Hello.
Posting in the wrong forum again, as there is no Tech/Performance section for C2s.

I am confused about spring rates. According to the folks at Viking, who seem to have a popular product, for a semi-coilover a C2, street/small block car should have a #350 spring. And, according to them a street driven, small block C3 should run a #450 spring.

This confuses me, because it is my understanding that it is the same front suspension and searching the web reveals little weight difference between comparably equipped C2s and C3s. Does a big block 4speed manual everything car weight that much more than a smallblock with A/C, PS and PB?

This will be a street driven, occasionally beat on car. No autocross. No track days (mostly because my local track mandates roll bars on convertables... so no 8/10ths track cruising even.
But it will mostly be driven on rural back roads at extra-legal speeds. So I need compliance, but nothing too soft.

Opinions?

 

350# if you want to run stock ride height, and want to use the suspension travel to soften the bumps.

If you lower it, you may need more spring to keep the front end off the bump stops.

Most vette guys think you need to stiffen it up, and more is better. BTDT. I ran Pro-Solo for 20+ years. Wound up loosening mine back up for bump compliance. went faster.

You have about 2.0" of front compression available. A 350# spring only uses 1-1.25" of it while on max brakes. Why would you want it stiffer?
Now if you lower the front, things change.
Buy good shocks.

Wrote this too:

 

Thanks for the response.

I definitely don't want it over sprung on the street where nothing is smooth. But, I did want to lower it about an inch. My limited suspension tuning experience has mostly worked best with soft springs and bigger sway bars/stiffer shock valving. I guess I'll go for the #350 springs. They are easily upgraded if I keep bottoming out.

 

Make sure you get adjustable shocks. Either single or double.
Then very likely you will never hit the bump stops.

I am going to cut down my lower bump stop to 0.6" high as recommended in the chassis prep section of the GM Powerbook.
That will give you back an inch of travel.

I am lowering mine an inch in the front also.
And going with ~400# springs on a fully optioned but al head BB.
ps, pb, pw, AC, 5 spd, etc.
Glass spring in the rear, 330#, keeping that one light too. Even with 550 ft lbs.
Oh yeah and bigger sway bars, double adjustables, and delrin a arm bushings. borgeson, 18*9s, etc.

Gotta make it handle better and still be as comfy as possible.

 

I would just use a 400#. You can tune it with the adjustable shocks. I run 500 on my 69 BBC with alumnuim heads.

I can set it to super soft or stiff as a board.

 

For what it's worth, as a reference point, I bought some QA1 coil overs for the front of my '66 and a call to the QA1 tech guys got me a recommendation for 450# springs. I bought them a couple years ago and have been very happy with the ride and handling in street use. The height is adjustable so I was able to dial that in to suit the stance I was looking for. Note that the rear sits a little higher on a TRW fiberglass spring on adjustable bolts, when the car is empty. With two butts in the seat, rolling down the road, it sits level.


Front set on 450# QA1 coil overs

 

Then I would go for the 450# springs.
They should be fine.
If you get the Viking shocks, they are all double adjustable, and you will want to start off with the adjustments almost at zero.
There is still plenty of shock dampening, and they get stiffer very quickly.
Or 2 & 2 could be a good start.

The shock settings are responsible for near 75% of the ride quality, versus that little difference in spring rate.
Maybe a 20% difference in spring rate.
Going from 2 clicks to 19 clicks is near 10 times stiffer on the shock. Not 10 %, but 10 times, or 1,000 %.


Keep your numbers under 50 on compression and 50-100 on rebound for comfort.
That's like 2 clicks compression and 4 clicks rebound. Or less. Not more.

I am getting the same shocks and front spring rate on my 72 BB.
What rear spring rate?
Tha matters also.
They should be similar in wheel rate.
I have a 330# glass one.

550/360# is a very popular suggestion.
I want mine just a little softer.
So 450/330#
You need to get the front and rear spring rates balanced nicely or you will get a funny head bob or rear hop over road bumps. It can be very unsettling if they are off enough.

Why are the C2s lower?
That is a great question.
It is not because of their weight, they are only say 100# lighter.
I am assuming they must have a higher ride height.
The GM Z & D the GM suspension heights.

Or it could be that GM just upped the spring rate on the C3s, because of all the racing going on back then?
In general the C2s do ride a little better, with their softer springs.
I am lucky enough to have driven several of each.
Two just last week.

The main thing is all the factory springs are long & variable rate, with progressive coil spacing.
They have a very light 210-280# rate at and before ride height.
But then they stiffen up in turns or on bumps to some unspecified number.
Almost all the aftermarket springs are single rate, so it is very difficult to compare.
My BB "HU" 284# springs at ride height appear to go as stiff as 421# by the time they hit the bump stops.
So the 400-450#s seem like a reasonable compromise.

But the shock settings matter MUCH more.
And you have to be willing to experiment with them.