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The "Fifth Spring" effect. Yes, it's a thing!

1543 Views 10 Replies 5 Participants Last post by  tossass4370
For those of you waiting to hear about your 2019 Bullit order, and you track rats, here's a little distraction.
Back in the day, when I was involved with a small GT1/Trans Am team, we did everything ourselves. We actually built the whole car from chassis to engine, hung our own bodies, brakes etc. We learned a lot and laughed a lot. The cars were pretty fast and we won our share of trophies in Canadian GT1 and placed well in the few Trans Am races we ran.
Race car chassis' are built to be stiff, very stiff, high torsional rigidity. We used high rate Springs, as high as 1200Lb/in on the front and 800lb/in on the rear. These cars were not designed for bumpy roads and had maybe 3 inches suspension travel.
Bullits do not possess overly stiff chassis', the SN-95 being less stiff than the S-197's. No knock on the 01's just progress made through the years. So what is this fifth spring you talk about Pat? I'm sure you've guessed by now that the chassis is the "fifth" spring. If you were to pick up a tube frame race car with a floor jack, in front of a rear wheel, more than likely you would pick up the whole side of the car and sometimes 3 wheels. Very stiff. On a street car, do the same thing and you pick up one wheel and maybe lift the other wheel on the same side somewhat. Much less torsional rigidity. The fifth spring effect makes the handling of a street car somewhat sluggish as the chassis has to catch up with driver inputs.
But help is on the way, we can improve the chassis stiffness for very little money and reduce fifth spring effect. And, you can feel the difference. You 01 guys, for $150 you can buy a set of bolt on sub frame connectors from BBK and in about 3 hours improve the handling of your car. Good bang for the bucks.
For you 08 guys, since your chassis is a bit stiffer, you will have to up your game with sub frame connectors from Competition Engineering for about $315 and with an afternoon's work you are there.
There are weld in connectors, but you have to really remove seats and carpets and they are permanent once installed. The others are removable and you won't burn through other critical components.
If you want more, and don't we all, a roll bar with removable door bars, tied into the frame connectors will really add some rigidity and a safety factor for you dedicated track hounds. The roll bar install is best left to the pros.
All this may affect your track set up, but it's not a big deal to compensate.
I had a 1996 GT convertible, and when I first bought it, I called it "frame on a hinge." When you would corner the thing or go over a rail crossing, the doors would wiggle up and down and the cowl would shake. After installing frame connectors and a roll bar, it was better than a hardtop coupe. If you are having fun with your Bullit and don't track it then all of what I just wrote may not be for you. If however, you want a little extra then maybe...
The fifth spring is real and it's out there! Good news for you 19 Bullit buyers, the chassis is a lot stiffer!
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Prior to the Bullitt we (wifey and myself) had a '70 Convertible. I have seen those cars rusted in the middle and actually pull apart. For this car, we installed a good set of frame connectors (along with solid sheet metal). The difference was amazing. One of the biggest plus was having the front wheel alignment staying in place. Without the connectors, an alignment was good only until the first bump in the road.
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