Thunderbike Speedborn 140 customized Harley-Davidson Fat Boy

Respect for the icon. Courage to change.

There’s no question that an anniversary bike is something special in itself. But that wasn’t enough for the customer—he had this bike taken to a whole new level, first by Sohn Motorcycles and then by us.

In the first step, SMC already made a clear statement with an expansion from 114 cui to 140 cui. But that wasn’t all: in the second step, we got to work and put the finishing touches on this Fat Boy.

Original Anniversary FatBoy

Thunderbike custom conversion of the Anniversary FatBoy

The one in 3000

It wasn’t just going to be one of 3,000 – it was going to be the one of 3,000.
We turned the 2023 Anniversary Fat Boy completely inside out, without losing sight of the original style and iconic lines.

The paint concept was deliberately retained, because it is precisely this anniversary look that makes the Fat Boy so special. At the same time, we wanted to give the bike our own signature. With our steel rear and front fenders and the front spoiler, we have therefore deliberately set new accents without breaking the original lines.

To ensure a perfect transition and a seamless design, we gave the original parts directly to the painter. This allowed him to analyze the factory look down to the smallest detail and transfer the finish exactly to our components. The result is a harmonious mix of original character and Thunderbike style—classic, but with clear new highlights.

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Original at heart—Thunderbike in detail

Number 906 of 3000

Chrome as a common theme – from mirrors to forks

Front fender in the iconic red of the Anniversary

Thunderbike style from front to back

The combination of the 21″ Big Speed rim, the Thunderbike single-sided swing arm, and perimeter brake creates a powerful rear end. The 260 rear tire is neatly enclosed and showcased by the steel fender.

And at the front? The 23″ Big Speed rim is a must, of course. Thanks to our adapter, we can install the 340 mm brake disc here – in the matching Big Speed design, of course. The result: a look that is as consistent as it is confident.

Thunderbike and Harley-Davidson original accessories also give each other a real high five on this bike. Our Hollywood Light handlebars with StripeLED turn signals and the Old Style risers set clear Thunderbike accents – and together with the Harley-Davidson Empire series, the overall picture is impressive.

Hollywood light handlebars with old-style risers in chrome

Footboards and brake pedals Empire series from Harley-Davidson

Mirrors and handles from the Empire series with stripe indicators

Brake caliper adapter 300 to 340 mm

No large gap thanks to Thunderbike front spoiler

Handcrafted seat by Spirit Leather

Parts & Details

Harley-Davidson Speedborn 140

Model: Fat Boy | FLFBS | Softail ● Engine: Milwaukee-Eight 114ci/1868ccm

Thunderbike Big Speed wheel
Thunderbike Brake Disc Floated Big Speed 340mm
Thunderbike Brake Caliper Adapter 300->340mm left polished
Thunderbike Pulley Big Speed Single Side
Thunderbike Ignition Coil Relocation Kit black
Thunderbike Single-Side Swingarm Kit 21′′ Alu
Thunderbike Belt 1′′ heavy duty
Thunderbike License Plate Frame Inside Plate
Thunderbike License Plate Lighting
Thunderbike Reflector Inside Plate
Thunderbike Rear Fender Steel 260 mm
Thunderbike Seat Foam for TB Seat Plate
Thunderbike Seat Plate
Thunderbike Cover Tank to Seat, black
Thunderbike Front Fender 23′′ Fiberglass
Thunderbike Chin Fairing GP Style Long
Thunderbike Tele Fork Lowering Kit
Thunderbike Riser Kit Old Style with wireholes
Thunderbike Turn Signals Stripe LED with Housing
Thunderbike Turn Signal Load Equalizer
Thunderbike Handlebar Hollywood Light
Thunderbike Front Axle Cover-Set
Thunderbike Axle Cover Swingarm left
Harley-Davidson Empire Mirrors chrome
Harley-Davidson Empire Rider Footboard Kit black cut
Harley-Davidson Empire Hand Grips chrome
Harley-Davidson Cover Battery Right, Gloss Black
Harley-Davidson Cover Battery Right, Gloss Black

From 114 to 140 – displacement instead of restraint

At Sohn Motorcycles, the journey from 114 to 140 cui doesn’t start with a few add-on parts, but right at the foundation of the engine. The original crankcase is CNC-machined and precisely prepared for the larger displacement. At the heart of the conversion is a reinforced and finely balanced SMC crankshaft, designed for sustained high loads and maximum torque.

The S&S oil pump with control plate ensures a reliable oil supply. The valve train components are consistently designed for performance and durability: an SE camshaft with special bearings, roller-bearing rocker arms with additional valve oil lubrication, and reinforced push rods guarantee precise timing and high stability.

The cylinders are constructed with high-strength SMC liners made of ductile iron and combined with SMC precision forged pistons. High-strength cylinder studs and cylinder head bolts permanently withstand the increased combustion pressures.

To ensure that the power is reliably transferred to the road, the drive is equipped with a reinforced SMC clutch. An SE air filter kit provides the necessary air flow, while fully synthetic engine oil with ceramic additives minimizes friction and provides additional protection for the engine.

The result is a cleanly built 140 cui engine with 140 hp and 215 Nm.

Big Speed in Contrast Cut

The process doesn’t end with milling for our rims. After the initial CNC machining, the surfaces are carefully sanded before the rim is sent for anodizing. Only then does the crucial step for our signature Contrast Cut begin.

Back from the anodizing bath, the contours are precisely re-milled. To do this, the rim star is clamped in place, milled, then turned and milled a second time. This is the only way to create the edges and clean lines that perfectly highlight the contrast between the anodized surface and the bare aluminum.

This is no shortcut, but genuine craftsmanship and CNC work – made for rims that not only look good, but are just as impressive at second glance as they are at first.

Thanks to state-of-the-art CNC technology, we achieve precision down to the last detail – every time. Perfect contours, clean lines, and results that are not only identical but also exactly the same for every rim. The result is uncompromising quality that you can see!