I am translating an Italian technical manual, and I'm having problems translating one word in particular: "planaritá". The literal translation of the word is apparently "planarity" but I've never heard of planarity before and from online research it doesn't appear to be a common engineering term for wheel/rail contact.

The direct translation of the relevant section is:

problem = incomplete wheel contact

solution = restore wheel and/or rail planarity

I wonder whether "alignment" would work instead. Is there another more technical/engineering term to describe the relationship between wheel and rail?

  • What's the source of the instruction? Jan 30, 2020 at 9:48
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    An Italian technical manual that I'm translating. They've used the word "planaritá" which translates as "planarity" which I can't find evidence as being the appropriate English term - so I've used alignment. But possibly my ignorance of engineering is letting me down
    – Kevin Ryan
    Jan 30, 2020 at 9:52
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    I suspect that in everyday English alignment might well be the best term. There could be a better term in rail industry jargon but that could vary from region to region. Jan 30, 2020 at 10:02
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    What is the nature of the incomplete contact? To my mind the contact between a rail and a steel wheel is either a point or a line across both the wheel and the top of the rail. I can imagine four causes of incomplete contact: a wheel mounted out of vertical, a misshapen wheel, a wheel mounted with its centre out of line with other wheels and deformed track. Of these I would only describe the first one as "out of alignment"
    – BoldBen
    Jan 30, 2020 at 11:12
  • @BoldBen - sorry that's all I have. The direct translations are "problem = incomplete wheel contact" and "solution = restore wheel and/or rail planarity". I've never heard of planarity and from online research it doesn't appear to be a common engineering term for wheel/rail contact, so I'm going with alignment...unless I hear better
    – Kevin Ryan
    Jan 30, 2020 at 11:30

2 Answers 2


I have found This book on the net having seen a reference to the term "Contact Patch" in one or two other searches. It seems to me that "contact patch" might be the technical term you are looking for.

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    The contact patch appears to be providing evidence of the problem. The question is what action do we take when we correct it. I doubt we can do better than alignment in this case. It covers all the problems you mentioned in your comment. Wheel alignment in vehicles covers two axis angles, caster angle, and axle spacing as applicable.
    – Phil Sweet
    Jan 30, 2020 at 12:22
  • Lexico defines "planarity" as The quality or condition of being planar so "incomplete planarity" sounds like a description of the effect of the fault rather than a description of the cause or a way of correcting the fault. I still think that "an incomplete contact patch" is more likely to be an accurate translation of the Italian than "incomplete alignment". I accept that a rail engineer might know a better term, though.
    – BoldBen
    Jan 31, 2020 at 14:19

We'll start off with considering your possible suggestion of alignment. Lexico (from Oxford dictionaries) gives:

1 [mass noun] Arrangement in a straight line or in correct relative positions.

‘the tiles had slipped out of alignment’

1.1 [count noun] The route or course of a railway or road.

‘four railways, all on different alignments’

while Cambridge gives:

an arrangement in which two or more things are positioned in a straight line or parallel to each other:

The problem is happening because the wheels are out of alignment with each other

I don't like the Lexico wording of "in correct relative positions". That wording would suggest that it's appropriate for some situations where it's not. Take for example four points on a plane, if they were in a straight line they wouldn't be "in the correct relative positions" to make a square, but they would be in alignment.

Cambridge's wording for this meaning is much better: "straight line or parallel to each other". If you draw a line through each of the four wheels of a vehicle, from the back of the wheel through the middle to the front, then compare these lines with each other, are they all parallel? If so, then they're in alignment. Applying this logic to a wheel/rail interface - as long as the line through the wheel is parallel to the rail, then they're in alignment.

But consider these situations:

image showing A: a wheel over a rail correctly and B: a wheel which has overturned slightly

The image is a 2D representation of a 3D situation, but we can see that in both A and B we can see that the line created by running the blue points into and out of your screen will show that the wheel is in alignment with the rail, as the blue lines would be parallel. But while A is clearly the correct contact between the wheel and the rail, B is not. This is because in A the plane of the rail (caused by running the red line into and out of your screen) and the plane of the wheel are the same; in B the rail and the wheel are in different planes.

So what we need is a word which means "the state of being in the same plane" or "the state of being co-planar". For your literally translated word "planarity" Lexico gives us:

The quality or condition of being planar

and Cambridge gives us: a page titled "Did you spell it correctly" and some search suggestions for similar spellings or pronunciations.

So "planarity" is clearly not a common word: you hadn't heard of it and neither has the Cambridge dictionary. But it does exist (as shown by Lexico), and it's meaning is easily understood from the word "planar" plus the suffix "ity". As a Chartered Engineer myself (though not directly in the right field for your question) I recognise the word.

So, the word you originally rejected planarity is the word you're looking for, assuming that situation B is the correct type of "incomplete wheel contact" anyway. And given that "planarity" is the literal translation, it would seem likely that that's the case.

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    Thanks for editing my question Andy. I don't find that in the least bit patronising.
    – Kevin Ryan
    Jan 30, 2020 at 14:19
  • @KevinRyan. Good good. Welcome to the Stack Exchange model, where editing of other people's content to improve it is encouraged, and also where comments are considered ephemeral and liable to deletion. If you don't like my wording, do feel free to edit it to fit your own "voice", though I did try to re-use your own wording from the comments where possible. I would not encourage you to remove important information from the question though, as that would be considered vandalism. (And yes, I am aware of how hypocritical that probably sounds, but there's logic in there somewhere...)
    – AndyT
    Jan 30, 2020 at 14:24
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    just to be clear, in addition to re-writing my question for me, you next delete most of my comment above and then you encourage me to edit my original question so that it reflects my own 'voice'. Oh, and if I don't like it, I need to remember that my comments are ephemeral and delete-able. Nice little community you've got running here.
    – Kevin Ryan
    Jan 30, 2020 at 17:27
  • @KevinRyan - I've not deleted any of your comment(s)...
    – AndyT
    Feb 3, 2020 at 15:26
  • PS I've had a thought that "camber" might be the word you're looking for. I've not got time to write it up into a full answer right now, will hopefully do so later.
    – AndyT
    Feb 3, 2020 at 15:27

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