Exhaust Pipes and Sadness


Exhaust Pipes and Sadness 

Gavin Liggett ponders the aesthetics and nuances of a finely tuned motorcycle exhaust as he laments the passing of an old friend.


Motorbikes need exhaust pipes for many practical reasons. However, an exhaust on a motorbike is different. It has the vital task of producing the one thing that gives a motorbike all its character. NOISE!

A motorbike has to sound like a motorbike, whether it is the howl of the multi-cylinder sport bike, or the thump, thump of a single. With few exceptions, motorcyclists will tolerate all the disadvantages that sometimes go with an exhaust pipe that sounds right. A bike that sounds right will be able to shake off any criticism relating to its other features. It’s a biking rule of thumb.

The sound of a motorbike is in many instances very distinctive. In the early nineties Harley Davidson, no doubt feeling threatened by the invasion of Japanese motorbikes in the Harley Davidson style, attempted to secure trademark rights to the "potato potato potato " sound that their bike exhausts make. They claimed it was a symbol associated only with Harley Davidsons. The courts, probably in an attempt to avoid a "poTHAto poTHAto" versus a "potato potato" argument, dragged their heels and after six years in the courts, Harley Davidson threw in the towel. Exhaust notes are important!

Good specialist exhaust makers are few and far between. They practice an art that combines an intimate knowledge of the physics of gas flow, the science of resonance, and the art of forming metal plate and pipe into complex compound shapes. To build an efficient exhaust, the exhaust maker must take into account the critical dimensions of the engine like bore size, stroke length, valve diameter, carburettor design, camshaft timing and the operating rev range. These parameters will determine the initial primary pipe dimensions. The product of their labour has to be pleasing aesthetically. Aesthetics and the constraints of physics are mutually exclusive. An exhaust that sounds right but looks wrong won’t cut it. It has to look mean and business like. Every biker knows a good-looking exhaust when he sees one. Every biker knows an exhaust that sounds right when he hears one.

Motorbike manufacturers spend a lot of time and vast amounts of money developing exhausts to suit and match their products. They have the very demanding task of designing an exhaust that performs well under a wide range of conditions. Rather than make a lot of noise, they have to conform to stringent government laws that regulate noise levels, emissions of carbon monoxide and a host of safety regulations. The exhaust has to perform well over a wide range of conditions and cater for the average rider riding on the average road with average ability. They have to do all this and make the exhaust look right. Despite their vast resources, they face an impossible task. As a result, original equipment exhausts are very average creations in most respects. Very average creations that, although bristling with technology, are with few exceptions and for various reasons, the first thing a motorcyclist swops on his new steed.

Every exhaust maker knows that the original equipment exhaust is a hard act to follow. To make an exhaust that is better than the original in every respect is extremely difficult if not impossible.

Exhaust makers owe their existence to their ability to cater for the specialist needs of motorbike owners that fall outside the average category. A racing machine has no need for an exhaust that performs well at town speeds. The converse is true of the commuting scooter that has no requirement for an exhaust that delivers maximum power at full throttle. Like every design, exhausts are a series of compromises. An exhaust that extracts the maximum power from an engine at maximum speed will perform dismally at the legal speed limit. Every specific motorcycling application provides a new set of challenges.

Internal combustion engines are nothing more than pumps, pumps that pump air in through the carburettor and out through the exhaust. A well-designed exhaust enhances the flow of air through the combustion chambers by taking advantage of the sonic pressure waves that are a product of the violent combustion taking place in the engine. These waves usually travel in the opposite direction to the airflow. By manipulating the exhaust shape and length, the exhaust maker will endeavour to get the pressure wave that travels at the speed of sound and the airflow, travelling at a few hundred feet per second, to synchronize and pull or scavenge the spent gases from the combustion chamber thereby making the pumping action more efficient.

The pressure wave travels between the exhaust valve and the far end of the exhaust pipe, and this wave is interrupted by the exhaust valve rapidly opening and closing hundreds of times a second. It is also affected by the piston travelling up and down the bore thousands of times a minute. The speed at which this sonic wave travels is also dramatically affected by the varying temperature of the air within the exhaust pipe, the higher the temperature the faster the speed of the pressure wave. The air temperature in an exhaust fluctuates tremendously. One can now begin to understand the complex task faced by someone building an exhaust that works. A well-designed exhaust can increase the output of an engine by as much as twenty five percent in a given rev range. That is one good reason to swop exhausts.

Design parameters obviously influence exhaust shapes. An exhaust designed for racing not only has to produce maximum power at high engine speeds but it also has to be designed in such a way that the motorbike can be leaned over at extreme angles, allowing the rider to negotiate corners at the best possible speed. A poorly designed exhaust that touches the track when cornering can at worst lift the rear wheel off the ground resulting in loss of traction and a crash and at best, it will reduce lap times and make the motorbike uncompetitive. In racing, weight is an all important factor and exhaust systems have to be built with this in mind.

Designers will resort to incorporating exotic materials like aluminium, carbon fibre and titanium in their design to save weight.

High revving multi cylinder engines usually work best with an exhaust that collects all the individual exhausts into one siamesed outlet a short distance from where they exit the cylinder head. This allows the exhaust maker to take advantage of the flow of the exhaust gas and pressure wave from one cylinder to aid in the scavenging of the next cylinder. The more cylinders involved the more complex the calculations around pipe length and diameter are. Each header pipe has to be of equal length for maximum efficiency. Given the restraints imposed by the frame, body work and ground clearance, this further complicates matters to the point that sometimes collecting all the pipes into a common collector is not physically possible and the task of designing the exhaust is more demanding. Many factors play a role in the combination of compromises that is the final product. Exhausts can win and lose championships. Every racer knows that.

With the development of motorbikes, many exhaust designs have become synonymous with particular marques. Most motorcyclists will recognize the slim upswept design of a Norton Commando exhaust or the peashooter like pipes that grace a Triumph Bonneville. Indian and Velocette, as well as the bike Captain America, Peter Fonda, mounts in Easy Rider, adopted the fishtail tailpipe and made it their own. Easily recognizable are the long gently curved header pipes of the Vincent Black Shadow as are the four separate exhausts of the Honda 750 Four. Many exhaust makers have produced exhausts that have gone on to become as unforgettable and Dunstall is one of them. His creations graced many race-winning motorbikes from Norton in the sixties to Suzuki in the eighties. Motorbikes and their exhausts are in many instances as inseparable as a Mercedes Benz and its bonnet emblem.

I have been fortunate enough to know someone who built exhausts.

He built me exhausts that transformed very ordinary motorbikes because suddenly they sounded right and the exhausts looked right too. He created shiny works of art delicately bent and shaped, the welds almost invisible: lighter, better and more handsome than the originals. Exhaust built from a recipe that contained all the exhaust maker’s secrets down to the last pinch.

I will have to find someone else to build me exhausts.

Brett Clark died yesterday on my birthday. He died doing something that he had a passion for; motorcycling.

His motorcycling exploits on his "tin" legs are legendary. Brett lost his legs in a motorcar accident as a young man but that didn’t deter him from riding bikes and building exhaust pipes. I will remember him every time I start one of my motorbikes and hear the exhaust note or run a rag over the shiny glow of the stainless steel.

You go well, Brettski.


Brett Clark Exhaust
Brett Clark custom exhaust bolted onto a Honda CBX six cylinder 1000cc

(photo courtesy Gavin Liggett) 


Gavin Liggett
Cape Town
South Africa
Jan 2013