Could be lack of electrolytes. Things like Sodium & Magnesium are the most important. Potassium and Calcium are lower order as electrolytes. If you feel tight after the ride, try walking a mile or two. I find that more effective than stretching, it's far safer and has the added benefit of a weight bearing exercise.
I used to have that happen all the time when I fenced saber, allways in my calfs and usualy in the morning, sometimes it would last pretty long and it would be hot to the touch. My mother had high blood pressure so there was never any salt in the house. After making myself eat salty food it went away. My feet will still cramp up sometimes though.
As Al Canoe says, could be magnesium deficiency. Many people are deficient, and magnesium takes a while to build up in your system. Youalso have to take calcium to aid its absorbtion. But better than taking suppliments to fix a potential problem, try brazil nuts - chock full of magnesium, selinum also, no side effects (such as stomach upset - many laxatives contain magnesium). Drink milk or eat cheese as well, though to get calcium.
I came across a Monique Ryan piece on the VeloNews site.
The Feedzone with Monique Ryan: The all-too-common cramping calves
By Monique Ryan, MS, RD
This report filed April 11, 2007
Unfortunately muscle cramping is an all too common problem, and several other VeloNews.com readers also asked about this same concern. Any cyclist who has suffered from a muscle cramp remembers it well, as these rapid muscle spasms are painful and inconvenient. Some cases of muscle cramping in cyclist have been more difficult to solve than others indicating that causes are likely multiple. Research data on cramping are limited, but cramping is more commonly seen during longer training efforts in the heat and is associated with overexertion. While this column addresses the nutritional aspects of muscle cramping, improper bicycle position that results in overuse of the calf muscles can be a cause- this would need to be addressed by a bike fit expert.
You have identified yourself as a salty sweater. Sweat does contain a number of electrolytes, including not only sodium, but also chloride, potassium and calcium. A quart of sweat (960 ml) can contain anywhere from 500 to 1,800 milligrams of sodium, 700 to 2,100 milligrams of chloride, 150 to 300 milligram of potassium, and 40 milligrams of calcium. Obviously the main electrolyte lost in sweat is salt or sodium chloride, though the amount of all these electrolyte losses can vary widely among individual cyclists. Sodium is an important consideration in the coming months as you will be training in the heat and humidity, and some cyclists will continue to have high sodium losses in their sweat, even when they are acclimatized to warmer weather.
While you mentioned that you are a salty sweater, you may also have a high sweat rate. This means that you could not have only high sodium losses, but high fluid losses as well. Make sure that you begin any training ride adequately hydrated. During rides, try to consume enough fluid from a sports drink to minimize your sweat losses. Make sure that you mix the sports drink at regular strength to ensure that the mix empties quickly from your stomach so that the fluid, carbohydrate, and sodium in the sports drink do reach your muscles as quickly as possible. Super concentrated drinks will empty from your stomach more slowly and compromise your efforts to hydrate, fuel, and replace the electrolytes lost in sweat. If you have a high sweat rate, your best drinking efforts are more likely to somewhat offset sweat losses, rather than match sweat losses. Losing more than to 2 lb. after a training ride, indicates that you have a high sweat rate and/or that you are not keeping up adequately with your sweat losses.
It is also important to pay attention to sodium intake when you are experiencing muscle cramping. Consume a higher sodium sports drink to increase your sodium intake. Other than scientific lab testing, there is no way to know just how much sodium you lose per hour, but consuming a higher sodium sports drinks in combination with increased drinking volume could effectively increase your sodium intake during training. Even higher sodium intakes can be reached with electrolyte tablets, but it is recommended that you experiment with the higher sodium sports drink first. If you do add in electrolyte tablets, make sure that you consume them with at least 8 ounces of fluid, and don't over consume these products.
You can also look at addressing some nutritional strategies in your daily diet. It maybe prudent to ensure that you consume higher sodium foods in your diet, (this is contraindicated if you have been advised by your physician to reduce sodium intake for health reasons such helping to manage high blood pressure). Foods that contain sodium include soup, pretzels, various cheeses, crackers, and yogurt. You can also salt food prior to training in the heat.
There has been some speculation that potassium losses can lead to muscle cramping, as indicated by your efforts to consume more high potassium foods such as bananas. Your training diet can easily provide ample amounts of potassium. Excellent sources include all fruit juices, fruits, and vegetables, with some of the best sources being grapefruit, orange, and pineapple juice: apricots, bananas, papayas; greens, carrots, potatoes, and tomatoes. Milk, yogurt, and dried peas and beans are also good sources of potassium. Though sodium is the more likely culprit, generous portions of high potassium foods also supply plenty of carbohydrate, and plenty of other vitamins and minerals. Many varieties of sports drinks provide potassium, as do some electrolyte tablets.
A lack of calcium is also blamed for some cases of muscle cramps, and increasing calcium intake has benefited some endurance athletes suffering from these spasms. Low calcium levels are more likely to play a role in muscle cramping for athletes competing in ultra-distance events. Calcium, the most abundant mineral in the body, plays a role in muscle contraction, and athletes suffering from muscle cramps should at least correct inadequate calcium intake. If nothing else, it is good for your bone mass. Good sources include milk, yogurt, and green leafy vegetable like bok choy, kale, and collard greens. Calcium fortified foods such as orange juice can also add to your calcium intake. A calcium supplement can also be added to the mix, if you fall short of 1,000 milligrams (over 1,200 milligrams for women) daily. You can also pay attention to magnesium intake, a mineral that plays an important role in muscle relaxation. Good sources of magnesium include nuts, seeds, legumes, green leafy vegetables, and unrefined grains.
Test out some of these strategies in training, first focusing on your hydration and sodium replacement strategies. The right plan for preventing cramping will depend on your medical history, usual diet, and own individual sweat, sodium, and other electrolyte losses.
My husband has major cramping issues after marathons. In addition to the things mentioned above, two other items that may help quinine (actually, I think it's the only medically proven aid for cramping, but I could be making that up - thought I heard it though), and someone else suggested taking a couple of Tums every day (probably related to the calcium issue mentioned above).