My experience of hard riding is that most of it comes from run flats more than low profile tires. X5 on 20 x 35/40 run flats had a punishing hard brittle low speed ride. When these were changed out to air tires there was a marked step improvement and acceptable ride comfort. Z4 on 20 x 35/40 has run flats and a hard ride along with a hard suspension but as it is a sports car it is something that I got to and have accepted. Last month I checked tire pressures and shock that all were only 24 PSI! I pump them up to 36 PSI but there was NO difference to ride comfort. I have driven the X5 home 40 km with one rear run flat tire with no air pressure. Tire fully up on the rim and ride and handling was no different with no air pressure. No heat in the tire when I got home with it. The very thick, heavy and expensive side wall supports most of the vehicle weight whereas as an air tire most of the vehicle weight is supported by air and not the tire wall. Vehicles fitted with run flats have a tire pressure monitor as the driver has little to no indication that a tire has lost air pressure and potential overheating. Tire shops are reluctant to repair run flats as it may have been driven with no air and overheated the side wall. Another problem with low profile tires is guttering as there is little tire wall height to take the contact. Air tires provide air suspension and low profile and or run flats tires are driving vehicles to now option air suspension due to the loss of air suspension tires. We can all list many so-called advancements in vehicles that have come and gone (rubber timing belts) but that is another subject but hopefully low profile tires and run flats will be one of them. Remembering Michelle metric diameter tires that came and went and everybody that had them had to re-rims their vehicle. I am 100% happy with the air 16 x 60 tires on my Accord. I can live with air 20 x 35/40 on the X5. Hate the run flats 20 x 35/40 on the Z4.