It is clear that day by day dual suspensions are gaining market share and by now there is one that will fit you, but hardtails will always have that old taste of "this is the way it was before".  Their ability to transfer direct power to the wheel, their amazing acceleration ability, the finesse needed to ride them (hopping and avoiding to beat your partners in a downhill), their steering precision. Yes, it is clear that hardtails will never die.

If you like hardtails or can afford to have more than one MtB there are some frames available that should be considered as the the base of your dream bike. We tested three of them that August evening: Cannondale CAAD 5, KHS Alite 4000, and Specialized Stumpjumper M4 .

Three aluminum frames, but that is not a coincidence. It would be possible to build carbon fiber composite frames to be even lighter than the top alloy ones. A carbon frame could feature interesting damping or shock absorbing abilities as well as bomb proof lateral stiffness, but it seems no one but Trek (and Scott out of the hardtail field with their Strike range) is building them now. Anyways, carbon frames don’t like too much to interface with stones. They can bear the heaviest extension and compression loads, but they are quite fragile.

Titanium is a very interesting fame material also. It has good damping characteristics, natural spring, and almost impossible to wear out if well worked. But, as aluminum it has a lower density, unless you can achieve the technology needed to differentially taper and shape titanium tubes on each of the three dimensions, a top alloy frame will be lighter and way stiffer than either.

Steel is something that was used in top frames of the past. These days, the best alloy tubes are tapered and shaped the same way the best steel tubes are, so there is no way you can build a steel frame to have such a good stiffness-to-weight ratio as high end aluminum ones do.

Before we tested these frames we needed to know the true weight of each. Only the KHS could be called "Ultralight" (and by the way that is the name of its tube set) with 1,400 g (3.086 lbs) in Small- 15" size. The Specialized reached 1,530 g (3.417 lbs) for a 18" frame, while the Cannondale marked 1,598 g (3.523 lbs) for its Medium size.

It is clear that manufacturers prefer to sell safe and reliable products instead of offering stunning marks that could result in failures. Pro riders are also on this very aware of this even though they could use a frame for one race and then just throw it away. Two hundred grams will never mean a victory, but a reliable bike is something essential, so most of World Cup bikes are a bit over 10 kg (22 lbs).

Cannondale buys customized 6061 tubes from Alcoa. There is not a single tube in the CAAD 5 that is completely cylindrical. It does not matter if the frame follows the classic diamond geometry, when you look this bike from less that 10 meters you can see this frame is very distant from the classic MtB frame.

Starting from the head tube, the folks at Bedford take an alloy tube and remove material from the outer central part of its wall at 2/3 of its circumference. This way it becomes thicker at both extremes to hold the headset cups, and at its rear side to create a strong area where to weld the down and top tubes. These tubes do not need any additional reinforcement welded at its outer side as other top frames do. Cannondale argues that conventional reinforcements create stress areas and their welds can decrease the tubes overall endurance. They prefer to create reinforce areas by softly tapering each tube. This way the stresses are better distributed throughout all the frame.

This is why their Power Pyramid Downtube has a thicker wall at the side where it contacts with the head tube, and why that thickness decreases to the BB shell, while the outside diameter increases. That BB shell is also machined to remove aluminum from its central, inner part.

The rear triangle is maybe the most special part of this frame: It is developed only for disc brakes, so seat stays can be made slimmer and lighter. They only have to play their structural role in the frame and do not have to avoid energy losses at the brake system. There are no cantilever bosses found on the frame. There is neither a need for symmetry now at this point, so the stays are shaped to allow certain vertical compliance to make the ride smoother.

The Alite 4000 is a much more classical frame, outwardly at least. The folks at KHS just take one of the best aluminum tubing available today and join all its components with flawless welds. But that is not so easy as it seems: At Rancho Dominguez, CA they have developed a nice geometry for this frame, it features interesting details such as a snake-shaped chain and seat stays, a downtube welded gusset, and their unique way of joining the seat stays.

The frame features a good sloping that makes it easier to steer in hard conditions without losing stiffness. Other sloping frames are stiff and light just because they are smaller, but if you measure the weight to stiffness ratio of the frame plus seat post set, they are not as good as bikes with higher top tubes with their seat post cut to the correct length.

The Alite 4000 is as compact as a sloping frame but as stiff as a conventional one. You will not need to use a super long seat post that would add too much stress to the top of the seat tube.

And, of course, to weld such a light tubing as the Eatson Ultralight 7005 is not easy.  These tubes are extremely thin, so any temperature excess results in an unrideable frame. For example, the down tube of the Alite 4000 has a wall thickness of 2.4 mm at the side where it joins the head tube, while it has just 1.0 mm at its central part and it reaches 1.3  mm at the BB shell side. Interestingly, the transition parts of the tube (those where the wall thickness increases or decreases) are longer than those with the thickest wall to avoid stress areas.

The Specialized Stumpjumper M4 is halfway between the KHS and the Cannondale. It is not as classical as the KHS and not so "aerospatial" as the Cannondale, but is a class of its own. Its "M4 fully manipulated" tubing is in fact a custom product from Columbus based on their Starship series. After a decade where manufacturers proudly declared all the specs of their tubing, these days they all seem to follow the Trek philosophy: You don’t need a aerospatial technology lesson, all you need is a rocket. So, it is impossible to know how they are tapered unless you carefully cut a frame. We tried to talk "Mikel into cutting his frame, but he would not agree.

This is a very nice frame indeed. Pretty paint and cool decals apart, details arise everywhere: Top tube gusset with an stamped S, downtube box-shaped reinforcement, no bridge between chainstays (so Mikel will not break this frame at this point as he did with the M2). The rear triangle is remarkable. As the chainstays use no bridge between them, they have to be stronger than usual to keep the frame stiff enough at the BB shell. They are oversized, ovalized, and shaped the way they don’t interface the chainrings and the rear wheel due to their large diameter. Add two nice and tiny dropouts, and a pair of snake seat stays "a-la-Specialized".

What happened to that M2 metal matrix composite used before? Wasn't it so far ahead from the rest of the aluminum alloys? Yes, it was in the early nineties when it first came out and was used on Specialized in bikes. During the first years, the folks at Morgan Hill used a 6061 based composite. The difference between this composite and a regular 6061 alloy is the fact that the composite has ceramic oxide particles among the whole alloy structure. That ceramic particles are not a part of the alloy, and that is why this matter is called a composite. It is like concrete: Concrete is just cement with crushed stone, gravel or other dry goods. But not the cement nor the gravel share at single part of their molecular structure with the other, so they are not an alloy (it would be impossible since none of them are metals). They are just a composite, this is, a physical joint of different materials that create a matter with different characteristics from the ones of its components. It would be impossible to make a building not with gravel, nor with cement, but it is possible to make the most resistant ones using concrete.

In the case of M2 metal matrix composite, the resulting characteristics are not so far from those from pure 6061 aluminum, because of the very low percentage of ceramic oxide that is used. Otherwise, the resultant matter would be not suitable to build bicycle frames. What adds the ceramic oxide to the aluminum? Hardness and  vibration damping abilities, above all. On the other hand, it is almost impossible to know exactly the mechanical characteristics of a MMC piece at a given point if not directly observed. If you are dealing with a large aluminum piece, you know its characteristics are almost the same throughout all its structure. With MMC you should apply computerized models to predict the average properties throughout all the structure, or scan with a tunnel effect microscope the whole structure, and then determine its exact properties point by point, what is in fact impossible.

When you use that computerized models to make a Finite Element Analysis to see how far you can lighten structure (i.e. a frame), it will not allow you to use the best properties the MMC can achieve, but the average characteristics the model predicts. So, it is not possible to fight the weight of the latest ultralight alloys.

Specialized’s latest attempt to save a few more grams without leaving the MMC path was the use of 7005 alloy as matrix instead of 6061. When the M4 frames substituted the M2 ones, this meant the good-bye to the MMC.

It is not Specialized that was wrong when they started to use MMC for bicycles. During a decade, the M2 frames have been among the best available What has happened is that bicycle tubing manufacturers have developed very special alloys to design the lightest and toughest frames that have surpassed the MMC abilities, and Specialized has recognized it.

The alloy used for the M4 frame is a perfect example of how far the aluminum technology has gone lately. It contains silicon and magnesium as main components apart from aluminum, and there are also little percentages of Cu, Mn and Zn. All this means a 6000 series alloy that is very resistant to high temperatures the way few millimeters far from the welds the tube reaches its highest mechanical alloys. After a complete cycle of heat treatments, the result is a extremely resistant matter, so it is possible to lighten the tubes although they have to suffer welding processes to be a part of a frame.