The frame geometry is designed to fit the tyre size we use as standard and we generally do not recommend using wider tyres as we cannot rule out problems with mudguards and frame.
Wider tyres can only fit if the mudguards are removed. The tyres we offer for Curt are 32 mm wide and by removing the mudguards it is possible to fit a maximum of 40 mm tyres.
For the Stout and Stellar models we would generally advise against using wider tyres - with 42 mm they are already quite big.
Please note that the dimensions of the tyres are not always the same, i.e. the nominal size of the manufacturer and the real size often differ. Even if wider tyres might fit at first glance, problems can quickly arise if the rim is out of true/uneven.
Thank you for your understanding.