I had the opportunity earlier this year to drive my GT5 Bullfrog at speed on the Texas World Speedway. I had driven the car into triple digit speed zone previously without incident. However, the TWS front straight invited going a bit quicker than previously.

I'm hypothesizing my speed was 120MPH plus when the front end of the car began oscillating up and down. I would opine that the condition seemed to become more exaggerated, although, I immediately backed off the throttle to bring the car back under control. This was a bizarre experience that I had never encountered before.

It reminded me of an documentary that I saw on TV about the design and construction of the Ford GT. When they were tuning the shocks for the car, they mounted the GT to a moving platform that would simulate various irregular road conditions. The initial shock set-up created a similar oscillation that, once started, continued to compound and get worse.

On my Pantera, I am running Carrera shocks. They came with silly soft springs. I am working from a bad memory when I guess that I have 700lb springs in the rear and 800lb springs in the front.

I realize that stiffer springs up front are not the norm. The stiff front springs were to reduce wheel/tire travel in the front to reduce the likelihood of the front tires hitting the flares. The combo has felt fine around town. Yes, I should create a better bump stop for the shock.

Regardless, do any of you suspension gurus have an insight as to what is going on and causing the up and down oscillation? Suggested cures?
Original Post
The front of #6018 bounces up and down on roads that were "poured" in sections, with seams or high spots going across the lane. 6018 probably has the spring rates mentioned by Rocky.

It feels like its pivoting on the rear suspension, rather than being absorbed by the rear suspension. However, problems are often just the opposite of what our perceptions would tell us. If that's the case with 6018, then I'd suspect that the front was too soft (springs or dampening) rather than the rear being too firm. I never got around to testing or fiddling with the adjustments.
JT, if I had to guess, I'd say the problem you experienced was due to your shocks not being able to control your spring rates. They might have even overheated while trying. Super stiff springs are not the best way to keep your tires from interacting with your fenders. You need to limit wheel travel. Shock absorbers resist/control up and down movement; coil springs merely support the weight of the car.

I'd suggest switching to 450/550 lb springs as a starting point. Keep in mind, those are still pretty stiff. Limit wheel travel by putting an extra set of bump rubbers in your shocks. If your shocks did overheat, you should have them rebuilt or toss them in favor of something better. Dennis Quella's been selling Koni shocks paired with springs with the rates I just mentioned, for decades. I'd talk to Scott at SACC too. He seems to have the latest technology in shocks.

Good luck!
John,

I think you have successfully accomplished what you set to achieve! Reducing wheel/tire travel in the front relative to the body, thus the reaction is now taken up through the body and not the shock as you are over sprung.

If you corner weighted the car you wouldn't have 800 lbs per corner in front, there is just not enough mass on an undulating surface to have that shock/spring combination work.

Julian
Thanks Julian - That is a scary thought that the body is flexing. Fortunately, no cracks at the seams. The Carrera shocks are very soft compared to similar rated springs on the Koni's. I ought to bite the bullet and switch my Quella, Koni's from my yellow Pantera over to the Bullfrog for comparative purposes. I do believe that my solution will be a set of Konis or the IPSCO set up eventually. - JT
don't know how this is even helpful but
the natural frequency of a spring and weight is

Hertz = 3.1 X sqrt( spring rate # per inch / weight #)

just roughly 800 rate with 800# at a tire would be the car wants to oscilate at 3 times per second.

from 1970's text book, a sedan should have a Natural freq of 1 and a race car/truck have 2

However I don't have a thought what could be exciting the as a function of speed
John, I agree with the input so far. Oscillation means you have an "underdamped" condition; your unsprung mass, spring rate, and damping (shocks) are mis-matched. Less spring rate and / or more damping will change the natural frequency.

If you have the opportunity, you might try just removing the rear wing, without changing anything else, see if the issue changes or goes away. Downforce applied behind the rear axle would tend to lift the front end, which might be what is starting the oscillations. That would tell you if the wing is involved or not. Rod.
John,

The shock setup is completely incorrect for the car. Do not take the wing off or anything silly like that. Playing with the wing would be fine if the shocks were close to being correct but they are so far off that it would be a waste of time. Plus, there are many that would debate the functionality of our wings at the speeds we typically run. But let's not go down that rat hole.

We can setup a shock package for you that meets all of your requirements. All we need is the shock length at the desired ride height. Also need the shock length you want at full compression. With this data be could put together a shock package that actually works. What you are running now is basically a car with no shocks. There are so many issues with the combination on your car right now but I won't go into that now.

Most vendors sell a one-package-fits-all shock setup. Yes, they offer different spring rates but other than that, the shocks are the same units that they have been selling for decades.

SACC Restorations sells new technology shocks that are manufactured by FOX Racing and sold through Ridetech. Fox is a leader in off-road shocks and has been so for years. The beauty is we can put a shock on your car that has the correct lengths for the specific setup on your car. This is why we need specific shock length desires to get them right for your car.

A Stock Pantera is a bit easier as we have all the information that most people want.

Let me know if you would like help with a custom, high quality shock setup!

Thanks, Scott
quote:
Originally posted by JTpantera:
.. my speed was 120MPH plus when the front end of the car began oscillating up and down.
..the condition seemed to become more exaggerated
...backed off the throttle to bring the car back under control
...Carrera shocks.
...700lb springs in the rear and 800lb springs in the front.
...The oscillations caught me by surprise
...hey were fast, not slow.


quote:
Originally posted by ZR1 Pantera:
...The shock setup is completely incorrect for the car.
...basically a car with no shocks.

There are so many issues with the combination on your car right now but I won't go into that now.



Is there any chance you could?

Is it that the Carrera is too small no matter what the spring?

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