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electrical interaction between aluminum and carbon fiber(15 posts)

electrical interaction between aluminum and carbon fiberDougSloan
Apr 21, 2003 9:34 AM
"Due to the electrical interaction between aluminum and carbon fibre, we recommend brass nipples for enhanced durability"

http://www.amclassic.com/Carbon_Wheels.html

Huh? I never heard of electrical interaction with carbon fiber? I thought carbon fiber was actually an insulator. What is this?

Doug
re: electrical interaction between aluminum and carbon fiberboyd2
Apr 21, 2003 10:47 AM
Because of the different nobility of the materials involved, graphite to aluminum contact is a bad idea. You are basically creating a battery and the aluminum will corrode away. This is really a problem in presence of salt water (road salt and rain). Couple this with the other alloy nipple problems (stress-rupture and stress corrosion cracking) and you will break nipples all the time. Perhaps lots of nipples will break at once. A good barrier coating (epoxy paint) will solve the problem, but the barrier will never survive the spoke tightening process without breaking up. Go with brass nipples.
re: electrical interaction between aluminum and carbon fibermicha
Apr 21, 2003 11:22 AM
Well, boyd 2 answered your question in detail. But the short of it is that carbon fiber is an excellent electrical conductor. For example, most instructions for carbon fiber fishing rods tell you never to use them anywhere near overhead electrical lines.
Carbon fiber is a good conductor...alansutton
Apr 21, 2003 12:58 PM
Carbon fiber is used as conductors in most ignition wires in motor vehicles.
carbon+aluminum+salt = batteryterzo rene
Apr 21, 2003 1:41 PM
Galvanic corrosion will destroy aluminum carbon joints in a hurry. As a veteran of 6 aluminum lugged carbon frame failures in less than 3 years in the late 80's when I was living in Hawaii, I know very well how fast it can happen in the wrong conditions.

I don't think it's a serious problem in most locations with anodized nipples in a carbon rim since the nipples will probably not be roughed up in the building process (I recently dismantled an AC carbon wheelset to replace the wheelsmith spokes with CX-rays and the original nipples showed no surface damage). But if you live near the ocean I would worry. AC is located next to the Gulf coast so they have probably seen it firsthand.
Hmmmm.Spoke Wrench
Apr 21, 2003 4:47 PM
Is there a way that I can take advantage of this to eliminate the battery on my computer?
Maybe....terzo rene
Apr 30, 2003 10:06 AM
but it would really depend on the current requirements of the device. Perhaps a potato would be better.
So what about carbon forks with Al dropouts? nmPdxMark
Apr 21, 2003 2:00 PM
Near the ocean I wouldn't trust them...terzo rene
Apr 30, 2003 10:10 AM
but if the fork dropouts did detach they wouldn't fall out unless you were bunnyhopping or something, so the safety risks from failure are pretty minimal. Still I have Ti dropouts on my forks.
Here is something neat too!the bull
Apr 21, 2003 3:44 PM
Get your test light(12 v)put alligator clip on positive of battery.
Then touch light to ground it should light.Now touch it to a radiator clamp on ENGINE that is isolated by hose!What happened?Is rubber a conductor?Hmm!Why does it light up?
A friend of mine had his AL seatpost fuse to ...Live Steam
Apr 21, 2003 3:51 PM
his Trek 5200. I haven't had a chance to call him to see if he's been able to remove it, but he was one unhappy camper when he discovered the problem.
The CF has to be "bare" for this to be an issueKerry
Apr 21, 2003 5:38 PM
With a normal situation, the CF itself is imbedded in epoxy and then has a clear coat over it. When things are glued together (fork dropouts, rear stays, steerer tubes) the glue layer serves as another insulator. As other posters noted, CF itself is a good conductor. Some early frames with Al lugs and CF tubes had problems because they didn't account for this problem. If you have a situation where the Al nipple could be gouging a CF rim and exposing the fibers, you'd worry. Of course, some of us wouldn't use Al nipples in any case due to corrosion worries, but that's just us.
these are Zipp rims, and Zipp uses alloy nipplesweiwentg
Apr 21, 2003 7:38 PM
like Kerry said, maybe bare CF would conduct, but alloy nipples with CF rims shouldn't be a problem. and you should know, because you've got dozens of nice carbon wheels. in fact you've got so many, that one of them's going to end up as a super high-end fixie ;)
(btw, what is to become of that Ascent Pro rear?)
these are Zipp rims, and Zipp uses alloy nipplesboyd2
Apr 22, 2003 6:34 AM
Everyones comments about electrical isolation by the resin on the fiber and anodize on the nipples is right on target on the macro scale, but:

1: In real world composites building not all of those fibers are fully encapsulated (wet out) with epoxy. There are going to be a few loose ends pokeing out. They make the galvanic cell and there is your corrosion.

2: The anodize may look intact, but microcracking is going to occurr at at the point where the spokes contact the rim as it is tightened. These micro cracks become perfect initiation sites for stress corrosion cracking and galvanic problems.

I personally do not believe in Alloy nipples. I have stripped too many of them over the years, and they just don't seem to last. Of course I come from a MTB background and I am kind of a retro guy. The last wheels I built (last year) were 36 hole, 3 cross, mavic rims, brass nipples with (get this) DX hubs! Talk about bomb proof!
I confused thermal insulatorDougSloan
Apr 22, 2003 6:48 AM
I assumed that a thermal insulator would be an electrical insulator, too. I was told by Velomax that carbon is very poor conductor of heat, thus melting brake pads (but good for tubulars/glue). (Velomax said you could dip a carbon rim in boiling water and then immediately grasp it in your hand, no problem.)

A guy at the Davis 24 hour last weekend had a Comotion tandem with Zipp 404 all carbon wheels, with 32 spoke Al ALLOY nipples both ends. He snapped off an alloy nipple on the rear drive side. Duh. Said he intends to keep them that way, though.

I would think there are much better reasons to use brass than electrical conductivity, particularly on low spoke count, high tension all carbon wheels.

The nipples in my Velomax/Lews are hidden, so I don't even know what they are. I never thought to check the Zipps.

Anyone know what to look for, should this be an issue?

Doug