900 classic tyres grinding? You need AIR ASSISTORS
Are your tyres grinding in the arches? You have 16" rims?
Then this could well be the answer

These airbags fit inside your rear springs and you blow them up with a compressed air gun (such as the one used for pumping up your tyres)

So under normal circumstances you have a small amount of air in (to help you over those speed bumps) but when you have 4 passengers or a good load in the boot you pump additional air in. When you're done you let the air back out to your standard setting.

They are a reasonably easy installation, drop the rear wishbones, pull the spring and insert the bag and reinsert the spring and bolt the wishbone up. Then choose an accessible place for the airline to enter the boot space.

They're remarkably similar to the OE original ones supplied by the dealers in the 80's as seen in the accessory picture I have scanned

In truth it's best done with some new springs but they'll extend the life of your old ones.

They cost £150 per pair

Replacement turbo (not 16S but 20mm lower than 900i) springs are £40 each
16S springs (30mm lower than standard 900i) are £140 for a set of 4


Having fitted them to my T16S I would say that they are a " must " to fit. Benefits include

Better load handling when fully loaded

They stop the car from dropping on it's haunches during acceleration.. improving traction

They limit the dive on the front springs when braking hard improving braking

They do all this whilst maintaining decent comfort because the air spring works on significant compressionleaving the spring in softer mode for the first part of its' travel

You're welcome to try mine out if you're local to me