Crab Comet conversion

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Mark Tatlow
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Re: Crab Comet conversion

Postby Mark Tatlow » Sun Oct 01, 2017 11:20 am

Hi Julian,

I am subscribing (having not previously before) to the view that the sound in a model loco should come out of the smokebox/cylinder area; because this is where it came out of in real life. Not my idea, I think it was Chris Pendlenton that suggested it to me first - so I am giving it a go!

Although I did have an eye to the fairly significant resistance of the springs needing a fair amount of weight to depress such that they achieved the correct suspension, I am packing a lot of weight in to give me the best possible haulage. It has been a bit of a bug-bear of mine for a while (largely because my locos have been light-footed!).
Mark Tatlow

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Will L
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Re: Crab Comet conversion

Postby Will L » Sun Oct 01, 2017 5:18 pm

Mark Tatlow wrote:...Although I did have an eye to the fairly significant resistance of the springs needing a fair amount of weight to depress such that they achieved the correct suspension, I am packing a lot of weight in to give me the best possible haulage. It has been a bit of a bug-bear of mine for a while (largely because my locos have been light-footed!).


But you should be aware that any sprung chassis is sensitive to where the weight is and with Brassmater individual springs if the CofG isn't central over the the sprung wheelbase it won't sit level, performance will be compromised and in extreme cases there can be a tendency to go strait on at corners.

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Re: Crab Comet conversion

Postby Julian Roberts » Sun Oct 01, 2017 7:43 pm

Le Corbusier wrote:I do like these lessons :thumb
Tim


Here here, and so many thanks to all the people putting their knowledge for us to all learn from, including updates from you Mark. Maybe if you do a CSB Crab you could show here what you do? On a quick look at the links I don't see how the fulcrum points are worked out but I know the info is out there in all your threads on the subject Will. There's an awful lot of ironmongery for the wires to have to get through, and a plethora of spacers, if doing it by the instructions... Will be interested to know how much you can get the loco to weigh.

One question I have is how much downward travel to assume is needed on my front and rear wheels that have stops to prevent them rising in the slots higher than 1mm below the top of the travel. (Easily altered if I find eventually that 1mm is too much) I am assuming 1mm but is 0.5 sufficient I wonder. Obviously it is a "how long is a piece of string" question as it will depend on how awful the track is, but on a reasonable assumption...? (and the track of our layout is of course perfect!!)

I will arrange an extra 0.5 up and down for the centre axle

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Will L
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Re: Crab Comet conversion

Postby Will L » Sun Oct 01, 2017 10:56 pm

Julian Roberts wrote:One question I have is how much downward travel to assume is needed on my front and rear wheels that have stops to prevent them rising in the slots higher than 1mm below the top of the travel. (Easily altered if I find eventually that 1mm is too much) I am assuming 1mm but is 0.5 sufficient I wonder. Obviously it is a "how long is a piece of string" question as it will depend on how awful the track is, but on a reasonable assumption...? (and the track of our layout is of course perfect!!)

I will arrange an extra 0.5 up and down for the centre axle


Tricky question. Using the Brassmaster system, the answer will depend on how heavy your loco is and so how much it compresses its springs.

If you've got the loco the right weight so that when static the loco sits on partially compressed springs at the correct right height, then 0.5mm either side of the static potions on all wheels is quite enough for any track work likely to be deemed fit for P4 use. In practice you will then only use 0.25 either sides, unless you really have to have 1mm high steps in your track. Fortunately, it is the unused 0.25mm either side of the perfect solution that means that you don't have to be totally precise with the loco weight.

This is the setup that CSBs allow you to plan for and guarantee, by allowing you to fit a spring wire that suits the loco weight, rather than have to weight the loco to suit the springs.

If however you loco is significantly above or below the weight that suits the springs (which is?) you are in an rather different situation. In these circumstances the axles will either be force against either the top stop (if too heavy) or bottom stop(if too light). Then you have to set the stops that the wheels are being forced against so that they give you the correct ride height and then provide additional clearance for the middle axle so it can move beyond the stop point on the outer axles. Which way will depend on whether your loco too light or too heavy. O.5 should again be sufficient.

Its precisely the fact that you get this sort of issue with individual springs that makes CSB the better solution.

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Re: Crab Comet conversion

Postby Julian Roberts » Mon Oct 02, 2017 8:40 am

Will thanks for answering. However what I am after is to not leave the ride height to chance. The idea is to bottom out the front and rear bearings. Here is what Dave Holt wrote about making his Crab, a picture of which I started this thread

On the loco, which sits on coil springs, I have had a bit of fun and games getting the weight in the right place. Initially, I added too much in the front portion of the boiler, causing the front driver springs to be nearly fully compressed. It was a mighty struggle to get the weight back out, having glued it in rather too well. That made things much better, especially after I managed to squeeze a bit more lead in the firebox and some under the cab roof, but after I fitted the the front pony truck, which is also lightly sprung, I found the reverse problem! Now the rear driving springs were fully compressed, so I had to re-fit part of the boiler weight and remove the lead from the cab roof to get a reasonable ride attitude. Compensation is a lot less bother!!!



Here is a picture of how I've arranged the bearings "stop blocks" - it just shows the centre and front bearings. The centre bearings don't need stops to prevent them rising to the full height, but I wanted to have the spigot aspect to retain the coil spring. So I've shortened the spigot that is part of the chassis etch which retains that end of the spring.
20170925_170848 (Small).jpg

The boiler has now got a trial fill of lead flashing as in the picture, and it weighs 155g. Rearwards from the flashing will be the motor.
20171001_222928 (Small).jpg

After much time was spent on Saturday endlessly trying different configurations of the Lo Loader gearbox, and getting into a muddle about what I'd tried and hadn't tried, I find I can just about manage the 1024 plus a small flywheel!!! (after all that advice!) (This flywheel is small but has its weight mostly at the rim as can be slightly seen here). This is approximately how it will sit, but the whole thing a little further back and the motor front tilted a little more upwards. Interesting (as in doh!) that Mark's way of mounting the motor never occurred to me
20170930_162654 (Small).jpg

So quite a lot more lead can be fitted into the firebox, and I can get more in little nooks and crannies in the body towards the front and between the frames on the chassis, but it won't really help the front wheel weight materially, so I'm doing some head scratching. I am assuming though that your reply means I can consider I only need to allow 0.5mm downward travel on the front and rear wheels (there will be none upwards on these).

What is becoming clear is that a weighted tender hanging off the back will be counterproductive...unless it could be arranged to have its hook acting near the centre of the loco, a challenge I would think.

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Russ Elliott
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Re: Crab Comet conversion

Postby Russ Elliott » Mon Oct 02, 2017 10:11 am

Julian Roberts wrote:The idea is to bottom out the front and rear bearings.

Why?

100g axles will deflect the Brassmasters springs about 0.7mm to 0.8mm. Which seems fine.

What John Brighton did regularly with the Brassmasters was to take off about 0.5mm from the spring tongue and aperture top of the middle axle. This considerably lightened the middle axle, and made the outer two sit down more on their springs as a consequence, so there was a resulting large disparity of axle weights, but it was a quick route to aid the pitch stability without having to finesse the CofG position too much (which is the essence of the frustration Dave Holt encountered).

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Re: Crab Comet conversion

Postby Russ Elliott » Mon Oct 02, 2017 10:42 am

Julian Roberts wrote:What is becoming clear is that a weighted tender hanging off the back will be counterproductive...unless it could be arranged to have its hook acting near the centre of the loco, a challenge I would think.

A symmetrically weighted tender of say 100g will typically impose 40g onto the back of a loco. (And would therefore need counterbalancing by extra weight in the front of the loco.)

weighted-tender1.png
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Re: Crab Comet conversion

Postby Julian Roberts » Tue Oct 03, 2017 3:57 pm

Great idea Russ, thanks for that. It is along the lines of my intention, reduced weight on the centre wheels, and as I have made all my compensated locos so far.

Re the tender however, the problem I see ahead is getting enough weight towards the front of the loco in the first place without any extra weight at the back.

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Russ Elliott
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Re: Crab Comet conversion

Postby Russ Elliott » Tue Oct 03, 2017 4:11 pm

Julian Roberts wrote:Great idea Russ, thanks for that.

I'm not so sure it's a great idea at all. I was merely reporting what John Brighton did, as a professional builder who couldn't afford to spend much time thinking and faffing about.

If you are bottoming out your front and rear drivers, what's the point in going to all that constructional effort in measuring up those pesky springs?

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Re: Crab Comet conversion

Postby Julian Roberts » Wed Oct 04, 2017 3:04 pm

Russ Elliott wrote:
100g axles will deflect the Brassmasters springs about 0.7mm to 0.8mm.


That is what I had measured Russ. Do you say that taking my word for it or by calculation from the forbidding looking formula on the CLAG page on these hornblocks.?
- I assume the latter, in which case I'd be interested to know how it works.

You ask why I want to know the spring force - firstly because I want to understand springs and secondly I need to know how much weight the loco requires to overcome them. Not sure that the formula can ever be more than mumbo jumbo to me, a musician not mathematician! - unless it's understandable in very simple terms.

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Russ Elliott
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Re: Crab Comet conversion

Postby Russ Elliott » Wed Oct 04, 2017 10:38 pm

Julian Roberts wrote:That is what I had measured Russ. Do you say that taking my word for it or by calculation from the forbidding looking formula on the CLAG page on these hornblocks.?

Both.

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Re: Crab Comet conversion

Postby billbedford » Thu Oct 05, 2017 9:18 am

Julian Roberts wrote: Not sure that the formula can ever be more than mumbo jumbo to me, a musician not mathematician!


Here we go again, someone who makes sense of dots on lines gives up when there are letters involved.....
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Re: Crab Comet conversion

Postby Knuckles » Thu Oct 05, 2017 9:58 am

Complex mathematics and Algebra is beyond some of us. I always struggled with it, and when I see the algebra in the S4 digests my mind literally shuts down and comprehension is zero. Effort to understand results in a fried brain, cognitive dissonance and no progress....always. It's a stumbling block hard go overcome.

Feel free to call me thick and others thick who fit the description. Where we lack we make up for in other areas. That's how people generally are.

Good on you and the others who can write and read it. Obviously it has benefit and is needed but we can't all excel in exactly the same areas in life. If we could there would be no concept or need of friendship and helping each other out.


Bad init.
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Re: Crab Comet conversion

Postby Julian Roberts » Thu Oct 05, 2017 3:17 pm

Thank you Knuckles my feeling exactly. Being an 11+ failure hasn't disqualified me from a career in music nor from understanding things eventually when people have the kindness and patience to explain things clearly, both qualities Russ has shown many times on this Forum. Some people's brains are wired differently e.g. dyslexics who can be geniuses yet total failures at school.

However maybe understanding that formula is unnecessary for me to make certain conclusions.

Somehow I hadn't seen the link on the CLAG page about these bearings to another page on springrates etc.
http://www.clag.org.uk/beam-annex1.html#caveat

I have played around with the really tiny Exactoscale 50g springs. 1/2mm from fully compressed is 40g, 1mm ditto 20g, 1.5mm ditto 8g, full travel is 1.8mm. However these are almost invisible to the naked eye so would pose practical problems in their use.
20171002_135100 (Small).jpg

The reason I'm looking at them is that the total weight I can get in the loco is 215g. There may be room for a little more but it will be scraps of lead here and there. The good news is that the Cof G is about half way between the front two drivers so some tender weight will be useful after all.

The Lo Loader gearbox I got from Chris a month ago. It proved to have a very close clearance for the gearwheel on the 3rd stage (driven axle). There has been a redesign for a new range of motors and now this box has a grubscrew fixing and slightly higher ratios. I had to remove any trace of solder (from where I had strengthened the fold) for the gearwheel to revolve freely, which it now does with the absolute minimum fag paper clearance. Reported this to Chris who said the redesign is not yet signed off and said somebody else had reported something like this.

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Re: Crab Comet conversion

Postby Julian Roberts » Tue Oct 10, 2017 7:37 am

It takes me a long time to get motor/gearbox orientation "just so" and then lock it with a touch of solder without locking the shafts in place at the same time (the plastic gears being nowhere near! and safely in their bag.)
I got the orientation just right so that with the flywheel it could be deftly slotted into the boiler space - just - and filled the firebox with lead thus:
20171008_184252 (Small).jpg

20171007_111956 (Small).jpg


However it was only then that I then realised the 1st stage shaft would be unremovable as it would be behind the wheel, in the case of the motor needing to be replaced or easily worked on should that ever arise. So - back to the drawing board, and I found that with the gearbox in a higher orientation I just couldn't fit it into the same space with the flywheel in any way that was easy to take the chassis on and off. And typically the lead in the firebox now took up too much room.

So it was off with the motor shaft and off with a few layers of lead, fortunately the 70deg solder melts quite easily without getting hot enough to melt the plastic body. And re-configure the gearbox. Now however it is possible to create a false bottom of the boiler, though not unfortunately all the way back to the firebox. With the current layers of lead the body weighs 240g with CofG still forward of the centre wheels. Still only 40g per wheel...

20171010_080838 (Small).jpg
New gearbox configuration

20171010_080604 (Small).jpg
Boiler false bottom, some lead removed from firebox.



SPACERS
I did not mention earlier that having decided (much earlier) that the motor would hang from the rear axle, not the centre one as per instructions, the large spacer meant to go in the firebox space could not fit there if I wanted to be able to easily remove (download?) the rear axle with its gearbox and motor, so it was put forward of the centre axle as per this photo. If I had been an 11+ pass I might have noticed the nut above the front axle could have been placed directly on top of it and then the screw that retains the motion bracket etc assemblies could have been height adjustable just in case compensating the front axle might prove to be a better option...

2017-09-10 22.47.03 (Small).jpg
2017-09-10 22.47.03 (Small).jpg (120.07 KiB) Viewed 4536 times

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Re: Crab Comet conversion

Postby Julian Roberts » Sat Oct 14, 2017 11:46 am

A couple of pictures of David Franks' Crab (EM gauge) on his layout Burley in Wharfedale. On the mainline and a detail. Note the original type FB rail with BR MK1 baseplates and elastic spikes, contrasting with the typical BH rail in the foreground siding. An etch by Bill Bedford. Very time consuming but subtly worthwhile representation of what was typical throughout Britain for a while and until more recently on secondary mainlines e.g. the Settle-Carlisle.
DJH kit with scratchbuilt compensated chassis made many years ago. Nowadays he uses CSBs.
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20171011_201628-1.jpg
20171011_213834.jpg

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Re: Crab Comet conversion

Postby Julian Roberts » Fri Nov 17, 2017 2:34 pm

The time since last able to progress the Crab shows why my total completed locos is only 6 in 10 years!

With the loco weighing 240g I concluded the supplied springs could not give me the downward force I want on the centre axle which needs more up and down movement than the outside driving wheels. The spring gives 30, 60 and 80g for each half mm of compression. So if it was to exert the minimum 30g half a mm below the ride level it would exert 60 at ride height and 80 half a mm above ride height. Meanwhile 240g gives me just 40g per wheel. 

It looked unlikely I could get what I want, a consistently lower weight on the centre wheels. Exactoscale springs might do but are so tiny they are very difficult to use if I need to take wheels on and off the chassis.

I had started out meaning to do this as quickly and simply as possible. I have (just a wee bit haha) of a neurotic perfectionism streak that I can indulge in this hobby where it is impossible in ordinary life! - and it has taken over again with this project....  :twisted: So I'm not saying following the normal course with this kit doesn't work because obviously it does - but I just like doing things my way. 

Compensating the centre axle with the rear one gives a consistent weight relationship [Edit - whatever irregularities the track may have] and I could see the brake shaft gives a perfect pre existing assymetrical fulcrum point where the result would be a centre axle with just that little bit less downward force than the rear one. 

Fortunately having opted for a rear wheel mounted motor I had happened to have only one spacer between middle and  rear wheels which would only require a little filing to make room for a twin beam arrangement.

20171109_223739.jpg
Slots filed in spacer for beams. Extra square tube spacer where frames getting flimsy


A 2mm by half mm piece of strip would do for beams. The only difficulty about them is fitting them between chassis and gearbox without fouling on the box shafts. 2mm strip should be just right there. The centre axle bearings have extra pieces of tube soldered on to support the beams.

I didn't get the design quite right and had to add little pieces for the centre wheels. They should give a ride height that will need adjusting downwards by filing them.

20171111_091858.jpg
Compensating beams

20171110_105905.jpg
Tubes fixed to bearings.

20171110_170940.jpg
Beams in place



PONY TRUCK 

One of the many debates on the practicality of P4 was prompted by an article in MRJ  where the problem was a front pony truck regularly derailing, and this is no rare occurrence whatever the gauge if it just trundles along for the ride.

20171112_083020.jpg
2mm hornblocks to fit inside pony truck

20171111_105933.jpg
Just the right space for 2mm hornblocks with a layer of 0.5 strip front and back

20171112_224918.jpg
Retaining wires needed still


The pony will bear some weight and determine the loco front ride height. The loco will have two rollers resting on the pony truck. Thus the half mm clearance between the pony wheels and the loco frame will be maintained and it will be able to swivel enough for 3 ft 6in curves. The pony wheels are sprung with the little Exactoscale 40g springs. The truck will be weighted with as much lead as practical between the guide frames that come from the pivot.

The front driving wheels will be sprung sufficiently to take as much weight as possible without lifting the loco front off the pony, but if it does so momentarily from a track irregularity the weight of the truck will keep it on the rails.

Whether this turns out to work as intended will remain to be seen!
Last edited by Julian Roberts on Sun Nov 19, 2017 8:56 pm, edited 1 time in total.

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Re: Crab Comet conversion

Postby billbedford » Sat Nov 18, 2017 10:10 am

Julian Roberts wrote:The pony will bear some weight and determine the loco front ride height. The loco will have two rollers resting on the pony truck. Thus the half mm clearance between the pony wheels and the loco frame will be maintained and it will be able to swivel enough for 3 ft 6in curves. The pony wheels are sprung with the little Exactoscale 40g springs. The truck will be weighted with as much lead as practical between the guide frames that come from the pivot.


This just confused.

Springing the truck wheels means that you are expecting the truck to carry some some of the body weight, adding lead to the truck will depress the springs so that they will not be able to carry ny body weight. You have to decide what you want here one or the other.
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Julian Roberts
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Re: Crab Comet conversion

Postby Julian Roberts » Sun Nov 19, 2017 8:45 pm

Yes Bill. Spring plus weight is a belt and braces thing. Maybe unnecessary - I'll try to explain my thought process there sometime, but it won't do any harm. The idea is the loco will rest on the pony with the pony wheels at the top of their travel, so the rollers will have to be exactly the right height.

I forgot to say that all the driving wheel hornblock bump stops will now have to be removed.

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Re: Crab Comet conversion

Postby billbedford » Mon Nov 20, 2017 9:14 am

It's always a good idea to look a the prototype and model that. It usually save an awful lot of time from not going down the road of fantasy configurations.
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Re: Crab Comet conversion

Postby Julian Roberts » Mon Nov 20, 2017 9:38 pm

Well Bill as it happens, being in England for work I was able to call into the NRM yesterday morning. I can put a few of the many shots I took to illustrate as I go along now! The Crab at the NRM is only visible from platform height upwards but you can get much closer than visiting the ELR and seeing one in operation.

Here are a couple showing the tender mismatch, width and level, apropos my current concern with getting the ride height right - I'll have to get the tender chassis made before I can finalize it on the loco.

20171119_120156 (Small).jpg
Can't get this photo to go upwards.


20171119_104834 (Small).jpg
This seems to confirm the accuracy of the drawing that comes with the Bachmann model at least in this respect.



Don't want to get into any more theorizing about springing etc, there's plenty of that on the Forum, but I will be honest and say what works and doesn't work as I go along.

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Re: Crab Comet conversion

Postby Julian Roberts » Mon Nov 20, 2017 10:02 pm

A couple of photos showing details of the reversing shaft. There is no link as shown in the instructions which I referred to in the OP.

What does happen is that the frames curve down and the boiler cladding goes down vertically immediately behind. Doesn't matter for our model but does explain how the reversing shaft gets to the other side, not through the frames but on top of them and through the cladding

20171119_101816.jpg

20171119_102640.jpg


An issue that arose - was the motion bracket assembly meant to immediately abut the footplate as the instructions say? - the answer is clearly, no.

20171119_101940.jpg


Another one that I didn't mention here is, how long is the expansion link? The Comet one seems rather small, while the real thing seems to me so long and curving as to be part of the reason for the "Crab" nickname in that it looks like one of the creature's claws. Seeing the real thing here, probably in its most far back position, shows why the model doesn't, I think, make it the proper length - it goes so far up and back that it seems to clout the footplate though in fact just disappears behind its edge or valance.

20171119_103759.jpg

20171119_104949.jpg
An exactly side on view would have made my point rather more clearly.


I haven't got the patience to fabricate a longer one myself, not sure how to do so anyway, probably would take hours.

Here are shots of the boiler and footplate just in front of the firebox
20171119_104017.jpg

20171119_104003.jpg

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Re: Crab Comet conversion

Postby Julian Roberts » Sun Nov 26, 2017 10:22 am

CRAB MODEL PONY TRUCK SUSPENSION DESIGN

Before I started making this bit I wrote the following to a pal.

The real locomotive pony truck may not go round such sharp bends that the flanges will encounter the frames. But if it did the springing would in any case make the frames rise with the pony wheel on any humps in the track, keeping the frames clear of the flanges.

On the model in all gauges there are various ways round the problem that, scaled down, the distance between wheel and frames is tiny. The normal way to get round it is to file away the frames so that there is no likelihood of any fouling. However, the following assumes that this drastic and visually naff approach, though not very very noticeable, is unacceptable. Alternatives are to narrow the frames, unnoticeable from an average side-on view, or to arrange the frames to be part of the truck, the subterfuge being invisible again from normal perspectives as it is hidden behind the valance. On the Crab neither of these approaches is possible because the framing is already on the Bachmann model, moreover it is very thick and strong and can’t be modified without a whole host of other problems, and as it is accurate it would be a shame for it to be modified.

The normal 00 gauge type basic design is for a free riding truck that contributes nothing to the suspension or direction of the loco. The weight of the wheels and truck is supposed to keep the assembly on the rails which is all that is required. Even in 00 this is not wholly reliable.

However this model cliché neglects the likelihood of the pony wheel flange catching on the frame cut-outs, which are only 0.6mm vertical distance clear in this case. They are 17.2mm wide over the outside. In 00 gauge the flanges will thus be inside this width but in P4 they will be 0.2mm outside.

This is the typical situation where problems will be blamed on the track being less than billiard table smooth and level.

I adopt the attitude that it is hopelessly unrealistic to expect a billiard table level layout and easier to design in suspension that can cope with probable humps and dips in the track.

In addition to such humps and dips the model will probably be required to go round sharper than prototypical curves. Thus the pony truck will definitely swing so that the flange comes directly under the frame cut-outs, and probably will even if restricted to prototypical radii bends.

So what is needed is for the pony wheel flange to be always clear vertically of the frames.

The usual P4 approach is to spring the pony downwards and horizontally to keep the wheels down and give some directional control. However it seems to me that this neglects the problem of the flange being trapped by the frame on a curve if one of the wheels rises just that half millimetre, and pony truck derailments are frequently a cause of trouble.

I propose to arrange for the front of the loco to actually rest on the pony truck. The front driving wheelset will be sprung sufficiently that there is not a great deal of weight on the pony truck, but not so much that the loco is lifted off it except momentarily.

I think this will be easier than attempting some means of springing the pony truck to work in the same way as the prototype

So then the question is how to design this? What is needed is a roller so that the truck can move from side to side completely freely while bearing some of the loco weight.

Now the question is, where should the roller(s) go?

Here is the Bachmann drawing of the loco, showing the 0.6mm gap between flange and frame cut out. And a sketch of the front view. (Unfortunately loads onto here upside down. I'll edit with another, clearer, drawing here tomorrow) The gap between the frames could be filled in to suit the physical size of the roller arrangement chosen. All will be hidden behind the front buffer beam, so there is no visual realism question.
img090.jpg
Upside down. 17.2mm over the OUTSIDE faces of the frames.

img086.jpg
Wrong orientation


There are, it seems to me, two possible configurations, each with their problems. First, highlighted in yellow, are two rollers, one each side. Or second, highlighted in orange, a central roller.

The problem with the orange central roller is, would a rise of one wheel on a track hump of say 0.5mm translate into the frame lifting sufficiently? The frame would rise 0.25 - would that be enough?

If the answer is no, then two side rollers would not have that problem, but here would a seesaw effect be created, where if one wheel encountered a track hump, in lifting up the frame could that cause the opposite wheel to rise into the air and lift off the track?

After some thought, I have decided that the pair of rollers is the right approach, with springs to keep the wheels on the track whatever the pony truck does. In addition I will make the truck as heavy as possible to help keep the wheels down, and also hopefully to balance out the spring force, as I don’t really want the loco to be sprung upwards here and reduce the weight on the front driving wheel any more than necessary.
____________________________________________________

Well I decided to go for a single roller. As that will tilt the springs aren't needed but they won't do any harm
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Re: Crab Comet conversion

Postby billbedford » Sun Nov 26, 2017 1:17 pm

Julian Roberts wrote:CRAB MODEL PONY TRUCK SUSPENSION DESIGN

Before I started making this bit I wrote the following to a pal.

The real locomotive pony truck may not go round such sharp bends that the flanges will encounter the frames. But if it did the springing would in any case make the frames rise with the pony wheel on any humps in the track, keeping the frames clear of the flanges.


I've put original drawings of both the Crab and 8F pony trucks on my dropbox:

https://www.dropbox.com/s/dltytub7ywvn08e/Crab%20pony%20truck.png?dl=0

https://www.dropbox.com/s/7mjj6rahpchztk9/LMS%208F%20Bogie.tiff?dl=0

The Crab truck is complicated by the fact that it has a swing link suspension, but essentially both drawings show that the truck frames were held parallel to the main frames and any difference in lateral wheel height was taken up by the carrying springs.

I make the clearance between the wheel flange and the main frame on the Crab to have been 3.5 inches. So there seems to be an error in your frame cutout.
Bill Bedford
Mousa Models
http://www.mousa.biz

Julian Roberts
Posts: 749
Joined: Sat Jan 09, 2010 11:33 pm

Re: Crab Comet conversion

Postby Julian Roberts » Sun Nov 26, 2017 6:59 pm

Fascinating Bill! Beyond my powers of comprehension. The ride height will be determined by the match with stock that is correct and adjusted till correct with that. Whether 0.5mm becomes 1mm....won't be an issue. Here is the sketch I mentioned earlier
Attachments
20171125_105008.jpg


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