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Post Info TOPIC: Renault FT Idlers.
Lothianman


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Renault FT Idlers.
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Wood is a perfectly good material for wheels especially if metal is used for the hub bearing and the actual rim - indeed, it was much used in this way for disc type railway coach wheels in the early 20th century (e.g. "Maunsell wheels"), and that was as critical a component as you could get.

It does strike me that if you are trying to keep the weight down and the consumption of strategic metals down in a non-critical area then wood could have been the best choice for the FT17 idler. For an expansive area of relatively thin but stressed material, a sheet of wood (or plywood, in particular) may well be better than metal sheeting of equivalent weight and/or tensile or compressive strength if it is less prone to buckling (another important failure mode). And in this area, bullet-proofing was probably/perhaps of little relevance.

That is not to say you are wrong concerning the specific case of the 'poulie de tension' of the FT17, or that keeping mud out wasn't (also?) important, but it does not seem to me as clear cut a priori as one might think. Perhaps someone has something like a detailed sketch from the manual. I will be interested to know the answer too!

It should also be added that the Fiat 3000 knock-off had skeletal sprockets but a disc front idler too ...



-- Edited by Lothianman on Wednesday 15th of August 2012 09:09:08 PM

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James H


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It has recently been put to me that the idea that the FT idlers were made of wood is preposterous. Thinking about it, I realise it probably is. They wouldn't have lasted five minutes.

The alternative suggestion is that the wheel was steel, with six spokes, and that plywood discs were bolted on to keep the spokes clear of mud and obstructions. Anyone agree/disagree?



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Tanker


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Bonsoir James,

Here is only two photos.

The first one in Billancourt. This Renault FT "pot de fleur", is always in front the old Renault's offices.

The second one was taken in Norway

It was a very good wood . . . . !

Michel



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CharlieC


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So all those wooden spoked wheels used on artillery pieces and on heavy trucks in WW1 are just propaganda images.

If you look closely at the FT idler it's made of shaped wooden blocks so the grain of the wood runs radially just like the spokes of a wooden

wheel. Since the idler doesn't have to bear the weight of the tank but only the weight of the track it would have been easily strong enough.

A wooden idler with steel rim and hub would have been easer to manufacture for French industry since making large steel castings was a 

difficult and expensive process in WW1 (you really need induction or arc furnaces for steel casting - both of these technologies were in their

infancy in WW1). The problem with wood is that it deteriorates over time - the FT idlers were replaced with steel cast

idlers in the early 1930s.

Regards,

Charlie



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Lothianman


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Exactly so re the grain of the wood - the wooden discs in the Maunsell wheels on railway coaches were, if I recall rightly, also made in sectors to give the same radial distribution of the grain of the wood.

I suppose that the question of wood rot was not much of an issue for a FT in 1917 given their likely lifetime in action! Also that there may well have been woodworkers going spare and it all helped reduce the need for skilled metalwrights.

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James H


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Well, I seem to have been soundly thrashed on this one. Of course, I knew all along the man was talking nonsense. I must disabuse him before he makes an even bigger fool of himself.

Thank you for your input.



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CharlieC


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Not really - the first image tanker supplied shows the construction - the idler is built in the same way a wooden wheel is - steel hub and rim.

Rather than spokes and felloes the space is filled with shaped wooden blocks with triangular filler pieces. I'd bet the rim was heat shrunk on

just like a wooden wheel. I'm not sure about the bolts in this image - period FT images don't seem to show these - I suspect they were added

because the idlers started to fall apart when the wood shrank through neglect. The usual practice would be to  treat the wooden parts with

linseed oil to stop shrinkage and embrittlement.

Don't knock the strength of wood under compression - on a gun carriage like a 15cm sFH 13 each 40mm diameter spoke was loaded up

with 1.1 tons end load (the gun weighed about 2.2 tons) when the spoke was directly below the axle. I guess you should also add in

some shear load because of the dish in the wheels.

Regards,

Charlie



-- Edited by CharlieC on Friday 17th of August 2012 04:52:15 AM

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TinCanTadpole


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My understanding (sorry, can't remember where from) was that the wooden idler was a steel frame with wooden inserts - which I think shows in the first of Tanker's pics, though I may be wrong.
On the matter of strength, I don't see a problem if a reasonable thickness of wood is used, even though wood is better used under tension than compression - I remember a university lecturer saying that wood is typically four or five times as strong as steel when placed under tension.

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Tanker


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Bonjour,

Here is somme other photos from Renault FT idlers. These two models were used during the First World war.

First model (wooden model) :

Second model (steel and wood model) :

You can see the 2 models, on post war Renault FT's postcards, and even on Renault FT used in 1940.

Bonne journée - Michel



-- Edited by Tanker on Friday 17th of August 2012 02:50:48 PM

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Ironsides


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Renault FT wooden Wheels, two show the rim Rivets/bolts? the third is not clear but they seem to be originally flush so would be hardly visible on sound wheels.... unfortunatly I cant credit the original pic owners so are here for study purposes only... as to the type of wood, oak or ash seems likely...

 

Cheerssmile

 



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James H


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Ah. I might be able to remove some of the egg from my face.

Thanks for those photos, Michel. Most grateful. These photos (scroll down) show the FT with a seven-spoke steel idler and triangular steel panels to fit.



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Ironsides


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There appears to be another type of wooden wheel made from equal segments of solid timber so far Ive noticed this on two contemporary survivors (Brussels right hand idler only and Norway) and a 1920s TSF pic see below...

At first I thought possibly repaired wheels( the norwegian one may well be repaired) but the TSF pic would confirm use of this type from an early date... now Im thinking is it possible to identify the manufacturer?

 

Cheerssmile

 



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MK1 Nut


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Looking at the wheels as purely a wood working task, I much prefer the equal segments one. The wedges are going to expand and either lift the metal rim away from the spokes, or shrink and fall out, shortly followed by he now unsupported spokes. Hence all the bolts. I do love world war one ingenuity and how they are prepared to design with time, material shortages and building skills in mind. Clearly making a wooden rim to hold the spokes in place takes time and well practiced skills, using bolts to prevent the spokes slipping by the rim is a nice neat compromise in design. The fact some FT's still have their wooden wheels, shows it to be a very good bit of design.

Sorry I can't add anything to the history side of things.. just like the wheels. :)



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Ironsides


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I would think that the metal rims would be shrunk on which would take up a certain amount of shrinkage of the wooden parts over time, in Europe with the relatively wet climate I dont think this would be a major problem... to me the multiple equal wedges seems less sound then the more complex construction dare I say "Renault" Idler where the triangular pieces are acting more as a filler.....

I think it was Tanker who said the Wood used was French Oak , this is likely to shrink a lot less down the grain then across it so although gaps may appear the wheel would probarbly still be quite solid, the wood grain would run from the rim to the hub in both cases....

The equal segment idler may appear solid when new and I think could easily be mistaken for a cast wheel...

Cheerswink



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MK1 Nut


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Hi,

Yep, have to agree about the French Oak, know a guy who built a model boat out of it... very tuff and stable wood.

I would disagree that the wedges are just infills, but a clever bit of design to get round the lack of a wooden wheel rim to hold the spokes in place at that end. The wedges enable the use of less spokes, and bolts needed to stop the rim sliding sideways. So basically I'm disagreeing with myself earlier and now think the wedge type wheel the better one. biggrin

Helen x



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