Steel vs. Aluminum in flexability

Author	Message

t-j-b

Posted: Oct 25, 2008 at 7:30

Quote

cheapbike wrote:

DavidMakalaster wrote:

thatguydave wrote:

Huh, well I stand corrected. The shameful thing is I know that too, total gap moment.

There are numerous types of 6000 series aluminum.

The only bikes I've seen with alu frames were 6061.

What is the "best" type of aluminum? 6061? 70##? 50##?

Application specific...

AirForceOne

Posted: Oct 25, 2008 at 7:32	Quote
It's in the new mbuk. i dont know much about it, but it certainly looks different.

pbuser102331

Posted: Oct 25, 2008 at 8:03

Quote

cheapbike wrote:

DavidMakalaster wrote:

thatguydave wrote:

Huh, well I stand corrected. The shameful thing is I know that too, total gap moment.

There are numerous types of 6000 series aluminum.

The only bikes I've seen with alu frames were 6061.

What is the "best" type of aluminum? 6061? 70##? 50##?

santa cruz use 6069.

cheapbike

Posted: Oct 30, 2008 at 15:58

Quote

theoutpost wrote:

cheapbike wrote:

DavidMakalaster wrote:

There are numerous types of 6000 series aluminum.

The only bikes I've seen with alu frames were 6061.

What is the "best" type of aluminum? 6061? 70##? 50##?

Application specific...

If you had to use aluminum in a dirt jumper frame- (lets say steel was discovered to cause AIDS, cancer, and insanity all simultaneously)

For a hardtail dirt jumper or low travel (3") dual suspension bike for slopestyle or 4X- what would be the best for each of these types of bikes?

t-j-b

Posted: Oct 30, 2008 at 16:15

Quote

cheapbike wrote:

theoutpost wrote:

cheapbike wrote:

The only bikes I've seen with alu frames were 6061.

What is the "best" type of aluminum? 6061? 70##? 50##?

Application specific...

I'd say 6061 T6

duanecressy

Posted: Sep 21, 2009 at 3:00	Quote
could anyone reccomend a bike for me (a hardtail that i will use for dirt jumping and just riding the occasional trail) i am thinking of getting a specialized p3 but half u guys reckon steel is waayyyyy better so yea

yo13bo

Posted: Sep 21, 2009 at 4:28	Quote
it is! cromo is alot flexier than the aluminium used for bikes. this aluminium alloy does not tend to bend, just snap, with cromo rrames you will bend it before it snaps, so its alot safer and it looks alot cleaner. im not sure on this aluminum stuff though, might have some different properties than aluminium but i am unfamiliar with this element that only seems to exist in america

WasabiJim

Posted: Sep 21, 2009 at 5:22	Quote
its the design of aluminum that makes it stiff not the properties of the material. its actually 'flexier' than steel but once flexed its useless, because its much lighter they can use more of it and go with big oversized tubes that are strong n stiff, so stiff they buzz your teeth out.

pbuser102331

Posted: Sep 21, 2009 at 5:55

Quote

WasabiJim wrote:

its the design of aluminum that makes it stiff not the properties of the material. its actually 'flexier' than steel but once flexed its useless, because its much lighter they can use more of it and go with big oversized tubes that are strong n stiff, so stiff they buzz your teeth out.

they use oversized tubes to stiffen the frame other wise it would be like a wet noodle,but this in turn makes the frames more harsh and less forgiving.

alloy for full sus frames where maximum stiffness is needed and steel for hardtail for that steel is real feel.

WasabiJim

Posted: Sep 22, 2009 at 3:15	Quote
yah, and because aluminum has a very limited fatigue life, noodle = snapage, often out of nowhere

hampsteadbandit

Posted: Sep 22, 2009 at 6:24

Quote

cromo is alot flexier than the aluminium used for bikes

this is a common misconception about materials

cromoly steel alloy is actually much stronger, stiffer and tougher than aluminium alloys (such as 6082 or 7005)

cromoly steel does not fatigue in the same way as aluminium, with aluminium - every high stress creates stored fatigue which can eventually lead to frame failure

as long as you don't overload a cromoly steel beyond its yield strength (i.e. run into a car) it will last for many years - its often corrosion (rust) that actually causes a cromoly steel frame to fail

the chromium in a cromoly steel frame is not present to inhibit rust, but for its other strengthening and hardening properties - cromoly steel is actually
"chromium-molybdenum steel alloy"

aluminium alloy is non-ferrous and so does not corrode like cromoly steel - aluminium does oxidise, but this is different to corrosion experienced by steels

oxidisation is a chemical reaction between the surface of the aluminium and the atmosphere, and aluminium creates its own protective layer that prevents further damage - this is why a "raw" aluminium alloy frame will go dull in appearence, unless sprayed with a clear coat laquer after being rawed - the laquer prevents the aluminium from oxidising

regarding the use of both materials for bike frames:

cromoly steel is much denser than aluminium, so its best to use thinner walled, smaller diameter tubing so that the frame does not become too heavy to be useful

as cromoly steel is stiff and strong, you can use these smaller diameter, thinner walled tubes and still maintain a good degree of strength and stiffness whilst keeping the weight manageable

with a cromoly steel frame you can actually build a bit of "bounce" into the tubeset, and due to the fact that steel does not fatigue despite tube movement, you can end up with a more "comfortable" frame - this makes cromoly steel idea for hardtail mountain bike frames

for very high stress frames like a BMX used for dirt jumping and skatepark, the design of the tubeset and frame layout is created to remove any flexure, and create a very strong, stiff structure that is responsive to ride, and very durable to withstand repeated crashes and high loads

aluminium alloy is actually weaker and more flexible than steel, so you must use larger diameter tubes with thicker wall thicknesses to provide adequate strength and stiffness - this is why aluminium frame end up commonly being "stiff" compared to a steel frame - basically from using bigger tubes

as aluminium is substantially less dense than cromoly steel, you can use big tubes with thicker walls and still end up with a lighter frame than one made from steel

one of the problems with tubing design is to prevent "buckling failure" (known as beercanning) and if you make a steel tube wall too thin, a high impact will actually kink the tube

this is why aluminium alloy frames can be lighter than steel despite having thicker walled tubes that resist buckling failure

aluminium frames do not benefit from being flexible as it generates fatigue stress in the material - so a more rigid aluminium frame is generally longer lasting than a more flexible one

aluminium alloy is also much easier to manipulate than steel for creating complex tube shapes and structures as you often require for designing suspension frames

modern techniques like hydroforming allow design engineers to create the ideal tube structures to create light, strong and rigid suspension frames

with all materials, including titanium and carbon fibre, the best results are got by a careful understanding of each materials properties, and manufacturing techniques - a good design engineer will use the right material for the type of bike frame, and design the tubeset to achieve his / her design engineering goals for that frame

something to remember about aluminium alloy frames, is that they have a finite (limited) lifespan - probably a couple of seasons of hard use for an average frame, and as little as a season for a lighter weight "race only" frame

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