Le Mans motorcycle exhausts
Endurance racing has much to recommend it as a spectacle, and it is a pity that there aren’t more countries holding 24-hour races. I have been to all kinds of motorsport events, and there is little to compete with the atmosphere of racing taking place at dusk, through the night and into the dawn hours. Motorcycle endurance racing is singularly impressive, with the riders and their machines being brutally exposed to the elements and, in the event of an accident, to damage.
In the case of an endurance bike, the most exposed engine-related component is undoubtedly the exhaust system. As you may be able to see in the photo below, the exhaust system on this Suzuki has already suffered a mishap – the silencer has certainly seen much better days.
What we might find surprising though is that much of the exhaust system is titanium, as shown by the colour of the pipes, owing to the thin oxide films that form on the material when they operate at high temperatures. We might have expected it to be made from steel, stainless steel or perhaps a material such as Inconel. However, most motorcycle exhaust systems benefit from the fact that they are relatively short and are well supported. As can be seen from the picture, this system is typical of many motorcycle exhausts in that it is connected to the chassis via a bracket attached to the silencer.
There is something unusual about this particular exhaust though, in that the ‘lug’ on the silencer which attaches to the bracket is not a welded lug but is riveted to the exhaust can. The problem with having the bracket welded is that the weld geometry is far from ideal. The two sheets of titanium from which a welded bracket would be made, are folded with a 90º bend and welded around their periphery to the silencer. In doing so the design has effectively manufactured a lug with an inbuilt crack, as it is impossible (unless using diffusion bonding) to weld over the whole contact area.
Providing the rivets are of a suitable size, this should be a reliable method of attaching the lug. The rivets are in a relatively cool area; the silencer is of the perforated tube type, with the volume between the inner perforated tube and the wall of the silencer packed with fibrous material which acts to attenuate exhaust noise.
There are further advantages with this method compared to a welded joint. The design of the lug becomes completely free of the need to design it as a sheet metal component. It may be designed with any type of machined feature in order to reduce stress concentration to a minimum, and can be manufactured from any material. In the case of this exhaust, the lug has large machined radii where the ‘upright’ meets the flat plate which is riveted to the silencer wall. Additionally, with welded lugs, we are constrained to making the lug from the same type of material as the part to which it is welded and in a compatible alloy.
Fig. 1 - The SERT Suzuki endurance team has raced at Le Mans for more than 30 years; during that time it has helped to develop exhausts that can remain reliable for 24-hour racing (Courtesy of www.suzuki-racing.com)
Written by Wayne Ward