I would also be interested as to how you set up a magnetic force instrument, could be a valuable diagnostic tool.
Here's my downforce measurer. Not pretty, but it works. It's composed of three elements:
1: The wife's kitchen scales.
2. A rigid aluminium baseplate to which are screwed wooden side blocks, which in turn have a steel crossbar screwed to them. The crossbar is mounted in such a way that its height can be precisely altered using shims and is sufficient to allow the scales to fit underneath without contact. It has a hole in it for the car's guide. The hole must be large enough so that no part of the car or guide touches the crossbar.
3. A smaller piece of aluminium plate which sits on the scales and onto which are placed wooden blocks that, when in situ on the base, exactly match the height of the upper surface of the steel crossbar.
My primary aim was to make the measurer rigid enough to withstand the forces of Carrera magnets and to give consistent readings. Whether the steel crossbar accurately mimics the magnetic attraction of a track rail is debatable, but the readings the measurer gives are very consistent with how magnet adjustments affect a car's on-track performance.
Away from the base assembly, place the smaller aluminium plate, central wooden blocks and car onto the scales and zero them. Then reassemble it all on the base; the reading shown on the scales is the magnetic downforce.
The Scalextric BMW here is fitted with its standard brown bar magnet which, despite being substantially weaker than modern neodymium magnets, still produces a whopping 94g downforce. For modern Scalextric cars fitted as standard with the thickest neodymium magnets, I've occasionally seen readings of over 700g. Everyone has their own taste, but cars stuck to the track by that much aren't for me.
By far the best set-up tweak for magnet cars is to position the magnet as close to the rail as possible, usually by truing the tyres down until the chassis is almost scraping the track. The downforce difference between having a magnet 1mm above the rail rather than 1.5mm is substantial.
I take it the paperclip sits on the inside of the car?
Yes, the ferromagnetic ballast used to tweak the magnet's magnetic field goes inside the car, touching the magnet or very close to it. In Carrera cars, I had good results by placing one of Carrera's standard rectangular magnet shims at an angle on top of the magnet so that it also touched the can of the motor. Sometimes two shims were better than one, sometimes a round washer had better results. Experimentation is the key.
The contraption.
Zero with car on scales.
83g. The car's weight.
Ready for measuring downforce.
94g. The amount of magnetic downforce.
Anything more than 150 g downforce is sad.
In my book, traction magnets are sad full stop, but the solution is simple: remove them. Harder to deal with are motors with high magnetic downforce. The cars which are fastest in standard form on my track are Avant Slot Mirage GR8s and Spirit Peugeot 405 Silhouttes, both of which have highly magnetic motors. Fun for a handful of laps, but thereafter a bit dull.
The best place for magnets.