ally frames and bending

I take the odd tumble every now and then when cycling (slid on some mud the last time) and have
gotten used to checking over the frame for damage (as I do each time I clean the bike) although I
guess most bikes are fairly resiliant.

My bike is an aluminium alloy one - 7005 T6 HT (whatever that means) and as I am fairly new to
cycling I've been reading up on stuff and I get the impression that aluminium alloy is a fairly
brittle material with a tendancy to crack and fracture rather than deform.

Does this mean that ally frames that have not been cracked as a result of a crash will retain their
alignment or do they bend leading to possible mis-alignment issues? I was just wondering really.

Thanks for any info. Paul

 

"Paul" <[email protected]> wrote in message news:[email protected]...
> I take the odd tumble every now and then when cycling (slid on some mud
the
> last time) and have gotten used to checking over the frame for damage (as
I
> do each time I clean the bike) although I guess most bikes are fairly resiliant.
>
> My bike is an aluminium alloy one - 7005 T6 HT (whatever that means)

It's quite meaningful, actually. 7000-series is an alloy of zinc and aluminum. T6 refers to a
specific tempering process, which strengthens the metal. HT means "Heat Treated," a process which
improves strength at the welded joints. Welding weakens most alloys.

and as
> I am fairly new to cycling I've been reading up on stuff and I get the impression that aluminium
> alloy is a fairly brittle material with a
tendancy
> to crack and fracture rather than deform.

Complicated issue. It will deform. I've experimented with semi-trashed lightweight MTB frames that
have been bent. I've ridden them without trouble, even though the rear stays were about 10mm out of
alignment. Never had a problem. Don't know how long they could have gone until failure.

> Does this mean that ally frames that have not been cracked as a result of
a
> crash will retain their alignment or do they bend leading to possible mis-alignment issues? I was
> just wondering really.

Aluminum *will* bend, dent, kink, ovalize, etc without cracking, or even weakening. I've seen super
lightweight (2.9 lb) Klein taper-walled aluminum alloy frames that were deeply dented; but Klein
inspected them and said they were safe to ride. This "plastic deformation" is not good. You wouldn't
want to repeatedly bend the same area - that would lead to failure.

When you think about aluminum, think airplanes. Aluminum airframes have been in service, almost
continuously, for going on 35 years now. All airliners are made of aluminum. Watch how much the
wings flex on those babies. They are engineered not to fail. With aluminum as with any material,
good engineering prevents premature failure, and overengineering allows some latitude for damage (as
with your bike).

-Barry

 

In article <[email protected]>,
B. Sanders <[email protected]> wrote:
>
>
>When you think about aluminum, think airplanes. Aluminum airframes have been in service, almost
>continuously, for going on 35 years now. All airliners are made of aluminum. Watch how much the
>wings flex on those babies. They are engineered not to fail. With aluminum as with any material,
>good engineering prevents premature failure, and overengineering allows some latitude for damage
>(as with your bike).

Airplanes are also engineered to be inspected, and they are inspected regularly in great detail and
at tremendous expense, so I suppose they are not content with "overengineering" as a safety plan.

 

Paul who? writes shyly:

> I take the odd tumble every now and then when cycling (slid on some mud the last time) and have
> gotten used to checking over the frame for damage (as I do each time I clean the bike) although I
> guess most bikes are fairly resilient.

Bicycles get damaged mostly from running into something rather than falling, as in losing traction
or balance. I don't think you need to be as meticulous in damage assessment as you say.

> My bike is an aluminium alloy one - 7005 T6 HT (whatever that means) and as I am fairly new to
> cycling I've been reading up on stuff and I get the impression that aluminium alloy is a fairly
> brittle material with a tendency to crack and fracture rather than deform.

"That" doesn't mean much and you are not assumed to be a metallurgist who can assess the merits of a
specific alloy or heat treatment. The designation is hype and is supposed to make the bicycle sound
high-tech. Aluminum is not "fairly brittle" although it has a lower strength than steel. That is why
aluminum bicycles of any worth have larger tubes than steel ones. The fatigue failure of metals is a
widely misinterpreted characteristic, discussed here often.

> Does this mean that ally frames that have not been cracked as a result of a crash will retain
> their alignment or do they bend leading to possible mis-alignment issues? I was just
> wondering really.

A dented or wrinkled tube, be that steel or aluminum, has residual stresses in the deformed zone
that usually cause fracture with continued use unless the area is annealed. This is not something
commonly done. On steel bicycles the tube is usually replaced.

Jobst Brandt [email protected] Palo Alto CA

 

In article <[email protected]>, Phil Brown <[email protected]> wrote:
>You're both missing the mark. In aircraft the wings (or any other part of the structure) return to
>their original position. The original poster wanted to change the return position -ie: spread the
>rear triangle. Due to the properities of aluminium this will result in cracking and failure.

Yes it's called going off topic.

I don't recommend spreading any aluminum rear triangle.

--Paul

 

Thanks for all the info.

The reason for the question was partly interest and partly down to ignorance and being rather new
to cycling.

I took a fall a while back, nothing major really, the rear wheel just slid out from under me on a
country road (probably hit mud) although I was going fairly fast (for me anyway). After checking
that my own 'damage' was superficial I was more concerned about my 'pride and joy' the bike. I
managed to get a foot down pulling the bike down with me and it took a glancing blow to the left
handlebar and pedal (panniers protected the bike) and slide a few feet before coming to a halt with
nothing more than a torn grip and scuffed pedal.

Riding back I was a bit shaken and occassionally felt that the back wheel was pulling slightly to
the left (probably a result of being a bit shaken up as that was the direction the wheel slide out
when I came off). The bike has had a service since and, short of putting it in a jig to check
alignment, is fine (although I occassionally imagine that I can feel the rear pulling slightly to
the left - I worry to much about my poor bike ). I can hold the rear of the saddle and push the
bike along and it runs straight.

However, it did lead me to thinking about whether an ally frame could be bent in a crash and what
sort of impact it would take, or whether it an ally frame that hadn't cracked would retain it's
original alignment.

Thanks again to all who posted a response.

Kind Regards to you all

Paul.