To avoid a mess in car interior, we've always put a large piece of carpet remnant on the floor.
When we transport tandem/single/luggage we put a blanket between the layers of 'cargo'. Being retired folks we don't have kids to take with us anymore (been there and done that). Having bike (externally) added to the vehicle it will reduce fuel efficieny. Did a 1,000 mile haul end of May from Tucson to northern Utah and got 36.2 mpg on the old Honda wagon.
With today's petrol prices (+/- $4 here) we consider that excellent. Of course we getting even better mileage on the tandem!
We all find out what works best for us!
Pedal on TWOgether!
Rudy and Kay/zonatndem

Time to eat my words!

Update: One Week Later -- I'm making some changes.

The rack has been working well enough. I've had the bike out four times on it now -- two different days of shooting the video, and two rides -- and, well...

- I shimmed up the Yakima High Roller. It was a little shakey in the original video because I was missing some spacers. But on Sunday's outing, we drove about 40 miles out to Mount Tam, including a stretch across the Richmond-San Rafael Bridge, with a pretty good crosswind. While I generally had confidence in the integrity of the rack, I was seeing a little too much movement as I peered at the bike through the sunroof.

Pondering this, I figure the bike is close to the 50 pound limit of the High Roller, and it's stretched out two feet further than a solo bike on this same rack. It's bound to react with some extra movement.

- So today, I went to my contingency plan: I bought a Thule Sidearm. Yeah, the whole rack. I dismantled the Sidearm, and installed it on one of the round steel tubes. It will serve to hold down the rear wheel while steadying the bike. I've done the installation and test fit, but I haven't driven it yet. However, shake tests on the car look great!

- Another "gotcha", the Rocky Mounts R4 wheel trays are not designed for 2.4" tires. Not a huge problem, but the tire stradles the tray rather than resting comfortably in it. And the Rocky Mounts SL wheel strap was on the short side -- I could only engage the ratchet two clicks. I should have purchased the SLX.

- So the Sidearm serves a second purpose: I'm going to slice its wheel tray into a 26" length, since it's compatible with tires up to 2.6" in width. I'll also use the included Thule wheel strap, which is longer than the Rocky Mounts SL. Really, I ought not need the wheel strap: the hooked arm will both stabilize the load and hold it down against the tray. But it's really such a minor task to cinch down the wheel strap, I'll likely keep using it.

- That means the Sidearm adds about $110 to the project, after subtracting out the $30 cost of the Rocky Mounts wheel tray and tire strap. (Sidearms retail for $150, but many retailers will knock 10% to 20% off that just for asking. There are even better deals on line.)

- Last item (important): The Sidearm "bearing" (it's a 2-piece nylon collar that the arm pivots on) is designed to fit around a 1" tube. The tubes I used are 1.125" (1-1/8"). Needless to say, a bit of effort was required on my part to sand down the nylon "bearing" halves to fit, and it was slow going, dusty, and imprecise work. Nonetheless, it worked out ok. But I suggest if you're following these instructions, you purchase (1) 1" tube and (1) 1.125" tube. I am absolutely positive it will be far easier to shim the rail clamp to fit down to a 1" tube, than to go through the hassle of sanding the nylon Sidearm bearing.

- The Sidearm bearing does have a couple of holes to be drilled into the steel tube to keep it from rotating. I haven't done that bit yet, and I haven't decided what I'll stick in those holes to secure them. I'll explain this with photos in a follow-up post.

Nevermind that I reshot parts of the video and had almost completed editing it prior to Sunday's drive & ride. I'll hopefully have the motivation to shoot it again, from scratch, to show the changes.

For now, here are a couple of pictures to look at.

http://www.booboodog.net/images/tandem-rack-20.jpg

http://www.booboodog.net/images/tandem-rack-21.jpg