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Bentley 4 1/2 litre Blower in 1/12th scale - may contain Airfix.


nick

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I just looked at the real-world chassis and I see it's a "C" cross-section so you'll want it recessed.   Carry on.

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Brass inserts in the C frame as they will be edge on will certainly give very strong vertical support even for short sections.

Along the entire length will give each chassis member very good strength.

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8 hours ago, albergman said:

I just looked at the real-world chassis and I see it's a "C" cross-section so you'll want it recessed.   Carry on.

Frank, the brass plates are CNC cut and drilled and are .4mm under size compared to the 3D printed C section so they just drop inside the profile.
so the vertical face of the chassis rails is double thickness - resin on the visible outward side with nice, regular ‘folded’ curved edges and brass on the inside where the reinforcement Is. As both ends are occupied by cross members it will be almost impossible to see this unless someone takes a depth gauge  to the chassis rails!
 

Having said all that, I need to reprint these as the print just failed due to lack of supports, so I will re-run in the morning. The brass cut OK though. Photos soon!

 

thanks again for the idea 👍🧐💡

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After what a seems like a lifetime of messing about with this, I've finally cracked it and even actually made something!

 

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I am just so impressed that I can 3D print something about 400mm long with a load of 0.8 mm holes all along its length and then CNC drill and cut a sheet of brass from the same CAD drawing and the holes just perfectly align along the entire length, I didn't even run a drill through them, they are exactly the same diameter and just perfectly aligned. I shouldn't be surprised I suppose but this is just hobbyists kit really and that level of precision from additive and conventional machining which couldn't be more different, is mind blowing to me.

 

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It even fits the airfix body

 

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With the way that you've split the chassis, it should be easy to produce a long chassis as well. The extra length is in the parallel section in the middle.

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Now I'm producing some of the larger parts, like the chassis, I've found more errors with the airfix version. The most major one being that it's quite a bit underscale. 

 

I scanned (not photographed) the airfix chassis rail and scaled it up by a factor of 12 to give me full size for my CAD drawing. I scale this back down at 8.33% to print to get me back to 1/12 scale. My print matches the Airfix item size in the real world, so I know I'm working at 12 X the Airfix kit in my CAD. You can see that here, my 3D model overlayed on the 12x airfix scan.

 

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However, when I calibrate a blueprint with the true 10' 10" wheelbase, you can see the airfix version is well undersized. look at the rear suspension, the front is in alignment with the drawing and you can see how short my airfix sized chassis is at the rear.

 

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I happen to know this figure is 93.48% of full size as I had to scale everything because I have been working on the 10' 10" wheelbase being my God Dimension and had scaled the airfix scan to match this rather than multiplying by 12 as I suggested.

If my maths is Correct, this makes it about 1/13th scale not 1/12th (it's actually 1967/2500 scale! :) )

 

I need to think about this now. I guess it wouldn't kill me to just produce the body as well as everything else and just make the thing correct having gone this far. This is a big error to suck up after going to these lengths.

 

Nick

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7 minutes ago, Malc2 said:

Thats interresting. Does this explain why your 3D printed engine does not fit the original Arfix chassis?

 

Malc.

It doesn’t help. Some of the proportionality is out as well as well as some things just being the wrong shape. 

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Hi Nick,

 

your coachbuilder's drawing shows the long (3302 mm) chassis, the Airfix model is based on the short (2994 mm) chassis. I'm following your build with fascination. Great work.

 

Klaus

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1 minute ago, klaus said:

Hi Nick,

 

your coachbuilder's drawing shows the long (3302 mm) chassis, the Airfix model is based on the short (2994 mm) chassis. I'm following your build with fascination. Great work.

 

Klaus

I was just looking for that info! Thank you, that has saved me a great deal of work potentially!

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Klaus has saved the day there, when I split the blueprint around the centre and make the wheelbase 2994 by overlapping the two halves, everything snaps back into place. Apologies Airfix!

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So here's how it looks when I chop the centre out of the wheelbase to make it 2994

 

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Much better. I must admit I couldn't understand how Airfix could get something as fundamental as the wheelbase wrong. Turns out they didn't - I did! doh.

 

In my defence that drawing is taken from the Haynes manual and is titled 'Coachbuilders Arrangement 4 1/2 Litre Supercharged 10'10" wheelbase' I did have a vague recollection of seeing something about a long wheelbase version, I hadn't realised that this drawing was it.

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It is not generally known that there were short chassis variants made of the Blower.

So very easy to get misled when you get a drawing of the chassis with a longer wheelbase without already knowing.

Don't beat yourself up about it Nick!    At least you found this out quite quickly to be able to correct it.

 

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5 minutes ago, Noel Smith said:

It is not generally known that there were short chassis variants made of the Blower.

So very easy to get misled when you get a drawing of the chassis with a longer wheelbase without already knowing.

Don't beat yourself up about it Nick!    At least you found this out quite quickly to be able to correct it.

 

So does that mean there are 3 chassis lengths then? The 10’ 10” (3302mm) being ‘standard’, the Birkin/Ralph Lauren car ‘short’ @ 2994mm and then a third ‘long’ (>10’ 10”) version?

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As far as I am aware there were two wheelbases.

I have a very old booklet 'Where have all the Blowers Gone?'  I think it might be a BOC publication. I will dig it out and see what I can find out from it.

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The devil, as they say, is in the detail. Now I’ve got properly into the chassis everything is lining up with the drawings, I’ve disappeared down a rabbit hole again piling detail into the many fittings on the chassis. Photos soon. 

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I dug out my little Where Have all the Blowers Gone booklet, It was compiled back in the late seventies by Stanley Sedgewick.  Most of the cars have probably changed hands and possibly had conversion or restoration work done on them since,

Anyway, this is what I could gleam.  the short wheelbase cars were 117.5 inches and the long wheelbase cars 130 inches.

 

UR 6571 is a short wheel base car and this registration no was the original used on the Airfix Kit when released. The original I believe was built as a short wheelbase car and became known as The Pau Car.  Ralph Lauren is the current owner.

UU5872 is the number plate now on the current Airfix kit releases. It was originally a long wheelbase car that was converted to SWB. This car I believe now belongs to Bentley Motors.

I would imagine that the plate number was changed on the model kit due to current licencing arrangements between Airfix and Bentley Motors.

GY3905 is the car in the National Motor Museum at Beaulieu in Hampshire. It was rebuilt as a replica Birkin Team car by restorers Elmdown Automobiles.

DS240 (previously BGH55) was rebuilt as a replica Birkin Team Car.  Current owner unknown to me.

GT8774 This car had its wheelbase shortened to 117.5 ins in 1946.  Current owner unknown to me.

 

As far as I can ascertain the above listed are the short wheelbase Blowers, unless of course any other original cars have since been converted.      There are of course possibly also ground up built replica copies out there.

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1 hour ago, Noel Smith said:

I dug out my little Where Have all the Blowers Gone booklet, It was compiled back in the late seventies by Stanley Sedgewick.  Most of the cars have probably changed hands and possibly had conversion or restoration work done on them since,

Anyway, this is what I could gleam.  the short wheelbase cars were 117.5 inches and the long wheelbase cars 130 inches.

 

UR 6571 is a short wheel base car and this registration no was the original used on the Airfix Kit when released. The original I believe was built as a short wheelbase car and became known as The Pau Car.  Ralph Lauren is the current owner.

UU5872 is the number plate now on the current Airfix kit releases. It was originally a long wheelbase car that was converted to SWB. This car I believe now belongs to Bentley Motors.

I would imagine that the plate number was changed on the model kit due to current licencing arrangements between Airfix and Bentley Motors.

GY3905 is the car in the National Motor Museum at Beaulieu in Hampshire. It was rebuilt as a replica Birkin Team car by restorers Elmdown Automobiles.

DS240 (previously BGH55) was rebuilt as a replica Birkin Team Car.  Current owner unknown to me.

GT8774 This car had its wheelbase shortened to 117.5 ins in 1946.  Current owner unknown to me.

 

As far as I can ascertain the above listed are the short wheelbase Blowers, unless of course any other original cars have since been converted.      There are of course possibly also ground up built replica copies out there.

That's great info thanks for digging it out! so that does correspond to the two metric numbers we have then 3302 and 2994mm. That's good news!

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There's a lot going on here when you start rummaging down in the detail.

 

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All the crossmember flanges have subtle angles to match the chassis which is quite time consuming. I've figured out the various lay-shafts for the braking system now, which is quite frankly terrifying for something this heavy with 250BHP or so on skinny cored tyres. 

 

Getting close to finishing the chassis now anyway and I must admit I think it was worth doing now I've discovered all the stuff missing or simplified.

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Hi Nick

 Something else to think about: the big wing sump was a one- off and originally fitted to the Birkin single seater Brooklands car. If you want to model the car as it raced  in period, you will need a standard sump.

 

The story was... after Tim Birkin died, Dorothy Paget kept the single seater and refused to sell it. Peter Robertson-Roger owned Blower no.4 - UR6571 (the original subject of the Airfix kit) trashed the engine, and eventually managed to persuade Dorothy Paget to sell the single seater (which was Blower No.1). In the summer of 1939 the engines from No.1 and No.4 went to Louis Giron, who got No.4 going with parts from No.1, including the finned sump which he switched with No.4...
What this indicates is that the Airfix kit was wrong if you wanted to build it in it's 1930 Pau guise, as it had a smooth sump until 1939...

 

Now that sump has been copied and every other blower seems to have one.

Edited by Jo NZ
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34 minutes ago, Jo NZ said:

Hi Nick

 Something else to think about: the big wing sump was a one- off and originally fitted to the Birkin single seater Brooklands car. If you want to model the car as it raced  in period, you will need a standard sump.

 

That’s interesting. As it happens I have been pondering the sump as it’s very difficult to print.
Usually there is non-essential face somewhere that can be hidden - the mating surfaces of the block, rocker cover etc and that’s the area where all the print supports go, so any scaring can be sanded flat and won’t be visible. Because of the huge horizontal fin just under the mating face of this sump it’s inevitable that scaring occurs somewhere very visible. I have just decided to split the sump into two parts to avoid this. 
 

this all goes away with a standard sump. I think I will make both now I know this. There is more visual interest with the finned one I guess.

 

This is the classic modeller’s dilemma- it’s not just WHAT you are trying to model, but also WHEN. Very often a photo of something we work from is just a point in time, and especially with something highly competitive and in constant use, like a racing car or fighter, enhancements, changes, damage,  repairs etc happen all the time. 

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