In designing the suspension for the UC Davis Formula SAE we've settled on a
KPI of 11.3 degrees and caster in the 5.7 degree range. Not that were
experts. I'd call this a decent amount of KPI and caster, and surprisingly
falls under the caster=1/2 KPI that you meantioned that I'd never heard. We
have alot of camber gain when turning, something like 2-2.5 degrees from
straight to full lock.
I wouldn't imagine your KPI causing your wheels to go positive. It may be
more your positioning of your upper control arm. The relation of KPI and
static upper control arm [UCA] position (relative to Lower control arm
[LCA]) made the greatest effect on camber gain during roll when I was doing
my calculations. Its kinda hard to describe over email but static if your
LCA is near parallel to the ground and your UCA is angling upward and
outward of the car, then the UCA arm will go through a greater change in
angulation during suspension compression. This pulls the top of the upright
inward (remember that the UCA inboard mounts are also being pushed out due
to body roll) and creates roll camber gain.
I'd say leave the KPI where it is for now and move the inboard UCA pickups
downward to provide more roll camber gain. In the case that you may have
downwardly angled UCA's move the inboard pickups up.
Also are you using any software? I picked up Susprog3D from
http://www.susprog.com . Its about $100 after the austrailian currancy
conversion. Its not the most intuitive program but it probably has some
templates that will get you started and if you take carefull measurments of
your car the calculations should be fairly acurate.
Good luck,
Ian Green
2003 Team Coleader
UC Davis Formula SAE
http://mae.ucdavis.edu/~fsae
97 Honda Civic CX
http://www.geocities.com/stscxr
"I never let school get in the way
of my education." - Mark Twain
> -----Original Message-----
> From: owner-ba-autox@autox.team.net
> [mailto:owner-ba-autox@autox.team.net]On Behalf Of James Creasy
> Sent: Wednesday, February 19, 2003 8:22 AM
> To: Rick Brown; ba-autox@autox.team.net
> Subject: Re: fatal flaw in my car?
>
>
> it would be the same thing as the KPI, although i think technically i dont
> have a "king pin".
>
> i can change the KPI by remounting the upper control arm lower
> and closer to
> the wheel.
>
> the reason a large KPI is bad is that it causes the camber to go
> positive as
> the wheels are turned. someone suggested the caster should be 1/2 of the
> KPI. scrub radius is something i am just learning about, so not sure what
> to do about that yet.
>
> thanks!
>
> -james
>
> ----- Original Message -----
> From: "Rick Brown" <rbrown7@covad.net>
> To: "James Creasy" <james@thevenom.net>; <ba-autox@autox.team.net>
> Sent: Tuesday, February 18, 2003 4:26 PM
> Subject: RE: fatal flaw in my car?
>
>
> > James,
> >
> > Are you talking about king pin inclination (KPI) or Caster?
> > KPI determines KPI trail, also called scrub radius. KPI is the angle
> > between the upper and lower ball joint centerlines as viewed
> longintudanlly.
> > Increasing KPI trail increases kick back through the steering wheel when
> the
> > tires
> > are subject to aysmetric forces. For this reason, when
> > ABS became real common most cars went to KPI angles that gave very
> > small scrub radii to minimize the kickback to the steering
> wheel under ABS
> > activation.
> > KPI is usually not adjustable and that is why when you change the
> > stock wheel offset and theefore change the KPI trail you can get
> > more kickback from the ABS (and more torque steer on front driver cars).
> >
> > Caster is the rearward inclination of the axis between the ball joints
> > and is adjustable, within limits, on most cars. This is what causes
> camber
> > gain when
> > the wheel is turned (and also lifting you mention).
> > It's primarily there to increase straight line stability and the higher
> > the caster trail the more restoring force returning the wheels
> to straight
> > ahead while
> > in motion. The camber gain is ususally a side benefit for handling
> whereas
> > the lifting is a side detriment.
> >
> > So, I don't see where a large spindle inclination is in and of itself
> > going to cause handling problems. Look at the scrub radius,
> > caster, static negative camber, toe, roll centers, springs,
> shocks, camber
> > curves, tires, rim sizes etc. to determine why your car isn't
> handling the
> > way you want it to.
> >
> > --- Rick Brown
> > BP Corvette
> >
> >
> >
> >
> > > > 650-704-0771
> > > >
> > > > -----Original Message-----
> > > > From: owner-ba-autox@autox.team.net
> > > > [mailto:owner-ba-autox@autox.team.net] On Behalf Of James Creasy
> > > > Sent: Sunday, February 16, 2003 11:53 PM
> > > > To: ba-autox@autox.team.net
> > > > Subject: fatal flaw in my car?
> > > >
> > > > someone pointed out that my car has a very high Spindle Inclination
> > > > Angle,
> > > > about 15 degrees. this apparently causes my negative camber to go
> > > > positive
> > > > when i turn the steering wheel (plus it jacks up the front of the
> car),
> > > > and
> > > > is likely the cause of the huge amount of understeer i get exiting
> > > > corners
> > > > (recently i noticed it entering corners too). this is really
> > > > depressing,
> > > > since i think my only choice is to remount the upper a-arm lower and
> > > > make a
> > > > new bracket for the spindle to position the ball joint inside the
> > > > wheel...
> > > > which also means i cant use my 15" wheels.
> > > >
> > > > what is an acceptable amount of SIA for an autocross car??
> any other
> > > > ways
> > > > to compensate for it?? thanks,
> > > >
> > > > -james
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