[Closed] Carbon wheels = Deathwish

CBA to watch all of that.

Is it the usual big hill, carbon rims, lots of brake dragging and heat, inevitable failure?

Presumably you don't fly on aeroplanes then? 😉

Yep, i didnt even realise that was a thing. Im amazed you can buy a bike like that, when Raul explains it it seems madness. Like buying a car which will explode if you brake too much..

Massive spectacular failure with the front rim snapping in half - rear wheel also failed just seconds before but not so bad.

100kg rider doing about 45mph and having been going down a long descent and then catching a pot hole.

Tbh I wouldn’t fancy carbon wheels with rim brakes - it just seems like a bad idea. I am considering carbon wheels
With disc brakes though, as it seems heat is the major factor in all these type of things.

Yep i do fly on planes where carbon is clearly used in the right application..

You could argue that carbon rims have good applications, he just wasn't using them for that.

Skinny climbers - tick
TT'ers - tick
People who don't drag their brakes - tick

100kg rider, riding down a 1in10 hill - nooooooooooooooo

Pretty sure you're missing the main advantage of carbon wheels in that they look cool.

Just ordered some of these which must be the most pointless purchase ever. Not even deep section so no aero benefit. Barely lighter than aluminium rims but, again, they're going to look great on my bike.

https://www.chainreactioncycles.com/prime-blackedition-28-carbon-wheelset/rp-prod166634

Presumably you don’t fly on aeroplanes then?

Why do they use carbon wheel rims with brakes acting diretctly on the rim?

Crikey, I feel like I've dodged a bullet...

Seen similar in the Alps, a group of us rented a ski chalet in Alp d'Huez village for a cycling holiday (daft idea as every day you finish your ride by ascending the bloody hill).

Anyhow, day 1 first ride a short one over Gallibier and back as a warm up. On the first decent of Alp d'huez we get to the bottom and the last rider, who was over 100kg and was quite nervous of the hairpins had been dragging his brakes a lot. Both tyres delaminated, luckily not rapidly and at the bottom the tread was just peeling off the carcass and the sidewalls were bulging right out. Luckily no injured but end of riding that day as he needed to buy two new tyres...

If you're heavy and drag brakes a lot on big hills you can cook your rims / tyres quite easily.

Ok it just me or are videos these days "why take 3m to say something when you can take 15m?"

I would have thought that carbon wheels with disc brakes would be signficantly stronger than alumninium wheels of the same weight.

Just a thought - but some carbon wheels seem to have a metal brake track. What’s the disadvantage of that - it seems a better idea than running brake pads along something where the resin will lose strength as it gets hot?

Ok it just me or are videos these days “why take 3m to say something when you can take 15m?”

Yep, very tedious....

Just a thought – but some carbon wheels seem to have a metal brake track. What’s the disadvantage of that – it seems a better idea than running brake pads along something where the resin will lose strength as it gets hot?

Pretty rare now. In the old days they struggled with sidewall strength for clincher tyres and some carbon wheels had Al sidewalls (my Zipp 404s being an example), but not seen any for years, 100% carbon now.

it just me or are videos these days “why take 3m to say something when you can take 15m?

Not just you. I find explanatory videos of anything that doesn't need you to see something moving are frustrating - I can skim a paragraph of words and couple of images in 10s and decide whether I'm interested, but I've watched too many videos when I get to the end and realised it was waste of time.

Ok it just me or are videos these days “why take 3m to say something when you can take 15m?”

They think they're pictures, trying to get in the 1000 words.

cynic-al
Subscriber

Ok it just me or are videos these days “why take 3m to say something when you can take 15m?”

Its to do with getting more money from youtube.

Yep video is too long tbh but thought i would share as i had no idea this was a thing. I figured there must be a fair few people riding carbon rims with rim brakes and are oblivious to it...

The default style on YouTube is just insanely slow. As a teacher I do a fair amount on YouTube, particularly at the moment. You can get through alot in 12 minutes

Just a thought – but some carbon wheels seem to have a metal brake track. What’s the disadvantage of that – it seems a better idea than running brake pads along something where the resin will lose strength as it gets hot?

I don't see any benefit there. The heat build up in the carbon won't be any different. I think the metal layer was just for greater friction and or sacrificial wear

I was going to say that it is crazy that there are bikes where if you get the braking wrong you can die. Then I remembered that this applies to all bikes on almost all rides

MTBs are hardly innocent either. We’ve all heard of folk “boiling their brakes” off road in the Alps. Of course generally it’s larger folk dragging brakes again too

Didn't Dura Ace wheels have an alu track that lasted one winter at best (from the boobs riding race kit in winter, not me!).

I guess there's a heat dissipation benefit.

Just a thought – but some carbon wheels seem to have a metal brake track. What’s the disadvantage of that – it seems a better idea than running brake pads along something where the resin will lose strength as it gets hot?

These, if you get the info from any good bike shop, are actually just alloy rims with a carbon sheath built over the rim to make them look cool or to give the aero affect, they essentially aren’t carbon rims / wheels.

Carbon wheels with rim brakes are adequate enough for me, although I only weigh 68kg and I’ve not ridden down an alpine pass on them yet. Saying that, TDF riders did perfectly well on them up until the last year or two that discs have come in.
But come to think of it, if it’s in any way wet or moist and I’m doing over 60kmh (getting up to 80+) , my stopping distance is probably about a kilometre.

Probably a few things in favour of pro riders.... 1) they're more skilled than us mortals 2) they're using tubs which (I think but may be wrong) means the rims can disperse heat better 3) I doubt there are many tour riders at 100kg....

Pros aren't using discs cos they're better for them or that they need them. Bike companies need to sell disc bikes. And for a lot of the people they sell to, discs probably are better. Not for everyone though. Good rim brakes are still absolutely fine.

mashr
Member

MTBs are hardly innocent either. We’ve all heard of folk “boiling their brakes” off road in the Alps. Of course generally it’s larger folk dragging brakes again too

While that's true, if you overheat your disc brakes, they just stop working well, they don't explode. And I think in the UK especially our reviews, user experiences etc are very biased towards short sharp descents, so it's easy for people to spec kit that is good for that but falls apart when tested harder (and bloody nonsense like vented pads, icetech etc has set a bit of a weird expectation too I think, as if it's normal or sensible for a brake to need that sort of bollocks to work properly, when frankly hydraulic brakes have been well sorted for at least a decade and any mtb brake that can't function with standard pads, fluid and discs is just pissing about
)

I don't think there's anything wrong with carbon rims personally; but there probably needs to be more caveats about their suitability. Possibly there's a bit of an inbuilt assumption that £1000 bike wheels should be a big, high involvement, enthusiast's purchase with lots of informed thought involved. Kind of like how a chainsaw used to have some big barriers to entry which helped reduce the real world dangers but now you can buy a rechargable one from Lidl and it just seems like any other battery tool.

2) they’re using tubs which (I think but may be wrong) means the rims can disperse heat better

A tubular rim doesn't have sidewalls which are being pushed apart by the tyre, which is one weakness of clincher rims. A tubular tyre is pushing the rim body together in compression.

You have less mass in a tubular rim, so less mass to dissipate the heat it, which means they would get hotter for the same braking effort.

; but there probably needs to be more caveats about their suitability.

Why, how many rims fail in this way, maybe one every year or two? Hitting a pot hole at speed with a heavy rider is always going to push a bike to its limit; he was still unlucky to get such an outcome.

If he was doing 45mph, then he surely wasn't dragging his brakes.

So I think he just hit a pothole going very fast, and smashed his wheels and then stopped very suddenly.

You would need to take a more detailed look at his rims to know for sure.

I don't know that disc brakes would have fared any better in that case.

Hitting a pot hole at speed with a heavy rider is always going to push a bike to its limit;

The man really needs to learn how to bunny hop

The first Marmotte I did was an insanely hot day. Climbing the Alpe at the end it was 38*C.

It was pretty weird how many exploding inner tubes I heard that day. Plenty on the descent to the start, the decent down the other side of the Glandon was carnage.
There was a guy at the bottom selling inner tubes to all those who were walking down.

I heard of 2 guys who had to stop due to delaminated carbon rims that day, but aluminium didnt help that much.

Video didn't tell me anything that anyone who's had the misfortune of riding carbon rims with rim brakes didn't already know.

Heaven forbid riding them in the rain sodding horrible.

Lucky I got to ride a decent set for a season without having to pay to find out coupled with many test rides on carbon on rimmed bikes pre disks and subsequently avoided then like the plague on anything that goes down steep hills in the rain or needs to stop in a hurry.

The brakes on the TT bike (carbon 82 /carbon disk ) are merely suggestive. ...and even more so in the wet.

Disks on my current road bike mean that the carbon rims don't suffer all the downsides of old.

Whenever I've touched the rims on my carbon wheels after doing a big (for me) descent, they've been no more than gently warm, about 45C max. But maybe it depends on the rim.

I have managed to deflate inner tubes by getting them too hot, but that was on an aluminium wheelset, and probably dragging the brakes.

What difference does dragging the brakes make? Surely it doesn't matter whether you drag them or apply them on/off or whatever, if you want to maintain a certain (avg) speed, you've still got to turn the same amount of potential energy into the same amount of heat in the same amount of time?
The only way to reduce the heat generated is go faster, so the air slows you down instead.

This time last year I gave a roadie a lift home after his carbon rim exploded outside our house. He’d just come down the Rake, a 25% gradient, dragging his brakes. It sounded like a gunshot when it went. The rim was completely shattered. 😳

Being a fat bastard, there’s absolutely no way on earth I’d trust a set of carbon rims

What difference does dragging the brakes make? Surely it doesn’t matter whether you drag them or apply them on/off or whatever, if you want to maintain a certain (avg) speed, you’ve still got to turn the same amount of potential energy into the same amount of heat in the same amount of time?
The only way to reduce the heat generated is go faster, so the air slows you down instead.

I think a few things that count against dragging brakes vs dabbing of braking. This is based on my knowledge of physics not specifically cycling

If you pull the break lever hard for a few seconds there will be a massive increase in surface temperature of the braking surfaces. But that high surface temperature will lead to very high rates of heat loss into the surrounding environment. Heat loss by Black body radiation is the fourth power of the surface temperature so getting the surface hot has a big impact on heat loss. Some heat will of course conduct into the rim increasing its temperature.

If you drag the brake you might have a lower surface temperature. That actually means less heat loss from the braking surface per second. The heat now has time to conduct into the braking surface and then tyre for a rim brake or disc pads and fluids in a disc brake. So although you have a lower rate of heat input you also have a lower rate of heat loss. But for longer hence the whole thing heating up

Dragging one brake only is of course the worst as all the heat is in one brake

If out brake dabbers averages 30 mph but brakes from 35mph to 25mph they will on average lose more energy to air resistance than dragging at a constant 30mph. Of course, if our brake dabber travels faster on average then they will lose even more energy to air resistance

Modern carbon rim brake wheels should have a special wear braking surface that helps breaking. Although they can still heat up on big Alpine descents especially if they are even a little out of true since that will general localised heating that can melt the carbon resin. Also most carbon rim brake wheels are still a little crap in the wet. That’s why I used Alu rims in the Alps/Pyrenees etc.. or disc brakes and leave my carbon wheels in the UK.

I wiegh 100kg ish and have done 45,000km on my old rim rake carbon wheels. Never even needed to true them (15,000km a year for three years)

you need to rake a it earlier, so a rotation or two clears the rims. Everyone has got used to on/off disc brakes.

MTBs are hardly innocent either. We’ve all heard of folk “boiling their brakes” off road in the Alps. Of course generally it’s larger folk dragging brakes again too

Heavier rider here, only time I've boiled brakes in the Alps is stuck behind cautious skinny folk. Physics go against you

Used to ride a bit with a Movistar mechanic who told me one year almost every rider of the team delam'd a front rim in the Pyrenees stages, Bora 1s that year iirc. So not just brake dragging 100kg mamils.

My wheels are 2013 CCUs (tubs) and have seen some big days in the Pyrenees, Picos plus the steep hills round here and they're fine - other than they're atrocious at stopping in the wet.

I bought a cheap Roval aluminium front wheel to replace my beautiful 18 spoke deep section Corima carbon one a couple of years ago. I enjoy my ride much more now. I don't get the fear down fast descents, riding on potholed lanes in dappled sunlight and honking out of the saddle, with my svelte 95kg body trying to rip the thing apart. I've still got the Corima, it's in perfect condition, but it spoiled my enjoyment of the ride.
And that's without the lack of braking. I'm very glad the bloke in the car stopped on the roundabout at the bottom of the hill into Ironbridge that time I shot across it out of control.

The other factors that counts against dragging:

Wheel will be turning more slowly. Heat transfer will be worse due to lower air velocity past the hot surface.

Drag coefficient increases with velocity squared. So a rider that goes faster and brakes hard but infrequently will be losing more energy to friction with the air.

A lot of the Alpine stuff referred to is 6% to mebbies 9% which means braking through air resistence/sitting up is a lot more effective than the short sharp jobbies we get in the UK. Carbon or not, that has a significant effect on how much energy the brakes have to deal with.

The only way to reduce the heat generated is go faster, so the air slows you down instead.

Hmm I’m not convinced this is really correct. Anyone??

Physicians in the house?

Yup, I've spent a fair amount of time riding alpine descents, fast, in packs, on me tod...have always done that thing almost out of habit where you get out of an aero tuck at the end of a straight and get the body upright into the airstream before a hairpin to add to the braking effect, in practice this means you do this pre-braking and lets you brake later and minimise the time on the brakes...seems to stabilise and shift the weight balance rearward which can be a benefit for some corners. Plus in my head it makes me think it looks proper and dynamic like a Moto Gp rider at the end of a fast straight, the reality may be slightly different.

I should add...this works on shallow gradient but fast sweeping alpine descents braking from 50+mph into hairpins..it has little effect on steep UK 25%ers which is where a high mass heavy braker may exceed the carbon resin's glass transition point.

The only way to reduce the heat generated is go faster, so the air slows you down instead.

Hmm I’m not convinced this is really correct. Anyone??

Physicians in the house?

100% correct. Although of course there maybe reason why going fast isn't a good idea (sheep, corners, rocks etc.)

If you were rolling down a 5% gradient and you and the bike have a mass of 100kg then there is a force of 50N pulling you down the hill. Let’s say you are sat up in the wind to create drag and going a constant 70km/h which is about 20m/s. That means that the air is dissipating about 1000W of energy. (Power is force x velocity)

If you break to a constant 35km/h then the air resistance force drops to roughly a quarter, 12.5N. So the brakes have to provide a force of about 37N. Your velocity is now 10m/s. So the brakes now need to dissipate heat at about 370W.

Of course if you went really slowly things turn round again. At 2m/s we could ignore air resistance and the brakes provide all 50N of force. But as you are so much slower so the power dissipated by the brakes only needs to be 100W. Although the total energy to dissipated is the greatest at low speeds. But it might be possible for the brakes to loose the heat at this lower rate? Anyone know? We use 30 W heaters at work and they can get a kg of Aluminium hot enough to burn you so maybe not

Health warning. I make this stuff up all day every day. But as i teach physics rather say designing nuclear reactors my assumptions can be a hit and mis. So let’s call my work a rough guide that needs checking

The first Marmotte I did was an insanely hot day. Climbing the Alpe at the end it was 38*C.

It was pretty weird how many exploding inner tubes I heard that day. Plenty on the descent to the start, the decent down the other side of the Glandon was carnage.
There was a guy at the bottom selling inner tubes to all those who were walking down.

Etape du Tour a couple of years ago was the same. I thought initially when I heard all the popping that someone had spread tacks on the road but it was just people dragging brakes and the wheels getting super hot. I was running aluminium rims (Mavic R-Sys) with the Exalith braking surface and even they were getting warm. Terrifying cross over of wanting to control speed but not wanting to apply hot brake pads to an already hot rim, while being surrounded by people losing control when their tyres burst.

Those Prime wheels that @londoncommuter linked to above ^^. I've got a set of them, they're really good and with the supplied brake pads, the braking isn't bad. Not up there with Dura Ace calipers on an alu rim, but far from the terrifying "will they/won't they stop" feeling you get on some cheapo carbon wheels.