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Edited Date/Time
10/31/2012 1:57pm
Hellion posted this in another thread and I figured it would be a decent discussion topic: "I wonder why everyone thinks aluminum frames are the way to go. Listen to guys who test bikes for a living, and some very well known pros who have said they prefer the forgiving nature of a steel frame." Obviously, there are pros and cons to either frame and it ultimately comes down to personal preference. So, what is your preference and why?
When KTM stuck with steel frames for their 350 and 450 it kind of made me question the general consensus that an aluminum frame is superior to a steel frame. This past year during the Phoenix supercross, Ricky Carmichael was speaking very highly of the benefits of a steel frame on slick and hard pack track while Dungey was out front. So much, that I kind of got the impression he prefers steel frames over aluminum.
I really do not have enough riding experience on an aluminum frame dirt bike to have much of an opinion. I have developed a strong preference for steel frame mountain bikes over aluminum due to the flex, which has kind of caused me to question replacing my steel framed YZ125 for a newer aluminum framed version.
When KTM stuck with steel frames for their 350 and 450 it kind of made me question the general consensus that an aluminum frame is superior to a steel frame. This past year during the Phoenix supercross, Ricky Carmichael was speaking very highly of the benefits of a steel frame on slick and hard pack track while Dungey was out front. So much, that I kind of got the impression he prefers steel frames over aluminum.
I really do not have enough riding experience on an aluminum frame dirt bike to have much of an opinion. I have developed a strong preference for steel frame mountain bikes over aluminum due to the flex, which has kind of caused me to question replacing my steel framed YZ125 for a newer aluminum framed version.
Hmm, for me (not being fast enough to know a difference)
I will say i like the rigidity of the aluminum frames on milestones vet track and comp edges vet track. I also like the feeling of the big frame on the inside of my boot... why i have no idea lol
Now my steel frame 2001 yzf feels more plush all the way around. Not suspension wise, it just feels more forgiving if that makes sense.
The Shop
Even the steel framed YZ's had smaller forks (46mm open cartridge) and softer springs compared the AF's (48mm closed cartridge).
To say I liked my '02 YZ 250's steel frame better than my '09 CRF 450's aluminum frame doesn't hold water to me because there way too many other variables that make these two bikes different than just the frames.
The YZ was way to stiff and rigid even though the suspension is consider the best out there (SSS).
Since getting the rm250 I haven't look back once.
Aluminuim doesn't flex as much and the hours it takes to scotch brite the rails to tune it aren't worth the work.
Bonds OTB
However, I noticed an ENORMOUS difference when I got my '05 YZ125 (it was my 7th YZ125 since 1997.) YZs are very stable and don't usually turn as well, but the aluminum frame woke that bike up. It was still solid as a rock in a straight line, but now it was more flickable and turned better. That bike was 10 pounds lighter than its predecessor, so that couldn't have hurt!
For those reasons, I prefer the aluminum framed YZ 2-strokes. I have never noticed the "harshness" of aluminum vs. steel.
,
Can you imagine being able to tell such small differences, and details? I was blown away by it, wish i remembered it more clearly.
2005 KX250F
I'm gonna test an Ubber new "steel" 500 this week; so, we'll see (2013 chassis/500cc 2t).
I've been fortunate enough to have REALLY well suspended modern thumpers; so, I really dig the modern Al chassis...
As I get more involved in some new "Development" work that's Very Influenced by some ol'school guys that INSIST that KTM's new frames are awesome...well...we'll see...
Pit Row
The only 'big 4' bike i had was an 2005 r6.
The steel framed bikes that i have riddin' never gave me that 'klick' handling wise that i had with my KTM's.
it all comes down to personal preference i guess. It's quite funny all the japs hop on the same boat though.
Again neither is bad these days but personally I think steel is an inherently better material for the job of an MX bike frame. It's strong yet can flex, whereas it seems the manufactures once had to go to great lengths to work some flex back into their aluminium frames. The weight savings don't exist either - KTMs steel 450 SXF frame is the lightest frame in the class.
Steel is also easier to weld if necessary and usually easier to work around.
To get the same strength with aluminium you need more material, this means larger tubing, castings, forgings etc.
Larger tubing means a higher stiffness because of its higher area moment of inertia.
Using advanced aluminium alloys an aluminium frame can be a lot lighter (6065, 7075 in T6 annealed condition), because of their higher specific strength. This also brings down the stiffness because you can use smaller tubing.
But remember that proper engineering meaning clever placement of materials can make a steel frame very light.
Note that stronger alloys aren't stiffer, there is no change in the Youngs modulus.
Steel generally is more resillient, which means it can take more energy and still return to its normal shape. This also is a factor for handling.
Note that both steel (KTM uses 25CrMo4 steel) and aluminium frames need to be carefully heat treated after welding. After repairing these frame you need to properly anneal and stress relieve the frame to achieve proper strength.
A lot of homegrown poorly converted frames (change in stress concentrations, flex charasteristics, not properly annealed) hold up because most people aren't fast enough to really stretch a frame to its limits. But this can happen if you are.
Al is cheaper to make because they can use castings. Also machining costs are lower because Al is softer.
Al is more difficult to weld , but this is not a factor on a production line.
Young's modulus for Al is 69 Gpa , steel is 300 gpa, Ti 105.
If two beams are the same dimensions in cross section and are loaded the same, the lower modulus beam will flex more.
Even though Al should be more flexy due to the lower E mod, the frames are always more rigid than steel.
Al does not have an elastic yield point. It begins plastic deformation as soon as it starts bending. The frame must be designed to be rigid because any amount of flexing would fatigue it.
Aluminium is not cheaper to produce per se. Aluminium can be a pain in the ass to machine because aluminium sticks to your tools, many cutting tools for aluminium are polished to reduce this effect. There are more factors to consider which make up price of a frame.
The Youngs modulus for steel is between 200 and 210GPa.
Aluminium does have an elastic yield strength like steels, it is considered a linear elastic material. The magnitude of its yield strength depends on your alloy and annealed state.
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