re: "Stepping out" over bumps

[Simon Favre <simon@domain.elided>]

The big reason they do this is a simple matter of unsprung weight. The
classic live axle setup (found on all but the latest Spiders we can't
get in the US) is right at the top of the list for unsprung weight.
The problem with a high unsprung weight is that once you get it moving,
it keeps moving. It's actually the mass, not the weight that kills you.
Once you launch the wheels upward from a bump in the road, the axle
keeps on going up until the tires lose contact, causing what's known as
rear axle tramp, or axle hop.

The other problem with the 105/115 Alfa specific rear end geometry, is
that the Trunnion (T-arm) places the rear roll center at an extremely 
high location. This is not good for the handling. this is what may have
led some experts to call the live axle setup on some American cars better.
Many Alfa racers convert to a Panhard rod for this reason. It lowers
the rear roll center. The GTA used a sliding pillar rear suspension,
but that's not much better than a properly setup Panhard rod. If it was,
there would be more of them on the track.

The other typical problem causing axle tramp on Alfas is having the
rear shocks dialed up too tight. What would actually be better is to
have double adjustable shocks, or shocks that have been re-valved for
a specific car weight, spring rate, etc. This is generally beyond the
amount of effort most street-driven Alfas need. I just run Koni reds in
the rear dialed all the way loose. It works well enough on our bumpy
ARA Autocross track. ;=)

Jay Negrin wrote:

>A good friend who is much more into vehicle dynamics than myself has said
>often that this behavior is common with live axle cars.  If this is indeed
>the case, I wondered, then why?
>
>Could be, that if the bump(s) are sharp enough, that the rear tires could
>lose their grip on the pavement.  Loss of traction at the back end equals
>oversteer, does it not?  It may be transient in nature, and control, i.e.
>traction, is restored in very short order.
>
>If this were the case, and the rate of travel were near the limit of 
>adhesion, then a bump like this could cause the car to spin, back end 
>first.
>
>Am I all wet on this, or does it sound as reasonable to the experts out 
>there as it does to me.

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