Which one is better?

Cyrus:

Hi! Can anyone give me advise about that 3 model of MTB? Which one is
better? All same price AUD$299.
Thanks a lot!

Melbourne
Cyrus


Special Edition Malvern Star - Aspect
Frame: 7005 Oversized Aluminium frame with replaceable rear drop out.
Forks:Adjustable RST 191TL Suspension fork, alloy crown.
Shifters: Shimano 24 speed indexed gears with Rapidfire Shifters.
Rear Derailleur: Shimano Acera 8 speed.
Brakes: Alloy Linear pull brakes.
Wheels: Alex Alloy rims; Stainless steel spokes & quick release alloy hubs.
Features: Alloy cranks, alloy seat post and alloy kickstand.
************************************************** *****

Give this one a miss, mainly for the 7005 Al tubing. 7005, while
exhibiting a longer fatigue life than 6xxx series Al, is very
susceptible to corrosion. Since scratches will be unavoidable on your
frame, there will be scratched spots that may act as stress risers when
they corrode.

GT- Palomar
FRAME: GT Triple Triangle design frame, with protaper down tube,
hi-tensile steel tubing with semi forged dropouts.
FORK: RST Omni CL
CRANKSET: Suntour CW-128, 48,38,28
BB: Sealed Mech
PEDALS: High Impact nylon ATB
FRONT DER: Shimano TY-32
REAR DER: Shimano Acera
SHIFTERS: Sram MRX
FREEWHEEL: Shimano 7 Speed 14-28 HG
CHAIN: HG-50
RIMS: Alloy ATB
TYERS: Tioga Factory XC 26 x 1.95
FRONT HUB: Alloy QR, with sealed
system cro-mo axle
REAR HUB: Alloy QR, with sealed
system cro-mo axle
SPOKES: Stainless steel 14 gauge
FRONT BRAKE: Tektro Linear Pull
REAR BRAKE: Tektro Linear Pull
BRAKE LEVERS: Tektro
HANDLEBAR: 20 mm Rise, 6 degree bend, anatomically sized
STEM: GT ATB, Forged threadless 25 degree rise with two bolt alum face
plate
GRIPS: GT ATB
made in China
************************************************** *******
Mongoose- Switchback SX
Frame: Comes in Gents & Ladies (ladies small only) designed HiTen
steel frame with CrMo seat tube.
Forks: MOZO USA C-710 suspension forks.
Gears: 21 Speed; Sram MRX 7 speed shifters matched with Shimano
derailleurs.
Brakes: Alloy linear pull brakes.
Wheels:Alloy Alex Z-1000 26" rims; alloy quick release hubs; &
stainless steel spokes.
Features: Comfortable Selle Royal Freedom Gel seat and alloy
suspension seat post.
made in Taiwan


Ride these last two and see which fits you better. Replace components
as they get worn; this is a reasonable way to upgrade a bike. You may
also want to see if the shop can do a deal for you on component swaps: I
find grip shifters a right PITA.

Jose Rizal wrote:

Give this one a miss, mainly for the 7005 Al tubing. 7005, while
exhibiting a longer fatigue life than 6xxx series Al, is very
susceptible to corrosion. Since scratches will be unavoidable on your
frame, there will be scratched spots that may act as stress risers when
they corrode.

What is the source of this myth? Aluminium alloys does not corrode,
because they instantly form oxide layer on the surface when exposed
to air. Perhaps this could be an issue when you rode your bike on the
beach only, but still it would take years to weaken the frame.

--
Best regards,
Rado bladteth Rzeznicki
http://www.widzew.net/~bladteth/rower.html

MCM #252

Jose Rizal wrote:

Give this one a miss, mainly for the 7005 Al tubing. 7005, while
exhibiting a longer fatigue life than 6xxx series Al, is very
susceptible to corrosion. Since scratches will be unavoidable on your
frame, there will be scratched spots that may act as stress risers when
they corrode.

What is the source of this myth? Aluminium alloys does not corrode,
because they instantly form oxide layer on the surface when exposed
to air. Perhaps this could be an issue when you rode your bike on the
beach only, but still it would take years to weaken the frame.

--
Best regards,
Rado bladteth Rzeznicki
http://www.widzew.net/~bladteth/rower.html

MCM #252

Rado bladteth Rzeznicki:

Jose Rizal wrote:

Give this one a miss, mainly for the 7005 Al tubing. 7005, while
exhibiting a longer fatigue life than 6xxx series Al, is very
susceptible to corrosion. Since scratches will be unavoidable on your
frame, there will be scratched spots that may act as stress risers when
they corrode.

What is the source of this myth? Aluminium alloys does not corrode,
because they instantly form oxide layer on the surface when exposed
to air. Perhaps this could be an issue when you rode your bike on the
beach only, but still it would take years to weaken the frame.

Read up on your materials science. Different Al alloys exhibit
different properties. 7xxx alloys are prone to stress corrosion
cracking due to soluble alloying elements copper, magnesium, silicon,
and zinc. Copper-free 7xxx alloys such as 7005 exhibit better corrosion
resistance, but as a group, copper-free 7xxx alloys are less resistant
to SCC than other types of aluminum alloys when tensile stresses are
developed in the short-transverse direction at exposed surfaces (see
reference below). Care must also be taken in properly heat-treating
7005, as it can lose up to half its strength on welding temperatures.
Can you be assured that a sub-$500 bike frame has been heat-treated
appropriately?

For the layman: http://www.key-to-metals.com/Article17.htm

Rado bladteth Rzeznicki:

Jose Rizal wrote:

Give this one a miss, mainly for the 7005 Al tubing. 7005, while
exhibiting a longer fatigue life than 6xxx series Al, is very
susceptible to corrosion. Since scratches will be unavoidable on your
frame, there will be scratched spots that may act as stress risers when
they corrode.

What is the source of this myth? Aluminium alloys does not corrode,
because they instantly form oxide layer on the surface when exposed
to air. Perhaps this could be an issue when you rode your bike on the
beach only, but still it would take years to weaken the frame.

Read up on your materials science. Different Al alloys exhibit
different properties. 7xxx alloys are prone to stress corrosion
cracking due to soluble alloying elements copper, magnesium, silicon,
and zinc. Copper-free 7xxx alloys such as 7005 exhibit better corrosion
resistance, but as a group, copper-free 7xxx alloys are less resistant
to SCC than other types of aluminum alloys when tensile stresses are
developed in the short-transverse direction at exposed surfaces (see
reference below). Care must also be taken in properly heat-treating
7005, as it can lose up to half its strength on welding temperatures.
Can you be assured that a sub-$500 bike frame has been heat-treated
appropriately?

For the layman: http://www.key-to-metals.com/Article17.htm

Rado bladteth Rzeznicki:

Jose Rizal wrote:

Read up on your materials science. Different Al alloys exhibit
different properties.

Nice reference, but I've got one question. What do you consider as
"damaging" environment?

What I consider damaging environment is irrelevant. It's what the Al
alloy sees as damaging environment which matters.

From http://www.key-to-metals.com/ViewArt...=17&SessionID=

"Well-known specific environments include water vapor, aqueous
solutions, organic liquids and liquid metals. Stresses sufficient for
crack initiation and crack growth can be far below the stresses required
for gross yielding, especially in those alloy/environment combinations
that are of practical importance, e.g., high strength aluminum alloys in
air."

7005 used on bike frames fits the bill for these conditions.

Can you be assured that a sub-$500 bike frame has been heat-treated
appropriately?

No and the alloy grade doesn't change this assurance at all.

Non-7XXX series alloys do not possess as heat-sensitive characteristics,
hence some do not require heat-treatment at all. Another point is that
the 7005 alloy, although having a narrower melting point range, still
possesses a higher crack sensitivity during welding than other Al alloys
and hence require more care during welding. I don't think you read the
reference at all.

Rado bladteth Rzeznicki:

Jose Rizal wrote:

Read up on your materials science. Different Al alloys exhibit
different properties.

Nice reference, but I've got one question. What do you consider as
"damaging" environment?

What I consider damaging environment is irrelevant. It's what the Al
alloy sees as damaging environment which matters.

From http://www.key-to-metals.com/ViewArt...=17&SessionID=

"Well-known specific environments include water vapor, aqueous
solutions, organic liquids and liquid metals. Stresses sufficient for
crack initiation and crack growth can be far below the stresses required
for gross yielding, especially in those alloy/environment combinations
that are of practical importance, e.g., high strength aluminum alloys in
air."

7005 used on bike frames fits the bill for these conditions.

Can you be assured that a sub-$500 bike frame has been heat-treated
appropriately?

No and the alloy grade doesn't change this assurance at all.

Non-7XXX series alloys do not possess as heat-sensitive characteristics,
hence some do not require heat-treatment at all. Another point is that
the 7005 alloy, although having a narrower melting point range, still
possesses a higher crack sensitivity during welding than other Al alloys
and hence require more care during welding. I don't think you read the
reference at all.