pH and hardness are two different things. pH measures from 1-14, hardness measures by ppm. water above 20 ppm are considered "hard" for detailing.
You'd get better foaming/cleaning power/slickness with soft water.
Foaming also doesn't always mean ph > 7 , a good foaming shampoo like optimum car wash is actually ph 5-6 and IMO not only foams the best but also clean the best.
I realise they are different, I am a chemist and actually make detailing products comparable to those many will use.
As I said, modern surfactants tend to be rather insensitive to water hardness. We do not use 'soap' anymore for this very reason - stearates/oleates or similar
do tend to be very sensitive to hardness and you get very notable scums. In practice, you would be using a very very old fashioned product if you actually had this problem. The main problem one should have with water hardness is mineral deposition, something exaggerated by the use of highly hydrophobic finishes.
pH really is as I have said. 5-6 is still broadly considered neutral. The chemistry of the situation is such that the majority of road soils (actually soils in general) will be acidic in character and will require alkalinity to easily remove them. Oily traffic films are not at all effectively removed by neutral or acidic aqueous products - think back to your school days and you may well have learnt about making soap. Fundamentally you react a fat or an oil with something like lye. This saponification process is why alkaline cleaning dominates in the world - oil/fat is not only removed by alkalinity, the result of the reaction between the two produces a soap which actually
increases the effectiveness of the system. For evidence of this, look to industrial vehicle cleaning. This is simply not done with pH neutral products unless forced by some factor (generally some busy body regulatory body) - it just does not work well enough. Most road going vehicles are cleaned with strongly alkaline (caustic) products since this will act against both particulate matter and oily matter. Trains often are cleaned by acidic products but this is because metallic contamination is of more consequence than the oily films which result from automotive fumes on the road. Some road vehicles are cleaned with acids but these tend to be special cases. For example, quary vehicles or building vehicles often have contamination resulting from cement and concrete. In opposition to the mainly acidic soils most vehicles will encounter, cement type residues are themselves strongly alkaline and hence an alkaline product will have very limited effectiveness. Acidic cleaners are generally kept for wheel cleaning on automobiles where metallic residues are again more important than oily ones (coming from the brake system).
For more info, I would recommend doing some reading on UK detailing forums. We have a much greater range of foam type products and you will find that even a pH neutral snowfoam will typically be formulated very differently than a shampoo. In actual fact, the right snowfoam product can exceed the performance of shampoos which cost many times more. I cannot comment on quite why the foam lance/cannon is so much more common with us.
