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Re: What is he saying?

To: Edwin McCarroll <emccarroll@mco.edu>
Subject: Re: What is he saying?
From: James Nazarian Jr <jamesnazarian@netzero.net>
Date: Thu, 22 Mar 2001 13:45:29 -0700
On Thu, Mar 22, 2001 at 03:14:13PM -0500, Edwin McCarroll profoundly declared:
> But can anyone tell me the answer to :   'What is bump steer?'   

Change in toe settings during suspension travel.

> Is it something I want?   

NO

>Something I need?  

See above

>Something I need to acquire?    

no

>If so, where do I get it?  

You already have it.

>How do I know if I have it?

All front suspension systems have some

>Please help with my edification.    Is this topic mg list compatible?    

I hope so

>So many questions ... so few answers.

Ok, now that I've addressed your questions.  You set your toe when the car sits 
at normal ride height,  if you measure it at any other position in suspension 
travel it will be different, maybe not noticable to you but it will be 
different.  If you were to draw these measurements on a piece of paper, you 
will see an arc of toe settings, the one you wanted will be in the center of 
the arc, and you will be at max toe in/out (depending on design) at full droop 
and compression.  The idea is to make the radius of the arc as large as 
possible, i.e. make the line look straight, and keep your suspension as close 
to your toe setting as possible during suspension movement.  If you hit a bump 
or rut in the road and the two sides of the car compress equally then you won't 
feel this, but if you hit a bump and don't compress equally (i.e. most bumps) 
you will feel this as the car sharply pulling off the line you were traveling.  
The second place you feel this is also in braking.  Assuming that your brakes 
work evenly, i.e. you keep going straight on a smooth road when you apply the 
brakes.  In both cases the problem comes from your toe setting changing due to 
bump steer, and your unequally loaded front suspension has more grip on the 
heavy side and that tends to determine where you go.

Ideally toe is intended only to compensate for forces incurred in driving so 
that your tires stay square.  When traveling in a straight line, there is 
rolling and aerodynamic resistance to your tires,  these things compress rear 
a-arm bushes, and stretch fronts, etc.  The toe setting is ideally intended to 
compensate for that movement, so that when all forces set in, the flex will be 
cancelled out by the toe in.  It can be used for other minor influences in 
handling, but that is/was it's primary intention. 

In summary, bump steer is bad, but you have to live with some, just like the 
rest of us.  It this is one reason that people always tell you not to lower a 
car with cut springs, as you get further away from the design ride height, the 
rate the bump steer sets in increases, this means near full compression the 
rate might change 10x more per inch of wheel travel, than it did near design 
ride height.
-- 
James Nazarian Jr
71 MGB roadster
71 MGBGT - with V8 parts
01 Impreza 2.5RS

A complex system that does not work is invariably found to have
   evolved from a simpler system that worked just fine.

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