Musings on snap oversteer and GM suspension tuning

[The Dark Side of Will]

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.

[Series8217]

For reference, stock rear 88 spring rate is about 142 lb/in. GM put a softer spring in for the 88s, added a rear swaybar for the Formula and GT. The 88 struts are a different part than the early ones, even though they are partiall interchangeable. I wonder how much different the stock damping is on the 88 struts compared to 84-87? I do have some OEM 88 struts with 35,000 miles on them.. but I don't have a strut/shock dyno.

My 88 feels soooooo much better with the rear swaybar off but really stiff rear springs... and I don't even have proper damping for them (yet). I currently have Monroe Sensatracs with 350 lb/in springs on adjustable coilovers. Once I get some Koni Sport struts worked out, all should be well. Definitely not putting the rear swaybar back on there.

[Pyrthian]

much of the problem is not the suspension
it is the short wheel base, and how far forward you sit for the rear axles.

in the vette, you are longer, which lets you feel it getting loose sooner, and you sit (proportionatly) ALOT closer to the rear axle. this lets you feel it alot more.

in a Fiero, the rear is much further behind you.

and, as you mentioned - the friggen steering.....it is so easy to "loose your wheel"

[Series8217]

Will, here's a good test.. you can disconnect the swaybar link on one side so the bar isn't doing anything but wont fall down.. then test drive and see if it fixes the problem. It's a lot easier than trying to remove the whole bar.

[Aaron]

My 88 feels soooooo much better with the rear swaybar off but really stiff rear springs... and I don't even have proper damping for them (yet). I currently have Monroe Sensatracs with 350 lb/in springs on adjustable coilovers. Once I get some Koni Sport struts worked out, all should be well. Definitely not putting the rear swaybar back on there.

Per your advice, I left my ear swaybar off (Yay weight savings), but I am on the stock 115k rear suspension with new poly bushings. I can't comment on how it handles compared to all stock yet as I haven't been able to drive it that spirited. My next mod will be suspension though.

How do you think the car will react with all stock suspension and the rear swaybar removed? Better?

[Blue Shift]

How much does stock tire size effect this? I ass-u-me that you're running stock GT sized rims/tires? Though we're talking stock 14's and pre 88 suspension, the sidewall flex on my duke car was nothing short of laughable - the whole car will yaw back and forth for a couple cycles after ziging right or left due to sidewall flex. :la:

[Series8217]

215 front 255 rear completely changed the feel of my car. It's MUCH more controllable in powerslides/etc now. I used to not be able to keep a handle on it before. It does like to push way more than before though. I upped the rear spring rate since (from 150 to 350) but haven't had a chance to drive it hard so I am not sure how that affected the balance.

[The Dark Side of Will]

I figured that disconnecting the rear bar would make it push like crazy. It doesn't have enough oomph to overcome that with power oversteer.

It does, however, light the inside tire up rather than braking both loose in tight turns in the rain. At the stock power level and with cheap tires, I think that's an indication that it's over-barred.

If you already have Koni reds, don't bother adapting the sports from something else... just send the reds back to Koni and have them re-valved. Discuss with a Koni engineer (spring rates, vehicle weight, etc) and figure out how much of an increase you want. They can also modify the reds to be double adjustable for a price that comes out to about $500 a corner.

The damping/stiffness effects we're talking about don't have much of anything to do with wheelbase. Porsche 997 has a shorter wheelbase than the Fiero, but nobody has heinous problems with it...

[p8ntman442]

Will, Take a look at 2005+ dodge power wagons, they have an electronically actuated sway bar disconnect, which could probably be adapted to fit. It wouldnt solve the problem, but it would allow you to have the bar for daily driving, and flip a switch to autocross without it. Or, with two different size bars, you could modulate between the two.

[The Dark Side of Will]

Electronic sway-bar disconnect? Wow... talk about gizmology.

I was thinking that once I put bigger rear tires on it, it will need stiffer rear springs. Maybe by then Steven will have driven his car enough to know if 350 without a sway bar is enough.

I have 325's on the back of the Northstar car, along with a front bar linked to the hub carriers instead of the control arms. It's been so long since I drove it hard, though, that it's difficult to remember exactly how it was. When I drove it recently after getting used to the Formula, it was obvious that the Koni reds were not enough for the 325 springs.

[Kohburn]

IMO the fiero worked best with stiffer springs and damping in the rear with no rear saybar. was much less prone to sudden changes like snap overstear.