I can get 2 in, mounted on a homemade slide out. It’s a GC680 but I think the space under the bed is similar for the GC600. They are a medium 29er Specialised Stumpjumper and a M/L Trek Remedy, the Stumpy has 780mm bars and the Remedys’ are 800mm. The fork mounts are Rockymounts Droptop. The rear axles mount through a Rose XL dummy axle which just sit in a slot at the back of the slideout. The strap up to the stem of the Remedy compresses the fork by 20mm so that it fits. That’s less than the sag when the bike is being ridden.
The slide out rails are 53mm high so the cupboard doors still open above and they are rated to 125kg or 50kg point load at full extension.
I remade the central divider with a small extension into the cab so that the derailleurs fit and to allow a longer slacker bike to fit in future. They derailleurs did fit without this but had to be held forward.
I’m pretty certain it would be possible to fit larger bikes. The most restrictive dimensions were the handlebars, which still have a bit of spare room either side and the wheelbase which can just extend ever so slightly into the cab.
All the wheels fit under there too, 2 on the drivers side, 1 in the middle and 1 on the passenger side.
The wheel bags are homemade. each one has a couple of internal pockets either side of the brake discs that hold a bit of cut down foam pool noodle and keep the discs away from anything that might bend them.
I hope that’s useful..
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