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-   -   A couple of my mechanic's observations on tubeless and carbon (https://www.bikeforums.net/classic-vintage/1165774-couple-my-mechanics-observations-tubeless-carbon.html)

squirtdad 02-05-19 01:21 PM

A couple of my mechanic's observations on tubeless and carbon
 
Not bashing, but sharing a couple of interesting (or so I thought) observations from my mechanic (don't use a lot, but buy parts from)

This guy as been wrenching for 20 years and works at a shop that builds frames, The sell Rivendell, Jones, etc. the guy works on everything and is not a carbon hater

On Road Tubeless he noted that he saw a tech article from USA cycling noting that there is increased rim cracking without tube usage, due to the pressure of the bead on the rim IIRC, he noted he has seen some rims this way

On carbon, for a while he and a partner sublet space to a guy who did carbon frame repair. The guy did a lot of top tube repairs, right where you rest you leg on the frame at a stop, apparently a lot of high end frames are not designed for this stress

noglider 02-05-19 02:29 PM

Interesting. I would not have predicted either.

nomadmax 02-05-19 02:41 PM

I'm fully aware that my aversion to carbon fiber isn't rooted in science; for me it's an irrational phobia that it will somehow collapse on me.

shoota 02-05-19 02:42 PM

Bull. Both things.

Spoonrobot 02-05-19 02:57 PM

USA Cycling releases tech articles?

Link please

Spaghetti Legs 02-05-19 03:02 PM


Originally Posted by squirtdad (Post 20780589)

On carbon, for a while he and a partner sublet space to a guy who did carbon frame repair. The guy did a lot of top tube repairs, right where you rest you leg on the frame at a stop, apparently a lot of high end frames are not designed for this stress

All the more reason to track stand.

crank_addict 02-05-19 03:05 PM

There's is some truth to the carbon 'road' 'clincher' tubeless wheel ~ high pressure. If one wants to be the experimenter, that's the price and risk. Wheel makers are in continuous evolution.

The problem does not reflect road tubular carbon, nor MTB carbon clincher, high volume low pressure.

KLiNCK 02-05-19 03:10 PM

Oh that nasty carbon fiber!


squirtdad 02-05-19 03:20 PM


Originally Posted by shoota (Post 20780724)
Bull. Both things.

Based on???? I am not saying 100% this is correct, it is info from my mechanic.who is pretty knowledegable. He did not give me technical citations or links But the frame is based on observation of hands on technical persib,

I can see both as possible, based on design issues and unexpected stresses.

shoota 02-05-19 03:24 PM


Originally Posted by squirtdad (Post 20780785)
Based on???? I am not saying 100% this is correct, it is info from my mechanic.who is pretty knowledegable. He did not give me technical citations or links But the frame is based on observation of hands on technical persib,

I can see both as possible, based on design issues and unexpected stresses.

Well to be fair, based on observation and the fact that I've never heard either of the "issues" before. You would think that if they were real problems that they would be talked about on the forums quite a bit. I've never seen anything like either issue mentioned. Carbon is waaaaay too strong for a limp leg resting on it to break it. That just sounds crazy.

CliffordK 02-05-19 03:28 PM

There is this thread about a failed Dura Ace rim.
https://www.bikeforums.net/bicycle-m...tal-epoxy.html

https://cimg3.ibsrv.net/gimg/bikefor...34f302eed.jpeg

Obviously one can't read too much into a single anecdote. And, the rim was apparently purchased used.

However, I do wonder if there is, in fact, a mechanism of risk to a rim with tubeless (and tubeless ready profile rims).

In general, riding shouldn't add a lot of extra stress. Low pressures, same outward pressure, etc.

Bead seating, however, could be a problem. Pump the wheel up to 120 PSI or so. More? Then "pop", hitting the rim with a high dynamic load.

And, of course, a push for lighter and lighter builds.

Perhaps also moving to larger tires. So, 28mm? Larger with MTBs? One thinks of pressure as a single measurement, but there is a tearing force on a balloon dependent on the size. So, the oversized tires would put more stress on the rim, especially if inflated to the same pressure, or inflated to a high pressure to seat the beads.

I can't say about the frames. Growing pains? My foray into Carbon Fiber has been with a vintage Carbon Fiber frame that I think uses homogeneous thickness, and isn't affected by that problem.

squirtdad 02-05-19 03:58 PM


Originally Posted by shoota (Post 20780794)
Well to be fair, based on observation and the fact that I've never heard either of the "issues" before. You would think that if they were real problems that they would be talked about on the forums quite a bit. I've never seen anything like either issue mentioned. Carbon is waaaaay too strong for a limp leg resting on it to break it. That just sounds crazy.

fair enough, carbon is strong with proper design and thickness.....my understanding of high end carbon design is that you put the material where it is needed and keep it minimal where it is not needed. and many bikes come with notices to not use bike racks where the bike is supported by the top tube or racks that clamp the tubes so I could easily see someone who completely supports them selves on the top tube pushing design limits....... Interesting engineering question

aggiegrads 02-05-19 04:09 PM

I think that the reason for cracked rims could be more behavioral than scientific. My guess is that most early adopters of road tubeless were MTB converts or cyclocross racers. Both try to run as low as possible pressures (one of the promises of tubeless is the ability to run lower pressures without pinch flats), which will inevitably lead to more rim hits. I can certainly see rim hits leading to more rim cracking.

RobbieTunes 02-05-19 05:01 PM

Don't know much about either. I've only wrecked a Technium, and the bonds held. The rest of the front did not fare well, and the front wheel pretty much blew up.

I've seen a couple of Kestrels crack on the top tube, no idea why, but it wasn't riding. I saw one hit a guard rail and break behind the head tube. Rider was fine (over the guard rail).

As far as tubeless, I have one set, and have not got the nerve to go commando with it. The cross riders I know say they're great, but they're running a lot lower pressure.

I rebuilt a Madone so a lady could ride across the country. I put liners in the GatorSkins and tubes that were extra thick on the face. She had one flat. She said those riding tubeless had constant problems. I attribute some of that to not having a wrench along (but they did have a chef).

It will be a while until I can get my head around putting 2oz of liquid in a tire and just riding off on it. That's all. Purely psychological, and since I'm pretty much nuts, well, there you go.

repechage 02-05-19 05:26 PM


Originally Posted by Spoonrobot (Post 20780746)
USA Cycling releases tech articles?

Link please

They must be tracking something, certainly not PED use.

The Carbon problems... the age of the Colin Chapman bike.

radroad 02-05-19 05:30 PM

Carbon frames forks and components have been in production for over 30 years and some goofball mechanic discovered the material's Achilles heel: brushing your thigh against the top tube.

tyrion 02-05-19 05:30 PM

Is your mechanic named Grant Petersen? (;) sorry, couldn't resist)

shoota 02-05-19 05:42 PM


Originally Posted by squirtdad (Post 20780851)
I could easily see someone who completely supports them selves on the top tube pushing design limits

Again, no. There is a big difference between clamping a carbon and resting on it. And who the heck supports their entire body weight on one leg, on a top tube? Sorry, but nope. I'm willing to bet your guy is seeing damage from handlebars hitting the top tube, not from resting a leg on it.

Salamandrine 02-05-19 05:51 PM

I've noticed some sunscreens can be pretty corrosive, enough to damage paint. That sounds like a more reasonable explanation for TT damage to me.

squirtdad 02-05-19 06:01 PM


Originally Posted by tyrion (Post 20780986)
Is your mechanic named Grant Petersen? (;) sorry, couldn't resist)

Tahn (for real).... but they do sell Rivendell, and honjo fenders..... I went in to get a chain and they were working on a rinkoed fender.... as shop most of c&v would like

squirtdad 02-05-19 06:02 PM


Originally Posted by radroad (Post 20780984)
Carbon frames forks and components have been in production for over 30 years and some goofball mechanic discovered the material's Achilles heel: bruising your thigh against the top tube.

guy who fixed cabon frames and fixed a lot of top tubes, not a 20 year plus mechanic. the mechanic related the story

bikeaddiction1 02-05-19 06:29 PM

I spent my career doing design engineering in the aviation industry. If you do not trust carbon fiber, you better not get on a modern aircraft. It has been used for a few decades now and is often used in flight controls and other critical structure and components.

There is a difference in that the aviation industry has much higher standards for analysis and testing, and required in service inspections that the bicycle industry doesn't. This could account for early adopter failures like early carbon road wheels. However, I would think any of the established bicycle manufacturers have their design and manufacturing procedures down pretty well.

I have been riding carbon fiber mountain bikes since 2013. I am not a free stile rider, but I use them for what hey are designed to do and (unfortunately) have had the odd spill and have seen no signioficant damage, and I inspect the bike closely every time I wash it.

That being said, I do love my steel bikes too, and think it is a great material.

clydesdale65 02-05-19 06:38 PM

https://cimg4.ibsrv.net/gimg/bikefor...ac27966e11.jpg


I personally had a carbon frame crack on the top tube during my first Belgian Waffle Ride. It cracked in the place where I've seen guys sitting on it, The manufacturer replaced the frame in due time, but I was told by someone at the shop not to rest on the top tube of carbon bikes because that is where the manufacturers can save some weight. This was about four years ago, and led me to start buying steel bikes for my more adventurous rides. I've attached a photo of the crack.

old's'cool 02-05-19 06:39 PM

All the inner tubes I've ever seen are not pressure vessels. I.e. they cannot contain any appreciable pressure on their own. Therefore I'm skeptical that inner tubes are somehow taking any appreciable amount of the hoop stress in a pressurized tire/tube/rim assembly.

bikeaddiction1 02-05-19 07:31 PM


Originally Posted by old's'cool (Post 20781063)
All the inner tubes I've ever seen are not pressure vessels. I.e. they cannot contain any appreciable pressure on their own. Therefore I'm skeptical that inner tubes are somehow taking any appreciable amount of the hoop stress in a pressurized tire/tube/rim assembly.

I agree.


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