The trouble with these interactions is that, unless there is irrefutable evidence to present, you end up in an idiotic conversation along the lines of:
“This rod goes soft and casts poorly when the temperature gets above 30°C”
“Eh? Don’t be ridiculous, the effect of temperature is negligible”
“No it’s not, I can feel it soften and my casting goes crap”
“Well I’ve never felt any fly rod soften with temperature”
“That’s because you’re not as sensitive a caster as me”
“My casting is more highly tuned and I notice these things, you don’t”
“But you can barely make 60ft and you tail consistently”
“Yeah – that’s because my rod’s gone soft as I was telling you”
One of the problems with dealing with such beliefs is that the person promoting them has some knowledge that all materials soften as temperature increases, so why shouldn’t their fishing rod? (All materials don’t actually soften, but that’s not for here). As far as a typical carbon fibre/resin composite goes they’re essentially right, it does soften with temperature – but by how much?
A piece of laboratory equipment I have is a Dynamic Mechanical Analyser (DMA). This essentially applies an oscillating force to a sample while simultaneously heating (or cooling) it. The most common set-up is to rest the sample on two supports and have the machine’s probe press down in the middle, this is called a three-point bend (because there’s 3 points of contact). By ensuring the probe flexes the sample within its elastic response region a measure of stiffness (along with other parameters) can be made upon every oscillation, typically set at 1Hz (1 per second). Thus by programming the equipment’s furnace to heat or cool, the material’s properties can be assessed over a wide range of temperatures.
Now a fly-fisher like me can’t have access to a DMA without sticking a piece of fishing rod in it at some time or other, so here are the results:
This chart shows stiffness vertically versus temperature along the horizontal axis. I chose to perform the test between -20°C and 50°C, the lower temperature being the coldest I could ever imagine anyone wanting to go fishing in (it was -5°C when Tracy and I fished the Dee last time, which wasn’t so bad) and the upper temperature being about the highest air temperature ever recorded. At first glance it appears that a significant softening has been measured, however it should be pointed out that the chart has been ‘zoomed in’. Without the zoom the results look like this:
A quick analysis of the data shows that over the extent of a 70°C temperature range the flexural stiffness changes by less than 5%, which equates to ~0.07% per °C. Now if you can detect that in your casting you truly are more highly tuned than me. Personally, I think the change in clothing I’d need to cope with the temperature extremes would have far more effect on my loops than the change in rod performance (not to mention the change in the line).