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 Post subject: Campy Bora heating up
PostPosted: Sat Jul 07, 2012 9:12 pm 
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So I started riding Campy Boras this summer... by far the best ride I've ever had, literally feels like butter with Veloflex Carbons. I'm 195lbs but can't really tell there's any flex or issues with my weight.

Did some serious climbing today and there's a 25% downhill at one point for about 0.25 miles that turns really sharp 90 degrees into a major road with little or no visibility. So my group really had to go inch by inch down the hill, some of the guys even got off their bikes and walked down (yeah, weenies).

At the bottom I touched the Boras and they were definitely very hot, I'd say at least 140F or so. The question is how dangerous this is? At which point one can expect structural defects in carbon/resin or perhaps tubulars' glue (and in a nightmare scenario, have the tires come off)? Just wondering if in these kind of situations, where one kinda has to be on the breaks the entire time (rather than break hard for a short period of time), if I should go back to Al rims...


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 Post subject: Campy Bora heating up
Posted: Sat Jul 07, 2012 9:12 pm 


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PostPosted: Sat Jul 07, 2012 11:49 pm 
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Joined: Tue May 03, 2005 2:20 am
Posts: 5784
Location: Belgium
Hi,

Most rim cements will start to liquify at around your guestimated temperature (60 degrees Celcius approx) so it is potentially dangerous.

Bora rims are pretty tough cookies but a sustained (as opposed to peak temp) highish (around 200 degrees C or 390 degrees F) temperature can cause the break track to show signs of burning. The brake pads would fry even much sooner depending on their particular formulation.
I have no idea at what exact temperature this would occur but without air cooling I guess it would still need to be much higher than 140 defrees F.

Ciao, ;)

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PostPosted: Sun Jul 08, 2012 12:02 am 
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To help with your estimation on the road.......if you can hold your hand/finger to the surface without a burn injury but still......"by golly it hurts a whole heck of a lot".....that's about 120 degrees F. If it's above that you won't be able to keep touching it for more than a second or two.

Might help put the issue you experianced more into perspective.

T


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PostPosted: Sun Jul 08, 2012 3:06 am 
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They will cope with a LOT more temperature than you should put your finger on :)
If you want to do a sensible and easy check....try lifting ( rolling really ) the tubbie off after you've given it 30seconds to let the heat dissipate a little. If your otherwise hard/solid cement "strings up" and it lifts/rolls with comparative ease, then you have your answer re your tyre safety. Re rims blistering/epoxy failure (expensive and dangerous!) try not to stop fully and leave the pads in contact with the rims as the heat build-up at this point is what I've witnessed cause the destruction of 3 different carbon wheels (i.e. get your hand off that brake lever once you have done your stopping/slowing!)

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PostPosted: Sun Jul 08, 2012 6:31 am 
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im sure it varies wildly, but 240degreesF is what ive been quoted by industry insiders as their resin's temp limit. carbon is very insulative, so you can get the surface hot in spots but it wont dissipate, which is the nature of the problem when it comes to rim bulges, etc, but it would be hard to transfer that heat to the tubular.


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PostPosted: Sun Jul 08, 2012 12:55 pm 
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So thisisatest, which will fail first, the rim or the glue job?


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PostPosted: Sun Jul 08, 2012 4:56 pm 
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You guys are right, my 140F guesstimate was based on the fact that the outside air temperature was about 100F (yes, we had one of the worst days yesterday here in NY) and the fact that holding my finger on the rim for 4-5 seconds was very uncomfortable.

So it sounds like 120F was more likely the overall rim temperature (as 140F would have created burns on my skin).

I think tubulars glue/cement is fine, i just inspected the tires. But perhaps it was failing right after the descent. Has anyone here had their tubulars come off due to excess heat?


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PostPosted: Sun Jul 08, 2012 5:10 pm 
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The people who've has their tubulars come off due to excessive heat may not be around to post about it.


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PostPosted: Sun Jul 08, 2012 5:49 pm 
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Nicely said... but I'm gettin really worried now (weenie!)

It looks like i might have to ride Al clinchers on my climbing rides which means a) switching brake pads and b) giving up that buttery Bora/tubulars ride... I guess small price to pay for my neck/collarbone.

Do any big boys (TdF) ride carbon rims on mountain stages? Ha, I know they dont ever use their brakes... but still.


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PostPosted: Sun Jul 08, 2012 6:00 pm 
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dudu wrote:
Nicely said... but I'm gettin really worried now (weenie!)

It looks like i might have to ride Al clinchers on my climbing rides which means a) switching brake pads and b) giving up that buttery Bora/tubulars ride... I guess small price to pay for my neck/collarbone.

Do any big boys (TdF) ride carbon rims on mountain stages? Ha, I know they dont ever use their brakes... but still.
More likely their braking technique is more hind than yours or mine. Long sustained braking as opposed to braking just for the turns is more likely to build heat up in the rims and affect your cement. Mind you 25% descents in races are rare.


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PostPosted: Mon Jul 09, 2012 1:53 am 
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Only tire I ever blew due to heat was an aluminum clincher. You can alway stop, spray a bit of water on them if your that worried. I'd be moe worried riding at 100deg F with the current air quality conditions

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PostPosted: Mon Jul 09, 2012 4:02 am 
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Braking technique is a factor that varies amongst riders - but for me I'd take a carbon tubbie over carbon clincher on a steep downhill - anyday.


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PostPosted: Mon Jul 09, 2012 4:38 am 
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Hm, it turns out prof Chip Howat of Kansas University did a study on thermal perfomance of rim cement, here is the paper for anyone interested http://www.engr.ku.edu/%7Ekuktl/bicycle/Part6.pdf

Basically at 140F (60C) the strength of Conti cement (which is what i used on the Boras) reduces about 50%... I reckon that's quite significant hit. Vittoria Mastik (according to Howat the best performing cement he tested) has about 30% decrease in strenght at 140F and at that temperature it's also 30% stronger than my Conti cement.

Apparently Conti has carbon rim specific cement which is - Conti claims - about 3 times stronger than the regular cement at 160F (70C), but it requires somewhat different gluing technique. Anyone here played with that cement?


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PostPosted: Mon Jul 09, 2012 5:12 am 
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But why use conti tubbies at all? Higher rolling resistance and all.....and therefore also "stuck" using their rim cement?

Veloflex or Vittoria's with Mastek cement is an optimal low rolling resistance/high security solution.


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Posted: Mon Jul 09, 2012 5:12 am 


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PostPosted: Mon Jul 09, 2012 6:28 am 
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conti glue "for carbon" is a pain imo. it dries so fast, there's no chance to reposition a slightly crooked tire. if you do try, the glue tends to fully release from the rim cleanly. this happens no matter how thoroughly the rim is cleaned, or sanded, dried, etc. i tried multiple techniques. even the glue on the brush starts getting too tacky to move smoothly across the rim surface before going all the way around the rim.
mastik is the way to go. i have no problems using it on any tubulars, including contis.


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