|1909: Krupp-Daimler Plattformwagen, 75 mm,|
SPAAG with light armour and AWD
|1910: Krupp-Daimler Plattformwagen|
|1911: Krupp-Daimler Plattformwagen, 75 mm SPAAG, light armour|
|75 mm Krupp SPAAG (before WWI)|
|French 75 mm SPAAG, WWI|
|Russian truck SPG, 1915|
|autocannone da 75/27 SPAAG, WWI|
|another WWI SPAAG|
|German WWI SPAAG|
|Krupp-Daimler Geschützkraftwagen 7,5 cm Kw 19 (or Sd.Kfz 1),|
late WWI and Reichswehr era
What did these have in common?
Well, all these photographs were done without colour, they all show trucks with quick fire guns and they're all older than 80 years. Some of them do in fact show vehicles which existed before there was any "tank" built.
Most show self-propelled anti-air (mostly "balloon") guns, but at least a few of them were more meant and employed for normal artillery fires. In fact, the last one was considered as an important anti-tank asset.
Interestingly, some of the very earliest examples were actually armoured trucks (pre-WWI German armoured trucks were either armed with an anti-balloon gun, a machinegun or a self-loading rifle, the latter being meant as vehicles for generals).
Truck-mounted artillery is no novelty. There was plenty such artillery during WWI, again plenty during WWII (and also on half-tracks) and there were also Cold War-era vehicles of this kind (such as DANA, G6).
The air deployability craze after the U.S.Army's "relevance" panic post-Kosovo Air War led to a huge reignition of interest in truck-mounted artillery. The 1990's design CAESAR became the poster child, closely followed by a Swedish competitor. Suddenly, truck-mounted artillery projects were immensely fashionable and appeared all over the world. The main marketing hype was about the lower weight compared to the Cold War's typically tracked self-propelled guns (SPGs) and thus deployability by C-130 and similar aircraft.
This was a fashion driven by ignorance and immaturity - or at least do I think so. Self-propelled artillery base don trucks is older than tanks - how could it suddenly be such a much better idea than only a few years earlier when almost nobody paid attention?
Let's look at truck-mounted artillery in general (but with focus on the SPG role).
Firing to the rear is usually no issue unless the traverse is restricted.
Firing forward may cause trouble at low elevation because of muzzle blast (unlikely to still be an issue if you have gun overhang).
Firing sidewards is a big problem, and the recent 155 mm SPGs on trucks typically don't do it. The Italians used 65 mm mountain guns with soft recoil (the first such gun) on 4x4 trucks during WW2 and the recent Hawkeye project uses the same concept - both times firing sideways is no issue.Reminder:
There's another solution to the problem of firing sideways, though: Two or four stabilizers lowered to the ground.
All-round traverse can thus be had with moderately powerful guns on a truck, while powerful guns such as modern 155 mm howitzers tend to have a limited traverse similar to most towed guns.
These trucks typically carry some ammunition, but usually not as much as a towed artillery battery would be allocated per gun. So you either employ a second truck for ammunition and maybe additional personnel, or you end up being very mobile, but incapable of sustaining fires for long. This was certainly discouraging at a time when vehicle costs were a major concern and horse-drawn artillery a serious alternative (till well into WW2).
Concerning mobility; even some of the very first examples employed a 4x4 drive, and one of the Italian SPGs of WW2 wasn't only 4x4, but one of the very few dedicated vehicles ever specifically built for sand deserts. So some degree of off-road mobility can be achieved with truck-mounted artillery, and it's apparently usually satisfactory for the indirect fire role.
A major concern of the soft-skinned truck SPGs was certainly that the loss of the valuable motor vehicle was more likely if it was stuck with the gun than if it was able to withdraw away from the battery position. Then again, the ability to simply drive away once counterfire arrives was invaluable. It was the reason for the breakthrough of tracked SPGs in late WW2. So lightly armoured truck SPGs have this strength, but at the expense that the truck is additionally laden with about a ton of steel.
A truck is - even if lightly armoured - probably more lightweight than a tracked vehicle, but this is not necessarily so. Caesar weighs in at more than 17 tons with a 155 mm L/52 gun while the about 40 years older Mk F3 155 mm howitzer (shorter barrel) from France weighed about the same on a tracked chassis. A completely new lightweight 155 mm SPG on tracks and with similar traverse restrictions as CAESAR may easily be feasible at much less than 20 tons. Then again, light truck-based SPGs with 105 mm howitzers are down to much less than 10 tons nowadays.
I think truck-mounted SPGs are a justifiable concept and choice, but they clearly didn't deserve the hype they enjoyed during the early and mid-2000's. Ironically, some of the truck-based SPG designs with the best-visible niches (compared to tracked SPGs) got very little attention.
An army such as the French one which moves a few battalions with lightly armoured trucks to some place in inner Africa for meddling in some civil war could make excellent use of such truck-mounted howitzers. There's also little reason to pursue concepts akin to Panzerhaubitze 2000 for European-style inter-state war any more. Truck-based SPGs may actually be more, not less, survivable despite less armour, less offroad mobility and usually less range: Long-range radars can easily discern tracked vehicles (and their traces on soft soil) from wheeled vehicles. There would be thousands of tracked vehicles (high value targets) in such a conflict, but hundreds of thousands of trucks. You better hide among the latter kind. The ideal European SPG of the future could actually be more like an AGM on a semi-trailer, moved by an easily replaceable commercial truck and undistinguishable from low value logistical vehicles most of the time.
On the other hand, maybe main battle tanks, medium tanks, light tanks, airborne AFVs and the like produce a couple future designs in which guns of 75-155 mm calibre have a dual direct and indirect fire role not only for guided, but also for dumb or merely trajectory-correcting munitions.
Quite the same could happen to the armoured wheeled vehicles, of course. Light mechanized groups (~armoured recce in French style) might have a lot of use for such versatility, and airborne forces might as well.