coach building

[darrene]

One of the many tasks I have on the list at the moment is to start making a chassis for some 5’ fibreglass coaches.

Each coach will have a maximum load of 2 adults or 3 children. The base of the shell leaves me with a 20mmx20mm 'perfect' size for the beams.

Rather than use steel angle do you think I could use 3mm thick aluminium C section on its side? I could then use stringers to brace the middle to gain some torsional rigidity and hopefully some strength to mitigate against drooping when sat-upon.

The aluminium will make the final coach a bit more manageable and would also be easier to construct as I don’t have any welding skills. I’m just wary it’s likely to flex more easily than the steel.

Darren

[92220]

Hi Darren.

I think you would definitely have a problem with the aluminium C-section bending. I'm pretty sure a 5' length, even on it's side, would flex far too much, as you thought. You would need a much greater vertical height to limit the flexing to a realistic amount.
The extra weight of a steel C-section should not make it so heavy that it couldn't be lifted easily by 2 people. You would need 2 people anyway, whether ally or steel is used, due to the length and cross sectional area, and under-frame detail, making it very awkward to carry single handed.

Bob.

[uuu]

How stiff is the fibreglass body? You might find it takes most of the load. Or you could reinforce it to make sure that it does.

Wilf

[kipford]

On a rough calc you are going to get between around 0.7 - 1.0 mm deflection with steel channel section 20 x 20 x 3 assuming two adults at 90 kg and 1200 mm between the bogie pivots. With Aluminium you can treble that because Al is only a third as stiff as steel. However if you follow the the prototype and use a proper underframe (which also looks correct) you will be able to use Al without any problem.
Dave

[darrene]

Oct 7, 2021 12:10:46 GMT 92220 said:

Hi Darren.

I think you would definitely have a problem with the aluminium C-section bending. I'm pretty sure a 5' length, even on it's side, would flex far too much, as you thought. You would need a much greater vertical height to limit the flexing to a realistic amount.
The extra weight of a steel C-section should not make it so heavy that it couldn't be lifted easily by 2 people. You would need 2 people anyway, whether ally or steel is used, due to the length and cross sectional area, and under-frame detail, making it very awkward to carry single handed.

Bob.

Hi Bob,

Thanks, that's a very good point which I hadn't considered. In the overall picture,  the chassis is only contributing a proportion to it and given the size of the thing, in it's finished form it's not going to be very easily transportable by one person in any case.

Darren

[darrene]

Oct 7, 2021 12:44:29 GMT uuu said:

How stiff is the fibreglass body? You might find it takes most of the load. Or you could reinforce it to make sure that it does.

Wilf

Thanks Wilf. It's very stiff as-is - no noticeable movement when carrying it around on my own. I will measure the fibreglass thickness but I'd hazard a guess it's about 2-3mm based on this picture.

Darren

[darrene]

Oct 7, 2021 13:20:54 GMT kipford said:

On a rough calc you are going to get between around 0.7 - 1.0 mm deflection with steel channel section 20 x 20 x 3 assuming two adults at 90 kg and 1200 mm between the bogie pivots. With Aluminium you can treble that because Al is only a third as stiff as steel. However if you follow the the prototype and use a proper underframe (which also looks correct) you will be able to use Al without any problem.
Dave

Thanks for crunching the numbers Dave. I was going to try and work out a calculation by researching it this event. I really appreciate you pointing me in the right direction.

I have just trialled something I'd initially ruled out on the ground of aesthetics - I have some 2020 aluminium profile which is really handy for mounting and jointing and which given the profile is inheritanly rigid. Now that I've mocked that up I don't think it would look too out of character once everything is painted..

If I were perhaps to use three spreaders - one a few inches in at each end and one in the middle - I could mount some vertical posts off them,with a beam across the top which will take the direct weight on the roof? This should mitigate the risk of the fibreglass flexing and cracking when sat upon.

The end spreaders could also be where the bogie platforms are. A thin aluminium sheet for the buffer beams would suit the ends and the buffers can mount through this onto T nuts in the profile?

As you suggest Dave, some triangulated bracing underneath will add to both the ridigity and a more prototypical look.

Darren

[kipford]

That body will take the loads without a problem. So it negates any issue with the chassis frame stiffness, it just needs a couple of transverse members to prevent the sides spreading. Under frame trusses will still be needed to make it look visually acceptable. Are you proving some padding for the passengers, that roof is mighty hard!
Dave

[darrene]

Thanks once again Dave

I did think that even with sprung bogies (intention at the moment is to run a set of Aristocraft ones), the ride is going to be rather...agricultural.

My next thought, I'm ashamed to say was 'well it is a passenger problem' which I'll admit was rather uncharitable

I think a decent solution may be a thin padded cushion, in a roof-colour. A friend runs an upholsterers next door to us, which could be just the ticket

[darrene]

A productive few minutes were spent scaling-up a few buffer designs which I had. These were then 3D-printed (in the only colour filament I had to hand, hence the fact they're green!) and used to mock up how they might look.

I've also scaled-up a drawbar coupling hook and lengthened the shank with a view to mounting it through the end bars making up the bufferbeam.

The aluminium profile isn't available in anything other than anodised silver at these lengths, so I've prepped those parts with panel wipe, primered and sprayed them satin black.

Sitting the chassis/body on some (rather coarse-scale) bogies, to see how it looks.

The next steps are to

1) wash, primer, mask and and paint the body

2) Take some interior photos shot through the windows of the real thing, scale appropriately and get them adhesive-vinyl printed so they can then be  stuck into the window apertures. I've seen this done to good effect on another coach and it creates a passable interior view using the 6'-rule.

3) Add some exterior details and somehow fabricate some underbody parts, starting with a frame

4) Add three 3mm aluminium L-brackets to each side - these can be easily attached and removed to T-nuts in the chassis side and will allow provision of a footboard,

Darren

[steamer5]

Hi Darren,
Coming on nicely!
A suggestion for you, make the foot boards so they fold up for traveling in car / trailer or what ever....save you a heap of room...ie 4" a side.

Cheers Kerrin

[darrene]

Thanks Kerrin!

That’s a great idea and one I hadn’t considered, so thank you

Thinking aloud, I could hinge them, or the 2020 profile accepts 5mm studs which I could use to mount the footboard L brackets to. If I drilled these to accept some P clips, it’d allow for a quick mount/dismount.

Hinging the boards would be a neater approach for storage. I’ll give it some thought..

[britannia]

With similar coaches, I built an internal wooden frame with a long wooden spine across the top.  The frame 'fits' the boy internally, and sits inside the bogie frame.  This allows the body to sit on the frame without any strengthening showing outside the body.  10 years later it still works well.  I did triple varnish the wooden frame to ensure it is preserved when sitting outside.

[darrene]

Thanks for this. It's just what I had originally imagined as being a good approach for it, so it's great to hear the design is proven.

I tentatively sat on the shell, sat on the outline chassis and bogies. It took the weight okay but I think an internal spine is the way forwards; it'll reduce the risk of the unsupported roof flexing, especially over any bumps or with additional passengers.

This is the current design. The dark grey plates are the bogie mounting points. The spine is supported at 3 points and could either be a wooden beam or an aluminium one. At the moment, the fit is so good that the fibreglass shell can be 'clipped' on and off of the frame without the need for fixings.



Paint has arrived so the shell needs washing and primering next. In the meantime I am printing of some corridor end details and playing with more buffer designs. I also have a contact designing some Mk1 battery boxes and voltage regulators to go underneath. Things are slowly coming together

Darren

[92220]

Hi Darren.

You say "wash" the shell. If you are thinking of using washing up liquid to clean the surface DON'T. Washing up liquid is difficult to fully wash off and paint does not like adhering to any surface that has been "cleaned" this way. Always use a solvent to degrease....and the stronger the better, depending on the material being degreased.

When I ran the paint business, 99% of all adhesion problems that customers came to us with, came down to the use of washing up liquid to 'degrease' before painting. If you have seen the Fairy Washing Up Liquid adverts on TV, they say that there is a 'sparkle' left on the surface, after using it. Yes there is, and that is a film that is left after it dries, and all washing up liquids leave this coating to greater of lesser degrees. That 'sparkling' coating is what paint doesn't like adhering to.

Bob.

[Gary L]

Oct 15, 2021 8:16:25 GMT 92220 said:

Hi Darren.

You say "wash" the shell. If you are thinking of using washing up liquid to clean the surface DON'T. Washing up liquid is difficult to fully wash off and paint does not like adhering to any surface that has been "cleaned" this way. Always use a solvent to degrease....and the stronger the better, depending on the material being degreased.

When I ran the paint business, 99% of all adhesion problems that customers came to us with, came down to the use of washing up liquid to 'degrease' before painting. If you have seen the Fairy Washing Up Liquid adverts on TV, they say that there is a 'sparkle' left on the surface, after using it. Yes there is, and that is a film that is left after it dries, and all washing up liquids leave this coating to greater of lesser degrees. That 'sparkling' coating is what paint doesn't like adhering to.

Bob.

Yes, worse than washing-up liquid is the release agent that will have been used to mould your shell. Some people use silicone wax as a mould pre-coat before the release agent… nuff sed?

Also, International (marine) paints make a GRP primer that might be worth investigating; I don’t suppose they are the only ones, nor the cheapest. In the absence of a primer, a thorough matting with wet-and-dry is usually recommended.

Gary

[darrene]

Ooo. Thanks for that timely advice Bob!
Yes, I'd have used some water and washing up liquid. I'm very pleased you warned me before I did...

Normally I'd use a decent panel wipe for preparing metal parts. Would that usually be expected to be suitable for fibreglass?
I'll be sure to test it on a small part and leave it for 24h anyway, just to play it safe

Darren

[92220]

Hi Darren.

Panel wipes are used by paintshops for preparing car bodies for spraying. As car bodies can be either metal or fibreglass, I would expect panelwipes to be suitable for both, but, for safety, I would always still carry out a small test to check. I would also, still do a last wipe-over with a clean solvent dampened rag.

Bob.

[darrene]

I realised I hadn't posted pictures of past progress. Time for an update!

The support frames were constructed such that the wood touches the underside of the roof and the bogies mounted to 4mm aluminium plates



The sides and ends were sprayed and some window detail photos sourced. These were sized for the window apetures and printed on adhesive vinyl.





The next task will be to fit door handles, battery boxes and underframe details, plus the identification and numbering decals!

[Deleted]

Nice work Darren...they look great..🙂

Pete