We’ll start off with the Vehicle Identification Number, or VIN. Every bike should have one, and they are the source of all knowledge — or leastways, after 1970 they are. Before then, frames weren’t stamped, and the title to the vehicle passed with the motor. These early bikes were identified by a numeric year code, an alphabetical model designation and the serial number but information on that is thin on the ground so with apologies to Flathead, Knucklehead and Panhead riders we’ll skip straight to the seventies. Those with post 1970 bikes, get a pen and paper, go to the shed and write down the VIN number from your frame — generally on the right hand frame downtube beneath the headstock. Go on, we won’t start ’til you get back.

Too easy to understand, perhaps, because they changed it to an alphabetic character and buried it deep within the VIN from 1981 onwards.

Easy isn’t it? Oh yes, model designations. Ah!

There will be those of you who, seeing the listing above, would have been able to put a name to every single model listed. I had to show off and write the names next to them because there’s nobody else here to prove I didn’t cheat — but I didn’t need to cheat because for all their apparent complexity, they follow a very well established pattern — which is what the timeline was trying to demonstrate before it got complicated.

If the truth is told, I did need to look up the FXRD to be sure, because the one I thought it was turned out to be the FXRDG.

The alphabet soup of the first half of the 20th Century settled down with the arrival of the Sportster and with the notable exception of the two-strokes, Aermacchi’s and Servicars, it all comes down to knowing what an F-series and an X-series bike is. And you do know if you think about it.

Hurtling through the early years, and in no way attempting to be detailed because we’ve already done that elsewhere, A, B and C models are singles, Ds were the first V-twins and Es came to represent a 1000cc V-twin in the modern age. When the litre engine grew to 1200cc it became the F-series because it ran alongside the E for a number of years. So anything beginning with an F is a big twin and has been since the 1200 Panhead was launched. It can trace its ancestry back to the 1936 Knucklehead E-series, and there is more than a passing resemblance today.

A second series of model designations came to represent the ‘lightweights’, which is weird because the sequence started with the letter U, and the U-series was anything but lightweight: the first 1340cc model was a U and it was a big, solid sidevalve. What it was, was a new sequence for the flatheads when they came out — putting clear blue water between them and the F-head Js. The U begat the V, which begat the W, the ubiquitous 45, which became the baby of the v-twin range by the time it was replaced by the … X?

No, the K.

The flathead K soldiered on for half a dozen years from 1952, introduced the bottom-end for the Sportster that is immediately recognisable today and that became the X. Anything beginning with an X is a Sportster and has been since 1957.

In both those cases the L can be assumed, although it first came along to identify the high-compression option.

Bringing us up to date, anything post-war that begins with a V will be a waterhog, born from the VR race bike though it remains to be seen whether its racing "R" will stay in place. That R as a second letter is from a rich heritage stretching back beyond the KR that most of us have heard of, and most associate with the XR — a racing X-series … well, nearly: the details are different but there is an obvious common ancestry. As far as I know there hasn’t been a factory FR.

So you’ve got an FL and an XL. The former a big twin with separate gearbox and destined to cover miles with ease, and the latter a livelier, sportier, unit-construction lightweight to keep the post-WW2 British parallel twins at bay.

And then Willie G happens along and puts a pair of XL forks into an FL and creates an FX, because FXL would have been cumbersome.

Intentionally or otherwise, an FL is now deemed to be a big twin with traditional big twin forks — ie heavy heavyweight — and an FX is a big twin with XL forks and is a custom — ie light heavyweight. The XL continues as the lightweight.

Having adopted the L as an integral part of the series moniker, a new letter was needed to denote the sportier option, and H for high-compression (or highway, depending on who you believe) was added for the flagships, before eventually also becoming absorbed when low-compression options dropped out of popularity. Big twins went from Fs to FL/FXs to FLHs — the FX wasn’t given a choice of compressions so it never got the H — while Sportsters went from XLs to XLHs with a C dropped in to denote a more competition-oriented version: the XLCH.

After that you can add letters to identify models. An FXE was an FX with an electric leg, an FXS was the first Low Rider. An FXE/F was an electric start FX with Fat Bob tanks, called the Fat Bob, the all-black and belted Sturgis was an FXB — although whether B was for belt or black is a moot point — and the FXWG was the first Wide Glide.

Next up, throw a frame into the equation. This was first done with the touring frame, the rubberglide 5-speed FLT, which served to differentiate it from the 4-speed FLH.

It wasn’t immediately seen as such because FLT also identified the Tour Glide model with its frame-mounted fairing. It got a bit messy with the FLHT, which was an Electra in a Tour Glide chassis, but we’re used to that now. An Electra is still an FLH though.

Having already used R for racing models, we can only speculate that Harley were trying to move up a gear in creating the FXR — the 5-speed street/custom range, which are still held in the highest esteem for their European handling traits, but of course it is just as feasible that they readapted R to mean Road. The FXST was next, with ST denoting Softail and as if to prove an earlier point, when the Heritage Softail was introduced off the back of the Softail Custom, complete with heavyweight forks, it became the FLST. Latterly we have the FXD, with the D denoting the Dyna chassis.

There are a few exceptions to the rules. The Dyna chassis was introduced in 1991 but there was an FXDG in the early eighties, but rather than being an FXDG it was an FXDG: the Disc Glide. A Low Rider, originally an FXS, is now an FXDL rather than an FXDS but that’s probably because S denotes sport to most people today, while the original factory custom was probably using S to denote special. They are generally straightforward though, and having identified the model from the VIN, you can take an educated guess at the rest — and even make them up.

Personally I want an FLDR, which I reckon would be a Dyna Road King, and I’ve seen something approaching an FLSTD which looked absolutely stunning but you’ll have to guess at that until we can get pictures. In Sportster terms, an XRCR would keep me on the streets for hours at a time, picking on poor defenseless Sportsbikes.

I’ll leave you with a quick final table to give a rough idea — and it isn’t meant to be definitive — of the logic behind it:

A Army (WLA)
B Black, Belt (FXB/FXDB Sturgis) or Beige and Blue (FXDB Daytona)
C Canadian Army (WLC), Competition (XLCH) or Custom (FXDC) Classic (FLSTC)
D Disk (FXDG/FXRDG)
E Electric Start (FXE)
F Fat (FLSTF)
G Glide
H High Compression
I Injection
L High compression (FL)
N Notalgia (FLSTN)
P Police (FLHP/FXRP)
R Racing (KR/XR/VR), Road (FXR, FLHR)
S Special (FXS), Sport (FXRS), Springer (FXSTS/FLSTS), Street (XLS/VRSCA)
T Tour Glide (FLT), Touring Frame (FLHT), Touring (XLT)
U Ultra (FLHTCUI)
X Sport (FXDX), Basic (XLX)
Y Aeroplane (FLY)
Z Bored yet? (FLZZZZzzzzzz)