All the motorcycle in motoGP uses aluminum spar frames!
Iron frames and other old-fashioned relics of the past! What's with the trellis frame, you idiot? There was a time when I felt that way too.
Aluminum twin-spar frames aren't necessarily the best frames.
WebiQ sends you a little bit of happiness and motorcycle knowledge. In this article, I will reconsider the advantages and disadvantages of each frame shape.
The word "evolution" is misleading.
The motorcycle frame started as a bicycle frame with an engine forcibly mounted on it, and the flat frame became three-dimensional, and the engine itself was used as a strength member, evolving over time to the modern aluminum frame.
That's true, but this gives the impression that the aluminum frame is an evolutionary reach, and the rest of the frame is a compromise where aluminum frames cannot be adopted due to cost and other factors. In fact, the majority of people think so.
With the exception of off-road motorcycle, a liter motorcycle with a liquid-cooled, 4-cylinder, DOHC, 4-valve, ultra-high-powered engine mounted on an aluminum twin-spur frame is the highest achievement of a motorcycle, and if you don't meet these requirements, you're not the latest and greatest! Don't you think so?
Let me be the first to assure you, that's a big mistake. Just because the aluminum frame was the last thing that came out of the evolutionary process does not mean that the aluminum frame is an almighty god at the end of the evolutionary process. In fact, there are many areas where it is inferior to steel frames in performance.
There are many people who would say, it's about performance, isn't it, like, more expensive to manufacture or something? (lol) That's not true, and sometimes steel frames are more expensive to manufacture.
If you think that the aluminum frame is the latest and strongest frame, you will be wrong in many ways.
What is required of the frame?
In the olden days, the only purpose was to mount the engine, but today's frames are very important for driving performance, as you know.
If we look at it a little more closely, it can be broken down into "strength", "rigidity" and "cost". If you apply power to the engine, it shouldn't break off, right? Strength is necessary to prevent breakage.
You can't have a gnarly bend when you apply power to the engine, can you?
Rigidity is necessary to prevent bending.
On the other hand, if you give it strength and rigidity to withstand the power of the engine, you can't afford to spend 10,000 USD on production, right? "Cost" is to keep the price of the target vehicle within the price range.
Cost may seem like a cliché, but it is an unavoidably important factor when talking about the frame.
However, it's interesting to note that it's not as if all motorcycles would be aluminum twin-spar frames if they were infinitely costly.
From here, we'll take a closer look at the characteristics of each typical frame shape.
Double Cradle Frame
The double-cradle frame was the first motorcycle to have an engine mounted on a flat bicycle frame, but as the engine output increased, the frame became more three-dimensional. This is a masterpiece of a large, high-powered engine, with pipes arranged around it to provide incomparably greater rigidity than a flat frame.
It is formed by bending steel pipes, and when viewed from the side, the part surrounding the engine can be seen as a cradle, and since there are two loops on each side, "double" + "cradle" means double cradle. If the cradle that surrounds the engine is flat and not split on either side, it is a single cradle frame, but this is almost non-existent today.
The double-cradle shape was first introduced to the world by Norton in the UK, and was a huge success in races around the world, including Grand Prix. It is also called a feather bed frame because it is more comfortable to ride like a featherbed compared to the conventional flat frame.
The image of "good riding comfort" is prevalent, and since they are often used in older steel-framed motorcycles, people tend to think that they are characterized by low rigidity, but this is not the case. Compared to a flat frame, it has overwhelming strength and rigidity, and the body (and engine) is solid, so the suspension can move more effectively and the unstable behavior due to the lack of frame rigidity is dramatically reduced. It seems to have originated. If you're thinking "squishy, comfortable ride = feather bed = double-cradle", change your perception.
But time is cruel, and the shape is not particularly rigid or strong nowadays. But it's also a masterpiece shape that delivers the necessary performance, and that's why there are still a number of new motorcycles that use this double-cradle frame.
For example, the very popular Naked is still almost always this double-cradle frame. CB-SF, XJR, INAZUMA, and ZRX are all in this shape. The double-cradle steel-framed cars in the old motorcycle races are setting ridiculous times, and it's clear that they're not just squishy frames that focus on comfort.
There is also a variant, the "semi-double cradle," which is a double-cradle only for the second half and a single cradle for the first half. It does not require as much strength and rigidity as the double-cradle and is employed when lightness and surmount ability are more important than that. A typical example is an off-road motorcycle. In many cases, even for full-scale competition models, there is only one frame that extends down from the steering head, but if that's all it takes to ensure strength and rigidity, then light and slim is the best way to go.
Benefits
- High rigidity compared to single cradle
- The frame is completed to some extent just by bending the pipe.
- Easy to control rigidity by simply adding reinforcement
Disadvantage
- Heavy compared to single cradle
- The engine is difficult to install and remove.
- Skilled production is required.
Backbone Frame
The backbone frame is an evolution of the double-cradle frame. The feature is the absence of the entire underside of the double-cradle frame. Backbone because it's only the spine.
The underside of the frame, which used to be there to ensure rigidity, is gone, so it looks like it's going to be squishy, right? Indeed, the frame alone is squishy compared to the double cradle. So how do you keep it rigid? The engine itself is used as part of the frame.
Rather, it is the first time that the engine itself has been designed to be strong and rigid from the start.
The engine itself seems to be more rigid than a bent steel pipe, doesn't it?
So, by using the engine aggressively, we've been able to reduce the weight and increase the rigidity of the frame by eliminating part of the frame. Because it uses a very hard engine as a strength member. It is also called a diamond frame.
His representative model is the Kawasaki GPZ900R Ninja. The fact that it was the fastest car in the world at the time of its introduction shows that this format had an advantage over the double cradle. Today, this shape is used in many 250cc class road sports motorcycles, such as the CBR250RR, YZF-R25, GSX250R, and Ninja250.
There are exceptions to this rule, where the engine is not used as a strength member, but the backbone frame alone provides strength. Typical examples are the Super Cub and Monkey.
The engine is only suspended and has nothing to do with the strength of the frame, but if it is enough, there is no need to install under pipe.
Benefits
- Lighter than double-cradle.
- Easy to install and remove the engine.
Disadvantage
- It is necessary to design the engine to be used as a strength member
- The precision of the engine coupling part is required.
Twin-spur Frame
This is a further development of the backbone frame. Also known as a twin-tube frame.
While the backbone frame is basically formed by a single spine, we wanted more rigidity and added two spines. If you're going to have two engines anyway, there's no need to bend the top of the engine like the backbone frame, so it's more efficient to swing it to the left and right of the engine.
This frame is based on the idea that by passing the engine left and right, it is possible to connect the swing arm pivots and the steering head in a straight line, which is the most efficient way to connect the parts that are under the most stress.
The shape was introduced at a time when the use of aluminum materials in addition to steel pipes became widespread, so it evolved almost entirely on the premise of an aluminum frame. (A twin-tube frame of steel pipes also exists.)
If you make a box-like object out of aluminum plates and sandwich the engine between two pieces of aluminum plate, it will inevitably become a twin-spar frame. Frames made of extruded wood rather than aluminum sheets were also widely used.
Most efficient at the moment, with very few exceptions, all of the motorcycles that are called Supersports have this frame shape. Almost all of the motorcycles in motoGP have this type of frame.
In recent years, it has become more and more common to control the rigidity of materials by changing their thickness in different locations, which is increasingly advantageous for aluminum twin tubes.
Benefits
- Ideal straight shape when viewed from the side
- Fewer processes such as welding
- Easy to control rigidity
Disadvantage
- When the engine is wide, it will have a curved shape that is far from ideal when viewed from above. It will be wider.
- In the case of aluminum, the material is expensive.
- In the case of aluminum, production molds are very expensive.
Monocoque Frame
This form of backbone frame was developed from a different direction than the twin spar. The basic idea is the same as the backbone, but the frame itself serves some functions, such as the exterior and air cleaner box.
It is no longer made up of pipes, but a box shape that looks like a combination of boards. It can be said that in order to ensure rigidity, the frame is formed in the shape of a huge box, and various functions are combined by taking advantage of its huge size.
Typical examples are the ZX-12R and ZX-14R, where the frame itself can be used as an air cleaner box or battery storage.
Further developed is the DUCATI Panigale 1198 series, which completely utilizes the engine as a strength member, and as a result, there is no longer a frame-like object. There's a frame that connects the steering head to the engine, but it's the air cleaner box itself, and I'm not sure if I can call it a frame.
The air cleaner box is connected to the engine, and the steering head is integrated into the air cleaner box, so it is treated as a monocoque, but in effect, it may be called frameless.
Benefits
- You can combine various functions.
Disadvantage
- It is difficult to change the design.
Underbone Frame
As opposed to the backbone, the underbone is the loss of the double-cradle spine. It's underbone because it's only on the underside of the frame.
No particular advantage in strength or rigidity.but since there is nothing on the top side, it has been adopted by scooters that want a walk-through footrest, after all. You can think of it as a scooter = underbone.
Underbones are also widely used in utility vehicles, especially in Southeast Asia, to provide more legroom. Priority is given to the ability to walk in the crotch without lifting the leg and to carry a load. This is the result of doing so.
Benefits
- Easy to keep the leg space.
Disadvantage
- No rigidity.
Truss Frame
The truss frame was used by DUCATI for many years and became famous for its truss frame, which looks more like the birdcage frame of a four-wheeled vehicle than a motorcycle frame, and was developed for motorcycles. It is also called a trellis frame (Italian reading of truss frame).
It is rarely used in Japanese motorcycles, but it is a fairly common frame shape for high end, high sporting motorcycles, such as Ducati, KTM, and BMW.
It is characterized by the ability to pursue ideal strength and rigidity freely by assembling small pipes into a triangle (truss shape) and combining them with large and small trusses. Although the pipes are conspicuous, the triangular surfaces combined with the pipes can be considered as a large monocoque like structure. The other feature would be that it is "beautiful to look at" (although this area is subjective to the individual).
The performance is very high because you can place the ideal pipe in the ideal place, but the biggest weakness is that it requires a great deal of skill in manufacturing because it requires welding short pipes together precisely and incomparably to make the whole thing. Also, since there are many pipes, they tend to be heavy, and if we try to keep them from becoming heavy, the diameter of the pipes will become extremely thin and the wall thickness will become very thin, which will further increase the difficulty of manufacturing, which is already difficult. To begin with, there are so many welding points. It is not suitable for mass production at all.
The reason why there are almost no examples of its use in Japanese cars is that it is not suitable for mass production. On the other hand, the reason why there are so many cases of adoption in foreign motorcycles is that they are better suited for small-volume production. f we rely on skilled workers to produce the car, can we produce the planned number of motorcycles? This is a fork in the road.
If you have to produce a certain number of motorcycles, like Japanese motorcycles, you can't adopt them even if you want to.
Benefits
- Very high performance
- It's beautiful to look at.
- Can be made very lightly.
Disadvantage
- Manufacturing requires skillful hands.
- Not suitable for mass production
The right person in the right place at the right time
For example, a 50cc shopping scooter with an aluminum twin-tube frame would be less convenient because it would not be flat underfoot. You don't usually need a shopping scooter that's too rigid instead of being able to straddle it quickly, do you?
If the ZX-14R uses an aluminum twin-spar frame instead of a monocoque, it will have a ridiculously wide body. You won't get the sportiness you're aiming for if you keep it wide, and if you pass it over the engine to reduce the width, you're throwing away the benefits of twin tubes. You don't need a ZX-14R like that, do you?
If you use a truss frame on a 250cc sport, the price of the body will skyrocket. It's wonderfully rigid, supple, and lightweight, but there are few people who want 250cc for 30,000 USD, right?
If you convert your off-road vehicle to an aluminum twin-tube, the rocks will hit the engine when driving on the gully. If you need an under guard to avoid a rock hitting the engine, and you need support to put a guard on, then the frame should exist under the engine in the first place.
As you can see, the shape of the frame you need depends on the purpose (aim) of the vehicle. Even if you adopt the highest peak shape and material, you can't make the best use of it.
You say so, but why not aluminize it?
Aluminum has a lighter specific gravity than steel, so it can be made lighter in the same shape. That's the best part about aluminum! However, the same shape cannot provide the same strength as steel.
So, if you make the pipe thicker to give it the same strength as steel, you should be able to make it slightly lighter with the same strength as steel in the end.
There it is! Aluminum is still the best! Steel is a compromise to reduce costs! But the problem is that the volume is larger than steel at that time. The reason it's bigger is that it's at a disadvantage in other areas, such as aerodynamics and mobility. Moreover, the increased frame area makes the frame extremely rigid, and the stresses that were previously absorbed by the frame are now transferred to other parts of the body.
As you can see, the use of steel frames is not just a matter of cost, but there is a reason why steel is used. Not everything is better in aluminum.
As a matter of fact, there are many examples of 4-wheeled vehicle wheels that become heavier as a result of being manufactured with the priority of "being made of aluminum". It's not so simple that you can make them lighter by using aluminum!
