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

1539 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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For 01's make sure you get the connectors with the seat brace. Those are the $150 ones.
I know the 01's are supposed to come with SFC's, but by now they are probably pretty "crispy."
re-post for west coast
That's what I understood Brad. I've heard some guys say they rust out after a while? And the seat brace should really add to it.
Webster Racing #95, Charlie Webster owner/driver.
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We had some sponsorship, but Charlie sponsored the car through his business and wrote it off as advertising etc, all legal.
The first car we built took us 2 years! After that we bought rolling chassis' and just put our body and drive train in/on.
We used Chevy small blocks, 301 cu in, and 355 cu in with a weight penalty. One year we bought a set of Petty Enterprise Nascar heads and that darn engine made nearly 800hp on the dyno, @8500rpm which was the max allowed in Trans Am at the time.
We never had any cash, so I would buy tires on my credit card and Charlie would pay me back Monday from winnings. We were motivated to do well.
Lots of great stories. We had our trailer stolen. One of the guys who volunteered with us was pretty shady. He found it and we got it back with a new checkerboard floor and 4 new wheels and tires. Hilarious.
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