Good advice from Andrew and clarification from Ali B.
Most of the Sahara is gravel, not soft sand which cross-routes avoid where possible.
But it only takes a few metres of soft sand to flip a bike as you can see below.
When I met Al Jesse* in Senegal in the mid-80s, he'd just crossed the Sahara the same way I had: Tam-Agadez, but 2-up on an ST (lightish BMW boxer road bike).
Back then it was 600km of sand and gravel - now it's 220 (currently closed).
As he says "
Our worse day was from the [Niger]
border towards Arlit [200km],
4 miles in 10 hour day. We had 10 gal. Water and 15 gal gas."
Four miles is close to my all-time-low on a bike in the Sahara and I can tell you, you end the day barely able to lift a spoon.
I think Al was a skilled off-road rider and this - plus tyre pressures as mentioned - makes a huge difference.
The problem with bike riding in soft sand (I read once) is the in-built 'forward-pointing' castor effect of the steering geometry (desirable) gets eliminated as sand builds up in front of the wheel. The wheel wants to flop to the sides so the answer is to gas it, raise the front end + gain momentum. That's fine on an RM250 or even a Tenere, but with half-a-ton of GS12 + gear + unpredictable passenger movements, it becomes dangerous, not least because that weight landing on either of you is what usually causes the injury, not the falling off (which is often a form of relief).
Even without a passenger it can
end like this (an R80
morning after in the Laouni Sands about 100km north of Niger border - now sealed).
IMO the worse terrain a bike must manage are twin sandy ruts with rocks or bushes on either side so you can't break out. Like the Gao-Tim piste (too risky these days) or indeed the French Line in the Simpson, old Gunbarrel or Telegraph Track in Au.
Tam-Agadez was not like this, it's a wide open plain, but even here you end up either paddling at 4 miles/day or as pictured below (Route A6, KM628 west) because the ever-changing terrain which you were blasting over to avoid getting stuck catches you out.
That's the nub of it on a bike in soft sand: you need to go fast to gain stability and maintain momentum, but that will soon catch you out on 500 kilos of GS/AT/CBX and pax. So you paddle at 2kph at which pace in the Sahara your water supply (time) becomes as critical as fuel (distance).
Ch
*Al went on to steal my luggage idea and became a millionaire