When I look at pictures of so called gravel bikes - as per the attached - loaded up with "backpacking" luggage, they look positively unstable to me. All the weight is high up on the bike, contradicting the basic laws of physics regarding centre of gravity. Whatever happened to good old, common sense panniers on relatively low mounted front and rear pannier racks!? Is this newish trend basically marketing tosh? Could someone put me right if I've got it all wrong? Thanks
I assume you meant bikepacking, not backpacking.
I think the early "bikepackers" were all on mountain bikes and trying to carry enough gear to sleep in the wild, without racks, etc. A lot of mountain bikes lack rack mounts.
Some people say that "bikepacking" has to be done off-road. Some (including me) think of "bikepacking" to be using minimalist luggage on a bike with no racks, whether you are on or off roads.
But now we see some of the "bikepackers" are using small racks too, which adds confusion to the destinction. And most bikepackers I see are wearing a backpack because they could not get all their gear onto their bike, but I never wear a backpack when on a bike, I want my luggage on the bike instead.
The total weight and volume carried is the big difference. Most "bikepackers" have less than roughly 50 liters from what I have seen. That 50 liter distinction is mine, I have not read of any clear distinction from others. But I think if you have a pair of rear panniers and a handlebar bag, you are at about 50 liters, then add rack top bag and maybe front panniers and you clearly then are up in the bike touring volume range.
Example, the first two photos attached are of bikes I saw on my bike tour in Iceland in 2016. That is bikepacking gear. In this case they were on road bikes with no racks, specifically Ritchey Break Away bikes that allowed them to carry their bikes on an airplane without paying an oversize fee. And all their gear including their bikes only needed per person one checked bag along with their carry on bag and personal item.
I personally need a lot more gear than that to enjoy myself on a trip, but for that couple that were staying on roads in Iceland, they really were enjoying themselves for two weeks even though they had almost no gear with them.
Regarding the new "gravel" bikes. That is marketing. The bike companies were looking to create a new type of bike that people would theoretically need to have. And it appears to have worked. They have sold a lot of new bikes.
I slowly am developing another distiction between touring and bikepacking. The bikepackers that I have seen were usually out for no more than about four days, or if longer they always re-supplied every few days. The point is that with that small amount of luggage, they can't carry enough food, stove fuel, etc., to last more than a few days. But bike touring, it is common to be able to have resupply intervals that are a week or more apart.
When I go camping, I usually carry almost 0.9kg per day for weight of food, that often takes more than one liter of volume per day, unless I am really skimping on supplies. And sometimes I carry up to 1.5 liters per day for food. But bike touring, I might carry a lot more volume of food for things like baked goods because I have the room for it. And I might even buy a lot of canned food which is heavy too. Bikepackers would have to avoid that.
Third photo is my bike from my last bike tour. The 31 liter rack pack in back varied from almost empty to completely full, depending on how many days it had been since I was grocery shopping. A bikepacker can't do that, but they don't want to carry that much weight either.