Is 20.5 tubular rim too narrow for a 33mm tyre?

[Zoro]

The tyre makers say 20.5 rim width is OK for a 33mm wide tyre. I'm a good glue guy, but how is the performance? Is it too squishy?

This would not be for a mud tyre, more a faster tread on dry/grass/sand/dirt.

I like it because it is a lot ~250g or so lighter than the wider options.

[jeroenfl]

I still use the Ambrosio Nemesis rim for cx tyres, even for disc wheels, and those are about 21mm. That's with dugast pipistrello 33 and challenge chicane 33.

Love those classics.

Also have carbon 40mm high/25mm wide rims for sandy courses with those pipistrellos.

[Zoro]

I still use the Ambrosio Nemesis rim for cx tyres, even for disc wheels, and those are about 21mm. That's with dugast pipistrello 33 and challenge chicane 33.

Love those classics.

Also have carbon 40mm high/25mm wide rims for sandy courses with those pipistrellos.

So you are saying...33 on 20.5mm are OK?

I also have other 25mm options (4 tubular sets now, this will be 5th). Just that I want an uber light set.

These will be silk 33s and <800g wheels (the ax Ultra - I own the road Ultra already and they are very strong wheels). So this is both a weightweenie project, but also something to really use.

The rider is a 70kg strong kid, so if it is stupid - I will save my money.

[jcrr]

I would say 20.5mm is totally fine for a 33mm tire- just think back, oh, 5-7 years ago, and all the pros were probably riding and winning on rims of similar size...

I would be more concerned with how fragile a <800g wheelset may be under a 70kg rider, presumably at race efforts. AX wheels look beautiful, and if you think they can take the abuse,

[Calnago]

The issue with mounting a big tubular on a narrow rim is that the radius of the rim bed does not match up very well with the larger radius of a bigger tire. So, once inflated, the center of the tire will tend to want to pull away from the center of the rim. Creates a good pressure on the edges (most important) but not so great in the middle. Hence what people used to call the “Belgian” tape method, whereas the center of a narrow rim gets built up a bit with tape and glue to help form a rim bed radius that more closely matches the bigger tire. When you pull the wider tire off your Nemesis rims, can you not sort of see this effect a bit, where the center doesn’t seem to have as good a bond as the edges? I run 25’s on my Nemesis time and have noticed that upon removal but it’s not so much that I bother to do anything differently to compensate for it. And that’s for road with higher pressures. It may be that for cross and lower pressure that your tires base tape stays pretty well glued to the rim despite the mismatch in rim and tire raidus. Hope that makes sense. But you could try it I suppose and see how it works. Like was just said, it’s not like it hasn’t been done before, but there’s better rim/tire matches available today that kind of make it unnecessary.

[Kurets]

I race an extremely old school setup with Mavic Reflex (now Open Pro T) rims, 32 spoke. While the manufacturer claims 22mm max tyre width I've had no issues with properly glued tyres. Ripping some old tyres of the same rims (bought a used set) I could see that the previous glue job had just the problem Calnago describes: No adhesion in the centre.
What I have done to avoid that problem is to aggressively press the basetape down into the channel when gluing and then leaving the tyre only barely inflated overnight (just enough so that you can strap down the valve). I have only had to remove one tyre of my gluing so far and there I could see adhesion between centre of the channel and the base tape. But, that was with a tyre I mounted backwards and had therefore never ridden so who knows how it would have looked after a race or two. Still, no rolled tyres so far even without the "Belgian" method.

[Zoro]

I would say 20.5mm is totally fine for a 33mm tire- just think back, oh, 5-7 years ago, and all the pros were probably riding and winning on rims of similar size...

I would be more concerned with how fragile a <800g wheelset may be under a 70kg rider, presumably at race efforts. AX wheels look beautiful, and if you think they can take the abuse,

I do. I have a few of theirs, and several of the Extralite hubs. Their stuff is strong and stiff.
But...that root/rock could break a lot.

[Zoro]

I race an extremely old school setup with Mavic Reflex (now Open Pro T) rims, 32 spoke. While the manufacturer claims 22mm max tyre width I've had no issues with properly glued tyres. Ripping some old tyres of the same rims (bought a used set) I could see that the previous glue job had just the problem Calnago describes: No adhesion in the centre.
What I have done to avoid that problem is to aggressively press the basetape down into the channel when gluing and then leaving the tyre only barely inflated overnight (just enough so that you can strap down the valve). I have only had to remove one tyre of my gluing so far and there I could see adhesion between centre of the channel and the base tape. But, that was with a tyre I mounted backwards and had therefore never ridden so who knows how it would have looked after a race or two. Still, no rolled tyres so far even without the "Belgian" method.

Thanks. I was more concerned about side to side swish...

I have a gluing method that makes me unconcerned about bonding in the centre.

[jcrr]

I would say 20.5mm is totally fine for a 33mm tire- just think back, oh, 5-7 years ago, and all the pros were probably riding and winning on rims of similar size...

I would be more concerned with how fragile a <800g wheelset may be under a 70kg rider, presumably at race efforts. AX wheels look beautiful, and if you think they can take the abuse,

I do. I have a few of theirs, and several of the Extralite hubs. Their stuff is strong and stiff.
But...that root/rock could break a lot.

Do you use extralite hubs for racing cross? if so, can you share your thoughts on maintenance and longevity of extralite hubs when racing in really muddy conditions?

JC

[Zoro]

I would say 20.5mm is totally fine for a 33mm tire- just think back, oh, 5-7 years ago, and all the pros were probably riding and winning on rims of similar size...

I would be more concerned with how fragile a <800g wheelset may be under a 70kg rider, presumably at race efforts. AX wheels look beautiful, and if you think they can take the abuse,

I do. I have a few of theirs, and several of the Extralite hubs. Their stuff is strong and stiff.
But...that root/rock could break a lot.

Do you use extralite hubs for racing cross? if so, can you share your thoughts on maintenance and longevity of extralite hubs when racing in really muddy conditions?

JC

No, but about to order. To get the pronouns right, I used to race, don't now. My son does and these are for him. I like buying and playing with the stuff, and seeing it used. So "I" means I bought and own it. The rider is normally my kid 63-70kg.

I have a 3 year old Extralite hubs that did quite well on the Belgium Pave. The rims were destroyed, and rebuilt on same hubs. I've replaced the ceramic bearings twice.

I then bought the 780g ax set for hill climbing - with hubs. Still perfect.
I bought their stem - also used on Pave, 3 years old - great.
I bought their seat post.

Then my buddy owns a wheel company. He does about 50 a year on Extralite. So far - 0 issues.

My thoughts are the seals are a bit light. They will let in more mud, grease and dust. It has turned out pretty easy to replace the bearings and off-brand ceramic with labor 120EUR/year. So not daily rider hubs.

My only hesitation is rim width of the ax @ 20.5 vs the 33mm silks I'm putting on. As I'm an old guy, if it does not work so well, I'll hang them on the wall.