Carbon rims and descending big hills………..

The problem with carbon fibre

The problem with carbon fibre composite rims as compared to Al alloy rims is
that CFC does not have a specific heat capacity anywhere near that of Al alloy.

What this means is that if you apply a certain amount of energy by way of heat
to a certain mass of CFC, it’s temperature will rise considerably more than
the same amount of energy applied to the same mass of Al alloy.

That scenario is what happens when you’re going down an alpine pass. You’re
scrubbing of your speed big style and the energy you have is proportional to
your mass and velocity. You “lose” that energy when you apply your brakes but
it is largely gained by your rims resulting in a rise in temperature of the
material that they’re fabricated from.

Al alloy: No probs. Large specific heat capacity. No failure of the rim or
glue attaching tub to said rim.

CFC: Big probs. Low specific heat capacity so the rim gets hot enough for the
matrix material in the composite to melt – it’s just some shitty old polymer,
after all. Result? The walls of the rim fall apart, your tyre comes off and
you faceplant…if you’re lucky. If you’re unlucky, you’ll go over the edge
and an ambulance will come along and scoop up the bits of you they can find at
the bottom of the valley.

I hear people saying: “But the pro’s ride CFC rims”.

Sure they do. But they’re paid a pile of money to do so; you’re not.

My recent experience (last

My recent experience (last weeked, and multiple times over the last 3 yeaars on a set of FFwd carbon tubs) is that they hold up just fine to long (7-12km) descents with frequent braking from 80+kph to +/-40 kph for bends. I’m 80 kg and descend very quickly. I haven’t ever had noticable brake fade on these rims (Swiss black pads BTW). My descents aren’t alpine descents, but the largest is a Cat 1 climb, 500m vert descent in 7 km with multiple corners and top speed of 80 – 85 kph. I’ve braked hard from 100 kph to stop on them with no issues too.

It is unlikely you will die due to the brakes’ performance. Your performance I can’t comment on 🙂

I do agree however, that

I do agree however, that people should probably avoid riding terrain they are not comfortable in, on equipment that has been optimised for racing conditions. 

So… if you can’t descent without iding teh brakes, use brakes that won’t suffer from over heating.

BehindTheBikesheds wrote:

Jimmy Ray Will wrote:

it all depends on how you descend and how heavy you are, and how hot it is. 

I’d also imagine that tyre and rim width will play a factor. 

If you are relatively heavy, and like to drag a brake, then yes, there is a real risk of you blowing tubes on long, hot descents. if however you don’t letting the bike go, before hitting the anchors  for turns etc. then you shall probably be fine and dandy. 

The problem with carbon rims is that they don’t shed braking heat quickly. Different resins, and using more effective braking surfaces can reduce heat build up a bit, but fundamentally a carbon rim is going to get hot and stay hot when being braked. 

There is no fool proof solution to this sadly… which to me is another reason why the industries are pushing discs so hard. 

 

I agree with all that you’ve said bar the disc aspect. How often do the pros, well actually anyone crash on high speed wet descents due to actual inadequate braking power on carbon rims?

Aren’t incidents a combination of not understanding the limitations of your kit in X conditions and failing to adapt to that, AKA pushing the boundaries and going beyond and then most of all find that the limiting factor is in fact the human in their judgement and the tyres not the brakes themselves?

Yes, in the wet, discs have a clear advantage (over rim brakes) in ultimate braking power and how much later you can brake to stop at the same point, also for some who don’t bother to learn how to control the amount of braking will be helped out by discs in that respect too (modulation).

However similar with motorvehicles, you’ll end up with people pushing the new ‘envelope’ same as before, gain more confidence so braking much later and finding that they no longer have sufficient thinking time in certain instances and braking later means the tyres are going to lose traction more often due to much harder, later braking from higher speeds in the wet thus we will still have the same amount of crashes if not more due to these factors.

The ‘solution’ is not go with a system that is cumbersome/ugly and adds weight/changes where the stress points on the frame and forks are, and loses a significant amount aerodynamically (all negatives IMHO) but to learn when you need to brake in x conditions at x speed, learn how to navigate descents and bends better/safer without having to be on the brakes all the time, learn how to modulate your braking and that if you can’t descend to a reasonable standard and be safe with respect to yourself and other road users then maybe think long and hard about where you cycle and if the conditions/terrain is beyond your and your bikes capability just stay away and choose something less risky.

Just because one has a bike similar or even better than a pro doesn’t mean you have any business being in certain environs trying to copy them, particularly in not so great conditions. Doing so has catastrophic outcomes all too often and discs will not resolve this at all.

If discs were so great why do we still have millions of crashes in motorvehicles every year in the UK alone with 185,000 killed or injured despite all the protective systems in modern vehicles? it’s not the brakes themselves but the higher speeds, later braking, loss of traction of tyres and smaller window of thinking time for the human.

The ONLY reason manufacturers have come up with discs is to make money, they can try to sell it as a safety feature all they want but it’s totally bogus.

If the OP is concerned and doesn’t have the required experience/skill (it sounds to me they don’t) then I suggest they simply stay away from the descent mentioned and go ride somewhere where they are within their experience/skillset until they are sure they can do so safely with the kit they have, this should apply to all. I know and have seen even in the pro ranks of too many people riding beyond their capabilities and coming a cropper, great athletes but not so good at understanding X, Y and Z.

 

Yar, you may have missed the point I was making about discs. One reason the industry is keen to push them is because there is no ultimate ‘fix’ for carbon clinchers. Carbon will always get hot, which in turn heats the tubes, which in extreme scenarios causes blow outs. These are common enough to be a recognised problem in ral world riding as; not everyone is under 75kg and not everyone has the ability and desire to let a bike go on a steep / long descent. 

The only definitive fix for carbon clinchers is to remove the braking surface away from the rim… hence the disc requirement.

carbon rims are more expensive, and more desirable than alloy rims, so naturally something the industry wants to keep. 

That said, a carbon tubless set up could get as hot as it likes, it wouldn’t blow any tubes.

Agree with the above about

Agree with the above about riding within your skill level and equipment limitations. That’s the key point.

As for rims overheating, there are still valid concerns about carbon clinchers on long descents  overheating and blowing the tubes,  but just adjust your speed and style accordingly and you should be fine.

Mavic’s coments from one of

Mavic’s coments from one of their recent wheel launches:

“At the 2015 Etape du Tour, for example, Mavic assisted with 100 wheels, of which 52 were failures and 38 were carbon clinchers that had delaminated, product manager Maxime Brunand said.”

Tour magazine did a big carbon clincher test last year and a couple failed under heat duress but none of the failures were either Fulcrum or Campag.  Campag Boras won the test overall so I think its safe to say that Fulcrums will be just as well put together.

it all depends on how you

it all depends on how you descend and how heavy you are, and how hot it is. 

I’d also imagine that tyre and rim width will play a factor. 

If you are relatively heavy, and like to drag a brake, then yes, there is a real risk of you blowing tubes on long, hot descents. if however you don’t letting the bike go, before hitting the anchors  for turns etc. then you shall probably be fine and dandy. 

The problem with carbon rims is that they don’t shed braking heat quickly. Different resins, and using more effective braking surfaces can reduce heat build up a bit, but fundamentally a carbon rim is going to get hot and stay hot when being braked. 

There is no fool proof solution to this sadly… which to me is another reason why the industries are pushing discs so hard.