Checking Wheelset Back to Backs

As well as producing 3D printed parts I also help solve problems on layouts, and one problem which arises is locomotives and rollings stock not running smoothly through turnouts and crossings. This is often due to the distance between the wheels, or back to back, not being correct.  In this post, I’ll show you how I check and adjust this.

Railway wheels, irrelevant of size and design, all have the same basic parts.  An axel, tire, rim, and flange.  The tire rides on the rails, and is held in place by the rim.  The flange sits inside the tire and the axel holds both wheels together, forming a wheelset.

The tire is actually tapered which causes the wheelset to sit centrally between the rails.  As the wheelset rolls along it will naturally center itself due to gravity.

But as wheelsets round a curve a centrifugal force will try to push the wheelset off the rails towards the outside of the curve.  As the wheels go faster the centrifugal force gets larger and will eventually overcome gravity, but the flange prevents the wheel from going past the rail.

The distance between the rails is fixed, as is the distance between the flanges. This distance is called the back to back.  Wheelsets will run if this distance is wrong, providing the flanges fit between the rails, but the problem comes when the wheelsets need to pass through a turnout or crossing.  The distance between the parts of the turnout is specifically set, and if the back to back is not right the one wheel will run in the correct place and the other will bind, jam or ride up over the rails causing a derailment.

With model trains, unlike the real thing, wheelsets are either made entirely from plastic or metal where the wheels have to be electrically isolated from each other so as not to short.  The real railways like electricity to pass through the wheelsets as they use that for train detection. The plastic wheelsets, as shown below, are usually injection molded in one piece and very accurately.  So unless they’re damaged the back to back dimension should be correct.

Metal wheelsets, as shown below, either have one or both wheels isolated from the axel.  This wheelset has a plastic isolator between the far wheel and the axel.

The problem here is the wheel on the far side can sometimes move on the axel as the plastic isolator is only held in place by friction.  This changes the back to back dimension.  Also some manufacturers have better quality control than others and it’s not unheard of for a brand new item of rolling stock to be incorrect right out of the box.

So how can this be fixed?  As always in model railways, there are several ways of doing the same thing, but for me, I like to use a gauge.  The NMRA (National Model Railroad Association) supply gauges for all the major scale and these include back to back checks as well as many other things for checking turnouts etc.  The gauges are also the same size as the loading gauge for that particular scale so you can check tunnel heights and platform clearances etc.

Another tool I tend to use for N scale is the Micro-Trains coupler height gauge. I’ve written about this before in my post about fitting Micro-Trains body mount couplers to older N Scale freight cars, which you can find here.

As well as being a coupler height gauge, it also has a wheel back to back check and a rail spacing check.  In the image above the wheel back to back check is on the near side and the rail spacing check is on the far side.

To use the gauge, simply put the wheelset into the slots; if they fit they are correct.  The set below is clearly out of tolerance.

As the wheel at the bottom of the image is fixed to the axel it’s the one at the top with a plastic isolator which will slide, and using a pair small pliers I can easily slide the wheel up until it’s in the right place.

You don’t have to take the wheelset out of the truck to check it when using either the NMRA or Micro-Trains gauge, but if you need to adjust the wheel back to back I would recommend taking it out as the pressure could easily damage the plastic truck.

With all your wheelsets back to back correctly adjusted you should find your trains run nice and smoothly through your track work.