[radz28]

I have a recollection that someone (maybe like Mountain, for example) posted, a comment about spring rate change, in a thread where part of the conversation was about switching rear control arms from stock to rod-ends. The comment stated, something to the effect of, when switching from rubber bushings to rod-end bushings, that you loose spring rate. I believe I understand the mechanics of this, so I won't go into it and make this post longer, though, anyone is welcome to comment on it. I don't track the car much (but do enjoy driving in the twisties in the mountains nearby), but would like to, and would like to have confidence I haven't screwed the car up in the meantime much (especially in the mountain roads), lol. The car is pretty much all street, but I'd like to try a little track work eventually.

My question is if there is a way to calculate this spring rate loss? I've tried searching and don't see anything (I admit I could very well have missed it). The way I thought I would start by trying to mitigate some of this loss was by upgrading the rear sway bar to a stiffer one, because I didn't want to change the ride quality. I bought an AFE adjustable bar and thought by starting with the softest setting, it would likely be higher than the OEM-bar I came from. I understand my car has a little understeer baked-in from the factory, and that I'd just be adding to that (basically, with lessening spring rate through the switch to rod-ends), so I figured a low percentage of rate loss from the rod-ends combined with a higher rate stab' bar would likely net a little higher rate than the OEM-bar I started with.

It seems the consensus of spring rate for an FE4-bar is from around 150-250 lbs/in (at least that what was posted here in a few threads I found). I averaged to about 200 and compared that to the softest setting of 245 from the AFE bar I installed. Does that seem reasonable to start? I understand I'd have to just track it to ultimately determine what's best (and what my driving style prefers), but I was just looking for a fair jumping-off point. The middle setting is 280 lbs'in, just for reference.

Thanks for any comments you are willing to share.

[Scargoes]

I would think it would be the reverse. Replacing rubber with spherical bearing will reduce deformation effects and give the car a tighter/stiffer feeling. A number of the links in the FE4 suspension already have spherical bearings in place. But you need to think of bind if you replace all the pivots with spherical bearings. Something has to give as the suspension travels thru its normal motion. Since you already have the ZLE subframe bushings, everything else aside from springs and dampers should be equal spec as the ZLE in the rear. Using the FEA front compliance arms will add some feel and reduce deflection in the FE4 hydraulic bushing arms.

[Msquared]

The spring rate loss the OP is talking about is due to the fact that stock rubber bushings are bonded to their inner and outer sleeves. The inner sleeve is captured by the pivot bolt and held solid to the frame, while the outer shell rotates with the control arm. So when the arm moves through its intended arc, the rubber bushing has to twist, and its hysteresis adds to the wheel rate. It is accurate to say that switching to rod ends - which add no wheel rate at all - will reduce wheel rate by eliminating the rubber bushing's hysteresis.

The problem with quantifying it is that every rubber bushing in every car has different dimensions, and some rubber bushings are even partially hollowed or fluid-filled. So we can just give a blanket rate spec for each bushing. Also, I think the rate from a twisting rubber bushing would be progressive and therefore much less for the initial amount of travel than it would be near the end of suspension travel. The only good way to quantify it would be to unbolt each link and literally put a scale under it as you move the unbolted end of each link...and you'd have to do this for each end of each link.

My guess is that the added wheel is fairly low, especially in the first couple inches of suspension travel in each direction. But it's not nothing.

[radz28]

Originally Posted by Scargoes

I would think it would be the reverse. Replacing rubber with spherical bearing will reduce deformation effects and give the car a tighter/stiffer feeling. A number of the links in the FE4 suspension already have spherical bearings in place. But you need to think of bind if you replace all the pivots with spherical bearings. Something has to give as the suspension travels thru its normal motion. Since you already have the ZLE subframe bushings, everything else aside from springs and dampers should be equal spec as the ZLE in the rear. Using the FEA front compliance arms will add some feel and reduce deflection in the FE4 hydraulic bushing arms.

I could very well be wrong - I will admit that. I replaced all the available arms in the rear, including the toe. I did notice the toe arms seemed to be spherical, but it seemed, pretty much, the remainder of the arms had rubber on the chassis side (the knuckle side seemed to be typical rubber-type, unlike FEA, so far as I understand).

I had a similar logic to you, it seems, at first. But after the comment I was referencing, my logic evolved to what I believe now. Installation manuals for these kinds of arms (OEM rubber or poly-bushings) say to final-torque with the suspension loaded, because they will go into bind if torqued-unloaded. That, to me, implies there is a point where they at least have some influence on spring rate (as best as I understand it). And as the bushing goes into "bind", spring rate starts increasing. At least, that's what it seems.

I feel like I'm on the right track, because after I installed the arms, I've noticed when I jack the car up, I get a lot more travel than before. So the tires I'm not lifting stay in contact with the ground for longer than they used to, because the suspension seems to flex-out slightly better than before.

Or I'm wrong, lol.

[radz28]

Originally Posted by Msquared

The spring rate loss the OP is talking about is due to the fact that with stock rubber bushings, the inner sleeve is captured by the pivot bolt and held to the frame, while the outer shell rotates with the control arm. So when the arm moves through its intended arc, the rubber bushing has to twist, and its hysteresis adds to the wheel rate. It is accurate to say that switching to rod ends - which add no wheel rate at all - will reduce wheel rate by eliminating the rubber bushing's hysteresis.

The problem with quantifying it is that every rubber bushing in every car has different dimensions, and some rubber bushings are even partially hollowed or fluid-filled. So we can just give a blanket rate spec for each bushing. Also, I think the rate from a twisting rubber bushing would be progressive and therefore much less for the initial amount of travel than it would be near the end of suspension travel. The only good way to quantify it would be to unbolt each link and literally put a scale under it as you move the unbolted end of each link...and you'd have to do this for each end of each link.

My guess is that the added wheel is fairly low, especially in the first couple inches of suspension travel in each direction. But it's not nothing.

I forgot that it might have been you that made the statement, too. If so - my apologies. I've read many of your posts.

Thank you for your comments. You must be onto something because I can't really find a reference for any of this, so maybe using a scale, as you've stated, might be the closest thing to me finding any good numbers. Hmm...

At least, now, if my OCD won't let this go, I have one way to try to figure this out. Thank you.

[Msquared]

Originally Posted by radz28

I forgot that it might have been you that made the statement, too. If so - my apologies. I've read many of your posts.

Thank you for your comments. You must be onto something because I can't really find a reference for any of this, so maybe using a scale, as you've stated, might be the closest thing to me finding any good numbers. Hmm...

At least, now, if my OCD won't let this go, I have one way to try to figure this out. Thank you.

Don't feel bad, I don't know if it was me who made the statement either!

This is easy to experience: just unbolt any link or control arm at one end and then try to move the unbolted end by hand. You'll feel the spring rate it has. It doesn't have to be a Camaro: it could be any rubber-bushed link/arm in any car. This is not the case for typical poly bushings, however, because in those the inner sleeve is not bonded to the poly but rather is intended to spin inside the poly bushing.

[radz28]

Excellent. Thank you Matt.

[Scargoes]

Originally Posted by Msquared

The spring rate loss the OP is talking about is due to the fact that stock rubber bushings are bonded to their inner and outer sleeves. The inner sleeve is captured by the pivot bolt and held solid to the frame, while the outer shell rotates with the control arm. So when the arm moves through its intended arc, the rubber bushing has to twist, and its hysteresis adds to the wheel rate. It is accurate to say that switching to rod ends - which add no wheel rate at all - will reduce wheel rate by eliminating the rubber bushing's hysteresis.

The problem with quantifying it is that every rubber bushing in every car has different dimensions, and some rubber bushings are even partially hollowed or fluid-filled. So we can just give a blanket rate spec for each bushing. Also, I think the rate from a twisting rubber bushing would be progressive and therefore much less for the initial amount of travel than it would be near the end of suspension travel. The only good way to quantify it would be to unbolt each link and literally put a scale under it as you move the unbolted end of each link...and you'd have to do this for each end of each link.

My guess is that the added wheel is fairly low, especially in the first couple inches of suspension travel in each direction. But it's not nothing.

Oops, I forgot about the rotational load.