All this while, I’ve been pretty intrigued by the unique triangular pattern on the frame of my KTM Duke 125. I always wondered why it was shaped that particular way and the engineering rationale behind it.
So after some research on motorcycle frame and chassis design, I learnt that there are actually only 4 frame types used on most production motorcycles. Here’s a little bit about each frame type and the bikes that use them.
The cradle frame consists of a single top tube and a down tube (two down tubes for a double cradle) that runs from the steering head to underneath the engine. The tube loops round the engine, then goes back up to the swingarm pivot.
A single cradle frame has one down tube from the steering head while a double cradle has two, supporting the engine on either side. A variation of the single cradle, known as the split single, has two small tubes at the bottom to support the engine and exhaust pipes.
Cradle frames are the simplest of all the frames and are strong and cheap to make and are usually found in off-road motorcycles.
The trellis frame consists of several short straight steel or aluminium tubes welded together into a series of ‘triangles’. These series of triangles give the frame its strength and stiffness.
The engine that bolts below the frame acts as a stressed member (meaning the engine bears stress like the other tubes and is considered part of the frame) further increases the frame rigidity.
The advantage of the trellis frame is that it is lighter than the cradle due to the use of the engine performing structural duties. However, it is more expensive to manufacture than a cradle frame. The engine also needs to be reinforced as a result.
Bikes that use the trellis frames in their chassis design are the Ducati 848 Street Fighter and KTM 690 SMR.
Like the name suggests, the spine frame comprises a large diameter tube which acts as the ‘spine’ of the bike, upon which the engine and other components are hung.
The advantage of this frame type is that it is easily concealable, thus allowing for flexibility in the design of the bike. Notice the frame is not visible underneath the engine on the Triumph Thunderbird.
Beam (aka Perimeter)
The beam or perimeter frame was made possible with advances in material technology. As aluminium alloy use became more widespread in frame materials, manufacturers exploited the stiffness of the material and created the beam frame.
The beam frame consists of two ‘box section beams’ joining the steering head with the swing arm in the shortest possible length for maximum rigidity and stiffness. The engine then fits into the frame as a stressed member.
Beam frames have the advantage of being very light and strong at the same time. They are now the most popular frame design for sport bikes, being used on the Aprilia Tuono V4R and the Buell Lightning XB12.
The Future – Frameless?
The steering head transitions into a box section that connects directly to the engine. At the rear, the configuration is similar, with the swingarm and seat also connecting directly to the engine. This frameless configuration was used on the MotoGP bike raced by Casey Stoner to 4th place in the 2009 World Championships.
Do you have any comments or feedback for my article? Most welcome below!
This ‘Type 2’ was developed in collaboration with 2012 French Supermono Champion, Jean-Claude Paul with the sole objective of defending his title in 2013.
What’s new about this second generation you ask? How about a self-supported aluminium rear that also doubles as a fuel tank? Pretty innovative if you ask me. And of course race-proven Akrapovic exhaust completes the set-up.I’ll ask the Mototech guys for more details and post again if I get any news here! Comments and feedback below!
Engine torque calculation
The torque that an engine develops is a result of the torque exerted by the piston on the crankshaft. (For multiple pistons, remember to add the torque for each piston).
Picture 1 below shows how the downward force of the piston is translated into a torque ‘force’ on the crankshaft via the connecting rod.
We can see that torque and piston are related in someway, but how so? Like this:
Torque = Piston Force x Crank pin offset — (Picture 2)
Still with me so far? Good.
So how do we calculate this Piston Force and Crank Pin offset?
Piston Force = Pressure on piston face x piston face area = P x π x (Bore/2)2
Crank Pin offset = Stroke / 2
See the Picture 3 below for a clearer explanation
So back to the equation
Torque = [Piston Force] x [Crank pin offset] = [P x π x (Bore/2)2 ] x [Stroke / 2]
This is the equation I need to prove that Torque (and hence horsepower) is independent of Bore and Stroke.
Lets go back to the table.
Since pressure is constant in both these engines, we can leave it as P. Plugging in the bore and stroke values into the above equation:
Torque of Engine 1 = P x [π x (58/2)2 x 47.3 / 2] — [formula for capacity]
= P x 125 / 2
Torque of Engine 2 = P x [π x (52.37/2)2 x 58 / 2] — [formula for capacity]
= P x 125 / 2
So there you have it, a mathematical proof that torque is independent of Bore and Stroke, and hence has no effect on horsepower.
I hope you’re still with me, and that you benefited from my learning. More to come next time.
If I made any mistakes, comments and critique are most welcome below!