From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: Hollow Forged Crank Arms??
Date: 22 Aug 2000 21:17:03 GMT

Shawn Amir writes:

> How does one forge a crank with a hollow inside--a la Shimano?

Forge a U-shaped crank and weld a lid on it.  I find this an
ultimately stupid idea considering the weight it saves and the
expense, not to mention the metallurgy of the composite forging and
welding.  These have been reported to crack at the weld.

Making a durable crank is difficult enough without added hazards such
as long welds.  I'd be a lot happier if they would redesign the pedal
to crank interface to prevent breakouts there.  Better yet, come up
with a BB that does not have the flaw of the current hollow splined
end design, that assumes no reverse torque on the spindle.  As we have
seen here in this newsgroup, they come undone if you stand, right foot
forward, over rough stuff (the only occurrence of reverse torque on a
crank spindle).

Jobst Brandt      <jbrandt@hpl.hp.com>

From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: welding cranks
Date: 23 Jan 2001 21:17:24 GMT

Nick Maclaren writes:

>> There are or have been welded steel cranks, Sweetwings are one
>> which I have seen.

> Good Lord, why?  A crank is a sufficiently simple object that I
> should have thought that there was no point in making it in multiple
> parts.

I sense that you do not own a sport or racing bicycle or you would be
aware of the attention to weight savings, be that hollow crank
spindles, variable wall thickness seat posts and many other items that
have material removed where it does little good except add weight.

In that pursuit, cranks are made hollow because the core material of
the beam remains unused in bending and torsion, where a tube is a
better cross section.  Shimano Dura Ace has made hollow aluminum
cranks by forging a channel onto which a cover is welded.  More
recently a method for forging tube shaped cranks has been developed.
Steel cranks, as the ones mentioned, are made of tubes that require
solid ends for pedal and spindle attachment.

Jobst Brandt      <jbrandt@hpl.hp.com>

From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: welding cranks
Date: 24 Jan 2001 00:04:23 GMT

Helmut Springer writes:

>> a better cross section.  Shimano Dura Ace has made hollow aluminum
>> cranks by forging a channel onto which a cover is welded.  More

> So 74** is already hollow?

Yes.

>> recently a method for forging tube shaped cranks has been
>> developed.  Steel cranks, as the ones mentioned, are made of tubes

> Are they able to forge the crank in one piece (Shimano's Hollowtech
> for example)?  Any pointer for some details regarding the method?

I have not seen the sequence of events but am told that the crank is
forged with a hollow cross section with an open pedal end that is
subsequently forged shut laterally leaving a witness seam that is
welded before machining the crank, thereby removing the seam that lies
in the neutral axis.  The tubular crank is formed around a core pin to
have an external fat end that, when forged shut, leaves a nearly
uniform thickness crank.  This explains why these cranks are
expensive.

Jobst Brandt      <jbrandt@hpl.hp.com>