I've read some recent articles in dirt bike magazines that have said that trials tires are better than knobbies in many off-road riding situations. They recommended IRC TR-11 and Michelin trials tires, but said trials tires are not made larger than 4.00-18. Is this true? Are there any larger size trials tires? Do you have other recommendations besides IRC and Michelin? Where is a good source to purchase trials tires?