Myths of Streamlining: The 1937 Joe Petrali Story
From my upcoming book, "Stories of Motorcycle Design"
Much has been written about how, on March 13, 1937 on the sands of Daytona, Joe Petrali’s Knucklehead Harley became un-rideable at 124 mph with its unique streamlined tail, but set an American record of 136 mph without the tail.
You might wonder why Petrali’s bike had a tail at all. This will require a brief explanation about streamlining.
|Streamlining is one, smooth, continuous shape: round at the front, pointed at the back. If you want to go fast, you need to make yourself real small and be streamlined. If you want good stability at speed on a motorcycle, you want the center of pressure as far back as you can get, which is one of the functions of a tail. Another function of the tail is to help smooth out the air as the motorcycle passes through. Equally important, you want the weight at the front.|
I think of a stable, streamlined motorcycle as an arrow (as in bow and arrow). An arrow is inherently stable because the weight is at the front and the fin is at the back.
All this was understood by airplane designers of the day. But it was not well understood by the Petrali team in the winter of 1936-37.
As a result of Joe’s well-publicized problem, a tail was considered to be unsafe on a motorcycle. In 1937, Motorcyclist Magazine[i] reported: “at approximately 124 mph into the wind, the machine had literally started to fly.”
Removing the tail: An infamous moment in history of motorcycle streamlining (picture courtesy Charlie Lecach)
1952, Cycle Magazine [ii]wrote that Joe was “quick to realize that some aerodynamic force built up around the tail, so finally took it off.” Almost 40 years after Petrali’s ride, Cycle Guide[iii] wrote: “The streamlining was lifting the front wheel.”
It must be true. Everyone said it was. But Petrali was heard to comment that the machine ran “more smoothly” with the tail in place and he was sure that “with the bugs out of its tail” the cycle would have done at least another ten miles per hour.”
|After years of designing streamlined motorcycles, I had a hunch that his “bugs” were “pounds”. I reasoned that his tail was probably made of steel because aluminum was not in common use and fiberglass was not yet invented. The tail was probably very heavy. If true, it would explain the handling problem. Remember, the weight needs to be at the front, not the rear.|
|2005: Harley-Davidson’s Historian, Marty Rosenblum helped solve the mystery|
|Was the tail made of steel? Was it heavy? Was this the real reason Petrali had to remove it? Harley-Davidson’s Historian, Marty Rosenblum, was intrigued by the question and put me in touch with “Chicago Ray” Schlee, the man who had restored Petrali’s machine. Ray confirmed that the tailpiece was indeed made of steel, riveted together. Further, he said that the tail weighed around forty pounds because it was full of lead at the rear where it was used to smooth over the rivets.|
The Petrali bike in 2005 Lookin’ for lead in all the wrong places.
Petrali did not have an aerodynamic problem. He had a weight problem. The weight was at the wrong end of the motorcycle. Just resting on the ground, the heavy tail was trying to lift the front wheel!
If Petrali had time to figure this out, he probably could have gone the additional 10 mph, which would have put him at 146 mph! But that was his one and only run. Wrong conclusions were drawn and the damage was done. For 70 years, we have believed that a tail is dangerous. The FIM even banned tails from racing in 1956. Consequently, there are no tails on motorcycles today.
Today, we want to burn less fuel. Today we have a whole new reason to streamline
What it looks like burning 20% less fuel
|For the same reason streamlining allows a vehicle to go faster, streamlining allows a vehicle to burn less fuel. For the past year, I have been designing a bike with a goal to go 70 mph into a 20 mph wind, carry 4 bags of groceries and get 100 mpg. To achieve these goals, my bike must be streamlined. And streamlining means a tail.|
A motorcycle with a three-foot tail is something popular culture believes is dangerous. But we have new materials today. My tail is filled with air so it weighs only 3 1/2 pounds. Further, I have 15 pounds of lead bolted inside the nose.
This is just the opposite of the Petrali machine, seventy years ago!
I have ridden this thing thousands of miles in real highway conditions: On Midwest freeways, and through Hurricane Point in Big Sur, California. I know from personal experience that the tail has no detrimental effect on handling. I can’t really tell the tail is there. (I can't see it in the rear view mirror, either.) But it does have very positive effects on mileage and storage capacity.
In direct, side-by-side comparisons at 65-70 mph into such headwinds, the Streamlined Helix burns 20% less fuel than a standard Helix. Is this a good reason to take another look at Streamlining?
An unexpected bonus is that the storage space in the middle easily holds 6 bags of groceries, out of sight, out of the wind and weather. So, besides burning less fuel at the same speed, we have yet another function for the tail: providing storage space.
You may follow the complete story, chapter by chapter on my webpage: www.craigvetter.com.
|So much has been missed in motorcycle design because of the Petrali Myth. It is time to make up for lost time.|
|Next: Freddy Ludlow responds with an Indian and the myths of streamlining continue...|
Page updated Jan 15, 2011
Page posted Jan 30, 2009