1984 BMW K100 flat-tracker

Guide price: £5,000-£8,000

Highlights


・Rebuilt engine
・Low mileage – only 40,000
・Custom flat-track conversion

The Background


BMW’s K100 tourer was the bike that snuck up on the market. It didn't grab any of the headlines but it was a superbly competent package that featured much of the technology that would go on to set the standard for the following decades. It used an efficient, inline-four cylinder engine turned flat so the weight was carried low. BMW wasn’t overly fussed about the width of the engine – it was more famous for its flat-twin ‘boxer’ engines at this point, so owners and riders were familiar with low-down and wide weight. 

The engine was a stressed member of the bike and it used fuel injection to help with that efficiency. It also used BMW’s trademark low-maintenance shaftdrive and was water-cooled. The output was deliberately kept conservative at around 90bhp so it wasn’t overly stressed and combined with a five-speed transmission and conventional fork suspension at the front and a single coilover damper at the rear, it gained a ‘Flying Brick’ nickname for its presence and performance. 

Later, there was an ‘sporty’ RS version but it really came alive on the custom scene; it's reliability, the fact the engine formed part of the chassis and the abundance of parts made it perfect for a generation of scramblers, café racers and flat trackers. 

The History 


This bike is being presented to auction from a private collection and was built in 2020 from a 1984 K100. It has been turned into what the owner described as a flat-tracker, aping the style of oval racers primarily from the USA. However, it also has a distinct scrambler feel to it, reminiscent of the bikes ridden in post-apocalyptical movies such as Mad Max and MM: Fury Road. 

The K100’s bodywork was removed, including the radiator cowling and the front mudguard was cut down to just around 300mm in length. Wider handlebars were fitted with the original bike’s grips on both sides, retaining the K100’s odd – and controversial – split indicators switches, with one on each side. These are paired to custom indicators and the headlight was changed for an LED/Projector round unit. The gauges were replaced with a single unit featuring a GPS speedometer, which is why there is no mileage reading. 

The rear bodywork was also removed and the subframe modified to accept a dual-level seat which was upturned at the rear, giving room for a pillion though there are no passenger footpegs present. The rear hoop of the subframe incorporates an integral LED tail and brake light built in to the tubing. The engine was removed from the frame to be rebuilt and the latter powder coated during the build.

The Paperwork


Very little paperwork accompanies the bike. There is, of course, its V5 registration document in the current owner’s name and a selection of its previous MoT certificates as well as several internet printouts of general information on the BMW K100. There is no current MoT, as the last one expired in November 2019 and the owner has not had a new one carried out since the bike has been converted. 

The Condition


Despite its aggressive and custom look, this bike is actually in good condition. The tank has been painted in what appears to be metalflake paint and the frame is nicely powered coated. The bike does appear to have suffered from being stored outside however. There is evidence of water marks on the alloy foot-peg assemblies, the brake-disc bells and the engine covers while some of the welding on the exhaust system and inlet filter shows signs of rust. However, one could quite easily say that this is part of the bike’s patina and its character.

The seat is in very nice condition, clearly made for the job and is finished in quilted, double-seamed leather. The wheels were powder-coated at the same time as the frame and are in very nice condition and were fitted with brand new Heidenau K60 Scout dual-sport tyres. 

The Mechanics 


According to the owner, the inline-four motor was stripped and rebuilt during the course of the overall bike build and has covered 40,000 miles – a fairly low mileage for these reliable and well-engineered units. It was fitted with a fabricated alloy coolant header tank and a mechanical coolant temperature gauge and a performance-style air filter to the side of the engine, along with silicon coolant hoses. 

The exhaust system takes four headers and joins then in a single collector and runs a fabricated pipe to a single silencer sat high, next to the seat. The bike retains the original K100 running gear – front suspension and brakes, shaftdrive and rear brakes - while the rear suspension was custom-built for the application. 

Both the engine’s ‘top’ and ‘bottom’ are protected by crash bars to prevent damage should the bike fall over (which it does not appear to have done) and the rear brakes use a short section of pipe as the fluid ‘reservoir’ – clearly minimal fluid is needed to activate the rear caliper. 

The bike is fitted with alloy aftermarket custom footpegs and gear/brake lever assemblies finished in gold anodising for some relief to the overall dark aesthetic. 

These bikes are renowned for smoking on start-up as, is common with many flat-twin and four engines, oil can run into the cylinders when they are stood. However, when the owner fired this example up, there was no evidence of smoke. The exhaust note is surprisingly subdued but it has a very visceral character to it, much like the bike as a whole. While it does not currently have an MoT, and hence will need to be trailered away or taken in a van, the owner confirms that he recently rode it off-road at a friend’s farm and that everything operates as it should. 

The Appeal


There is little that we can do to explain the appeal of a bike like this to you; you either get it or you don't. If you don’t, then that’s fine – the whole concept of customs is that they aren’t for everyone. 

But if you do get it – and there is plenty of evidence around that lots of people do – then this is a great example of what can be done with a reliable, strong and ultimately dependable machine. It looks great – a real gnarly off-road aggressive stance that according to the owner, actually gives a very nice and comfortable upright riding position. The exposed mechanics – the air filter and header tank/temperature gauge and the fabricated exhaust with the deliberately unfinished welds give it a genuine scavenger, Max Max feel. 

We would hope that the new owner would get a fresh MoT for the bike and continue its presence on the roads. It certainly deserves to be used and not just for chasing fresh-faced Australian future policemen or tanker lorries full of water.


Photos provided by seller




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Vehicle specification
  • Year 1984
  • Make BMW
  • Model K100
  • Colour Black
  • Odometer 40,000 Miles
  • Engine size 1000
  • Location Hertfordshire
  • Country United Kingdom
Bidding history
3 bids from 2 bidders
  • Robmort•••• £3,990 04/10/21
  • john.ho•••• £3,500 04/10/21
  • Robmort•••• £3,250 01/10/21

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