Variable rate springs

[erlihemi]

I'm sure somebody has tried variable rate springs on the rear, but I don't recall seeing it posted.

So here's how I got there. Back in October I was having various traction issues not all related to throttle position. I didn't feel the car had that much "lean" on the corners, but, more than one person mentioned I was getting air under the inside rear tire. So now its post holiday, pre Superbowl, Daytona, boredomshire and I took action. Somewhere between other events I took some measurements on the coilovers. At the time I was running 285/35/18 Hankooks on the rear and my car came with the 350lb springs on Konis. During black Friday/Cyber Monday madness I picked up some new wheels/tires to match offsets and fenders better. (including 12 X 18 425 TT's). I also got a set of 295/40/18 Hankook Rs4 for the rear. My thinking was I wanted a little more sidewall and diameter in the rear at least for the street. This would reduce gear ratio (3.73) and soften the rear a little to take out some of the chatter.

While I had the wheels off I took center to center measurements on the Koni's. My height is set so the differential misses the 3rd link support by about 1 1/2" static. That means the car is low. After doing the math I had less than 1 1/4" rebound. So I started pondering dynamics. If I had Stiff springs and stiff sidewalls then I really wasn't compressing much on the outside of the corner. I was lifting the inside which meant I was rolling on the outer edge of the tires and my scuff marks reflected that. So even with the car low I did not have the rebound I wanted from the Koni. I think I was unloading the inside rear more than was apparent because the front was doing so well.

So I looked up some part numbers and built my traditional Franken combination. Maybe it will work or maybe not. I picked up a pair of QA1 DD601 and a pair of 225/450 variable rate springs along with the nice bearings. This gives me another inch or more of rebound and puts the piston travel somewhat closer to center. The DD601 is supposed to have longer springs, but you cant get those in the FFR chassis. I got 10" springs and they fit, because they compressed a little more than the 8" 350lb. 225/450 was the only viable variable rate in the QA1 pile.

Some Math: FFR Koni springs are 8" free height and the 350lb compress about an inch static. The 225/450 QA1 are a 10" free height and compress almost 2" at static load. The DD601 has 6" travel of which I'll probably use 4". The DD501 would only work if I used the 1" extension on the rod and I didn't dare buy 12" springs. So, with the car set back down I use up almost all the low rate portion on the spring and I figure I'm around 275-300lbs for the next inch of travel before I get to the stiffer coils. With the additional rebound I should be getting the 3 link to work as intended.
If I didn't go too soft I wont rip the fenders off or drop the diff on the 3 link, but I now also have double adjustable dampening, so we'll see.

Of course I've broken my rule of changing one thing at a time. I put softer tires, softer springs, different shocks, and added 1/2 degree negative camber. If it works though I'll put 345/35/18 Hoosier A7 on those big rims...

[NAZ]

Let us know how that set-up works. With less than 1 1/4" of rebound I could see you lifting an inside rear so gaining some rebound travel is where I would have suggested you start. And like you say, one change at a time.

I've never used variable rate springs but I have used reduced spring rate and more preload to tune suspensions, usually for weight transfer issues. Good luck, sounds like you're having fun tuning your ride. Can't wait to get mine to that stage.

[GoDadGo]

I'm running Eibach HELPER springs with COUPLERS with the standard Koni set up; however, these are designed just to take up space when the suspension is unloaded.

https://www.summitracing.com/parts/eib-helper250

https://www.summitracing.com/parts/eib-spacer250

They do offer TENDER springs is various ratios which would give you a progressive rate and maybe a smoother ride.

https://www.summitracing.com/parts/eib-01752500150

https://www.summitracing.com/parts/eib-02252500200/overview/

https://www.summitracing.com/parts/eib-01752500250/overview/

https://www.summitracing.com/parts/eib-02252500300/overview/

You would likely need shorter primary springs, but it might give you a PROGRESSIVE OPTION!

[erlihemi]

GoDad,

Yes the helper springs would fit as long as I cut the lower mount "NAZ" style to have room for the adjusters. Initially I did not think I would have enough room with the 10" springs and I didn't consider that I could have more rebound(extension) than the spring would take up. I may be getting the Eibach Helpers and Couplers. I did use stainless safety wire on the top cup to keep the spring and cup together. ONE BAD PART about these QA1 is they do not positively secure the cup to the eyelet. This is something I would have caught earlier if I was doing a four wheeler, but my brain wasn't in catching air with this car. Running the DD501 with an extended rod end and the tall cup may have been a better solution. It is really tough to mock this up with the body and sheet metal installed. I almost took the aluminum back out of the trunk to do this. Fortunately I did put my top bolts in the correct way on the initial build!
Thanks for the link, hopefully this will help the next guy too.
I was so focused on the front of the car I took it for granted FFR had the back correct with the 3 link, but alas, the assumption corollary arose. Kind of glad the IRS wasn't available when I got my kit now. That would really be driving me nuts.

[erlihemi]

The tender springs would probably work also, but I would guess 250lb is the lowest rate I would go, otherwise its just a stiffer helper and will normally be bottomed out like a helper. I would then go back to the 8" 350lb with the tender. Looks like I will spend as much time with my coil over springs as I did with my valve springs. Sure would be nice if FFR had put some basic suspension and chassis dimensions in the manual. The car handles good as is, but, I want greaterness.

[1932]

Go back and look at tom veales old posts, he did a lot of spring and shock work.

[NAZ]

Don't forget that rear roll center is a tuning aid that affects body roll. The FFR Panhard bar adjustment is limited but it's easy to modify that to gain more adjustment.

[erlihemi]

I'm thinking the Koni is a bigger issue than the spring rate. Panhard can't fix that. Probably be awhile before I get some good test runs in. Little cold in the N.E. tires wont stick to the ice...

[gumbya55]

Has anyone attempted to fit the Jegs double adjustable shocks?

http://www.jegs.com/i/JEGS-Performance-Products/555/64915K3/10002/-1

[DaveS53]

I'd want some good measurements of the shock length, at the desired ride height before buying any new shocks. It doesn't make sense to have a car "sitting low", with the shock over-extended. That would mean that the original shock was too short.

When I build a car, I use mock-up bars in place of the coil-overs, and set them to a length that yields 60% of the travel in compression. If that setup doesn't work, then the shock length is wrong. Unfortunately, most shocks take a big jump of 1.5-2.0 inches between sizes. I have adjustable lower shock mounts on my rear end, but even those have 1" jumps between positions. You may find that a different brand of shock just happens to have an in-between length.

Those Jegs coil-overs would have a ride height of 16.9 inches at 60% compression. I also notice that they use 1/2" mounting bolts. I need 5/8" for my car. Some brands offer a 5/8 rod end with a 1/2 reducer bushing.

Often overlooked is the need for bump stops between the frame and rear axle housing. I added some to my car, after it was finished.

[erlihemi]

I bumped up Tom Veale's post in this section for everyone to take a look at. Koni problem is a common theme. Add this to the list of parts purchased in the Kit that got pulled back off.

Dave, I said the car is low to emphasize I did not "Jack it up" to get larger tires on it. It was generally within the FFR range given that the only dimensions they supply are floor to frame. Which is essentially meaningless. The shocks pulled off matched the dimensions in the manual. Essentially I hit the I believe button during assembly and now for my application I need a different shock. The Koni became the mock up. If I wasn't picky the set up would work fine, but, I didn't get a 3 link to have it bind up on shock extension or the lack thereof. A basic mustang 4 link may not have reacted the same.
There is probably better options, but, the point is to hear from the FFR builders as to what they are experiencing and how they are resolving the issues with FFR pieces.
I found that on the drivers side the double knobs had to turn toward the wheel to clear the 3 link mount. 12" springs will get close to the axle. Soft 12" springs will likely clear the axle tube. I have not moved my axle back 1/2". That changes fitment of the coil over also.

[erlihemi]

Has anyone attempted to fit the Jegs double adjustable shocks?

http://www.jegs.com/i/JEGS-Performance-Products/555/64915K3/10002/-1

Those appear about 3/4" longer Than the ones I just put on, which is probably OK, but everyone seems to like a little different set up. The FFR is 1/2" mounting bolt. There is multiple ways to do this. Make sure you can swap ends for different styles, offsets, and changing the center to center length and then you can customize it. Probably ALDAN has the most configurable set up.

[gumbya55]

Has anyone found the optimal extended length and compressed length using the stock 4 link setup? Meaning swapping out the Koni's for the jegs?

[DaveS53]

Optimum length of coil-over would depend on the tire diameter being used. Some folks use the "recommended" 26 inch, others have used 29-31" to get the real hot rod look. There's a very narrow range of about 1/2" either side of nominal, before you need something longer, or a relocation of the coil-over.

Ideally, you want 60% of the travel in compression, but 50% is tolerable. With a 5 inch travel, that's 1/2" of play. If you have a shock with excess travel, like 6 inches, you can waste some of it.