Stuart, I posted a similar question on the SoCal board on Road Bike Review. One responder mentioned he had ridden up CA-18 ad CA-138. I ended up riding from Pasadena to San Diego rather than ride from Pasadena to Arrowhead. And given the temperatures that day I'm really glad.
My plan was to ride east on Foothill to Cajon, which parallels the 15. There is one stretch of the 15 that you have to ride as there are only dirt roads in the area. From there you take CA-138 to Crestline.
When we drove up CA-18 to Arrowhead for Labor Day weekend I was looking at the shoulder to determine ones ability to safely ride the road. The road from the bottom of the mountain to where it narrows in Crestline looks pretty darn safe. While the shoulder is not huge it is appears far wider than say something like CA-2. The road past Crestline however seems quite treacherous. In the lower part there are two lanes and a decent shoulder, in the upper part there is one narrow lane and basically no shoulder. I would look into an alternate route through Crestline if you can fine one. The problem is finding a suitable route through rather hilly terrain when using maps that show no relief.
Good luck with your ride and let us know what you end up doing.
Last edited by MTBMaven; 09-19-07 at 08:01 AM.