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We have a few trips planned in the camper, with bikes.
We will of course put the bikes inside the van and throw a tarp over when required. The bikes are locked onto the rack at all times with more than the rack handle lock.
However I am wondering if a bike cover adds anything to things such as camping overnight (in the van!) or in areas I am less worried about thievery (such as Southern Ireland, Hebrides, Norfolk etc)
Or are they flappy waste of time because the scroates will nick anything without looking?
Something like:

Had one like that with our T5. Bought with the idea of protecting the bikes from road grime when driving but it was just a huge big sail - T5 punches too small a hole through the air to shield the cover from airflow (unlike bigger vans like above). Every little helps when it comes to anti-thievery, so I'd still use one now when parked up if bikes had to stay outside the van.
They shred quickly when driving as well. By all means have one for when on site but not when driving. Also lock bikes to van not rack obvs. Dont leave van when "outandabout" with bikes on rack,or just put them inside. I have 2 U-locks and a big chain (all heavy Kriptonite) when driving. On site lock them to one of the alloy wheels. Sounds a faff but quickly get into the drill.
I vaguely remember some saying some racks advise against covers as the add extra load when they catch in the wind while driving along
I use one on the back of my Trafic, to protect against the road grime that gets sucked up behind a big, flat-ended van. It's just a cheapo off Amazon and, though it's really big enough for two bikes, I wrap a couple of bungees round it to stop flapping. It works just fine.
Make sure you buy the right type for your rack - either to fit "wheel gutter" types or for the racks that hold the frame under the top tube.
I've bought a ground anchor for the bikes when camping - called a Portalock. Will chain to the alloy wheels too, but this is for when we aren't with the car. PS cheap on ebay these days.
Use here on a large coach built motorhome. Less about theft deterrent more rain protection. They are a faff to fit (4 bike rack here) I find the first use of the season takes me ages til I remember sweet spot of how it fits. Got a fiamma cover and for the money £50+ it’s still just a fairly thin sheet of material. Most also not really meant for 780 wide bars and long Mtb frames. I just get way with the 4 bike cover as two bikes are for 9 yos.
Gave up on using one on our motorhome. It was a PITA to get on, collected water ie large pool of water in the bottom of it, and only just managed to work its way around 2 x wide MTB bars. When the second (Fiamma) one ripped (a warranty replacement for the first one, which lasted 2 months), I gave up.
Had a taylormade 2 slit one for fiamma rack, (*edit, it was much thicker than the thin silver fiamma ones) impressed, still in great condition when 4 years old (bit faded but fine). Would do the same again.
Bikes locked together and to rack under cover, I used a very long kanulock (wire reinforced lockable tie down) that was in a sort of figure of 8 round the wheels , over the frames and thru the rack mounts on the outside, tightening this up pulled the cover round the bikes to stop flapping and was meant to act as a deterrent,
Never bothered on my old T5 with factory bike rack. One of the reasons is I couldn't reach!
You see some and they're just billowing about in the wind....surely that adds extra stress to the fixing points. And half the road grime comes from below anyhow.
Current van the bikes are on a tray inside.
Would often - and still do - lock the bikes outside with a tarp thrown over then with a lock going through the wheel/ladder. That only really happens if we're somewhere where lots of other riders are too, like bike parks.