Framebuilders - Carrying a front load...

My next frame will be a city bike with a front rack. Generally, what adjustments are made to standard road geometry to carry 15-20 lbs up front?

I've mostly made touring racks and they don't require anything, since the load can be moved for and aft since it is mounted laterally. With basket/porteur racks sometimes there are adjustment made in the fork reach. There was a lot of stuff on this when Kogswell did a Porteur bike and offered it with various forks. I think the basic principle is that the weight out front makes the fork want to overstear or flop to either side. This would be less pronouced to the extent that the axle was relatively further forward, adjusted for the new weight. That affects steering generally and I don't recall what was the verdict at Kogswell, you can check for yourself.

The whole discussion is actually pretty complex. If you want the whole story you'll probably need to find a bunch of back issues of Bicycle Quarterly. The French began using front racks in the 1930s and apparently learned, through decades of experience, what it takes to make it work.

The short version is that a 73 degree head angle with approximately 6 cm. of fork rake (that's not a typo) gives you the appropriate trail figure for stable-yet-light handling with a 5-10 pound front load. That's assuming 700c wheels and +/-25mm tires. Wider tires like less trail, which means more fork rake -- up to 7cm. As far as I know, it isn't possible to make a bike handle really nicely with a 15-20 pound load on a front rack, and low-mount front panniers may be a better option in that situation.

The whole discussion is actually pretty complex. If you want the whole story you'll probably need to find a bunch of back issues of Bicycle Quarterly. The French began using front racks in the 1930s and apparently learned, through decades of experience, what it takes to make it work.

The short version is that a 73 degree head angle with approximately 6 cm. of fork rake (that's not a typo) gives you the appropriate trail figure for stable-yet-light handling with a 5-10 pound front load. That's assuming 700c wheels and +/-25mm tires. Wider tires like less trail, which means more fork rake -- up to 7cm. As far as I know, it isn't possible to make a bike handle really nicely with a 15-20 pound load on a front rack, and low-mount front panniers may be a better option in that situation.

It's interesting that some of the modern sport-tourers which are commonly weighted in front are only being built with about 4.8 cm of rake on a 73 deg angle in an effort to keep them nimble feeling like consumers want.

In my experience this works ok for generally light loads (sub 5 lbs....we're talking a parka, some energy bars, maps, maybe a patch kit and levers in a handlebar bag), but gets squirrely with loads heavier than that.

6 cm. is the target I regard for absolutely sure stability. You can get away with hairs less, but it's a losing battle.