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page trail:  Templot Club > Forums > Templot talk > P4 Wheels on EM-SF track
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                 P4 Wheels on EM-SF track
     
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1st message | this message only posted: 10 Aug 2019 19:14
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Rob Manchester
Manchester



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Hi Martin,

Just had an email from a old friend I haven't spoken with for years. He is just starting to put a layout plan together after being abroad for a few years. He has a stock of kit-built EM locos with back-to-backs set around 16.6-16.7mm so looks like maybe he had my idea of using EM-SF too :) The problem is he has a quantity of high end unmade kits that have Ultrascale P4 wheels with them. He asked about the chances of using these on EM-SF. I reckon it could be done but he would loose the ability to run them on standard 1mm flangeway EM. Do you concur ?

Thanks
Rob


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2nd message | this message only posted: 10 Aug 2019 19:39
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from:
Martin Wynne
West Of The Severn, United Kingdom



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Rob Manchester wrote: Hi Martin,

Just had an email from a old friend I haven't spoken with for years. He is just starting to put a layout plan together after being abroad for a few years. He has a stock of kit-built EM locos with back-to-backs set around 16.6-16.7mm so looks like maybe he had my idea of using EM-SF too :) The problem is he has a quantity of high end unmade kits that have Ultrascale P4 wheels with them. He asked about the chances of using these on EM-SF. I reckon it could be done but he would loose the ability to run them on standard 1mm flangeway EM. Do you concur ?

Thanks
Rob
Hi Rob,

P4 wheels have 0.4mm effective flange thickness, so need to be set at 16.8mm max back-to-back for both EM-SF and regular EM at 17.2mm check gauge.

The issue will be wheel width:



For EM-SF the minimum wheel width is 2 x 0.8mm + 0.25mm blunt nose = 1.85mm.

P4 loco wheels are 6" scale width (2mm) so should be good on EM-SF.

P4 wagon and coach wheels are 5" scale (1.7mm) so might be a bit bumpy over the crossings, unless constructed with sharp-nose vees.



For regular EM the minimum wheel width is 2 x 1.0mm + 0.25mm blunt nose = 2.25mm.

Even EMGS wheels at 2.3mm wide are marginal with prototypical blunt noses (that's why EM-SF improves the running). P4 wheels are going to be very bumpy on regular EM.

If built with very sharp-nose vees, P4 loco wheels will be marginal. Wagon and coach wheels will be bumpy whatever you do in regular EM.



Hope this helps. Summary: Back-to-back 16.8mm. It's doable with P4 loco wheels in EM-SF. Wagon wheels would be a bit bumpy.

Regular EM is a problem all round, but just about doable, for locos only, if crossings are built with ugly sharp-nose vees.

cheers,

Martin.

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3rd message | this message only posted: 10 Aug 2019 20:08
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from:
Rob Manchester
Manchester



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Brilliant Martin, thank you very much. I hadn't considered the implications in that depth. Sounds like he will be OK on his locos. I don't think he has any rolling stock yet.

Maybe you will have 2 EM-SF builders soon :D

Rob


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4th message | this message only posted: 11 Aug 2019 11:58
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from:
Jim Guthrie
United Kingdom

 

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Martin Wynne wrote: P4 loco wheels are 6" scale width (2mm) so should be good on EM-SF.

P4 wagon and coach wheels are 5" scale (1.7mm) so might be a bit bumpy over the crossings, unless constructed with sharp-nose vees.
Martin,

Do P4 wheel makers actually have different tyre widths for loco and rolling stock wheels?  I know that in S scale we only produce a wagon wheel profile from official drawings and that does for everything. :D

Not all loco wheels were 6" wide.  I have a drawing of a Drummond Caledonian 0-4-0 pug where the wheels are 4 3/4" wide.  I reckon this was to squeeze the motion behind the outside cylinder slidebars.  I've also got drawings of six-coupled and longer wheelbases where the profiles of the central drivers are quite different to normal.

Jim.

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5th message | this message only posted: 11 Aug 2019 12:33
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from:
Martin Wynne
West Of The Severn, United Kingdom



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Jim Guthrie wrote:Martin,

Do P4 wheel makers actually have different tyre widths for loco and rolling stock wheels?  I know that in S scale we only produce a wagon wheel profile from official drawings and that does for everything. :D

Not all loco wheels were 6" wide.  I have a drawing of a Drummond Caledonian 0-4-0 pug where the wheels are 4 3/4" wide.  I reckon this was to squeeze the motion behind the outside cylinder slidebars.  I've also got drawings of six-coupled and longer wheelbases where the profiles of the central drivers are quite different to normal.

Jim.
Hi Jim,

As usual, as soon as you try to find definitive information, everything turns to jelly. I certainly remember measuring some P4 wagon wheels at 1.7mm wide, but where they came from I can't remember. I know that P4 loco wheels are 2.0mm wide the last time I measured any, although the P4 standard allows them down to 1.85mm minimum.

Here's some stuff from the old Scalefour Digest:

"Prototype TW  Tyre width: 5" min, 6.1/2" max  = 1.67mm, 2.17mm
(see Note 1 below)

1.Some early wagon tyre widths were specified as 5", but wagon tyres are generally 5.3/8" or 5.1/2" wide, coach tyres are 5.1/2" wide, and locomotive tyres are towards the upper end of the specified range."


cheers,

Martin. 

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6th message | this message only posted: 11 Aug 2019 17:00
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from:
Nigel Brown
 

 

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I may be talking through ignorance here, but been having thoughts about wheel drop at crossings recently and have been wondering if the problem isn't overstated. Here's my (probably ignorant) reasoning.

You're likely to get some degree of wheel drop anyway, due to the fact the wheel treads are coned and so the outer rim of the tread, which is what gets supported (by the wing rail) as the wheel passes across the crossing gap, is of a smaller radius than the inner bits of the tread, which is what gets supported by the crossing nose and knuckle.

So what happens if the tyre width is sufficiently small that the bottom of the tread doesn't make contact with the wing rail. What will happen is that, because the wing rail moves inwards as it approaches the knuckle, contact will occur with a bit of the tread which isn't right at the bottom. So a further drop may happen.

The point is, as we're talking about very fine adjustments here, if the wheel width is reduced from 2.25 to 2.0 mm, is that further drop going to be significant? My guess is that it isn't.

Thoughts?

Nigel



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7th message | this message only posted: 11 Aug 2019 18:16
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from:
Martin Wynne
West Of The Severn, United Kingdom



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Nigel Brown wrote:You're likely to get some degree of wheel drop anyway, due to the fact the wheel treads are coned and so the outer rim of the tread, which is what gets supported (by the wing rail) as the wheel passes across the crossing gap, is of a smaller radius than the inner bits of the tread, which is what gets supported by the crossing nose and knuckle.Hi Nigel,

The vertical movement caused by the coning angle on the wheels is taken up by the vehicle suspension, and the wheel remains fully supported on the rails. The nose of the vee is taken down a fraction below the level of the wing rails so that the coned wheel can roll smoothly off the wing rail onto the nose. Here I have drawn some yellow lines on a new V-crossing so that the low nose can be seen:



It can be seen from the rail wear marks that the wheels make contact with both the wing rail and the vee nose simultaneously, and are always supported by one or the other. It's worth doing the same thing with model vees. Just a few thou off the top of the vee nose can make a big difference.

What is normally meant by "wheel drop" on model crossings is different -- a situation where the wheel is not supported on the rails and is expected to leap across fresh air by magic. If the flangeway is 1.0mm each side of the nose and the nose is 0.25mm wide, the width of fresh air immediately in front of the nose is 2.25mm. If the wheel is narrower than that, it is going to fall down the hole with a bump.

Not very far on most short-angle crossings because the gap narrows quite quickly towards the knuckle, and the wheel will jam down between the wing rails and vee nose only momentarily. But enough to cause very bumpy running.

The 2.25mm dimension is the same for all angles of V-crossing. So provided the wheels are wider than that they will run just as well over a 1:20 crossing as over a 1:4 crossing.

But on a longer crossing the gap narrows much less quickly towards the knuckle, and there may be a length of several mm over which the gap is wider than a 2.0mm wheel. They will fall in it most noticeably on longer crossings.

Despite the best efforts of modellers over many years, no-one has yet found a way to support an object 2.0mm wide over a gap of fresh air 2.25mm wide. :) There is the dodge of filling the crossing to support the wheel flanges, but this works only if all wheels have the same flange depth -- which tends not to be the case. Also the filling needs to be metallic, otherwise there is a risk of momentary loss of pick-up as the wheel is lifted fractionally off the rail.

The solution is to use wheels of the proper width for the track standard, or alternatively to design your track to properly support the actual wheels in use.

No-one takes their track to use on someone else's layout, so the best option is to modify your track to suit existing wheels -- rather than the other way round using your own design of wheels, which won't then run on other layouts.

Hence the many track standards listed in Templot -- something for everyone. :)  

cheers,

Martin.

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8th message | this message only posted: 11 Aug 2019 22:41
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from:
Nigel Brown
 

 

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Thanks, Martin

The bit I'm interested in  as follows. If the wheel is too narrow, as in your example a 2.00mm wheel where the gap is 2.25mm, it doesn't just drop. The point of contact remains on the nose rather than being transferred to the wing rail. That point of contact on the wheel will be more or less the last point of contact on the wheel, although that point is now behind the lowest point of the tread rather than at the lowest point. So the wheel moves down gradually until the gap between wing rails decreases to 2.00mm, at which point a point on the tread ahead of the lowest point of the tread makes contact with the wing rail. At this point the wheel ceases to drop.

Take a 1 in 7 crossing. The distance from the nose of the crossing to the point where the gap has decreased from 2.25mm to 2.00mm is 0.25 x 7 = 1.75mm. The point at which the wheel reaches its lowest point is half that, i.e. 0.875mm.

Take a wheel of radius R. The amount the wheel will drop is R - sqrt(RxR - 0.875x0.875). E.G., for a 6mm wheel radius this works out at 0.064mm. If my maths is right. Not a lot.

Ok, am I wrong?

Nigel


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9th message | this message only posted: 12 Aug 2019 07:38
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from:
Martin Wynne
West Of The Severn, United Kingdom



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Hi Nigel,

I did say that the wheel would not drop very far on short-angle crossings. But the drop increases rapidly for longer crossings.

Also, there are a few other factors to take into account:

The nose of the vee is already below rail level, say by 0.1mm. That will add half of that, 0.05mm, to the unsupported drop.

There is a chamfer between the wheel tread and the face of the wheel. Not much, but if we say 0.05mm, that reduces the effective wheel width to 1.95mm for full support.

There is a top corner radius on the rail head. It should be a 1/2" scale rad (0.17mm in 4mm scale), but most model rail has a sharper corner, and because it is a radius rather than a chamfer it won't have so much effect. Say 0.05mm before there is effective loss of support. Only on one side, because the flange prevents any support on that side.

So the gap becomes effectively 2.30mm and the wheel is effectively 1.95mm, a difference of 0.35mm. The combination of chamfer and radius reduces that a bit, let's say 0.3mm.

So at 1:7 the wheel drop is in effect over 7 x 0.3 = 2.1mm. For a wagon wheel at 6mm radius that means a drop of 0.09mm. Adding half of the low nose increases that to 0.14mm or 5 thou.

Not much, but enough to be noticeable in bumpy running. The effect is made worse because the combination of chamfer on the wheel and radiused corner on the rail cause the wheel to wedge itself into the space, and need to be dragged out by the couplings, creating much more of a bump than the drop alone.

But on longer crossings the effect increases significantly. Suppose you have a long curved crossover needing 1:12 crossings to stay within your radius limit. The wheel drop is then in effect over 12 x 0.3 = 3.6mm, and the drop is more than doubled to 0.33mm or 13 thou. With greater wedging effect between the rails. That's enough to create a very noticeable bump.

What all this means is that the effects are small, but noticeable. And easily avoided by using a track standard which provides full support for the wheels to be used:

Measure the wheel width. Subtract a thou or two for the front chamfer. Subtract the width of the blunt nose on the vee. For traditional bullhead that is 3/4" = 0.25mm in 4mm scale. Divide the answer by 2, and subtract another thou or two for the rail top corner radius. The result is your maximum flangeway gap. Add that to the check gauge for the wheels, and the result is the track gauge.

Notice that the track gauge is arrived at last. Rather than being the starting point because so many modellers regard it as the critical dimension. It isn't. Start with the wheels.

(The check gauge for the wheels is found by adding the effective flange thickness to the back-to-back.)

cheers,

Martin.   

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10th message | this message only posted: 12 Aug 2019 12:18
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from:
Nigel Brown
 

 

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Hi Martin

Thanks a lot for that breakdown. Shows how all the little bits add up.

Cheers
Nigel

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