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I've very pleased with the performance versus ride compromise
of the Dinan springs and Bilstein sport shocks. For me, it's
a nice balance, able to handle both normal street driving
and BMW driver's schools.
Bilsteins do take a little while to break-in. At first they
seem a little harsh. Then after a short time they feel much
better and seem to keep improving (in compliance) over the
first couple thousand miles.
Original Ride Question
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Jon Posner asks:
>I just had Bilstein comfort ride shocks installed on my
>E34 1991 535i. Actually, just the rears were replaced...
>New bilst. rears, worn and possible slightly damaged oem fronts..
>All of a sudden I find the ride very "jiggily" with every
>imperfection in the pavement (yes, we have those) causing a
>seemingly complex series of adjustments-- it's literally making
>me motion-sick!
>... does this sound like some off-shoot of the mismatch between
>fronts and rears not being matched (that will be fixed next week)
Jon,
Yes, it sounds like a mismatch between the worn original front
shocks and the new rear Bilsteins. The matching front Bilstein
shocks (strut inserts) should make your car feel much better.
It may take a little while for the Bilsteins to break-in and
feel as compliant as the original shocks. I think you will be
very happy once it's all together and a few miles are accumulated.
Here's a more detailed explanation/guess about the motion-sickness
nature of your presently mismatched suspension.
Usually, for best ride, rear springs are a little stiffer than
front springs. 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.
Of course, you haven't changed the springs, so the relative front
to rear rates are just fine. The other part to this is the dampening
of the suspension provided by the shocks.
The "damped natural frequency" is proportional to the:
square root(spring rate / (damping factor x sprung mass))
So more dampening (new shock) decreases the damped natural
frequency. Your front end probably has a higher frequency than
new (worn shock) while the rear end frequency is lowered by
the new shocks. This means the rear suspension can't catch-up
to the front anymore, making the car pitch over bumps.
Your original shocks probably degraded at approximately equal rates.
The ride may have been sloppy, but didn't affect the damped natural
frequency balance. Putting new shocks on the rear-only has thrown
off that balance.
Performance Ride Question
- -------------------------
Dennis Ignacio asks:
>I just had my 535i equipped with a Dinan Stage-I suspension
>which includes 4 Bilstein struts and 4 "matched" springs.
>Prior to this mod I put on a set of AC Schnitzer Type-Is
>(17x8.5) mounted on Dunlop D40M2s...
>Sure it stays flatter on the curves and steers quicker, but...
>I actually miss the smoothness of the stock suspension.
>On top of that, the rear tires are slightly covered by the
>lip of the rear wheel wells. In the front it has about 1.5 - 2
>inches of clearance. Overall it looks as if the car has some
>heavy people in the back seat. All I was looking to do
>was to reduce the leaning and improve the appearance.
>...Supposedly the Bilsteins that I had installed are HD
>for heavy-duty and are adjustable.
Dennis,
Dinan shocks are specially valved Bilstein sports, but
should be pretty close to the regular Bilstein sport valving.
There is no dampening (valving) adjustment except to send
the shocks back to Bilstein for custom valving.
Bilstein's marketing material says the shocks are "self-adjusting".
It's a misleading statement -- all shocks are self-adjusting,
in that they are velocity sensitive. The faster a shock moves,
the higher the dampening force. Shock adjustment or revalving
changes the dampening force at a fixed shock piston speed.
Bilstein rear sport shocks are height adjustable. You say
the rear of your car seems too low. Usually Dinan springs
drop the car equally, about 0.75 inches, at each end.
The front to rear rake should be about 0.5-0.75 inches
measured along the rocker panel, just behind the front wheel
and before the rear wheel.
My E28 Bilstein sport rear shocks have six adjustment grooves
10mm apart. Height (and rake) is changed by moving the circlip
that holds the lower spring perch. With Dinan springs, my car
needed the circlip set at the fourth groove down from the top.
If the rear seems too low, the circlip may be set too low on
the shock body. Check with Dinan 415.962-9401 for the specifics
on your (E34) car.
As far as the ride being less comfortable than stock, welcome to
the world of performance versus comfort tradeoffs. Your choice
of tires and wheels makes the car stiffer and more responsive.
The increased spring rate and firmer shock valving stiffens it
further. The good news is, as I mentioned earlier, that
Bilsteins break-in and get more comfortable with some mileage.
Regards,
Pete Read
'88 M5