Helix diameter

[Crawfy]

G'day

Just wondering, what would roughly be the tightest practical diameter of a helix for reliable operation, suitable for large modern locos and rolling stock in HO scale with a grade of roughly 2.5%? I've heard that the tighter you go, the more dramas your more likely to have with derailments. Is there a rule of thumb or something?

Thanks

 

[Rico]

Well the smaller the diameter the higher the grade of course.
Also depending on what you need for clearances, modern trains will need more than old timers do.
Another factor is what material you use to build it, the thinner the material the less you'll have to duck under to make the turn.
My double track helix is roughly 2.5% and is 25.5" and 28" radius, making it a 56" diameter. My clearance is about 3.5" from railhead to plywood to get doublestacks through.
You could go a little tighter and still have the 2.5% if you don't need the clearance.
If you search the forums for Helix you should find more info.
Clear as mud?

 

[Crawfy]

Thanks Rico, you pointed something obvious out to me that I forgot completely, and that is that the grade would increase as the radius shrinks. This is why I didn't take advanced math in final years of high school.

Hmm, I was aiming to keep the clearance at 100mm (3.9") for the purpose of double stack well cars, but if you're successfully running them with in 3.5" that could help me keep the size of the thing down.

Anyway, my current plan is only single level, so I don't have a need for a helix yet, but was just thinking what I could do later.

Cheers.