Converting a 30-inch mountain bike (MTB) to 28 inches is not a common or straightforward task as wheel sizes are typically fixed for a specific frame size. To achieve this conversion, you would need to replace the entire wheelset, including the rims, tires, and tubes, with 28-inch components that are compatible with your frame and brakes. It is important to consult with a bike mechanic or specialist to ensure that the new wheelset will fit properly and function safely on your MTB. Additionally, consider the potential impact on the bike's geometry and handling before proceeding with the conversion. **

Bicycle tires come in a variety of sizes, but the most common sizes for adult bikes are 26 inches and 700c. Mountain bikes typically have 26-inch tires, while road bikes often have 700c tires. However, there are also other sizes such as 27.5 inches and 29 inches for mountain bikes, and 650c for some road bikes. It's important to check the specific tire size of a bicycle before purchasing new tires or tubes. **

The bicycle is 26 inches. **

The cost of a bicycle helmet with 30 varies depending on the brand, features, and quality of the helmet. Generally, a good quality bicycle helmet with 30 can range from $30 to $150. It is important to consider the safety standards, fit, and comfort when purchasing a bicycle helmet, rather than just focusing on the price. **

Converting a 30-inch mountain bike to 28 inches is not a common or straightforward task. The wheel size of a bike is a critical component that affects the overall geometry and performance of the bike. To convert a 30-inch mountain bike to 28 inches, you would likely need to replace the entire wheelset, which includes the rims, tires, and tubes. This can be a complex and costly process, and it may be more practical to simply purchase a new bike with the desired wheel size. **

The jeans size for a man with a waist of 27 inches and an inseam of 30 inches would typically be a size 27 waist and 30 inseam. However, it's important to note that sizing can vary between brands, so it's always best to refer to the specific brand's sizing chart for the most accurate fit. **

The size of a bicycle can vary depending on the type and model. However, a standard adult bicycle typically has a wheel diameter of 26 inches. This measurement is taken from the outer edge of one tire to the outer edge of the opposite tire. Keep in mind that there are also bicycles with different wheel sizes, such as 24 inches for smaller adults or 29 inches for mountain bikes. **

I do not have a preference between 11 inches and 129 inches as I am an AI and do not have physical form or size. The choice between the two would depend on the context or purpose for which the measurement is being considered. **