Ride UK Tech Columns
#71 Lard Inspired
quest for reliability and tranquility
(This article first appeared in Ride UK issue No. 71 and is reproduced
here by kind permission of Ride UK.)
The last tech column had a stupid name. It's not the name that I gave it
so please don’t blame me. It was entitled "Truth" originally but for some
reason it got changed to "Build Your Own Wheels", which, although a good
idea, was very little to do with the article which was just about truing
wheels. This month the column is called "Lard inspired quest for reliability
and tranquillity". If it has been changed then you know it's not because
of me. I don’t think Mark ever proof-reads these articles, I think he just
does a "search and replace" on fuck piss shit twat etc so this paragraph
should escape editing and you will all know the real title…
So Lard sent me a lovely e-mail all about how "The other day my pedal snapped
as I cranked out in front of an Audi TT at a 3 lane roundabout."
So he wanted to know if there were any standard checks you should do before
riding your bike.
Well yes. But I really want to talk a bit about how things break before I
mention them. Unfortunately Lard made it pretty clear that I waffle on like
a dick way too much in these columns so he isn't going to be too pleased
about it if I do.
Look away now Lard.
I just can't see how I can talk about avoiding part failure without talking
about how things fail. Failures basically fall into two categories which
I will call "catastrophic" and "progressive".
In Lards example his pedal spindle snapped clean off as he accelerated. This
usually causes you to throw yourself over the bars under the approaching
bath tu- , sorry Audi TT and could safely be called catastrophic. One minute
the pedal was doing its job fine, holding up Lard's manly figure and transmitting
all the power of his lithe young legs to the cranks, the next moment, complete
breakage. So what changed?
A pedal axle is usually steel, and in the case of a "loose ball" pedal (ie.
One that doesn't use cartridge bearings) it has to be at least surface hardened
to resist the wear of the balls. As steel gets harder it generally gets stronger
but you never get owt for nowt so it also gets more brittle. In a low temper,
steel may start to yield or stretch and bend at a stress of say 400 Newtons
per square millimeter, as we increase the hardness of the steel its strength
may increase to over 1000 Newtons per square millimeter. The downside is
that while the low temper steel would stretch and bend a lot making it obvious
that it is breaking, the harder material will barely move and give little
sign of anything being wrong right up to the point where it suddenly snaps.
This is the dilemma that faces every engineer using almost any material to
make bike parts. For safety's sake he wants the part to make it obvious that
something is going wrong before it breaks but on the other hand he wants
it to be as strong and light as possible.
To complicate matters he can do a lot of fancy juggling with the tempers
and hardnesses of the component. He can harden the surface of a component
like an axle so that the outer layers, that have to deal with bearing wear,
are much harder than the inner core of the axle.
A typical pedal axle is therefore "case hardened" with a tempered core which
is more resilient.
So what happened to Lard? Did he simply pedal so damn hard that the pedal
snapped off in one go? Perfect then snapped in a fraction of a second? Probably
not. The likely explanation is that the pedal had been slowly breaking for
sometime. My guess would be that Lard had had his pedals a while and that
they bent almost as soon as he fitted them. I am sure that we have all experienced
the feeling of riding someone else's bike and thinking that the pedals and
cranks are bent. The reason for this may be that they are; or that yours
are and you are used to it, so straight "feels" bent; or most likely both
sets are but in a slightly different way.
What I would guess happened is that a while ago, Lard bent his pedal, at
which time the brittle outer "case" cracked through and the tempered inner
core bent slightly. But it didn’t break completely because the movement of
the inner core absorbed the energy of the impact. Since then repeated impacts
will have made this small crack grow slowly across the pedal, maybe repeated
impacts slightly work hardened the axle as well.
Over time a tiny crack opened up across the pedal, then on that fateful day
it was so weakened by the crack that Lard's explosive accelerative effort
snapped it clean through the last part and dumped him on his head in the
street. If Lard had gone back and looked at his snapped pedal spindle he
probably would have seen that the broken cross-section of axle had two distinctly
different surfaces. A smooth flat surface where the crack had been slowly
growing and a rough crystalline looking surface where it finally broke. The
crack might only have been small, maybe just a tenth or a twentieth of the
way across but that is the crucial stage.
So now we know how and why it broke how can we catch it before it happens
again? Well the key is to identify the time when it first bends and replace
it then. This is easy enough to do, just remove the pedal from the crank
and spin the axle in the body. If the end of the axle "wobbles" as it turns
(and the bearings aren't loose) then you know the axle is bent. Unfortunately
most pedals seem to bend slightly within a few rides yet are quite capable
of lasting years before the crack grows large enough for them to snap. We
have very little option but to hope they need replacing for some other reason
before then or just wait to slam your knees into the bars…
The really worrying one is forks. If your forks snap you really know it,
the chances are it will happen on a big jump or other hard landing and you
will already be heading towards the ground with some speed. At an approach
speed of say 30mph if the forks break you probably have less than a tenth
of a second from the point they break to your face hitting the ground (or
your bars), no time to prepare for the slam of your life. Luckily forks do
not need a hardened bearing surface for the headset (because there is a separate
race) so they do not need to be surface hardened, crack development is much
less likely and being a big hollow tube it is much more likely to bend before
it cracks. But if you ignore a bend it will crack eventually. Never risk
it with forks, if they are bent replace them immediately!
It should be fairly easy to spot a bent fork, they nearly always bend on
the steerer tube just above the bottom headset race so any bend is obvious
just by looking at the bottom headset parts. You don’t need to take anything
apart to do it just look at the bottom of the headset from the side. If the
bottom of the fork crown is not perfectly parallel to the bottom cup then
chances are the forks are bent.
Cranks are another case where fatigue will be the big worry. Small cracks
will develop and grow slowly then faster across the arm until it is ready
to snap all the way. These will usually start on or near a weld so keep an
eye out for them. If you see them then you can usually weld them back up
fairly safely but if you ignore them they will eventually snap under you.
I am not suggesting that you should get down on your hands and knees after
every trick and examine every inch of your bike with a magnifying glass for
cracks, but if you have a little look round now and again it may save you
some pain down the line.
There is one other thing that can help a lot, noise. As a crack grows it
will often make a clicking noise or a creak or groan. If your bike makes
any funny noises keep an ear out for more. I once had an Auburn race frame
which I rode steeet on. At one years KOC I noticed it was making a really
annoying click every time I cranked hard on the pedals. Sprinting at the
pyramid banks it would go click click click on every pedal stroke.
Later that day I tried to find the cause, I started looking the bike over
and wobbling things. Is the headset tight? Check! Back end bolted on tight?
Check! Pedals and cranks tight? Check! Stem tight? Check! Massive crack in
downtube? Check! Ah. That might be something to do with it I suppose…
The crack was three quarters of the way round the downtube and open a good
three millimeters at the bottom, one more big impact and I probably would
have torn the headtube clean off, with the inevitable injuries.
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