Someone shows you a bike with internal routing, and you marvel at all the advantages:
- you're not upsetting the tension by lifting the bike from its top or down tubes,
- there are no exposed sections of the cables, and hence no risk of chipping to the lacquer,
- cleaning the bike will be that much easier,
- there is no risk of dirt entering the cable housing near sections where the frame is used as the cable tensioner,
- the aerodynamics are better,
and perhaps other good factors. All in all this more than makes up for the added complexity of changing cables, which is in any case an uncommon maintenance event.
But then you ride the bike, and you hear very clearly the sound of a cable rattling with the frame when you're in certain gears. It's not loud, and it's not alarming, but, aside from tyres, you expect well-tuned bikes to be basically silent.
Is rattling unavoidable with internal cable routing?
Are there styles of internal cable routing?
A good, but now-deleted, answer suggested that there is more than one style of internal cable routing. The less-well designed (and offered at lower price points) bike frames apparently let the cable(s) loose inside the frame, as illustrated in the cross-section on the left of the figure. In this case rattling noises are inevitable, at least some of the time.
Better, higher-value frames introduce a special channel that enables securing the cable housing through foam or a similar method. I improvised how this might be done in the figure on the right.
If this is accurate, the question becomes how we can determine (from the holes, perhaps) how a given frame routes the cables, with the objective of predicting whether a silent ride is possible.