[Patrick]

If you do change to bigger tyres, make sure you change the gearing to make up for the larger circumference, other wise just putting big tyres straight on will over stress things. (extra weight and geared for a higher speed)
Maybe use BrianG's speed calculator to find out what gearing you'll need to get if you want the same speed. But remember the big tyres might be a bit slower, because of the extra weight.