spazegun2213

Alright guys, I'm no suspension engineer and I'm far from smart. I know there are far smarter people on this forum, and with all the suspension shops that offer their products, I hope someone can chime in.

At the moment I have Penske 8100 series shocks, with a threaded body. Over the winter i want to convert them into coilovers. However, I'm wishy washy on the spring rates i want. I've heard two different theories, one is the typical stiffer in front, and ~100lbs less in the rear. The other is the reverse! I've heard it from both sides, and even found out that the t1 springs actually have a stiffer rear rate, than front. So, I want to find out why someone would run a stiffer rear spring on a corvette. I know the tranny is in the rear, but is the wheel rate just that much different in the rear for a stiffer spring to be used?

Now, I'm not asking what rates i should run, I'm simply asking what the wheel rates are for the c5, and why someone would run a stiffer setup in the rear. What do you all think?

Z06Fix

I know the autox guys like a much stiffer front spring than the rear.

Maybe some of the road race guys will chime in. But for some reason I was thinking that most of the off the shelf coil-over kits were setup with the front stiffer than the rear.

ltborg

I hear this all the time too, but I think you can run stiffer all around with the same proportions front and rear and be in pretty good shape. I think it boils down to what works best with your driving style. I flipped the springs on my LG coilovers so they were 600 lbs/in front and 700 lbs/in rear at the advice of Aaron Pfadt. I thought the car handled much better this way in the autocrosses I attended.

To get back to answering your questions, running a stiffer rear spring is useful when you have a decent amount of power and are getting a large amount of squat under acceleration. This is why the C5 Z06 has a stiffer rear spring. Just as you do not want the car to roll too much because the suspension can get into a non-linear range, you also don't want too much squat or dive because the same thing can happen.

The spring rate selection for the rear (and front) will also depend on the anti-roll bars you run and the tires you are using. I do not know for sure but it appears that the people running stiffer front springs make up for it by using stiffer rear anti-roll bars. The purpose of this choice is that it gives you the same roll resistance between the two set ups when you are looking at steady state cornering, but it gives you a soft rear spring for more mechanical grip when you are accelerating. The ride quality might also be slightly better.

The flip side with the stiffer spring/softer bar set up in the rear is that an anti-roll bar transfer load from the inside tire to the outside tire. This reduces the overall lateral cornering capacity of that axle. That's why my personal preference is to get a little more of the cornering stiffness from springs than anti-roll bars.

Sorry that got so long winded, I got on a roll. Anyway, I hope that helped answer your question.

Independent1

I am not very smart either!

I have read a few books about suspension systems, including Carroll Smith's, "Tune to Win". I highly recommend you buy and read the book before doing anything.

The springs serve two purposes. To keep the tires on the road and to isolate the car and driver from road imperfections. Carroll Smith is of the view that you should use the softest springs you can get away with, and use the dampeners (shocks) and anti-roll bars to fine tune the suspension.

In the suspension world they talk about roll rates, which helps to describe how the car rotates when driven through corners. There is a front roll rate and a rear roll rate. You don't want them to be too far apart otherwise you will get an ill handling car.

Let us know what you do.

spazegun2213

I'll be sure to let you all know! The reason I'm asking is that my experience has been on other cars with different suspensions that normally the wheel rate is higher in the front of every car. However, this may not translate into high spring rates in the front due to the suspension geometry. My 944, with a trailing arm rear would need something like .74*spring rate in the rear for its wheel rate, thus the rear spring was always stiffer than the front. However, they were normally close enough that the wheel rates were higher in the front than in the rear. My subaru, the wheel rates are nearly the same, so the front springs were always stiffer than the rear.

I'm sure someone has figured out what the wheel rates are for the c5

97C5inSD

My car is setup with 450lb fronts and 550 rears. DRM coilover setup. Commonely DRM sends them out with 400fr. 600 rear.

ghoffman

So can you! But be aware that what the motion ration at full bump is different at full droop, especially in the rear. That is why we run a dual rate setup to compensate for the non-linearity and falling rate.

spazegun2213

While i have taken a lot of math classes, I dont trust myself to dive into suspension geometry. I'd rather have someone i trust give me an explination that i can understand and go from there

0Randy@DRM

[edit] Wheel rate
Wheel rate is the effective spring rate when measured at the wheel. This is as opposed to simply measuring the spring rate alone.

Wheel rate is usually equal to or considerably less than the spring rate. Commonly, springs are mounted on control arms, swing arms or some other pivoting suspension member. Consider the example above where the spring rate was calculated to be 500lbs/inch, if you were to move the wheel 1 inch (without moving the car), the spring more than likely compresses a smaller amount. Lets assume the spring moved 0.75 inches, the lever arm ratio would be 0.75 to 1. The wheel rate is calculated by taking the square of the ratio (0.5625) times the spring rate. Squaring the ratio is because the ratio has two effects on the wheel rate. The ratio applies to both the force and distance traveled.

Wheel rate on independent suspension is fairly straight-forward. However, special consideration must be taken with some non-independent suspension designs. Take the case of the straight axle. When viewed from the front or rear, the wheel rate can be measured by the means above. Yet because the wheels are not independent, when viewed from the side under acceleration or braking the pivot point is at infinity (because both wheels have moved) and the spring is directly inline with the wheel contact patch. The result is often that the effective wheel rate under cornering is different than it is under acceleration and braking. This variation in wheel rate may be minimized by locating the spring as close to the wheel as possible.

This was straight from this Wiki http://en.wikipedia.org/wiki/Vehicle_suspension

You can test this your self!!! Super easy actually. Get the car up on jacks, use a bottle jack or even a floor jack. Measure hub to ground, vs shock travel. You can check it as often as you feel is prudent. Every half inch of hub travel is a good start. You can do a graph, or just look at the numbers. This will give you the actual wheel rate.


What exactally are you looking for???

Randy

0Randy@DRM

Disclaimer, you must remove the spring before jacking up the car. Let out all the pressure in the shock too.

Randy

spazegun2213

Randy, as always i can trust you to give a great explanation of what i was asking. However, I'd really just like to know what the front and rear wheel rates on the vette are, and/or a justification for running a stiffer rear spring on coilovers.

thanks
-Ross

ghoffman

Randy is right, just use the bump rubber or a tie wrap to measure shock travel versus wheel travel. What you find is that the rear is very different beginning to end. The rear is a falling rate so that if you have a spring that is correct at full bump so it does not bottom easily, it will be too stiff initially and vice versa. The rear shock angle will change about 14 degrees and you will see why poly bushings on top put tremendous bending on the shock shaft. If I can find the spreadsheet for the C5 somewhere on my backup drive I will let you know.

0Randy@DRM

Ross,
Here is about the most layman calulator that you can find. It is not exact for the C5s but it will give you a idea why we run a stiffer spring in the rear.
http://www.stockcarproducts.com/pstech8.htm

Fronts are right around 20 degrees, rears are 30 or so. Gary is right the angle increases pretty fast in the rear requiring even a heavy spring.

Randy

spazegun2213

woohoo, that is what I wanted to know. wow, thats a really weird rate in the rear. So Gary, you are saying with upper poly bushings that the shockshaft will actually bend at full droop? eek! thats not good at all! How would one combat this? or is it not a problem you can fix?

ghoffman

The best solution is a clevis like Hardbar and LGM provides, but if you have to run the pin tops the poly should not have a straight hole in it but should look like this:

The pin is 10mm but when you do the math, it wants to be 14mm out at the edge of the taper. You could modify the poly bushings that VBP sells to look like this.

bowmanized

The simple answer is, it depends.

Depends on a number of different factors.

1. Drivers Goals Street vs Track
2. Driver Preference and Skill to determine Neutral-Understeer-loose bias
3. Sway Bars Bigger is not necessarily better. I am from the school where you use spring rate and not sway bars to achieve the correct rate at the wheel. With less brake dive and roll you'll acheive better over all grip on most tracks. We can accomodate for stock, Hotchkis, T1, or Pfadt Race.
4. R-Comps limit the spring rates you can use. At some point the tires' sidewall will become the suspension. With the full race slicks you can use much higher rates if needed since they have fantastic sidewalls.
5. Cars weight
6. Aero
7. Home Track, the guy who hangs out at Sebring on weekends vs the brand new buttery smooth track is going to have different rates.

The double adjustable shocks like your 8100s can use a wider range of spring rates.

If you need springs I'd be happy to help.

ghoffman

Wow, I published this model last year, it seems like someone has copied me again! I really need to stop giving out information, because it gets stolen by other vendors and then they try to patent it!

Z06Fix

Can some one explain to me why the autox guys like a much stiffer spring up front? Also would this help or hurt the car on a road course?

Thanks

Silverton

How do you like that set up? Can you comment on why you went +50 front and -50 rear?

Here are some spring rates (not wheel rates) I've collected from this forum:

Spring Rates (cannot verify accuracy):
• 440 lb/in, front; 577 lb/in, rear --- C5 FE-1/Base
• 660 lb/in, front; 633 lb/in, rear --- C5 FE-3/Z51
• 650 lb/in, front; 680 lb/in, rear --- C5 FE-4/Z06
• 600 lb/in, front; 700 lb/in, rear --- LG Coilovers (+- 50 lb/in)
• 400 lb/in, front; 600 lb/in, rear --- DRM Coilovers (+-50 lb/in)
• 650 lb/in, front; 800 lb/in, rear --- T1

EvilBoffin

This isn't specific to spring rates, but maybe it will help...

Whenever I've discussed this with Pat Salerno, Gary Thomason or Mike (JR) Johnson, they've consistently indicated that the front needs to be stiff to reduce roll, keep the tire patch as big as possible and aid quick turn-in when a Vette is optimized for the low speeds and violent transitions of an autocross.

As an example - even on my old softly-sprung, VERY camber-limited Porsche Boxster S with skinny (7-inch wide) wheels, Mark Daddio suggested running my GT3 adjustable front swaybar on FULL stiff to help keep the front flat, the tire patches in contact with the surface and actually reduce understeer. That seemed bas-ackwards since you can't use a different rear swaybar to balance out that stiff front bar OR change springs in SuperStock - - but it worked. At the time, I didn't even have adjustable shocks (we tried Moton doubles, which didn't work at all for my application, but that's another story...).

Running softer in the rear (along with ~1/4 inch toe-in) can be helpful in putting power to the ground effectively, which is REALLY important in an autocross since you're constantly trying to get back on the power as soon as possible when exiting these low-speed turns with all that torque. Such compliance especially helps power application when the pavement is lumpy, undulating or broken-up (like ALL the event sites I seem to go to lately ).

Of course, this all varies by driving style, course design & speed, surface conditions and class rules, so someone here will probably dispute everything I just said. But it comes from the best Vette drivers in the autocross biz, so take it for what you will. Hopefully at least it makes some sense for you.