The diameter is changed due to the reason of expansion.

• The iron tyre kept slightly smaller because when they heated up the tyre shape will be changed easily and can be fitted into the wooden wheel.
• Also there is a reason that in summers the iron things expand due to heat.
• The wooden tyre will not expand, so the iron tyre will remain fitted in it.
• The heating the iron tyre will make it to mount in any shape you want.