Thanks to jagamajajaran, we now know about the C7 spring rates:

https://www.corvetteforum.com/forums...post1592998290



Assumptions:

• I assumed stiffer rates are for GS/Z06 are Z07 package, and FE3 on Z51 on the table.
• Assumed the stab bar referred to is front sway bar (Stingray doesn't have rear bar anyway).
• I assumed spring rates for base, Z51 and Z06 did not change for 2017.

Remarks:

• The spring rates between Stingray and Z51 are almost identical! Looks like the difference in comfort is mostly from sway bars (or lack thereof at the rear).
• As was also reported by the drivers, Z51 w/MRC spring rates are very low relative to the other models. What is interesting is that the regular Z06/GS rates are not that low, yet some regular Z06 forum members reported it as very soft, too. Perhaps the MRC tuning has more range and less compression for better comfort in non-sporty modes, or stickier wide-body tires may cause more lean.
• Except Z06/Z07, the front/rear ratio all over the place across the board. This is very confusing to me.
• The F/R ratio is
  • 0.56 on FE3,
  • 0.62 on FE4
  • 0.80 on GS
  • 0.66 on GS w/Z07 (is table missing front spring info???)
  • 0.80 on Z06
  • 1.07 on Z06 /Z07
• GS with Z07 is confusing. Sure, it will be a bit lighter up front, but should be less than 100lbs difference in total car weight. Is the chart missing the front spring rate for GS w/Z07? Another theory is that GM needed something softer than Z06 w/Z07's 190, but stiffer than 117, but we all know they hate new part numbers, so they fixed it up by using much thicker front sway bar at 32mm diameter, and 5mm thickness.

In summary, questions:

• Does GS w/Z07 really come with 117 front springs, remedied by a much-stiffer sway bar? Or did they just miss the additional spring rate in that table, to be used by Z07 option. I first thought for sure they must be missing the data on the chart, but then realized the sway bar is much stiffer.
• Instead of getting coilovers, perhaps one could buy the Z06 w/Z07 springs, and just buy shock bodies with no coil over them, so the additional stress on shock mounts would be eliminated? I was told this scenario allows cross-talk of the OEM springs, but I'm not sure how big of an issue it is. Until a vendor tunes their shocks specifically for this, though, this would be hard to do.

 

You are a techie, and know that suspension tuning is a huge black art... Manufacturers must take into account a wide range of performance variables... and if you are tuning a high performance car... well...

My guess is that 90% of Z06 buyers will never drive their car on the track, but will drive the car to the coffee shop and brag to their buddies that it will break 7 minutes on the 'Ring... and then complain that it hurts their back.

 

Another thing to consider are the stab bar bushings. We know those are different from FE6 to FE7 even though the bars are the same. It's likely they did something like that with the GS with the Z07 package.

 

These spring rates make all the sense in the world....thanks for posting this....this confirms what I have seen putting a micrometer to the springs at the dealership. When you think about it, there is not a lot of "black art" here. Here is a rule set for basic suspension tuning...
Rule 1) The grip of the tires sets the spring rate
Rule 2) the spring rate sets the damping of the shocks
Rule 3) Sway bars are used to fine tune the cars balance
For anyone that needs shocks for these springs on the other cars, just call the Bilstien tech center, they can re-valve the z51 non mag shocks with these spring rates...Last time I had this done( on another car) it was $75-100 bucks a shock.

 

I didn't know our shocks were rebuildable! It's a bit too late for me now, since I am switching to LG G2 coilovers, but I think it's a great idea to just have track valving on the shocks with whatever springs you'd like.

To elaborate on my comment re: too high rear spring rate, if the effective wheel spring rate is indeed higher at the rear than front, you don't see this happening unless the car is rear-biased in terms of weight, like a mid-engine car (Lotus Elise, Cayman, etc.) or 911. Due to this, I still assume the leverage of the spring is lower than the actual spring rate suggests (it's pushing down on the control arm from the middle of it, not from the edge of it, losing leverage), and perhaps front springs have more leverage, changing the balance of (effective) spring rates.

 

Switching the base model over to the Z51 sways makes very little difference in ride quality. I think the primary reason for the difference in ride is the shock valving. The Z51s seem to have much more compression damping.

 

As a follow-up I checked GM front and rear spring part numbers and found that there are different spring part numbers for Stingrays without MRC (FE1 suspension), Stingrays with MRC (FE2 suspension), Z51's without MRC (FE3 suspension), Z51's with MRC (FE4 suspension), Z06's without Z07 (FE 6 suspension) and Z06's with Z07 (FE 7 suspension). Different spring part numbers are also listed for all of the FE 1, 2, 3,4,6 and 7 suspensions based on transmission type (manual or auto).

An earlier "Ask Tadge" post indicated that there are different spring rates for Z06's without Z07 (FE 6 suspension) and Z06's with Z07 (FE 7 suspension). The same is likely true for GS's without Z07, GS's with Z07, Stingrays without MRC (FE1 suspension), Stingrays with MRC (FE2 suspension), Z51's without MRC (FE3 suspension) and Z51's with MRC (FE4 suspension).

 

Here is the thread with some specifics on FE6 vs FE7
https://www.corvetteforum.com/forums...fferences.html

 

Ok, I went into my notes on suspension tuning and these spring rate numbers little are a crazy.

How many pound/inch in 1 N/mm? The answer is 5.71014716277. So If you take 92.4 N/mm X 5.71 = 527.6 lb/in is the spring rate on the front of the z51 without mag ride....sorry, I am a old guy that thinks in pounds per inch.... That is not a lot of spring rate for a car of this weight and tire size....the question is...

What are motion ratios are for the front and rear suspensions at the spring mounting points and the shock mounting points?

Has anyone gotten a corner weight numbers for a Z51 and alike?

Is there a curb weight per car listed some place? How much heavier is the lt2 than the lt1 ect....?

 

1LT Z51 is about 3350 lbs.

What is very confusing to me is that Z07 suspension comes with much much stiffer front springs, which makes the front stiffer than rear by about 7%, but every other setup comes with much softer fronts, where the rears are substantially stiffer in comparison. Considering how the weight distribution is almost identical among these cars, and how he suspension geometry is also identical, having different spring F/R ratio does not make sense. I would understand much stiffer springs, but a huge shift in ratio is hard to explain.

 

There's a few different things going on, here. First, note mustclime's question about motion ratios. What really matters is wheel rate, not spring rate. The front and rear do have different spring motion ratios. Next, knowing wheel rates is only defining the ride motion frequencies (assuming you also know masses). If you want to understand changes in cornering balance, you also need to know effective wheel rates for the bars, as well as RC/IC. This is a fairly complex thing to understand and predict, which is why most people work it through trial and error. If you can calculate it, you may find that those spring rate changes aren't as large as they initially appear.

I can't explain or understand how an OEM arrives at such small differences between models, like we see here between the base and Z51, or GS and Z51. My racecar tuning experience tells me those differences are in the noise. You need very large spring rate changes to effectively change a car's overall capability or cornering balance, especially true when the bars are as big and with such short lever arms. If you look at the competitive cars in SP classes at autocrosses, for example, it's common to see cars with 2x - 4x the OEM front spring rates, and rear rates in the 1x - 2x range. That front Z07 spring is about 2x the Z51 spring, so swapping in just that front spring could really be the hot ticket.

I do have shock angles, distances from ball joints to spring pads and to shock mounts, and wheel weights for my Z51, but haven't gotten around to crunching numbers yet. For whatever it's worth, I think the car feels incredibly soft. The only harshness seems to come from high-speed compression damping on the shocks. For motions where low-speed damping is in play, the car feels like it wallows and bounces off the bumpstops. So I definitely agree with your "not a lot of spring rate" comment.

-michael

 

Right, we are all aware of wheel rates vs. the actual spring rates, which is why I focused on F/R balance of various different models. Considering they're both supposedly meant for track, I'd expect the Z51 and Z06 to have similar F/R spring ratios, but it doesn't seem to be the case. Whatever the wheel rates are, they are essentially a function of the spring rates, so the change in F/R balance will be same even if you use the wheel rates.

In short, I'd have expected F/R balance of Z51 to be similar to Z06, if not Z07, which is not the case.

On a side note, C6 spring rates are much different, and C6 T1 springs are much stiffer, and much more balanced front to rear. Considering the mounting points being very similar, it makes you wonder.

 

Well, no. Contradicting that thought was exactly my point. Note, I wasn't trying to say they don't have a different bias in wheel rate, just static wheel rate alone doesn't tell the whole story.

If you're looking specifically at cornering balance, even static wheel rates won't give you what you need to compare. I'm going to set up an example that illustrates this, but keep in mind I have no idea what the actual numbers are. We'd need to measure and calculate to know what differences exist front to rear.

Example:

Let's say the spring motion ratios are equal front and rear, and defined as y=0.5x. Let's additionally state the bars act on the same general area as the springs, making their motion ratio equivalent. Let's say the front bar rate is 500 and the rear bar rate is 400.

Z51 springs are 528/942, and the Z07 springs are 1084/1011.

Let's calculate what the effective wheel rate would be with high cornering loads, where the bar is in full effect:

Z51 front wheel rate = (500 * 0.5) + (528 * 0.5) = 514
Z51 rear wheel rate = (400 * 0.5) + (942 * 0.5) = 671

Using your F/R ratio comparison, this is 514/671 = 0.766

Z07 fwr = (500 * 0.5) + (1084 * 0.5) = 792
Z07 rwr = (400 * 0.5) + (1011 * 0.5) = 706

Using your F/R ratio comparison, this is 792/706 = 1.122

They are different ratios than what you calculated from the spring rates. They also do not compare to each other the same way.

1.07/0.56 = 1.91

1.122/0.766 = 1.46

There's too much bar effect to claim there's a linear wheel rate relationship with spring rates, when looking at what happens with cornering load.

Even GM doesn't bother with comparing ratios, nor with much in the way of precise rate calculation. They calculate ballpark figures, then use trial and error to zero in on what behavior they want. R&D is a huge part of every vehicle life cycle.

Also, doesn't the FE7 have the larger rear bar? That rate increase dwarfs the spring rates, and would make spring rates in the above calculations even less relevant.

All that said, again, I do think swapping in the front Z07 spring is probably a good thing. If I didn't race in a class where it's disallowed, I'd try it in a heartbeat.

-michael

 

OK so you're saying it is essentilly function of both sways and the spring, sure, makes sense : ) It still doesn't explain the discrepancy between the spring rates of Z51 vs. Z06. The point here is that the front spring rate probably needs to be much stiffer to be competitive for a Z51, as you also alluded to by suggesting Z07 springs as a possibly great option to try.

I had incredible lean in corners with 315 NT01s up front and T1 shocks/sways (with FE3 springs). The tires moved up about 3", evidenced by the rub marks. If they did not gain so much dynamic camber, they would also destroy my fenders; I guess I'm lucky. I will get to try the very same set of tires with LG coilovers and sways. I will put a camera by the fender to watch the suspension movement with the new setup; I think it will be fun to watch : )

 

FE6 and FE7 use the same bars, just different bushings. See the link in post #8.

Also. FWIW my 3LZ Z06 weighed 3559 full of fuel so you'll looking at ~200 lb delta from a Z51.

 

I thought the Z51 had a weight of ~3,450 lbs.

 

As far as what I've found:
Z51 1lt is ~3350 lbs. Fully loaded test examples hit 3444lbs.

 

Just a fyi, As of October 2016 Koni makes adjustable rebound shocks for non MRC cars. I called their tech dept and gave them the spring rates for the Z06 springs and was told that the shocks would control those rates......so if you want adjustable shocks or you want to control your body roll with z06 springs and would need the shocks to control those springs....this is a good option IMO

http://www.koni-na.com/en-US/NorthAm...s/C7-Corvette/

 

Indeed, this would be a great solution with very low cost!

 

FYI Part numbers from my 2017 Grand Sport
Rear Spring 23344288
33mm Rear anti-roll bar 22936510
Front spring 22892342
28mm Front anti-roll bar 20966781