The Podbike and the Quadracycle Quandary Part 2

 From the previous post, we concluded that the Podbike, with a few refinements, was an environmentally responsible vehicle for a single individual to take short trips using pedal and battery power. The problem was that having four wheels, if did not constitute an e-bike in the US and Canada. 

 If we were to convert the design to static three wheeler and maintain the roll-over resistance, ROR, the width would have to increase by 50%, making the trike too wide to share the road with autos of share a bikeway with other cyclists. And converting the design to a leaning trike would increase the cost and complexity.

So, can we eliminate the electric assist and match the top speed of a Class 3 e-bike, 28mph? Since extremely streamlines recumbent bikes have covered a flying 200m at over 90mph and were ridden over 57 miles on a track in an hour, this should be very doable.

Let's take another look at my all-time favorite all-weather, pedal-powered commuter vehicle, the Pedicar. It was designed by two aerospace engineers, Robert Bundshuh and Lionel Martin and produced in 1973. The Pedicar is discussed in detail in "Pedicar Technology" in a previous post.

https://www.youtube.com/watch?v=zvh44wzhw9c

Several factors prevented the Pedicar from becoming popular. Only 20 vehicles were produced. At $550, it was expensive compared to an average bicycle and had a top speed of only about 18mph.

How would one make the Pedicar more efficient so your average cyclist could pedal comfortably at 28mph?

The main speed limiter was the drivetrain followed by the aerodynamics.

The Pedicar had a linear-motion constant-treadle pedal drive (see " Lure of the Linear Pedal Drive" in a previous post) connected to a 5-speed transmission. Bundshuh incorrectly assumed that the constant treadle would produce a 50% power increase over circular pedaling. It could produce a 50% torque increase, but the deadspots at the extremes of the pedal stroke prevented even rotary-pedaling power levels from being realized. The pedals were not coupled together and that prevented a smooth pedaling stroke. One benefit of the linear pedal stroke was the nose of the Pedicar was lower than it would have been with circular pedaling. This resulted in a very unobstructed view of the road ahead of the vehicle.

Possibly an even bigger problem was the transmission. It had five speeds and allowed a range of 1:6.8, but the step size was 60%. The size of the steps never allowed the rider to get in a comfortable gear. A derailleur system has significantly smaller steps and combined with a rotary crank was considerably lighter. The weight of the Pedicar as 125lb. So some weight reduction is in order. 

So we could replace the Pedicar transmission with a 1x12 mountain-bike drive that has a ratio of 1:5.6 and a step size of 17%. To minimize the swept pedal volume at the nose of the vehicle we could use 160 to 165mm cranks with a bottom bracket having a low "Q" factor. 

The other area for improvement is the aerodynamics.

The exposed wheels are the biggest problem. The top of the tire is entering  the airstream at twice the speed of the vehicle. So the airdrag of that tire is 4X the drag of a tire that is not rotating. The solution is to enclose each wheel in what the aviation world calls a wheel pant. 

A place to look for ideas for streamlining a single-person, four-wheel vehicle is Extreme Gravity Racing.

Many of the racers use a bod-pod + wheel-pod approach for their layouts. Even though the drag coefficient, Cd, is larger than if the driver and wheels were enclosed in a single body, the cross-sectional area is significantly lower and the product of Cd*A is low enough to  produce a noticeably more streamlined vehicle.  

The picture at the beginning of the post is a corporate entry gravity racer from General Motors. Now the picture is deceptive because the GM car is very low and the wheels are only 12" in diameter.

The height of the GM racer would need to be raised to the 1.15m minimum recommended in the previous post. The width over the wheel pants would need to be no more than 1m. For the front wheel-pods, the wheel pants would need to be enlarged to accommodate 16"dia. tires. They would also be wider to house the steering kingpins and disc brakes. The front support beams that attach the wheel-pods to the bod-pod must house brake cables and the tie-rod linking the wheel together. The rear support beam only needs to house the drive axle, since the brakes can be located inboard in the bod-pod. This would result in the rear wheel-pods being narrower than the front. The nose of the bod-pod would need to be greatly enlarged to house the swept volume of the feet on the pedals.

In the end you might wind up with something that looks like the doodle below.

Now there is an interesting consequence of having our all-weather commuter vehicle use four wheels. In a previous post, "Pedaling Along the Skyway", I talk about elevated bikeways. If these bikeways use wheel tracks to simplify construction, then a four-wheeler only requires two wheel-tracks.

 

Another consequence is that the tracks could supply electric power to the vehicle. The vehicle could incorporate an electric motor to drive the rear wheels but it wouldn't include a battery. Thus, when not on the bikeway, it would not be an e-bike. It would be an e-bike only when is is using the electricity from the bikeway. Getting to and from the bikeway would only be on pedal power. This approach should allow the quad to use regular bike ways and streets like conventional bikes and trikes.

Hephaestus

  

 

The Podbike and the Quadracycle Quandary Part 1

I have been reading Bill Gates' book " How to Avoid a Climate Disaster". Now one of Bill's remedies for the intolerable levels of CO2 in the atmosphere is to use electric cars. I am sure he would applaud the design of the Podbike as an environmentally responsible means of single-person short-distance travel.

https://www.youtube.com/watch?v=6OSSMteU4qY

As the commentator above points out, the vehicle he is testing is only a prototype. As such, we can assume that the issues he encounters will be addressed in the production version. Things like the lag between pedaling and vehicle motion, the harsh jolts from the front wheels when going over curbs and a clear canopy that will heat up like a greenhouse on a warm days. 

The Podbike has a very futuristic but functional appearance an should be very weatherproof. 

The most novel feature, however is its totally electric drive. The pedals drive a generator and there are motors in each of the rear wheels. You can drive the motors by pedaling, with the battery or both. You can brake and regenerate by backpedaling. You can also back up by pedaling backwards. You don't need any gearing and having a motor for each rear wheel eliminates the need for a differential.

The biggest drawback with this design, if you are a US or Canadian customer, it it is legally not an electric bike or trike and must be considered a small car. The three wheel regulation is archaic and is a simplistic interpretation of what constitutes a pedal-propelled vehicle.

Given the option of designing a vehicle with either three or four wheels, what would be the reasons for picking three instead of four?

There are legal reasons. If you are building an automobile, having three wheels allows it to be classified as a motorcycle if the weight is less than 1500 lb. With this comes reduced safety regulations, which in turn result in a lower-cost vehicle. And if you are designing a light-weight pedal-electric vehicle, it allows it to be classified as an e-bike.

From the technical standpoint, there are a few reasons. If you steer the single wheel, the steering can be very simple. The vehicle can be more aerodynamic if one chooses the tadpole layout. And a suspension is not required for all the wheels to touch the ground simultaneously. 

But there is a huge disadvantage selecting three wheels when you could use four wheels, roll-over resistance, ROR for short.

For a given center-of-gravity, c.g. height and an equal weight distribution on the wheels, a three-wheeled design has 2/3 the ROR of a four-wheeled design.

Consider the Aptera,  a three-wheel, two person electric car that is getting a lot of press lately.

The width of a Honda civic is about 70in. The width of the Aptera is 88in. 18 additional inches to compensate for the reduced ROR for a three-wheel layout.

The width of the Podbike is 33in. You could expect this to increase to 48" it maintain the same ROR. This brings the width to that of the Organic Transit Elf, which I consider to be too wide to share the roadways with cars and even bicyclists on bike paths.

As a tool for comparison let us look at a very well, if not the best designed velomobile available, the Leitra. Designed by Prof. Carl Georg Rasmussen in 1980, over 260 units had been produced by 2015 and it has been continually improved. The Leitra is an all-weather tadpole trike that is about 1m wide and about 51in high. The rear wheel is driven and the front wheels are steered with all wheels having suspension. The ROR is probably close to 2/3gees. 

In Washington State, where I live, a new class of e-bike has been added. Class 3 allows a top speed of 28mph with both the rider pedaling and e-motor assist. The pervious top speed, Class 2, was 20mph. For a given turn radius, a 28mph turn experiences twice the gees of a 20mph turn. So ROR is an important performance metric. 2/3gees might have been good for a 20mph top speed but it is probably inadequate for 28mph. A turning radius for a 1gee turn at 28mph is 52ft.

The ROR for the Podbike is probably about 1gee. I the design was converted to a three wheeler, the vehicle width would become too wide and like the Elf,  make the velomobile impractical.

Those of you who are not new to my blog know I have spent a lot of time designing and building leaning trikes. Theoretically, a leaning trike can have 33% more ROR than a four-wheeler and double that of a three-wheeler for a given track and c.g. height. This performance increase comes at the price of greater complexity. If the vehicle is completely weatherproof, entry and exit require the vehicle be in a statically-stable mode and then transition to a leaning mode when in motion. The mental adjustment going from static to leaning is not trivial and can be confusing. Although there are several leaning trike designs that have been produced, IMO, none are up to the task of an all-weather pedelectric commuter vehicle like the Podbike. 

So what is the answer? I think that four-wheelers (quads) should be legal e-bikes. In addition to regulating power, (mopeds can have up to 1500 kW motors) regulate the vehicle dimensions. If the e-quad is ridden along the edge of a roadway or on bikeway adjacent to said roadway, limit the the width to 1m and require the height to be at least 1.15m. If the e-quad is lower than 1.15m, require the use of a bike flag at least 1.5m high. Don't try to regulate speed. Let the speed limits of the infrastructure be the controllers. 15mph for bikeways and the posted speed limits of the roadways. Just like regular bicycles.

It is counterproductive to let antiquated regulations eliminate environmentally-responsible transportation solutions. 

Hephaestus