Mudflaps: A Elementary Treatise on Their Design and Construction

After last weekend's very wet ride through rural and exurban Maryland, I thought it time to collect my thoughts on the subject of the seldom used, oft-misunderstood mudflap. Not merely a place to display lewd or political images on your semi truck or bike, they actually serve a purpose. Material choice, shape, and placement all affect how well they fulfill that purpose.

What is the purpose of a mudflap?

• Keep as much water off you and your bike as possible when riding on wet roads
• Keep water off the people riding behind you

The photo in this post by Jan Heine show how much of a difference a good mudflap can make in keeping your bike clean. But we are interested in going even further into the fluid dynamics that allow your feet to get wet.

Water leaves the tire tangent to the wheel in the direction of rotation. 

Another Jan Heine post demonstrates how much extra protection a longer fender provides.

But we are also interested other paths the water takes. When riding through standing water (anything deeper than 1/8"), the tire plows through the water sending it simultaneously back toward your feet and outward. Anyone who has watches a car go through a puddle at full speed will appreciate how much water can be moved and how far. A narrow mudflap will fail to stop the water that isn't thrown straight back. Result? Soaked feet.

I have also found that wide tires (more than 38mm) going through puddles throw up enough water into a fender that a significant amount leaks around the edges and onto your feet. So, the less water that is directed into the fender, the better.

Front fender wraps around the sides of the wheel, offering maximum protection when riding through standing water. (from book Rene Herse by Jan Heine)

So what is the ideal mudflap for very rainy rides? It should hang as close to ground as possible (less than an 1" away), it should be wide enough to provide protection from puddle tidal waves, and it should direct as much water as possible toward the ground and not up into the fender.

The result:

hangs low and wide and is attached behind the fender, not in front

Which can also be seen in countless classic photos of cyclists riding and racing on fully equipped bikes.

One of the lower-rung teams of the VCCA competes in the Coupe Herse in 1956. Lucien Detee leads Rene Delahaye. Marcel Pineau is in third position, apparently pushing a rider going through a  difficult patch.
(from book Rene Herse by Jan Heine)

I use 1/16" Neoprene on my fast bikes because it is more flexible in the wind while still providing good protection. On my utility bikes I use thicker 1/8" Neoprene.

Now that we've addressed the design and placement of the front mudflap we should note that the rear mudflap has different design considerations.

If you want to be courteous to the rider behind you, you would like to protect them from as much of the water off your back wheel as possible. This means a low hanging mudflap. Since the rear fender typically ends 12" off the ground, 12" of Neoprene flapping in the wind doesn't offer much protection. In this case, it is better to go with a stiffer material, usually thin, hard plastic. Perhaps a piece of a trashcan or something similar. It also doesn't need to be as wide because the following riders feet are considerably further back (roughly 3') than in the case of the front fender and your own feet.