I've got about 8 rods stored in my garage right now, mostly leaning against the wall in a corner. A couple of them have started to develop a slight curve, especially my older Shakespeare spinning rod. Should I be using a wall mount, rod holder, or keeping them horizontal on a rack instead?
Leaning them against a wall is basically asking for trouble, especially with older rods that have already started curving. I've got my collection stored several different ways depending on the rod, and the ones that are clamped vertically in a rod holder or mounted on the wall with proper supports stay straight year after year. The key is that you need support at multiple points - not just at the base or the handle. A good wall mount with cradles every couple feet prevents the rod from developing that permanent bow that your Shakespeare is getting now.
Horizontal storage on a rack actually works really well if you've got the space. The rod sits flat and supported along its entire length, which is ideal for preventing warping. You can pick up a basic horizontal rod rack fairly cheap, or even DIY one with some PVC pipe and wood. Just make sure whatever you use doesn't pinch the rod or put pressure on the guides and reel seat. If you go vertical with a wall mount, look for one with cushioned cradles that holds the rod loosely - tight clamps can cause stress fractures over time.
Temperature swings in your garage are also worth thinking about. If you're getting big temperature shifts from season to season, that causes expansion and contraction that can warp rods. Keep them away from heaters, direct sunlight through windows, and damp areas. Given that you've already got some curve developing in that older rod, you might be able to straighten it slightly by storing it horizontally under light weight for a while, though a permanent bend usually stays. For the rest of your collection, get them off that wall corner now and into either a horizontal rack or a proper vertical holder.
Log into answer.
