Theoretical question on braking with carbon rims.

[Globalti]

I've only tried carbon rims briefly and no, they don't offer much friction to the brakes so softer compound rubber brake blocks are needed. Rain makes them even less effective and they wear fast once gritty water gets between rim and pad.

I have some wheels with "Plasma Electrolytic Oxidation" of the braking surfaces, which I believe means hard-wearing oxides are fused to the alloy, somewhat in the way of the best abrasive papers, and they work extremely well although again they need softer compound pads to prevent expensive and unsightly wear to the braking surface.

So I was wondering if it would be possible for wheel manufacturers to incorporate a fine grit into the resin, which encapsulates the carbon weave, creating a more abrasive and hence more grippy and wear-resistant surface. If so, why hasn't it been tried? There is plenty of knowledge of abrasive and wear-resistant materials in industry; my guess is that it would be difficult with production techniques still in evolution, to incorporate two different qualities of resin into the rim, assuming the "grippy" resin was not considered appropriate for the structural part of the rim.

Any thoughts?

 

[dan_bo]

They want your shoot to wear out so you buy more of it?

 

[raleighnut]

With carbon rims disc brakes are the answer (but fugly)

View: https://youtu.be/ET1jRVynOBA

EDIT - the discussion on disc brakes starts at @ 17 minutes.

 

[Yellow Saddle]

I've only tried carbon rims briefly and no, they don't offer much friction to the brakes so softer compound rubber brake blocks are needed. Rain makes them even less effective and they wear fast once gritty water gets between rim and pad.

I have some wheels with "Plasma Electrolytic Oxidation" of the braking surfaces, which I believe means hard-wearing oxides are fused to the alloy, somewhat in the way of the best abrasive papers, and they work extremely well although again they need softer compound pads to prevent expensive and unsightly wear to the braking surface.

So I was wondering if it would be possible for wheel manufacturers to incorporate a fine grit into the resin, which encapsulates the carbon weave, creating a more abrasive and hence more grippy and wear-resistant surface. If so, why hasn't it been tried? There is plenty of knowledge of abrasive and wear-resistant materials in industry; my guess is that it would be difficult with production techniques still in evolution, to incorporate two different qualities of resin into the rim, assuming the "grippy" resin was not considered appropriate for the structural part of the rim.

Any thoughts?

Carbon rims are a primary driver in the move towards disc brakes and for good reason.

When two materials slide over each other without lubrication (as happens in brakes), then heat is generated. Specifically, the heat is generated in the softer of the two materials. Although the harder material (the rim in rim-braked wheels or the disc in disc-braked wheels) also get hot, this is because the heat was transferred through contact from the softer material to the harder.

Aluminium is an excellent conductor of heat and readily accepts the pad's internal heat and distributes it over a large surface and dissipate it harmlessly. Most of us don't melt pads on our everyday rides on aluminium rims, no matter how hard we make the brakes work. However, once you substitute aluminium for carbon composite, which doesn't conduct heat, you have a problem. Now the heat generated in the pad is trapped in the pad and cannot go anywhere. The pad overheats and melts. Hence the stop-gap measure of substituting rubber for something that doesn't melt - like cork or sawdust or such.

However, the other part of the problem is that the coefficient of friction between carbon and most soft materials is small - so small that we actually need power-assisted brakes or hydraulic brakes to increase the friction. This is obviously not practical. Hence most people put up with the poor braking on carbon rims and some companies attempt to add an aluminium brake track to carbon rims, which only partially solve the problem but brings about a far more serious problem. leave that aside for now.

Should manufacturers now start to put grit in the carbon matrix in an effort to increase braking friction, they will still have to cope with the heat management problem.

Various hard oxides such as ceramic and it's derivatives have been added to aluminium rim brake tracks in an effort to prevent premature rim wear in wet conditions. These are not successful for exactly the same reason as it won't work on carbon either - heat management. These ceramics are such poor conductors of heat that the rim cannot dissipate the heat at all and the pads melt. Ceramic stove tops illustrate the problem perfectly. Right above the halogen element the plate is hot but 2mm outside the hot ring the stove top remains cold. Compare that with old steel stoves.

Melting isn't as dramatic in appearance as it sounds. Only the surface of the rubber melts and is smeared off by the revolving rim where it immediately solidifies in the airstream and lands on your thighs as rubber dust. You need white, hairy legs and a long downhill on carbon rims to illustrate this nicely. The rest of the pad doesn't melt because it too is a poor conductor of heat. All the generated heat remains at the contact interface and does its damage there.

In summary: Carbon rims are an abomination. They're a problem looking for a solution. I get it that for specific applications one needs deep rims and the only way to deliver them is with carbon, but you and I don't need them. Let's stick to aluminium. It is the most appropriate material for the job.

 

[winjim]

Carbon rims are an abomination. They're a problem looking for a solution. I get it that for specific applications one needs deep rims and the only way to deliver them is with carbon, but you and I don't need them. Let's stick to aluminium. It is the most appropriate material for the job.

Word.

 

[Globalti]

I knew you wouldn't disappoint!

 

[mythste]

Carbon rims are a primary driver in the move towards disc brakes and for good reason.

When two materials slide over each other without lubrication (as happens in brakes), then heat is generated. Specifically, the heat is generated in the softer of the two materials. Although the harder material (the rim in rim-braked wheels or the disc in disc-braked wheels) also get hot, this is because the heat was transferred through contact from the softer material to the harder.

Aluminium is an excellent conductor of heat and readily accepts the pad's internal heat and distributes it over a large surface and dissipate it harmlessly. Most of us don't melt pads on our everyday rides on aluminium rims, no matter how hard we make the brakes work. However, once you substitute aluminium for carbon composite, which doesn't conduct heat, you have a problem. Now the heat generated in the pad is trapped in the pad and cannot go anywhere. The pad overheats and melts. Hence the stop-gap measure of substituting rubber for something that doesn't melt - like cork or sawdust or such.

However, the other part of the problem is that the coefficient of friction between carbon and most soft materials is small - so small that we actually need power-assisted brakes or hydraulic brakes to increase the friction. This is obviously not practical. Hence most people put up with the poor braking on carbon rims and some companies attempt to add an aluminium brake track to carbon rims, which only partially solve the problem but brings about a far more serious problem. leave that aside for now.

Should manufacturers now start to put grit in the carbon matrix in an effort to increase braking friction, they will still have to cope with the heat management problem.

Various hard oxides such as ceramic and it's derivatives have been added to aluminium rim brake tracks in an effort to prevent premature rim wear in wet conditions. These are not successful for exactly the same reason as it won't work on carbon either - heat management. These ceramics are such poor conductors of heat that the rim cannot dissipate the heat at all and the pads melt. Ceramic stove tops illustrate the problem perfectly. Right above the halogen element the plate is hot but 2mm outside the hot ring the stove top remains cold. Compare that with old steel stoves.

Melting isn't as dramatic in appearance as it sounds. Only the surface of the rubber melts and is smeared off by the revolving rim where it immediately solidifies in the airstream and lands on your thighs as rubber dust. You need white, hairy legs and a long downhill on carbon rims to illustrate this nicely. The rest of the pad doesn't melt because it too is a poor conductor of heat. All the generated heat remains at the contact interface and does its damage there.

In summary: Carbon rims are an abomination. They're a problem looking for a solution. I get it that for specific applications one needs deep rims and the only way to deliver them is with carbon, but you and I don't need them. Let's stick to aluminium. It is the most appropriate material for the job.

Today I learned...! Out of interest, how does having an aluminium brake track only partially solve this? Is it to do with the bond between the aluminium and carbon?

 

[Yellow Saddle]

Today I learned...! Out of interest, how does having an aluminium brake track only partially solve this? Is it to do with the bond between the aluminium and carbon?

The carbon brake track is relatively small so it doesn't have lots of heat capacity nor does it have lots of surface for heat dissipation.

In other words, it heats up quickly (because it is small and it's mass is low) and, it doesn't have exposed surface area to dissipate the heat. Further, it expands at a different rate to the carbon substructure and therefore frequently breaks loose from the carbon rim.
The bond per se is not an issue but you can imagine a material where there surface expands more than the interior. It will be stressed and delaminate. For a friction component you really want a uniform material.

 

[Scoosh]

@Yellow Saddle - !