[TR] tr3 best improvement

[Jim Muller]

On 20 Jul 2008 at 12:14, Dave1massey at cs.com wrote:

> When I mashed the gas the back end would squat and the camber
> would change in the positive direction and the rear tires would
> grab and the front end would plow.  Releasing the gas would
> cause the back end to pop up and the back end would come around.

And therein lies a lesson in geometry and genetics.  A semi-trailing 
arm suspension is somewhat like a swing axle, but with reduced 
effect.  With a swing axle the axis about which the wheel swings with 
suspension travel is longitudinal (mostly) so that the wheel tilts 
inward or outward.  A pure trailing arm has a transverse axis so that 
suspension travel rotates the wheel forward or backward while moving 
it up and down, thereby keeping the camber identical to the roll of 
the car's body.  When the inside end of the trailing arm is moved 
backward, thereby making it "semi-trailing", the wheel takes on some 
of the swing axle's camber behavior.  The purpose is so that as the 
body rolls in a turn the outside wheel, on moving up in the 
suspension travel, will gain less positive camber.  The tradeoff is 
that when the wheel moves down in the suspension, as a rear wheel 
will during decceleration, the camber will go correspondingly more 
positive.

By contrast, with a good unequal length A-arm setup such as is used 
in front the camber will go negative as it rises in the susepnsion, 
be nearly zero (or some chosen value) at normal ride height, but go 
also negative if the wheel moves falls below that position.  When you 
back off the gas you necessarily lose any negative camber you would 
have gained from rear-end squat, but you then gain it back again as 
the car pitches further forward.

The moral of the story, of course, is don't look down your nose at 
the humble swing-axle!  It is the genetic foundation of many a more 
pretentious IRS! :-)


-- 
Jim Muller
jimmuller at rcn.com
'80 Spitfire, '70 GT6+