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.
Hephaestus
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.
I agree, four wheelers(quads) should be legally recognized as e-bikes in USA and Canada. The two planned delta and quad velomobiles profiled in article 28, issue 12 of the Human Power eJournal provide an illustrative point to support your argument that vehicle dimensions(width/height) should be used to determine vehicle suitability(functional fit with cycle lanes) rather than the number of wheels. The current regulations would permit the more prone-to-roll-over delta model to be eligible for e-assist while the equal-sized quad model would not be eligible. From a road safety perspective, the current regulations are counter productive.
ReplyDeleteA clarification can be made to my previous comment. Instead of "two" velomobiles I should have written two "forms" of the same velomobile which can be readily transformed from a quad into a delta or vise versa. Since both "forms" carry more than 98% of the same DNA (read interchangeable parts), I believe this clarification enhances the point that the current regulations hamper the vehicle "form" which is more conducive to road safety- the quad.
ReplyDeleteThere is much more to stability of a quad formfactor than 'mere' RoR. A tire blowout, a large pothole or a rumble strip can cause a typical tadpole velo to lose control, do a 'ground loop' and immediately roll over. Also, during cornering, weight transfer further robs the rear tire of traction - making such a scenario all the more likely. A quad can have more equal weight distribution and have a 'spare wheel' thus making a tire blowout or quick puncture completely uneventful.
ReplyDeleteWell thought thru arguments in this blog.
ReplyDeleteAnd thank you for evaluating our Podbike FRIKAR design. I documented my reasons for using four wheels, as well as other engineering decisions, in my thesis that can be found here: https://www.podbike.com/wp-content/uploads/2020/10/Sorensen_Per_Hassel.pdf