Pics?
What I'm envisioning is that for the front of the bot, from the ground up to the height of a stair, the first thing encountered would be treads. above the height of the stair, the chassis would be encountered.
At the same time, why is this a problem? Do you plan to mount a touch sensor (i.e. button) on the wall-hitting part of the chassis? Or are you planning on driving around blindly and hitting stairs by pure chance? o_O