what is the big-est turn in ho

Back when I was modeling in HO, the largest radius curve in HO sectional track was 24" radius, typically larger than that was made using flex track, bent to the desired curvature. Bachmann, offers many curves larger than that now, in their EZ-track line, up to 35.5" radius.

Doug

My HO model railroad club uses a minimum radius of 40 inches.  This would be equivalent to O-80 in 3-rail.  The club strives to use larger curves whenever possible.  Sixty inch radius curves are common even in yards.  

The minimum switch is a number 6.  We try to use number 8 switches whenever possible.

All the club's HO track is either hand laid to fit a particular space or is flex track.   Two rail code 83 or code 100 HO flex track is much easier to bend to a given curve than most O gauge 3-rail track.  It is also easier to cut to length.  

I think that you should always strive to use the largest radius curves that your space allows when building a model railroad not matter what the gauge or scale.  

Joe 

how big is 18 radius in o scail

18 inch radius is O-36 in 3-rail.   It is a very sharp curve even in HO.   Only small HO switchers and 40 foot box cars operate well on 18 inch radius.

Joe

Unless I've been doing math all wrong these 40 plus plus years. 18" radius in HO would be 0-72 in O

You haven't been doing math wrong, you've just been using the wrong formula and a (common) wrong assumption.  You can't be blamed for the last part, since even a famous on-line editor gets it wrong when he claims that "H0 is just one half the size of 0."

The ratio between 0 and H0 is not 2:1.  0 is 1/48 and H0 is 1/87.5, so the ratio is about 1.8 to 1.  Put another way, H0 equipment is slightly larger than one half of 0 scale equipment.

I understand what you're trying to say...that the EQUIVALENT curve, taking into consideration the differences in scales, should be twice as big (actually, 1.8 times as big.)

So you are almost correct...A curve that is 18" in radius (36" in diameter) in H0 would be about 64" in diameter in 0 scale ("0-64" in curved sectional track.)

Historically, 0 ("zero") scale was a European size that was about 1/43 to 1/45 the size of real trains. Half-zero (H0) is indeed half of that size, at about 1/87.5 the size of the real thing. The science gets muddy when you consider that at some point, when model makers wandered away from metric towards English measurements, the scale (ratio) of 1/48 was an easier choice, since 4 feet to the inch is easy to visualize and to lay out on plans.

probably can be blamed for drinking the wrong beer too.  Our O gauge trains are not true 1/48th either. so Half O is close enough. But Arthur is correct

"The wrong beer" ??? What a concept !!!  That's like breathing "the wrong oxygen."

Well it must be remembered that Maerklin was really defining the track gauge, not the scale, when they established the numeric gauges, later including 0 at 32 mm.  When HO came about commercially in Germany it was 16.5mm, and the rolling stock was non-scale tinplate.  In both 0 and H0 the proper scales were then defined by the width of the track, the US using 1/4" for convenience instead of 1:45.   The 43.5 for 0 only came about much later when Francophones arbitrarily declared that if HO is 1/87, 0 should be twice that !

The fact that 32 mm and 1.25" are almost the same for track width was rather fortunate.

SZ