<E30> Instability on Bumps at High Speed

[madsman@xxxxxxxxxxxxx]

>Date: Sun, 6 Dec 1998 05:02:58 -0600
From: "Mike Theuri" <mikesoft@domain.elided>
Subject: <E30> Instability on Bumps at High Speed 

On I-40 my 86 325 seems to "loose it" slightly 
when it goes over a series of bumps at about 70
to 80 mph, it feels as though the car would go to
either left or right if you didn't steer just a little to 
put it back straight. My shocks have a little bit 
of life left in them. Could this be a result of worn 
out bushings ? Or could the front suspension have
something to do with it ? In in-town driving the
symptoms don't seem to be apparent at all
probably due to lower speed. Thanks for your 
insights. 

Mike  

Hi Mike,

You could be experiencing "bump steer", where the toe setting of the rear wheels changes as the 
rear suspension moves up and down in response to road undulations . Imagine a straight line 
running through the center of your tire, parallel to the side of the car. If the line points 
towards the center of the car, that's "toe in", if it points out, that's toe out.All cars are 
designed with a specified amount of toe, to compensate for the forces of acceleration and 
deceleration, etc. Since our ultimate driving machines are designed not to do this ( highly 
refined suspension geometry, independant rear suspension, etc. ), I'd say you're on the right 
track looking for worn rear suspension parts, unless something was damaged (bent)due to a recent 
unfortunate episode. If the front end toe is out, you could also experience this, but in my 
experience, you'd also feel some front end wander, especially when changing lanes on a highly 
crowned road. I'd start with an inspection of the rear end, making sure that there are no 
obviously worn or broken parts, such as rear suspension mounting bushings, shock bushings, etc.,
and no damage to the suspension mounting points ( I recall something in the Roundel about this 
about a year ago). If everything checks out ok I'd do an alignment, making sure that the shop 
that does your alignment follows BMW specs re a full tank of gas in the car and whatever amount 
of weight necessary to be put in the car prior to the alignment ( my 5 series calls for 600 lbs 
to be distributed throughout the interior, for example ).

A fellow named Fred Puhn wrote a book called "How to make your car handle" that supplies a great 
explanation of how all this stuff works, including instructions on how to make a simple toe 
gauge to see if your toe  is shifting. If you can't find the book, e mail me, and I'll send the 
details...good luck,

Mike Capaldo

(happily in geezerville now that the M3 is sold)
97 528i
96 454 Suburban ( the ultimate Bubba-mobile<g> )
98 Sebring Convert ( wondering if I'm gonna get a 3 pointed star in the mail now that the merger
has gone through)

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