Expert: Paul Soderman - 11/4/2008

Question
QUESTION: THanks for previous answer - So safety was indeed a concern for the Dymaxion car afterall. Bucky himself talked about difficulties in a crosswind, but thought that drivers could compensate for it.  There is one more think niggling at me;  with the 75 years of improved understanding, why is there not a 10 seater bus with 50 miles to gallon fuel efficiency and speeds of 120 miles an hour around today?

ANSWER: Judy
I think you are asking a market force question rather than a technology question.  First, a bus that went 120 mph would be a hazard on the road and a liability to the owner once he killed 10 people.  In other words, who would want a bus to go that fast.  A sports car drive might want to go that fast, but he is only interested in carrying one or two people (who are not paying him).  Maybe in 75 years the lawyers have gotten so good that certain technologies are no longer feasible.

Second, a 10 seater bus would be heavy enough that to get 50 miles per gallon would take some very advanced technology, which means cost.  A bus owner would probably be glad to pay less for a somewhat lower performing bus and simply pass the fuel costs on to the customers.  

Are we really sure Fuller got that mileage ?   I don't think engine technology was up to that level in 1930.  There is more to getting good gas mileage than improving aerodynamics.  And if Fuller claimed to get good mileage at high speed I would be really dubious since we know that aerodynamic power required and engine power delivered goes as velocity cubed.
Paul

---------- FOLLOW-UP ----------

QUESTION: Just writing about the crosswind problem. Did the tapered end, with the lighter weight that implies, also make the vehicle more susceptible to crosswinds? Or is it just the rearwheel steering that is the problem?

Answer
Hi Judy
A tapered car should have reduced side forces on the aft end where the side surface area is less.  However, when I look at the photos of the Dymaxion I don't see a large difference in shape between the front and rear.  I assume most of the taper shows in the view top-down.  Therefore the side forces might be distributed along the body with only somewhat larger forces forward where the taper is smaller.

More importantly, it appears that in crosswind the Dymaxion would pivot around the front wheels because the two front wheels have twice the ground contact area of the single back wheel.  There is probably more weight forward as well if that is where the engine is located. (Which is where you want the weight for stability.)  Therefore, the front wheels should resist side force better than the back wheel so the car will tend to turn into the wind.  This is even more likely at speeds where the car's angle of attack will allow air to lift the aft end and reduce aft wheel ground contact force.
Paul