switching bikes on a turbo trainer

steve68

I have a turbo trainer (tacx sirius) which I use with my road bike, with 700c wheels and 23mm tyres. My wife would like to be able to put her own bike on to the turbo. The tyre dimensions for her bike are 40-622 (700x38c). However when I put her bike into the turbo, the rear wheel is in contact with the roller before I press the lever down to bring the roller up to the wheel. How can I adjust this?