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Discussion Starter #1 (Edited)
From discussion on another thread (http://www.ffcars.com/forums/showthread.php?p=1871126#post1871126), it seems that some people have noticed the very limited rear suspension travel on pin-drive IRS cars, depending on what shocks are used.

The problem lies with the design of the pin-drive lower control arm, which has the spindle mounting tabs angled up vertically at 90 degrees instead of horizontal, which is done to narrow the rear track by 2.5" per side for the pin-drive. But it effectively raises the wheel relative to the lower control arm by that same 2.5". The lower shock mount is thus lower, relative to the wheel, so that at normal driving height, the shock absorber is more nearly extended relative to the wheel, reducing the amount of shock travel remaining when the wheel moves to full extension. Many of you may have noticed that when you jack up the rear end, the wheels hardly drop down at all.

Not only does wheel droop become limited by shock extension, but the shock is never fully compressed when the suspension is fully loaded: the upper control arm (UCA) hits the frame long before the shock is fully compressed. My car often bottomed out hard withe the stock Proshocks and 450# springs, but stiffer springs and shocks cured that.

The lack of "droop" is probably not a big deal for normal driving; if you unweight the rear enough while driving, you may lift one or both rear tires off the ground: but that's not a problem IF you are going in a straight line--if you're in a corner, though, watch out!

I noticed the problem when I started to autocross: when cornering hard, the car would suddenly spin without warning. I think that this happened when the suspension travel limit was reached; restoring full travel has cured this completely, and the car is very predictable now.

Looking at my rear suspension, it was clear that with the pin-drive arms, my Konis were never close to fully compressed, and topped out at full extension much too soon, limiting my rear wheel travel.

One solution would be to fit longer shock absorbers, to increase extension.

Another would be to relocate the lower shock mount on the lower control arm. That looked very difficult, and also I didn't want to weaken the LCA for fear of catastrophic failure.

My solution was to relocate the upper shock mounts 1.6" lower, so that the shock works through it's full range of travel and allows the wheel to go through it's full range. This works fine with the standard-length shocks because none of the travel is "wasted".
So that I wouldn't have to weld on the completed car (and upside down!), I did this by making a custom bolt-on bracket. This bolts across both rear shock mounts, with a tube connecting the two sides for rigid stability. There are tabs with holes to relocate the shock mounts "lower" by 1.6".

Why 1.6"? The rear LCA motion ratio is 1.70 (in my car, my measurements, YMMV), so actually 1.47" additional shock travel would have given me back the 1.47x1.7=2.5" of droop at the wheel, but I found that I could go a little beyond that and gain 2.7" at the wheel. This position also allowed me to use the rubber bumpers inside the Konis as a bump stop, rather than letting the UCA bottom out against the frame on a hard bump.

Before mods, I had about only 3.5" rear wheel vertical travel. Now I have 5.75" (front wheels have 6.875", for comparison).

This has worked really well with Gordon's DA Konis for autocross and open track. With these mods and the VPM swaybars, my car is very much faster and far more predictable than it was with the IRS as factory-delivered.

Here is the bracket installed in the car, (with Gordon's DA Konis and VPM swaybar), the bracket before installation, and some detail shots:

There are some more photos available in the gallery:
http://www.ffcars.com/photopost/showgallery.php?ppuser=197&cat=500

I hope this is helpful.

Forrest
 

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Trying to understand what you did. Am I right...You moved the lower control arm spindle mount to be in-line with the arm. Right?
 

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Never mind. I understand what you did
 

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Exactly what I was going to do with mine. But with the FFR Koni's only being $274 at the time I bought the longer ones and sold the shorter to someone with std width IRS for $250.
Great solution though for those stuck with the shorter shocks..:thumbup:
 

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Discussion Starter #6
wow Forrest.
when is the next autocross event you are going to?
Sergio
Sergio!

I actually haven't autocrossed for at least a year. It's good driver training, and a good way to sort the car, but open track is SO much more fun! At COCOA, the 20 minute run groups are just right: plenty of track time, and I'm always tired out by Sunday evening.

I'll definitely be at Willow for the COCOA event on Mother's day; I hope that you will join us. It's a great bunch of guys, and really a good time!

Forrest
 

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Could someone please define the length of the shorter Koni shock and the longer Koni shock. I have the complete kit with pin drive IRS. I am curious if mine has the shorter or longer Konis. Thanks.
 

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Thanks Convincor. Looks like mine came with 15 1/2 front and back.
 

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I'm still building so they haven't seen any action yet. I'll give FFR a call this week. Thanks again.
 

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Discussion Starter #12
Never mind. I understand what you did
I could have been a little more specific with installation. If I knew how to draw a diagram I could explain it better.

Basically, the upper pair of bolt holes are lined up with the frame-mounted upper shock mount tabs, and are bolted through, using a short length of pipe inside as a spacer so that the tabs do not get bent inwards when the bolt is tightened.

Then the shock itself is mounted in the lower pair of holes, using the original FFR shock mounting spacers.

Forrest
 

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Very interesting.

I just finished the first assembly of the rear suspension in my Coupe and found that I've got excessive droop if I have the rear suspension set up for 4-1/2" frame-ground clearance. The 750lb springs appear to be way too high a spring rate and the springs are way too short. I exchanged the shorter Konis for the longer ones. If I jack the chassis up and let the suspension go to full extension I've got over an inch from the top of the spring to the upper seat. I'm hoping this will change as I put more parts on the car. When you guys go with softer springs do you also order longer ones?

The problem that I did have with my Konis was that I have to mount them body-down as the non-threaded spring seat was hitting the lower control arm at full droop. With the body down and with the spacers I made up for the lower mount (about 0.785" long on both sides of the lower shock mount) I now have clearance and full travel of the shock at the lower. Now the thing that's limiting suspension travel is the upper control arm at the inner mount position hitting the triangular mount at full suspension compression. It looks like I have some camber adjustment to make to get closer to -1 degree so this clearance may improve. Once I get it closer to being set at the correct camber setting I'll be able to see if I have to grind some material away on the upper control arm mount.

Thanks for the info!
 

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Pin Drive IRS Droop Problem

There's a company that recently started building IRS systems for the FFCarss that has solved this problem with a complete redesign of the IRS system. The lower control arm doesn't drop down and the lower shock tabs are placed where they should be. They make a complete system that is lighter and a better design than FF's. It's Blue Dragon Metal Works in Kansas City,MO. They have a website www.bluedragonmetalworks.com. The system you'll see on the site (Pictures Page) is thier prototype.
 
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