Hey John,
I'm with Nick on this, if the car were getting loose going with the
rear wheels trying to pass the fronts under braking, I'd say dial
out some rear brakes and your done. But "darty" behavior under brakes
useually eminates from the front end. While you could start by checking
bump steer (not too hard to do after all), it would be helpful to know
a little more about your set up. Alignment (including ride hight), tire
and wheel combo (including wheel backspacing), front and rear spring
rates, car weight and F/R %, and shock selection.
Specific items that might cause darty behavior under braking include:
- Excesive bump steer (as mentioned below).
- Excesive nose dive due to soft front springs or
insufficient rear reabound damping, this will
dynamicaly take positive caster out of the font
suspension robing stabilty and reducing side force
when you crank the wheel over.
- To much static toe-out in the front end alignment.
Mustangs have a fair amount of akerman built in
so 0 toe to 1/8" in is fine for the track.
- Static caster settings of less than 2 degrees
positive and/or more than 0.25 degrees varience
side to side.
- Static camber not the same within 0.1 degrees.
Also negitive camber much beyond 2.5 degrees
may negitively (Heh!) affect braking at the
front depending on tire choice (radials are
much more tollerent of this).
- Wheels with a back spacing that results in a
large scrub radious (i.e., the tread center
line has moved much out board of the point on
on the ground where the line defined by the
upper and lower ball joints lands.
- Excessive compliance in any of the front suspension
pivots, especially the strut rods or tie rods, but
also the steering box.
Heh... that's the end of my brain dump. Hopefully some of
this blather will help.
At 09:23 PM 10/13/04 -0400, you wrote:
>I am looking for some opinions on how to improve (if possible) my car
>set up. It's a bit darty under threshold braking, most noticeable
>when braking on a downhill such as turn 1 at Watkins Glen. The calipers
>are not hanging or anything and I don't think I have any real mechanical
>issues - its just that its hard for me to press braking zones when I don't
>have confidence that the car will slow in a straight line.
>
> I suspect I need to back off the rears with the proportioning valve,
>front disks - rear drums. Anyone have any guidelines on setting the
>proportioning valve? Would shock adjustment help any? Anything else I
>should consider?
>
><snip>
>
>John-
>
>Is it actually lack of brakes or a "darty" car? The bump steer on those ole
'Stangs was not the best and if you modified the a arm mounts ala Shelby, it
can become downright diabolical. Pop a spring out and check your bumpsteer,
the alternative will really give you religion at speed!
>
>20-20 hindsite...sold that Shelby for peanuts...
> Nick in Nor Cal
All the Best! Dave ;^)
'66 Formula S Auto Barracuda, CP "Light"/Vintage (GoldFish), Gold
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'66 C code 5 Speed Mustang Coup, CP (Ol'Beastly), Red
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