C4 Thread: triple prime

[Joe C.]

What was the first year that zero scrub radius steering geometry was used on a C4 Corvette?
What steering/handling concern did it address?

"There are several reasons that the '85-87 C4s make wonderful autocross cars. The old L98 just can't be matched for bottom-end torque-it'll damn near pull stumps out of the ground. The LT1s, LT4, LT5s, LS1s, and LS6s are all great horsepower engines, but each and every one of them lacks the raw low-end grunt of the old Tune-Port Injection 350. Fifteen years of technology has produced some great engines, but none of 'em can match the low rpm of a well-tuned L98."

----'Vette Magazine

[Brandon T.]

Re: C4 Thread: triple prime

What was the first year that zero scrub radius steering geometry was used on a C4 Corvette?
What steering/handling concern did it address?

"There are several reasons that the '85-87 C4s make wonderful autocross cars. The old L98 just can't be matched for bottom-end torque-it'll damn near pull stumps out of the ground. The LT1s, LT4, LT5s, LS1s, and LS6s are all great horsepower engines, but each and every one of them lacks the raw low-end grunt of the old Tune-Port Injection 350. Fifteen years of technology has produced some great engines, but none of 'em can match the low rpm of a well-tuned L98."

----'Vette Magazine

True but after owning a 95 and driving many older c4's with the l98..they do infact run out of breath after around 4000-4500 rpm where the lt1 and up keep pulling.

no contest in a race.

[Joe C.]

Re: C4 Thread: triple prime

True but after owning a 95 and driving many older c4's with the l98..they do infact run out of breath after around 4000-4500 rpm where the lt1 and up keep pulling.

no contest in a race.

That depends.

Absolutely NOT in an autocross. Early C4's with Z51 were essentially gymkhana cars right out of the box. 1984 Z51 cars were so stiff, that spring rates were changed for 1985, leaving the new L98 TPI Corvette still stiff enough to loosen all of your fillings.

Driving an early C4 is what the Corvette experience is all about. They handle like slot cars, and although the later C4's are nice cars and have marshmallow smooth suspensions, I prefer the brute torque (especially when coupled to the 700R4) and the lightning quick reflexes of the early C4 equipped with Z51.

The early C4 stands as the watershed of Corvette chassis development as far as race technology adapted for street use. The early C4 represented the Phoenix rising out of the ash heap that had been Corvette for the previous 12 years. The early C4 was to steering and handling what the L88 and ZL1 were to horsepower.

How quickly we forget.

[Bill M.]

Re: C4 Thread: triple prime

What was the first year that zero scrub radius steering geometry was used on a C4 Corvette?
What steering/handling concern did it address?

1988

Wheel fight during an ABS event.

[Joe C.]

Find the Answer Here:

http://www.caranddriver.com/reviews/car/archive/chevrolet_corvette-road_test/counterpoint_page_3

The term "zero scrub radius" refers to a steering geometry whereby the wheel's pivot axis intersects the tires contact patch at its side-to-side center point. In other words, a line drawn through the "king pin axis" (thru the upper and lower ball joints) will intersect the latitudinal center of the tire's contact patch.

Scrub radius is to a wheel's side-to-side geometry as caster is to its fore-and-aft geometry, because caster refers to the same imaginary line thru the king pin axis and its intersection with the tar's longitudinal center of its contact patch.

Positive scrub radius produces forces in the tire which tend to cause the tires to toe-out while subjected to rolling friction. Negative scrub radius has the opposite effect. Zero scrub radius produces a much more stable steering geometry, and it becomes increasingly important as the front tar width increases!

[Clem Z.]

Re: C4 Thread: triple prime

bump steer is a very important to good handling at speed and most production cars do not have it because the steering and suspension are a compromise. bump steer is the changing of the toe in/toe out setting while the suspension compresses while cornering. i believe they sell aftermarket kits to help correct this problem on corvettes

[Bill M.]

Re: C4 Thread: triple prime

From Corvette from the Inside by Dave McLellan, page 172: "in 1988 we made several changes to make the car work better with antilock brakes. We introduced a zero-scrub front suspension so that braking with one wheel on gravel and the other on pavement would not induce a difficult-to-control torque in the steering system." That difficult-to-control torque is called "wheel fight" for short.

[Joe C.]

Re: C4 Thread: triple prime

bump steer is a very important to good handling at speed and most production cars do not have it because the steering and suspension are a compromise. bump steer is the changing of the toe in/toe out setting while the suspension compresses while cornering. i believe they sell aftermarket kits to help correct this problem on corvettes

Clem,

Bump steer affects toe as a function of loading/unloading a particular wheel/tire, and is minimized by lowering unsprung weight and increasing roll stiffness. It is not the same as "bum steer", however.

Scrub radius affects toe as a function of the tar's rolling resistance.

[Clem Z.]

Re: C4 Thread: triple prime

Clem,

Bump steer affects toe as a function of loading/unloading a particular wheel/tire, and is minimized by lowering unsprung weight and increasing roll stiffness. It is not the same as "bum steer", however.

Scrub radius affects toe as a function of the tar's rolling resistance.

bump steer is caused by the tie rods not moving in the same plane as the bottom "A" frame when the suspension moves during cornering causing the wheel to move in or out changing the toe

[Brandon T.]

Re: C4 Thread: triple prime

That depends.

Absolutely NOT in an autocross. Early C4's with Z51 were essentially gymkhana cars right out of the box. 1984 Z51 cars were so stiff, that spring rates were changed for 1985, leaving the new L98 TPI Corvette still stiff enough to loosen all of your fillings.

Driving an early C4 is what the Corvette experience is all about. They handle like slot cars, and although the later C4's are nice cars and have marshmallow smooth suspensions, I prefer the brute torque (especially when coupled to the 700R4) and the lightning quick reflexes of the early C4 equipped with Z51.

The early C4 stands as the watershed of Corvette chassis development as far as race technology adapted for street use. The early C4 represented the Phoenix rising out of the ash heap that had been Corvette for the previous 12 years. The early C4 was to steering and handling what the L88 and ZL1 were to horsepower.

How quickly we forget.

From the point of view I was talking about there no depends about it line them up and at 4000 rpm the l98 looses steam big time compared to the LT engines which pull hard to redline.

ya in auto cross the l98 does a good job I agree

2 different situations there though. I'm not downing the l98 I think it was a good cruiser, good torque around town just don't go out to the strip thinking you're going to run down LT engines stock for stock.

[Joe C.]

Re: C4 Thread: triple prime

Hey Brandon. It's great to see a young whippersnapper like yourself take a likin' to an "old man's car" like the Corvette.
We need more young blood here, so invite yer friends, yer younger brothers if ya have any, OK?

Listen, yer dead nuts on about an L98 vs an LT1 in the 1/4 mile. But you never said what kinda race you was takin about, ya know? So, I figgered I'd defend the L98 by sayin its better'n an LT1 in an autocross........which it is.

Ya know, my bone stock '85 with 700R4 trannie runs 14.20's and 98 MPH on its original Gatorback tars. An' any self respectin' LT1 with marshmallow suspension an valid up-to-date man-card should do the quarter in at least 1/2 second less. Now, the 1/8 mile is a different story. I''ll be lookin' fer ya next summer at the National. How about a "heads-up" in a 1/8 miler?
Ya on?

Joe

[Brandon T.]

Re: C4 Thread: triple prime

Hey Brandon. It's great to see a young whippersnapper like yourself take a likin' to an "old man's car" like the Corvette.
We need more young blood here, so invite yer friends, yer younger brothers if ya have any, OK?

Listen, yer dead nuts on about an L98 vs an LT1 in the 1/4 mile. But you never said what kinda race you was takin about, ya know? So, I figgered I'd defend the L98 by sayin its better'n an LT1 in an autocross........which it is.

Ya know, my bone stock '85 with 700R4 trannie runs 14.20's and 98 MPH on its original Gatorback tars. An' any self respectin' LT1 with marshmallow suspension an valid up-to-date man-card should do the quarter in at least 1/2 second less. Now, the 1/8 mile is a different story. I''ll be lookin' fer ya next summer at the National. How about a "heads-up" in a 1/8 miler?
Ya on?

Joe

Cool deal thats a strong running l98!

I had a 95 vert auto with 3.07 G92 option it usually would do 13.4ish.

ever heard of Mr. mojo? he clicked off a 12.9 at I believe 106? with a stock 95 6sp! he does some mean shifting..he was in a magazine I believe for doing that.

[Joe C.]

Re: C4 Thread: triple prime

Cool deal thats a strong running l98!

I had a 95 vert auto with 3.07 G92 option it usually would do 13.4ish.

ever heard of Mr. mojo? he clicked off a 12.9 at I believe 106? with a stock 95 6sp! he does some mean shifting..he was in a magazine I believe for doing that.

No. But I Googled him and see he's on CF. His profile sez he lives in Lakewood, NJ, about 20 miles from me, so I'll try to remember to watch for him at Raceway Park.

That's a strong run for an LT1. Watching the ZO6 C6's run in the mid 11's @ 125 MPH steals EVERYBODY's thunder though.

[Bill M.]

Re: C4 Thread: triple prime

Listen, yer dead nuts on about an L98 vs an LT1 in the 1/4 mile. But you never said what kinda race you was takin about, ya know? So, I figgered I'd defend the L98 by sayin its better'n an LT1 in an autocross........which it is.

Joe

Not so fast, there, Joe. I have to stand up for the LT1. In 1999 and 2000, a friend and I ran about 40 autocrosses. He has a '90 L98 Z51 6-speed. I have a '92 LT1 Z07 6-speed. Both cars were dead stock (required to run NCCC street tire class.) We both ran new BFG KD tars.

We found that we could tell who was going to be faster during the course walk. If the course was tight, it was the L98. If the course was more open, it was the LT1. I'm not sure his advantage was all L98, because the '92 Z07 front springs and rear bar were softer then the previous years' Z51 and Z07. We did a rally in his car, and I could tell his Z51 springs were significantly stiffer (ouch) than my Z07 springs.

We figured it was a draw. The cars were very close, and it was a ton of fun.

Based on SCCA National Championships in A Stock, the '89 seems to be the winningest, so it would be hard to argue the LT1 is faster, but it's very close...and it might get closer if the LT1 had Z51 springs!

[Joe C.]

Re: C4 Thread: triple prime

Not so fast, there, Joe. I have to stand up for the LT1. In 1999 and 2000, a friend and I ran about 40 autocrosses. He has a '90 L98 Z51 6-speed. I have a '92 LT1 Z07 6-speed. Both cars were dead stock (required to run NCCC street tire class.) We both ran new BFG KD tars.

We found that we could tell who was going to be faster during the course walk. If the course was tight, it was the L98. If the course was more open, it was the LT1. I'm not sure his advantage was all L98, because the '92 Z07 front springs and rear bar were softer then the previous years' Z51 and Z07. We did a rally in his car, and I could tell his Z51 springs were significantly stiffer (ouch) than my Z07 springs.

We figured it was a draw. The cars were very close, and it was a ton of fun.

Based on SCCA National Championships in A Stock, the '89 seems to be the winningest, so it would be hard to argue the LT1 is faster, but it's very close...and it might get closer if the LT1 had Z51 springs!

Hi Bill,

Well, maybe yer friend with the L98 was tar'd when he autocrossed so he couldn't get the most out of his tars.

It DOES make good sense that the L98 would have a slight edge in a tight course, since the prodigious torque would be more beneficial in such a situation.

It's interesting to hear a comparison such as yours, which was obviously compiled over a long period. It's also great to hear stories of folks flogging their C4 Corvettes, too! Honestly, this series of Corvettes does NOT get the recognition that it deserves. Most 'Murrican car guys are the drag race type...........myself included. So horsepower and speed rather than agility and quickness are of primary importance. I was basically stirring the shi* pot when I boldly stated that the L98 would be faster than an LT1 in autocross.......basically an educated guess. I'm happy that you provided proof that my suspicion is essentially correct, or if not, then at least worthy of a "photo finish".

Maybe I'm just "nuts", but I didn't have anybody else on my tail while I was thrashing my '85 on the "twisties" during the 2009 Road Tour. Then again, C5's, C6's, and Z06 C6's are much more expensive to replace if you end up going over the wall.

Joe