Rear tires rubbing over bumps!

[mechanicalmoron]

Every wheel that has a toe change rotates around a steering axis, also known as king pin inclination or king pin angle. It's the vertical line between upper and lower pivots. On a macP it's the line between the ball joint and the upper mount for the strut center. On a double a arm like our multi link rear, the steering axis is between the ball joint and the virtual intersection of the traction and camber links. Scrub radius is the distance between where the steering axis hits the ground and the center line of the tire contact patch.

A larger scrub radius is fine, but there are generally accepted limits. Most suggestions say to stay near factory, but for RWD, up to 3" is an acceptable upper limit. The reason being that the tire doesn't pivot on the center line of your tire, but at the steering axis where it inclines into the ground. The more scrub you run, the more effects of bump steer, torque steer, and fidgety wheels from road irregularities will be apparent. Scrub is more of an issue up front since the wheels turn more up there than in the back, but knowing the effects of scrub can alter grip in the rear under power while cornering.

Hopefully I'll be able to get the rear suspension mapped enough that I can finally publish an article I've been dying to get out concentrating on S chassis suspension.

Ahh..... you're amazing.

I've been reading through the zilvia thread on suspension/geometry, and trying to understand it. I want to set my s13 up as ideally as I can with stock arms and coilovers, it seems like it should only need one or two rear ones. But I want to get my roll center as good as possible, for both the front and rear, and so that the front and rear play nice (I think I've read this being called roll-couple, or something?).

It seems like putting the rear roll center in the right place (judging by LCA being horozontal) puts the rear medium low (where tein suggests) but putting the front anywhere near to horozontal would be like stock, and like a monster truck - which makes me worry about the difference between the two, and why they originally came with the roll centers how they did. Tein's suggested height seems to make the rear perfect, at the expense of the front..... maybe intended, as the front has different springs, sway bar (I have SE, so larger still), and weight? Maybe it balances?

Clearly, I'm willing to put function before form (the thing's not pretty anyways).

I really need to take some pictures and make my own thread, I suppose.

[Hijacker]

The roll couple is the length between the ceter of gravity and the roll center. You can treat each end seperately for sake of the roll couple. However, there's an issue with that, because there's an axis line drawn between the front and rear CGs. The slope of that axis plays with how the car transfers weight under braking and acceleration.

As for the roll couple, you want a shorter one. The best way to counter the roll center from lowering is to adjust the position of the ball joint. The line drawn between the center of the ball joint and the inner control arm pivot. There are some "roll center correctors" out there by moonface and megan racing that are ball joints that have different base heights. They don't change crap cause the ball joint stays in the same place. The shank needs to lengthen to reposition the ball joint down. PBM and GKtech make knuckles that so the same thing using the stock ball joint by lowering it's mount.

The front rc is the worst offender. It lowers way quicker. I haven't done calculations of the rear yet, but most people say it doesn't lower that fast. I would also say that using a drop knuckle is better than just extending th ball joint shank. Lowering that pivot would probably throw off the camber and toe curves by taking the upper a arm further away from parallel

[mechanicalmoron]

So with stock arms, lowered 2in the back of the rlca is horozontal, but the front is still at a slope.... I assume this is good? (or good enough)

But then for the front, a 2in drop will probably mess up the roll center, since the arms are sloped down a bit (when drawing a line from the center of the bushing to the center of the balljoint, the balljoint being the higher end)?

And if flipping the eccentric over could give +2 camber, it could put the rear within camber spec, but on the fun side of the spec - you think it would be a bad idea, and throw other specs off enough to worry about?

I don't mind going to stock height, but I've heard that this is not ideal, either, and that a small drop is good for the roll center (especially considering the stiffness of my springs, compared to stock). I just want to adjust it for handling, so much the better if it happens to look good too. (I mean, I think being 2in lowered looks great, and again, it's what tein suggests. but I'll change either end any way that I need to for better performance)

*edit* I was just reading through this and what it says about the lca being at a negative angle giving positive camber makes sense. So at the least, it looks like (roll couple be damned) I have to make sure that the front LCAs allow normal cornering deflection without going past horozontal (so probably, I get to raise the front an inch).

So yeah, would you have any suggestions? I just want the best handling possible with stock arms and tein basics (and again, SE, so 25mm front sway bar). Would you forsee any problem having the rear lowered 2in (assuming I can get the alignment hammered out properly) and the front only 1in, as it looks like that will put my tie rods and LCAs in nearly ideal position? I mean, other than looking silly, which it probably will....

[Mr240dude]

So my steering wheel is crooked after having the rack bushings installed. Anyone have any idea why? The old one was cracked and dried.

[mechanicalmoron]

So my steering wheel is crooked after having the rack bushings installed. Anyone have any idea why? The old one was cracked and dried.

Did you do it yourself?

Sounds like either the rack is seated wrong and off center, or, if you also did tie rod ends (or removed them, like if you had a single-piece type bushing or something) you didn't get them quite right (which is kind of hard).

For instance, I just did my tie rod ends and rack bushings, now it always looks like I'm going aroud a turn. I didn't get the tie rod ends on in exactly the same spot, and messed up my toe a bit (luckily, I think I did it pretty uniformly, but you might not be so lucky).

Or, maybe it was previously shifted because of the bad bushings, and it was aligned like that, and then you centered it right and now you have your tie-rods off center, even if your toe is set right (harmless, if you don't mind the wheel looking funny)

An alignment will fix it, but make sure they know it's an issue for you, so they pay attention to it. But if you're doing a bunch of suspension stuff, better wait on the alignment, as long as it feels safe. Make sure all your bolts are torqued right, the ones on the rack girdles ane 65-80ftlb (so, I personally went 80, to make up for having to use an extension to get to the higher one).

[daemonyk]

So my steering wheel is crooked after having the rack bushings installed. Anyone have any idea why? The old one was cracked and dried.

If the steering shaft was removed from the rack at any point during the procedure, it may not have been put back on the splines aligned the same (easily done, very common, esp if you don't mark things first). Also like mechanicalmoronsaid, possible that the "right" alignment on the old bushings, is now the "wrong" alignment now that things are new, among other things. Center the wheel, then turn it full lock in both directions. If it locks at the same relative point in both directions, check your alignment. If it locks at noticeably different points, it's likely the splines clocked wrong on the shaft on the rack.

[Mr240dude]

Alright thanks that could be it.

In addition I also had rear poly bushing set installed too. I really don't notice a difference with both these kits installed...aalmost dissapointing.

Is this due to the fact that I may need an allignment to get the most of of them?

[daemonyk]

Alright thanks that could be it.

In addition I also had rear poly bushing set installed too. I really don't notice a difference with both these kits installed...aalmost dissapointing.

Is this due to the fact that I may need an allignment to get the most of of them?

To be brutally honest, your suspension sounds pretty janky with the spring and shock problems you're already having, I'm not surprised your car doesn't feel better / different, especially if those are the only things you've changed and everything else is still old and worn. Rack bushings don't help much if the tierods balljoints and arm bushings are ragged and loose - same goes for rear poly arm bushings, they won't help too much if you have worn and torn subframe bushings and unknown bouncyness. Are those the only bushings you've done? If so, that's probably why it feels the same.

[mechanicalmoron]

Alright thanks that could be it.

In addition I also had rear poly bushing set installed too. I really don't notice a difference with both these kits installed...aalmost dissapointing.

Is this due to the fact that I may need an allignment to get the most of of them?

They're just going to make the ride rougher, you will feel the road more.

I just did flca, steering bushings, and tie rod ends, so I don't know which did the most, but I feel every rock compared to before, some roads are tiring/obnoxious to drive on for prolonged times, now. Especially washboarded ones.

I mean, they won't "improve" your ride in the luxury sense, quite the opposite.

Rear poly bushings are supposed to really suck, unless you grease them regularly. They don't like the multiple directions that multilink requires them to move, and bind up, or something like that. If you "got them done" instead of doing them yourself, it would have made more sense to get OEM or performance type rubber bushings pressed in. (but I guess you can't be blamed for not knowing that, poly bushings, like going as low as you can, are part of the drift religion, probably because they can be cut and installed without a press)

Maybe if you did them yourself, you could find a creative way to channel the bushings and drill the arms for grease fittings...... I don't think they offer greasable bushings for normal arms. But energy suspension does offer greasable sway bar bushings, which only move on one axis, which probably tells you how bad the binding can be.