For the maximum wheel diameter, simply take the value given above and divide by 87 then by pi. This gives 3989 / 87 / pi = 14.6 mm. Our HO car wheels rarely exceed 12 mm, so this is perfectly suitable.
To me, the most appropriate type of wagon to contain the device seems to be the open wagon, but a flat wagon may also be suitable. For track holding at high speed, a bogie wagon would be preferable, but the wheels are too small in diameter in relation to the magnet thickness. A solution could be to fit larger wheels, but this will change the coupling height and possibly cause uncoupling. Moreover, in the tight curves, the axle will move with respect to the reed switch mounted in the body: one can expect malfunctions.
So I elected a brave old Hornby Acho two-axle open wagon ref. 7080: the body is large enough to hold the device, and I have equipped it with 12 mm diameter wheels. Obviously, it must also be equipped with at least one coupling of the same type as that of the locomotives.
The tricky point is to fix a magnet on an axle of the wagon. The one delivered with the tachometer is too bulky. It is necessary that the magnet comes near and then go away from the reed when the wheel rotates, so it must fit the space between the axle and the wheel radius, otherwise it may stall on turnouts or crocodiles. It must also be powerful enough to operate the reed, but not too much, which could cause the reed to stick permanently. Finally, it is preferable for the axle to be balanced, especially if one wishes to use the device at rather high speeds.
The axle has been milled on 0.5 mm (1) to ensure that the magnet (2) ø 5 × 13 is above the plane of the rails. It is much easier to machine the axis than the magnet itself.
The approximate balancing is carried out by a flyweight (3) glued on the axis opposite the magnet. All metals are suitable except iron or steel! In fact, since they are magnetic materials, they will extend the magnet action symmetrically around the axis or short-circuit the magnetic flux, and I don’t think that the reed can then be operated correctly (but I did not test). So lead or brass seem to be the most appropriate.
In my particular case, the magnet has a mass of 2 g. A cylinder of lead ø 4 × 14 gives the same mass when placed at the same distance from the axis.
Here is a view on the axle equipped with magnet and balancing mass, held by electrician tape. The wagon was equipped with a Profi coupling on Roco elongation mechanism. You can see the programming button in the tachometer’s lower right corner.
All I have to do then is to store the connection cable between sensor and display, because I wanted not to cut it. It is therefore wound on a piece of tube on which the display is attached like on a bike handlebar. Inside the tube is placed a ballast: indeed, the poor measuring wagon will probably be subjected to speeds that were hitherto unknown to him.
Here are two views of the arrangement in the wagon body.
My tachometer’s manual is multilingual, with an instructions flyer like a medicine’s one, but much larger. The hard work is to find the interesting part in the suitable language. Then, after entering the setting mode (small button at the back), you just indicate the wheel circumference, converting it from 1:87 to 1: 1. For my wagon’s wheels of 12.0 mm in diameter, the actual size circumference is 12 × pi × 87 = 3279.8 ≈ 3280 mm. This value is to be entered into the device.
The sensor (reed) is mounted into the wagon body, just above the magnet carrying axle. It is fixed by a thin plasticized iron wire formed into a stirrup. The display is placed next to it.
If everything’s right, rolling the equipped wagon on a table end, you will see the instantaneous speed rise fast enough: God's hand can easily push the wagon at 100 km/h! You should take into account the fact that the magnet opposes a certain resistance to the forward movement when the wagon is pulled by a loco, which is not a drawback in DCC if the decoder has a load compensation. But be careful in DC, and especially on steel rail: with a strong magnet, it brakes hard! We reinvent the (electro) magnetic brake!
Here is finally the wagon equipped, ready to begin a campaign of measures.
Some trains, not equipped with couplings, cannot be tested: EAD, RGP, VT11. Or by pushing the wagon, but only in a straight line.
For DCC tests, it is better to disable the inertia (acceleration and deceleration) settings of the tested machine, with CV3 and CV4 = 0, or more simply by pressing the key which inhibits this inertia, usually F4. Otherwise, we spend more time waiting for the speed to stabilize than to make the measurements.