cbfight

Hi all,

Would it be trivial to reposition some top tube cable stops by stripping the paint, taking an OA flame to the stops, then removing them and reattaching them to run cable along the top of the tube instead of the along the bottom? It's a CrMo frame but fairly modern (welded tubes), so I'm worried the cable stops might also be welded. If they are welded, I'm assuming I can't heat them up and "pluck" them off and I'll have to instead use a cut off wheel and new cable stops?

Thanks for any help.

unterhausen

depends on who made it. You should be able to tell if it's welded because it's really hard to do a neat job welding stops. Stops are cheap, however

You could post pictures

Andrew R Stewart 

The lay out of braze ons and cable routing is one of my pet peeves. Excepting the paint issues i'd reposition any bit that didn't meet my requirements. Try grinding off the stops. Limits the heat cycles and paint damage and will work reguardless of their attachment method. Don't be surprised if there's tubing distortion where the old stops were placed (or clamped during brazing).

unterhausen

if you do try to use heat to remove the stops, just heat up the stops. You don't need to heat the tubes. This is the secret with any task like that, dropouts for example. The brazing filler will let loose, and then you can clean the tube with a file.

bellweatherman

People are going to get all up in arms, but JB Weld from Home Depot is more than strong enough for gluing on cable stops to a frame. Just make sure the paint is completely taken off in the areas that the epoxy will adhere too. The good thing about JB Weld and other epoxies is that there is no extra heat that would pull a frame out of alignment that occurs when you weld or braze new cable stops on. Just glue them on.

JohnDThompson 

Position the frame so the piece you're removing can drop off on its own when the braze melts. A bucket of water below the piece will catch it safely. A couple swipes with a wire brush will remove almost all of the molten braze from the tube.

JimBeans83

I wouldn't doubt this for front/rear derailleur use, but for braking I'm not sure. It has many advantages. Difference for derailleurs will be that a failure won't cause an accident. Braking may, though the front gives the redundancy one hopefully isn't not needing.

What forces (quantities) are you taking into considering for making this recommendation ?

Brazing is a minimum of some 15 times stronger (jb @ 4kpsi ), although the surface area used will be much smaller. JB weld can be brittle and break off - I'm not sure how it's going to stand up to temperature change, bumps, surface liquids, UV, etc. JB weld is not as easy to know if you've applied it perfectly well enough. One advantage with the JB weld though is that you could easily come up onto the sides of the cable stop and get a good 5x surface area increase.

Of course some empirical experience would be nice to hear for front/rear derailleur and brakes.

bellweatherman

I wouldn't use epoxy on brake bridges, if that is what you are asking. However, epoxy is more than adequate for brake bridges. The tensile strength far exceeds the braking forces that part encounters. I wouldn't do it though, unless carbon was involved. Anyway, that is a diversion from the real topic. The original poster inquired about moving cable stops. Brake or derailler cable stops can definitely be done with epoxy like JB Weld. The yield strength is very much within the range of epoxies.

JimBeans83

On that other note : that´s what´s being asked - what force are you using for saying that's adequate, for both brake and derailleur ? As this is entirely dependent on the lever being used and the strength of the person squeezing/shifting.

For braking, there is a considerable difference between what will lock the rear wheel and what one can squeeze beyond this point. For gears - certain levers continue to apply a reasonable amount of force once the front/rear derailleur has met one of the adjustable stops. During the inner gears, of course tension changes are minimal. Another failure will be if something snags an exposed cable.

On longevity : I'm also not sure of the long term viability of the epoxy considering the system as a whole under normal bike usage. Temperature changes (different coefficients of material expansion)/not knowing if it's been applied perfectly/contaminants entering, chipping, vibration, etc. JB weld seems to work quite well when it's penetrated something. I've had mixed results using it purely as a surface glue.

Not a criticism - the idea is much better for many reasons, especially DIY placement and avoiding painting issues. Just sorting this out out-loud quantitatively rather than relying on gut feelings.

tuz

Say the max. squeezing force is 75 lbs, and the lever has 3:1 mechanical advantage. Force on the cable stop is 75x3=225 lbs. Say the surface of the brazed/glued stop is 0.25x0.5=0.125 in2. The stress would be 225/0.125 = 1800 lbs/in2.

unterhausen

I suspect that the strength of epoxy goes down pretty radically with increased thickness. Most things designed to be glued have a lot of surface area. I wouldn't do it

bellweatherman

Nonsense. Epoxy is used in all of the high end carbon bikes for cable stops without any issues. Using it on an alloy like steel or aluminum would matter not. Cable stops do not see a lot of force acting on them in the same way that a metered joint does. The strength of an epoxied cable stop would far exceed the forces imposed on it from the pulling of the brake cable.

MassiveD

I don't buy it. I have a ton of epoxy experience though none in this application, so there is that. My experience with glueing stuff to steel is that it is not long term reliable. There is no chemical bond, which is more than possible when you make a composite bike from scratch, you can, and probably will in a production setting, ger a chemical bond. You can also make your attachments to have the correct surface area. You can choose your glue to be non-generic. Over time epoxy seems to peel off steel. Maybe this is due to some of the discussed factors, and also due to the expansion ratio of metal.

The thing is that while something may work, it is a big step from there to say that it is up to the job. Brazing deals with the basic forces, abuse, and all know environmental factors, and seems to have a significant level of redundancy also. I'm thinking a very light tap with a hammer would send a stop flying, and there are plenty of things that can have that effect in daily life.

Am I imagining it, or were stops one of many things that were banded in the old days. I remember when BOs were not standard, or only a few were standard, on average road bikes. OK, I don't actually remember the details that well...

Edit. There are all kinds of steel gluing aps that are reliable like the heads of golfclubs, hardware bonding on yachts, etc... I am talking here about surface mounts, where the glue is sheared, or under tension, and there are no fiber supports.

veryredbike

Usually bands were used with full length housing, they'd slip as stops. There are now clamps that'll let you add stops, but they don't make a lot of sense to me, as the clamp weights more than the housing for that run, the friction won't be high on a straight run of housing, and using some good compression-less brake housing will compensate for... well... compression.

Edit: er... ok, on second thought, I do remember seeing a few bikes with bands and a brazed on little heart shaped bump to prevent sliding.

unterhausen

the stop was a nice touch, but usually only on downtube because some people used downtube shifters and others needed a stop for barends or something else on the hbars. BITD, only cheap bikes had brake stop braze-ons. The only bolt on stop I remember were shifting related.

JimBeans83

If such nonsense, please tell me why it's not done by fabricators. And please back your theory. Epoxying a series of brazeons is much cheaper, faster, causes less HAZ concerns, etc. etc. So if it's so adequate, and better in every other respect? Restrict the domain to steel::steel applications, as mentioned, with all of the other concerns at hand.

I've just made a simple test rig to calculate pressure, a few points: I'm not that strong, these are not good levers, the cable bends are not ideal. I'm able to easily get to 45 kgs squeezing, the digital scale is rated only to 50kgs+/-50gs.

How are you calculating maximum possible force? And your calculations on how epoxy would hold up? Thanks, it matter would.

JimBeans83

I think that's inline with real world expectations, based on a quick test.

JB weld tensile shear lap strength is 1 kpsi, less than half of what could be encountered above, and not incorporating safety margins, under the best of application characteristics, new, no other stresses experienced over lifetime, temperature stable, perfect shear force, etc. etc.

So, without doing a destructive test for this, which I may do for curiosity's sake, it can easily fail even in a laboratory. Let alone pass normal expected lifetime test or any certification body testing.