Lightweight aluminum frame transported via trunk rack I've been riding steel for a while now and recently added a CAAD10 to the fleet. I am planning on taking it with me on our upcoming vacation. 

I have a Thule gateeway 2 (trunk rack) that I've been transporting the steel bikes on very happily. My question is, should I be concerned carrying the lightweight aluminum frame in such a manner?

I brought the bike home from the shop initially this way and I felt that I had it secured on there without placing too much undue pressure on the frame itself. However, that was a short 5-10 mile transport.

Update: I have found it and included here. Upon re-inspection, I see that it says "not recommended" for either, but "do not use" is for CF.

"hAnGeR RACk
On this type the bicycle hangs on two arms projecting aft from a trunk mounted rack: Not recommended for either carbon or aluminum bikes. Do not use on bikes with carbon fiber tubes. The point loading where the arms contact the underside of the tubes may crack the tubes. Additionally the lower part of the bike is difficult to secure, and it may move, causing chafing and structural wear. Additionally when two bikes are mounted on these racks the contact points between the bikes may be cracked or chafed, causing structural damage."

PDF here page labeled 101 (103 of PDF)
As far a aluminum goes, do this experiment. Get an empty soda can with no dents and stand on it. It will support your weight. Now put the tiniest dent in the side, notice how easily this happens. Stand on it and it collapses with a fraction of the weight it previously supported. 

How does this translate to bikes? A bike frame is built much much stronger than it needs to be to support the forces generated when riding. They are built to withstand the forces that will cause dents, the even a dent in aluminum can be catastrophic. 

When placing a bike on a rack such as yours, there is a chance it can be dented - probably not from straps being too tight, most likely from other bikes on the same rack banging into it. If you secure the bikes firmly, they are fine. You must use padding between them of some sort - "Dog Bones" are great (plastic spacers with a strap at each end), or foam.  If the bikes can swing around freely you damage them. 

I used this style rack for years with aluminum bikes, as do most of my mates. The main reason to discourage the use of such racks is every bike I carry has paint damage from rubbing against another bike where I was a little less careful than I could have been putting them on. I have seen derailleurs knocked off from them as well. Roof racks or rail based racks are much better at protecting the bike in transit and virtually ensure no forces are generated that will break a frame.