Holiday Cycling - strapping a bike box to roof bars with ratchet straps

Ive happily used 25mm straps on a tandem kayak on the roof for fairly long journeys. I always use more than needed though - I'm paranoid about having a strap disaster - but never had anything go wrong.  I'd thread through the handle of the case as one of the straps though - then it's not going anywhere. 

You'll be fine.  I've done this plenty of times.  As someone has said, you can get some neoprene pads to make absolutely sure it doesn't slide around causing damage to the hire car.  Incidentlally, I usually travel with a padded soft bag, further padding it up to the max weight limit of 23-30kg with clothes bags, etc.  My holiday bike is titanium, but no problems to date.  Must have done it 30-odd times.  Three to four Thule 25mm straps will do it.  I usually thread one over the top and through each door, and then two under the bike rack and over the other straps to make sure the bike bag is firmly attached.

I'm in holiday mode now  

Shouldn't be a problem. As others have said, 10cm is overkill. Halfords do 4cm straps for not much which should be fine. Just don't drop the buckle on the roof! Make sure that you tie it down in such a way that the straps don't become slacker it if moves a bit, and don't underestimate the damage that a vibrating strap can do.

One of these:

https://www.handiworld.com/handirack-194/handirack.html

Plan A involved a soft bike bag which could be folded in half.  That went into the boot of a hire estate car, luggage on top of it and the bike went on the roof using a seasucker bike rack that travelled in hand baggage.

However,  the soft bag was inadeqate in many regards and has been sacked to be replaced with a box and an inflatable rack for getting to and from the airport and the seasucker if needed  during the course of the hols.

Seasucker remains the best bike rack i've ever had (it's a long list) and ideal for travelling.

http://www.chainreactioncycles.com/seasucker-talon-15mm-bike-rack/rp-pro...

Seem to hard to get hold of at the moment though

Yep, it's no prob, but I'm with Last boy scout, 100mm is too wide adn cumbersome. I use some I have which are about 4cm (1 ton breaking strain) and they hold it on just fine. I use 2 straps, 3 m  long each. I always just check the straps each time I stop to make sure it hasn't shuffled around or something. 

Rope or string will do the job fine.

Edit - should have said "I've never needed more than 25mm".

You'll probably need 5m, rather than 3m long straps - the extra can be wrapped somewhere and tied off. Last set I bought were Typhoon ones, which have a nice little neoprene pad under the buckle to stop scratches.

You only need to tighten the straps enough to stop movement - you don't need to try your strength on them. Depending on the shape and how slippery the finish of the box is, you might need to be creative with routing the straps or add a 3rd strap to make sure the box can't slide out.

Just for info, I happened to go out to the garage and, out of interest, weighed my kayak - it's just shy of 19kg and I can't imagine your bike box is more than about 25kg.

I regularly strap a bicycle hard case to the roofbars, sometimes it even has a bicycle in it. Never a problem but just remember you can create extremely high tension with a ratchet strap so don't overtighten.

Firstly, you won't need 10cm wide ratchet cargo straps for that. I've spent years carrying assorted loads on car roofs, mainly surf, windsurf and kayaking kit, but occasionally other stuff, and I've needed more than 25mm cam-buckle straps. I do have a set of 50mm straps, but I bought them for strapping multiple kayaks on a trailer.

Have a look at the various soft racks, either foam padded or inflatable - they will be ideal for what you want and usually come complete with straps. They can be a bit fiddly to install, but better than nothing - the only real downside to them is that they can wick water into the car if you get a lot of rain, so take them off when you're not using them.

You can take your pick on eBay or most watersports shops.

I like a bodge as much as the next person but why not just stump the cash and get a proper bike rack for the rear. You might get problems with lifting/vibrations as the air passes around/under if there are any gaps or the box stands too proud.

That said if you really want to give it a go do some practise runs at lower speeds and build it up, if it works for you then who are we to say don't do it.

worst case it damages the vehicle and or comes off hitting another vehicle and all the fall out from that, you'd certainly get done by the police for not securing the load correctly.

Is all that worry worth it over £30-40?