Musings on snap oversteer and GM suspension tuning
Posted: Sun Feb 03, 2008 10:10 pm
I have Koni dampers all around and rod end lateral links in the rear of my Formula. Otherwise the suspension is stock. The struts are set full stiff and the shocks one turn from full soft. Tires are cheap street tires in stock sizes.
This seems to be a pretty good combo for most things, BUT I've noticed that when I hang the back end out and am not perfectly smooth reeling it back in, the rear end can "snap" back in the opposite direction of the initial slide. This is the reaction of a vehicle that is *underdamped* in roll and is tough to catch with the Fiero's slow steering. Since the Konis at full stiff seem to be very well matched to the spring rate in jounce and rebound, the only thing that could be putting the extra roll stiffness into the system is the rear anti-roll bar. I surmise that the struts were developed for the early cars and then not updated for the '88's with rear bars.
This returns me to previous conclusions about GM suspension tuning. GM, as all MFG's, tunes cars for a compromise between ride and handling. GM's corporate compromise involves fairly soft ride quality with pretty flat cornering (flat cornering is NOT the same as good handling). To do this they use soft springs and stiff roll bars. This creates a significant mismatch between the chassis' jounce/rebound stiffness and roll stiffness. The dampers can only correctly match one of these stiffnesses. In practice this means that they match neither but are themselves a compromise.
This is obviously the case in the '88 Fiero because the chassis' behaviour indicates that the rear springs are relatively soft compared to the size of the rear bar.
I've also observed this behaviour in a C5 Corvette I test drove. The car cornered very flat and had lots of grip, but bumps, uneven pavement or undulations in mid-corner induced a relatively large amount of vertical chassis motion. Compared to the flat cornering attitude, the vertical motion seemed unsettled. This may be an indication that the vertical motion was somewhat overdamped, while the roll stiffness was somewhat underdamped.
The car didn't snap transition like the Fiero because the damping compromise was different. The Fiero had the jounce/rebound correctly damped and the roll stiffness underdamped.
This seems to be a pretty good combo for most things, BUT I've noticed that when I hang the back end out and am not perfectly smooth reeling it back in, the rear end can "snap" back in the opposite direction of the initial slide. This is the reaction of a vehicle that is *underdamped* in roll and is tough to catch with the Fiero's slow steering. Since the Konis at full stiff seem to be very well matched to the spring rate in jounce and rebound, the only thing that could be putting the extra roll stiffness into the system is the rear anti-roll bar. I surmise that the struts were developed for the early cars and then not updated for the '88's with rear bars.
This returns me to previous conclusions about GM suspension tuning. GM, as all MFG's, tunes cars for a compromise between ride and handling. GM's corporate compromise involves fairly soft ride quality with pretty flat cornering (flat cornering is NOT the same as good handling). To do this they use soft springs and stiff roll bars. This creates a significant mismatch between the chassis' jounce/rebound stiffness and roll stiffness. The dampers can only correctly match one of these stiffnesses. In practice this means that they match neither but are themselves a compromise.
This is obviously the case in the '88 Fiero because the chassis' behaviour indicates that the rear springs are relatively soft compared to the size of the rear bar.
I've also observed this behaviour in a C5 Corvette I test drove. The car cornered very flat and had lots of grip, but bumps, uneven pavement or undulations in mid-corner induced a relatively large amount of vertical chassis motion. Compared to the flat cornering attitude, the vertical motion seemed unsettled. This may be an indication that the vertical motion was somewhat overdamped, while the roll stiffness was somewhat underdamped.
The car didn't snap transition like the Fiero because the damping compromise was different. The Fiero had the jounce/rebound correctly damped and the roll stiffness underdamped.