On 19 Mar 2012 22:49:37 GMT, John Doe wrote:

I want to put a spring under the heel of my inline skate. But
there is only a 1/2 inch clearance, and it needs to be a
heavy-duty spring. Should I consider using a spring steel flat bar
or similar, instead?

Thanks.
You could use belleville washers. They are a type of spring. See the
link:
http://en.wikipedia.org/wiki/Belleville_washer
Quite common and available from many sources.
Eric