Re: Rear Shock Info???

[STEADIRIC@aol.com]

Bob Damato asked on friday night:

>Can you explain bump steer to me better than other people? I have a good 
>idea, but not positive. Thanks!

Nerd Mode On......

Bump steer is caused by the suspension operating outside of it's optimal 
designed stroke limits.  All supension members operate in arc's, 
unfortunatly all the members do not operate in the same arc's, this is 
what cause's our problems. (hold on a second need to open up another 
"Anchor Steam"... Thanks Scott.......)
The steering rack operates through tie rods, which operate a steering arm 
mounted to the front or rear of the "Upright" or strut tube.  Front 
mounted steering arms are referred to as "Front Steer" and Rear mounted 
are know as "Rear Steer"  this is important to know because this defines 
our bump steer curve.

Lowering or raising the car will place the steering arm in a different 
location relative to the steering rack.  Lowering the car will raise the 
steering arm's relative to the rack and vice versa. 

The factory placed all of the suspension parts according to several 
factors....  Some Money, some ease of build at the factory, some Stress 
loads that they want to design for (Which open's up another can of worms 
in-itself) and some suspension geometery. Some times it's more cost or 
space effective to design a car that has a very small normal operating 
range for it's suspension, like +/- 1.00". Some cars (Like Ferrari's and 
F-1 cars) have VERY well designed (Read big $$$$) suspension which may 
have a larger overall operating range.

Okay you ask, What does all that have to do with the two evil words "Bump 
Steer" .... LOTS!

If you lower a front steer mass produced car more than, oh say 1" You 
WILL get bump steer. you can actually rasie that same car 2" over static 
ride height before things get to ugly.  A rear steer car will behave in 
the exact oppisite. In a front steer car when the suspension deflect's up 
(Bump) the steering arms get "Longer" causing toe-out, at normal ride 
height this effect will be very small something like 1/16" per 1" of 
travel per wheel.  If you lower the car 1.5" this effect could change to 
something like .75" per 1" per wheel (Total toe-out 1.5").... Not good.  
In a rear steer car the toe-out becomes toe-in.

Toe-out is not very desireable because it cause the car to dart in the 
direction of most traction or weight, Toe-in on the other hand does not 
cause the car to change direction, it just cause more tire, suspension 
and driveline wear. Rear steer is obviously more desireable, but auto 
makers like front steer for many reason, like it turns tighter, has more 
leverage for less power assist, better Ackerman charicteristics, better 
flat tire response etc, etc.

If you have ever been in a front steer car (I think somthing like 90% of 
all new cars are front steer) that has been lowered to much you will 
notice that the car is VERY nervous.  A rear steer car will just feel 
dead and not want to turn.

Hopes this helps.  If you or anyone else has any questions fire away.

Nerd mode off.

Hmmmmmm maybe that's where my ride height is where it is......



Later!

Eric Fletcher
'87 5KCSTQIA2RSR2B

STEADI RIC@aol.com