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...on engine/car etc
sounds like a silly question but it needs clearing up !
Our bike rack is a Thule tow bar bike rack one of these
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when loaded with 4 bikes including the rack must be a fair old weight (2 isla bikes 2 adult hybrids) on the tow bar/nose weight therefore adding strain to the engine - do you think its more/less than towing a very lightweight caravan ?
or am I complete plonka in thinking this !
Alot, lot less with the bike rack. The set up will be lighter and more aerodynamic. The pulling force due to the aerodynamic drag on the caravan at 60mph will be significant. Bikes are light, caravan's not so. Bikes are quite small and have lots of holes in them, caravans not so much.
The only real concern with the bikes on a rack is the tow ball weight as it will lift the nose of your car which makes handling dodgy and will affect the aero by increasing drag. I would think from your description you will be fine, 4 big DH bikes might be something to worry about though.
Pure weight wise in terms of engine power a caravan will be much worse and there will be lots of drag and side wind loading.
There will be some drag from the bikes but not that much (unless you have a 1 litre or something).