Isn't a BH 90 a bit large for a 3130 ???
Welcome to TBN. Maybe you didn't notice, but this discussion was ten years ago, when the
L3130 was a current model. At that time, the BH90 was the only factory subframe backhoe available and it could be added to an
L3130. The BH90 was a heavy backhoe, but the
L3130 chassis was large enough to handle it, mostly because the subframe design puts most of the loading and stresses from the backhoe to the rear stabilizers and the FEL and not so much to the tractor itself. The tractor does provide hydraulic power for the hoe and carries it around, but when you're using the hoe, the tractor wheels are generally off the ground or just barely touching.
The subframe installation is considerably more robust than the 3 point type backhoes, particularly on a compact type tractor.
I don't think the BH90 is still available new from Kubota, but there are some excellent aftermarket models, if you're looking. My favorite is Bradco, but Woods and some others also have good ones.