super-duper awesome handling: any spring and sway bar recommendations?

[Sagaris]

I would like some real world input. Tell me if my logic is correct:


Assuming that the information I have obtained is correct:

The stock sway bar size for my 325i is
Front: 20mm Rear: 12mm (Front = 40% larger)

The stock spring rates for a 325i are (lbf/in)
Front: 106 Rear: 245 (131% higher spring rate in the rear)
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Stock E30 M3 sway bar size:
Front: 19mm Rear: 14.5mm (Front=31% larger)
Stock E30 M3 spring rates (lbf/in):
Front: 114 Rear: 314 (Rear= 175% higher spring rate)

Assumption, with everything else being equal (or assumed to be): more proportional sway bar size and less proportional front:rear spring rate in the M3 helps reduce understeer compared to the 325i

H&R Race springs are (lbf/in)
Front: 315 Rear: 570 (Rear= 44.7% higher spring rate)

H&R Sport springs are (lbf/in)
Front: 180 Rear: 380 (Rear = 111% higher spring rate)

My car as it is, in stock condition, handles neutrally with slight understeer. I want to chose springs that will help improve the handling as much as possible. I am not a stance-whore and don't give a crap about being hella flush. I know that stiffer /= better handling in every case and it depends on the road. It is not a track car and I am replacing all my bushings with stiffer rubber, not polyurethane, but I intend to run the grippiest tires I can on the street.

1)So if my logic is correct, if I were to upgrade to H&R springs, the proportion of front:rear stiffness is lower than stock which would cause understeer, this makes me believe that a thicker rear sway bar is needed in order to bring the car back to neutral balance. Is this right?

2)For those with H&R sports + Bilstein sports, is the handling improvment worth it? or do you wish you went a different route?

3) What other springs can you recommend that are engineered for great handling, not just for looks?

[LivingLegend06]

Your making a whole lot of assumptions in your post that may not be valid.

First off the stiffness of a sway bar is affected by many things and the rigidity between front and rear cannot be compared due to the differences in sway bar geometry and mounting front and rear. It should also be noted that the torsional stiffness of a solid bar is proportional to its diameter to the forth power.

While the spring rates you provided may be somewhat accurate most stock style springs will be somewhat progressive in nature and any spring rates must be taken with a grain of salt. It should also be noted that when a car is lower the front and rear roll rates will be changed and for a different ride height different spring rates (and front to rear spring rate ratios like your looking at) will be required to give the car a stock balance. I don't pretend to fully understand it all by any means, I just want you to understand that there are a lot of factors involved if one were to actually do in depth kinematic suspension analysis.

To answer your questions though

1. Yes a stiffer sway bar will give you more oversteer.

2. The actual handling improvement is actually small, especially for street driving but the perceived handling difference can be very large and your cars behavior will become more predictable.

3. Most lowering springs on the market will give your car improved handling over stock worn out components, especially with fresh shocks. From what I've read the H&R cup kit is excessively low and soft and would make a less ideal track suspension, other than that its really just personal preference.

[jlevie]

As stated above, there are more factors at work than just spring rates. Springs, shocks, and sways should be thought of a package. The easiest solution is to use a combination that has already been developed and known to work well together. The Spec E30 suspension (H&R Race springs, Bilstein Sport shocks, 22/19mm sways) is one relatively low cost example, though it might be a bit low for areas that have poor roads.

[Sagaris]

Thanks for the input. I had a thought. I was going to be installing my new control arms and bushings today. Due to the whole "30-minute rule", wouldn't it be better to wait on installing the bushings after I install the springs I intend to use?

For example, the bushings bond to the ends of the control arms. If I install them today, then when I install springs and dampers, the car will sit lower and the bushings will have a torsional load on them since they bonded at a stock height. Any insight?

[jlevie]

Given the work involved, I'd hold off on the control arms & bushings until you have the new springs & shocks in hand unless the control arms on the car are really bad. To change the front springs you have to remove the struts, which means separating them from the control arms. If you have a ball joint press, that can be done w/o damaging the boots. But if you use a pickle fork on the new control arms you'll destroy the boots.

There probably would be a slight pre-load on the bushings if you install them now and then change the springs.