In preparation for an upcoming suspension refresh on my Beast, I've gathered some info that I wanted to share with the group and hopefully spur some conversation and additional data points. THANK YOU to the following members for taking time out of their lives to take a couple measurements on their cars and contribute:
I've owned my 01 M5 for nearly 2 years and from the beginning, the one thing I disliked about it was the handling. I came from the E46 M3 camp and had become accustomed to the tight and responsive steering, lack of body roll, and confidence in a bumpy corner that the E46 chassis delivers. Unfortunately, my M5 exhibits lazy steering response, lots of body roll, and feels like a boat in rough seas on a rough freeway off-ramp.
I've recently developed a bad ball joint on my pass-side lower control arm, so I've decided now is the time to step it up and improve the handling of this otherwise magnificent car.
Due to a variety of factors that are outside the scope of this thread - I made two decisions:
- I wanted a conventional strut/spring combo rather than an adjustable coilover
- I wanted Bilstein dampers rather than Koni
Now that I've decided on dampers, that left me with one decision point left...Springs:
There appears to be several spring sets available for the E39 in general, but only a couple for the M5 specifically. I'm focusing on two sets in particular: Dinan & InTrax
Dinan Performance Spring Set for E39 M5: Dinan Performance Spring Set
Available From: Dinan directly or several of their authorized distributors
Price (as of May 2016): $308, free shipping
Linear Rate Springs
Front Spring Rate: 197 lb/in
Rear Spring Rate: 250 lb/in
Advertised lowering: 0.25" / 6.35mm
InTrax BMW1251: InTrax Racing
Available From: Evolve in the UK or InTrax direct in Holland. There are no US distributors for this product, per InTrax customer service.
Price (as of May 2016): $350 shipped to the US (Evolve)
Linear Rate Springs
Front Spring Rate: 191 lb/in
Rear Spring Rate: 228 lb/in
Advertised lowering: 1.18" / 30mm
Aside from the obvious difference in how much the InTrax lower the car vs. the Dinan, it's also worth noting that the Dinan springs are approximately 3% stiffer up front and 10% stiffer in the rear.
The ride heights are listed above, so what's the problem?
Bilstein B8 are a nitrogen-charged, high-pressure monotube damper. Due to the internal pressure, it's very well documented that these dampers add a touch of ride height to most cars when installed. Combine that with the fact that the E39-specific rear dampers allegedly have the spring perch in the wrong spot and you can see that trying to calculate ride height from a pile of specs is tough.
As a means of establishing a baseline of sorts, here are some measurements from my 36K-mile 2001 M5. My car is completely stock with the exception of Dinan mufflers in the rear, and a steel space saver wheel/tire in the trunk. Measurements were made with a full tank of gas and sitting on level ground.
Stagger: 16mm lower in rear
A note on ride height measurements:
The conventional means of measuring this on our cars has been to measure from the center of the wheel/hub to the top (12:00) position of the wheel arch. While this certainly works in theory, I've found in practice that it introduces a lot of variables that make it tough to compare results from one car to the next. While watching a video on YouTube from Redish Motorsports in the UK, the owner James measured ride height differently and I think it's a more accurate, repeatable, and easier measurement to make. The caveat here is that you must have stock Style 65 wheels...
The idea is to measure from the wheel arch to a fixed point on the wheel. The center is a great point in theory as it allows people with different wheels to compare. The reality is that it requires some "eyeball" work and depending on how two different people eyeball it, you may come up with different measurements. The way I've listed measurements here is shown below:
Measuring is a piece of cake - just hook the end of the tape between the wheel & tire, make sure the tape passes through the center of the BMW Roundel, and read the measurement at the arch. **If you would like to convert these measurements to the "wheel center" method, just subtract 246mm (the radius of the Style 65 wheel)**