Part 2
Written By Rickey M. Horwitz
Version 7.0
The written material
contained in this section is protected by
Steering systems come in two
basic flavors: Over Seat Steering (OSS) and Under Seat Steering
(USS). Each of these steering systems has several configurations.
Over Seat Steering-
Advantages
- Lower weight
- Lower
complexity
- Allows a
narrow track wheels configuration
- Lowers
overall frontal area, improved aerodynamics
Disadvantages
- Rider cannot
use the tiller for support, requires a seat with lateral support to keep
the rider from falling out.
- Not as
popular as Under Seat Steering, due to arm fatigue or lack of intuitive
design.
- Fatigue
level is higher than an Under-seat steering system.
The Over Seat Steering (OSS) system is
normally configured as a 'T' or 'Y' bar Single Handle Tiller. As for the
better steering handle configuration, it is a matter of personal preference.
From an inconclusive observation, the Single Handle Tiller (or 'Y' bar)
is geared towards sport riding, as the rider's arms have limited support, but
tight control. The 'T' bar fits the more traditional role as it is both
user-friendly and a bit more comfortable than the 'Y' bar. Whatever the
preference, the design of the
Under Seat Steering-USS
Advantages
- Intuitive
control makes it easier to master
- Provides
comfortable support for arms
- Gives the
rider support during high G turn, precludes the use of lateral seat
support.
Disadvantages
- Heavier weight
compare to
- Increases
the Frontal Area making the trike less aerodynamic.
- Places the riders hands dangerously close
to the wheels or ground
- Requires
ample room for U bar clearance that may compromise wheel track or seat
width.
As for Under Seat Steering, the actual
steering mechanism is either a U-bar configuration or dual lever design. Again, the choice is up to the rider, as to
which configuration is better suited. The dual lever design is best
suited for ultimate comfort, while the U-bar gives the vehicle a sportier and
lighter feel. Additionally, the U-bar system tends to be simpler and cheaper as
it requires fewer parts for operation. However, the expensive, dual-lever
system offers superior linearity and better flexibility for adjustment.
Steering Linkage Systems and How They Work
The steering linkage is another factor in
the equation. Although more than a dozen steering systems exist, I'll
mention a few widely used steering linkage systems as they would apply to the
Thunderbolt. These are shown and explained below:
Single Tie Rod and Drag Link
System
This type of steering system was common on
early automobiles and eventually found its way to farm tractors. A
knuckle-to-knuckle Drag Link provides continuity between the wheels, while the
Tie Rod provides linkage to a Bell Crank (Pitman Arm). The best attribute is that the main linkage
consists of only two rod-end bearings.
This allows the steering to remain relatively tight. Although this system uses more parts than the
other two steering systems, it provides superior flexibility for adjustment and
provides adequate Ackerman compensation. However, the system weighs slightly more than
the other two systems mentioned. Misalignment
of the Bell Crank orientation (caused by the Tie Rod deviating from 90°) causes
a slight non-linearity throughout the steering range. This is compensated
by applying Ackerman to the steering knuckle control rod that links it to the Bellcrank.
The above drawing depicts an application for
As with the first example, this design also
offers good Ackerman compensation.
Dual Drag Link System
Note
This system offers lower weight, less parts than the Single Idler Arm system and is optimized
for Over Seat Steering, as the Bell Crank is mounted almost at the kingpin
plane. The major advantage to this system is that it provides near perfect
Ackerman compensation. This design was
used on the Volkswagen Bug over 50 years ago. The Bell Crank orientation
and length must remain constant to maintain proper Ackerman. Adapting a
USS steering system requires a U-bar mounted aft of the king pins. Unfortunately,
the steering linkage becomes increasingly complicated as a second Pitman Arm
(Bell Crank) and Tie Rod are required (refer to the drawing below). The Bell
Crank length (from arm pivot to axle) must equal the Steering Knuckle Lever
length (measured from the arm pivot to kingpin axle). Deviations to this
relationship can diminish the Ackerman compensation.
The example above was
initially used on the production Thunderbolts. Obviously, you can see why
we are using the Single Tie-Rod steering system.
Crossed Dual Drag Link
The Crossed Duel Drag Link is optimized for
a USS (U-bar system), as the
The science of maintaining a linear rod
linkage system requires the application of the Right Angle Rule. The Right
Angle Rule requires that both rod ends maintain a 90° angle to each linked
lever arm when the wheels are in a neutral, forward position. Not only does
this practice insure that both rods maintain a linear arc throughout the full
range of motion, it also insures the stability of the linkage. As the rod ends
approach an angle close to 0° or 180° in relationship to either of the lever
arms, the linkage rod loses its ability to hold and control the arm. The Right
Angle approach guarantees the steering linkage force is optimized throughout
the 90° arc of steering travel. This principal is applied to the Crossed Dual
Drag Link steering configuration shown above. To achieve the 90°-angle
relationship with the above example, the two drag link rods require separate
mounts on the Pitman Arm. To prevent tire scrubbing during turning, these
mounting locations are angled back further on the Pitman Arm to provide the
necessary Ackerman compensation.
Greenspeed has used this system for several
years and has only recently changed it to a dual draglink system for better
stability for braking.
Summary
Each steering system has its advantages and
disadvantages depending on its application. My personal choice is an aft
lever system using a Single Tie Rod and Drag Link System. My decision is
based on cosmetics and practicality.
Although I have described the mechanics, it
is well worth the time and effort to experiment with Peter Elands magical
spreadsheet.
Note
I do not have quantitative data that specifically
addresses this subject. However, I have conducted numerous experiments in
sling/webbing and rigid back seat design.
Seating is a preference. Basically, an HPV
seat is divided into two types: Mesh or Rigid. In some
incarnations, a combination of the two can be had. Each has its virtues
and disadvantages. Some people indicate that foam-back rigid seats have
the greatest efficiency, but none have
substantiated their claim. The same applies to mesh seats.
Performance Criteria
The critical performance issue of an HPV
recumbent seat is the provision for firm support of the rider's lower back.
Deflection for the lower back of the seat should be less than 1.5
inches. Deflection for the buttocks and upper back can be
exaggerated without much efficiency loss. Please note that this performance criteria may not apply to low angle
seating.
Another important performance issue is seat
weight. Lately, graphite composite materials have made rigid seats as
light as the nylon mesh on aluminum frame seats.
A lower seat angle allows better
aerodynamics. Aerodynamics play a critical role
in an HPV's overall performance. With a low
slung seat, the trike rider can cut through the wind like a hot knife through butter....right smack into a car! Low slung
seats compromise the rider's vision, so beware!
Comfort Criteria
As for comfort, take your pick. My
personal preference is a Mesh Sling seat. The fabric is breathable and is
void of uncomfortable hard spots. A well designed, quality made sling seat
has several adjustments that can fit the most discerning buttocks.
The virtue of a rigid seat is that it can
provide firmness. However, in my experience, they have been scorned by many
racers, as they retain heat and moisture and many are not adjustable.
Easy Racer has addressed some of these problems by using a set of contoured
pillions. These pillions are designed for minimized area, but allow maximized
support and comfort.
Seat angles and head rests are subjective
topics. As for an efficiency advantage (disregarding aerodynamics), I have
heard pros and cons from each camp. As for head rests, they're
great. However, bicycle helmets are designed for upright bicycles and not
for recumbent trikes. Consequently, back rests and helmets don't seem to
get along these days.
Side/Lateral Support Criteria
A trike seat is unique in that it must
provide lateral support for the rider. However, this is not always
true. In the case of the Greenspeed GTS and GTR model trikes a piece of
nylon mesh (simple potato sack) is stretched over the seat frame and offers no
lateral support. In this design, the Under Seat Steering 'U' bar provides
the lateral support for the rider. In other cases lateral support is
built into the seat. A primary example is the Windcheetah. The
Windcheetah or Speedy was designed as a narrow track, Over Seat Steering
configured trike. Thus lateral support is required to keep the rider in
the cockpit during high G turns.
As mentioned, everything is a compromise,
even the seat.
Before I go off the deep end on this
subject, I must mention that most tadpole trikes use only front brakes. A
back brake is rarely used. Therefore, this discussion focuses on front wheel
brakes. I'll elaborate on the rear brakes later.
Basically, three types of braking systems
are employed: Drum, Disc, and Caliper.
Drum Brakes
The standard drum brake used for the
majority of recumbent trikes is the Sachs VT5000. This drum brake is
classified as a single, leading shoe brake. The basic drum brake uses
two brake shoes inside a cylinder drum. When the brakes are applied, an
actuator rotates an oblong cam that forces both brake shoes outward against the
cylinder drum. See the illustration below.
Drum Brakes, Pros and Cons
The major advantage of the drum brake is
that it provides solid and reliable braking and is optimized for Tadpole trike
designs. The disadvantage is that a drum brake performs poorly in wet weather
conditions (if moisture gets into the drum) and is susceptible to heat fading.
Additionally, the self-energizing drum brake action is nonlinear and may be
slightly unpredictable.
Single Leading Shoe Drum Brake
The single leading shoe drum brake is a self-energizing
brake system. The principle behind the self-energizing brake is that when the
brake shoe is applied to the drum, the brake mechanism diverts some of the
rotating energy and applies it to the shoe for additional contact force to the
drum, hence more friction and stronger braking force. In essence, the
self-energizing mechanism operates as a positive feedback system. The chief
component of the self-energizing system is the leading shoe. As mentioned, the
shoe moves on a stationary axis. On the opposite end, a cam is used to
push the shoe against the drum. The leading shoe is designed in such a fashion
that when the cam pushes that shoe against the rotating drum, the initial
friction grabs the shoe and forces it even harder against the drum. As the name
implies, only one of the two shoes is self-energized. The direction of rotation
dictates which shoe is leading (self-energizing). In most
cases. the drum brake manufacturer designs the
leading shoe slightly larger and heavier than the passive shoe. The Single
Leading Shoe Drum Brake is widely used for bicycles and HPVs.
Dual Leading Shoe Drum Brake
The dual leading shoe self-energizes both
brake shoes. In this configuration, the stationary axis is replaced with
another
Drum Brake Fading
Brake fading is the degradation of braking
power over a defined time of constant usage. An example is traveling down a
steep and long descent, applying the brakes constantly to maintain a safe
speed. During the descent, the brakes may appear weaker, requiring extra force.
Contrary to popular superstition, brake
fading is caused by the expansion of the brake shoes and drum that occurs
during extreme heat. When brakes are cold or at room temperature, the brake
shoe fits flush against the drum. When both of these components get warm, they
began to expand. Consequently, the brake shoe no longer fits flush against the
drum and braking is impaired. The brake shoe material 
does not compromise due to heat, and hence
does NOT cause brake fading! Over the last century, scientists and engineers
have perfected several composite materials that stand up well to excessive heat
and wear. Braking is a science, not voodoo magic.
Disc Brakes
The problems with brake fading and
sensitivity to moisture have both been remedied by the advent of the Disc Brake
system. The disc brake applies a set of flat pads on opposing sides of a
revolving rotor. Since both brake pads and rotor surfaces are flat, the brake
is infallible to fading or moisture buildup.
Disc Brake Pros and Cons
The major advantage of a Disc Brake is that
they provide excellent and reliable braking and are optimized for Tadpole trike
designs. The disc brake action is proportional and provides smooth braking even
during the harshest weather conditions. As for disadvantages, the
majority of disc brakes are heavy as compared to drum brakes. Lighter disc
brakes are available, but are very expensive. Performance reliability for
disc brakes is another problem, as most disc brakes are prone to rubbing. Not
only is this rubbing an annoyance, it is also a performance robber.
In recent times, the disc brake systems
adapted for bicycles have advanced dramatically. In the past, the bicycle disc
brake had a negative reputation as being heavy, noisy and having lackluster performance. However, due to many technological breakthroughs (chiefly in material
science), disc brakes are now smaller, stronger, and quieter.
Disc Brake Characterization
Several variations of the disc brake exist.
A disc brake is categorized by Actuation and Execution.
Actuation
Actuation is how the brake is activated.
Three types exist: Mechanical Cable, Hydraulic, and Hybrid.
Mechanical
Cable Systems use much
of the same hardware as a standard bicycle caliper brake. The brake is actuated
using a conventional handle and cable/housing. A levering or cam system
constricts two brake pads against the rotor in order for braking. On the Avid disc brakes, a cam and ball
bearing system is used for even braking. The advantage of a mechanical disc
brake is that the cabling is simple and parts are always readily available. The
major disadvantage of this system is that the inherent cable-stretch and cable
housing compression reduces the overall effective force that can be applied to
the brake mechanism.
Hydraulic
Systems rely on a
Master and Slave cylinder system to provide the actuation. As with all
hydraulics, the medium is a lightweight oil that is
moved through a semi rigid line from the brake handle (Master Cylinder) to the
disc caliper. The amount of force developed by the Master Cylinder depends on
the cylinder’s displacement. The direct force that can be applied by a
hydraulic system is awesome! However, no system is without its problems. Replacement parts are difficult to find, and
if you don’t like the handles that came with your brakes, well... too bad.
The
Hybrid System uses standard brake cables, but actuates a
mechanism that contains both Master and Slave cylinders. The reason for this
system is it allows a cable linkage system to be optimized by the caliper.
Additionally, the hydraulic actuator provides better performance than a total
mechanical system. As a mixed blessing, conventional cabling and brake
handles can be used. This type of disc
brake is slowly being phased out due to the unneeded complexity.
Execution
The mechanics of a disc brake are
simple: Squeeze two brake pads against a
turning rotor and voila! However, preventing the brake pads from rubbing
against the rotor (when the brake is not engaged) has always been a
problem. I'll describe two methods how this is accomplished.
In the Floating
Rotor design, a Caliper containing the actuator
and brake pads is situated in a fixed position (e.g., mounted to the steering
knuckle). The Rotor is mounted to a spline shaft on the wheel hub where it
has restricted horizontal movement. When the rotor is rotating, it can
brush up against either of the two opposing pads. When this occurs, the
rotor bounces off the pad and is resituated (hopefully) in a position where it
is not touching either pad. The premise of this design is that rotor and pad
rubbing cannot be avoided, but can be reduced to a tolerable level.
Advantage: no calibration or adjustment and the system is
light and simple. Disadvantage: always slight rubbing and the spline is
prone to wear out quickly. We can be thankful that these types of brakes
are all but obsolete.
In a
Floating Caliper design, the caliper is either floating or is
biased to a location where neither pads contact the rotor. On the
Practical Innovation's disc brake, the caliper was designed so that it was in a
fixed or biased position during no braking. During braking, the caliper
became free-floating so that both pads could contact the rotor with identical
force. Advantage-least susceptible to rotor/pad rubbing. Disadvantage- many adjustments and weight penalty. As with the floating rotor, this design is
quickly sinking to obscurity.
The Fixed Caliper design is built around the
assumption that the rotor is perfectly true and will remain so. As the
name implies, this caliper design is stationary mounted using either a
concentric washer or spacers to make the final adjustments. Once adjusted, the pads should not touch the
rotor until the brake lever is actuated. These disc brake systems are used
for both mechanical and hydraulic and are usually the lightest and most common
system available.
Caliper Brake System
The venerable caliper brake offers adequate
performance and reliability. Since several books exist on this subject, I will
not elaborate much. The caliper brake system can only be used with the
Steering Stirrup that supports a standard BMX wheel. This additional
ancillary can compromise weight constraints. However, the caliper brake is
readily available and so are the BMX standard wheel sets. The economy and
practicality of this system makes it a very attractive alternative for the home
builder.
Summary
As a designer and innovator of disc brake
systems for tadpole trikes, my opinion stands as an authority on this subject.
Currently, I feel that the drum brake is the most practical choice (not always
the best) for recumbent trikes. The drum brake is inexpensive and easy to adapt
to tricycle needs. In addition, the wheel can be easily removed. Even though the hydraulic disc brake beats
the drum in almost all categories, the price is normally higher for most
people.
Side Note
As the former owner of
Practical Innovations, my mission was to produce a product that was
technologically ahead of its time. I spent many months and thousands of dollars
developing a practical disc brake system. My first two generations of disc
brakes were utter failures. However, perseverance prevailed and I finally
developed a high performance disc brake system that was reliable. The disc
brake was the main selling point for all models of Zephyrs sold. Although there
are now disc brake systems that offer better performance than my own, I am the
only manufacturer that has successfully implemented a proprietary brake and
linkage system to a recumbent.
The biggest misunderstanding in designing a
recumbent trike is the requirement for both front and rear hub
bearings. For years we have been tantalized by all the great custom hubs
built by Phil Wood and countless other manufacturers. Most of these hub
builders use seal
Radial
loading is the amount of
weight placed vertically above the axle. Sitting on a trike places an
axial load on the bearings. The drawing below is a cross-sectional view of
a radial bearing.
Axial or Thrust loading is the amount of force placed against the horizontal
plane of the axle. Negotiating a tight corner at high speeds places a radial
load on all three wheels. A cross-sectional view of a cup-and-cone axial
bearing is shown below.
Although all bicycles use a combination of
both axial and radial loading, the recumbent tricycle places much more emphasis
on axial loading. Therefore, the venerable cup-and-cone bearing
arrangement is still the most effective.
If an axial load rated bearing is the best,
why are they less reliable than the seal
Another advantage of the cartridge bearing
is the easy serviceability. In most axial bearings, the cup is an
integral part of the hub and cannot be replaced.
Summary
Although the cartridge bearing appears
attractive, it is not always the ultimate solution.
Polyethylene Tubing
In today's trike market, the polyethylene
garden sprinkler tubing is widely accepted as the standard. This tubing
is light, offers a slick surface, and is inexpensive. Another important
feature about this tubing is that it protects the chain from grime and dirt,
hence extending its longevity. However, efficiency can be compromised if
the wrong lubricants are used on the chain. It's best to use very thin
oil when using this tubing, as it offers the least amount friction between
chain and the polyethylene surfaces.
The reason why a chain can pass through this
type of tubing with little friction is very elementary. In a straight
path, only outside edges of the chain contact the polyethylene. What is
important is finding the correct diameter of tubing to use, as three basic
sizes exist, along with several thicknesses. If the tubing is too small
in diameter, the chain surfaces have more contact with the tubing creating
higher friction. A diameter of tubing that is too large makes the system
heavy and allows too much chain slop. Polypropylene tubing with ~.7
OD and an ID of ~.6 inches seems to work the best. However, those who
have an endless supply of money and resources can purchase the ultimate
friction free solution; Teflon, tubing goes for $5.00/ft. BTW- We sell this stuff on our website!
The application of this tubing should be
restricted to a straight chain line. This is especially important when
applying the tubing to the drive side of the chain. On the non-drive side
(the side of the chain where the derailleur pulleys take up the tension)
routing the tubing in an arc may increase friction. If an arc is
required, make it as gradual as possible, as to decrease the chances of
friction. Another way of increasing the efficiency is to keep the chain
tensioned, as to make it self supporting so that it never has to rest fully on
the inside of the tubing.
Chain Pulleys
Ok, so they're actually skate board
wheels. They work great if you get the right ones. Look for a
durameter of at least 98 and a diameter of 50 mm. Almost every recumbent
uses a chain pulley. In most successful trike designs the pulley is
used almost exclusively on the drive side of the chain to change the chain line
angle.
Mid-Drive Systems
To my knowledge,
s
Performance
Considerations
The
most frequently asked question I get is, “What makes a high performing
recumbent trike?” This answer assumes we
want ultimate efficiency and handling on both flat and hilly roads. The first and foremost criteria for
performance are aerodynamics followed by weight and rolling resistance. I’ll elaborate each of these attributes and
explain how they are addressed in a performance trike design.
Aerodynamics
The
majority of energy lost is through wind resistance. The science of aerodynamics is very complex
and sometimes controversial. I confess
that I only understand the basics of this field. However, applying these basics to an
un-faired trike design provides 90% of what we need to know.
Note
The mention of a front
fairing and tail box has been omitted form this discussion. Although both accessories provide an ultimate
solution to reducing aerodynamic drag, I don’t have the expertise to elaborate.
Reducing
the overall frontal area make the vehicle more aerodynamic. There are several ways to reduce the frontal
area of a trike. I have provided a few
of the major ones here:
Increasing
the seat angle provides an aerodynamic advantage. Most sport recumbent trikes have seat angles
less than 35 degrees. Some have angles
down to 25 degrees. As mentioned earlier
in this chapter, a low angle seat does have controversy as to comfort and
visibility.
Decreasing
the wheel track or the overall width of the trike is an obvious way of reducing
the frontal area of the trike.
Tucking the rider’s
arms towards their torso decreases the amount frontal area. This is accomplished using an OSS Joystick.
Using
smaller front wheels reduces the frontal area.
This is a paradox. Although
reducing the wheel size down 20% (20” Vs. 16”) will obviously yield better
aerodynamics, the performance edge is completely eroded away by the decreased
roll-over efficiency of the smaller wheel.
We will explain this later.
Note
In several periodicals and articles, I have found
conflicting definitions of "Sprung" and "Unsprung"
weight. In the automobile industry, Unsprung
weight refers to the weight not supported by springs (e.g., wheels, steering
linkage, etc.). People talking about bicycles have contradicted terms
referencing both Sprung and Unsprung weight as the
same. Consequently, I am avoiding the semantics of both definitions, as they
officially do not apply to HPV's (according to my
Webster's).
Frame Weight
As
a preface, I wish to explain the virtues of weight or lack there of. A light-weight trike allows faster
acceleration and the ability to climb hills much easier than a heavier
trike. An out-of-shape, grossly obese
rider on a light trike is like ordering a diet coke with a super size of fries
and a triple cheeseburger. A light trike
is best suited for an athlete, as it is they that best benefit from this
performance attribute. Obviously, a
light weight trike will not compromise on stiffness or reliability.
Dynamic Weight
The performance merit of any bicycle or HPV
is bas
Weight residing in non-moving parts (e.g.,
rider's torso, HPV frame, and accessories) presents less of a performance
penalty, as it only plays a factor during acceleration, up-hill riding and
added wheel resistance.
In summary, lighter wheels and drive train is
the key to optimum performance. Weight
loss in non-moving components should be of secondary concern.
Decreasing Dynamic Weight
Throughout
this chapter I mention that a trike design is the sum of many compromises. Consequently, lowering the dynamic weight of
any HPV will be a fine balance of compromise.
Lighter Components
Assuming
money is no object, an ultra light chain, Crankset,
pedals, and wheels provides the most effective means of reducing Dynamic
weight. Lighter tires are perhaps the
most effective and cheapest way of reducing weight while switching to Titanium
spokes provides the most expensive method.
Smaller Rear Wheel
Decreasing
the size of the rear wheel from 26” to 20” appears as an easy and convenient
method of reducing the rotational weight of a trike. Although there is a 90 gram difference in
wheel weight, a smaller wheel requires a larger chain ring to achieve the high
gear inch range as the 26” wheel. A 68
tooth chain ring increases the overall weight by at least 42 grams, not to
mention 16 extra links of chain would add yet another 41 grams of weight (2.55
grams per link x 16). The total weight
saved diminishes down to only 10 grams. The
greatest attribute using a smaller wheel is where the weight is saved. After all, all weight is not created equal
when we throw angular velocity into the equation. Are we confused? Let me explain:
I have two rear
wheels that weigh 4 lbs ea. However, one
wheel uses a heavy hub while the other uses a heavy rim. Which wheel has a higher rotational mass? The
wheel with the heavier rim of course!
Rolling Resistance
I
may be splitting hairs on the hierarchy of this discussion. Some may claim that rolling resistance is
more paramount than vehicle weight.
Having three wheels instead of two may be a valid point to this claim.
The
subject of rolling resistance is a very touchy subject, as many people have
their own strong preconceptions.
Therefore, I’ll present all the factors that affect rolling resistance,
but will not place them in a hierarchy.
Tire Rolling Resistance
Several
years ago, Ian Sims built a rolling machine that was intended to test the
rolling qualities of wheels and tires. Although
his test could not measure the efficiency of a tire size, it was able to
produce some interesting results with different brands of tires. More importantly, some European groups had
also conducted empirical testing of tires with results that paralleled much of
the results conducted by Ian. Some of
the winners in this group of 20”size tires include Tioga Comp pools and Schwalbe Stelvios. Although the rolling resistance for some
tires may be extremely low, it is best to consider other important factors too,
such as tire weight, and application.
Roll-Over Resistance
Warning!
There are
people that will always believe that small diameter wheels have a rolling
advantage to larger diameter wheels. This
article is not to evangelize, but to educate.
Although my method and conclusion of research are open for debate, I
don’t spew self-serving propaganda to promote my product or design.
Small tires small minds? Not exactly, up until ten years ago, almost
all tadpole trikes used larger 26” or 700c rear wheels. The Greenspeed trikes became an almost
instant success with their 20” rear wheel design. Not only did the 20” rear wheel make the
trike slightly smaller and more convenient, but it made the rear much stiffer
too. As the popularity increased, the
question concerning the efficiency of a 20” rear wheel came under scrutiny. As mentioned previously, Ian Sims attempted
to defend his position by using rolling test data that favored the 20”
wheels. However, many discovered that
the test methods used by Ian were only conducive at determining tire rolling
resistance and not for comparing wheel size.
At the time of these results I too, strongly questioned the validity of
testing, as his test results for larger wheels greatly contrasted data
generated from other empirical testing published elsewhere.
The subject of larger vs. smaller wheels is
a very controversial subject. The answer
to this is understanding the simple concept of Roll-Over
Resistance. Roll-Over Resistance is the
ability or inability of a wheel to roll over an uneven or aggregate
surface. Example I have a skateboard
that rolls fast over a smooth sidewalk, but doesn’t roll well over the coarse aggregate
of asphalt. Apply this principal to
trike wheels. Over a smooth gymnasium
floor the difference between a 20” and 26” tire is very little. As the surface becomes increasingly coarse,
the 26” wheels will roll better than a 20” every time.
Roll-Over Resistance is measurement that can
only be accurately measured by comparing two wheel sizes over a known aggregate
surface. Although rolling resistance of
a tire can be accurately measured in a controlled laboratory experiment,
Roll-Over Resistance is best measured by empirical road test methods. In my expert opinion, Roll-Over Resistance is
as important if not more important as Rolling Resistance.
The problem applying this to a recumbent
trike is that using smaller wheels in front is a requirement, as they reduce
wind resistance, handle side loading, and allow adequate room for
steering. The same applies to many
recumbent bicycles, as a smaller front wheel allow easy foot clearance from the
crank set and reduces wind resistance.
However, on a trike we have three wheels instead of two. Since the front two wheels cannot change for
the reasons mentioned, we have but the rear wheel to be concerned with. Consequently, changing the efficiency of one
out of three wheels will not have the amount of success as that of a
bicycle. Regardless a larger wheel will
provide better roll-over resistance.