Advice Needed

[The Saint]

How to explain this - here goes. Today whilst desending in Suikerbos I ran out of brakes due to the rim becomming so hot that it started to melt the brake pad - yes some heavy braking I know to cause this. On stopping I put my finger against the braking surface of the rim which is alu and it was hot as all hell. The rest of the rim is a carbon deep section.

 

I am not to worried about the heavy braking which caused this but what I would like to know is how does carbon disperse heat as obviously something like this could run the risk of bursting a tube should the carbon not disperse the heat quick enough.

 

Is this a common problem with Carbon rims with an Alu braking surface?

[Johan Bornman]

How to explain this - here goes. Today whilst desending in Suikerbos I ran out of brakes due to the rim becomming so hot that it started to melt the brake pad - yes some heavy braking I know to cause this. On stopping I put my finger against the braking surface of the rim which is alu and it was hot as all hell. The rest of the rim is a carbon deep section.

 

I am not to worried about the heavy braking which caused this but what I would like to know is how does carbon disperse heat as obviously something like this could run the risk of bursting a tube should the carbon not disperse the heat quick enough.

 

Is this a common problem with Carbon rims with an Alu braking surface?

 

Welcome to the world of inappropriate materials.

 

Heat is generated in a friction interface like a bike brake in the softer of the two materials from the rubber stretching and chemical bonds between molecules broken. Note that the heat is not generated in the rim, but in the pad and only at the interface, not deep inside the pad. The heat is then transferred to the rim by plain old contact. This heat, in an aluminium rim, is then dissipated by air and to a small extent by transferring some of the heat to the spokes through the brass nipples. Brass and aluminium are excellent conductors of heat, stainless steel less to and carbon very, very poorly so.

 

In your case, you had an aluminium braking surface there which served well to remove the heat from the pad but did very poorly in getting rid of the heat, since the carbon and foam inside the hollow section, is an excellent insulator and didn't allow the air to do its work.

 

Your risks are a tyre blow-off - something that is quite common in tandem riding or if you are lucky, total brake failure - which is better than tyre blow-off when it comes to choosing the lesser of the two evils.

 

You can avoid this by getting approrpriate wheels - i.e. made from aluminium. However, your braking style also has a bit to do with it. You should do all your braking at places like Suikerbosrand with your front wheel - that way you get maximum braking. Rear braking serves no purpose here unless you are short of traction due to gravel or sand on the tar surface.

 

Dragging a rear brake causes it to heat up very quickly and it doesn't do anything to reduce your speed that a front wheel can't do 400% more efficiently.

 

Carbon wheels are terrible for mountain passes. But they don't tell you that in the bike magazines or the shops where you buy the stuff. They only tell you how little they weigh.

 

Something else that fares quite poorly at Suikerbosrand is Shimano pads. I'm talking about pads of 8 years ago or so. They melt rather quickly and you can actually feel the point of melting - one second you're still controlling your speed and the next you're free-falling with lots of hot black crumbs blowing onto your thights from the front brakes cooking.

 

In that case, switch to Koolstop Salmon pads and your problems are over.

 

Unfortunately very few people ride in the mountains so the dirtly little secret about carbon rims remains a secret.

 

You could have had it worse - your wheels could have been all carbon.

 

 

JB

 

 

 

 

 

 

 

 

 

[The Saint]

Thanks so much Johan your advice is much appreciated, I will swap the brake pads as mine are Shimano pads. I will give them another try and if I still have the same problem riding in Suikerbos will be back to Alu rims and Alu rims only.

 

[Tumbleweed]

hang on, are those the rims delboy was complementing when i met you guys?

[The Saint]

Yup those are the ones.

[Tumbleweed]

they're purdy, man! but sometimes you gotta go for function over form. sh!t i'm late for dinner. see you guys!

[Johan Bornman]

Just yesterday I had to rebuild a deep section carbon/aluminium rim for someone. I therefore had the opportunity to put a hacksaw to an expensive rim. Have a look at the cross-section:

 

 

If you think about it, the heat has nowhere to go. The foam is an excellent insulator as is the carbon. As I explained above, the heat is transferred from the pad to the braking surfaces - the two parallel surfaces you can see here. The heat can basically only go into the tube. The little bit that is dissipated by the exposed braking surface isn't enough to allow the wheel to cool by airflow alone.

 

That's why these wheels don't work well in the mountains, as the original poster experienced. The best coolilng would come from a deep section aluminium rim but normal box section rims perform well enough to give none of us, other than tandem riders a problem in the mountains.

 

However, it is always easy to blow off a back tyre by dragging that brake for a kilometer or two down a long hill, no matter what type of rim.

 

JB

 

 

[Wonder Woman]

MuXmAn, the answer here is stop being chicken and dragging your brakes, only brake when needed and use you front brakes.

 

Oh, and maybe buy those other brake pads!!