Date: Sun, 24 Dec 1995 14:37:58 -0500
From: read_at_engr05.comsys.rockwell.com (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
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
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
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
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
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
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
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
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
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
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
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
- If the car feels good, keep it stock because it is quite
- 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.
- 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.
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