eeuuugh

Rivendell used to sell, maybe still does, a rim made by Velocity with the box section reinforcements in the corners only, called the Twin Hollow. FWIW I think that's a snappy enough name to be a general term.

ThermionicScott 

Here's a post from T-Mar that describes it well. It wasn't like a tubular or single-tube tire at all.

Feldman

Having built wheels as part of my job for 45 years, I will be rude and tactless about this and say that old single wall rims are terrible product and should be discarded if at all possible. Rims are a vastly improved product over the years, even something inexpensive like today's Weinmann or Sun rims will build more easily, hold a tire beyter, and make a stronger wheel than any 1970's clincher rims. Tubular rims haven't changed as much--what's to change?--but cluncher rims have. There are even quality 27" units still in production. Want to simulate vintage? Build a pair of Sun M13's and peel the labels off--polished silver for a bargain price and better than anything made in 1975!

T-Mar

Based on what I read somewhere, a long time ago, I call them channel section rims, as the small enclosed section in the corners look like channels. . It's may not be the best description but in my mind it provides distinction from single wall and box section.

This style rim looked to combine the advantages of both the single wall and box section rim. The small channels increased the rigidity and allowed for a simpler pinned joint, though at a weight penalty over box section. The one big advantage they retained was easier tyre mounting, which is considerable when you are touring in a remote region with only basic tools. Channel section rims have the deep trough between the channels, that the tyre bead can drop into, making it much easier to easier to get fit last section of bead over the sidewall.

I remember when Specialized introduced their Expedition grand touring model. For the first year, maybe two, they spec'd spec'd it with 700C, box section Mavic Module 3 rims. Even though these were the heavier/stronger Module rim, they were dropped relatively quickly in favour of the Super Champion 58. Shop talk was that they weren't strong enough for a fully loaded touring bicycle.

Well, at least I've made one convert after all these years of preaching!

True clinchers may actually still be in use some parts of the world. They retained their popularity in Asia into at least the mid-1980s, as Araya was still cataloguing them at that time. The Asians preferred to called them beaded edge tyres and I imagine that some Asian tyre manufacturers continued to offer replacement tyres for quite a while after that. It wouldn't surprise if there are still a couple of small Asian companies still catering to this market segment with new rims and tyres.

branko_76 

I built a pair of wheels with SUN M13, I also built a couple of pairs with RIGIDA 13/20 from the early 1980's (maybe earlier). I don't recall having any issues building any of them and they are all still true. What is it about the SUN rims that makes them better than older rims?

steve sumner

originally in the 70s box section wired on (or clincher) rims were a response to the
then new x-narrow tires. borrowing a design from sewup rims it was how to make a rim
this narrow and light strong enough. box section rims weren't made in wider touring sizes
MTB sizes don't count. I far as I know there were no 700c/27" wide box rims ever.

Salamandrine 

There is another advantage of older style channel section rims (). I've noticed that they seem to be more resistant to fatigue cracking around the spoke holes. I haven't done any scientific study about this, but it's been my observation that box section rims often crack and fail around the spoke holes. FWIW I've never seen this happen to for example a mod 58. Not saying it never happens, just that I've never seen it. It seems that this type of twin tunnel channel section layout tends to have thicker material in the middle, along with less span across the width. IOW there is one small thick wall instead of two wide thin ones.

A bit of googling informs me that it is fairly common for people to misidentify this type of rim as single wall. It's still wrong. Single wall rims don't have any hollows at all.

steve sumner

I chose Super Champion mod.58s to use on my tandem bcux their strength
is legendary but keep in mind that ultimately the strength of a wheel lies
with the builder. and only the builder knows how well the rim is made.