Framebuilders - Internal Cable Routing?

Internal cable routing for the rear brake cable through the top tube.
Personally, I think it looks fantastic.
My question is, does it weaken the top tube?
How do you reinforce the entry and exit holes to maintain strength?
Does it cause additional friction on the rear brake cable?

If the holes punched are along the dead centreline of the tube underside, they'll have very little effect, seeing as the top tube tends to get little torsional loading.

Structural aspects aside, I've always disliked the idea of introducing more holes into the frame than is absolutely necessary. You don't want to be giving water, road salt, dirt, grime and extremely small mammals any more ingress points to the inside of a tube than you need to, were they can go about their nefarious business out of sight and out of mind...

the top tube is under compression, so reducing its strength is no big deal. There are many bikes out there being ridden with dents in the top tube without any problem.

most people put a reinforcement on the entry points to the tube. There is also a tube that reinforces the hole. It may actually be stronger than the original tube considering the loads. I suspect there is some more friction, but not a lot.

Here is one part that is made to do this:
tube (http://www.novacycles.com/catalog/product_info.php?cPath=61_137_139&products_id=763)
reinforcement (http://www.novacycles.com/catalog/product_info.php?cPath=61_137_139&products_id=762)

Well, that's not entirely true...
The underside of the top tube is in compression, and the top surface is in tension, but what you're saying about strength not being an issue holds. I've just always thought three big holes in a frame is already more than enough...

Thanks for the information. Basically, if the holes are reinforced properly then you should not have to worry about creating stress risers in the tube?

I think Falanx is right, but dang! it's been a while since I've had my Physics course in college which I've conveniently forgotten since I've graduated. Isn't the top surface of the top tube under both compression and tension? Maybe, I'm just not clear on the differences.

That said, I prefer the holes to be at about the 7-8 o'clock position on the left side of the top tube. Kinda in the same place as normal cable stops are brazed in. I don't think it weakens the top tube noticeably. The braking is better though with cable stops.

since the joints can withstand a moment, my previous statement was over-simplified. Having trouble seeing how the top of the top tube can be in tension though.

Me to, if one is just taking about the major loads, there are other effects in every imagineable direction. Normally a hole is drilled at an angle, and a tube like stainless brakeline is introduced in the top tube and brazen in place, so there shouldn't be any access for anything into the top tube, though it ads complexity and weight for little discernable benefit, but it is a clean look.

Also, I thought they were mainly punched dead center at 3 or 9 oclock, not the underside. Which has nothing to do with where they might best be punched if structure was a major concern.

The seat tube is loaded such that it is 'rotating' away from the top tube and the headtube in the mirro of this, too, both placing the top of the top tube in tension when the bicycle is being ridden, or at least to a first approximation. The true stress states are way more complex, but I'm trying to avoid worrying too much about the loading regime here, because it's not the major issue.

In short, don't worry about holing the tubes from a stress point of view. If you're going to worry, do it from a corrosion POV. Peter's solution is about the only way I'd like to see it.

/Falanx out :-D

Hi All,

Do take a look at: http://www.novacycles.com/catalog/product_info.php?products_id=763 I have used this on several bikes. The first has been on the road for about 15 years with no problems. The good part of this system is that the cable housing isn't fished through the frame; it terminates at both ends while the cable runs through the smaller brass tube. It takes some work to shape the entry spot as you are making a slot for the end tube so it angles into the top tube. It is also a trick to calculate where to cut the internal tube before brazing it onto the end piece before brazing both ends onto the top tube. I will add some pictures of the results tomorrow.

Thanks,
Tom