A tiny question
 

In search of marginal gains in bicycle appearance (I already have color-matching presto stem caps...)

I have external cable routing, in which the rear brake cable runs along the bottom of the top tube. The little rubber rings (i think they are called donuts???) Always slide back toward the seat tube, and so (1) don't do their job of keeping the cable away from the paint, and (2) look silly.

What can I do?

Are they really necessary? I don't have it on any of my bikes and don't have problems with the cable contacting the TT. Get rid of them. They're like the dork disk on your rear wheel.

and (2) look silly

Cut them off maybe, especially if they don't work?

If it was me, and I wanted it to look neat and keep the cable from touching, I'd slide a short piece of shrink tubing over it. But, anything you put on the cable, if it rubs very much it will still eventually scuff the paint off so maybe you're better off with just the cable.

You could...
Position them correctly...that is far enough away from where the cable goes into the housing...and then apply just a very small (tiny) amount of rubber cement to hold it in place. That shouldn't interfere with the operation, and would be easy enough to remove if/when you need to remove the cable.

Dan

or maybe paint them the same color as your matching prestos ...

This is the answer.

I dealt with sliding donuts for >15 years and really anything will do - nail polish, rubber cement, a little hot glue, RTV silicone gasket sealer... Whatever is handy.

The right answer when looking for marginal gains is to ditch the valve caps.

But I have purpose for valve caps! Valve caps indicate tubeless conversions. No valve cap is for inner tubes, where you don't care about it getting gunked up (because it's disposable).

1. I'm going for marginal APPEARANCE gains, hence the shiny blue aluminum valve caps.

2. I don't do "marginal" performance gains. I work hard for 3-4 weeks, make LARGE gains, then slack off for a few weeks and lose it all...and repeat...

That doesn't sell it as a gain, appearance or otherwise.

This, or remove them and put some tape on your frame between the cable. 3M make a nice thick clear tape.