Thunderbolt Mk IIII Plans

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The most advanced and comprehensive trike on the market today can be built in your own shop using these comprehensive plans.  Over 200 illustrations and detailed instructions make these plans second to none.  For a detailed example click HERE.

(Details and dimensions omitted)

The drawing above is of a Thunderbolt Mk III, 20

The Background of the Thunderbolt

My success with the first Zephyr back in 1994 was credited to elegance. I was now challenged to design a recumbent trike that was ultra-light with nimble handling characteristics.  The only example of a high performance trike was that of the legendary Wincheetah Speedy designed by Mike Burrows.  I had the honor to pilot one of these fine machines and it left an everlasting impression  on me.  After several months of research I designed a sports trike with characteristics of those of the Speedy, but at a much lower price.  The newly designed trike was refined and named the 'Thunderbolt'.  The "T'bolt" as I nicknamed it, was a very high performance and low cost trike in it's day.  Although a capable machine, I sold only a handful before I closed my first business in 1996.  In retrospect, the Thunderbolt was technically superior to anything available.  

In 2002, I went back into business and released the Spitfire.  The Spitfire was based upon the success of Thunderbolt, but had several improvements including a reduced wheelbase and track, 38° seat angle, disc brakes and a smaller 24" rear wheel.  This new trike design weighed less than any tadpole trike on the market including it's micro-sized contemporaries.  Not only was it lighter, it was stiffer, more stable, faster and much better looking than the venerable Thunderbolt or anything else on the market.

The New Thunderbolt Mk III 20 and 26

The Thunderbolt Mk III is a home-project version of the production Spitfire ST with all design option included.  The differences between these two names are minor. 


The Mk III is offered in several configurations:


20 inch or 26 inch rear wheel


29 to 33 inch wheel track options


Single or two section (segmented) frame


Direct Steer or USS steering systems


38.5 inch to 42.75 inch wheelbase


Mk III 20

For optimized performance, the Thunderbolt Mk III incorporates all the features and interchangeability of the 26 with the newly designed 20. This includes an Ergo Seat design that has increased lumbar support making it more comfortable and better looking.  The reduced track option make this configuration as nimble as ever. 


Mk III 26

The larger rear wheel offers unsurpassed roll-over efficiency, better ride and a higher Gear/Inch range.  The Mk III 26 features an Ergo Seat design that has increased lumbar support making it more comfortable and better looking.  The reduced track option and increased wheelbase puts this trike on par with the ultra-high performance Spitfire RS. 


A side view comparison of the Thunderbolt Mk III 20 and 26



 The table below is a preview of the many build options available.

Option number

Option Description

Option Compatibility

Comments / Caveat

Chapter(s) affected


Narrow Track Width

3, 5, 6, 7, 8, 9

29.5” wide wheel track.  This option can only be used with the tiller steering option!



Standard Track Width

3, 4, 5, 6, 7, 8

Although an industry standard, this track width of 32 inches does not fit through standard doorways



Single Tiller Steering

1, 2, 5, 6, 7, 8, 9

Although compatible with any option combination, single tiller steering is an acquired taste that’s not suited for everyone.  Steering is an improved version used on the AVD Windcheetah



Direct Knuckle Steering

2, 5, 6, 7, 8

Direct Knuckle Steering is similar to the Catrike and is only compatible with the standard wheel track which has the added clearance between the wheels and seat rails



20” Rear Wheel, Single Piece Frame

1, 2, 3, 4

The 20 inch rear wheel option has a 39 inch wheelbase and is aimed towards touring and commuting.  7005 tubing length for the main tube is very scarce.



26” Rear Wheel, Single Piece Frame

1, 2, 3, 4

The 26 inch rear wheel option has a 42.8 inch wheelbase and offers excellent performance and comfort.  7005 tubing length for the main tube is very scarce.



20” Rear Wheel, Segmented Frame

1, 2, 3, 4

A segmented frame can be broken down and stored into a small area, e.g., large trunk.  Both 20” and 26” rear sections are interchangeable and have a 39 inch and 42.8 inch wheelbase respectfully.  More complicated than a single piece frame.



26” Rear Wheel, Segmented Frame

1, 2, 3, 4

A segmented frame can be broken down and stored into a small area, e.g., large trunk.  Both 20” and 26” rear sections are interchangeable and have a 39 inch and 42.8 inch wheelbase respectfully.  More complicated than a single piece frame.


Work Required 

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The art of bicycle frame building is not for the faint hearted. Additionally, building a complex Cycle Car is no easy undertaking either.  To complicate these matters, the Thunderbolt design requires lots of fabrication details.   I have made several changes to simplify the design of this project and  offer pre-fabricated parts at a reasonable price.  Still, novice frame builders may find this project challenging.  Improvising is a necessity for success, as materials and tools may not always be available.  Since a well-designed jig requires advanced skills and machinery, I have designed the trike so that elaborate fixtures are no longer required.

The frame is constructed out of either 6061 or 7005 aluminum with some 5052 alloy aluminum sheetmetal.  7005 aluminum offers excellent strength and only requires a simple artificial aging to achieve near maximum strength.  On the downside, tubing is hard to acquire.  This frame can also be made of 6061, but an expensive heat treatment is required for maximum strength.  Likewise, a heat treatment can be waived, but the tube wall thicknesses must be increased which also increases weight.


Welding aluminum is not for the first time novice.  Therefore, take this project at your own risk.  This trike can also be fabricated out of muffler tubing, but the performance of the frame will be greatly compromised.    Therefore, the frame Aluminum is extremely easy to work with can be formed and machined without difficulty.  In summary, aluminum is one of the best frame materials to use for building a trike. 



The Thunderbolt Mk III plans now include Redlines for building this project from steel!!!

Project Features

Over the last 12 years I have built an international reputation for outstanding detail of my instructions.  The Thunderbolt Mk III, represents detail and accuracy on a much higher level.  Each chapter is structured to include a Bill Of Material (BOM), Tool Requirements, Chapter Objective,  comprehensive Instructions, Illustrations, Drawings, and a Final Review.  Attention to every detail is included. 

These plans are not a bunch of ambiguous drawings.  This is a book that fully details every step of the building process.   Created by a professional engineer and technical writer, these plans offer detail not found in any plans for any price.


Design Features

Single Piece Cross-Member

In most kits, the cross-members consist of two pieces of tubing that requires mitering and elaborate fixtures to properly mount it to the main tube section.  The Thunderbolt Mk III uses a single formed piece of tubing that represents both cross-member tubes.  This assembly is shown below:


The cross-member tube is securely placed into the forward main section tube using a hole saw cut-out, as illustrated below:

Adjustable Steering Geometry

Almost all production trikes use a fixed geometry system. A fixed geometry means that neither Caster or Camber is adjustable. The most common method for securing the Kingpin to the tricycle frame today is by using a standard bicycle headset. Obviously the tube interfacing with the inner race of the headset represents the Kingpin. The Kingpin secures the steering knuckles using a single-ended cantilever design.  Although the materials for these steering systems are universal, they do not allow any geometry adjustments to the steering.

The Thunderbolt Mk III uses Rod-End Bearings as a way of securing the kingpin to the tricycle frame. These bearings have proven to be extremely reliable and effective. I first got the idea of using rod-ends from a local Go-cart shop.  I reasoned that if these bearings were tough enough for Quarter Midgets, they must be good for recumbent trikes too.

Another reason I chose to use the adjustable rod-end bearings was to compensate for the frame distortion caused by welding or improper alignment.  Since the usage of these bearings allows very loose tolerances, extreme trike frame alignment will not be a critical issue.

Rear Wheel Triangulation

Most trike designs do not use a seat tube and/or seat stays to add support to the rear wheel.  The flimsy chain stays results in a wheel that flops during hard cornering (commonly known as side loading effect). Although all these manufacturers dismiss this anomaly as benign, it does compromise the trikes handling capability and can lead to early frame failure. 

To reduce rear wheel flop, the Thunderbolt Mk III uses a fully triangulated rear stay system. The weight penalty is only 10 oz, and the overall rear stiffness is excellent.  Anything less is unacceptable.

Webbed Gusset Reinforcements

Web gussets add structural integrity to the trike design preventing it from flexing and from early frame fatigue. CroMo trikes are allowed to flex without fatiguing, so these structural supports are not as common on steel frames. The Thunderbolt  Mk III uses a total of three of these structures to ensure extended reliability of the frame.

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Chain Management

The T'bolt uses a single roller system.  The single roller allows lower friction, lower cost, lower weight and lower noise.  The passive side of the chain is routed through polypropylene sprinkler tubing, protecting it from dirt and grime.


Removable Seat

The upper seat is mounted using three fasteners to the trike frame. This upper seat can be separated from the frame allowing it to folded over.  With the wheels and BB boom removed, the new T'bolt Mk III becomes a compact package that can be shipped UPS.  The length of the frame is 56" for the 20, and 59" for the 26" model.  However, if this is still too long look below.


Segmented Frame

Not sure whether you want a 20 inch rear wheel or 26 inch?  How about building two rear ends that support both wheel sizes and share the same frame? You can do this with the Segmented Frame option.  The Segmented Frame consist of a Telescopic rear section that slides into forward frame in the same fashion as a Bottom Bracket Tube.  The advantages are:


Ultra compact storage and shipping - no part longer than 30 inches


Easier Heat Treating- the frame is in smaller sections that are easier to handle


Better selection of 7005 tubing - Most 7005 tubing comes in sections shorter than 36 inches


Dual purpose - As mentioned, you can build a 20" rear wheel section for commuting and a 26" rear wheel section for sport and touring.      


Multiple Steering Configurations


Direct Steering

The Thunderbolt Mk III is available with a comfortable Direct-Steering Side Sticks (these are clip-on style handles that attach directly to the Steering Knuckles) that are fully adjustable. 

OSS Joystick Style Tiller Steering

 The sporty Over-Seat Steering Club Tiller is the ultimate for performance riding.  This tiller reduces the overall frontal area providing better aerodynamics.  The fully articulated steering shaft allows the rider to lean into corners resulting in phenomenal handling.   

Although the example above shows a RS Spitfire, the tiller system used on the Thunderbolt Mk III is very similar and just as effective. 







(These values can differ depending how the trike is built and the components used)

Frame Description:
TIG welded 7005/6061 aluminum, artificially aged (T5 condition for 7005) (T6 for 6061).
Tube web-gussets on main boom (2 ea.) and on aft section with fully triangulated rear stays.  Telescopic Bottom Bracket section for rider adjustment.  

Total Length        : For average 5'10" rider configuration 74.5" (1,892 mm)
Total Width          : Mk III 20    32"-35"(889 mm)        Mk III 26 32"-35"
Wheelbase           : Mk III 20    38.5"(1,016 mm)      Mk III 26 42.75" 
Wheel Track         : Mk III 20    29"-32" (813 mm)       Mk III 26 29"-32"
Ground Clearance : Mk III 20/26   3.8" (91.5 mm) (Ultimately depends on tires used)

Description-Two section aluminum frame (fully re-enforced) covered with a Nylon Lanoloc mesh sling. .  
Seat Angle                 : Mk III 20 35°,  Mk III 26 35°
Seat Width at base    : Mk III 20/26 18.5" (470 mm)
Seat Height               : Mk III 20/26 ~32" (or around neck level or below) (813mm).
Seat to ground height: Mk III 20/26 7.5" to 8" (at low point) (190mm)

Steering System (for both 20/26)
Ackerman Compensation
Center-Point Steering 19° angle
Adjustable Camber
Caster set at 12° (can be adjustable)
Kingpins rotate on two 3/8" Rod-End Bearings
All rod-ends and bearings are industry standard.
OSS Club Tiller-Dual Drag Link steering linkage tied to an OSS single/dual handled fully articulated club tiller (similar to a speedy). 
Direct Steering-Single Drag Link between both steering knuckle levers.  Each steering side stick is mounted directly to each Steering knuckle
Turning circle: ~15 ft (4.57 m)
29.0 Lbs (14.1 Kg) to 34.0 Lbs (16.3 Kg) Depending on thickness of material used for frame and components.
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