A few months ago I started this thread for comments on a homemade bike rack that I kludged together by bolting an additional tray onto the end of a Rocky Mount car top rack that was designed for factory aero bars
I ended up using this rack to transport the tandem from San Francisco back to Texas with no issues at all. I did follow one piece of advice and used tie-down straps to add extra security and rigidity which I hooked to the stoker seat rails and rear crossbar as shown here:
Essentially this saved me about $500 because to install a proper roof top rack would have required buying one of the standard Yakima, Thule, or Rocky Mount racks and then buying Yakima crossbars and attachments to mount to my existing side rails. Normally for riding around Texas I just roll the bike into the back of the minivan but that wasn't going to work for this cross country trip because I had the wife, 3 kids and luggage inside.
A few comments for anyone contemplating this.
1. This is probably only a viable solution for people with minivans, SUVs or other long roofed vehicles and factory racks. When I spread the factory crossbars on the Sienna to their farthest apart position they measure 51" from center to center which is MUCH greater distance between bars than the typical setup on a sedan or smaller car roof. That meant the rear wheel on the tandem was only 18" behind the rear crossbar so there wasn't a huge amount of flex. On a car with a shorter roof or setup with crossbars that are closer together the rear of the tandem is going to stick out several feet and the beefier frame of the Yakima or Rocky Mount racks is going to be more important. In fact I had the crossbars spread so far apart I had to drill new holes in the bottom of my Yakima rocket box to mount it on the other side because it was designed for crossbars that were about 1 ft closer together.
2. After observing and inspecting the stress points and flex with the setup I concluded that most of the flex actually came from the factory crossbars not the bike rack. I could flex the tandem from side to side fairly easily but it was mainly the crossbar flexing. The factory crossbars are wide aero bars that are pretty flat so they have a lot more flex than a standard Yakima crossbar.
3. Using the tie down straps is the key to making this setup work. I used a cheap pair of tie down straps I picked up at Wal-Mart and cinched each side down snugly and inspected them each time we got gas. This eliminated all of the flex and really locked the bike down tight onto the roof. I inspected the setup each time we got gas but it stayed tight and snug the whole way home. I would not want to use this rack for more than short trips around town without using the tie down straps. They are the key.
4. I spent some time poking around hardware and metal shops looking for some sort of aluminum channel that I could use to add strength to the bottom of the rack but ended up concluding it wasn't necessary as the flex was coming from the crossbars not the rack.
5. With a small 2-step folding step ladder I was easily able to load and unload the tandem myself with no trouble whatever. So I don't think the pivoting business on the more expensive racks is really necessary unless one is using a rack all the time. Then again, I have a pretty light higher-end tandem. An older heavier bike might be more of a problem.
6. One advantage to this setup over going the conventional route and installing Yakima crossbars and a dedicated tandem rack is that it is substantially lower. The Yakima crossbars mount a couple of inches higher than the factory crossbars and the Yakima and Rocky Mount tandem racks also add additional height. So my tandem was perhaps 4-6" lower than it would have been otherwise.
Bottom line, this solution seems to work great for those who have factory racks with crossbars that can be spread at least 50" or more apart and who happen to have bike racks that are designed for factory racks. I already had 3 of the Rocky Mount racks that are made for attaching to factory crossbars so my only investment was just buying an extra piece of rack channel material, some stainless steel bolts to attach them together, and the cheap tie down straps. I would probably not try to do something like this on a car with crossbars that are much closer together as you will run into more issues the farther the rear tire extends out beyond the rear crossbar. But for a Toyota Sienna it seems to work just fine.