I'd avoid tailgate racks like the plague. Too many chances of damaging car, bikes or both.

Towbar is possibility but as you note it'll cost you on a Beemer (or anything else). Also consider the noseweight limit for towing. If it's low a family set of mountan bikes might be marginal. A lot of tow bar jobbies still seem to leave bikes wobbling and rubbing agains each other as well, plus you cannot get to the boot.

My suggestion is to put them on the roof. A pair of transverse bars and a set of mid range Thule bike racks, the sort that clamp the bike's downtub, will be less than a BMW approved towbar. If you're a cheapskate Halfords value racks are £20 or so each. Glitch may be a low weight limit for roof. Four 12kg bikes plus the racks etc won't leave much change form 60kg.

There's a knack to lifting the bike on to the roof and you need to set the rack clamp and open it first. Much, much easier though than faffing about with a cat's cradle of straps on a tailgate fit.

And you can still get in the boot.

Edited by Bromptonaut on 17/11/2013 at 18:09

I'd agree with Brom, put them on the roof. Hanging 60+Kg off the back end of any car (possibly in addition to weight in the boot) seriously affects the cars stability by making it tail heavy and nose light. The weight is further rearward than would be the case with a tow bar and trailer, and most cars would have a max ball weight of only about 40 anyway. Not to mention you'd probably still need a light board.