yes I have an idea about this, because I calculate stuff before I build it
Short answer to your friend would be: Counting calipers does not help and means exactly nothing.
I will try a medium answer for some more insight )
For one: Yes you need a brake dimensioned like ours "only" for cars up to 4 tons. That's what the calculation is based on. So on a 3.5t 4x4 you could go with a ~12.5% smaller setup.Since the difference is so small, we have not developed a second configuration for obvious reasons.
Secondly, the whole setup did not come out of the blue, but is a direct copy (in terms of overall dimensioning) of the Pinzgauer 716/718 disc brake setup that has been adapted for a) larger tires, only 4000kgs, c) the brake booster setup present on the Volvo and d) legal requirements of maxiumum pedal force for specific amount of brake force delivered.
d) has been calculated with the Pinzgauer setup aswell and it can be seen, that their setup also complies with this rule. (500N force on the pedal for a 50% braking procedure, here calculated at 4000kgs)
a) The pinzgauer tire size is pretty small, only around 430mm in diamater, we use 470mm diamater for 37" tires. That means just for the tires we need to come up with about 9% more braking power to be within legal limits.
is the compromise to make it possible at all, and to be on the safe side since most of us have down-rated their 6x6 to 3500kgs for various legal reasons.
c) the Volvo has a weirdo 2x7" booster setup, which gives more power to the brake system than the Pinzgauer 8" booster, so that is good. In case of failure of one booster, you would not be within legal limits anymore, but the pedal force needed to stop a 4000kgs truck with be well within humanly possible limits, so consider that a safety factor of the Volvo brake circuit.
Now let's see some OEM cars and their calipers:
* Sprinter 906 (4.6tons): Double piston brembo calipers, 52mm, wheel size pretty small but 9/10" Tandem booster (!)... piston diameter per wheel: 73mm
* Volkswagen T5 Multivan (2.8tons), single piston calipers, also 9/19" tandem booster and "small" wheels, 60mm piston per wheel
* Pinzgauer 718 (4500kgs), larger tires, 2 calipers with 54mm each, only 8" booster, equals 76mm piston per wheel.
Even though there are some factors that influence size of calipers etc., one thing should become pretty clear by now: The heaver the car, the larger the pistons/calipers needed. The big differences in booster size are due to the fact, that the overall setup nowadays is a little diffferent from back then: Now we optimize cars for comfort, so we use pretty large main cylinders, that in turn need huge boosters to give the same system pressure as we would have reached back then with smaller boosters and larger main brake cylinders. We gain very small brake leverages and the car therefore feels more sporty. The overall dimensioning stays the same: pedal force multiplied by booster must equal enough pressure to stop the car within legal limits.
Now let's see what we've got on the THE BRAKE:
* Volvo 303/304 (3500-4500kgs), double piston double caliper, with 38.6 an 41.2mm pistons, dual 7" booster (basically just like 7/7" tandem), equals piston diamater per wheel of 79.8mm.
Notice something? 79.8mm piston diameter and all the other parameters (disc diameter etc.) equals about 10% increase of system "performance" over a standard Pinzgauer setup.
So to sum it up: THE BRAKE is not some crazy lunatic setup, way oversized for the job, but it is VERY comparable to other OEM setups and calculated to stop a 4t Volvo on 37" tires within legal limits with a proper safety margin, as you would expect from a OEM setup aswell.
GIven that, I hope your friend can trust in the brake setup dimensioning of Magna Steyr, Volkswagen and Mercedes Benz, because THE BRAKE has been designed based on their existing setups. Not more, not less
That was the medium length answer with no math involved, hopefully that will do it. I know THE BRAKE is expensive, but over-dimensioning is not the reason
P.S.: Almost forgot to answer your question: YES, absolutely, you could get some calipers made, that are big enough to have a single caliper setup in the front. But: I don't know such a caliper from any OEM, one reason why Steyr also used TWO calipers on their car to get to the needed piston area. Secondly, even if i managed to get a 6 or 8 piston caliper manufactured that lead to the needed piston sizes, it would not be cheaper and huge as f*ck. Secondly I would introduce a second type of caliper on the whole car, since of course you would need a smaller variant on the rear axles. The result would have 2 major disadvantages: You would reduce the number of calipers manufactured for each type, thus raise manufavturing costs per caliper again. Secondly you would end up with 2 different types of calipers of your rig, thus totally undermining the concept of identical parts to enable emergency repairs and servicing on the road. In summary: Does not make sense. Unless... unless you raise the overall system pressure to a point, that would allow using a smaller piston caliper. E.g. by introducing a hydraulic brake booster or some 9/10" tandem booster setup with different diamater main brake cylinder and so on. That would basically lead to overall dimensioning of a whole system, which I did not want to do and which many of you wouldn't want, either. So instead we decided to keep as much of the stock system as possible, copy the best parts of the Pinzgauer and the Unimog (basically only the double caliper idea and overall dimensioning) and modify to suit our needs.
Ok, that should do it for now... Good night
P.P.S.: Just for illustration's sake and to make you feel more comfortable with the double piston setup... we are not alone here: