soyabean

This isn't a question about wheter crossing shift cables under the downtube is better or not, I believe any owner can do whatever they want.

I want to know if there are ways to tell if a frame was made for crossed cables from the factory.

I think if crossed cables touch the downtube then it's a sure no. Because that surely adds friction which impedes shifting.

If crossed cables to not touch the downtube, that does not necessarily imply it was made for it.

And what about crossed cables that are almost touching? I've done setups where I swear were like half-millimetre away from rubbing. Is there a standard acceptable distance? I can't find this data published anywhere.

I've also seen frames where the rear brake cable runs along the left of the top tube, opposite from the rear brake lever on the right. The rear brake housing benefits being loose and open in front of the headset, giving a tighter and responsive braking. However, that does not imply this frame's shift cables are to be crossed. They don't require nearly as much force as brakes.

Maybe this is manufacturer specific? Some bikes were designed with crossed shift cables, others not?

Dan Burkhart 

For me, the deciding factor is how cleanly and neatly the loop from the levers to the down tube can be made. This can be affected by the shape and length of the handlebar, the length of the stem,and the position of the stops on the frame.
If it all fits neatly on the same side of the frame as the lever, I don't cross them
Otherwise, I don't hesitate to cross them under the down tube as long as it can be done without the bare cable touching the frame.

Kontact 

I don't think anyone is making bikes with crossed cables in mind. The ones where it works have the stops in the lower 90 degrees or so of the tube, but I have seen it work even higher up that that.

Generally, you can sight along the stop to the BB, and if you can see the opposite BB guide it will work fine.

Crankycrank

I'm not really sure any are specially designed to be crossed. IME routing depends on various combos of bars, bar height, stem lengths, type/model of brakes, etc., that make cross-routing function better.

Steel Charlie

Why is this a thing?

soyabean

I understand that even if the frame allows for it, a setup on the bar may not.

Here's an example of all cables entering at the other side, shift and rear brake.

Andrew R Stewart 

See post #2 for the answer. being a small guy (and making frames for other small riders) the amount of open casing between the handlebar tape exit to the frame stop is usually fairly short, less then on larger sized bikes for sure. I want a handle bar be able to swing fully from tup tube contact to top tube contact (or caliper to down tube) without the casing being stretched too far or having to be really droopy when steering ahead.

In my world there's no issues with cables touching each other at the mid point along the down tube. The cables are under low tensions and are not continuously moving. I'll thread them through a short piece of inner cable liner where they cross each other, to keep them neat and lessen any frame pinging.

I started doing this as I deemed better 30ish years ago. Had a friend that on unpacking their bike from my shipping it to them (after some service on it) couldn't figure out how to "untangle" the cables and ended up going to a shop My mistake was not telling them of this routing change I had made. Andy

KerryIrons

While you are surely correct that it adds friction compared to a cable that doesn't touch, the amount of friction it adds is surely negligible compared to all the other friction in the cable system. I cross cables under the down tube because it makes for better routing off the handlebars.

FBinNY 

You are w-a-a-a-ay overthinking this.

It's very simple. Feel free to cross them as long as they don't touch or rub. This isn't horseshoes, so close doesn't count.

Dan Burkhart 

I don’t worry if they touch or rub each other because the contact is very light. I prefer they don’t touch or rub the frame.

70sSanO

I use the rear derailleur cable loop as a guide. If my shifter to stop minimum radius is less than what I would use for an RD loop, I might consider cross cabling.

Of course this is at a minimum riding radius, not handlebars at 90*. That would indicate a whole lot of other catastrophic problems, especially if done at speed.

John