Axle Control Arms and flex - JeepForum.com
Search  
Sign Up   Today's Posts
User: Pass: Remember?
Advertise Here
Jeep Home Jeep Forum Jeep Classifieds Jeep Registry JeepSpace Jeep Reviews Jeep Gallery Jeep Clubs Jeep Groups Jeep Videos Jeep Events Jeep Articles
Go Back JeepForum.com > General Technical Discussions > Advanced Jeep Tech > Axle Control Arms and flex

Great prices on seating!--Oconee Off-RoadTJ 5.25" Speaker Adapters - NalinMFGTJ, YJ & LJ Drop Down Tail Gate Conversion Kit

Reply
Unread 04-09-2013, 09:45 AM   #1
JeeperDon
Patience, Grasshopper
 
JeeperDon's Avatar
1993 YJ Wrangler 
 
Join Date: Jan 2005
Location: Albuquerque, NM
Posts: 6,330
Axle Control Arms and flex

I have leaf springs, so this doesn't apply to me, but I always wondered...

Control arm axles usually use different length arms on the top and bottom, and usually 4 links per axle. With different lengths top and bottom, I suspect the axle would rotate (have a slight caster change) as the spring compresses/extends. My wondering is what happens when one tire goes up, say this increasing caster, and the other goes down to decrease caster. This causes a seemingly unsolvable torque on the axle assembly.

Assuming the OEM rubber bushings (say in a TJ) handle the discrepency, what happens whe you replace them with poly as many people do, or heims in a custom system?


__________________
'93 YJ, '02 GM 4.3 Vortec V6, SOA, dual ARB's, 8.8+D30(w/WarnHubs), 4.88 gears, 35" BFG KM2's, AX15, 15x7 wheels, NP231+4:1+SYE+2LO, York OBA, Warn M8000, no top or doors Apr-Oct.
Member and Web guy: New Mexico 4 Wheelers My Truck Page on NM4W Site
JeeperDon is offline   Reply With Quote
Unread 04-09-2013, 11:14 AM   #2
ratmonkey
R.I.P.
 
ratmonkey's Avatar
1997 ZJ 
 
Join Date: Dec 2006
Location: , Pennsylvania
Posts: 19,805
The axle follows an arc. Once the joints run out of flex, the axle starts to twist.

In extreme flex situations, caster really doesn't mean anything.

Rubber and Poly bushings allow less flex then Rod ends.
__________________
'97 zj 5.2, some stuff, some other suff, and some things that even work sometimes.

ratmonkey is offline   Reply With Quote
Unread 04-09-2013, 11:33 AM   #3
Imped
Web Wheeler
 
Imped's Avatar
2004 TJ Wrangler 
 
Join Date: Feb 2008
Location: Indianapolis
Posts: 19,219
The control arms define the arcs that the axle housing travels through and they locate the housing. Binding would only occur if the ends of all control arms were welded directly to the housing.....something would have to give. In control arms with standard rubber bushings that use bonded inner and outer sleeves, radial movement around the bolts is very limited (due to the bonded sleeves). Most deflection in bushings such as this come from the longitudinal direction and torsional misalignment of the inner sleeve and arm itself (stock TJ/XJ/ZJ arms). If the actual arm doesn't twist in a torsional manner (solid tube, square, etc) then the bushing is expected to take it all. In situations like that or in >stock travel scenarios, those bushings will wear out quickly.

With joints that can misalign and rotate in a circumferential manner, binding is essentially nonexistent unless you somehow reach the limit of misalignment. The axle housing simply 'floats' on the control arms. Your leaf spring suspension sees far more binding than a linked suspension with joints of some sort at all ends.

My answers are assuming a non-radius arm setup. That's a different subject and with such a setup, the axle housing is being 'twisted' in a torsional manner. That's one reason I'm not a fan of RA's in higher-travel suspensions that see articulation.
__________________
OlllllllO
Float Test Build
IndyORV
Imped is offline   Reply With Quote
Unread 04-09-2013, 12:08 PM   #4
JeeperDon
Patience, Grasshopper
 
JeeperDon's Avatar
1993 YJ Wrangler 
 
Join Date: Jan 2005
Location: Albuquerque, NM
Posts: 6,330
Maybe I didn't state my question correctly, and I'm mainly talking about a front axle, since most rear systems use a Y on the top.

The two misleading points apparently are that I'm not at all talking about the twisting that may occur at the link ends, and I was only using caster as reference to mean axle rotation. I'm talking about the actual length of the links causing stress. Since the uper and lower link lengths are different, up or down motion of the axle would cause the axle to rotate. In other words, the upper link will cause it's axle end link bolt to move fore/aft differently than the lower link bolt, resulting in overall axle assembly rotation (from a side view).
__________________
'93 YJ, '02 GM 4.3 Vortec V6, SOA, dual ARB's, 8.8+D30(w/WarnHubs), 4.88 gears, 35" BFG KM2's, AX15, 15x7 wheels, NP231+4:1+SYE+2LO, York OBA, Warn M8000, no top or doors Apr-Oct.
Member and Web guy: New Mexico 4 Wheelers My Truck Page on NM4W Site
JeeperDon is offline   Reply With Quote
Unread 04-09-2013, 12:45 PM   #5
Imped
Web Wheeler
 
Imped's Avatar
2004 TJ Wrangler 
 
Join Date: Feb 2008
Location: Indianapolis
Posts: 19,219
Correct. Why do you think this will result in anything but the housing rotating throughout travel? Remember, the control arms aren't fixed members welded directly to the axle. The allowable rotation around the bolts removes the 'bind' that you're referring to, even with metal-on-metal rod ends with no longitudinal deflection.

Also, even in a triangulated setup in the rear (the Y I believe you're referring to), the concepts are the same. Link length usually winds up different between the uppers and lowers, as well.
__________________
OlllllllO
Float Test Build
IndyORV
Imped is offline   Reply With Quote
Unread 04-09-2013, 01:08 PM   #6
ratmonkey
R.I.P.
 
ratmonkey's Avatar
1997 ZJ 
 
Join Date: Dec 2006
Location: , Pennsylvania
Posts: 19,805
link length may be different, but due to vertical and horizontal separation, they actually describe the same arc from the same virtual end points.
__________________
'97 zj 5.2, some stuff, some other suff, and some things that even work sometimes.

ratmonkey is offline   Reply With Quote
Unread 04-09-2013, 01:25 PM   #7
Imped
Web Wheeler
 
Imped's Avatar
2004 TJ Wrangler 
 
Join Date: Feb 2008
Location: Indianapolis
Posts: 19,219
Quote:
Originally Posted by ratmonkey View Post
link length may be different, but due to vertical and horizontal separation, they actually describe the same arc from the same virtual end points.
If they're describing the same arc then 1) center to center distance would have to be the same and that's not always the case nor is it always wanted and 2) the housing wouldn't rotate at all. Unless I'm not understanding what you're saying.....
__________________
OlllllllO
Float Test Build
IndyORV
Imped is offline   Reply With Quote
Unread 04-09-2013, 01:53 PM   #8
ratmonkey
R.I.P.
 
ratmonkey's Avatar
1997 ZJ 
 
Join Date: Dec 2006
Location: , Pennsylvania
Posts: 19,805
the end point at the axle are in the line the travel arc describes for the axle, the end points at the frame side continue on to intersect at a common virtual point that describes the radius of the arc the axle will travel through. in a perfectly designed and built system.
__________________
'97 zj 5.2, some stuff, some other suff, and some things that even work sometimes.

ratmonkey is offline   Reply With Quote
Unread 04-09-2013, 03:25 PM   #9
JeeperDon
Patience, Grasshopper
 
JeeperDon's Avatar
1993 YJ Wrangler 
 
Join Date: Jan 2005
Location: Albuquerque, NM
Posts: 6,330
Quote:
Originally Posted by ratmonkey View Post
link length may be different, but due to vertical and horizontal separation, they actually describe the same arc from the same virtual end points.
Lets say the upper link is 2' long and the lower is 3' long. I didn't run through the math, but I can't imagine where you could put two fixed center points of a 2' and 3' radius circle such that a 10" long or so line (the bolt to bolt distance at one end of the axle) would stay at exactly the same vertical angle. If the 10" line does not stay, one tire up and the other down would not work without binding, or something like rubber bushings to absorb the difference.

Using the TJ as an example again, does the geometry make it possible to replace all four front links with ones using heim joints (which allow zero length change), and not have binding with one tire up and the other down? I'm thinking it can not.

Is there really a geometry that indeed allows that 10" line to stay at exactly the same vertical angle?
__________________
'93 YJ, '02 GM 4.3 Vortec V6, SOA, dual ARB's, 8.8+D30(w/WarnHubs), 4.88 gears, 35" BFG KM2's, AX15, 15x7 wheels, NP231+4:1+SYE+2LO, York OBA, Warn M8000, no top or doors Apr-Oct.
Member and Web guy: New Mexico 4 Wheelers My Truck Page on NM4W Site
JeeperDon is offline   Reply With Quote
Unread 04-09-2013, 04:28 PM   #10
ratmonkey
R.I.P.
 
ratmonkey's Avatar
1997 ZJ 
 
Join Date: Dec 2006
Location: , Pennsylvania
Posts: 19,805
http://www.jeepforum.com/forum/f369/...ators-1301802/
__________________
'97 zj 5.2, some stuff, some other suff, and some things that even work sometimes.

ratmonkey is offline   Reply With Quote
Unread 04-09-2013, 04:40 PM   #11
ratmonkey
R.I.P.
 
ratmonkey's Avatar
1997 ZJ 
 
Join Date: Dec 2006
Location: , Pennsylvania
Posts: 19,805
Quote:
Originally Posted by JeeperDon View Post
Lets say the upper link is 2' long and the lower is 3' long. I didn't run through the math, but I can't imagine where you could put two fixed center points of a 2' and 3' radius circle such that a 10" long or so line (the bolt to bolt distance at one end of the axle) would stay at exactly the same vertical angle. If the 10" line does not stay, one tire up and the other down would not work without binding, or something like rubber bushings to absorb the difference.

Using the TJ as an example again, does the geometry make it possible to replace all four front links with ones using heim joints (which allow zero length change), and not have binding with one tire up and the other down? I'm thinking it can not.

Is there really a geometry that indeed allows that 10" line to stay at exactly the same vertical angle?
you don't need it to follow a perfectly straight line through the joint ends. you'll have some relative pinion angle change, but it's very very little. think about how much of the circumference of that circle the average suspension cycles through. 7" on a stock tj, 12" on many highly modified vehicles. a good system using arms at around 36" long for the lowers will have more droop before the arms can finally contact eachother(yes, they eventually will as they don't describe exactly the same arcs) than you can fit in shocks. the u-joints will bind at the tcase well before you get to that point even on a zj. and i have a 37" drive shaft in mine.

i'm explaining things poorly. when you have one side stuffed and one side drooped the axle is rotating relative to the arms. the joints have misalignment spacers that let the joint twist in the bracket. some can go 30-45 degrees at each end of the arm when using joints at both ends.
linked axles only get the twisting from one side to the other when the joints have been maxed out in the brackets or when using a front radius arm design.
__________________
'97 zj 5.2, some stuff, some other suff, and some things that even work sometimes.

ratmonkey is offline   Reply With Quote
Unread 04-10-2013, 10:30 AM   #12
Imped
Web Wheeler
 
Imped's Avatar
2004 TJ Wrangler 
 
Join Date: Feb 2008
Location: Indianapolis
Posts: 19,219
I believe this guy was thinking the same way you are Don.

http://www.pirate4x4.com/forum/gener...link-bind.html
__________________
OlllllllO
Float Test Build
IndyORV
Imped is offline   Reply With Quote
Unread 04-10-2013, 06:25 PM   #13
JeeperDon
Patience, Grasshopper
 
JeeperDon's Avatar
1993 YJ Wrangler 
 
Join Date: Jan 2005
Location: Albuquerque, NM
Posts: 6,330
Quote:
Originally Posted by Imped View Post
I believe this guy was thinking the same way you are Don.
That thread was talking about double triangle, which I can see wouldn't bind. With a standard 4 link (no triangles), I still don't see it, but I'm very open to any thoughts.
__________________
'93 YJ, '02 GM 4.3 Vortec V6, SOA, dual ARB's, 8.8+D30(w/WarnHubs), 4.88 gears, 35" BFG KM2's, AX15, 15x7 wheels, NP231+4:1+SYE+2LO, York OBA, Warn M8000, no top or doors Apr-Oct.
Member and Web guy: New Mexico 4 Wheelers My Truck Page on NM4W Site
JeeperDon is offline   Reply With Quote
Unread 04-11-2013, 06:57 AM   #14
Imped
Web Wheeler
 
Imped's Avatar
2004 TJ Wrangler 
 
Join Date: Feb 2008
Location: Indianapolis
Posts: 19,219
Quote:
Originally Posted by JeeperDon View Post
That thread was talking about double triangle, which I can see wouldn't bind. With a standard 4 link (no triangles), I still don't see it, but I'm very open to any thoughts.
Considering I've never seen a suspension setup under any Jeep with no triangulation, including stock, it's a non-issue in my mind. But for discussion's sake, in a setup in which the lowers and uppers are all parallel when viewed from above the bushings will bind when the suspension articulates....once the bushings can no longer travel, the bolts will be stressed and finally the mounts. Not ideal for an offroad suspension that needs to articulate.

So, travel definitely needs to be kept in check with such a setup, just as it always should in a system that presents bind when articulating. Straight travel will present no binding. The stock TJ setup binds due to it having 5 total links on each end, not because of a lack of triangulation....both the lowers and uppers are triangulated, just not much. Remove an upper or the track bar (I'm not recommending this, just bring it up for discussion's sake) and the bind factor is gone.

This is why many choose 3 link setups in the front when building their own suspension--two lowers, one upper and a track bar. Bind-free travel as long as you don't reach the limit of one of your links (track bar in my case), complete rotational constraint of the housing, and very nice manners if set up right.
__________________
OlllllllO
Float Test Build
IndyORV
Imped is offline   Reply With Quote
Unread 04-11-2013, 07:19 AM   #15
JeeperDon
Patience, Grasshopper
 
JeeperDon's Avatar
1993 YJ Wrangler 
 
Join Date: Jan 2005
Location: Albuquerque, NM
Posts: 6,330
I guess I didn't say it right before (again...lol), by triangulation, I meant the designs where say the two upper links mount to the same point, like on top of the diff case in the rear, basically any setup that does not need a track bar. Your 3-link v 4-link point pretty much is how I understood it when I started this thread, a 4-link system (one needing a track bar) has to consider flex in the design a lot to minimize binding at the designed travel limits. Pulling out the one link eliminates binding completely.
__________________
'93 YJ, '02 GM 4.3 Vortec V6, SOA, dual ARB's, 8.8+D30(w/WarnHubs), 4.88 gears, 35" BFG KM2's, AX15, 15x7 wheels, NP231+4:1+SYE+2LO, York OBA, Warn M8000, no top or doors Apr-Oct.
Member and Web guy: New Mexico 4 Wheelers My Truck Page on NM4W Site
JeeperDon is offline   Reply With Quote




Jeep, Wrangler, Cherokee, Grand Cherokee, and other models are copyrighted and trademarked to Jeep/Chrysler Corporation. JeepForum.com is not in any way associated with Jeep or the Chrysler Corp.