ATTN: Math Gurus...

JeepFreak21 · Feb 11, 2004

I'm crunching some numbers, trying to find out how long to make my lower control arms for my long arm setup. On no plane does my lower arm run perpindicular to either the axle or the crossmember. Taking the angles into account, I found the length of the lower arm on the vertical and horizontal planes (pics 1 and 2, respectively)... but how do I combine my results to get a real life length???
.
.
.

Top View (vertical plane)
.
.
.

Side View (horizontal plane)
.
.
.
TIA!
Billy :twak:

NCSUcherokee · Feb 11, 2004

this is rather fun... which is kinda messed up but i got a few questions-

1.do the brackets come off of the axle and crosmember perpendicular it or do they follow the same 19 degree inward slant?

2. in the first pic the crossmember bracket was measured to be 3 inches- is that the measurment of the actual bracket or is it the measurement of how far it comes off of the crossmember in the horizontal axis ( i ask becasue it is pointed up in the second pic which would require more calculations to break its dimentions up into the horizontal and vertical components

3. what on those two drawings is a measurement and what is a calculation? i calculated the actual length of the arms to be in the 31.xx range but i came up with some data that doesnt match yours exactly buy doing some of my own calc's ( such as the distance between the brackets in the second pic with those other values given is around 30.4 inches rather than 30...

give me some hard numbers and i will work it out for ya but remember... you can always make them 31.25 inches long(split the difference) and make them adjustable

MaXJohnson · Feb 11, 2004

JeepFreak21 said:
On no plane does my lower arm run perpindicular to either the axle or the crossmember. Taking the angles into account, I found the length of the lower arm on the vertical and horizontal planes (pics 1 and 2, respectively)... but how do I combine my results to get a real life length???

Your numbers don't jive. In the upper drawing (plan view) you show an arm length of 32" and a horizontal separation of 12.5". Use half the horizontal separation to create a right triangle.
therefore, using: C^2 - A^2 = B^2

side A = 6.25", side C=32", side B=31.38"

In the lower drawing (side view), you show the same fore/aft distance to be 30".

You should be able to extend this dimension from the plan view to the side view with both measurements being the same.

If 31.38" is the correct dimension, then the arms should be 32.6" long
If 30.00" is the correct dimension, then the arms should be 31.3" long

If you're making them adjustable, 32" with a few inches of adjustment should work either way.

JeepFreak21 · Feb 11, 2004

NCSUcherokee said:
1.do the brackets come off of the axle and crosmember perpendicular it or do they follow the same 19 degree inward slant?

They follow the same slant.

NCSUcherokee said:
2. in the first pic the crossmember bracket was measured to be 3 inches- is that the measurment of the actual bracket or is it the measurement of how far it comes off of the crossmember in the horizontal axis ( i ask becasue it is pointed up in the second pic which would require more calculations to break its dimentions up into the horizontal and vertical components

It's the measurement of how far it protrudes form the crossmember. I see what you're getting at, and I took that into account.

NCSUcherokee said:
3. what on those two drawings is a measurement and what is a calculation? i calculated the actual length of the arms to be in the 31.xx range but i came up with some data that doesnt match yours exactly buy doing some of my own calc's ( such as the distance between the brackets in the second pic with those other values given is around 30.4 inches rather than 30...

I actually measured all 4 brackets, distance between the axle and crossmember, the distance between the 2 sets of brackets, and the distance of where the lower arm would start, and where it would end respective to the ground (in the second pic). The degrees of angles and the length of the arms are calculated.

NCSUcherokee said:
give me some hard numbers and i will work it out for ya but remember... you can always make them 31.25 inches long(split the difference) and make them adjustable

The arms are already adjustable... I just wanna get as close as possible and just use the adjustments for "fine tuning".

Thanks a lot!
Billy

JeepFreak21 · Feb 11, 2004

MaXJohnson said:
Your numbers don't jive. In the upper drawing (plan view) you show an arm length of 32" and a horizontal separation of 12.5". Use half the horizontal separation to create a right triangle.
therefore, using: C^2 - A^2 = B^2

side A = 6.25", side C=32", side B=31.38"

In the lower drawing (side view), you show the same fore/aft distance to be 30".

You should be able to extend this dimension from the plan view to the side view with both measurements being the same.

If 31.38" is the correct dimension, then the arms should be 32.6" long
If 30.00" is the correct dimension, then the arms should be 31.3" long

If you're making them adjustable, 32" with a few inches of adjustment should work either way.

Hmmm... when I did the first drawing, I forgot to divide the 12.5" horizontal separation by 2 to form the 2 triangles. I recalculated and came up with a lower arm length of 30.5" (after subtracting the brackets)... which matches my numbers for the bottom... is this a coincidence or did I get it?
Thanks,
Billy

JeepFreak21 · Feb 11, 2004

*updated pic top picture to reflect correction*

SeanP · Feb 11, 2004

Billy, you are way over thinking this. Like I said last weekend, my arms are about 36" and they are more than long enough. I would say 30" is where you should be. Either way, your stock arms are about 16.5, adding just 10" to this would be a big improvement. Look at Goatman's set up: It works great and his arms are like 22-24 (can't remember). The big thing is to make sure you can easily make a crossmember that supports the tranny mounts and your frame mounts for the arms are tucked as far away from harms way, yet still clear the frame rail, drive shaft and exhaust. Don't even get caught up in the geometry. Do use rubbers bushings or JJs at the axle end do they can flex when the suspension is cycled.

The lenght of your arms will, however, affect the amount of slip needed in the front driveshaft. The closer to the same length the arms are to the driveshaft, the less slip (like a stock DS front has) you will need. Differences in lenght will scribe different arcs and thereby cause need for more slip.

Only when you get 4 linking the rear, do you really need to start worrying about geometry to control anti-squat and roll center.

JeepFreak21 · Feb 11, 2004

SeanP said:
Billy, you are way over thinking this. Like I said last weekend, my arms are about 36" and they are more than long enough. I would say 30" is where you should be. Either way, your stock arms are about 16.5, adding just 10" to this would be a big improvement. Look at Goatman's set up: It works great and his arms are like 22-24 (can't remember). The big thing is to make sure you can easily make a crossmember that supports the tranny mounts and your frame mounts for the arms are tucked as far away from harms way, yet still clear the frame rail, drive shaft and exhaust. Don't even get caught up in the geometry. Do use rubbers bushings or JJs at the axle end do they can flex when the suspension is cycled.

The lenght of your arms will, however, affect the amount of slip needed in the front driveshaft. The closer to the same length the arms are to the driveshaft, the less slip (like a stock DS front has) you will need. Differences in lenght will scribe different arcs and thereby cause need for more slip.

Only when you get 4 linking the rear, do you really need to start worrying about geometry to control anti-squat and roll center.

I'm not even sweating the geometry though... I'm just trying to run the numbers so I can finish, at least the lower, arms before having the axle under the Jeep to take measurements. I think if I make the right calculations, I can have this axle ready to "bolt in" (for the most part) before tearing my D30 out.
Billy

Majo · Feb 11, 2004

here' a good link

http://www.math.com/tables/trig/identities.htm

i learned alot about statics for design engineering, if you wanna know more according to what my statics books says about the angles and forces you'd have going on there PM me.

JeepFreak21 · Feb 11, 2004

Majo said:
here' a good link

http://www.math.com/tables/trig/identities.htm

Thanks. I used this one :rof: http://www.mathepower.com/english/sincostan.php

ATTN: Math Gurus...

JeepFreak21

NAXJA Forum User

NCSUcherokee

NAXJA Forum User

MaXJohnson

NAXJA Forum User

JeepFreak21

NAXJA Forum User

JeepFreak21

NAXJA Forum User

JeepFreak21

NAXJA Forum User

SeanP

NAXJA Forum User

JeepFreak21

NAXJA Forum User

Majo

NAXJA Forum User

JeepFreak21

NAXJA Forum User

Similar threads