RH vs LH threads

A few days I had a "zoom" get together with a few cycling friends. One topic of discussion was why do bicycles use left-hand threads in certain places? I let my friends discuss this then jumped in with the right answer.

When I say the "right answer", I mean what I always believed to be the reason. Listening to my friends who are not "engineers" try to work out what is not necessarily an intuitive thing was interesting.

So, over to you guys, whats the reason? I won't give my answer just yet so as not to influence any possible discussion.

Hi John it is obvious that if you used two RH threads on cycle crank ends one of the pedals would keep coming unscrewed. As you pedal and if the correct way installed with LH and RH threads they will tend to tighten as you pedal.

David

In some ways a bicycle is a bit like a bench grinder.

I don't know if it is the case with all modern cranks but I agree with David on this, when the pedals were the screwed in type they came marked LH and RH on both the pedals and cranks on ones I changed out.

I recently re-did the preload on my mill's spindle bearings and I have a written note on the mill saying LH threads and an arrow to the top nuts. I remember the first time I tackled the job I was puzzled over my inability to get the top locknut loose. There was no visible thread to give a clue so when it finally dawned on me that it was LH I put the note on to make sure I didn't go through the same head scratching in the future. Once again this makes sense from the point of is it tightened in normal use? With the potential for intermittent cuts causing vibration it seems to me that the importance of the nuts tightening in use rather than loosening is quite high.

Martin C

Same reason BB threads are left and right handed.

Before the advent of spigot-located roadwheels on large goods vehicles, the securing nuts on the left hand side of these vehicles were always left hand thread, but now with spigots they are all right hand. We were always told that the reason for the LH threads was to stop the tendency for the nuts to unscrew as the wheels rotated, but this no longer seems valid with spigot wheels. Initially with these wheels there was a rash of cases of wheels coming loose and sometimes falling off, but this was overcome by improved materials for wheel studs and nuts and the now extensive use of torque-limiting wrenches to tighten the wheel securing nuts to the correct tension.

This explains why a proper pedal spanner is relatively long; you need the leverage to loosen the pedal.

Corrosion will also add to the force required to remove pedals. Pedals are a sort of "fit & forget" component.

Well I've read 'Zen and the Art of Motorcycle Maintenance', which asserts there are two ways people view technology. The romantic view is all about appearances, imagination and creativity, and feelings matter far more than facts. The classical view focuses on reason, facts and design. Thus there's a collision between riding a motorbike, which is romantic, and maintaining it which is classical, made worse because many people can only think in one mode. Engineers who don't care about looks as long as it works, who delight in slag heaps, and Lay persons who can't cope with a screwdriver, but have earned enough money to fill their homes with expensive art work. 'Nude, Green Leaves and Bust' sold for $100M and there's no point in only owning one Picasso!

Unimaginative Classical and Romantic thinkers may be so extreme in their views as to despise each other, which is bad news to any endeavour relying on teamwork to make a profit!

Given the dichotomy between Classical and Romantic thinking, the engineer uses left-handed threads for engineering reasons but romantic users believe it's done to confuse and humiliate them - yet another reminder they don't understand how the physical world works, which contradicts their view that they are the masters and technology the servant.

Meanwhile, the chap who understands left-handed threads believes he should be running the world, despite copious evidence engineers don't understand the 'what we want' people issues driving politics, religion, economics, myths, pseudo-science, and fashion...

Schulz captured the collision when Snoopy decided he was the Red Baron:

Imagining himself to a heroic WW1 aviator with a life full of high-adventure, the truth is he's just a dog, sat oh his kennel, wearing a scarf and flying helmet. That's you and I...

Dave

PS. Reading the book, which is about values and the 'Metaphysics of Quality', I found the first half delightfully readable but the second half blew my mental fusebox. It plunges deep into the meaning of 'quality' and is too hard for me: I'd be grateful If anyone can explain part two in simple terms!

The book is one of the reasons I dislike 'quality' so much; either the word is meaningless without a specification, or the concepts are beyond my comprehension.

Edited By SillyOldDuffer on 14/03/2021 10:10:04

The only way to explain that book is the book itself. Good luck. You are trying to understand it in the classical way, like the first half. Try the Romantic way. Its all about how the book makes you feel.

Even Pirsig struggled to define Quality as he meant it. Something to do with subject and object becoming one. Maybe. EG when you become "totally involved" in machining a piece and forget about the surrounding world. Your hands are operatiing the lathe control handles, your mind is absorbed in the cutting of the metal by the tool. So you are probably doing Quality every day, just didnt know it.

Edited By Hopper on 14/03/2021 10:35:57

ISTR too that the bearing retaining ring-nut on the lefthand side of the old bicycles' bottom bracket (pedal axle bearing housing) was also left hand thread so it tightened as one pedalled.

re that bearing retaining nut - why are the spanner flats on them rounded off so they are impossible to grip?

So you have to buy the special spanner with the big hole in it that is rounded at the ends and flat in the middle. (Or use a hammer and chisel as our old man taught us on the "new" bikes we bought home from the tip.)

Left Hand threads are relatively more difficult to make, so are only used where really needed, but do have their uses.

Left Hand threads are also used in conjunction with right hand threads in such places as scissor jacks for cars, and turnbuckles to adjust the length of straining wires or links..

In other places, (Such as pulleys in vacuum cleaners, or one end of a bench grinder, as on bicycles ) they are used to prevent the item unscrewing during normal use.

The nut that retains the pulley on top of the quill on my Mill/Drill uses a Left Hand Thread.

Howard

It's utter drivel.

Is that simple enough?

And the little yellow plastic pointer tell tales.

Hence the rule of thumb to "grease all threads".

Hi, or better still, use anti-seizing compound, as grease can dry out over time and go quite hard.

Regards Nick.

Tony

Either that or you don't understand it. Bit like the Romantic with the left hand thread that SOD mentioned.