I have found lead also works to lessen the magnetic effect .
Giving one number for stainless steel is very misleading. Stainless steels covers a wide range of alloys. There is a very wide spread of magnetic properties depending on the alloy and for some grades. heat treatment can make a big difference to the permeability.Here's how the permeabilities of some common materials compare:
Stainless Steel ....... 200
Quite a lot of modellers would agree slower the better. Not sure how many racers would say slower the better. Arguably, good racing needs a field of cars with similar cornering speeds, individuals will have different takes on how realistic the speed is.I digress, but am I the only racer that wants the cars to corner at realistic speeds? The slower the better as far as realism is concerned for me. ...............................(New topic perhaps?)
Experiment now done on Ninco track. I installed the BRM motor in an anglewinder NSR Mosler with about 2 mm ground clearance. The static rear axle weight was 48 gms. With the unsheilded motor the axle weight registered 126 gm. With a single wrap of Magnet Sheild the value dropped to 71 gm. The magnetic effect was therefore reduced from 78 gm to 23 gm., a 70% reduction. This is now within the realms of motors which are not advertised as "magnetic effect"I will wrap a short piece around a can in a real car and see what happens when tested on a piece of track.