One of the biggest causes of derailments in P4 are having incorrect back to back settings on wheels, Even checking each axle in a quality back to back gauge can still leave significant variance.

A great new product designed to combat this is the Gravity Back to Back designed by Tony Sissions (modelmaker87) and available to members through the Scalefour Society's stores.

The basic concept is that you put a wheelset at the top of the incline and if it rolls down and stops between the lines, the wheelset is good.

The metal plate that forms the gauge has been stuck to a sheet of 12mm thick tufnol carp, giving a smooth hard base. I decided against screwing the plate on as the screws would stand proud of the plate (Which was getting a bit too near to the gears on a set of Ultrascales), instead I fitted it to the tufnol with double sided tape which seems to have held it on well. A couple of off cuts of card were glued under the tufnol at one end to make the incline (this was determined through trial and error to find an angle at which the wheel rolled all the way down, but not too fast that it jumped off.

One of the other advantages of this design is that it can test wheels when they are fitted to stock (as can be seen by the VAA shown below) in this case both axles were too tight requiring some adjustment. In the photo the 'safe' area starts about where the hand break leaver is, so the wheels (in this case Maygibs) are a fair bit under gauge.

This photo shows a bit clearer an out of gauge axle which is too wide(this time from Branchlines)

Finally this is what you are aiming for right between the lines (An Ultrascale set after adjustment)

I mentioned at the beginning there is a fair bit of variance even when the wheel is tested on a back to back first. So I have a little test, 10 axles (2 Gibsons, 2 Ultrascales, 2 Branchlines 12mm, 2 DC kits 14mm and 12mm, and 2 Maygibs 14,5mm) All were set to the 'correct' back to back prior to starting with an Exactoscale gauge.

The Results:

Gibson:

The first was too wide, the 2nd varied between too wide and just in gauge indicating a wobbly wheel

Branchlines:

1 was 3mm past upper limit

1 was 6mm past upper limit

DC Kits

14mm - just past centre - OK

12mm - Same as with Branchlines, too wide

Maygibs

1 was dead on the line

1 was 14mm past upper limit

Ultrascale

Both approximately 2mm past upper limit

So out of a batch of 10

70% were completely out of gauge,

20% were in gauge

10% were useless due to poor fitting wheels.

Seems that I have a lot of work ahead of me before I next exhibit a layout redoing the back to backs on the vast majority of my stock. The thing that was of particular interest to me was that all of these were a tight fit onto what is regarded as the 'best' back to back gauge on the market, yet when tested the majority fail to get a consistent result.