Schmucker

If our cars are pretty close to 50/50 wieght distribution, why do we put 460 or 550lb springs in the front, but only a 330 or 360lb spring in the rear? Would it be better for performance to have equal spring rates front and rear, or am I missing something?

MotorHead

Don't know the answer but I do know there is only one spring on the back and two on the front maybe that has something to do with it :D

Just thought about it a little more and if the whole rear spring is rated at 330lbs and only half the spring is connected to each rear wheel I think this would increase the spring rate at that wheel to be more in line with the front.





[Modified by MotorHead, 11:11 AM 5/28/2003]

Detroit Vette

Weight transfer during acceleration, and also the weight of a small block compared to a big block is different. Just to name a few.

turtlevette

Yes, Its its my personal opinion that it would perform better with higher rate rear springs in the rear. I hate cars that push like pigs. Thats why i have 550 front and 420 rear (the 5 leaf). This is the highest rate rear spring i could find. The higher rate rear sping will cause the car to oversteer but that can be corrected by adding a bigger front swaybar. Right now my front swaybar is not hooked up and it handles like a dirt tracker. Real tail happy (Oversteer). For low speed autocross this is good since i can steer the car with the throttle and slide around tight turns. From the factory all cars are biased towards understeer for safety, but cars that are neutral to biased slightly towards oversteer are more fun to drive although more dangerous because of the risk of spinning out.

below i have attempted to summarize.

Adding more PUSH or UNDERSTEER or nascar term TIGHT
higher rate front springs
more front swaybar
stickier or wider front tires

Adding more OVERSTEER or Nascar term LOOSE
higer rate rear spring
add a rear swaybar or bigger rear swaybar
stickier or wider rear tires




[Modified by turtlevette, 1:03 PM 5/28/2003]

Schmucker

Yeah, because QA1 recommended the 450 lb springs for the front and 350 lb springs for the rear (these are for coilovers) so I was thinking that if the front sits right with 450s, then I'd bump the rear up to 400s in a while. It seems to make more sense to be even all over. The shocks can handle weight transfer at the drag strip (they run stiffer rear springs than front too, btw).

Gordonm

If I remember right the stock springs for a small block are close to 300 front and about 300 rear. Very small front bar and no rear bar. Handling was not great but I guess they figured the racers would modify it anyway.

Jvette73

One must also consider the effects of braking. In hard braking, most of the weight is put to the front. Gotta have heavier springs up there to help support that load transfer.

Schmucker

That's a good point, but having heavy springs all around would work too. I might get 400s for the front when I put a 383 or 406 in since I'll be running aluminum heads and intake and such. It can really remove a lot of weight off the front. We tested the 450s in the front and it will work as coilovers, just barely. A 10 inch spring only compressed one inch.

I was happy with the handling before, but I know it can be so much better, so that is what I'm going for. I'm keeping the stock crap sway bar in the front for now, but I'll be making some big chrome moly solid mounted ones on the front and rear.

Tominator

Turtlevette; in your opinion is it better to go with the fiberglass rear spring or steel?

turtlevette

I believe the higest rate rear spring is the 5 leaf 420lb steel at $99 from VBP. The fiberglass costs about 4 times as much and i don't believe you gain anything except for the weight savings. When you go to a spring like this, be prepared for a harsh ride.

Schmucker

Which is another good reason to atleast get adjustable shocks to soften in out a little when putzing around.

Shark Racer

Re: understeer as posted by turtlevette.

Here's my setup:
550 lbs front, 360lb rear springs
1 1/8" front sway, 7/16" rear sway
Bilstein Sport Shocks
Rear Smart Struts

The 'vette as it sits has a tendency to oversteer. Ever since the smart strut upgrade, I've found myself steering left a couple times in a right hander.

The rear is supposed to squat/lift/etc, and its geometry is a lot more "weird" than the front. I've never had my fronts wheelhop, but the rear quite a lot. With the heavier springs, it would only emphasize that.

Perhaps some more racers will chime in...

-Steve

ZD75blue

Wheel hop happens when you twist the spring in the wrong direction... it would more then likely go away if ya bumped it up a few lbs, the side effect is you get to turn left in more of the right handers! :cheers:

75 BBC Stingray

Just started reading a book called "The Trans Am and Corvette Chassis - Design/Theory/Construction". In it when talking about Roll Centers on pg5, the author states
"Roll Centers are determined completely by suspension system geometry. Because of the design of today's stock passenger cars, the front roll center is slightly lower than the rear roll center. With this design built in, the front suspension is handling more of the inside-to-outside weidht transfer... The amount of body roll attained from a certain roll center height is dependent on the leverage effect between the roll center and the center of gravity height. The higher the center of gravity and the lower the roll center, the greater amount of body roll there is (because of a high amount of leverage between the two points). The higher the roll center and the lower the center of gravity, the less body roll there is ( because not much leverage is placed on the roll center by the center of gravity.)
If there is a high roll center and thus very little body roll and weight transfer, softer springs can be used to control what body roll there is... "

One of the complaints about the stock system was the high roll center in the rear suspension and the tendency of the rear to jack up from the lateral forces. Perhaps the above was one reason the springs were kept softer than the front. (I do apologize for the novel, but I thought most of it was necessary to get the meaning across) Not my words, or ideas, but perhaps something to think about...


[Modified by 75 BBC Stingray, 7:25 PM 5/29/2003]

ddecart

Ok...time for a brief vehicle dynamics lesson. :)

First let's distinguish between SPRING RATES and RIDE RATES.
The spring rate is...well...how stiff the spring is. It's expressed in lb required to compress the spring one inch. (lb/in)
The ride rate is the stiffness at the contact patch. It's expressed in lb required to move the contact patch one inch. (lb/in)
What's the difference? There's a 'linkage ratio' at work here. If the spring is located so it pushes 1/2-way out the control arm, the linkage ratio is 2:1. So a 100lb/in spring will take only 50lb/in at the wheel.

That's one big factor for the springs being softer in the rear than the front. The rear springs act much closer to the wheel and thus have a much better linkage ratio and less spring rate is required for a given ride rate. (Note: The stiffness of the tire plays into ride rate since the suspension and the tire are both springs working in series).

Ok, so it's ride rate that we're interested in as opposed to spring rate. So what do I want there? Well, as a vehicle dynamics engineer, I'll look at a couple of things. One is the ride frequency. What's the mass on the axle and what's the wheel rate and what's the ride rate? Square root of k over m and I've got the ride frequency. Generally speaking, the front ride frequency of a typical passenger car is in the 1.1-1.2Hz range and sporty cars are more in the 1.3-1.4+ Hz range.

Now, I don't have a total ride frequency for the vehicle. Instead, I have a front and a rear ride frequency. You might be thinking that I want them to be the same. Nope. When i hit a bump, the front moves first and then the rear. If the frequencies are the same, the car will pitch (rock fore & aft) which will make you seasick. So I set the rear spring stiffness so the rear ride frequency is a bit higher than the front. Generally that front to rear ratio is in the 1.1-1.2 range.

So now I have a car that has acceptible ride characteristics over a variety of road surfaces & inputs. To control body roll, I use those stiffnesses along with the added stiffnesses of sta-bars to get the amount of roll I want along with the front to rear roll stiffness (load transfer) distribution that I want for good balance.

I hope that helps. Let me know if you've got any questions.
:cheers:
Dave

turtlevette

Sounds great. Can you translate this therory into some practical recommendations on how to set our cars up? I like some oversteer built in so i went with the 550 lb front and a stiff rear spring, ie 420 lb 5 leaf. I don't know if there are any rear springs higher than that. The other reason i went with the big rear spring is that i like to run big tires and you can't run a rear sway bar due to the interference. So that option isn't available to me to make the car looser. I have the biggest front sway bar which makes the thing push like a pig when connected. I'm going to put the smaller front bar back on to attempt to get a balanced to slightly oversteering car. The other idea is to get some adjustable links (the ones with the springs) to use with the large front bar to make it phase in later as the car rolls.

ddecart

Well, that gets a little (ok, a lot) more complicated. I presume you're referring to push/looseness in transient events like lane changes, slaloms or whatever as opposed to the 'proper' definition of understeer/oversteer which is a steady state relationship.

To determine the proper spring and bar combination, I'll go through the process as described above for the spring rates. Then I'll calculate what additional stiffness I need out of the bars based on a few characteristics:
1) What are the front & rear roll center heights?
2) What are the front/rear track widths?
3) What are the axle masses?
4) What is the Center of Gravity Height?
5) What roll gain do I want? How much do I want the vehicle to roll?
6) What front/rear balance do I want? Generally I want the percent of load transfer per axle to roughly equal the percent of mass on each axle, with a little bias toward the front. So if I had a car with a 60/40 weight bias, I'd initially set it up with a 60-62 percent front load transfer.

As you can see, there are a lot of things here that i don't know about the Corvette suspension. If someone can provide them, I can quite easily determine some good starting points. Unfortunately, I've not done any development/evaluation work with different spring rates for a C3, so i can't make a judgement call from experience. Guys like Guldstrand can though.

OatBoy

don't forget that the motor is in the front, thats an extra 400-600 pounds.