What's an adequate height for an over \ under pass in OO scale?

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centenary
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What's an adequate height for an over \ under pass in OO scale?

Postby centenary » Sat Mar 06, 2021 1:33 pm

Im still toying with layout ideas. I like Topley Dale but it has some limitations so Im looking at a 'folded 8'(?) type layout in OO scale. This would require a higher pair of lines to cross one over a lower set in a couple of places.

What's the optimum distance from top of the lower set of rails to top of the higher, crossing rails? (Or is that not the way toapproach it?)

Instinct tells me it's the height of your tallest rolling stock plus a bit but what's people's thought because this will also impact the gradient for the cross over point?

I can probably get a maximum 4 metre straight for the incline with a curve at each end. If I could improve my woodworking skills, I could also try and work in one set of lines rising with the underneath lines on a slight decline and theoretically, ease the gradient.

Im not into magnets, traction tyre locos etc, just bog standard DCC operated locos hauling a max 6 carriages.

Thoughts?

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Re: What's an adequate height for an over \ under pass in OO scale?

Postby flying scotsman123 » Sat Mar 06, 2021 1:40 pm

centenary wrote:What's the optimum distance from top of the lower set of rails to top of the higher, crossing rails? (Or is that not the way toapproach it?)


Not quite, you really want distance from top of the lower set of rails to the bottom of the bridge deck of the upper line, as the thickness of the bridge deck can vary depending on how you make it, and as you say;

centenary wrote:it's the height of your tallest rolling stock plus a bit


Not much more to it than that, I suspect you'll probably want the clearance to be relatively tight to keep the gradient as shallow as possible, better to have the gap slightly less than prototypical than end up with a gradient too steep to run trains.
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Re: What's an adequate height for an over \ under pass in OO scale?

Postby Flashbang » Sat Mar 06, 2021 5:51 pm

Hi

All measurements should be taken from the lower level rail tops, as that sets the datum needed. Overhead clearance is never taken from baseboard as track height varies depending on what if anything is under the sleepers of the lower track!

In OO the minimum height lower rail top to anything passing overhead is recommended to be 60mm. But that doesn't allow any access for hands etc to remove derailed stock or clean track etc. So I would recommend as an absolute minimum 100mm rail top of lower track to the underside of anything above, higher the better! :D
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centenary
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Re: What's an adequate height for an over \ under pass in OO scale?

Postby centenary » Sat Mar 06, 2021 9:10 pm

Thanks both.

Bigmet
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Re: What's an adequate height for an over \ under pass in OO scale?

Postby Bigmet » Sun Mar 07, 2021 2:34 pm

centenary wrote:...What's the optimum distance from top of the lower set of rails to top of the higher, crossing rails? (Or is that not the way to approach it?)

Instinct tells me it's the height of your tallest rolling stock plus a bit but what's people's thought because this will also impact the gradient for the cross over point?...


There are two question topics here, first about clearance above the lower track, the second concerning gradient and what can be done to minimise it; and you have had good answers about the clearance above the lower track.

Gradient reduction. The plan to 'share out' the gradient, by having one line fall while the other rises on the approach to the crossing point is a good one, and needs to be designed into the layout plan from the start. It does require more design and construction work, no getting around it! If the four metre distance to the crossing point you mention is available on both routes, then allowing for all the transitions from level to gradient and back to level, then something in the range 1 in 60 to 1 in 80 applied to both lines should deliver: the steeper gradient for 100mm clearance, the easier gradient for 75mm. Testing your locos and rolling stock on 'bench test' gradients is worthwhile to assess if this will deliver. I would suggest that the simple criterion is 'will the loco start with the whole train on the test gradient?'. (If it will start then it will keep going when running.)

To reduce the vertical distance between the railtops of the upper and lower tracks - which is what determines the necessary gradient -the overbridge deck can be constructed in thin yet stiff material, (I have used 1mm thick aluminium sheet for this purpose) if the crossing is short. Slim support posts between the lower tracks, or bracing on top, are possible if the span is long enough that there is detectable sagging with your heaviest trains on the bridge deck: another aspect to test in advance of launching the construction project.

ChrisGreaves
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Re: What's an adequate height for an over \ under pass in OO scale?

Postby ChrisGreaves » Sun Mar 07, 2021 2:46 pm

Bigmet wrote:... 'will the loco start with the whole train on the test gradient?'. (If it will start then it will keep going when running.)
My physics teacher Mr Puzey would be delighted at this. Rolling and Static friction accounted for! :D
Cheers
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Re: What's an adequate height for an over \ under pass in OO scale?

Postby Bigmet » Sun Mar 07, 2021 5:19 pm

ChrisGreaves wrote:
Bigmet wrote:... 'will the loco start with the whole train on the test gradient?'. (If it will start then it will keep going when running.)
My physics teacher Mr Puzey would be delighted at this. Rolling and Static friction accounted for! ...

No prize for guessing in which of the O level courses I sat model railway items were used to demonstrate many of the concepts of mechanics.

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Stainsacre
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Re: What's an adequate height for an over \ under pass in OO scale?

Postby Stainsacre » Mon Mar 08, 2021 9:46 am

Figure 8 folded layout. Therefore finite length of track. Height set in stone...

Length equals one, height equals one. It is irrelevant if the track goes down whilst the other bit goes up, the average gradient is still the same.

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inoffapost
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Re: What's an adequate height for an over \ under pass in OO scale?

Postby inoffapost » Mon Apr 19, 2021 5:37 pm

My old Triang-Rovex Truss girder bridge had a deck height of 80mm so I kind of used that as the height basis going forward for my elevated sections. That said, as I added additional infrastructure I found the 80mm looked a little 'cramped' in some areas so on certain sections I've increased it to 95 rising to 100mm where the elevated section exits the layout to an elevated fiddle yard section.

I didn't do the math beforehand but my incline up to the 80mm measures 1152mm. I would have gone longer but was compromised through available space. It was very much trial and error with a great deal of the latter. I think next time I would come on here and ask someone cleverer than me for their advice and recommendations. But what I've got works fine for a first effort anyway :lol:

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RAF96
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Re: What's an adequate height for an over \ under pass in OO scale?

Postby RAF96 » Tue Apr 20, 2021 3:41 pm

This much is plenty.
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