Part 1
Written By
Rickey M. Horwitz
Version 7.0
The written material contained in this section is protected by
Introduction
This chapter focuses on the basics of
recumbent tricycle designs. In order to
keep the concept elementary I have simplified many of the terms and explain the
technology in detail at a level that most people will enjoy reading. If
the explanations contained in this document do not address your concerns or
doesn't present the 'big picture', please email me (Rick) at rhorwitz@hellbentcycles.com
Tadpole vs. Delta Design
Before elaborating on the design overview, I
thought it would be appropriate to offer some reasons why I prefer the two
wheels front, one wheel aft design commonly referred to as the 'Tadpole'
configuration. The most common tricycle
design is the single wheel in front, two-wheel aft configuration (1F2R)
referred to as the 'Delta' configuration.
Although a well-designed Delta trike has many merits such as reduced
cost, weight and complexity, it does not have the handling characteristics of a
Tadpole design. Without going deep into
physics to explain this comparison, I’ll only mention that it has something to
do with the Moment Of Inertia. In layman
terms, a Tadpole trike’s front-end exhibits less acceleration (less G forces)
than the rear end when turning. This
allows the trike to negotiate corners at great speed and stability. On the other hand, a Delta trike exhibits the
opposite condition; where the vehicle’s front-end exhibits a higher degree of
momentary acceleration in comparison to the rear. This condition results in over-steering and
can compromise the handling performance of the vehicle. Although the overall handling characteristics
are dependent on the actual design of the vehicle, the Tadpole design comes out
as the winner as for handling.
As for braking, again the Tadpole has the
major advantage. During braking, a
vehicle weight displacement and force goes forward. This is why on all vehicles the major braking
is performed on the front wheel(s). On
a delta, only a single wheel exists. It
doesn’t take a rocket scientist to figure out that stopping with two front
wheels is more efficient and effective than a single wheel.
A tadpole configuration uses the same
steering geometry design principles as an automobile. Geometry considerations
such as Caster, Camber, and Toe-End exist for this trike design just as they do
on an automobile. In contrast, a Delta design uses geometry similar to that of
a bicycle. The design objective for the Thunderbolt Mk II was to create
a 'Cycle-Car', not a bicycle with a third wheel.
Front Steering vs. Rear Steering
One gimmick that pops up every year or so is
a rear steering HPV. The rear steering concept has been applied to both
Tadpole and Delta trike configurations without any staying success.
Although the virtues of rear wheel steering include a simplistic design,
lighter weight and a smaller turning radius, the fact that the trike
drives like a forklift makes it a losing proposition every time. Since
people have a fascination with gimmicks, the rear steering trike will always
have a place in the HPV industry.
Lean Steering Trikes
A lean steering trike is a three wheeled
vehicle that steers by virtue of leaning the rider's body into the desired
direction of the turn. Although several variations exist, the most
notable implementation is where the seat moves in relation to the frame causing
the vehicle wheels to turn. A stationary horn or tiller is fixed to the
frame (this can also be a U-bar) allowing the rider to lean the seat left or
right (the seat is linked to a steering mechanism) , hence steering the
vehicle. Since the rider leans into the direction of the turn, the
center of gravity is optimized producing a trike with excellent low speed
handling. The concept is similar to steering a bicycle with one obvious
exception: the leaning is linear to the steering and not by the G forces
applied. Consequently, the steering in this implementation is not
optimized for higher speeds. Obviously, the example explained here may
not be characteristic with all lean steering tricycles.
Lately, there have been other
implementations of the lean steer trike that allow the front wheels to lean
into the direction of the turn. Not only does this enhance the handling characteristics
of the trike, it also relieves the wheels from side-loading allowing larger
diameter wheels. As with most emerging technology this concept is
relatively new, requiring future improvements to make it practical.
The main premise of a trike is to provide a
stable platform that does not require balancing. Although lean steering
has the potential of optimized handling at lower speeds, it does require
equilibrium to master. As with rear steering, this is yet another gimmick
that comes and goes. Those who are hopelessly hooked on two wheels will
be glad to know that there will always be a lean steering trike in at least one
incarnation or another.
Front
One of the virtues of the recumbent industry
is the ever-evolving, innovative change being made. The recumbent trike
market is undergoing even more changes. The recent technology making news is
front wheel drive systems. The main virtue of a front wheel drive system
is superior traction and a localized drive train. Currently, two trike
manufacturers build front wheel drive, tadpole-configured recumbent
tricycles. This technology is still in its infancy as both manufacturers
have several problems to address. Two outstanding problems preventing
this technology from instant success are weight and reliability
considerations. Ironically, one would think that a localized drive train
would have these points as virtues instead of liabilities. However, with
a limited slip differential, four universal joints and cantilevered wheel
bearings in the steering knuckles, this concept becomes overcomplicated. Perhaps in the near future, these problems
shall be worked out, making it a desirable alternative to the conventional,
long-chain and idlers of the rear drive system.
Suspended Trikes
A well-designed suspension system can offer
a featherbed ride regardless of the conditions of the road. Up until
recently, most implementations have been limited to rear suspension, as this is
extremely easy to implement. Since rear
suspension poses no severe adverse consideration to handling or steering
geometry, it is gaining fast popularity. With easy-to-route chain
management systems, and the cranks being so far forward from the swing arm,
it's obvious to see that rear suspension can be easily adapted to a
tadpole-configured recumbent trike with little compromise. However,
the virtue of a rear suspension system is refutable, as the rear wheel delivers
less than 1/3 of the total shock felt by the rider. Secondly, the swing
arm designs currently employed on these trikes are all more susceptible to
side-loading forces than conventional triangulated rear ends. Therefore, front suspension would be more of
a requirement than the rear. However, a
lightly dampened front suspension system hampers handling, as the trike is more
inclined to sway. More sophisticated suspension designs use a
parallelogram configuration that reduces sway and brake dive. Other
designs simplistic in nature, avoid these side-effects by restricting the shock
travel to under 25 mm (1 inch).
Basic Trike
Frame Design
In this section, I discuss the basic
elements of a Tadpole trike design. I also attempt to discuss the
hierarchy of each as they apply, so that the readers can understand that any
trike design is actually a compromise of all of these elements.
Weight Distribution
The weight distribution of a trike dictates
how well it handles. The more weight on the forward wheels, the better the
cornering and less over-steer. However, too much weight on the front wheel
causes the rear wheel to be too light.
This can lead to the rear wheel to wash-out during hard cornering or
cause the trike to end-over during braking.
Too much weight in the rear of the trike causes it to capsize even
during mild handling as the single wheel has the majority of weight. A trike with 70/30-weight distribution is
optimum.
Center of Gravity
Forget what I said about weight
distribution. If all the weight is
placed well below the axle, the trike is going to have excellent handling regardless
of weight distribution. Obviously, a low slung vehicle does have several
disadvantages including visibility, safety and comfort.
Wheelbase
The effect of the wheelbase on a trike
influences the steering, weight distribution and overall comfort. The wheelbase
is the length between the rear wheel axle and front wheel(s) axle. A short
wheelbase makes the turning radius of a trike much smaller while a long
wheelbase makes the turning circle larger. Additionally, a trike with a short
wheelbase exhibits faster steering than a trike with a long wheelbase. A short wheelbase trike generally places more
of the payload weight on the front wheels. On the other hand, a longer
wheelbase trike offers a much smoother ride, as the rider is not placed on top
of the wheels. Obviously, a happy medium is needed. For the Thunderbolt
and Zephyr, I used a 43-inch wheelbase. If tighter turn radius, faster
steering and convenience is required, a much smaller wheelbase should be used.
A reduced wheelbase is compromised (limited)
by two factors; the rear wheel size and the seat angle. Obviously a trike
with a short wheelbase, steep seat angle, and large rear wheel cannot
exist. This statement is repeated when I
discuss wheel size and seat angle, so remember it.
Wheel Track
The wheel track is the width between the two
front wheels. The wider the wheel track the less susceptible the vehicle is to
capsizing during cornering. However, if too wide, the vehicle becomes
impractical on most bike lanes. A 32-inch wheel track offers excellent handling
and is practical for all bike roads, too. Lately, several manufacturers
have released compact trikes that have reduced wheel tracks under
29 inches. The overall widths of these trikes allow them to pass through
a standard 32 inch doorway. On the
Hell-Bent Spitfire RS, the wheels can have a negative camber (we'll discuss
camber later) that allow a wider wheel track but the overall width compact
enough to fit through a door way.
Steering Geometry
The quality of the steering system and
steering geometry also dictates the performance of the trike. The steering geometry is outlined later in
this chapter.
Frame Design
The last element in basic trike technology
is the frame design. There are several issues
here that affect efficiency and handling. The most important issues of
the frame are weight and rigidity. Along
with rigidity comes stability, as any frame or wheel flex is always undesirable
especially at high speed. Beyond these basic requirements are other
elements that should be equally noted. Reliability, cost, ergonomic and
convenience are but a few requirements that the designer must
consider. However, these considerations go beyond the scope of
performance.
An excellent frame configuration is a
3-dimensional space frame (e.g., Greenspeed/Catrike/Spitfire), as the design is
both extremely strong and lightweight. The next choice is a web-gusset,
reinforced frame (e.g., Thunderbolt/Spitfire), as it is strong, relatively
light and compact. However, the lowest
weight, lowest cost, and least rigid of any vehicle would be a two-dimensional
frame without seat stays (e.g., Terra Trike and older Trice).
Summary
The success of a recumbent tricycle design
is a careful mixture of Weight Distribution, Low Center-of-Gravity, Wheelbase,
Wheel-Track, Steering geometry and frame configuration. In most cases the design will be a compromise
of all these attributes.
A recumbent trike is only as good as the
steering, as it behaves similar to an automobile. Therefore, the steering system is inherently
complicated, as more than a single geometry is used to define it. In this section, I'll discuss the fine art of
steering geometry.
Caster Angle
The first geometry is the caster angle. This angle is the kingpin plane relationship
to the wheel contacting the road (contact patch). Refer to the drawing
below. As the drawing illustrates, the kingpin points down in front of
the tire's contact patch. Since the steering system rotates on the
Kingpin plane, the relationship between the contact patch and the kingpin
forces the wheels to point inward as weight is placed on the wheels.
Increase the kingpin angle, and more force is applied to bring the wheel
inward. The resulting effect forces the
steering system to return back to a neutral (or straight) position. We use a
12° caster on the Thunderbolt project.
As a footnote, a standard automobile uses a 4-5° caster and a race car
or go-cart gets much steeper.
Camber
The next geometry is the camber angle of the
front wheels. If the wheels are at exact right angles to the ground (90
degrees) or the distance between the top of both wheels equals the distance
between the bottom of both wheels, the camber is said to be neutral. If the
distance between the top of both wheels is shorter than the bottom, the camber
is said to be negative. And, if the distance between the top of both wheels is
longer than the bottom, the camber is said to be positive. Normally, a negative
or neutral camber is desirable. The Thunderbolt project has adjustable camber
so that you can adjust it to your satisfaction.
Toe-In
The toe-In is actually the desired relationship
of the front wheels. Both front wheels
point inward very slightly when the vehicle is pointed straight. On a trailing arm steering system (such as
the Thunderbolt), both wheels have a natural tendency to point inward, as the castering effect forces them to do so. However, the forward motion of the wheels
counteracts this force. To reduce this effect, we bring-in, or toe-in, the
front wheels slightly. A toe-end of no more than 0.1" is sufficient.
Ackerman Steering Compensation
The Ackerman steering compensation provides
a way for a vehicle to turn without the front wheels scrubbing. In layman's
terms, this means that when the vehicle is steered in either direction, the
inside wheel shall always turn sharper than the outside wheel. Let's look at
this with an example: My Thunderbolt can turn around a 15-ft. circle. This
means that the outer tire is pointing at a particular angle that follows the
15-ft. circle. However, the inside wheel, which tracks 32 inches closer to the
inside, must turn at a sharper angle so that it can follow a 9.5 ft circle.
Obviously, if both wheels turned at the exact angle, they would scrub when the
vehicle turns. Not only would this wear out the tires, it would also cause the
vehicle to drastically slow-down when turning.
There is some consideration concerning
Ackerman that should understood. First, perfect Ackerman does not mean
always yield the best performance. Secondly, the accuracy of the Ackerman
compensation is dependent on the type of steering system used on the trike
design. Peter Eland has created a couple of spreadsheets that
accurately calculate the Ackerman Steering based on the steering linkage type,
wheel base and wheel track.
As mentioned, perfect Ackerman steering
compensation does not guaranty the best performance. In some cases it is
desirable to reduce the Ackerman during large radius turns as it makes the
steering less sensitive and less prone to over-steering. This
Anti-Ackerman actually prevents over-steering at high speeds. An
Anti-Ackerman is actually a partially compensated Ackerman implementation and
allows a small amount of scrubbing when turning a large radius, but it follows
the full compensation during smaller radius turns. The exaggerated
results is a vehicle that slows down in the corners, but allows the trike to
sustain faster speeds without steering instability. My personal T'bolt
can sustain speeds greater than 50 MPH without steering instability, as it uses
a 2° Anti-Ackerman offset. Therefore, perfect Ackerman is up
to the rider. Again, I strongly recommend using Peter Eland's
spreadsheet.
Kingpin Inclination (Center Point Steering)
The inclination of the Kingpin allows the
steering axis to turn precisely on the center patch of tire contacting the
pavement (hence the name Center Point Steering). This imaginary
intersection is commonly referred to as the Scrub Patch. Because the steering axis rotates directly
over (and front of) the contacted patch of tire, the steering is less affected
by defects in the road, hence reducing 'bump steering' and allowing the full
effects of the caster to work. Another by-product of kingpin inclination
and Caster allows the camber to change in relationship to the wheel steering
angle. This compensation allows the wheels to lean into the corner in which
they are turning. Ultimately, this dynamic orientation modifies the wheel
geometry resulting in slightly enhanced handling. The kingpin inclination
is at a 90-degree plane in relationship to the caster angle.
Deviations
of King Pin Inclination
Most automobile designers purposely reduce
the Kingpin inclination angle so that the projected intersect line falls short
of the center of the tire patch. This is done to give the steering an
enhanced 'road feel'. On the other hand, some trike designers
extend the angle so that the projected intersect line falls outside the center
of the tire patch. This over-compensation further reduces the effects of
brake pull, but can also cause over-steering. In my humble opinion
(and for what little it's worth), if concerned trike manufacturers used a
balanced braking mechanism, this practice could be avoided.
Some manufacturers refuse to implement
center point (Pointe for you over the pond) steering into theirs designs.
In some cases the designer has placed the King Pin axis so close to the wheel
that the king pin centerline becomes very close to the tire patch.
However, in most cases the designers or builders are just plain ignorant, as
their designs completely ignore this concept.
Kingpin
to Wheel Axle Orientation
The placement of the wheel axle, in
relationship to the kingpin, drastically affects the steering. If the wheel
axles are placed in front (lead) of the kingpin axles, the 'caster effect' is
defeated making the steering unpredictable and extremely unstable. However, if the wheel axles trail too far
behind the kingpin, the steering may be influenced by road shock and brake
steering. Again, this occurrence is referred to as bump steering and
brake pull. Ultimately, the wheel axle and kingpin should intersect or be
within 0.5 inches trailing.
Note
In this section, I address the stay
reinforcement issue by referring to a Seat Tube. The name originates from
descriptions intended for the standard diamond frame bicycle. Beyond this
discussion are other types of designs. Additionally, in terms originating
from the diamond frame bicycle, we refer to the 2 sets of tubes that support
the rear wheel as Chain and Seat stays. The chain stays are normally oriented
close to a horizontal plane, where else the seat stays
are normally oriented 30 to 70 degrees in relationship to the chain stays and
intersect at the wheel drop-outs. Recumbents are
weird, as these conventions sometimes do not apply.
Designing Rear Stays for a
Trike
The rear-end structure of a trike requires a
balanced combination of reinforcements to overcome direct vertical weight
loading, chain loading and torsional side
loading. Each of these forces is dynamic and some directly interacts with
each other. The designer or builder must make provisions to address each of
these loading forces as they play a critical part in the trike's overall
design.
Direct Vertical Weight
Loading
As mentioned, only 25 to 35% of the total
weight of the vehicle is placed on the rear wheel. Therefore, the need for
designing rear wheel stays for vertical weight loading is not a chief concern.
Most trike designs that do not include triangulated seat/chain stays (Dual
Cantilevered stays) still use seat support rods that attach to the chain stays.
Although this does not remedy torsional flex caused
by side loading, it does offset some of the vertical force applied to the chain
stays. Refer to the illustration below.
Chain Loading
Chain loading dictates that when high torque
is applied to the crank arms the force is transferred through the chain to the
rear wheel. The pulling action of the chain causes either a compression action
or cantilever action to the rear wheel stays. If the chain stays are relatively
parallel with the chain line, the energy exerted as a compression force. If the
chains stays are angled in relationship to the chain line (e.g., the base of
the stays are above the chain line), the stays undergo a cantilevered force
instead of compression force. As the angle increases so does the tendency for
this type of flexing. The vertical weight load on the stays help offset some of
this energy. In summary, the chain stays oriented to the chain line handle the
compression loading with more predictability than stays that are angled away
from the chain line.
Side Loading
Side loading is the affect of the rotational
side force placed on the rear wheel (although the front wheels exhibit this
too). This force is exerted during cornering or swaying of the trike; hence a torsional force is applied to the rear wheel stays. The
most common method of counteracting this type of force is to triangulate the
stays using a combination of seat and chain stays. Both sets of stays require
mounting to a firm seat tube (a term used to describe the base tube that both
sets of stays attach to) with a minimal space between each stay at the base.
The angle between the seat and chain stays is arbitrary as other compromises
exist. If the seat tube were a non-flexing structure, the optimum angle of the
stays would approach 90 degrees with the chain stays parallel with the chain
line. However, the seat tube is an integral part of the trike frame and is also
subject to flexing. Consequently, added reinforcement is necessary for the
frame to accommodate this configuration. A compromise would be to lower the
angle so that minimal reinforcement would be required for the seat tube.
Another method for reducing side loading
forces on the stays is to use a smaller rear wheel. Since a smaller wheel has a
reduced radius, the side forces have reduced leverage on the wheel axle.
Additionally, the smaller radius allows shorter chain stays decreasing the side
loading effect even further. On the down side, a smaller rear wheel makes the
ride of the vehicle even harsher. Special gearing is also an issue.
An Angled Cantilevered Stay is yet another
method for reducing side loading. I will discuss this design later.
Rear Stay Examples
Although many configurations exist, I have
compiled together a study of a few basic designs that summarize this section’s
discussion.
Dual Cantilevered Stays
The rigidity of a trike’s rear-end increases
both reliability and performance. Many trike designers choose to use only dual
cantilevered chain stays (or a single stay with a stub axle). Although this is
a violation of my trike design principle, it does reduce the overall cost and
weight of the trike. Furthermore, reducing the size of the wheel makes this
approach more attractive, as it reduces the cost and weight further and
increases the stiffness and reliability.
It is my opinion that the overall weight and
cost penalty outweigh the penalty for sacrificed performance and reliability.
Angled, Dual Cantilevered
Stays
Triangulated stays are not the only
solution. Back in 1995, I met Bill Haluzak who was
displaying a new, lightweight version of his popular short wheelbase Horizon
recumbent bike. Instead of using a triangulated rear stay, Bill opted to use a
single chain stay rear-end design using a modified BMX fork. At first glance I
thought the rear wheel would be susceptible to severe side loading. However,
after careful inspection I concluded that the rear end was adequately firm
proving my first impression absolutely wrong. What made Haluzak’s rear frame design so rigid that he could use a
single stay design? The answer lay in the geometry in which the stays were
designed.
The best way to explain this configuration
is to use the standard front wheel and fork as an example. The fork blades
point towards the ground in a vertical fashion. The supported wheel transfers
all the side loading forces to the fork crown. A dual cantilevered stays design
is identical to a standards fork assembly, except it’s mounted horizontally
instead of vertically.
Let us compromise and move the fork/stays to
45 degrees. We now have stays that can handle side-loading forces. This is what
the Angled, Dual Cantilevered Stays are all about. This approach can be
implemented for a trike, provided that a seat tube is added to compensate
for the low CG required for a tricycle design.
For any type of structure, the price for
rigidity is weight. The low CG required for a trike makes it difficult to
maintain angled chain stays without increasing the amount of main tubing
material. This can be seen in the illustration above. Another penalty that must
be paid is chain line routing. If the chain line routes directly from the main
tube to the rear wheel, the chain stays would flex under the demanding loads of
the chain. Consequently, either the angle of the stays must be reduced or
conventional triangulated stays must be employed.
Full Triangulated Stays
This classic design has been in use for over
a century. No surprises here, it works and there has not been a tube design
since that can rival the strength and reliability. Any trike manufacture worthy
of praise would design a trike rear end with full-triangulated stays. This
design solves all loading problems as well as chain and vertical loading. The
drawbacks of a fully triangulated rear end are that it complicates the design,
costs more and adds weight to the trike. However, these issues I regard as
trivial. See the illustration below.
Single Cantilevered Stay
The Windcheetah, AS32, and the Rubicon all
use a Single Cantilevered Stay design. In comparison to the Dual Cantilever
Stays, the rear wheel is supported on only one end of the axle. This chain stay
in most implementations is an extension of the main frame tube. Although I have
little experience with this design, I can confidently remark that the chief
redeeming feature is aesthetic quality and certainly not performance. As with
the dual cantilever stays configuration, the design is void of any side loading
support. Additionally, the single end support of the rear wheel axle is subject
to added cantilever forces. Again, a smaller rear wheel reduces these
problems, but does not eliminate them.
Summary
As with all aspects of the trike, the rear
end design is based on the builder’s specification. However, I have made the
decision for the Thunderbolt and have designed it using triangulated rear
stays.
Tadpole-designed trikes come in a wide
variety of wheel size configurations. If the center of gravity were not
an issue, I would be bold to mention that wheel size has little affect on
handling (if the rear wheel stays are sturdy enough). Chiefly, wheel size
affects efficiency, weight and quality of ride.
During the testing of my first prototype, I
discovered that large diameter front wheels were too weak and tended to fold
(taco) easily during hard cornering. BMX 20-inch wheels (ERTO 406) were tried
and found to work without any problems. Since the rear tire is under less side
loading, I was able to use 26-inch wheels that offered excellent rolling
resistance and made the ride significantly smoother than a rear 20-inch BMX
wheel.
The chief advantage of a small rear wheel is
that it offers better reliability and lighter weight than a large wheel. The
reliability aspect of a smaller wheel system is that smaller wheels tend to
take side loading better than larger wheels. The lighter weight virtue is
obvious, as a smaller wheel is lighter than a larger diameter wheel and the
wheel stays are smaller, too.
The chief advantage of larger wheels is that
they provide better Roll-Over resistance and offer a stable, comfortable
ride. Additionally, a large rear wheel does not require special gearing
(such as an oversized chain ring and extra chain).
Although I show more virtue for a small rear
wheel concept, I prefer using a 26” for the rear, as it allows an excellent
high gear inch range, provides a softer ride and has superior “roll over”
qualities that give it an overall performance edge over a 20” rear wheel.
The basis for a recumbent style HPV is to
provide comfort. Therefore, great care should be made to provide simple
ergonomics such as the placement of key controls. The seat should be somewhat
adjustable as to modify the orientation to suit the rider. In the case of a
recumbent, the height of the bottom bracket should require deliberation, as it
is a very important and subjective issue. It is best to use a neutral or
conservative approach to ergonomics as a baseline. After your experimentation,
the 
design
can be changed to accommodate your special needs.
The key advantage of a tricycle recumbent is
that the orientation of the rider has little impact on the handling
characteristics or performance of the vehicle (as long as weight distribution and CG are optimum).
A tricycle allows the designer greater flexibility in the design so that more
emphasis can be placed on the rider's ergonomics.