Lonnie-S wrote:
. . . <snip> . . . .
But, what's a good enough target for a street Locost? 3,000 lb-ft/degree? 4,000? 5,000? More? It's really hard to know. I do suspect there is a number above which no practical benefit is delivered for any extra stiffness added. With a race car you would expect to need a higher number than for street applications. Anyone know a real chassis engineer we can ask?
Cheers,
I don't usually answer my own questions, but I wondered how much I could glean from the reference books I have in my own library. It turns out there was more there than I realized. Here is an answer for both torsional stiffness and bending from the book “Vehicle Body Layout and Analysis” by John Fenton, Mechanical Engineering Publications, London, UK, 1980, page 7.
“In a typical family saloon 4500 lbf ft/deg (6100 Nm/deg) should be a minimum with 5000-5500 (6500-7500 Nm/deg) preferred.”Further on Fenton writes:
“Mid span bending deflections for a car should not exceed 0.05 in (1.27 mm) and door aperture deformation should not exceed 0.05 in (1.27 mm) for a 1500 lb (680 kg) mid span load.”Those requirements are higher than I expected. I thought 4,500 lb-ft/deg sounded pretty good for a street sports car. Additionally, I found some specific figures for both sedans and race cars in “Design of Racing Sport Cars” by Colin Campbell, Robert Bentley, Inc., Cambridge, MA. In the 1976 edition gives the following for torsional stiffness (as tested) for these vehicles:
On page 137:
Ford GT40, Mark I – 12,500 lb-ft/deg
Volkswagon 1500 unitized body – 7,000 lb-ft/deg
Rolls Royce Silver Shadow – 10,800 lb-ft/deg
On page 142:
For the Bobsy C Sports racer (USA, pop rivet aluminum, semi-monocoque) 12,000 lb-ft/deg
It looks like both FEA camps here, street car and race car, may need to increase their goals somewhat.