[200q20v] Re: Bump Steer, DIY

Bernie Benz b.m.benz at prodigy.net
Wed Oct 3 13:18:54 EDT 2001


DIY bump steer measurement is like DIY alignment but more so, long and
tedious.  But unlike alignment, you probably couldn't find anyone
knowlegable and willing to do it for you at any price.  This is a true DIY
only project.  Further, the correction or minimumization of BS can be a
major chassis redesign project inof itself.

BS characterization involves measuring toe, or more accurately the change in
toe, as the suspension, front or rear, is moved through its free travel
either side of the static ride height (SRH).  Obviously, this requires that
the exercise be performed without springs installed, so best that it be
programed as a part of other major suspension work.  With the wheels on the
floor, the chassis is jacked through the full range of suspension travel,
measuring toe change at approperiate intervals.  The results are best
understood when displayed as a plot of suspension displacement around the
SRH vs. toe change relative to the toe at the SRH.  Any book on suspension
tuning will cover the process and DIY tools used in more detail.  My
reference is an old paper back, H.P. Books #46, How to make your car handle,
by Fred Puhn.

> From: "Aaron Gjerde" <gjerdea at pconline.com>
> Date: Wed, 3 Oct 2001 11:44:51 -0500
> To: "Bernie Benz" <b.m.benz at prodigy.net>
> Cc: "audi-20v" <audi20v at rennlist.org>
> Subject: RE: Bump Steer
> Hi Bernie,
> I just thought that some folks with less of an engineering background (mine
> is light too) might benefit from a plain-language description.  Indeed, no
> corrections are needed for accurate definitions!
> I have not quantified the bump steer characteristics of my car, but I would
> be very interested in doing so.  I have been replacing suspension pieces for
> about 2 years now and most of them are new.  I am a bit of a purist and have
> kept my car very near to stock - the only exceptions (suspension-wise) are
> Boge Turbo-gas strut inserts (terminology flame not needed) and URQ blue
> strut bearings.  Both were put in by a trusted Audi specialist in Minnesota
> and finally aligned by a shop that made my car drive straight and true.
> However, the tires are now shot so it is still a bit squirrely.  I had my
> Speedlines refurbished and still run stock tire sizes.
> How does a DIY mechanic go about measuring bump steer characteristics?  I'd
> love to better understand why my car behaves as it does.
> Thanks,
> Aaron
> P.S.  What is your profession?
Retired Engineer, BSME, MSEE, MBA.  The only problem with retirement is that
one can't go back to work to rest up!

> -----Original Message-----
> From: Bernie Benz [mailto:b.m.benz at prodigy.net]
> Sent: Wednesday, October 03, 2001 9:44 AM
> To: Aaron Gjerde
> Cc: audi-20v
> Subject: Re: Bump Steer
> Aaron,
> You have restated or explained my definations, apparently without
> correction.  Thanks.
> Have you ever quantified the bump steer characteristics of your car?
> Bernie
>> From: "Aaron Gjerde" <gjerdea at pconline.com>
>> Date: Sun, 30 Sep 2001 12:12:21 -0500
>> To: "Audi 20v List" <audi20v at rennlist.org>
>> Cc: "Patrick Yam" <pkyam at bitstream.net>, <b.m.benz at prodigy.net>
>> Subject: Re: Bump Steer
>> Another way to put it is that when you go over a bump the toe setting
>> changes and it steers your car in a way you don't want.
>> I learned about this when snowmobiles started using independent suspension
>> on the front skis.  On a trail one ski would travel up over a bump and it
>> would run you off the trail even though you hadn't turned the handlebars
> at
>> all.
>> Similar deal for cars.  Hold the wheel straight and when you go over a
> bump
>> the car turns a little.  Very, very bad thing.
>> Roll steer is when your car rolls naturally in a corner and it changes toe
>> characteristics and it steers itself differently than where you were
> aiming
>> it.  Think about how bad that would be in a hard corner where you need
>> precision predictability.  Ouch!
>> Aaron
>> ----------------------------------------------------------------------
>> Subject: Re: Tire Monkeys revisited
>> From: Bernie Benz <b.m.benz at prodigy.net>
>> Date: Fri, 28 Sep 2001 07:51:21 -0800
>> X-Message-Number: 20
>> Bump Steer is the change in toe setting as the suspension moves through
>> its range of motion.  The ideal being zero bump steer, no change in toe
>> settings, front or rear, whatever the instantanious suspension condition.
>> A special condition of bump steer is called roll steer, having an
>> obviously more limited definition.
>> Bernie
>>> From: pkyam <pkyam at bitstream.net>
>>> Reply-To: Patrick <pkyam at bitstream.net>
>>> Date: Fri, 28 Sep 2001 09:01:27 -0500 (CDT)
>>> To: "Audi 20V" <audi20v at rennlist.org>
>>> Subject: [audi20v] Re: Tire Monkeys revisited
>>> Bernie,
>>> Can you please define Bump Steer? I am unable to extract the definition
>>> from your previous post. Thanks!
>>> Pat

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