"whoyakidding" wrote in message
...
On Wed, 11 Jul 2012 16:00:07 -0700 (PDT), Bob La Londe

Having a mismatched pair makes it difficult to specify the correct
matched-pair replacements. To obtain replacement springs for a
mismatched pair, you can either specify the same odd pair, try to
calculate the equivalent matched pair sizes, or (this is the best
method measure an accurate door weight and calculate the right
spring size(s) "from scratch". The spring seller should be able to do
the calculations from your accurate measurements of weight, height,
and drum size; or you can attempt the calculations yourself using my
engineering formulas below. "

Thanks. I have installed a few OH doors over the years, but they all either
had a single spring or a matched pair. I can see the thinking behind having
mismatched springs for over all load capability and so they can carry fewer
different springs in their service truck, but it still seems like the stress
on the shorter spring would be greater over time.

I've done low head room rear frame springs. I've down high head room jack
shaft setups with a "normal" top axle (usually solid for this application),
and I have done regular hollow shaft normal height top spring installations.
I've done some really heavy insulated commercial doors too. All had either
single spring for light hollow doors or two matched springs for heavier
doors.