Suspension Questions

Did some more adjustments on the Daewoo Nubira and got some unexpected results. It's a FF all strut car with 500 lb springs on all four corners and no swaybars. It was lowered 1 inch in the front and .5 inches in the back. It's now a little more than an inch lower than stock in the back.

I lowered the back because on really high grip surfaces in quick transitions the car would jack up a little in the rear. By lowering the rear suspension I also lowered the roll center, which was the theoretical cure to the jacking problem. I assumed this would lower the effective roll stiffness in the rear and cause understeer.

However, in my observations, lowering the car has cured the jacking problem, increased oversteer and lowered the total amount of body roll.

The alignment didn't change, I didn't change the shocks, the tires and pressures are identical before and after.

What is happening?

If you're now on the bumpstops because of the lowered ride height, you'll have a much higher effective spring rate. Do you know that the roll centered lowered with the car?

I'm racking this through my brain. Would a stiffer front end maybe help. An upper roll bar caused less over steer in my escort when it was in a similar stance. (after re-reading I see you have NO bars. This wouldn't work in that case.) Believe your roll center is higher, though.

Which bump stops are you suggesting, Keith? Front? I can imagine that causing some over steer. I'm not the expert though, just exercising my brain.

what's the weight distro look like?

Is that 2.2 swapped in? Does what's the weight difference from the (I'm assuming) 2.0 that was in there?

Autolex wrote: what's the weight distro look like?

^^^This

Lowering the car would lower the roll center . As Keith said, if the car is now running on the bump stops it will increase the spring rate which in turn will reduce understeer. How did you lower the car ? cutting springs also increases the spring rate. It boils down to the general rule:

Unders steer- stiffen rear or soften front. Oversteer-soften rar or stiffen front. Also, a FWD car tends to push (understeer) under power and over steer on trailing or closed throttle.

iceracer wrote: It boils down to the general rule: Unders steer- stiffen rear or soften front. Oversteer-soften rar or stiffen front. Also, a FWD car tends to push (understeer) under power and over steer on trailing or closed throttle.

Is this what you want to do to correct the over/under or is it the result?

That's what you want to do to correct.

(salesman voice)How To Build a High Performance Miata covers the theory of handling, starting from the basics of weight transfer and how to use it to adjust your car. It's not just for Miatas!

The bumpstops I'm referring to are the ones you hit when the rear suspension runs out of travel. Basically, once you hit the limits of the suspension, the expected behaviors changes. It could be that adding a rear roll bar would cut down on roll, which would keep the car off the bumpstops and get rid of the oversteer under hard cornering. It's the opposite of what you'd expect to see, but those boundary conditions can be odd.

Keith wrote: If you're now on the bumpstops because of the lowered ride height, you'll have a much higher effective spring rate. Do you know that the roll centered lowered with the car?

I am nowhere near the bumpstops. I'm pretty sure I could rally this car without fear of bottoming out.

I lowered the rear suspension because I wanted a less severe roll axis inclination. Theoretically, this should have decreased the relative roll stiffness of the rear, but I'm observing more oversteer and less body roll over all.

When I lowered the rear, I thought I was trading some of my trail throttle oversteer for a cure to the jacking problem. It seems I've gotten rid of the jacking and gotten more oversteer. I'd like to understand how.

I can provide full specs for the virtual arms etc, if that helps.

Autolex wrote: what's the weight distro look like?

62/38.

N Sperlo wrote: Is that 2.2 swapped in? Does what's the weight difference from the (I'm assuming) 2.0 that was in there?

I actually went from a 1.5 (you don't get this motor in the US but it is almost identical to the Aveo 1.6 DOHC) to a 2.0L because of classing restrictions in the time attack format I want to run.

The 2.0 is the same basic architecture with a forged crank, slightly taller deck height and better cylinder head. I didn't weigh the two engines, but I can't imagine they are that far apart.

The difference can't be that big, because the factory says a 2.0L Australian market 2.0 hatch is lighter than a USDM 1.6 sedan.

No front swaybar? Oversteer? I might possibly link those two facts, but thats just me...

With a nose-down roll axis, there is a small yaw component with roll. The nose points out of the turn slightly, relative to the four contact patches. This makes the car feel steady to the driver, rather than twitchy.

For the weight distribution the rear spring rate may be too high. Still trying to figure how you lowered the rear 5 inches and still have bump travel. Did you alter the spring seats ?

Reread your original post. How did the car get so high that you were able to lower it five inches and end up one inch lower than stock. ? So basically you lowered the car one inch, not five.

Is it possible that the lowering car lowered the CG more then roll center thus increasing the roll stiffness in the rear? The roll center and the CG don't necessarily move up and down at the same rate but I have a hard time believe that there would be enough relative change in .5" to have that much effect.

iceracer wrote: Reread your original post. How did the car get so high that you were able to lower it five inches and end up one inch lower than stock. ? So basically you lowered the car one inch, not five.

He said it was lower .5 inches.

Ah, I see said the blind man. Maybe if he had said 0.5"

N Sperlo wrote:

iceracer wrote: It boils down to the general rule: Unders steer- stiffen rear or soften front. Oversteer-soften rar or stiffen front. Also, a FWD car tends to push (understeer) under power and over steer on trailing or closed throttle.

Is this what you want to do to correct the over/under or is it the result?

That's what you do to correct.

iceracer wrote: For the weight distribution the rear spring rate may be too high. Still trying to figure how you lowered the rear 5 inches and still have bump travel. Did you alter the spring seats ?

That was supposed to be .5 inches, ie, half an inch.

93EXCivic wrote: Is it possible that the lowering car lowered the CG more then roll center thus increasing the roll stiffness in the rear? The roll center and the CG don't necessarily move up and down at the same rate but I have a hard time believe that there would be enough relative change in .5" to have that much effect.

It's possible, I guess. I'll have to calculate the new roll center.

Perhaps it's the yaw component McCarp mentioned.

"With a nose-down roll axis, there is a small yaw component with roll. The nose points out of the turn slightly, relative to the four contact patches. This makes the car feel steady to the driver, rather than twitchy."

My car had a pretty radical roll axis inclination, less so now, I would assume.

Streetwiseguy wrote: No front swaybar? Oversteer? I might possibly link those two facts, but thats just me...

It was understeer biased when it had no front swaybar and a higher rear ride height.

I'm not unhappy with the balance now, I'm just seeking to understand it.

iceracer wrote: Unders steer- stiffen rear or soften front. Oversteer-soften rar or stiffen front. Also, a FWD car tends to push (understeer) under power and over steer on trailing or closed throttle.

I find that (at least in <70mph autocross land) that TLLTD is more important than this general rule that people seem to follow. The closer you get to 50% Front, the more "loose" the car is. IMO, you can actually set up a car to perform well below or above 50%, just don't stay too close to that number.

Regarding the original post, it seems to make sense to me. By lowering the rear roll center, the "tilt" that occurs transferring rear roll weight to the front outside tire is reduced, making it less likely to overload that tire and more likely to overload the rear/slide.

A lot of that come from the reduced camber change due to roll reduction.