Quoted wheel sizes

[Steve Taylor]

On opening a loco kit, we are usually faced with buying some extras, wheels, gearbox, motor etc. If we are lucky the instructions detail what wheels are required, so off we go to the appropriate website or show stand and exercise the credit card. If no info is available off we go to the "reference library", ascertain what is required and then partake of retail therapy. However I am curious at what is meant by these given dimensions. When a wheel is quoted as being of a particular size, is that purely covering the machined wheel prior to it having the tyres pressed on, or is it a figure accounting for the increased diameter of the tyre too, and if so is it based on a brand new tyre, minimum acceptable thickness or a median figure?

 

I'm trying to workout if it might be feasible to rework some 00 rtr wheels by turning off the flanges altogether and replacing them with "tyres" scavenged from a suitable aftermarket replacement........ why you may ask? Well no-one does a proper 3mt wheel for p4....... and my hairy shirt is not itchy enough.

[Miss Prism]

The quoted wheel diameter is usually the diameter over the tyre for a new condition vehicle or loco.

[LMS2968]

The quoted wheel diameter is usually the diameter over the tyre for a new condition vehicle or loco.

Except on the LNWR, where it all gets complicated!

[Steve Taylor]

thank you both, that gives me something to go at.

 

In which case....... any idea of the nominal thickness of a tyre at the outer  face (non flanged edge) ?

Edited November 19, 2017 by Steve Taylor

[Mim]

Except on the LNWR, where it all gets complicated!

This one caused me some head scratching recently trying to make sense of a LNWR drawing and the quoted wheel diameter. There is an explanation on the LNWR Society web site. Basically the wheel size quoted for LNWR engines doesn't match any dimension on the actual wheel! Sort of, well a bit if you squint.

 

Mim

[Grovenor]

thank you both, that gives me something to go at.

 

In which case....... any idea of the nominal thickness of a tyre at the outer  face (non flanged edge) ?

If you are going to fit aftermarket tyres, for example from Alan Gibson, then obtain your tyres first , measure them and turn the centres to fit. I have done this a couple of times. Mim has given a prototype example above but you have to use available tyres unless you are going to turn your own.

Regards

[Buhar]

Very often a wheel that is correct over the tread (at some point on it's cone) will be over-sized at the flange (unless you're in 18.83) and can cause clearance problems with splashers or adjacent wheels.  If you go 1mm (3" in 4mm) down you may well be OK.  Tyres were re-turned over their life-time, if I recall correctly, by as much as an inch, ie 2 inches less than the original diameter, so you end up not far off.

 

I believe Mike Sharman's book has just been republished; "Wheel Specifications for The Modeller" and has got quite a few corrections from the little brown number of eons ago. 

[jim.snowdon]

This one caused me some head scratching recently trying to make sense of a LNWR drawing and the quoted wheel diameter. There is an explanation on the LNWR Society web site. Basically the wheel size quoted for LNWR engines doesn't match any dimension on the actual wheel! Sort of, well a bit if you squint.

 

Mim

Crewe's "nominal" diameter roughly equates with the minimum diameter before scrapping. How useful it is is a bit of a puzzle; the only thing that comes to mind is in calculating the maximum speed of the wheels and motion components. The maximum tractive effort is with minimum size wheels, but what matters more to the operators is the minimum tractive effort relative to the regulator and valve gear settings.

 

Jim

[DCB]

There are lots of references to GWR standard 4' 6" dia wheels becoming 4' 7 1/2" when thicker tyres were introduced in the 20th century but the same didn't seem to happen to the 5'8" ones.

 

On models there is often a wheel dia/ footplate and splasher,/ buffer height trade off.  Triang used a 1mm higher than scale buffer centre line to get large wheels onto some locos and that has carried over into more recent releases, H/D used reduced diameter wheels and correct buffer heights. you pays your money etc.. but I use the Hornby Dublo setting and turn down flanges and sometimes tyres to make the wheels fit. Plastic splashers and footplates can save a lot of grief on otherwise etched brass or monkey metal bodied locos.

 

You can turn some RTR wheel tyres down but they really need a mandrel as using the axles will usually rip the axle from the wheel center. Smokey Joe wheels are redeemable if you cut out the disc and reduce the flange using a mandrel to hold the tyre.

 

I fitted Romford 24mm tyres to Triang Hall wheels once, Big mistake, I since found turning the original iron tyres works a lot better and fitting H/D castle wheels works better still and you can turn those down in a black and decker drill with an old file if your hand is steady enough.

[Poor Old Bruce]

Except on the LNWR, where it all gets complicated!

 

Crewe's "nominal" diameter roughly equates with the minimum diameter before scrapping. How useful it is is a bit of a puzzle; the only thing that comes to mind is in calculating the maximum speed of the wheels and motion components. The maximum tractive effort is with minimum size wheels, but what matters more to the operators is the minimum tractive effort relative to the regulator and valve gear settings.

 

I have read somewhere recently that the wheel size quoted for LNWR engines is the size of the wheel casting itself without tyres. Add 2.5 inches for the tyre thickness which makes the dimeter over the tread to be 5 inches more than the quoted size. In LMS and BR days, the size given would be over the tread.