when i put the road bike on the roof carrier, the front wheel sits nearly right over the arm mounting bracket, so its difficult to tighten the ratchet strap.
I cant just move the arm mount along the rail as this doent help, i think its just down to the geometry of my bike.
Or is there an adjustment i'm missing?
I had to move the arm mount on mine. It means it sits at a bit of angle on the roof bar as the arm is now on the 'bend' of the rail but it hasn't fallen off (yet).
Before this the front wheel was hanging over the front of the rail.
Not sure why this wouldn't help you?
There's usually a bit of adjustment around where on the down tube you clamp it, which can allow you to tweak the position of the wheel trays.
Ah just re read the op. Think I understand the 2 brackets are too close together? (one on top of the other). Hand this problem on the back bracket where the strap caught the roof bar. I managed to just shuffle the bracket along enough to miss the bar. If you can't do this can you cut the strap so that it's short enough not to catch but long enough to click the ratchet - guess you then couldn't use a MTB so maybe buy a second strap and swap them depending on bike?
sorry not a lot of help
db - moving the arm mount didnt help because the horizontal distance between the ideal downtube clamping spot and the front hub (which is directly above where the wheel tray strap tightens) remains constant even if i move the arm along the rail.
Unfortunately for my road bike the front hub (and therefore the ratchet strap position) is nearly directly above the arm mount on the tray of the carrier, so even if i move the arm mount i then have to move the wheel holder and it ends up in the same position.
Probably not the best explanation!
I just wondered if there was some other adjustment on the carrier i was missing. I'll try clamping in a different position on the down tube.
Does it matter if the wheel strap is a bit forward of the front hub? Doesnt look as secure to my eye when i do this but i'm not sure how much load these straps really take, i guess they are just to stop the bike lifting at the front or back and stopping it coming out of the try.
Edit - you beat me to it db!
I'm a bit baffled by how this is happening tbh. Got a picture of how you are installing it?
ok not my bike / car / photo, but might help to explain:
[img] 
[/img]
The road bike has less horizontal distance between the front hub and the downtube clamping position (where thule say the arm should be as close to 90 degrees to the downtube as possible) than the mountain bike in that pic. That means the front wheel tray is further back than in the picture, and on my bike it sits very close to where the mounting of the bottom of the arm. This bottom mount plate is actually quite bulky, so it makes tightening the strap difficult.
I just had an idea though, i might try a soft webbing strap, it should be easily strong enough and i can then move the buckle on it so it'll be easier to tighten and the soft floppy end wont interfere with the arm mounting plate
edit: i'm the OP not ed34, just used flatmates laptop as mine died and cant find the mains lead!
you are clamping the down tube right behind the chainrings at the seatpost/downtube junction?
I know it says 90deg but that's not going to work. This sometimes happens when people flip the carrier around to have 2 bikes pointing forwards so you cant put the clamp behing the chainrings where it is most secure from a theft point of view.
Other than that I would grab a fork mount carrier and take the wheel off. I had to do that for my carbon mtb as I didn't want to clamp the down tube.
i'll have another look see if i can clamp lower down, i think i tried this but frame bulges a bit more lower down due to large BB area (carbon frame) and theres also the exit 'bulge' where the internal cable routing for the front mech leaves the frame.