Thanks for your help mickeg and RonS!

From a Google search I found someone on this forum who said the mk3 has a bigger tyre clearance but I just wanted to confiirm that.

I will consult the archive of Thorn brochures in the Thorn General section!

The first thread in the “Thorn General” section of the forum has a historical archive of Thorn brochures. Perhaps the info you need will be there if no forum members are able to assist.
Good luck and happy riding.

What is a gravel bike?
...

Andy, I assume you have heard the term gravel bikes before and also pretty much know what it means.

I mostly agree with Dan, the manufacturers needed another marketing idea for a pretty stagnant market. 

My thoughts on "what is a gravel bike", below.

In USA there has been a lot of growth in gravel bike trails over the past decade or two, I suspect more people wanted a bike that had tires wider than the common 25mm tires on road bikes a decade ago for such trails.  And presumably some people were tired of rough riding skinny tire road bikes on rough pavement.

From what I have seen, a "gravel" bike has to have a wide range really big and expensive cassette on a 1X drive train. 

• A lot of people did not like being confused by two shifters, wanted a 1X shifter to make life easier.  (Similar to a Rohloff.)
• Retailers found it was much faster to assemble a bike from the box if the front derailleur did not need any adjustments, because it did not exist.
• They could charge as much for a 1X bike as a 2X or 3X bike, that is fewer components, thus a bit more profitable.
• When the owner needs to replace a giant cassette like that, it is pretty profitable for the shop. 
• The giant wide range cassette was a newer concept, thus the latest fad.  If you had that, you were automatically cool.

A friend of mine had to buy a new gravel bike about five or six years ago.  His carbon bike had developed a fatal crack in the frame, and the gravel bike being a new fad showed he was cool to own the latest thing.  Within a year, two of my other friends saw that bike and they had to be even cooler, one bought the same model bike but with a carbon frame.  And the other bought the same model but with both carbon frame and carbon wheels.  I found it interesting that after a few years two of these three started riding their older bikes more often.

My light touring bike is not a Thorn, it has a titanium frame.  I built it up in 2017.  Has a 3X8 drive train, crankset has square taper crank (decades old tech), a pair of shifters that I first put into service in 2004, and a rear derailleur from the 1990s (but fitted with newer ball bearing jockey wheels).  Has disc rear and rim brake on front because I had an older rim brake fork in storage that had the correct crown race to axle length and rake (or offset), thus I did not need to buy a new fork for the frame, saved roughly $400 (USD).  Fitted with 37mm 700c tires.  But other than the drive train the only other difference between my light touring bike and a gravel bike is that I have fitted fenders to it, and since it was built for touring it has longer chainstays.  (Most gravel bikes have chainstays almost as short as a road bike.)

When I built up my light touring bike, why did I choose a 3X8 drive train?  It is robust, reliable, easy to replace parts, easy to adjust and repair.  Last time I bought a new cassette and chain, paid about $35 or $40 (USD).  It has more gears than a 1X and a wider range.  It is a half step plus granny system, which is an acquired taste, most people would not choose that but I like it.  When I avoid the two most cross-chained gears for each chainring, that gives me 18 usable gears, 558 percent range.  Compare that to a 10 or 11 or 12 speed drivetrain that at most is 520 percent on the 1X.  (Campy has a 13 speed, but it is rare.)

First photo, my light touring bike.  Strip off the fenders and luggage, and what you have is a gravel bike with longer (roughly 20mm longer) chainstays.  (It was early spring, the lake in the background had not yet thawed, was ice and snow covered.)

Second photo, my light touring bike being used for what it was designed for.

I could have put a Rohloff on that bike instead of a 3X8 drivetrain.  I have nothing against the Rohloff on my decade old heavy touring bike (Nomad Mk II), but there are both advantages and disadvantages to a Rohloff compared to a 3X8 drivetrain.  The trips that I anticipate using my light touring bike, I felt I would prefer the 3X8 system over the Rohloff 1X14 system.  I put new fenders (mudguards) on my Nomad this year, but they are a bit dusty in the third photo. 

There used to be a British watercolorist, name now escapes me, who used even in old age to go to Japan every year to paint and teach, who taught, and practised, this simple gem: Give the sky the upper two thirds or three quarters of the painting's height, and  viewers will be emotionally satisfied.

What is a gravel bike?

Andy asks a very good question here!

For my (somewhat cynical) money, much of the "push" for gravel bikes as a niche was initially for product differentiation and a way to sell more bikes, as with other increasingly thin slices of the pie to goose sales when the market goes a bit flat. Cynicism aside, a drop-bar road bike that has greater versatility to handle a wider variety of surfaces turns out to have a lot of appeal to many people and may come closer to being a "quiver-killer"/all-'rounder than other kinds of bikes currently on the market. I predict gravel bikes will become more specialized as have other niche-bikes. It won't be too long before we see more widespread use of front or front/rear suspension and then we'll have something akin to a full-sus 26in/650B MTB but with fat 700C tires and road/flared drop handlebars. Several makers have recently floated bikes with just that configuration, well received in recent cycling press.

All of my bikes (one from 1938, but mostly made 1970-2012) have been ridden on gravel, from back in late 1970s/early '80s America when tire sizing and marketing was driven by advertised weight. Some of the Specialized brand tires I rode at that time measured 1-2 (later) ETRTO sizes narrower than marked. Tires much arrower than labeled meant an automatic reduction in weight if you could believe the label and many did, thanks to marketing. I used them anyway, as there was a shortage of quality touring tires available in my area at a time when special-ordered Wolbers in true 27 x 1-1/4 could easily take 6-8 weeks' time to arrive in my locale.

A lot of my bikes are "old/er", made when more generous clearances were common, so really fit the more modern general ideal for gravel bikes: A road-bike frame with more relaxed road-bike geometry and larger 700C wheels shod with wider tires, typically starting at about 34mm and going upward (mine range from 32mm to 38mm and these have worked well for me on most gravel). If you find an older frame designed around 27in wheels, you can gain 4mm in tire/mudguard clearance by converting to 700C and longer-reach brakes.

A good question to also ask is "what kind of gravel?", for it ranges from what I regard as benign (finer than pea-gravel, tamped well into car tire-track ruts) to malign (fresh-pour in what we here call "three-quarter minus" to full-on ballast). For the latter, I prefer my bikes built with clearances to accommodate my 26x2.0 Schwalbe Duremes, as on my Nomad, tandem, and repurposed MTB née Enduro-Allroad (1987 Diamondback Transporter). While all my bikes "will" traverse gravel of one sort or another (including my 1970 Windsor Professional-based fixie on 25mm Continental road slicks), I am most "comfortable" doing so on those with wider tires. A fave for the more benign sort of gravel is my 41 year-old tourer, closest to Thorn's old Club Tour. It will happily accommodate 38mm tires with adequate, safe clearance for mudguards and run rackless and with short-reach/shallow-drop handlebars, gives a good impression of most modern steel-based gravel bikes. For the bad stuff, the lighter deraileur Transporter does as well as the Nomad for unladen or lighter loads when the advantages of a Rohloff drivetrain and/or expedition-grade carrying capacity is not essential.

I'll be a heretic here and suggest -- based on my own experience -- buying an older road bike with generous clearance for wider tires is a good way to test the waters before splashing out for an all-new bike. A lot of fun can be had at low cost while you learn and refine priorities and preferences on a budget now such bikes are going for peanuts at yard and boot sales. My 2012 Nomad is my latest bike, but might not have been had I not learned what I needed for a specific task -- extended, self-supported long-distance touring with expedition loads on the roughest of roads and tracks. It is also possible to retrofit an older bike with a Rohloff drivetrain, then swap components over if you later find a frame that better suits your needs.

Just some thoughts to ponder.

Best, Dan (...who always looks forward to members' "New Bike Day")