Off-centre bogie pivot?

[Bisto]

I'm re-wheeling a Hornby Dublo breakdown crane and I want to do it by using two 4 wheel bogies inside the frames. I have a problem with one of the bogies in that the crane cab pivot is very close to the centre point between the axles where the bogie pivot would normally be.  Is there any reason why I should not position the bogie pivot a few mm off-centre? Or would a configuration of 2 fixed axles  and one pivoting bogie work better?

[The Johnster]

With the bogie pivot off-centre, the vehicle will not be aligned with the track, which will not only look odd but also potentially lead to side clearance issues as it will be out of gauge.  Then on top of that, it will lean to the side opposite the off- centre pivot.  Sorry to come off a bit negative, but this is really a non-starter!

Edited October 9, 2021 by The Johnster

[Bisto]

 

Thanks!  I had my doubts, which is why I asked. I think two fixed axles in the middle with a pivoting pony truck at each end would definitely work, but would two fixed axles at one end and a pivoting bogie at the other end also work? - essentially the same arrangement as a 4-4-0 locomotive.

[proton]

Surely the off-centre suggestion means in the longitudinal direction (along the track) and not across the track?

[Nearholmer]

Assuming you mean off-centre along the axis of the track, the issue will be uneven loading of the axles, with that furthest from the pivot-point less-loaded, which can lead to a tendency to derail - the further the pivot from the bogie centre point, the greater the affect.

 

Now, this may not matter, the unloading maybe small, but if not, you will need to put a rubbing plate above the bogie to transfer load from the crane body to the less-loaded axle, which is tricky to do without interfering with the rotation of the bogie about the pivot.

[Bisto]

1 hour ago, proton said:

Surely the off-centre suggestion means in the longitudinal direction (along the track) and not across the track?

Yes, I'm sorry I didn't make that clear.

[Il Grifone]

I have some American 12 wheel tenders with off centre* bogie (or is that 'truck'?) pivots. This seems to present no problems, apart from setting them up for three point suspension. (One (Mantua) is very wobbly and requires this.)

 

*If they weren't off-centre, there would be access problems for the pivot/securing screws.

Edited October 10, 2021 by Il Grifone

[proton]

The Dapol Western has the bogie pivot set between the inner-most and centre axles.  Unfortunately under load it has the result of causing the outermost wheels to lift slightly, which in P4 results in derailments.  Not a concern here though, but attention to freedom to rotate in all axes may be beneficial.

[The Johnster]

Sorry, Bisto, I took your meaning to be positioning the bogie pivot off centre transversely across the vehicle from the longitudinal centre line, but apparently you were talking about moving it along the longitinal centre line to clear the crane slewing (rotating) pivot.  This put me in mind of my 1950s Rovex Triang Black Princess, which, like many locos with leading bogies of that era, had a loose swinging arm link which pivoted at both ends, the rear being pivoted from a point on the front of the chassis block, and the front acted as the bogie pivot, mounted of course at the centre point of the top of the bogie frame.  It was a simple and less than ideal arrangement, but with the steamroller wheels and deep overscale flanges of the day, most of  the bogies stayed mostly on the track most of the time...

 

Something like this might work for your breakdown crane, the swinging arm link being attached to the underside of the chassis a little way along the centre longitudinal axis, bent to clear the crane slewing pivot, with the bogie attached to the other end at it's central point/directly above it's centre of gravity.  The bogie will not take any of the vehicles actual load, and will be a free running 'dummy' in that sense; the load will be transferred to the other bogies and/or wheelsets.  How this will work in practice is moot, and predicated on a 'try it and see' basis, but for a vehicle that will spend much of it's time as a siding lurker or being hauled at low speeds, but if you try running at the scale 60mph that real breakdown cranes were allowed to run at around sharp curvature you may well run into trouble!

 

The track-holding performance of hauled or propelled stock on curved track, especially turnouts where there is a bit of sideplay available to the flanges at the crossing vee (frog) that has to be restrained by the check rails, is affected by weight distribution, and the original designers of the model will (or should!) have taken this into account, especially with a heavy die cast multi-wheeled item like a Hornby Dublo breakdown crane; you mess with this at your peril!  But so long as the load is fairly evenly distributed along the longitudinal centre line of the vehicle on to the other wheels and bogies, you should be ok...

[Penlan]

I have some old (Red Rose Kits) chassis etches for LNWR 42' Radial coaches and at each end of the coach the 'fixed' and outer radial axles are in fact on an inside bearing bogie with the pivot bearing point off-set to wards the outer (buffer) end - probably 3 - 4mm
This basically reduces the throw of the outer (Radial) axle and thus less end throw over the buffers.  I've never had a problem with the inner bogie axle not having enough weight transferred to it to keep it on the rails.  

[Bisto]

Thanks to all for your very helpful replies. I decided to offset the pivot point by 2mm and it seems to have worked.

[DCB]

Some Hornby Dublo coach bogies have  the pivot offset a few mm towards the centre of the vehicles.  Not sure why. probably to equalise the end throw and centre throw on 1st Radius curves.

I have offset the bogie pivots to the left or right a few tenths of a mm or so on various coaches and it looks absolutely awful.

I think the breakdown crane needs careful thought as a bogie version might be unstable.  Offsetting the pivots towards the ends so the end wheels take most of the weight might help stability.   Two inner axles fixed and the outer two in pony trucks sounds horribly unstable. Mine have been left standard but seem to spend a lot of time falling on their sides.  A lot of cranes seemed to have three fixed axles and a bogie, I have neve seen one with two fixed axles and a bogie.

[Il Grifone]

I would be tempted to have a rigid frame with play on the two centre axles, but this would depend on the minimum radius it has to traverse.

I have noticed quite a quantity of models with bogie pivots off centre. Before I hadn't paid much attention.

[DCB]

I cut the handles off my H/D crane and replaced them with spur gears from a clock.   This allows the crane to run without demolishing lineside structures and still allows the play value of having working jib lift and ability to lift on the hook.  

[IanG]

I’ve just come across this article and also plan to re wheel my crane with RP25 wheels is it possible to use the existing HD crane chassis but to use flanged wheels on the innermost axles somebody must be running a HD breakdown crane on modern fine scale track? What method are you using ?

[John Isherwood]

10 minutes ago, IanG said:

I’ve just come across this article and also plan to re wheel my crane with RP25 wheels is it possible to use the existing HD crane chassis but to use flanged wheels on the innermost axles somebody must be running a HD breakdown crane on modern fine scale track? What method are you using ?

 

Simply replaced the HD wheels with Markits wheels, and limited sideplay of the outer axles with washers.

 

CJI.

[IanG]

What about the inner wheels CJI as these move up in the slots as well I assume you would need to limit side play in these as well?

[John Isherwood]

1 hour ago, IanG said:

What about the inner wheels CJI as these move up in the slots as well I assume you would need to limit side play in these as well?

 

Why?

 

HD made them able to move vertically to accommodate uneven track, and to move laterally to deal with sharp radii.

 

What worked then works now, but with flanges on the inner wheels, lateral movement is essential.

 

Mine has the advised arrangement and works fine - but is a swine to put on the rails; once railed, it remains on the layout!

 

CJI.

[IanG]

Thanks for the reply as you say what works then would work now I was just unsure as to it working with flanged wheels that was all I will have a look and see what wheel sets I have going spare I’m pretty sure I have some Dapol ones 

[DCB]

The H/D crane gets round 15" radius curves with its non flanged centre wheels.   I have 2 neither to hand at present but I'm pretty sure the wheel holder is rivetted to the cast chassis  so removing and more awkwardly refitting the wheels is not easy.  You can grip the axle force it hard left and force off the left hand wheel and change it and vice versa but I think changing to metal rim flanged wheels will require drilling out the rivets as will shortening the axles so they limit side play and stop the wheels touching and shorting on the outside frame and on the metal inside frame, you coule use washers but I wouldn't.  It's so much hassle that I have neve changed either of mine despite changing the handles for gears etc. 

Edited March 10 by DCB