Left Hand threads!?

If you’ve ever done any bicycle maintenance, you’ll know about them, the majority of bikes have two, sometimes three. We’re going to talk about the Bottom Bracket here though….

My mate emailed me earlier and asked me which way to turn a drive side BB cup to remove it…. Go on then!?

In the middle of what I was doing, I distractedly applied my engineering head to the question, and replied; “Anti-clockwise to undo……Think about what’s happening when you pedal?….. On the drive side, the action of pedalling tightens it up, on the non-drive side, the same thread would get unscrewed, so has to be a reverse thread.”

His reply was; “This is why its driving me nuts, have a look at this article from park Tools http://www.parktool.com/blog/repair-help/cartridge-bearing-type-bottom-bracket-service-bbt

“My mistake!” I replied! And then over lunch it got me to thinking, what I had suggested seemed to make perfect sense, and yet I was wrong. It then made me wonder about all the times I have removed them myself, and why I had never noted this point before, and in fact why I had no recollection of ever heaving a spanner the wrong way tightening up the cup I was trying to remove!? True, some of the newer outboard BB’s have arrows on them showing you how to treat them, but not the older cartridge type. I can only assume that in practice I look at the thread of the replacement first, to ascertain the correct direction, and have never questioned what is right before my eyes!

Sheldon Brown talks about “precession”, a term I’m vaguely familiar with from the dim and distant past sat in University lecture theatres listening to professors talking about Mechanincs, this article goes into a little more detail:

Subject: Left Hand Threads
From: Jobst Brandt

Date: April 28, 2004

On bicycles, left hand threads are used mainly in three places, on left
pedals, right bottom bracket (BB) bearing cups, and freewheel cones, to prevent
unscrewing under operating loads. Unscrewing occurs from precession, in which a round object rolling in a circular ring in one direction will itself turn in the
opposite direction.

For a pedal, a rotating load arises from downward pedaling force on a spindle
rotating with its crank making the predominantly downward force effectively
rotate about the pedal spindle. What may be less evident is that even tightly
fitting parts have relative clearance due to their elasticity, metals not being
rigid materials as is evident from steel springs. Under load, micro
deformations, enough to cause motion, occur in such joints. This can be seen
from wear marks where pedal spindles seat on crank faces.

Precession of right side BB cups is less obvious because the rotating load is
only partial. The largest load being chain tension, that together with the
moderately large downward force on the right crank and the smaller upward force from pushing down on the left crank, make 3/4 of a fully rotating load. For this reason some right BB cups have used right hand threads and some with left hand threads have loosened. The left BB cup with no significant rotating load has
little tendency to turn.

Freewheel cones are more obvious candidates for precession, their load being mainly radial, and rotating continuously in the direction that would unscrew a right hand thread. There are other such but less common threads on bicycles.

Precession forces are large enough that no manner of thread locking glues,
short of welding, will arrest them. Mechanical fretting, the micro-motion of
tightly fitting parts moving against one another, is the mechanism of this
motion. Motion in these joints causes visible fretting rouge, red iron oxide, on
the shoulder of the BB cup and on the face of the pedal spindle.

Left hand threads would not be required on left pedals if a design common on
cars were used. Before the advent of conical lug nuts, many cars used left hand
threads on left side wheels. Today, stories of wheels rolling away from cars no
longer make news, the conical seat having solved this problem on car wheels as
it could on bicycle pedals.

However, unscrewing is not the main problem for pedals, but rather crank
failure caused by fretting erosion of the pedal eye. Fretting initiates cracks
that can cause sudden and unsuspected pedal separation when the eye of a crank
breaks. Because this occurs equally with right and left cranks it is the more
important reason for a conical spindle face and crank eye. This has been tested.

Jobst Brandt

So there you have it, something I really ought to have known, but had never really applied my mind to. It’s obvious now it’s been spelled out to me!

This entry was posted in Biking. Bookmark the permalink.

3 Responses to Left Hand threads!?

  1. Jimbo says:

    Lefty loosey Righty tighty innit.


  2. Dr Gonzo says:

    Jobst Brandt and Sheldon Brown in one post!


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