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At that time, the Stability Index of the CJ-7 was 2.04 and the design that had been chosen for the Ford Bronco was slightly worse at 2.03. Ford's early testing with the Bronco therefore also found regular problems in the danger of rollovers. But the Ford Executives did not seem that interested! Around 1981, Ford Engineers were challenged to find solutions to the rollover danger. They came up with five potential solutions. Three of the solutions would have provided substantial improvement in safety, one even increasing the SI to 2.25. But Ford Executives rejected the better solutions, because they would have required extensive re-design and also changed the overall appearance of the vehicles. So a design that had an SI of 2.03 was chosen. There were extensive testing programs planned for this design, but early on, test drivers became terrified at having to test the very dangerous Bronco, and the testing was soon stopped. For the first six years that the Bronco was sold on the market, essentially no actual safety testing had been done on it!
Later, the terminology changed, and the term became Static Stability Factor. Part of the reason for that change was that there were also DYNAMIC factors involved in rollover accidents. Specifically, larger Ford vehicles use a front suspension called a Twin I-Beam Suspension. That design has some advantages, but it causes dynamic disadvantages regarding rollover accidents. The action of that particular suspension has the effect of (dynamically) bringing the front two wheels closer together (reducing the Track) while also Jacking the vehicle UP higher (increasing the height of the center-of-gravity). BOTH of these dynamic effects INCREASE THE DANGER of rollover accidents (per the calculations presented in our ROLLOVER page). These added dangers of the Twin I-Beam Suspension were known as early as 1965. However, the dynamic aspects are much harder to experimentally measure. Therefore, it was apparently decided that simply describing the STATIC Stability Factor, would be sufficient. This new SSF is exactly half of the older SI value, HALF the Track divided by the height of the C-G.
Manufacturers seem to never want to describe the heights of the C-G of their vehicles! So even though the Track is easy to find or to measure, getting the correct value for the C-G height is hard to do! The government and some independent organizations present data from crash testing, which can include the number for the SSF for vehicles. For example, the 2009 Ford Escape has an SSF of 1.13. A 2009 GMC Envoy is 1.17. More standard shaped vehicles have greater values, which indicates their much better stability and less rollover danger. For example, a 2009 Chevy Impala is 1.39 and a Chevy Malibu is 1.41. These are all great improvements over that value of 1.02 for the older Jeep CJ-7 or the 1.015 for the older Ford Bronco. But notice that the stability factors for SUVs are much lower / worse than for sedan-type vehicles.
Consider the example vehicle we discuss (and calculate) in our SUV Rollover Accidents and the Physics and Analysis presentation. There, we consider a vehicle which has a Track of 60" and a center-of-gravity height of 30". We can see that such a vehicle would have an SSF rating of 1.00 or ((60 / 30) / 2). That value represents an extremely unstable vehicle, essentially like the early Ford Bronco. We will show below that even the horrific stability of early Jeep CJ-7s and early Ford Broncos could be amazingly improved!
My concept is to use an EXISTING technology from the 1960s and 1970s! Cadillacs of that era had small motors in the suspension of each wheel. When three heavy people got in the back seat, the mushy suspension tended to cause the rear of the vehicle to sink down several inches, which then made the headlights point up into the sky! So those vehicles had sensors for levelness. As soon as those three heavy people got into the back seat, you would hear the little motors start up and RAISE the rear suspension back to the height where the vehicle was again level.
That self-leveling feature was very popular in luxury vehicles.
So this is what happens. When stationary and when traveling at slow city speeds, the SUV is EXACTLY as customers want it to look and be! And it then had the unstable 1.00 SSF rating as we had calculated.
But the new feature is that whenever the vehicle speed rises above 30 mph, the suspension motors all LOWER the suspension by SIX INCHES (or even more). Let's look at the SSF calculation now. We have (60 / (30 - 6)) / 2. This is now enormously raised up to 1.25. No SUV ever made has had an SSF that was that REMOTELY that SAFE!
Let's consider a modern real SUV. We noted above that government test data gives the 2009 Ford Escape has an SSF of 1.13. Let's now imagine that THAT vehicle had my improvement where the vehicle body would be lowered by 6" whenever it was traveling at greater than 30 mph. The SSF rating would then rise to 1.46!
Please note that that simple improvement would then make that SUV MUCH MORE STABLE AND SAFE (1.46) than even the Chevy Malibu (1.41) or Chevy Impala (1.39) sedans!
Even the vehicle cost would only be MINIMALLY higher due to the addition of that antique technology, and a trivially simple vehicle speed sensor, which the existing computers in the vehicle already monitor!
It is a PERFECT SOLUTION! I'm just waiting until any or all of the manufacturers are willing to listen to a Physicist who has an answer that they desperately are trying to find!
I do not believe that they use as great a vertical range as I had Designed and Engineered when I invented this late in 2005.
It also appears that Citroen has also stolen my idea, with a new vehicle that they apparently intend to put on the market in 2014 or 2015. Their version only adjusts the vehicle height by 4 cm (or about 3 vertical inches) and only by the driver initiating the change, but at least they took my idea of only allowing that to happen at less than 40 km/hr (which is nearly exactly the same as my proposed 30 mph from my 2005 invention, and where my vertical shift is automatic, ALWAYS lowering the vehicle when the speed exceeds 30 mph.
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C Johnson, Theoretical Physicist, Physics Degree from Univ of Chicago