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Date: Sun, 24 Dec 1995 14:37:58 -0500
From: (Pete Read)
Subject: M5 Suspension -- Part 1, Overview

Over the past several months, I've installed the equivalent of a Dinan Stage 3 suspension in my 1988 E28 M5. First stiffer springs and negative camber plates, next Bilstein Sport shocks all around which eliminates load leveling, and finally adjustable sway bars.

My upgrades are nothing original. Dinan Stage 3 suspensions are on many M5s as reported in Stan Simm's M-register newsletter. Considering all the good reports, I made the changes mostly on faith. I'd have felt more comfortable knowing why the changes work and some numbers to describe "firmer".

I'll try to provide an explanation and give some numbers. The spring and sway bar numbers are calculated by measuring the components and then plugging the values into formulas from the book "How to Make Your Car Handle" (Fred Puhn, HP Books). If I'd originally planned to report my calculations, the measurements would have been more careful. The numbers aren't perfect, but they should be pretty close.

To make this more "digestible", I'll give the basics in this "Part 1". Then later, I'll post more "Parts" for the calculations and further details.

M5 SUSPENSION -- PART 1, Overview

I've tried to follow knowledgeable people's advice about improving car performance. That is, the nut behind the wheel is the most important upgrade. Driver's schools improve performance more than any modification to the car. This advice is certainly still true for me. M5s are awesome cars stock, so most modifications are "wants" not "needs". After nine driver's schools in my '87 535i and '88 M5, I "wanted" some changes. It's fun to experiment. Plus you feel so cool explaining the changes to others. Of course, the implied message is even the mighty M5 isn't quite up to snuff for your incredible driving skills.

BMW engineers are more capable car designers than any of us and probably most aftermarket suppliers. It comes down to tradeoffs. Suspension performance improvements generally compromise ride comfort for a stable, controlled ride -- read firmer and lower. Firmer and lower always feels better the faster you drive on dry, smooth surfaces. Firmer isn't always better though. Bumpy, wet, surfaces feel better with softer suspension.

BMW transformed the 535i into the M5 with a more powerful engine (256 hp vs 182 hp), wider tires and wheels (225/50-16 vs 200/60-15.35), bigger brakes (11.8 vs 11.1 inch front), and stiffer suspension. I'm not sure how much stiffer the springs are, but sway bar size increases dramatically (25mm/18mm vs 19mm/15.5mm).

Larger sway bars make sense for performance street cars. They reduce roll without the harshness caused by increasing spring stiffness for the same purpose. It seems the M5 sway bars can't get much bigger without overstressing the mounting points. Dinan uses adjustable sway bars very close to the stock size (25mm/19mm). Roll stiffness and response to bumps increases both through sway bar adjustment and stiffer springs. Then stiffer sport shocks cope with the increased spring rate.

My major complaint with the M5 is understeer (pushing) and related front tire rollover. Although the M5 is much faster at the track, it doesn't seem to have quite the balance of the 535i. This is most noticeable on wet skid pads where it's difficult to get the tail out (oversteer). The 535i understeers slightly, but can be changed to oversteer with the throttle. At the track, I usually end up with six psi more front than rear pressure to balance out the M5's constant understeer.

The different balance is, once again, a deliberate choice. M5s have extra understeer designed-in to compensate for the increased power and speed. Cars generally oversteer more at higher speeds. The front has more downward force while the rear has some lift. More power also causes oversteer. Rear tires can handle both cornering and acceleration forces at the same time, but they can only do so much. With more power, the total acceleration and cornering forces overcome traction at that end, so the rear tires slip (oversteer). To compensate for the power and speed related oversteer, front sway bar stiffness is increased more than the rear. This brings the car back to a strong understeering tendency.

Understeer is safer than oversteer. Most driver's natural reaction in either case is to let-off the gas. This transfers weight from rear to front that reduces understeer, but increases oversteer if done too quickly. So the extra understeer keeps people out of trouble but, for my tastes, they went too far.

1st Change -- Camber Plates and Springs

Dinan suggested front camber plates and stiffer springs, with stock shocks, for a minimum cost change.

Camber is the angle tires make with vertical. Negative camber means the tops of tires are closer than the bottoms. Like everything else, camber is a compromise. Zero camber works best for braking and tire wear in a straight line. However, during hard cornering, the outside wheel tucks under (camber becomes more positive) causing the tire to roll up onto its outer edge. Increased negative camber keeps the outer (heavily loaded) tire tread flat on the surface.

The Dinan plates produce a total of about 1.0 degree negative camber, 0.33 negative stock plus an additional 0.625 negative from the plates. The front strut upper mount comes off during plate installation so it's a good time to change springs (and shocks, in 20/20 hindsight).

Dinan springs are shorter and stiffer. Ride height lowers about 0.75 inch. Springs are 30% stiffer in front, (187 lb/in vs 144 lb/in) and a whopping 65% stiffer in the rear (267 lb/in to 162 lb/in). The front value seemed okay, but the rear increase didn't make sense until I thought about it.

First, the "effective" stock rear spring rate increases somewhat through the load leveling assist. Second is that rear springs are generally stiffer than front springs. This isn't intuitive because you'd think a car with approximately 50/50 weight distribution would have equal spring rates front and rear.

The reason front and rear springs aren't equally stiff is to absorb bumps without the car pitching. Front rates are less than the rear, so the rear can "catch up". The front hits a bump first and reacts slower so the rear can finish its cycle at the same time as the front.

This front to rear "natural frequency" difference is usually optimized for the speeds a car normally travels. It works out to about 50-60 mph with the Dinan springs.

After spring installation, front to rear rake is adjusted by changing load level height. Dinan recommends 0.5-0.75 inch front to rear rake measured at the rocker panel behind the front wheel and just in front of the rear wheel.

Adjust load level height by rotating the U-clamp at the center of the rear sway bar. As the rear suspension drops, the sway bar rotates and moves a load level sensor arm. The load level system pumps the rear ride height up until the sensor arm is back in it's original orientation. Reorienting the U-clamp and attached sensor arm changes ride height.

Well, the increased roll stiffness from the heavier springs and negative camber from the plates reduced front tire rollover and understeer, but the 100,000 mile original shocks were not up to the task. The ride was much too active or underdampended.

Dinan normally recommends at least front Bilstein Sport shocks with the spring set, but I tried to go cheap. Probably my stock shocks were worse than I realized, but it seems that the stock load leveling shocks, even in good shape, must be underdampened with the stiffer springs. However many people in the M-register report satisfaction using Bilstein Sport front shocks while retaining the stock load leveling rear shocks.

2nd Change -- Bilstein Shocks and Load Level Elimination

To prevent repeating the spring/strut/shock job yet again, I installed Bilstein Sport shocks all around which eliminates the rear load leveling feature. Load level elimination is nothing more than disconnecting the electrical connection beneath the spare tire and removing the stock rear shocks and hoses. Standard 535i size shocks, shock mounts, springs, spring pads and so forth replace the load leveling parts.

Load level rear springs are larger diameter (4.5 inch M5 vs 3.9 inch diameter 535i rear spring). So load level replacement required two 535i size short rear springs of the same rate (267 lb/in) and the following BMW parts.

Quan Part Number Description

2 33521 124 572 Spring Pad
2 33521 124 507 Spring Pad
2 33521 124 575 Protection tube
2 33521 124 573 Adsorber 2 33521 126 680 Guide Support
2 33521 125 651 Washer
2 33521 125 649 Support Cup

Bilstein Sport shocks work with shorter, stiffer springs (most aftermarket springs). Softer Bilstein Heavy Duty shocks, designed for longer, softer, 535i springs are available and fit the M5. Any standard 535i size rear shock installation, Sport or Heavy Duty, requires a 535i size spring and the load level elimination parts above.

As with load leveling, adjust front to rear rake by changing the rear ride height. Bilstein Sport rear shocks have six adjustment grooves 0.37 inches apart. Change height by moving the circlip that holds the lower spring perch.

Setting the Bilsteins at the fourth groove down from the top with Dinan springs, gives 0.75 inch rake along the rocker panel. Front and rear rocker panel heights are 7.75 inches and 8.5 inches respectively. Ride height lowers about 0.75 at each end. Clearance to the catalytic converter is 4.2 inches and about 5.2 inches to the oil pan.

At first the Bilsteins didn't have much compliance. Then after a few hundred miles the seals seemed to break-in, making the ride much more comfortable. Be sure to follow Bilstein's instructions for lubricating the shock seals. Push the damper rod _all_ the way down before installing the shock. Otherwise they will feel rock hard at first.

The Bilstein shocks made a tremendous difference, less pitching and faster settling time. Of course the huge difference was partly because the car was so underdampened with the stiffer springs and worn original shocks.

3rd Change -- Sway Bars

Sway bars (and springs) control body roll and the amount of weight transfer at each end of the car. More weight transfer causes greater tire loading and slip at that end. So larger, stiffer, rear bars cause more tire loading and slip at the rear (oversteer). Larger front bars generally understeer more. Front to rear stiffness needs balancing. Ideally you'd have slight understeer and the ability to induce oversteer with the throttle.

Sway bars are just torsion springs. Stiffness increases to the fourth power of bar diameter. So the easy way to determine sway bar stiffness increase is:

(new diameter/old diameter) to fourth power

So changing the M5's rear bar from 18mm to 19mm increases sway bar stiffness 24%. (19/18)^4 = 1.24

Suspension Technique (ST) bars have three adjustment holes in front and two in the rear, at one inch increments. Calculate actual bar stiffness by measuring the bars and plugging the values into an equation. Adjustment holes just change the measurements used for calculating the stiffness. Adjusting to the inboard holes or full tight increases front stiffness 22% over stock (285 vs 233 lb/in) and rear stiffness 58% over stock (139 vs 88 lb/in).

An interesting number is the front to rear sway bar stiffness ratio. It's 2.6 for a stock M5 (233/88) and 1.6 for a 535i (78/48). ST bars come out somewhere between at full tight (about 2.0, 285/139). This difference in ratios certainly helps explain why an M5 understeers more than a 535i. However, as mentioned earlier, the M5's extra power and speed require more front to rear stiffness than the 535i.

Sway bars are much easier to install than shocks and only take a few minutes to adjust. Just jack up the car up so both wheels are off the ground (no twist in the bar), and move two bolts to different holes. Adjustments are harder than changing a tire, but easy enough to encourage some experimentation.

Adjustable sway bars provide the final piece to car balance. After some experimentation, the balance now is moderate understeer with throttle controllable oversteer available. A wet skidpad is now lots of balanced, tail-out, fun instead of frustration.


The Dinan approach is fairly conservative but makes a noticeable difference. Ride is much more controlled, with less body roll and pitch. The Bilstein Sport shocks and stiff springs made the most difference. Adjustable sway bars help balance understeer versus oversteer to individual tastes. To put a number on firmness increase, I'd say about 30%.

Bilstein Sport shocks are quite firm for the first few hundred miles until the seals break-in. My car is a daily commuter and I'm very happy with the overall ride, but the roads are fairly smooth. Sport shocks and springs may be too stiff for people driving rough roads all the time.

Here are the choices in order of increasing commitment

  1. If the car feels good, keep it stock because it is quite


  2. If handling seems a little sloppy but fine otherwise,

    maybe just replace the stock shocks with Bilstein HD or equivalent Boge shocks. Unless you drive hard, don't bother with the camber plates.

  3. Aggressive drivers should go for it all, springs, sport

    shocks, camber plates and sway bars. Don't worry too much about getting rid of load leveling. With the stiffer springs, 300 lbs in the trunk only drops the car about 0.5 inch.


Dinan provided the springs and camber plates. They (Jeff Hecox in particular) were always helpful and knowledgeable -
  • much more so than the average aftermarket supplier. I bought shocks and sway bars elsewhere because I mistakenly thought Dinan was much higher than other sources.

I was using the 1993-94 Dinan catalog which lists the Stage 3 kit at $2059, but they changed their pricing recently. The 1995 catalog lists the same kit for $1579. It's a competitive price and they offer a 10% BMW CCA discount. At the new price, I would have made my life easier by buying everything from Dinan.

If you're more comfortable with another vendor, the equivalent parts should work fine. Here's some price information. BMP prices are used for comparison because I happen to have their catalog they are usually competitive.

Dinan Stage 3 Kit $1421 ($1579 - 10% club discount)

Equivalent Parts $1281 ($140 less or about 10% savings)

  Camber Plates                     $198 (Dinan $220 - 10%)
  Shorter/Stiffer Springs           $250 (BMP)
  Bilstein Sport shocks             $384 (BMP)
  Load level elimination kit        $ 63 (Stock BMW parts)
  Suspension Techniques sway bars   $386 (BMP)

The Dinan catalog suggests eight hours installation labor for a Stage 3 kit with load level elimination.

Merry Christmas,

Pete Read

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