Is there a write up on how to install the GM arms? I have the spacer and reducer set from shonut, is there anything else (besides the arms) I need? For some reason I remember washers being needed. Is this true or am I confusing this with something else?
GM Arms Install
- Thread starter NJSHO
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PROPHET
Well-Known Member
Have you gotten the H-brace?
If not might as well and do the job once.
If not might as well and do the job once.
luigisho
SHO Member
It's been a while, so I forget if there were enough washers or if I used extra. I know I bought some extra to be safe. Since they're so cheap, you might want to do that before it's in pieces. If you see rust or corrosion at the knuckle I would recommend getting a torch-- MAPP is a little hotter and a better choice.
Shoaz
Studly dood
I think you wind up needing either two or four additional washers, same size as what comes with the kit with the reducing sleeves. Be sure to get hardened washers, otherwise the sleeves can deform the washers.
When I did my '97 there was one side of the bolt that the washers did no good on. So the amount sent was enough. You should also look into getting a brace that keeps the two bars parallel as they will flex/bow a bit when stressed.
Shoaz
Studly dood
I don't see what good those braces are going to do. Unless the arms are hit by something there aren't normally any forces that will cause them to bend or bow, and even if they did those braces won't prevent that.
The stock stamped pieces are very weak in that direction, because they don't really need any strength that way. A lot of track cars use the stock arms without any trouble. IMHO the primary benefit of GM or BMR arms is that they're adjustable.
I have the beefy BMR arms, and Lance has pointed out to me before that it may be better to have weak arms there to protect other pieces. In other words, if something's going to give, let it be the arms which are easy to replace and relatively inexpensive, rather than twisting the torque box or breaking a spindle, strut rod, etc.
The stock stamped pieces are very weak in that direction, because they don't really need any strength that way. A lot of track cars use the stock arms without any trouble. IMHO the primary benefit of GM or BMR arms is that they're adjustable.
I have the beefy BMR arms, and Lance has pointed out to me before that it may be better to have weak arms there to protect other pieces. In other words, if something's going to give, let it be the arms which are easy to replace and relatively inexpensive, rather than twisting the torque box or breaking a spindle, strut rod, etc.
Shoaz
Studly dood
SHOZ123 said:
That's my point: during normal use nothing grabs them to make them bow. The forces are only tension and compression along the length of the arm. The only time I've ever heard of anyone bending one of these, stock or aftermarket, is during an accident or when something hit the control arm.
The braces as you've shown them will only reduce flex in one direction. If you really think they're going to bend, the can still just bend up and down rather than laterally.
What would cause a rear arm to bend in the direction constrained by the brace?
DHMag
Free At Last
Shoaz said:
the center of mass is nearly between the 2 arms. im speaking of the knuckle/wheel. on a yaw, the arms would bow out (front bows towards front, rear bows toward rear). i see both of your points and understand. but to prevent any flex in the GM arms, youd need 4 arms per side, all interlaced like Pauls to avoid/prevent bow/flex/deflection. the stock arms are designed as such so on a lateral yaw, bowing would not be an issue. it would take a serious impact to bend the arms, GM or OE Ford.
if you want zero deflection, make one out of solid 1/2" steel, and replace both front and rear arms with one solid arm. or design a single arm out of composite material with intrical webbing and reinforcement to prevent bowing.
Shoaz
Studly dood
I'm still not quite following what would make the arms bend.
If the ends of the arms were spherical bearings then there'd be no means for a bending force to get into the arms. I think the FPS arms are made this way.
As they are on the GM and BMR arms, the joint prevents any up and down bending force from getting into the arms. The only way that I can imagine a front and rear bending force getting into the arms like you're describing would be for the knuckle to distort significantly. Do you think the knuckle or torque box distort that much? Even without Todd's knuckle brace or an H-brace I have a little difficulty seeing how either could distort that much even under track conditions. I think both the BMR and GM arms have distortable bushings in the ends, which could be expected to attenuate much of whatever bending force did exist.
If the ends of the arms were spherical bearings then there'd be no means for a bending force to get into the arms. I think the FPS arms are made this way.
As they are on the GM and BMR arms, the joint prevents any up and down bending force from getting into the arms. The only way that I can imagine a front and rear bending force getting into the arms like you're describing would be for the knuckle to distort significantly. Do you think the knuckle or torque box distort that much? Even without Todd's knuckle brace or an H-brace I have a little difficulty seeing how either could distort that much even under track conditions. I think both the BMR and GM arms have distortable bushings in the ends, which could be expected to attenuate much of whatever bending force did exist.
TimZ
Member
Shoaz said:
Excellent observations.
It's also worth noting that the braces (especially the interlaced cross-bracing that was mentioned) are over-constraining the suspension movement.
When the knuckle gets deflected longitudinally (braking, hitting potholes, etc), the "box" shape defined by the arms is intended to flex into a parallelogram (the arms stay parallel and so do the torque box/knuckle, there are just no longer 90 degree angles between them) , so that the toe-in of the wheel stays the same. If you force the arms to stay in the box shape by putting bracing between them, the wheel will be forced to toe out under longitudinal deflection. This is almost certainly a far worse problem than the one you were trying to solve.
If you try to addess this issue by taking all of the compliance out, you will be putting potentially huge forces into the knuckle and torque box that that they were never designed for and more importantly, don't need to be there.
As you can see the arms are installed on my '97. I can also assure you that the point that the arms are installed at the knuckle is not flexing and neither is the box in the middle. Under hard cornering when the force is inward towards the center there will be some flexing and a toe change.
FWIW on the GEN 3, I consider the OEM arms to be stronger than the GM arms, just not adjustable enough.
FWIW on the GEN 3, I consider the OEM arms to be stronger than the GM arms, just not adjustable enough.
Lance Cheney
SHO Member
TimZ said:
Hmm. Hadn't thought of that before. The change is small (and dependent on the stretch in the tension rod plus its bushing compliance) but definitely there -- could be significant under heavy braking. Rear toe-out under trail braking could be very "interesting".
The other thing is that the braces are only keeping the two arms from moving front-to-back. In terms of compressive force, if you exceed the capacity of the metal it could buckle in any direction as Eric mentioned -- that bracing isn't going to help anything. Is the brace a slip fit or is torqued down? (I can't make out enough in the picture to tell if it is just a two-piece sleeve bolted together or something more).
As Eric knows, I bent one of the GM arms at the track in a slow-speed off-track excursion. I definitely don't consider them to be real strong. My main question now is how much they elastically deflect under hard driving. Having them go plastic before anything gets damaged on the car is good, but having them bow and cause alignment changes as I'm cornering is another thing altogether. I actually added material on mine for the 'long' part of the shaft to try to reduce the load deflection while hopefully still allowing for impact absorption in the rest of the arm, but it's likely not a better solution than just getting better arms (BMR or TCE) to start with.
The camber adjustability is definitely good though.
-Lance
Shoaz
Studly dood
Lance, it'd be cool to mount one of Mark's lipstick cameras on a GM arm near the torque box pointed down the length of the arm during a track event. Any flexing under normal track use would be evident.
Well you have me confused.
If you say the arms are meant to be independent to accommodate flexing of the suspension, what is it that is flexing? Are you implying the center mount box is flexing or the knuckle?
If you say the arms are meant to be independent to accommodate flexing of the suspension, what is it that is flexing? Are you implying the center mount box is flexing or the knuckle?
shojuan
New Member
The bushings.SHOZ123 said:
Even my solid rod end spherical bushings allow the arm a good degree of movement.
Lance Cheney
SHO Member
SHOZ123 said:
The main one that we were discussing was this case:
When you hit a pothole or brake heavily, the knuckle will tend to be pulled toward the rear of the car by that force. The amount that it is moved is dependent on the size of the tension rod and the bushings that are attached to it. Regardless the knuckle will move rearward relative to the body when this happens. If the arms are locked together then it will act like a box that is rotated (the end of which is attached to your knuckle), which will rotate the knuckle to create more toe-out. If the arms are not locked together then the bushing flex (on the arm) will allow the knuckle to remain at relatively the same angle with respect to the body. Essentially it looks almost as though the arms 'slid' with respect to each other. Hard to see without drawing it out.
The bracing you have doesn't appear to be real thick so it probably deflects under this case and doesn't end up causing much of a problem anyway, but if you actually locked them together (let us say you welded an X between the two arms) then you would cause a lot of problems.
-Lance
TimZ
Member
Thanks Lance - that was exactly what I was talking about.
It's also worth noting that as the suspension moves up and down in it's normal travel, it moves fore and aft through the arc that is prescribed by the tension rod. So, if you were to lock the two arms together with a cross-brace, you would essentially be adding bumpsteer to the rear suspension where it did not exist before.
Additionally, if you were to try to take all of the compliance out of the suspension by heim-jointing everything, the suspension would actually bind as a result - something would have to bend to allow the suspension to move. This is what I meant when I said it was "over-constrained".
As Lance mentioned, I am referring to the addition of more extensive cross-bracing than what is pictured on Paul's car. Someone else had mentioned making it into one solid arm, as I recall.
Paul - how are those braces installed on the arms? Are they rigid (i.e., welded on), or do they allow some amount of movement/compliance?
It's also worth noting that as the suspension moves up and down in it's normal travel, it moves fore and aft through the arc that is prescribed by the tension rod. So, if you were to lock the two arms together with a cross-brace, you would essentially be adding bumpsteer to the rear suspension where it did not exist before.
Additionally, if you were to try to take all of the compliance out of the suspension by heim-jointing everything, the suspension would actually bind as a result - something would have to bend to allow the suspension to move. This is what I meant when I said it was "over-constrained".
As Lance mentioned, I am referring to the addition of more extensive cross-bracing than what is pictured on Paul's car. Someone else had mentioned making it into one solid arm, as I recall.
Paul - how are those braces installed on the arms? Are they rigid (i.e., welded on), or do they allow some amount of movement/compliance?
Shoaz
Studly dood
TimZ said:
I can see that. I think, though, that heim-joints on the control arms and compliant bushings on the strut rod wouldn't be too bad. Anybody using the FPS control arms has essentially that system. I'm not sure what the old TCE arms used, but I *think* they were spherical ends as well. Such a system would eliminate any worries about bending forces in the control arms for those who care about such.
This idea with the compliance is making me happier about the BMR setup with poly bushings in the ends of the arms and stock bushings in the strut rod, too.
TimZ
Member
Shoaz said:
Agreed. Compliance isn't always such a bad thing.
Also, my "heim-jointed everything" example would only bind if you had cross-braced the two arms into a single piece. Rod ends on the arms without cross-bracing them together will work just fine.
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