Reflections on the potential of human power for transportation

Tuesday, February 7, 2012

The Drymer and Varna Lean Forward

In my previous post (“Rx for a Healthy Commute” below), I proposed ten criteria I felt were necessary in the design of a human-powered commuter vehicle, or HPCV. In rough order of importance they are listed below.
1.       Weather Protection
2.       Statically Stable
3.       Reasonable Cruise Speed
4.       Cargo Carrying Capacity
5.       No Wider than a Bicycle
6.       Same Height as an Auto
7.       Comfortable posture and ease of entry
8.       Two-wheel drive
9.       Car-type Wheels
10.   Electric Assist for Hills
Now in the pervious post we examined the fact that criteria 5 and 6 seem to be mutually exclusive if you don’t want your tricycle to overturn while cornering. The concept of a leanable trike was proposed as a suggestion to address both criteria. The trike would lean at moderate to high speeds but the leaning mechanism could be locked out at low speeds and when the road conditions were slippery.
The concept of a leanable trike is not a new one and many auto, motorcycle and bicycle enthusiasts have experimented with the idea and at least one motorized vehicle is commercially produced.
There are basically four approaches to designing a leanable trike, two for one-wheel forward, OWF, and two for two-wheels forward, TWF layouts. Again I am only addressing wheel layouts that are symmetric about the front-to-back axis of the vehicle and layouts where the front wheel(s) do the steering.
For the OWF layout, the most common means of providing leanability is to allow the front wheel and the majority of the vehicle mass to rotate about a near horizontal pivot with respect to the rear-wheel pair. The rear wheels don’t lean and maintain their relative position to each other and to the ground.  Let us call this configuration the fixed-rear wheel or FRW for short. This FRW is popular for motorized vehicles because it allows both rear wheels to be driven the same as a non-leaning trike. In most cases the motor is part of the un-leaning mass. Since the motor mass does not tilt, the effectiveness of leaning on rollover resistance can be significantly reduced if the motor is heavy.

The Lean Machine is a prototype FRW vehicle from GM produced during the 1980s. The rear-motor pod remains stationary while the body and front wheel lean.
The Carver is a contemporary FRW leaning trike similar to the Lean Machine. The Carver is in production.

The other approach to allow an OFW vehicle to lean uses articulated-rear wheels or ARW for short. Usually a parallelogram linkage is used that moves one wheel down while moving the other wheel up. This approach is not as common as the FRW approach and is more common for human-powered trikes than motorized versions. The reason for this is it is more complicated to power the wheels for an ARW vehicle as opposed to a FRW vehicle. When used in human-powered trikes one has the additional option of driving a single rear wheel, both rear wheels or the front wheel.

 The vehicle above is the EcoVia, a prototype pedaled trike that uses the ARW approach to lean.  The drive chain connects the cranks to a drive shaft located under the seat. A gear cluster is rigidly attached to the drive shaft and two single-speed freewheels are attached at the outer ends of the drive shaft. The rear wheels are mounted on beams that rotate about the drive shaft. Each wheel has a fixed gear cog that is connected to the drive shaft freewheel by a second and third chain. A parallelogram linkage connects the beams, so as one moves down the other moves up. The dual freewheels on the ends of the drive shaft act as a positraction-style differential between the wheels. More on the EcoVia in a future post.
For TWF layouts, the most common means of leaning the vehicle it to use articulated-front wheels, AFW. Similarly to the ARW approach the wheels are interconnected, again usually by a parallelogram linkage, so as one moves down the other moves up. Of course, the TWF AFW layout has the additional complexity that the wheels must also turn. In almost all cases each wheel rotates on its own pivot and is interconnected by some form of tie rod. In many cases, the linkages for independent front suspension can be modified to couple those linkages and result in an AFW approach.
The Mercedes Life Jet is an example of the AFW approach. The rear wheel is driven in motorcycle fashion. The Life Jet uses coupled parallelogram linkages that allow suspension to be incorporated as well.
The last means of leaning a TWR trike is the fixed-front wheel approach, of FFW. I could not find a photo for an example of a FFW approach but I do recall a three-wheeled scooter that had two front wheels mounted very close together on a common axle.  
There are three methods of controlling the amount a vehicle leans to result in a turn where the resultant of the weight forces and the radial acceleration forces are aligned with the midplane of the vehicle; the balanced turn condition.  The radial acceleration force is the squared linear velocity of the vehicle divided by the radius of the turn. Therefore, the amount of banking for a balanced turn is determined by the speed of the vehicle and the tightness of the turn.
The most complex method of lean control is used in motorized vehicles that have electrical systems. A microprocessor can combine the inputs from a velocity sensor and a steering-angle sensor to arrive at the appropriate amount of lean. This signal is sent to some form of actuator that leans the vehicle.
The next method is to manually control the amount of lean. The Lean Machine had foot pedals to lean the vehicle and a hand control to do the steering. This method of two-input control doesn’t work as well when the rider’s feet are required to turn the pedals. Several vehicles have combined the steering and leaning controls into one motion.  These vehicles used coaster-type steering where the steered wheels were located at the ends of a common axle and the axle pivoted at its middle, like a child’s coaster. The pivot axis was inclined from vertical so the steering motion resulted in a tilting motion as well. These vehicles are correctly leaned for only one speed per steering angle. Since the vehicle requires less steering and more lean as the speed increases, the control is moving in the wrong direction. The control provides more lean with more steering angle. So, this approach works acceptably only for slower vehicles or where the speed range is narrow. Several children’s riding toys used this single input approach.
If the leaning system is well designed, consisting of a mechanism that leans the vehicle about an axis at ground level (mimicking the tilting of a single tire at the ground plane), than the vehicle can be ridden like a bicycle with no leaning control required. The approach, which we can call free leaning, is probably the most popular with light-weight human-powered vehicles and clearly it is the simplest. The addition of a manual lean-lock mechanism for very low speeds and slippery conditions makes this approach work for all conditions. 
There is an ever increasing amount of interest in leaning trikes in the human-powered vehicle community. One is in production from Canada and another is nearing production from the Netherlands.

The Varna cargo trike is the brainchild of George Georgiev, designer and builder of the Varna Tempest, the world’s fastest HPV (refer to “Back to the Future” below). With his ability to come up with simple but elegant solutions, Georgiev has come up with a very simple design for an OWF tilting trike. The trike is made up of two portions. The front section incorporates the front wheel, steering, pedals and seat. The rear section incorporates the two rear wheels, the freewheel and space for a rather-copious-cargo rack. The two sections are connected by a horizontal pivot behind the seat support which allows the front section to tilt with respect to the rear section. The leaning is therefore accomplished by the FRW approach. Within the horizontal tubes making up the pivot is a torsion spring that resists leaning too far. So the system is basically a limiting form of free leaning. The chain is long enough that the misalignment between the tilted crank and non-tilted freewheel can be ignored. Only one of the two rear wheels is driven, the other just spins freely.  The rear-wheel track is 16”. There is also an hub-motor option for electric power assist. The cost is $3590 with electric assist and $2650 without.

The other leaning trike is the Drymer from the Netherlands. The intent is that it will be in production soon. 

I must admit when I saw the Drymer video I thought I was watching a contemporary version of the Pedicar. The Drymer is a TWF trike that uses a AFW approach to leaning. The weight distribution of the wheels is a bit odd. Most of the weight is on the rear wheel for reasons of increased traction. As a result, each front wheel is lightly loaded and more prone to lateral slippage than is desirable. As you can see it is semi-enclosed, has a moderate cargo capacity behind the seat, 20 Liters, and has the option of a hub motor in the rear wheel for electric assist that will allow for a speed of 25kph. Cost is to be 6000 Euros for the full-up model with the body and electric assist and 3000 Euros for the base model.
So how do these vehicles measure up against the 10 HPCV criteria? I will add some speculation as to what can be done to meet the criteria if they are not already met.
1.       Weather Protection:  The Varna has no weather protection. Its long wheelbase and high rider position would make a faring rather bulky but something approximating the Lean Machines body could be made to work.
The Drymer has a body that provides some weather protection but the sides need to be enclosed to make it practical for very wet conditions

2.       Statically Stable: Since each vehicle is a trike, they can be made statically stable by locking up the lean. It is not clear that the Drymer has any lean-lock mechanism. The torsion spring on the Varna acts as a lean-limiter.
3.       Reasonable Cruise Speed: Without a body the Varna is limited to bicycle like speeds when pedaled.  A bit more streamlining on the Drymer might bump up it cruise speed to closer to 25mph.

4.       Cargo Carrying Capacity: The Varna has a very large cargo capacity. It should be noted though that the cargo does not lean. The more weight that is carried the less effective the leaning is in preventing overturning when cornering. The cargo capacity of the Drymer seems adequate.

5.       No Wider than a Bicycle: I think a target number here should be about 24” or less. The Varna is no wider than the rider’s shoulders so it is definitely narrow enough. The Drymer’s track is about 28” but there appears to be room to narrow that somewhat.

6.       Same Height as an Auto: Both vehicles have a safe rider height.

7.       Comfortable posture and ease of entry:  Both vehicles have good ease of entry and a comfortable rider posture.

8.       Two-wheel drive: There is no simple way to give the Drymer two-wheel drive. The Varna, on the other hand could have the freewheel fixed to a continuous axle and put ratchets in each wheel to provide two-wheel-drive with a positraction-type differential effect.

9.       Car-type Wheels:  Both vehicles could be converted to car-style wheels. In fact the front wheels on the Drymer are supported on only one side. So if the Drymer’s rear wheels were cantilevered and the Varna’s front wheel was cantilevered, they could employ car-style wheels where each wheel was the same.

10.   Electric Assist for Hills. Both vehicles already employ electric assist options. However, I question if hub motors are appropriate for the high-torque low-speed requirements hill climbing.

In conclusion, then, the lack of a body on the Varna trike keeps it from having any real potential as a HPCV. The Drymer, on the other hand, has a great deal of potential with more of an enclosing body and some additional aerodynamic improvements.


  1. There's also a Japanese commuter cycle that uses a lean steer arrangement that's a bit like the Drymer, except the Japanese example is a bit more compact.

    The following link, which includes a video, is from, a Chinese shopping website:

  2. @Hephaestus
    Regarding Tilting Three Wheelers, (HPV)
    Have you encountered the message group Jetalk? Henry Thomas has attracted a fine bunch of people, including some lurkers such as myself.

  3. Thanks for sharing your research work. Comparisons used and explanations provided help a layperson understand the importance of proper design and application (in the expanding velomobile market).