Rear Lower Control Arms- why?
Thread Starter
1st Gear Member
Joined: Apr 2009
Posts: 66
From: MD
Rear Lower Control Arms- why?
I'm in the infant stages of learning about suspension setup- Lower Control Arms (specifically rear LCA's): what do they do, and what is the end-result reason for upgrading them? Thanks, please don't kill me for asking basic questions.
Steve
Steve
6th Gear Member
Joined: Feb 2007
Posts: 7,635
From: state of confusion
They perform two basic functions. Locating the axle ends so the axle stays under control, and isolating the chassis against whatever roadway roughness the tires encounter.
Their design, which is generally considered to include the bushings or other pivoting mechanisms at the axle and chassis ends, is a compromise between the accuracy of axle end location and the reduction of noise/vibration/harshness (NVH) transmission.
Problems arise when the OE compromise of softish bushings (NVH relatively important) gets in the way of axle location under heavy acceleration load, or in some cases where the soft bushings allow the axle to steer a little too much while cornering. When bushings are deformed, energy is stored in them, but this energy can be spit back out under certain conditions. Most folks know the resulting effect as wheel hop.
If you don't mind a bit more NVH, stiffer bushings help. But here's where you need to make a few of your own choices and compromises. Simply stiffening everything up with the usual polyurethane bushings will help the wheel hop issue, but the same poly stiffness also acts in other directions within the bushings and interferes with the car's handling balance. When the car rolls (and the axle doesn't, at least not as much), you actually want some bushing softness so that this roll does not generate large forces that ultimately reduce rear tire grip. Right about here is where one of the dragstrip vs cornering decisions may show up.
Generally, this is an oversteer-ish effect, and although you may not even notice this in most reasonably normal driving it will be there if for any reason you ever do find yourself up near the limits of lateral tire grip. For some, that's a fairly frequent intentional occurrence; for others, perhaps it never happens - at least not intentionally.
Fortunately, there are choices available that reduce this "bad-direction stiffness" handling interference while preserving the stiffness in the "good" directions. Not "something for nothing", though - the solutions are more complex/costly/perhaps less durable (poly/rod or rod/rod LCAs, Johnny Joint arrangements, etc.). It is also possible to "tune around" the handling issue, although that involves swapping the rear springs and/or the rear sta-bar for slightly softer pieces.
Norm
Their design, which is generally considered to include the bushings or other pivoting mechanisms at the axle and chassis ends, is a compromise between the accuracy of axle end location and the reduction of noise/vibration/harshness (NVH) transmission.
Problems arise when the OE compromise of softish bushings (NVH relatively important) gets in the way of axle location under heavy acceleration load, or in some cases where the soft bushings allow the axle to steer a little too much while cornering. When bushings are deformed, energy is stored in them, but this energy can be spit back out under certain conditions. Most folks know the resulting effect as wheel hop.
If you don't mind a bit more NVH, stiffer bushings help. But here's where you need to make a few of your own choices and compromises. Simply stiffening everything up with the usual polyurethane bushings will help the wheel hop issue, but the same poly stiffness also acts in other directions within the bushings and interferes with the car's handling balance. When the car rolls (and the axle doesn't, at least not as much), you actually want some bushing softness so that this roll does not generate large forces that ultimately reduce rear tire grip. Right about here is where one of the dragstrip vs cornering decisions may show up.
Generally, this is an oversteer-ish effect, and although you may not even notice this in most reasonably normal driving it will be there if for any reason you ever do find yourself up near the limits of lateral tire grip. For some, that's a fairly frequent intentional occurrence; for others, perhaps it never happens - at least not intentionally.
Fortunately, there are choices available that reduce this "bad-direction stiffness" handling interference while preserving the stiffness in the "good" directions. Not "something for nothing", though - the solutions are more complex/costly/perhaps less durable (poly/rod or rod/rod LCAs, Johnny Joint arrangements, etc.). It is also possible to "tune around" the handling issue, although that involves swapping the rear springs and/or the rear sta-bar for slightly softer pieces.
Norm
Last edited by Norm Peterson; Apr 22, 2009 at 09:13 AM.
Former Sponsor
Joined: May 2008
Posts: 167
Norm covered it perfectly. If you do chose to replace you control arms please consider BMR. We have several different options and our products can withstand race conditions or everyday street use. Here is a link to our site. If you have any questions please give me a call.
http://www.bmrfabrication.com/
http://www.bmrfabrication.com/
4th Gear Member
Joined: Dec 2007
Posts: 1,692
From: Ontario, Canada
To complement Norm's excellent explanation here is a link that provides a more graphical though less complete description - http://www.cherod.com/mustang/HowTo/LCA%20_adj.htm
In my case last year I put in poly/poly LCAs but this year I switched them to poly/rod LCAs. However I am one of those who is willing to trade off NVH for performance and do run the car hard on lapping days.
If you want to really get into the subject check out the Reading List thread.
In my case last year I put in poly/poly LCAs but this year I switched them to poly/rod LCAs. However I am one of those who is willing to trade off NVH for performance and do run the car hard on lapping days.
If you want to really get into the subject check out the Reading List thread.
2nd Gear Member
Joined: Jan 2007
Posts: 403
From: Plant City FL
I have BMR LCA (tubular w/urethane bushings), relo brackets (welded), adj. panhard rod and rear sway bar w/billet endlinks. The improvement over stock is huge. The rear isn't harsh by any means, just noticeably firmer and stays planted. I can't say enough about the stuff and will be installing their front sway bar soon.
Former Sponsor
Joined: May 2008
Posts: 167
I have BMR LCA (tubular w/urethane bushings), relo brackets (welded), adj. panhard rod and rear sway bar w/billet endlinks. The improvement over stock is huge. The rear isn't harsh by any means, just noticeably firmer and stays planted. I can't say enough about the stuff and will be installing their front sway bar soon.
2nd Gear Member
Joined: Jan 2007
Posts: 403
From: Plant City FL
And for folks who aren't aware, BMR is a class operation - great bunch of guys who know their stuff. Oh and Dennis, a buddy of mine from West Palm Beach is coming up here in June and we'll be installing your rear sway bar w/billet endlinks and adj. panhard rod on his car, as well as your front sway bar on my car 
Last edited by MTAS; Apr 23, 2009 at 11:09 AM.
Former Sponsor
Joined: May 2008
Posts: 167
Thanks Dennis, I figured you're recognize it And for folks who aren't aware, BMR is a class operation - great bunch of guys who know their stuff. Oh and Dennis, a buddy of mine from West Palm Beach is coming up here in June and we'll be installing your rear sway bar w/billet endlinks and adj. panhard rod on his car, as well as your front sway bar on my car
And for folks who aren't aware, BMR is a class operation - great bunch of guys who know their stuff. Oh and Dennis, a buddy of mine from West Palm Beach is coming up here in June and we'll be installing your rear sway bar w/billet endlinks and adj. panhard rod on his car, as well as your front sway bar on my car Last edited by BMRFabricationSales; Apr 23, 2009 at 02:21 PM.