So rim brakes on Carbon wheels are better ...

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trailgumby
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Re: So rim brakes on Carbon wheels are better ...

Postby trailgumby » Mon Dec 11, 2017 4:24 pm

djw47 wrote:I have carbon rims with disc brakes. They look as clean and beautiful as the day I bought them. I don't need to concern myself with grinding huge chunks out of the rim when I brake heavily in the wet anymore which in itself is a huge bonus. From what I've read, delaminating carbon rims is fairly unlikely however carbon rims and rim brakes on a steep descent, wet or dry, doesn't fill me with coonfidence. I have no concerns ripping down Donna Buang, Baw Baw or Hotham at full throttle on the discs, knowing that the chances of anything failing are virtually nil.

This is definitely a bonus associated with discs. Won't be putting carbon rims on my roadie until such time as I go to a disc brake frame.

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Re: So rim brakes on Carbon wheels are better ...

Postby RobertL » Mon Dec 11, 2017 5:40 pm

So, what would it take in the real world to cause problems to a set of rims?

For example, I'm 100kg, so I'm going to be harder on the brakes than a lot of people. But what sort of speed, hill, braking duration would I need to do to actually cause any damage to a carbon or alloy rim?

I keep reading about "descending the Alps" or "Hotham" but how much braking do I need to do before I get worried?
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Re: So rim brakes on Carbon wheels are better ...

Postby human909 » Mon Dec 11, 2017 6:55 pm

uart wrote:Seven to nine pounds (32 to 41 Newtons) of brake force at the levers is not exactly feathering the brakes though. That's close to full on brake application in my opinion (think braking both wheels). With most modern braking setups I'd think that would stop you in under 20 seconds in most circumstances, let alone 20 minutes. Unless of course you have a 1200 watt motor driving you forward. :)


To a roughly close approximation the brake level application force shouldn't matter. If you have a 1200watt motor putting in 1200 watts then you have roughly 1200watts going into the wheels. That all might be erring on the high side so lets balance it out by ignoring rear wheel braking.

So what do you need to put 1200watts into your front wheel for 20minutes?

According to the bike calculator an 15% grade and an 80kg rider would do it, riding the brakes at a 66kph. So at that speed for twenty minutes you would cover 21.9km and an altitude change of 3285m. You can play with the figures a bit but you certainly need decent altitude to get 1200watts for 20 minutes.

*(didn't check the accuracy, but if the bicycle calculator is doing its thing correctly then it should be about right.)

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Re: So rim brakes on Carbon wheels are better ...

Postby MichaelB » Tue Dec 12, 2017 10:26 am

hamishm wrote: A lot of that was discussed in the article and the comments. They only ran a single test because they paid for the rims themselves, rather than having it provided. They used the same brake pads because they were trying to minimise variables supposedly. etc. etc.

I didn't care about the differences between wheels - the interesting part and the point of the article was that all of them are rubbish.


Just because they talked about it, doesn't mean the test didn't prove anything other than the Alto rims outperformed all others for that SINGLE test with that SINGLE type of braking for that SINGLE type of brake pad.

Minimising variables is one thing, but using it as an excuse for poor testing methodology from which to draw conclusions is worse.

Yep, raised some good points and the article was good, but it still is a really flawed methodology and you cannot draw any real inferences from the results. Yep, the Alto rims outlasted, but what is their braking like compared to the others ?

None of them are rubbish. Overpriced and overhyped, most likely.

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Re: So rim brakes on Carbon wheels are better ...

Postby uart » Tue Dec 12, 2017 12:23 pm

human909 wrote:
uart wrote:Seven to nine pounds (32 to 41 Newtons) of brake force at the levers is not exactly feathering the brakes though. That's close to full on brake application in my opinion (think braking both wheels). With most modern braking setups I'd think that would stop you in under 20 seconds in most circumstances, let alone 20 minutes. Unless of course you have a 1200 watt motor driving you forward. :)


To a roughly close approximation the brake level application force shouldn't matter. If you have a 1200watt motor putting in 1200 watts then you have roughly 1200watts going into the wheels. That all might be erring on the high side so lets balance it out by ignoring rear wheel braking.

Yes but the speed was almost a constant across all wheels, even when they increased the braking force to 9 pounds. So there had to be some form of speed control on the motor. This means it was actually just putting out whatever power it required to achieve the desired speed of about 20 MPH. The 1200W was just a nameplate rating. If the speed is a constant then the friction is a wildcard. The power is inversely proportional to the coefficient of friction for each rim.

So what do you need to put 1200watts into your front wheel for 20minutes?

According to the bike calculator an 15% grade and an 80kg rider would do it, riding the brakes at a 66kph. So at that speed for twenty minutes you would cover 21.9km and an altitude change of 3285m.


Yeah, the braking power under descent depends a lot on the areo power loss - so on both air density (elevation) and what figure you use for the CdA aero drag term. That 66 km/hr figure must have been under fairly low CdA assumptions. The more aero you are the faster your speed (at the maximum braking loss point) and the higher your braking power for a given gradient and mass, but I can't see any sane rider trying to hold a super tight tuck when they are trying to slow down.

I used CdA = 0.4, Crr=0.005, and an elevation of 2000 metres to get a maximum braking power of just under 1100 Watt (1079 Watt) at 52 km/hr on 15% gradient (m = 80 kg). So in the same ball park, but my simulation obviously had higher areo losses than yours. Going any faster actually decreased the braking power because of aero losses.
Last edited by uart on Tue Dec 12, 2017 3:22 pm, edited 1 time in total.

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Re: So rim brakes on Carbon wheels are better ...

Postby Comedian » Tue Dec 12, 2017 1:03 pm

RobertL wrote:So, what would it take in the real world to cause problems to a set of rims?

For example, I'm 100kg, so I'm going to be harder on the brakes than a lot of people. But what sort of speed, hill, braking duration would I need to do to actually cause any damage to a carbon or alloy rim?

I keep reading about "descending the Alps" or "Hotham" but how much braking do I need to do before I get worried?

Well.. I've ridden down the steep side of glorious in the wet (and the dry) with my CF rims and it was fine.

Thing is.. if you're really going to be climbing big mountains why not pop a set of light alloys in? You won't have to worry on the descent, and you'll have a distinct advantage on the climb over your friends with disc brakes and their deep section CF wheels. :mrgreen:

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Re: So rim brakes on Carbon wheels are better ...

Postby biker jk » Tue Dec 12, 2017 1:45 pm

Comedian wrote:
RobertL wrote:So, what would it take in the real world to cause problems to a set of rims?

For example, I'm 100kg, so I'm going to be harder on the brakes than a lot of people. But what sort of speed, hill, braking duration would I need to do to actually cause any damage to a carbon or alloy rim?

I keep reading about "descending the Alps" or "Hotham" but how much braking do I need to do before I get worried?

Well.. I've ridden down the steep side of glorious in the wet (and the dry) with my CF rims and it was fine.

Thing is.. if you're really going to be climbing big mountains why not pop a set of light alloys in? You won't have to worry on the descent, and you'll have a distinct advantage on the climb over your friends with disc brakes and their deep section CF wheels. :mrgreen:


Heavier riders have overheated both carbon and alloy rims descending Glorious so your advice against disc brakes doesn't make any sense.

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Re: So rim brakes on Carbon wheels are better ...

Postby human909 » Tue Dec 12, 2017 1:52 pm

uart wrote:Yes but the speed was almost a constant across all wheels, even.....


uart wrote:Yeah, the braking power under descent depends a lot on the areo power loss - so on both air density (elevation) .....

All very valid points.

I just wanted to run the rough numbers on a rough test and got very rough results. :wink:

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Re: So rim brakes on Carbon wheels are better ...

Postby g-boaf » Tue Dec 12, 2017 2:15 pm

One question, how did people ride downhill before disc brakes were put on road bikes?

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Re: So rim brakes on Carbon wheels are better ...

Postby P!N20 » Tue Dec 12, 2017 2:26 pm

^ They used alloy rims.

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Re: So rim brakes on Carbon wheels are better ...

Postby g-boaf » Tue Dec 12, 2017 3:05 pm

P!N20 wrote:^ They used alloy rims.


But, heavier riders overheated alloy rims as well, according to the statement above, so I'm guessing that nobody heavy ever rode downhill. :wink:

Rather, I think people just learned to descend properly because they didn't have disc brakes.

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Re: So rim brakes on Carbon wheels are better ...

Postby Comedian » Tue Dec 12, 2017 3:07 pm

biker jk wrote:
Comedian wrote:
RobertL wrote:So, what would it take in the real world to cause problems to a set of rims?

For example, I'm 100kg, so I'm going to be harder on the brakes than a lot of people. But what sort of speed, hill, braking duration would I need to do to actually cause any damage to a carbon or alloy rim?

I keep reading about "descending the Alps" or "Hotham" but how much braking do I need to do before I get worried?

Well.. I've ridden down the steep side of glorious in the wet (and the dry) with my CF rims and it was fine.

Thing is.. if you're really going to be climbing big mountains why not pop a set of light alloys in? You won't have to worry on the descent, and you'll have a distinct advantage on the climb over your friends with disc brakes and their deep section CF wheels. :mrgreen:


Heavier riders have overheated both carbon and alloy rims descending Glorious so your advice against disc brakes doesn't make any sense.

Are you 100% confident that those people with such poor technique that they overheated alloys won't overheat a road disc? I'm not. I've seen people overheat car brakes going down that hill...

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Re: So rim brakes on Carbon wheels are better ...

Postby RobertL » Wed Dec 13, 2017 10:13 am

Comedian wrote:
RobertL wrote:So, what would it take in the real world to cause problems to a set of rims?

For example, I'm 100kg, so I'm going to be harder on the brakes than a lot of people. But what sort of speed, hill, braking duration would I need to do to actually cause any damage to a carbon or alloy rim?

I keep reading about "descending the Alps" or "Hotham" but how much braking do I need to do before I get worried?

Well.. I've ridden down the steep side of glorious in the wet (and the dry) with my CF rims and it was fine.

Thing is.. if you're really going to be climbing big mountains why not pop a set of light alloys in? You won't have to worry on the descent, and you'll have a distinct advantage on the climb over your friends with disc brakes and their deep section CF wheels. :mrgreen:


Yeah - I've never ridden Glorious or Nebo. I'll make it one of my goals for next year.
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Re: So rim brakes on Carbon wheels are better ...

Postby g-boaf » Wed Dec 13, 2017 1:08 pm

RobertL wrote:
Comedian wrote:
RobertL wrote:So, what would it take in the real world to cause problems to a set of rims?

For example, I'm 100kg, so I'm going to be harder on the brakes than a lot of people. But what sort of speed, hill, braking duration would I need to do to actually cause any damage to a carbon or alloy rim?

I keep reading about "descending the Alps" or "Hotham" but how much braking do I need to do before I get worried?

Well.. I've ridden down the steep side of glorious in the wet (and the dry) with my CF rims and it was fine.

Thing is.. if you're really going to be climbing big mountains why not pop a set of light alloys in? You won't have to worry on the descent, and you'll have a distinct advantage on the climb over your friends with disc brakes and their deep section CF wheels. :mrgreen:


Yeah - I've never ridden Glorious or Nebo. I'll make it one of my goals for next year.


If you want a test, go overseas and ride up this climb, and then ride back down:

https://www.strava.com/segments/10205155

It has some of the most sketchy wooden bridges I've ever seen. I only rode up it and kept going, thank heavens. It'd be good on really light carbon wheels going up, but I wouldn't want to chance it going down.

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Re: So rim brakes on Carbon wheels are better ...

Postby RobertL » Thu Dec 14, 2017 9:53 am

Thanks, g-boaf, but I might just work my way up to that one!
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Re: So rim brakes on Carbon wheels are better ...

Postby Ross » Thu Dec 14, 2017 7:53 pm

Cycling Maven interviewing Raoul from Luescher Teknik

CARBON WHEELS - THE TRUTH!! (Part One)
https://www.youtube.com/watch?v=ET1jRVynOBA

CARBON WHEELS - THE TRUTH!! (Part Two)
https://www.youtube.com/watch?v=bFdG2NgIc7s

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Re: So rim brakes on Carbon wheels are better ...

Postby Comedian » Thu Dec 14, 2017 9:10 pm

Ross wrote:Cycling Maven interviewing Raoul from Luescher Teknik

CARBON WHEELS - THE TRUTH!! (Part One)
https://www.youtube.com/watch?v=ET1jRVynOBA

CARBON WHEELS - THE TRUTH!! (Part Two)
https://www.youtube.com/watch?v=bFdG2NgIc7s

Ssssh... he cannot be mentioned. :shock:

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Re: So rim brakes on Carbon wheels are better ...

Postby andrewjcw » Fri Dec 15, 2017 6:45 am

Now they've changed the racing rules it really doesn't matter much either way. If you ride very steep stuff or in bad weather or you want carbon wheels (but not tubs) than discs are pretty clearly superior.

If you feel a gag reflex coming on whenever you see someone with hairy legs or mismatched socks on a road bike then stick with rim brakes.

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Re: So rim brakes on Carbon wheels are better ...

Postby g-boaf » Fri Dec 15, 2017 8:01 am

andrewjcw wrote:Now they've changed the racing rules it really doesn't matter much either way. If you ride very steep stuff or in bad weather or you want carbon wheels (but not tubs) than discs are pretty clearly superior.


I think that's your view. The major thing on descents is the skill of the rider. You do not need discs to ride in steep conditions or bad weather, or both. Totally not necessary. You just need to learn how to use the brakes properly. Soon people will want to have anti-lock brake systems on their bikes and we'll see everyone saying "oh, I can't stop without ABS".

Even I left people behind on very fast/steep descents who were on these apparently superior disc brake bikes. And I was being pretty cautious.

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Re: So rim brakes on Carbon wheels are better ...

Postby andrewjcw » Fri Dec 15, 2017 8:56 am

The major thing in any cycling is the rider. I have no doubt you could descend faster than many disc braked cyclists. I'm sure many cyclists on bikes 5kg heavier than yours would beat you up ventoux as well.

However that doesn't mean a heavier bike is better for climbing any more than a rim brake bike is better for stopping.

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Re: So rim brakes on Carbon wheels are better ...

Postby outnabike » Fri Dec 15, 2017 9:33 am

Yep stir the pot. Nothing wrong with a decent tree branch for jambing into the ground under the BB either. Worth a few pages that one. :)

Me, I have a disc on the front and a caliper on the back. Someones abandoned attempt to sit on the fence. That idea discontinued these days. Funny how they did this to help folks in outback riding, but then rationalized away from the old "parts are hare to find" theory.

It might be the overall weight of me and the bike and I am only a fun get arounder, but the discs have been a constant maintenance item in a comparison between the two systems, and noisier. Mind you they are ally rims so maybe off topic.
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Re: So rim brakes on Carbon wheels are better ...

Postby human909 » Fri Dec 15, 2017 10:29 am

outnabike wrote:It might be the overall weight of me and the bike and I am only a fun get arounder, but the discs have been a constant maintenance item in a comparison between the two systems, and noisier. Mind you they are ally rims so maybe off topic.

That's what I've found. Some people claim otherwise but IMO rim brakes are low maintenance and just damn simple and extremely effective in most conditions. I have no desire to move away from rim brakes for road conditions.

Years back I did do a friendly mountain bike race in muddy forest conditions with a V-Brake mountain bike. Having limited grip in the mud was bad enough. Having poorly functioning brake because my rims were covered in wet gritty mud made it worse! Would have loved discs then and thankfully my MTB now does have discs.

That said, the guy who 'won' the race had a hard tail commuter with rim brakes, a pannier rack and no shocks. He won because most of the time to be gained is in the uphills and was the most fit. Which highlight andrewkcw's point:

andrewjcw wrote:The major thing in any cycling is the rider.

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Re: So rim brakes on Carbon wheels are better ...

Postby Thoglette » Fri Dec 15, 2017 11:23 am

outnabike wrote:Nothing wrong with a decent tree branch for jambing into the ground under the BB either.

I'd heard of that one but was trying to find a reference from someone who'd actually done it.
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Re: So rim brakes on Carbon wheels are better ...

Postby Comedian » Fri Dec 15, 2017 2:44 pm

And in braking news (or should that be breaking) :mrgreen: the other wheel manufacturers have said the test that started this discussion is rubbish..

https://www.bikerumor.com/2017/12/14/enve-mavic-boyd-respond-alto-cyclings-carbon-rim-brake-test/

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Re: So rim brakes on Carbon wheels are better ...

Postby human909 » Fri Dec 15, 2017 5:18 pm

Comedian wrote:And in braking news (or should that be breaking) :mrgreen: the other wheel manufacturers have said the test that started this discussion is rubbish..

https://www.bikerumor.com/2017/12/14/enve-mavic-boyd-respond-alto-cyclings-carbon-rim-brake-test/


I would agree. The test quite clearly is poorly done. Scientific method is pretty much non existent. As my (very vaguely roughly scientific) calculations show it would require some pretty big hills to produce the heat that these test supposedly generated.

That said. The risk is real. Plenty of anecdotal evidence to support it. Sure blame the user if you want, but seriously if cars wheels failed catastrophically on big descents there would be lawsuits all round. Is it really too much to ask for reliable and safe equipment?

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