I always find that many people have a fear of riding in the rain. What would you do if you are caught out in the rain while riding or are forced to ride through rain for any one of many reasons? What can you do?
You need to practice. I find when teaching that riding in the rain is one of a rider's biggest fears. One of the reasons I don't mind teaching the BRC or ERC in the rain is that it really helps the students out so much more than when learning in sunny weather. They learn right at the start that riding in the rain can be controlled. So find that parking lot, and get to work! (On a rainy day of course!) When you first start to practice in the lot there should be little to no traffic but it will still help you rebuild your confidence. As you get more comfortable, work up to residential areas where you will have some traffic and so on.
Now you may be saying: "Yeah, sure, but the feeling I have is that wet surfaces don't give any warning. One moment you've got grip, the next it's gone. How then, can you practice ...?"
You must realize that cars can hydroplane much more easily than a bike with good tires. The rounded profile and carefully designed tread patterns on modern motorcycle tires really limit hydroplaning- you would be down in a second if they didn't. Car tires with lots of the right kind of tread will resist hydroplaning, but as the tread gets worn, cars can hydroplane pretty easily. Being on the verge of hydroplaning creates the feeling that the vehicle isn't "planted" anymore whether in a car or on your ride. When this phenomenon happens, fear can rear its ugly head.
You can ask just about anyone that has crashed on a motorcycle about fear. What ever caused them to go down, sharp right hand curve in a downhill, decreasing radius turns etc, fear will cause them to be uneasy for a while when encountering a similar situation again. Just hang in there, practice, and practice and you will be fine.
First and foremost you have to get past the fear. It takes over your brain and does not allow one to make proper decisions and take corrective actions. I ride in the dirt as well as on the street and it can be firm, loose, slippery etc. You have to ride the bike, look ahead, think of what you want to do and do it all without fear. If you are scared you are not doing the many things that riding the bike requires.
Typically a bike will hold traction much better than most of us can ride it! Even in the rain!! You need to trust your tires. Try using your rear brake when it's wet to test how much traction you have available. Keep in mind that your mentality can be your worst enemy. You should be relaxed and be as smooth as you can on the throttle and brakes. Trust is a learned behavior so if you acquire some experience with the available traction in the wet then your trust of your tires (and your bike) will improve.
When riding in the wet, explore available traction with the rear wheel primarily, and only with the bike straight upright. You can test the rear brake to its limit as well as perform acceleration tests to investigate the limits of wet traction you have (in a controlled environment of course). I was impressed one time when I was able to loft the front wheel in the wet, not that I was trying... but it's just a testament to the grip of good tires these days. Of course I have spun up the rear as well when assessing traction in the wet, but this is all part of the learning experience. This is where you will build confidence in how much traction is there. Good tire tread depth is also very important for those of us who ride in the rain!
Other than working with some trainers and performing some practice there is not much else I can advise you to do. Keep in min though when you are out on the road and it's raining, don't let the drivers behind you "push" you out of a comfortable range of speed. I'd rather be slow and safe than appease the person behind me by speeding up.
I hope this helps.
