Setting Your C4 Up for Auto-X
01-14-2014, 02:24 PM
#22
Burning Brakes
Awesome write up!
Jacking occurs on an independent rear where there is a single lateral swing arm (the axle shaft). The cornering force applied to the bottom of the tire leverages the suspension arm and forces the rear tire into massive positive camber, while "jacking" the rear of the car vertically.
IMO, the only reason the '84 isn't a killer autox car is the engine. Not the stiff suspension.
Jacking occurs on an independent rear where there is a single lateral swing arm (the axle shaft). The cornering force applied to the bottom of the tire leverages the suspension arm and forces the rear tire into massive positive camber, while "jacking" the rear of the car vertically.
IMO, the only reason the '84 isn't a killer autox car is the engine. Not the stiff suspension.
Your description is exactly what I experienced with a 944 that I did a lot of work on in the 90's when I first got the car I could feel the back of the car rise and then roll. Horrible in sweepers. It was as if the back end was trying to climb over the top of the outside rear tire. When I started working on the car, first a M030 setup then aftermarket, getting stiffer especially in the back, fixed it. Unfortunately My Vette project has sat in a back corner of the shop because my oval track suff has eatin up all my motorsport resources. The limited time I spent in the 84z felt like the back was probly close but the front was way too soft. There is just to much that is to worn to really get a handle on where I need to go. My plans are to get close to the R9G setup. I have been able to buy some parts from Tommy Morrison's program from a CF member
."reason the '84 isn't a killer autox car is the engine" ouch! truth hurts! That xfire setup looks really cool hangin on the shop wall though
.btw Thanks to all the contributers. Threads like this are solid gold for me
N I will hang out with the Shop teachers before those geeky english teachers any day!
01-17-2014, 11:06 PM
#24
Safety Car
Thread Starter
Awesome write up! Wish more members contributed as well as you have or do!
You asked for input corrections...I have one: your explanation of "jacking" is not quite right; Jacking occurs on an independent rear where there is a single lateral swing arm (the axle shaft). The cornering force applied to the bottom of the tire leverages the suspension arm and forces the rear tire into massive positive camber, while "jacking" the rear of the car vertically. The "Jacking" is detrimental (relatively speaking) as much as the camber change is. The camber change reduces the amount of tread on the road, and the effectiveness of the tire. Grip goes away, car goes into massive oversteer.
The stiff springs on an '84 do not cause jacking, and in fact, would mitigate it; it's a rate and it decreases as fast though rebound as it increases through compression. If the pressure drops as the wheel moves in rebound, how is the stiff spring contributing more to 'jacking', then a softer one? It can't.
Also, the '84 (and all 'Vette's since 1963) use a two lateral arm rear suspension which does two things;
1. It reduces or eliminates the forces that cause jacking
2. It allows the tire to stay perpendicular to the road during suspension travel in corners.
IMO, the only reason the '84 isn't a killer autox car is the engine. Not the stiff suspension.
Also a stiff sway bar doesn't contribute to jacking either, It can't, and in fact it would also mitigate any jacking effect. A "too stiff" rear spring, a "too stiff" rear sway, or a "too stiff" combination of the two items causes or contributes to oversteer because the weight transfer is more dramatic on the stiffer end of the car...forcing that end of the car to rely on one tire (more so) rather than two, for grip. Imagine if you had no sway up front, and a very compliant front set up. In the rear, you weld the suspension to the frame of the car so it's like a go-kart. When cornering, ANY body lean will transfer 100% of the load onto the outside tire, in the welded rear. In the front, the suspensions travel and compliance allows a near 50/50 weight distribution- both tires are splitting the load. So the car looses grip in the rear. Oversteer. Not jacking, just too stiff a rear, which is tuneable in the swaybar.
You asked for input corrections...I have one: your explanation of "jacking" is not quite right; Jacking occurs on an independent rear where there is a single lateral swing arm (the axle shaft). The cornering force applied to the bottom of the tire leverages the suspension arm and forces the rear tire into massive positive camber, while "jacking" the rear of the car vertically. The "Jacking" is detrimental (relatively speaking) as much as the camber change is. The camber change reduces the amount of tread on the road, and the effectiveness of the tire. Grip goes away, car goes into massive oversteer.
The stiff springs on an '84 do not cause jacking, and in fact, would mitigate it; it's a rate and it decreases as fast though rebound as it increases through compression. If the pressure drops as the wheel moves in rebound, how is the stiff spring contributing more to 'jacking', then a softer one? It can't.
Also, the '84 (and all 'Vette's since 1963) use a two lateral arm rear suspension which does two things;
1. It reduces or eliminates the forces that cause jacking
2. It allows the tire to stay perpendicular to the road during suspension travel in corners.
IMO, the only reason the '84 isn't a killer autox car is the engine. Not the stiff suspension.
Also a stiff sway bar doesn't contribute to jacking either, It can't, and in fact it would also mitigate any jacking effect. A "too stiff" rear spring, a "too stiff" rear sway, or a "too stiff" combination of the two items causes or contributes to oversteer because the weight transfer is more dramatic on the stiffer end of the car...forcing that end of the car to rely on one tire (more so) rather than two, for grip. Imagine if you had no sway up front, and a very compliant front set up. In the rear, you weld the suspension to the frame of the car so it's like a go-kart. When cornering, ANY body lean will transfer 100% of the load onto the outside tire, in the welded rear. In the front, the suspensions travel and compliance allows a near 50/50 weight distribution- both tires are splitting the load. So the car looses grip in the rear. Oversteer. Not jacking, just too stiff a rear, which is tuneable in the swaybar.
The other thing your forgetting is that the C4 and earlier Corvettes have a FIXED length upper control arm. The rear tires DO NOT stay level with the road like C5 and up. There is camber change with suspension travel, has to be with the fixed shaft. This is kept to a low level thanks to the low pivot swing axle design of the C4. Which got lower in 88 with the new mount for the rear lower control arm.
That being said, C4's have a very good rear suspension just not as good as newer cars, but much better than its peers or past sports cars that had basic swing axles. Which jack like a SOB.
I'm no expert in suspension dynamics but I know what works and doesn't. The 84 Z51 spring would work just fine with a front to match. If you put it in my car, it would totally UN-driveable.
Last edited by 93Rubie; 01-17-2014 at 11:14 PM.
01-17-2014, 11:13 PM
#25
Safety Car
Thread Starter
I'm working on a Bachelors degree and so for, no issues.
At only 26 years old your lucky I'm speaking in complete sentences with punctuation. U lik thm appls?
01-17-2014, 11:18 PM
#26
Safety Car
Thread Starter
Thanks for the write up, myself and many others appreciate your efforts. My car has been a project for many years and many years ago I sent my FX-3 shocks in and they came back with new dark yellow "Bilstein" paint and decals and I had them revalved to the Morocca. Since you hurt my cars (91 ZO7) feelings, can you tell us what the "Morocca" valving is? I can't remember but I didn't send in my controller.
Hib Halverson mentions the valving in this article. http://www.corvetteactioncenter.com/...src_page2.html
01-18-2014, 04:51 PM
#27
Melting Slicks
The Morocca valving is a stiff valving set. Your Z07 will be just fine, its a good setup, just watch the rear roll stiffness, you may find its too much, when sticky tires come into play.
Hib Halverson mentions the valving in this article. http://www.corvetteactioncenter.com/...src_page2.html
Hib Halverson mentions the valving in this article. http://www.corvetteactioncenter.com/...src_page2.html
01-19-2014, 04:42 PM
#28
Max G’s
Inaccuracies as far as what? Grammar and so forth. Well pardon me. I'm not an English major. I work on cars for a living and the last real English class I had WAS in HS. I took a couple technical writing classes in college and no one had a problem with it.
I'm working on a Bachelors degree and so for, no issues.
At only 26 years old your lucky I'm speaking in complete sentences with punctuation. U lik thm appls?
I'm working on a Bachelors degree and so for, no issues.
At only 26 years old your lucky I'm speaking in complete sentences with punctuation. U lik thm appls?
Good job on providing some insight and help to new comers to the sport.
As far as the jacking issue, I myself have not really experience this in any of my C4's. My late 91 stock 6 spd was a stock suspension and the modded 91 started out stock and was modded over time. And anytime there was a change to the front there was also a complementary change to the rear. Tom mentioned shock rebound issues in a different post, Could this be the issue with the "jacking"? A person updates to a stiffer rear spring and/or anti-roll bar and doesn't make a change with shocks and experiences the "jacking" issue or feeling?
01-19-2014, 04:59 PM
#29
Team Owner
I read through a lot of stuff that doesn't pertain to me and a lot of stuff that I'm not going to do to run autox. However, I would like to hear some ideas on tire pressure settings with stock wheels and tires on my 94 convertible.
01-19-2014, 07:56 PM
#30
Safety Car
Thread Starter
Good job on providing some insight and help to new comers to the sport.
As far as the jacking issue, I myself have not really experience this in any of my C4's. My late 91 stock 6 spd was a stock suspension and the modded 91 started out stock and was modded over time. And anytime there was a change to the front there was also a complementary change to the rear. Tom mentioned shock rebound issues in a different post, Could this be the issue with the "jacking"? A person updates to a stiffer rear spring and/or anti-roll bar and doesn't make a change with shocks and experiences the "jacking" issue or feeling?
I'm reloading my document with some revisions. The whole jacking thing really isn't important the roll stiffness front to rear IS. I don't want to confuse newcomers so well simplify the issue to what is important.
01-19-2014, 08:00 PM
#31
Safety Car
Thread Starter
I'd start with about 40psi and lower it until you see the rollover be about optimal. Tire pressures are a bit of a art like a I said. They can change the feel/performance of a car quite a bit, but that is a whole other paper if I wanted to do that.
Long story, short. You NEED enough pressure to keep the tire from rolling over on the sidewall.
01-19-2014, 08:22 PM
#32
Le Mans Master
I also have never had any of this jacking effect nor know anyone else who has.. Is this caused by the rear suspension binding up?
01-19-2014, 08:25 PM
#33
Team Owner
Setting tire pressure is a bit of an art. To do it simply, get some crayon or some shoe polish and "paint" the edge of the tread where it meets the sidewall. Do this in about 4 places around the tire, do all 4. Then run the course and you will be able to see just how far the tire rolled over. You want it to roll close to the sidewall but not onto it. Look for the little triangles on the tread edge, that is about the limit where you should allow the tire to roll over.
I'd start with about 40psi and lower it until you see the rollover be about optimal. Tire pressures are a bit of a art like a I said. They can change the feel/performance of a car quite a bit, but that is a whole other paper if I wanted to do that.
Long story, short. You NEED enough pressure to keep the tire from rolling over on the sidewall.
I'd start with about 40psi and lower it until you see the rollover be about optimal. Tire pressures are a bit of a art like a I said. They can change the feel/performance of a car quite a bit, but that is a whole other paper if I wanted to do that.
Long story, short. You NEED enough pressure to keep the tire from rolling over on the sidewall.
01-19-2014, 09:19 PM
#34
Team Owner
Member Since: Oct 2004
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St. Jude Donor '05
Ah, another one who found an X or anything gives no real power gain. Post that often and break out the haterade 
Kills me thinking someone here sold their car due to obsessing over a "what if an X pipe gave me ___hp" number. lol
Kills me thinking someone here sold their car due to obsessing over a "what if an X pipe gave me ___hp" number. lol
01-20-2014, 10:57 AM
#35
Team Owner
Pro Mechanic
"Jacking" is the wrong term. "Jacking" is when the rear suspension geometry is such that forces imparted on the suspension, literally lift ("Jack") the body of the car up.
This has nothing to do w/spring rates, and in fact a higher spring rate would reduce this tendency. How? It's stiffer so it will "push up harder" than a soft spring. In compression, the spring will "push harder" than a softer spring, per inch of compression, but likewise, the rate will fall off faster when in rebound. So if you have a car with geometry that creates jacking, then the stiffer spring in rebound, will contribute to the jacking, to a lesser degree than a softer rate sping...per inch of rebound travel, or degree of body roll, in this case.
The 'Vette that goes into "snap oversteer" does so mainly because the roll stiffness in the rear is too much, as compared to the front, and is not because of spring rate causing "jacking". Suspension geometry is what causes Jacking.
01-20-2014, 08:08 PM
#37
Burning Brakes
fwiw Driver feed back and terminology can be very tribal. A common definition of "snap over steer" is when a car is pushing then hooks up and rotates. The back end can not follow the sudden rotation and snaps loose. The stiffer the setup the more the sudden rotation (that may be very subtle) will have a lateral force on the rear contact patch. A soft setup will delay and smooth the G spike on the contact patch. A setup in the back that is stiffer than the front will overload the back and pretty much be loose everywhere. One thing I noticed about production Vette setups is that some are intended for a square tire sizing where some are intended for a staggered arrangement with larger tires on the rear. The staggered set up will need more roll resistance in the rear. Also the roll resistance is heavily influenced by the moment arm of the CG to the roll center. The changes in geometry between the early and late cars may look slight but if the geometry was plotted out it my have a very significant influence on a good setup for an early car vs a late C4. The bottom line, If your fast you aint too far off!
01-20-2014, 10:36 PM
#38
Safety Car
Thread Starter
^Evidently the factory understood this as the spring rates from early cars to the later ones changed quite a bit mostly in the front for the Z51/Z07 cars.
@Tom, we are on the same page now.
@vinniemc, good luck with that. I've been down that road. Its not fun, trying to find parts, but I did it. Its worth it. The stock FE1 suspension also on the FX3 really sucks in comparison to the Z07 setup, SOOOO much tighter and flatter in cornering attitude and manners. If you have any ????'s ask away. I've probably ran into it with my own conversion. Honestly, its easier to just find a Z07 car. LOL.
@Tom, we are on the same page now.
@vinniemc, good luck with that. I've been down that road. Its not fun, trying to find parts, but I did it. Its worth it. The stock FE1 suspension also on the FX3 really sucks in comparison to the Z07 setup, SOOOO much tighter and flatter in cornering attitude and manners. If you have any ????'s ask away. I've probably ran into it with my own conversion. Honestly, its easier to just find a Z07 car. LOL.
01-20-2014, 10:37 PM
#39
Safety Car
Thread Starter
Thanks,, I ran 37 lbs last year (my first auto x in a c4) and it seemed ok. I got about 1/8" of scuffing on the side of the tread and it didn't seem to go up the sidewall past the slits any further than that. I could control the slide with the throttle, so I didn't try any other settings.
07-07-2016, 05:57 PM
#40
Racer
I'm bringing back an oldie but a goodie!
I have an 89 FE1 car that I installed a 32/24 sway bar combo on a few years ago along with Bilstein sport shocks. The car is also lowered. The improvements were very good and it doesn't push bad during the occasional autox's I've done. I scored a NYU spring today off an 88 Z51 and was wondering if used with the FHA (93.1) front spring, if I am going to be neutral, slightly oversteer or lots of oversteer. This total package is almost identical to a 92 Z07 other than my front spring being 93.1 vs 90.1 and my front bar being 32 vs 30. I'm still going to look for a front FHB Z51/Z07 spring but was wondering if this setup will be good in the mean time. Thanks folks!
I have an 89 FE1 car that I installed a 32/24 sway bar combo on a few years ago along with Bilstein sport shocks. The car is also lowered. The improvements were very good and it doesn't push bad during the occasional autox's I've done. I scored a NYU spring today off an 88 Z51 and was wondering if used with the FHA (93.1) front spring, if I am going to be neutral, slightly oversteer or lots of oversteer. This total package is almost identical to a 92 Z07 other than my front spring being 93.1 vs 90.1 and my front bar being 32 vs 30. I'm still going to look for a front FHB Z51/Z07 spring but was wondering if this setup will be good in the mean time. Thanks folks!