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ZR1NET  January 2003

ZR1NET January 2003

Subject:

Re: TECH: Coil-Overs for ZR1s

From:

Andy Denslow <[log in to unmask]>

Reply-To:

Andy Denslow <[log in to unmask]>

Date:

Mon, 27 Jan 2003 10:27:25 -0600

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (113 lines)

So how does one go about lowering the upper control arm mount on the front?

----- Original Message -----
From: "Jim Milstead" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Monday, January 27, 2003 9:46 AM
Subject: Re: [ZR1] TECH: Coil-Overs for ZR1s


> Mike Kryger wrote:
> >
> > Chris
> > Stiffer front bar equals more grip at the front with more understeer or
less
> > oversteer.
> > Stiffer rear bar equals more grip at the rear with more oversteer or
less
> > understeer.
> > Mike
>
> OK, this is how things work. Sway bars (or anti-roll bars) control the
roll
> angle of the car but do not control the weight that is transferred.
Changing
> to a stiffer bar will only alter the proportion of the weight transfer
split
> between the front and the rear. The roll moment arm at each end of the car
> combined with the position of the CG between them is called the Roll
Couple
> Distribution which should be balanced by roll stiffness distribution to
> achieve a neutral or balanced car. That is what you are changing when you
> are playing with the sway bars. The end of the car with the higher roll
> stiffness transfers a larger portion of the total weight transfer. When
the
> rate of a bar is stiffened, that end of the car will transfer more weight
> and the opposite end of the car will transfer less. This then alters the
> dynamic weight supported by the tires.
>
> Just a quick recap about tires, any lateral weight transfer between a pair
> of tires results in a net loss of traction. So, if we reduce the amount of
> lateral weight transfer we can increase the speed of the corner. The two
most
> significant influences in weight transfer is the car's track width and the
> height of the center of gravity. This weight transfer occurs as a function
of
> a car's mass being above the road surface, and above the point at which
the
> forces originate, and has nothing to do with suspension.
>
> So what is the best bang for buck? I think that is what you are shooting
> for, right? OK, you have to look at the suspension itself to make that
kind
> of determination. At the stock ride height, the wishbone geometry is
parallel
> and unequal length, so there is very little camber change or scrub for
> small amounts of suspension travel. The downside to this layout is that
the
> camber goes negative relative to the chassis in both bump and droop, so
the
> camber of the inside tire goes the wrong way when the car rolls. The
outside
> tire camber doesn't change much, which requires a lot of static negative
> camber but at least it goes a little in the right direction.
>
> The good news about the camber curve is that lowering the car improves
> the characteristics of camber control. The virtual swingarm length gets
> shorter as the car is lowered, which helps the camber change of both front
> tires in roll. Lowering the car 1" from stock would be the maximum
desirable
> change from the standpoint of suspension geometry. Another way to get this
> improvement is taller wheels and/or tires, which would raise the ride
height
> back up after the spring mounts are lowered.
>
> The roll center as measured is just above ground level, about 0.11" or
> so. As the suspension moves, the distance from the chassis to the roll
> center is reasonably well controlled. It is highly desirable for this
> dimension to remain constant in order to minimize the cornering balance
> change due to bumps and weight transfer. As the car moves down 1.00" in
> bump, the roll center moves about 1.22" in the same direction, and vice
> versa in droop.
>
> The front suspension geometry has too much anti-dive and the anti-dive
> percentage changes significantly with suspension  travel. The anti-dive
> is produced by angling the upper wishbone in side view. At static ride
> height, the car has about 44% anti-dive. At 1" bump, 38%. At 1" droop,
> 49%. Ride height dependent anti-dive makes the cornering balance
> unnecessarily sensitive to one wheel bumps, two-wheel bumps, and roll
> while braking, producing unpredictable, varying amounts of understeer
> depending on road surface irregularities and braking force.
>
> The most valuable modification is to lower the front upper wishbone
> forward mount point at least 1.00" from stock. This will do three things.
> First, it will reduce the anti-dive to 24%, which will dramatically
> improve turn-in response. Second, it will improve camber control for
> racing use, producing a 233" virtual swingarm length at stock ride
> height. Third, it will raise the front roll center to 0.83" above ground
> allowing a lighter front anti-roll bar for a given roll stiffness.
>
> Hope this helps
>
> --
> The name is Bomb...   James Bomb
>
> Problems? Contact Dave Bright at: [log in to unmask]
> TOPICS: TECH: or NONTECH:
> http://www.zr1.net
>

Problems? Contact Dave Bright at: [log in to unmask]
TOPICS: TECH: or NONTECH:
http://www.zr1.net

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