What is that little metal plate flapping on tractor’s exhaust?

A lot of tractors, bulldozers and some trucks use exhaust flappers – small metal plates that can be seen and heard flapping on top of vertical exhaust pipes. They are more common in older equipment. Do you know what the function of these flappers is?

It’s hard to say who invented the exhaust flapper (flapper cap, if you will). Its first patents seem to have appeared around 1948, although there is no doubt that this mechanism was used before that. Different exhaust pipe cover designs were patented by many different inventors at different times, and it is likely that many farmers independently have thought of this thing. Admittedly, a not-so-elegant solution was also often used – many farmers used to put empty coffee cans on the exhaust pipes of their tractors.

It is a heavy metal cover that flaps on a simple hinge. It is helped by a small counterweight, which is only slightly lighter than the cap itself. Vertical exhaust pipes of tractors, bulldozers, some older trucks, loaders, and other machinery can be covered this way. Similar accessories are sometimes installed on horizontal exhaust pipes, but much less commonly.

That cap is opened by the pressure of the exhaust gasses – it is a rather simple mechanical device. As soon as the engine starts, the flapper starts doing its little dance and rises with each stronger exhalation of the engine. When the tractor stops working, the cover simply drops down, closing the pipe.

That flapper is actually just a cover that protects the exhaust pipe from rain, dust and debris. Tractors often have tall exhaust pipes and their openings are sometimes pointed straight up, which means that rain would fall directly into them if there was no cover.

Water can cause a variety of problems. First of all, it can lead to corrosion of the exhaust system. If an old tractor stays parked without a cover for some time, water can even reach its exhaust valves. Water is definitely not desirable there, especially if it has a chance to accumulate and do its damage for a long time.

In addition, the exhaust system of a parked tractor may prove to be an attractive nesting place for some birds or, more likely, insects. Remember that after a period of intense operation some farm tractors can stand still for several weeks or even months. All kinds of creatures living in the exhaust pipe can completely clog the system, which could later prevent a tractor from starting. It would just require some extra maintenance.

Exhaust flappers can also protect against falling debris, such as clumps of soil flung up by the tires. In the case of excavators and bulldozers, that debris can be much more destructive – pieces of concrete or wood. Whatever falls in there, it will likely need to be removed manually, which can cause significant down time.

So why don’t all tractors use such flapping exhaust caps? Well, newer machines often have better corrosion-resistant exhaust systems. The angled tip of the exhaust pipe can protect against rain. Uninvited insects and birds are generally an uncommon problem to begin with, especially when machines are used more often or are kept indoors. In addition, that sooty cap is simply not to everyone’s liking, and it does not always work as intended – a wasp can slip through even a very small gap.

Those flapping caps are definitely still used today. Some plastic spacers can make them quiet (if the noise bothers someone at all), which prevents flappers from being percussion instruments. Some people like using them, while others hate their look. Exhaust flappers do serve a purpose and are still used today.