Motorcycle Frame (Courtesy: TVS)

Motorcycle Frame: Types, Design & Construction


What Is a Motorcycle Frame made of?

Firstly, let’s know what a motorcycle frame is made of. The material is a very important quality of the frame. The frame’s rigidity, cost, and capability, all these qualities depend upon its material. Conventionally, manufacturers use steel for budget-oriented motorcycles such as commuter bikes & mopeds. These frames make use of steel tubes. The tubes are bent or welded together to suit a specific purpose/design.

Steel & Aluminum Alloy

Motorcycles with low to moderate performance parameters use steel which is reasonably strong, cost-effective, and appropriate for its frames. However, modern motorcycles need their chassis to be stiffer, more lightweight. Besides, they should also look aesthetically better than traditional steel tubes. Hence, modern motorcycles make use of aluminum and alloys for the purpose. Aluminum and alloys deliver high performance to most of the mainstream motorcycles.

Exotic Materials in Motorcycle Frame

However, some high-performance sports bikes need even lighter and stiffer materials. For such motorcycle frames, manufacturers use exotic and expensive materials such as carbon-fiber, magnesium, and titanium. Some manufacturers also make use of composites made of these materials, and also newer materials which are even more costly. Such expensive materials improve the performance of extreme racing machines, like the ones in MotoGP. However, they have limited use in mainstream street bikes. For most general-purpose motorcycles, the performance of an aluminum or alloy chassis is more than satisfactory.

Following are the types of Motorcycle Frames by construction

Just like tires, different motorcycles need different types of frames. The motorcycle frames are categorized depending upon their construction and application. Various performance parameters of a motorcycle need different frame designs.

1. Backbone Frame

This one is one of the most basic and economical types of motorcycle frames. As the name suggests, this one resembles a spine. However, it is not a complete skeleton and keeps different motorcycle parts in place.

In this design, the amount of steel used is lesser than other types of motorcycle frames. Thus, it makes the construction of this type of frame very economical. In this design, the engine is bolted on to the backbone frame. Generally, the engine hangs off the backbone frame. It is neither cradled nor does it contribute as a stressed member.

The backbone type of motorcycle frame is very economical to produce. However, it lacks very good strength or torsional rigidity. It is suitable mainly for low-cost budget motorcycles with a low capacity engine.

Honda CB92 Benly & Honda Hornet made use of the backbone type frame.

2. Single Cradle Frame

This frame design is a very basic type, simple and cost-effective. There is only one down-tube in this design. This type looks more like the frame of a bicycle. It includes steel tubes of various sizes and strength capacities welded together. It forms an assembly that holds together various motorcycle components. Manufacturers often refer to these motorcycle frames as single cradle frames because they use single down-tube frames.

TVS Radeon
TVS Radeon

The factor that decides if it’s a cradle-type frame or not mainly depends upon the engine on the motorcycle is a stressed member or not. In some cases, the engine acts as a member of a chassis and bears the stress. However, in some other cases, it doesn’t. Mostly, manufacturers specify if the engine is a stressed member of the chassis or not in case of a single down-tube type chassis frame.

Honda CG125 aka Honda Stunner and TVS Radeon, Victor and Royal Enfield classic 500 use this type of frame.

3. Double Cradle Motorcycle Frame

Manufacturers also call it a double-down-tube chassis frame. A single cradle frame has just one steel tube going down to support the engine. However, a double-cradle frame has two tubes going down to support the engine. A double-cradle frame has a significant advantage over single cradle frames with regards to strength and rigidity. However, they don’t cost very high compared to single cradle frames. The double-cradle frame can handle the forces well during high lean angle and heavy braking much better than a single-cradle frame.

RE Continental GT 650
RE Continental GT 650

Furthermore, Indian manufacturers use this variety of frames in large quantities. This frame type offers strength and rigidity. However, it is still not the best suited for performance-oriented bikes and is a relatively old design.

Suzuki TS50ER and Royal Enfield Continental GT650 use this type of frame.

4. Perimeter Frame

It is the most commonly used frame types among the performance sports bikes. The perimeter frame is also known as the Twin-Spar frame.  It is one of the most popular choices for performance sports bikes. This frame type is established on research from motorcycle racing. It suggests that a bike’s rigidity improves significantly if you connect its steering head to the swing-arm in the shortest distance possible.

Perimeter Frame
Perimeter Frame

The beams join the steering head with the swing-arm. They need to be lighter without compromising stiffness. The two beams surround the engine as they stretch out for the swing-arm pivot. Earlier twin-spar frames were made of steel. However, almost all modern perimeter frames use lightweight aluminum. This motorcycle frame type is popular with performance bikes. Its success is proof of its suitability for high-performance applications.

Honda CB1000RR, Ducati Panigale V4, Bajaj Pulsar NS160 and Bajaj Dominar 400 use this type of frame.

5. Trellis Frame

The Trellis frame is quite similar to the perimeter frame. This frame’s primary role is to connect the steering head with the swing-arm as directly as possible. The main difference, however, is the way the two beams coming from the steering head are constructed.

KTM 390 Duke (Courtesy: KTM)
KTM 390 Duke (Courtesy: KTM)

The perimeter frame uses aluminum beams. However, the trellis frame uses numerous short sized steel or aluminum tubes welded together. They form a trellis-like structure. In most cases, the trellis frame is better than the perimeter frame in terms of rigidity, and light-weight construction. Another great advantage of the trellis frame is that it’s relatively much simpler to manufacture. Besides, it doesn’t require heavy machinery for its production.

Despite its simplicity, it’s very effective. Hence, it is the preferred motorcycle frame choice for many bike manufacturers. KTM 390 Duke, 690 Endure and most modern Ducati motorcycles use this type of frame. Read more about Trellis Frame here.

6. Monocoque Motorcycle Frame

Furthermore, monocoque type frames are typically used in cars. Manufacturers build them in the highly robotized and automated manufacturing units. Generally, these are more expensive to make unless you have a massive production scale.

However, some extreme-performance motorcycles require the entire skeletal structure made of a single, super-stiff piece of metal. Hence, it is termed as a monocoque motorcycle frame. Such frames are not very commonly made. Besides, manufacturers use it mainly for hyperbikes or motorcycles with extreme power. Such motorcycles demand unconditional torsional rigidity and lightweight construction.

Monocoque Motorcycle Frame (Courtesy: Kawasaki)
Monocoque Motorcycle Frame (Courtesy: Kawasaki)

Furthermore, some hyperbikes make use of this type of motorcycle frames. They have complex construction and a high degree of precision. Such frames regularly make use of exotic materials such as carbon-fiber and magnesium. Thus, they are very expensive as compared to other common frame types.

Kawasaki Ninja ZX-14 and Ducati 1199 use this type of motorcycle frame.

All Images - Courtesy: respective manufacturers

Watch a bike having Monocoque Frame in action here:

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