Newsgroups: rec.autos.simulators
Path: news.jprc.com!newsfeed.sgi.net!feeder.qis.net!newshub.northeast.verio.net!cpk-news-hub1.bbnplanet.com!news.gtei.net!newsfeed.atl!news.acsu.buffalo.edu!freenet.buffalo.edu!bd427
From: Doug Milliken <bd427@freenet.buffalo.edu>
Subject: Re: Understeer in open wheeled sims are gonna get better
In-Reply-To: <365C2E74.672E@mci2000.com>
Content-Type: TEXT/PLAIN; charset=US-ASCII
Message-ID: <F342s1.GDE@freenet.buffalo.edu>
Sender: nntp@freenet.buffalo.edu
NNTP-Posting-Host: freenet.buffalo.edu
Organization: Buffalo Free-Net
References: <738nn8$fei$1@nnrp1.dejanews.com> <F2usLo.2zD@freenet.buffalo.edu> <73g7sh$2oq$1@nnrp1.dejanews.com> <365C2E74.672E@mci2000.com>
Mime-Version: 1.0
Date: Sat, 28 Nov 1998 02:54:49 GMT
Lines: 66
Xref: news.jprc.com rec.autos.simulators:60453


On Wed, 25 Nov 1998, Pat Dotson wrote:

> Yes, that is _the_ Doug Milliken :)

Right -- Just got back from the latest NASCAR Silicon Motor Speedway (Nyack
NY).  It should be running today (they were giving out free test rides on
Wednesday before Thanksgiving).  Shameless plug--it's in the new Palisades
Center Mall, near the Tappan Zee Bridge. 

Pat's plot of lateral force (LF or Fy) vs slip angle (SA) (below) is good,
but I've changed the axis labels (to match standard terminology). 
Sometimes there isn't any drop-off at the peak at all, the peak lateral
force stays about constant at the peak value.  The lateral force is
measured (defined) at the road surface, and lined up with the axle
(perpendicular to the center plane of the wheel, the "y" direction.)


>      |         ---
>      |        /   \
>      |       /     -------------
>      |      /
> LF   |     /
> lbs  |    / 
>      |   /
>      |  /
>      | / 
>      |/
>       --------------------------- 
>          SA (degrees)


> When the tire passes the peak slip angle, the friction force drops, but not
> as drastically as you suggest.  There will still be considerable force
> rolling the tire sidewall, even while skidding at 90 degrees, so the tire
> will not "snap back" in any visually percievable amount.  In any event, the
> transition is gradual. 

Now to aligning torque (AT)--(another post in this thread).  AT vs slip
angle (SA) looks different than lateral force vs SA.  The AT rises to a
peak and then drops to near zero torque once the tire is at peak lateral
force.  The tire does get "twisted" at low slip angles because the front of
the print is just getting set onto the road, while the back of the print is
"stretched" sideways.  Once the whole print is sliding (above the peak LF)
then there isn't much difference between the sliding velocity (and force)
at the front and rear of the print.  Crudely like the plot below, where SA
is to the same scale as the plot above. 

 AT     |     /\
 lb-ft  |    /  \
        |   /    \
        |  /      \
        | /        \
        |/          --\
         ---------------\====\---------======
           SA (degrees)        \------/

The figures and explanation that we put in Chapter 2, of "Race Car Vehicle
Dynamics" are much more complete.  Another shameless plug-- Get RCVD now
from Classic Motorbooks at the old price of $85 -- SAE just raised all the
prices at their in-house bookstore, RCVD is now $89 from SAE. 

-- Doug
		Douglas Milliken  <bd427@freenet.buffalo.edu>
		Milliken Research Associates Inc.

