Hey there,
I'm building up a new bike. I have to cut and route my brake cable housing. I have nothing to go by, so I need input. When routing the rear brake from the right level, does the housing route between the front of the head tube and the left brake housing or does it route in front of the left brake housing? Does anyone have a good picture showing the proper length for the housing where it leaves the levers? thanks.

it will go between the front brake housing and the headtube.

search google for 'site:sheldonbrown.com cables' and read this page from their cache

https://www.sheldonbrown.com/cables.html

for some reason it has disappeared off his site within the past two or three days...oh, Sheldon?

I had to go downstairs and check my bikes to see how I've done it in the past.

On all of my road bikes, the rear brake cable goes behind of the front brake cable. On all of my flat handlebar bikes the rear brake cable goes in front of the front brake cable. I don't have any engineering-based principle for doing it like that. I just route the cable housings whichever way seems to produce the smoothest curve.

I just checked four different bikes. On three of them, the rear brake cable housing is routed behind the front brake housing, i.e. between the front housing and the headtube.

For the one exception, the wing-type handlebar puts the front brake housing so close to the headtube that I ran the rear brake housing outside (in front of) the front housing.

Make the rear brake housing just long enough that you can just turn the handlebars fully to both sides without binding the housing.

Interesting. I picked up the Bicycling maintenance book. The specifically say the left housing should pass through the loop of the right housing, ie the right housing goes in front of the left housing and the left housing passes between the head tube and the right housing. Yet, all the bikes I checked and most of the answers here say otherwise. That seems to say to me that it doesn't really matter.

what matters is how the routing affects performance. i do it my way because, in my opinion, it looks better, and there's no bad effect on performance. what drives me absolutely crazy is when rear shifter housing has to be routed on the right-side of the head tube, and front housing on the left side.

You don't have to do it this way if the bike has downtube shifter bosses. You can route the rear (right) shifter housing to the left of the headtube and the front housing to the left, then cross the cables under the downtube to get the operation to the right derailleur.

If the cable stops are headtube mounted you are out of luck.