Then why do they traditionally use chrome-moly for steel bicycle frames?
(These days they make them out of all sorts of exotic materials.)
Ordinary mild steel would be _much_ cheaper, but they use chrome-moly.
"Fatigue limit" is the reason. You first want to make the frame as
light as practical, but you want it to last for awhile without breaking.
Chrome-moly has excellent fatigue strength. Thus, you can make it thin
and lightweight, and stay within the bounds of the fatigue limit.
https://en.wikipedia.org/wiki/Fatigue_limit
In steel, the fatigue limit scales roughly to half the ultimate
strength. (It depends on a plethora of other factors such as surface
finish, stress concentration, the spectrum of load stress, etc. of course.)
A motorcycle frame built from chrome-moly steel or mild steel would be
_indistinguishable_ for the first few miles. They will handle
identically because the frames would flex identically. They would weigh
the same. Then after awhile cracks will appear in the mild steel frame,
and awhile later it will break.
They only make cheap bicycle frames out of mild steel. Same thing for
motorcycles.
Bill D.
On 12/4/2019 9:42 AM, Paul Compton via EV wrote:
Aircraft frames are built for high strength to weight ratios, but
vehicle frames normally target the stiffness to weight ratio.
If any part of your frame is getting close to the yield point of
ordinary mild steel, then it is, almost by definition, flexing too
much for geed handling.
Lotus, among others, built many successful race cars, even at the
Formula 1 level, from cheap ERW (electric resistance welded) tubing.
Looking for a lighter and stiffer frame than the Norton 'Featherbed',
Colin Seeley made his race frames from ERW, but with straight tubes.
When asked why he didn't use stronger Reynolds tubing, he's reported
to have said 'Nobody knows how to crash a motorcycle not quite hard
enough to bend the frame'.
Formula Ford cars at one time had a (cost control) rule that only ERW
could be used in their construction (roll hoop excepted).
It seems that at one time USA made ERW had a bad reputation for
failing at the seam and was banned by some motorsport sanctioning
bodies.
Brazing Chrome Molly can lead to brittleness due to inter granular
penetration if the metal gets too hot. That's why the Reynolds
Manganese alloys are preferred for Brazing and Bronze welding.
On Tue, 3 Dec 2019 at 19:59, Bill Dube via EV <ev@lists.evdl.org> wrote:
Chrome-moly steel is about double the specific strength (ratio of
ultimate strength divided by density) of mild steel. That's why they
often use chrome-moly in airplanes, and never use mild steel.
https://en.wikipedia.org/wiki/Specific_strength
As you can see from the chart, mild steel has a specific strength of
71-85 kN m/kg, while low-carbon steel has a specific strength of 46 kN m/kg.
Looking further down the chart, you can see why they use carbon fiber
composite (785 kN m/kg) to build modern airplanes.
Chrome-moly (4130) is often chosen for its ease of fabrication and its
forgiving nature when abused. It can be welded (or brazed) without
subsequent heat treatment, and it deforms ~25% before failure. (It
stretches like taffy before it comes apart.) It also has a high fatigue
resistance.
Bill D.
About double On 12/4/2019 7:31 AM, Paul Compton via EV wrote:
On Tue, 3 Dec 2019 at 03:27, Lawrence Rhodes via EV <ev@lists.evdl.org> wrote:
Using chromoly will reduce the weight.
Oh, is Chrome Molybdenum Steel somehow magically lighter than mild Steel?
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