b**2 + c**2)/(2 * a * c) = 0.20536151

so
B = 78.15 degrees
A2 = 11.85 degrees

Law of Sines: a2 / sin(A2) = c / sin(90)

so
a2 = (sin(11.85) / sin(90)) * 16.09
a2 = 3.3042667

x = a2 - a = 0.3042667

Pythagorean Theorem: b2**2 = c**2 = a2**2 = 16.09**2 - 3.3042667**2

so
b2 = 15.747

so our 2 constants x and b2 are solved:
x = 0.3042667
b2 = 15.747

You can also calculate the stock control arm angle from this - it comes out to 1.1 degrees...

Aw, dude, can we just talk about simple stuff like oil? :laugh: :laugh:

BTW, I'm a programmer too. Been doing smalltalk for about 12 years now...

Here is a javascript equivalent (you can save the quoted text to a text file as "calength.htm", then load that file in a browser, and it will pop an alert with the calculated length for whatever lift height is specified in the body):

<html>
<head>
<title>Control Arm Calculator</title>
<meta>
<script>

var a
var b
var c

function calcLiftHeight(liftHeight)
{
var a = parseFloat(liftHeight) + 0.30427
var b = 15.7471
var c = Math.sqrt((a*a) + (b*b))

var caLength = Math.round(c*100) / 100

alert(caLength)
}
</script>
</head>

<body>

<script>
calcLiftHeight("3.0")
</script>

</body>
</html>

DoctorD said:

Aw, dude, can we just talk about simple stuff like oil? :laugh: :laugh:

I can do math, but I'm a dummy when it comes to oil :rotflmao:

VjohnV said:

Wow, can you imagine a 10" SA lift -- talk about evil riding!!

At what angle below horizontal would the front lower CA be?

If my quick math is correct, 33.2 degrees :rotflmao:

wsm04rubi said:

VjohnV said:

Wow, can you imagine a 10" SA lift -- talk about evil riding!!

At what angle below horizontal would the front lower CA be?

If my quick math is correct, 33.2 degrees :rotflmao:

Since you're pretty much a genius, could you tell me the required length for 3.75" front lift and 4.5" for the rear lift? I want to see how close I am with my settings.

SRG said:

[Since you're pretty much a genius, could you tell me the required length for 3.75" front lift and 4.5" for the rear lift? I want to see how close I am with my settings.

:rotflmao: Not a genius, just having some fun with math :laugh:

3.75" - 16.26, or 16 and 1/4
4.5" - 16.46 or 16 and 15/32

I added a new calculator to the calculators page on my website that will let you calculate it yourself - just enter the size in inches and hit calculate:

http://home.comcast.net/~billandrobbin/calength.htm

wsm04rubi said:

SRG said:

[Since you're pretty much a genius, could you tell me the required length for 3.75" front lift and 4.5" for the rear lift? I want to see how close I am with my settings.

:rotflmao: Not a genius, just having some fun with math :laugh:

3.75" - 16.26, or 16 and 1/4
4.5" - 16.46 or 16 and 15/32

I added a new calculator to the calculators page on my website that will let you calculate it yourself - just enter the size in inches and hit calculate:

http://home.comcast.net/~billandrobbin/calength.htm

COOL! Well I was close with the front which I set at 16.25, BUT the alignment guy moved them out a bit more to get 6.5 on the caster. My 16.25 had me at 5.6, which I might set back because I think I have a hint more vibe on the front.
The back I set to 3/8's more than stock, so if I don't have any contact issues with the TB and gas tank skid, maybe I'll dial it in.

THANK YOU! 8)

CHIJPR said:

n4phth4 said:

I was certain this info was here somewhere already but can't fargin find it. :banghead:

Is this what you were looking for?

Just to be sure.....is this eye to eye or end to end? I'm going to be adjusting all my arms soon and doing so bumpstop adjustment as well.....just want to be sure.....

I'd imagine its eye to eye since different arms have different sized ends......