Originally Posted by Springer240
Invisible, if you took the width x 3.14 you would need a girth of 6.28” to fill the condom. Think about a rubber band; when laid flat, it’s width is great, but when made into a circle, the rubber band is not as wide now.
But nearly an additional 5% of an inch in girth by wearing a condom. That’s awesome!
That’s a good point. I wasn’t visualizing it like that, but you are right. I didn’t bother plugging any numbers into the formula to check as you did (sadly, not the first stupid thing I’ve done today).
So, let’s think of a better model. A rubber band is a useful analogy so let’s work with that (it’s helpful if you actually go and get one to look at). We’ll begin by making a few definitions:
G = girth of the rubber band when it is in circular form as measured from the outside (i.e. including the thickness of the band itself)
T = thickness of the rubber band
Wi = width of the rubber band when flat and not including its thickness
Wo = width of the rubber band when flat and including its thickness
Imagine the rubber band laid flat (or do it for real). The relationship between all these variables should be G = 2*Wi + 2*pi*T. To understand why, there will be a Wi length on the top and one on the bottom of the band, accounting for the 2*Wi component. The only portion of the circumference remaining is the corners. These corners essentially form a half circle of radius T, so when added together we get 2*pi*T. This may only be an approximation but it seems like a good one to my naked human eye.
The relationship between Wi and Wo is given by Wo = Wi + 2T. This is because each corner will contribute T to the Wi length on each side. Rearranging this equation gives Wi = Wo – 2T. If we plug this into the expression above for G, we get G = 2Wo + 2(pi-2)T. To use this equation in the context of condoms, just note that Wo will be the width given by the manufacturer and T will be the thickness.
I actually checked this formula against several rubber bands and it seems to give good agreement. It’s a little tricky since rubber bands don’t always stay in perfect circles so there will be experimental error in this.
Now, let’s check this formula against actual Magnums. I’m sorry to say that’s a pleasure I’ve never had the need for (hopefully PE will take care of that). For the thickness of the condom the 2(pi-2)T component will be negligible so we can basically use G = 2Wo. Does 5” in head girth, 4.5” in shaft girth, and 4” in base girth sound reasonable? It seems a little tight to me, but the condom will expand so maybe this is right.
(Now excuse me while I go smack myself for being such a geek. :green: )