Alluminum Frames? What's all the fuss?

8 replies to this topic
  • CharlesFP

Posted April 09, 2005 - 04:01 PM


I don't understand why its so important to have an alluminum frame. If you look at the weight specs of all bikes, KTM'S are always on the lighter side even though they still use steel. Alluminum may be stiffer, but it is far less durable than chromoly, and tougher to repair. Also, because its not as strong as steel, more of it is needed to accomplish the same task, and that is why Honda's alluminum frame is chunky and covers everything. I doubt Yamaha engineers really wanted to go the Alluminum route, but are being forced to because the CRF's are so popular and the consumers want trick looking Honda-like frames. As for the stiffness issue, I doubt all but the greatest riders who ride their bikes to the absolute limit would notice the differences in stiffness between the current YZF frames and the 06's or 07's, whenever they come. And do most riders really need so much stiffness? That is one of the nice things about chromoly, it is forgiving. Who needs to be beat up and the end of the day?
However, none of us would complain about less weight. If, at the end of the day, the next bike is 10 pounds lighter, I guess alluminum would be nice. If I had it my way, a lighter steel frame would be better, but I guess the current fad won't allow that.

  • simon@vic

Posted April 09, 2005 - 04:19 PM


dude, do you have any idea how "weak" the Yamaha frames are???? my WR is only a year old and it needs a frame already. they bend, dent, and get crushed, they also need paint witch wares off. i do like the "flex feel" of a cromo frame better then the stiffness of an AL frame. none the less AL is much better IMHO.

  • Woodzi

Posted April 09, 2005 - 05:14 PM


Aluminum has a better strength to weight ratio than steel - so you can theoreticaly make the frame lighter. It also has a lower modulus of elasticity (which is the same thing as a spring rate) so it could be less stiff than a steel frame.

The disadvantage of aluminum is that it has no fatigue limit, which means that if it goes through enough stress cycles, no matter how small the stress, it will eventually break. At low stress it takes a lot of cycles and at high stress, just a few. Steel has a fatigue limit. As long as the stress is below about half the ultimate strength it will never break no matter how many stress cycles it sees.

So when you are designing parts out of aluminum, they either need to be replaced at regular intervals (if we know the stresses, we can predict when the part will fail) or they need to be over-designed. With bikes, we don't want to replace wheels, swingarms and frames every few months, so the parts are made to keep the stresses low enough that the parts will last for a very large number of stress cycles. As a result, the parts loose some of their weight advantage over steel and they end up feeling stiffer because of the lower working stress. On the other hand, most people will accept the fact that a piston needs to be replaced at regular intervals. They go through hundreds of stress cycles per second, so they will reach their fatigue limit relatively quickly. The fatigue life of aluminum is one factor that goes into determining the recommended replacement interval for a piston.

  • MotoX780

Posted April 09, 2005 - 05:15 PM


It actually has less to do with wieght and strength then you would think. It is actually far cheaper to produce an aluminum frame than it is to produce the same strength/weight frame from cromoly or easton tubing. This became very obvious in BMX frames a few years ago. When aluminum frames first came out they were the best in BMX, lighter than the standard 4130 cromoly frames of the time, but were more prone to breaking, and cost way more. Now the cost has came down significantly because it became aware that the aluminum frames were far cheaper to produce yet cost far more. So very big names in the business went bankrupt over this(GT, Powerlight, Schwinn, Robinson just to name a few). Now there are some very good alumimum frame makers out there, but more frames are being made of tripple butted easton cromo tubed frames that are not only as light or lighter, but far stronger as well. You pay substantially more for these though. The higher grade steel tubing takes more time to weld, and costs more per pound than aluminum. The motocycle industry is now using hydroforming/casting to make the parts cheaper and quicker than what they can if they used the more expensive same weight/more strength high grade steel tubing. So they are actually making more money off of selling aluminum frame bikes than the cromo framed ones. They sell them for the same price making it seem as if the aluminum is a wonder material when in reality it is saving them money.

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  • wr450fyamaha

Posted April 09, 2005 - 05:24 PM


i would want a steel frame, aluminum is just to week!!!

  • RCannon

Posted April 10, 2005 - 05:11 AM


I have a question about this stress cycle thing in aluminum. I am flying to Dallas in a few hours. The airplanes are made of aluminum.......

Is the plane eventually going to break, or will it be retired after a certain number of hours???

  • backinthedirt

Posted April 10, 2005 - 06:01 AM


Go fly and don't worry. Plane manufactures design parts with modern CAD systems and use FEA to predict the life of aluminum (all) parts. They then assign a life cycle to them and during regular service that part must be replaced wether it is showing wear or not. The part may look like new with no signs of wear but they are require to replace it. Now if you are flying on something really old before the invention of modern computers or something made in some off breed country you might want to take a parachute as your carry on luggage!

  • elton

Posted April 10, 2005 - 09:57 AM


Ditto Charles;

-If the Aluminum frames are so great, why is Honda's "new fangled" 450X heavier than a 2005 WR450?

-I don't like the cluttered "fat frame" look of Honda's frames (new Suzuki RM-Z450, etc.).

-I read in Dirt Bike mag recently, that Honda had major issues with their aluminum frames thus Yam and Suzuki have been trying (for years) to avoid such pitfalls in their designs -- must not be an easy task.

-Don't recall seeing complaints from KTM or Yam owners of broken frames here or on KTMtalk.


  • beezer

Posted April 10, 2005 - 11:06 AM


I read in CYcle News that Ricky Charmichael liked his Suzook because it had a more forgiving steel frame. He said the suspension had to be perfect on a aluminum bike.


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