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steel frame longevity

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Since every bike is aluminum these days almost, I was curious about the actual longevity of a steel framed bike.

Does a certain type of riding put more stress on a frame??

For example...would a guy riding freestyle ramps (equivelent of riding an sx track) put more stress on a frame then say a top 5 B rider on the local level?

Occasionally i'll hear that the bike feels "clapped out" and that the frame is starting to crack or stretch.

i've never seen a frame crack or stretch...

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i just sandblasted my 96 cr frame and looked over everything well. and got it scaned. no cracks or anything. welds are still solid ect.

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Since every bike is aluminum these days almost, I was curious about the actual longevity of a steel framed bike.

Does a certain type of riding put more stress on a frame??

For example...would a guy riding freestyle ramps (equivelent of riding an sx track) put more stress on a frame then say a top 5 B rider on the local level?

Occasionally i'll hear that the bike feels "clapped out" and that the frame is starting to crack or stretch.

i've never seen a frame crack or stretch...

Theres nothing wrong with steel frames. "clapped out" just means the bike is in need of an overhaul (ie- new bearings, cables, etc)

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Theres nothing wrong with steel frames. "clapped out" just means the bike is in need of an overhaul (ie- new bearings, cables, etc)

this is what i thought..both of my RM's (03 125 and 04 250) felt fresh and they were ridden..both steel frames lol

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There are a lot of steel framed ktm's in freestyle. Aluminum frames are generally heavier to combat the work hardening.

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there are fatigue stresses that are introduced during cyclic loading. so who puts more stresses on the frame is a very broad question. a freestyle rider will probably put a larger load on the frame then a class B rider depending on the size of jumps they are hitting. however a motocross rider or off road rider will likely have way more cycles of smaller loads that can eventually induce failure. steel can be engineered to reach a fatigue limit in which a certain load cyclically loaded will never cause it to fail. where aluminum does not have a fatigues limit and even with a small load will eventually fail. fatigue failure modes are only considered for over 10^6 cycles so there is a long time before most bikes will get to that. areas near welds have residual stresses in them from the heat involved in welding that can cause them to break first. i believe the reason for the aluminum frame vs the steel frame is mostly that it can be made stiffer for the same weight as a steel frame.

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i believe the reason for the aluminum frame vs the steel frame is mostly that it can be made stiffer for the same weight as a steel frame.

Aluminum also gives one manufacturing options that steel can't match at the same weight and strength. With the '06-'09 YZ450 as an example, a single pair of castings incorporate the rear outside loops of the cradle, the swing arm pivot points, upper rear shock mount, 3 sub frame mounts, head stay mount points, and rear master cylinder and chain roller mounts into two pieces that are dropped into an automatic machine to drill, thread and face the hard points in a single operation, after which it needs to be welded in only 6-7 places to complete the whole back half of the frame. Contrast this with the cutting, fitting, jigging, and welding needed to fashion a similar assembly from steel.

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fatigue failure modes are only considered for over 10^6 cycles so there is a long time before most bikes will get to that.

Depends on the amount of force, type of force,...etc placed on the frame, and if it is above or below the endurance limit.

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i've never seen a frame crack or stretch...

The mid to early 90's KX frames cracked. The 94 I'm restoring, after sandblasting I found loads of cracks in the frame. Mainly under the engine. I vee'd them out, welded and blended.

I think alot of the "clapped out" feel has to do with the steering and suspension bearings. Also how well the suspension is. Even things like loose/worn kicker, shifter and other controls. I don't think the frame itself is the root cause of a bike feeling "loose", as I sometimes call it.

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Depends on the amount of force, type of force,...etc placed on the frame, and if it is above or below the endurance limit.

if there is less cycles then 10^ 5 or 10^6 im not sure the exact number to failure then it is not a fatigue failure, it would be categorized under a different failure mode.

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if there is less cycles then 10^ 5 or 10^6 im not sure the exact number to failure then it is not a fatigue failure, it would be categorized under a different failure mode.
I believe you'll find that variations in the amplitude of each cycle will change the cycles-to-failure number, will you not?

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I believe you'll find that variations in the amplitude of each cycle will change the cycles-to-failure number, will you not?

Yes that is true, when it is over the 10 ^6 it is the endurance limit for steels, i was thinking that its not considered fatigue failure under the 10^6 limit. i just looked it up in my textbook and it is fatigue under that limit but under 10^3 standard fatigue equations do not apply.

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