• OMGuar
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Topic: C4 Corvette in Hot Rod

Great article about the limits of weight reduction..  Started out at 3,243 pounds, reduced to 3,204 pounds lap times 29.7 seconds..  Once they got the weight down to 2502 pounds handleing was worse due to camber and spring rates. best lap times 27.7 sec. 
    As the weight of the car comes down the suspension needs more work.. springs need to be softened, suspension needs lowering, Camber curve needs adjustment.. Tire compounds need to be softer..
That year Corvette made a big deal of approaching 1 G cornering speed. in it's current shape I doubt it could do .75 G's

  • From: Choctaw, Oklahoma
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Re: C4 Corvette in Hot Rod

The springs on those Corvettes were harsh to start with. If, like most of us LeMoners, you start with soft springs, you're not so likely to get enough weight reduction to get to that point.

...but YMMV

  • JBgotM
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Re: C4 Corvette in Hot Rod

We pulled out several hundred pounds from our firebird. We shortened the extremely soft springs a bit on our car to get the ride heigh back closer to stock.  We also added a little more negative camber (-2 degreees). The balabnce of the car wasn't bad, but it wasa little pushy. There was alot of dive and roll which was causing problems. We destroyed the bump stops and shredded the outside of the tire. We are swapping out for a stiffer set of springs off of another model, even though we are significantly lighter than stock. A guy who raced firebirds a decade ago recommended 1000# springs in the front as opposed to the stock 350#. We will be close to 700# next race. For us, the rate is as much of car preservation than anything else.

  • From: Atlanta, GA
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Re: C4 Corvette in Hot Rod

Most road cars get a ride frequency (sqrt(k/m)) of ~1Hz.  On non-aero racecars the frequency is closer to 2Hz, so you need about 4X the spring rate.

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Re: C4 Corvette in Hot Rod

That's an excellant bit of information. I've never seen it expressed so simply..
    Usually I have to go into my computer program and  make a whole bunch of assumptions.. then run the program and see what sort of lap times it puts out.. then make more assumptions based on tire temps etc.. and rerun the program..
I've wasted a whole day calculating spring rates.. only to show up at the track and find out I'm way wrong and  wind up doing the spring swap   (ain't easy on some cars.) 
What those NASCAR boys lose in sophistication they make up in ease of spring change..

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Re: C4 Corvette in Hot Rod

JBgotM wrote:

We pulled out several hundred pounds from our firebird. We shortened the extremely soft springs a bit on our car to get the ride heigh back closer to stock.  We also added a little more negative camber (-2 degreees). The balabnce of the car wasn't bad, but it wasa little pushy. There was alot of dive and roll which was causing problems. We destroyed the bump stops and shredded the outside of the tire. We are swapping out for a stiffer set of springs off of another model, even though we are significantly lighter than stock. A guy who raced firebirds a decade ago recommended 1000# springs in the front as opposed to the stock 350#. We will be close to 700# next race. For us, the rate is as much of car preservation than anything else.

Careful about spring rate advice.. circle track calls for a differant mind set than road race.  Dirt track is at one end and banked ovals another..  Even relatively flat tracks call for differant springs than a road racer uses..  Finally things like a tube frame car will respond differant than a space frame car especially one without subframe connectors..
 
   Trying to make a spring do the job of a sway bar will produce that sort of result..  If you finally do get them to hold up the corner in a turn you'll find out they are now too still and your cornering speed drops as a result..  Cornering speed off?  Means you're slower in and slower out of the corner.  Lap times suffer a lot and trying to make up for it you tend to push the car too hard.  Not good in an enduro.
Check out pickup truck sway bars.   Find one of the right diameter and make it fit rather than trying to find a stiffer version of the bar you're using..   Your Firebird will be narrower than most newer trucks so look in older truck junkyards.   Check 3/4 ton and 1 ton chassis..
Hint,  most major towns have a spring shop someplace where truck springs etc are re-arched etc.. It used to be real cheap to get bars bent into the correct shape and reheat treated..

7 Reply by JBgotM (edited by JBgotM 2010-10-01 12:37 PM)

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Re: C4 Corvette in Hot Rod

OMGuar wrote:

Careful about spring rate advice.. circle track calls for a differant mind set than road race.  Dirt track is at one end and banked ovals another..  Even relatively flat tracks call for differant springs than a road racer uses..  Finally things like a tube frame car will respond differant than a space frame car especially one without subframe connectors..
 
   Trying to make a spring do the job of a sway bar will produce that sort of result..  If you finally do get them to hold up the corner in a turn you'll find out they are now too still and your cornering speed drops as a result..  Cornering speed off?  Means you're slower in and slower out of the corner.  Lap times suffer a lot and trying to make up for it you tend to push the car too hard.  Not good in an enduro.
Check out pickup truck sway bars.   Find one of the right diameter and make it fit rather than trying to find a stiffer version of the bar you're using..   Your Firebird will be narrower than most newer trucks so look in older truck junkyards.   Check 3/4 ton and 1 ton chassis..
Hint,  most major towns have a spring shop someplace where truck springs etc are re-arched etc.. It used to be real cheap to get bars bent into the correct shape and reheat treated..

The advice was from a guy who spent several years driving thirdgen f-bodies in SSCA road racing. I wasn't trying to cross advice from dirt to black-top. We have adressed some of the things he recommended, but definately not everything. We basically hit the things that could be done very cheaply or free. We have also swapped out for a bigger OEM swaybar off another car. That should help as well. Surely, getting the cheapest shocks on the market will help compared to the blown shocks we took off the car. In the corners, we get taken to the cleaners by the light weight cars anyway. Even if our balance isn't optimal, and our lap time suffers very slightly (which I highly doubt), then it will be worth it to actually get a whole race out of 340 treadwear rated tires.

The frequency response is a good rule of thumb, but alot of other factors come along with it. Considering the limitations of Lemons and the mismatch of valving rate to spring rate, we are just bumping the rate up high for cheap and calling it a day.

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Re: C4 Corvette in Hot Rod

EriktheAwful wrote:

The springs on those Corvettes were harsh to start with. If, like most of us LeMoners, you start with soft springs, you're not so likely to get enough weight reduction to get to that point.

...but YMMV

Corvette's too harsh?   Not when they were approaching 1 G cornering speed..  that was what was called for to get the car around corners well. 
As for riding around going to the grocery store etc.. it was stiffer than  the wallpaper paste suspension in some cars!  But it wasn't trying to be a old folks car..
However when they removed 891 pounds the suspension was no longer in it's best camber gain curve.In fact the tires rode on their edge. 
  They had to make major suspension adjustments  just to get the car anywhere near proper handling..

  • From: Houston (Spring) Texas
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Re: C4 Corvette in Hot Rod

That is the ying and yang of car tuning: for every modification, another is required. More power = better brakes. Better handling = gripier tires. Weight improvement = suspension modification

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Re: C4 Corvette in Hot Rod

Hoonatic Racing wrote:

That is the ying and yang of car tuning: for every modification, another is required. More power = better brakes. Better handling = gripier tires. Weight improvement = suspension modification

Excellant comment.. adjusting tire pressures required a softer suspension setting. which required a stiffer rollbar. that required a rear end ratio change
   Lap times went from 2.17 to 2.03 eventually.   (Old Brainard track)..
  Had a similar experiance with Elkart Lake..
  Biggest Change of all was in San Diego. the set-up from I had  was so differant that caster/camber, front and rear ride hieght had to be changed.. front  sway bar required 2 extra inches of stiffness and rear bar required 1 inch decrease.  Jounce in the front shocks increased 4 clicks  rear 2 click decreased..  rebound I softened up by 2 clicks all around..   Tires pressure down to 23 front 22 rear  (getting air borne as I crossed the intersecting runway followed with extremely hard braking for a buttonhook was the main cause of those changes).  Went from a 3.78 rear end ratio to  4.55,  two full needle changes plus the oil in the piston chambers {SU carbs}  Air density changed from morning to afternoon requiring 4 full flats on the jets.
  Since I had run previously at sea level in the Bahamas and both tracks were similar lengths I thought I went down there with the set up pretty close.
    Morning practice I was 12 fastest.. for qualifing in the afternoon I was on the pole.. that was with race tires 4 years old..

11 Reply by OMGuar (edited by OMGuar 2010-10-02 01:26 PM)

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Re: C4 Corvette in Hot Rod

JBgotM wrote:
OMGuar wrote:

Careful about spring rate advice.. circle track calls for a differant mind set than road race.  Dirt track is at one end and banked ovals another..  Even relatively flat tracks call for differant springs than a road racer uses..  Finally things like a tube frame car will respond differant than a space frame car especially one without subframe connectors..
 
   Trying to make a spring do the job of a sway bar will produce that sort of result..  If you finally do get them to hold up the corner in a turn you'll find out they are now too still and your cornering speed drops as a result..  Cornering speed off?  Means you're slower in and slower out of the corner.  Lap times suffer a lot and trying to make up for it you tend to push the car too hard.  Not good in an enduro.
Check out pickup truck sway bars.   Find one of the right diameter and make it fit rather than trying to find a stiffer version of the bar you're using..   Your Firebird will be narrower than most newer trucks so look in older truck junkyards.   Check 3/4 ton and 1 ton chassis..
Hint,  most major towns have a spring shop someplace where truck springs etc are re-arched etc.. It used to be real cheap to get bars bent into the correct shape and reheat treated..

The advice was from a guy who spent several years driving thirdgen f-bodies in SSCA road racing. I wasn't trying to cross advice from dirt to black-top. We have adressed some of the things he recommended, but definately not everything. We basically hit the things that could be done very cheaply or free. We have also swapped out for a bigger OEM swaybar off another car. That should help as well. Surely, getting the cheapest shocks on the market will help compared to the blown shocks we took off the car. In the corners, we get taken to the cleaners by the light weight cars anyway. Even if our balance isn't optimal, and our lap time suffers very slightly (which I highly doubt), then it will be worth it to actually get a whole race out of 340 treadwear rated tires.

The frequency response is a good rule of thumb, but alot of other factors come along with it. Considering the limitations of Lemons and the mismatch of valving rate to spring rate, we are just bumping the rate up high for cheap and calling it a day.

So what were your tire readings temp wise?  I used to use my hands on the tires as the car came into the pits but once I started racing with racing rubber they got burned too often so I switched to the less painlful method of using a tire temp reader..
You have to be careful though. If there are relatively long straights between the critical corners and the pits tires can cool off enough to fool you..   Don't replace shocks with cheap shocks, replace them with cheap HEAVY DUTY shocks.  Cheap heavy duty shocks typically have the sort of valving you will want.

  • From: Choctaw, Oklahoma
  • Registered: 2009-11-12
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Re: C4 Corvette in Hot Rod

Corvette's too harsh?   Not when they were approaching 1 G cornering speed..  that was what was called for to get the car around corners well.
As for riding around going to the grocery store etc.. it was stiffer than  the wallpaper paste suspension in some cars!  But it wasn't trying to be a old folks car..

I never said they were TOO harsh. My old daily driver RX-7 had a set of springs on it so stiff that I made it a point to hit the bathroom before I drove anywhere.

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Re: C4 Corvette in Hot Rod

EriktheAwful wrote:

Corvette's too harsh?   Not when they were approaching 1 G cornering speed..  that was what was called for to get the car around corners well.
As for riding around going to the grocery store etc.. it was stiffer than  the wallpaper paste suspension in some cars!  But it wasn't trying to be a old folks car..

I never said they were TOO harsh. My old daily driver RX-7 had a set of springs on it so stiff that I made it a point to hit the bathroom before I drove anywhere.

That's what I hate about this sort of discussion.. confusion!  A person can't get the usual visual clues you can in conversation, nor can tone and other critical communication tools be achieved..
  So words are taken out of context, humor missed, assumptions made and the net result is confusion..
I apologize for misunderstanding..
Great article never the less.
  I wonder what would have happened if they would have used longer bolts for the rear spring attachment to put the rear axle camber back where it belonged?  IT's what I used to do on my earlier Corvette. 
They could have done the same to the front springs to achieve the same thing   That's the ease of working with transverse leaf springs..

  • Registered: 2009-10-25
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Re: C4 Corvette in Hot Rod

so you want SOFTER springs...sure maybe for a street car, but no way for a RACE car....I really don't know how stiff the springs are on the Estate...but we cut 1.5 coils off of each one....and they feel pretty stiff.....

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15 Reply by m610 (edited by m610 2010-10-03 02:55 PM)

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Re: C4 Corvette in Hot Rod

I always thought the rule of thumbs on springs was as soft as possible as long as the car wasn't bottoming out, and since the car is as low as possible, the springs end up being pretty stiff.

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Re: C4 Corvette in Hot Rod

icemang17 wrote:

so you want SOFTER springs...sure maybe for a street car, but no way for a RACE car....I really don't know how stiff the springs are on the Estate...but we cut 1.5 coils off of each one....and they feel pretty stiff.....

If springs are too stiff the tires skip over the bumps and as a result the tires aren't in contact with the track all of the time. Traction when the tires are in the air is zero.
  Great handling is with springs soft enough to conform to the track, geometery that puts the tires vertical (Tread flat on the track) undercornering forces, and without excessive load transfer to the outside (loaded tire) tire..

17 Reply by OMGuar (edited by OMGuar 2010-10-03 03:53 PM)

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Re: C4 Corvette in Hot Rod

m610 wrote:

I always thought the rule of thumbs on springs was as soft as possible as long as the car wasn't bottoming out, and since the car is as low as possible, the springs end up being pretty stiff.

The rule of thumb is the tread needs to be completely flat in a corner to use as much rubber as you can.. It helps tremendously if the sway bars transfer the load to the inside tires to use all four tires to corner with instead of just 2.
To achieve that the suspension geometery needs to be in it's best working range.. excess lowering of a car results in way too much camber gain or loss.

  • Registered: 2009-10-25
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Re: C4 Corvette in Hot Rod

I think you are missing my point.....since just about ALL street cars are horribly UNDER sprung....this is why race versions (even Lemons ones) have stiffer springs....to help control the body motion when proper (probably used) shocks are used....   

While I agree that extremely over sprung cars is not a good idea....everything needs to work together....  too much spring-shock-swaybar-alignment can cause problems....

Richard Doty
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Porsche- "there is A substitute" Racing
Dirt Poorsche Racing #2