Suspension Question

Ingotsilver5liter said:

2012 GT koni orange str/shock, BMR performance springs, 285/35/19 Hankook Ventus V2 tires.
rear end is sloppy and abrupt over bad roads and not very comfortable

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hmmm . . . "sloppy" usually means loose and bouncy while "abrupt" usually means firm and tight so I'm not 100% clear what you mean or what you want, but . . .

I would expect that combination to give a fairly firm ride. The springs are stiffer than the original springs and the shocks have more damping to keep the springs under control. The tires have fairly low sidewall which makes them stiff also.

If the bushings in the rear control arms (lower and upper) are shot they can introduce some sloppiness but I doubt that is what you are feeling. You could check them out to see the condition of the rubber bushings and replace them if they are shot.

I doubt you are hitting the bump stops often, so those should not make any difference.

If you want a softer and more comfortable ride, you can change the springs to something with a lower spring rate, more like the stock springs. Check American Muscle, I think they have some lowering springs that have the same spring rate as stock. If you don't mind returning the car to factory ride height, you could go with the factory springs.

The control arms are simply the lever arms that attach the rear axle to the body of the car. The lower control arms allow the axle to move up and down; the upper control arm allows it to move up and down and also prevents it from rotating due to the torque from the engine/driveshaft. The bushings at each attachment point absorb some of the noise, vibration, and harshness ("NVH") from the axle; while at the same time preventing excessive movement of the attachment points. If those bushings are worn out, it can introduce excess play at those attachment points; but you are only talking about maybe 1/4" or so; might be noticeable if they are really bad but generally so small that you probably won't feel it.

Like I mentioned earlier, the reason people replace the lower control arms is usually to stop "wheel hop" ; which happens because the factory arms are relatively flexible and have soft rubber bushings, so on a hard launch they bend, then unbend, then bend again, very rapidly; and you get this pounding banging affect which is called "wheel hop." The upper control arms are sometimes replaced with an adjustable arm, to correct or fine-tune the pinion angle. There are a few other subtleties to this but they mainly apply to road course cars. Another reason to replace them, is if the bushings are shot.

Control arms generally cannot be "too stiff" because they have a rotating joint at both ends. The trade-off with the bushings is that the harder bushings in the aftermarket control arms transmit more NVH than the soft factory ones. If the bushings in the upper control are are so hard that they do not allow the axle to rotate when one wheel hits a bump, that can cause binding . . . but usually not an issue. If the bushings bind up they could prevent the arms from moving up and down like they should, but I think that would be extremely rare.

I am not sure of the factory spring rates, but they are probably available out on the interwebs somewhere. Be aware that progressive rates can be deceiving; the initial rate is much lower than the rate after some compression. 165 and 160 sounds fairly low to me, but I am not sure. Lowering the car reduces the travel available for the suspension, and combined with soft springs, that could cause the suspension to bottom out more than it should. You would feel this as a hard bump/thud over big bumps.

Standard disclaimer: "that's how I understand it and I'm explaining to the best of my ability."
You are welcome to get a "second opinion" or course, but those are the basics and I'll be interested to see if we hear much different.

Ingotsilver5liter said:

I’m looking for a comfortable ride, she’s my daily. The BMR performance springs are : 165 lb. - Front, 160 lb. - Rear
Not sure if that’s too soft compared to others, I want to hear others people opinion on LCA’s post install, if it’s too stiff or if it really helps the rear end stay planted over bumps and dips.

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That would be the SP009 kit, which is "stock-ish" in terms of stiffness. The few lb/in difference in rear spring rate isn't going to make enough difference to matter, and shouldn't be outside what Koni orange shocks can handle stiffness-wise . . . assuming that they're still in good condition. Which is something of an 'if' here, with that much lowering with stockish spring rates - I'm a little uneasy about that much lowering and Konis in general.

Personally, I don't think that 160-ish is firm enough for 1.5" lowering. It's throwing away too much of the available bump travel without reducing the amount of that bump travel that is going to happen by anywhere near that much.

I suspect that you may still be getting bump stop contact, and the Eibach bump stops are probably a bit stiffer than OE Ford so when you do hit them, you'll hit them harder than you would the OE Ford stops.

Can you measure the static clearance between the stops and the pads that they contact?

Does your car also have LCA relocating brackets? If not, the lowering will crank up the amount of axle roll steer enough in the 'understeerish' direction to where odd behaviors may become noticeable. If your car does have the relo brackets, the setting being used might be pushing roll steer far enough in the 'oversteerish' direction to make the car feel nervous for that reason. How does it behave in an emergency-like lane change maneuver on a smooth road?


Norm