HOW TO MEASURE THE Walkin` Wheels ? Fold of Flank measurement of a Dog for a Wheelchair To measure the height of a dog for our Walkin` Wheels®, measure from the ground to the "Fold of the Flank" as shown here. The Fold of the Flank (FOF) is a thin piece of skin that connects the dog`s upper rear leg to his body. Look at the webbing between your thumb and index finger. This skin is very similar to the fold of flank (FOF). * When measuring the fold of flank of your dogs leg, do not pull the leg tight. Leave a little natural bend. We do need the fold of flank (FOF) measurement to be as accurate as possible as that is how we determine the size of the wheels and struts your dog will need for his wheelchair. If your measurement is not accurate, you may need to exchange the wheels or struts at your expense. If you are unsure about how to measure, please call us before placing your order and we would be happy to assist you.