Back to back measurement for Gibson wheels

[David BernerParticipant]

Can anyone advise please on the correct back to back measurement for loco wheels in EM? I’m using Gibson wheels and have always set the back to back measurement to 16.5 mm but have just heard that 16.65 mm is recommended for finer wheels such as Gibson.

[Nick RidgwayParticipant]

I just use the EMGS back-to-back for all wheelsets and gauges for my trackbuilding. I have no idea about the tolerance on manufacture and I doubt if my track is right to within 0.15mm. In any case, I’ve never measured it.

[Nick RidgwayParticipant]

The individual giving the recommendation is the best source for that advice.

[David BernerParticipant]

Thanks – I’ll stick to 16.5 mm back to back. I hadn’t heard of any variation on this until a professional builder told me about the 16.65 mm idea for finer wheels.

[John CutlerParticipant]

If you increase the back-to-back, then you might need to increase the check rail gauge.
Otherwise you increase the risk of wheels catching/derailing/riding up at the crossing nose/frog.
My advice is to stick with the EMGS standard of 16.5mm unless you want to do some testing or further research.

Many years ago Bernard Weller sold an EM back-to-back gauge of 16.7mm (which I use!) so it is possible to increase the BB and apparently so do other EM users. But I use a check-rail gap of 0.9mm (aluminium strip) instead of the 1mm per the EM standard (even this is anathema to the purists). Supposedly the increased BB gives less ability for rolling stock to yaw (twist) on the track but I am not sure it is worth the extra effort (but it is not a lot of extra effort unless valve-gear clearances are involved!).

GW Models (George Watt) advertises an adjustable BB for £15 in MRJ266 (tel 01903 767231) if you really want to go down this route.

I certainly agree with Nick about the inability to measure track variations to within 0.15mm (well I hope I am within 0.1mm!!).
If you take 2 supposedly identical gauges they will often differ.
You need to spend an awful lot of money to produce or buy an accurate gauge.
There is a current debate about accuracy of gauges and the pitfalls of dependency on them in the ScaleFour Society.

Health Warning: Some people get quite vitriolic when people deviate from the accepted standards!

[John CutlerParticipant]

Just to clarify:
If you increase the BB then you need to do it for all wheels.

I run RP25 wheels as well as AG wheels at 16.7mm BB through checkrail gaps of 0.9mm with few issues (so far!).

I nearly forgot!

I now remember the main reason I adopted the bigger back-to-back is so that rolling stock is better centred on the track.
This means that AJ couplings work much better.

[Nick RidgwayParticipant]

@Nick Ridgway said:
I just use the EMGS back-to-back for all wheelsets and gauges for my trackbuilding. I have no idea about the tolerance on manufacture and I doubt if my track is right to within 0.15mm. In any case, I’ve never measured it.  

0.15mm on the track gauge scales-up to a little under 1/2″ in the 12″:1ft scale world, which is within acceptable parameters for continued use, according to various editions of “British Railway Track”, published by the Permanent Way Institution. However, such a dimension is way out-of-tolerance on the back-to-back measurement, which is in the order of 1/8″, resulting in a not-to-go application on the vehicle in question when beyond that limit.

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