Difference Between Rims

Tom,
I run dirt bike rims on all my road race bikes. They are fine if you choose the right size. Most dirt bike rims use 36 or 40 holes just like street rims. Dirt rims do come in a lot of 19 and 21" sizes which you wouldn't use on a street bike but their 18" rims are usually available up to WM3 in width...enough for most 500cc and below street bikes.
I run Excel 18" 36 hole WM2 rims on my race bikes....unflanged...the flange they speak of is on the inside (spoke) side of the rim...like original Akront's, British rims etc. Has nothing really to do with how the tire is mounted or retained...the flange is no where close to the bead of the tire. Flanges were a design aspect of alloy wheels back when materials and design made them necessary for strength....the design has carried over mostly out of momentum and the fact that there are still some cheapo spanish and chinese companies turning out crap rims that need the flanges for strength.

One thing about flanges to remember...they are a tad heavier and fill up nicely with garbage. Big hp 350-500cc bikes...I'd still use them. 350cc and below...no need. A good Excel unflanged dirt bike rim will work great.
JohnnyB

Geets,
To be honest I've never seen such a flange. I've seen some dirt bike rims with a bead area designed in such a way that you can run screws through it into the tire bead, I've seen the internal bead locks. I'd guess than any flange that extends outward around the bead would make mounting the tire next to impossible. I've heard of what you are talking about but never seen one.

About the fewest spokes I've ever seen on a wheel is 32, seems like there have been a few odd purpose wheels with 28. What can be misleading just looking at various spoke wheels is the different spoking patterns used. Typical of most Japanese bikes is the "cross 2" pattern, some Euro dirt bikes and Brit street bikes used "cross 4" pattern, and I'm sure there are others I haven't seen.

You know...now that I think about it...seems like I remember when bikes makers started touting "safety rims". Look at a mid 60's steel Honda rim...often very little height on the bead flange. I think in the later 60's or early 70's they started making rims with a taller bead flange, and they called these "safety rims" because the tire was less likely to come off the rim in a flat. I bet that's what you thinking of. To anyone under 50 years old the "safety" rims would probably be all they have ever seen.
JohnnyB