Frame Spacers for P4 and EM

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Frame Spacers for P4 and EM

Postby jjnewitt » Tue Feb 05, 2019 12:17 pm

As part of the steam loco chassis commissions that I've recently been undertaking I've some analysis in CAD using EM and P4 tyre profiles to derive the amount of sideplay necessary for a given minimum raidus (I usually design them down to 1m) and therefore set the frame spacer widths from this. What I've found with recent LNER J15 and MSWJR 2-4-0 chassis is that I can use the same frame spacers for both EM and P4. The P4 wheels will require some sideplay on the centre axle but the EM ones seem to need little or none for a 1m radius curve with alomst everything seemingly absorbed by the slop in the standards.

I wonder if anyone else who's drawn up loco chassis has done anything similar to set frame spacer widths or how they ended up with the dimensions they used? I am also curious as to where the 'standard' P4 frame spacer dimension of 15mm (assuming it is a standard) came from and why EM frame spacers are generally so narrow?


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Re: Frame Spacers for P4 and EM

Postby dal-t » Tue Feb 05, 2019 12:30 pm

I am so glad you have asked this question, because after many years of building locos in EM I got thoroughly fed up with the number of axle washers I had to use to get the sort of running I wanted (i.e. not swaying all over the track, or looking like a segmented snake) but thought it was just my building technique. Of course, I never though to use the spacers marked 'P4' if they were provided on the same fret, but then I am a bear of very little brain (and I would say practically everything I built then was designed before CAD allowed the sort of analysis you have undertaken),
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Re: Frame Spacers for P4 and EM

Postby grovenor-2685 » Tue Feb 05, 2019 1:06 pm

I recently assembled a chassis using the supplied P4 spacers and then discovered that the wheelsets had no sideplay whatsoever and would not turn freely. So have just dismantled it to start again with the EM spacers. One problem is that the rear bosses on wheels may not be what the chassis designer expected. In this case I am using Studiolith wheels.

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Re: Frame Spacers for P4 and EM

Postby Dave Holt » Tue Feb 05, 2019 4:02 pm

In more recent times, I've either used the "P4" spacers provided in the kit, where applicable, or used either society or Alan Gibson etched spacers, without much thought about their width. Looking through some of my model files, it looks as if a width over the main frames of about 16 mm seems to be the general case. Using mainly Gibson driving wheels and London Road Models horn blocks and axle boxes, I've generally had to use some washers, but not to excess (actually had to thin some down), to give running clearances on the outer axles. I tend not to restrict side play on the centre axle(s) as it will only move as far as needed by the track curvature.
on my earlier models, I cut and filed spacers from brass sheet. Again I appear to have aimed for a width over frames of 16 to 16,2 mm. I recall that on my first chassis - a BR Std 5 4-6-0, I went wider than this but can't find a note of the actual figure (a sketch mentions 15,5 mm spacers with 0,45 mm frames, but i can't be sure this is what I used), however, on this loco the frame to wheel gap does look much less than my other locos. It still negotiated a 900 mm reverse curve test track before fitting front steps and connecting the tender.
All that said, as Keith rightly states, the ideal frame width depends on the thickness of the boss on the back of the wheels, which probably varies between makers and even between wheel types from the same supplier.
From an appearance point of view, and to avoid excessive use of washers to limit side play, I wouldn't like to see frames much less than 16 mm outside, for P4 locos (except Britannias, Clans, 9F's and Bulleids, of course).

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Tim V
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Re: Frame Spacers for P4 and EM

Postby Tim V » Tue Feb 05, 2019 4:09 pm

15mm and 15.5mm date back to Studiolith days! They produced two widths of frame spacing jigs, the recommendation was to use the narrower spacing normally, but of your curves allowed it, to go for the wider spacing.
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Re: Frame Spacers for P4 and EM

Postby davebradwell » Thu Feb 07, 2019 6:21 pm

If the bosses on the back of the wheels are too long, don't you just shorten them so the wheels fit? Sorry, but it seems obvious.

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Re: Frame Spacers for P4 and EM

Postby Flymo748 » Fri Feb 08, 2019 7:12 am

davebradwell wrote:If the bosses on the back of the wheels are too long, don't you just shorten them so the wheels fit? Sorry, but it seems obvious.

That’s what I typically do with Alan Gibson wheels anyway. It allows the tyre to be closer to the frame, and is pretty much essential if you are going to use some form of “spider” to short out wheels for split axle chassis.

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Re: Frame Spacers for P4 and EM

Postby Enigma » Fri Feb 08, 2019 10:50 am

My home made chassis construction 'jigs' have a between frames width of 14.5mm which, with 'normal' thickness frame material provides anything from 15 to 15.5mm overall width. Some kit spacers fit perfectly, some are to wide, some are to narrow. All EM ones I've tried are far to narrow but I have soldered in a piece of 20thou to fill the gap at least once. As I've never built anything grander than a 6-coupled chassis I rarely have any problem with lack of side play even on small radius curves and a bag of Alan Gibson assorted washers provides plenty of options to take up the slack. I occasionally use Peco fibre washers if I find I have allowed to much sideplay after wheel fitting, thinning them by rubbing on fine emery, and slotting with a scalpel so that they can be slipped onto the axle. Taking wheels off axles to fit washers is not to be recommended. Far better to measure clearances accurately prior to wheel fitting, selecting washer thickness accordingly.

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