Lotus Elan

Knock-off Spinner Tightening Rationale (Yet Again)

PostPost by: abstamaria » Fri Jan 09, 2015 3:35 am

I know there have been various threads on why Elan KOs tighten clockwise on the right-hand side of the car and counterclockwise on the left-hand side. This is of course the reverse of the procedure for Rudge-Whitworth KOs found in MGs, Triumphs, and several other cars.

The reason given in the book "Colin Chapman Lotus Engineering" is that in the RW system pressure on a loose spinner (being female) is at 12:00 of its inside diameter (the gap being at the bottom), while in the Lotus system pressure on the spinner (being male) will be at 6:00 of its outside diameter. (I am stating this very badly). And then of course there is the cap within a roll of tape demonstration, which shows that if you move the cap or the roll in a circular motion, without rotating it, the following cap or roll will move either in the same or opposite direction. The relationship is called "(quasi) epicyclic."

I did the tape and cap demonstration, with satisfying results, but had the following questions:

1. If the roll of tape represents the wheel and the cap inside it the spinner, why shouldn't the tape be rotated on its axis, in the same way a wheel turns.

2. Why would the spinner be in loose contact with the wheel, when we've hammered it on?

3. Finally in the roll/cap demonstration, if the roll is moved (not rotated) in a circular clockwise direction (as I assume would be the case on the right-hand side wheel), the cap will rotate on its axis in a counterclockwise motion, thereby loosening the RH Elan spinner.

I read more about this (bicycle pedals, among other things, tighten as per Lotus spinners) and the phenomenon seems to be "mechanical precession," which is explained in Wikipedia (nice moving illustration there too):

http://en.wikipedia.org/wiki/Precession_(mechanical)

I think the answers to my questions are as follows, but please correct me as necessary (I studied the wrong things at school):

1. It is not the wheel rotation that loosens the spinner, but the pressure on the contact point with the spinner (always at 6:00 in the Elan spinner, I assume).

2. As the Wikipedia article states, there is always a minute, perhaps microscopic gap between two metal pieces screwed or jammed into each other, because of elasticity and other things. This minuscule "looseness" is enough for mechanical precession to take effect.

3. I don't have the answer here. Perhaps the correct way of replicating the effect of a wheel rotating on its axis clockwise would be to move the roll (without rotation) counter-clockwise, which is the direction of the point of pressure on the spinner as it rotates.

I would appreciate enlightenment here, please. If this has been explained at length, please point me in the right direction; a search wasn't very helpful).

Then I will have to apply all that to a RW spinner to see why it behaves differently.

Many thanks, and my best wishes to all for this New Year.

Andy
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PostPost by: abstamaria » Fri Jan 09, 2015 3:43 am

If the Wikipedia link doesn't lead you to the article, Google "Precession (mechanical)," which should should lead you there.

Andy
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PostPost by: jimj » Fri Jan 09, 2015 10:08 am

On a purely practical level; if you compare the number of people who`ve had a wheel fall off with the number who haven`t, I`d suggest that the Chapman system appears, statistically, to work.
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PostPost by: Chancer » Fri Jan 09, 2015 12:00 pm

Leaving aside epicyclic motion which is really concerned with a small wheel or gear Rolling Inside or outside of a much larger one and not two equal diameters.

A simplistic view is to consider what would happen if a Wheel was fitted without a drive stud or that it failed in use, Under braking the reaction would be movement between the Wheel and hub which would tighten the spinner, a safe situation.

This is true of both front and rear wheels although the rear spinners would also loosen Under acceleration, the front steering wheels should be the priority.

I have never understood why some trailers have left hand threaded Wheel nuts on the right hand side, or is it the left?
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PostPost by: elansprint71 » Fri Jan 09, 2015 2:51 pm

My Grandad used to be able to roll his cap...

I'll get me coat....................... :wink:
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PostPost by: abstamaria » Sat Jan 10, 2015 9:12 am

Thanks, Jim. l have no doubt Chapman was right. I'm just trying to understand why. It's not easy!

Best regards,

Andy
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PostPost by: abstamaria » Sat Jan 10, 2015 9:20 am

Thanks, Chancer. The example you gave (locating studs sheared off) may not be a good one, because that should apply to Rudge-Whitworth spinners too. And they are theaded dilferently (Left handed thread on right side).

There seems a different force at work. Perhaps precession.

Andy
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PostPost by: gordont » Sat Jan 10, 2015 10:49 am

I lost a wheel thankfully at sub 5kmh, anyone else?
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PostPost by: powellsmail » Sat Jan 10, 2015 11:26 am

My rear left will unscrew itself if I ignore it long enough. But only that corner. Gives a feeling a bit like the tyre going down, rear feels vague. I have one of those big sockets and just tighten them all up on a regular basis. I am running minilites and always wondered if the original steel wheels had a degree of spring in the wheel which pushes back into the spinner, which the minilight does not have.
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PostPost by: 69S4 » Sat Jan 10, 2015 11:43 am

gordont wrote:I lost a wheel thankfully at sub 5kmh, anyone else?


I lost a n/s rear very soon after buying the car. I was proceeding down the A1 south of Peterborough (my passenger friend was a police officer :lol: ), not particularly quickly - just keeping up with the traffic, when there was a loud clunk. The rear of the car sank down and we went into a slow spin, finally coming to rest (fortunately) at the edge of the road with both of us unhurt. The rear wheel and hub together had come off the taper. Closer examination revealed the nut rattling around inside the spinner and even closer examination revealed it to be some kind of alloy rather than steel. It had come off by leaving the threads behind on the taper. Almost certainly a D.P.O "modification" (he was a helicopter technician!)

No harm done and other than a flat ground on the bottom of the rear strut (still there) no damage done - but it could have been a lot worse :(
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PostPost by: jimj » Sat Jan 10, 2015 12:42 pm

A few years ago a scrutineer on a classic rally queried the spinners being threaded the "wrong" way. He had a roll of yellow tape for marking towing points so, with a small jar rotated inside the tape, I demonstrated how the jar turns the tape one way and the tape rotates the jar the other. He was convinced I was a genius.
I`m not, but..........it seems to me that the spinner is just a nut and if you imagine the wheel is like a washer being held by the "nut" on a threaded whatever, hub in this case, it`s no different to all the other rotating things secured by threaded nuts, which don`t come undone. In the case of a wheel, unlike a washer which could rotate if slightly loose, the pegs, or splines in other cars, prevent rotation. So, stay with me on this, the only potential for a spinner to loosen or tighten, under braking or acceleration, would be the momentum of the ears of the spinners changing relatively to the motion of the car. As braking is much more severe than accelerating the ears would tend to be thrown forwards at the top, clockwise on the right, anticlockwise on the left, so the Lotus way is the right way. But we already knew that didn`t we?
Having said that, the fact is that other cars, Healeys, XKs, etc. don`t suffer from wheels falling off either so it`s not a significant issue.
Jim
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PostPost by: rcraven » Sat Jan 10, 2015 2:21 pm

abstamaria wrote:The example you gave (locating studs sheared off) may not be a good one, because that should apply to Rudge-Whitworth spinners too.
Andy


Rudge Whitworth wheels and hubs usually have splines not studs, and the splines can eventually wear and cause slackness.
Robert
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PostPost by: abstamaria » Sun Jan 11, 2015 7:18 am

If the splihes on a RW wheel sheared off completely. on braking, the right wheels would continue to spin clockwise. That would seem to tighten a Lotus spinner, but loosen a RW wheel (which uses LH thread on the right side of the car). Why the difference?

Geodetic precession is perhaps the answer. Kidding.

http://www.popsci.com/fast-orbiting-pul ... fb&src=SOC
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PostPost by: abstamaria » Sun Jan 11, 2015 10:10 am

Seriously, now, the term is "mechanical precession, explained in relation to bicycle pedals here -

http://momentummag.com/features/pedal-precession/

That seems to explain Chapman's decision regarding the direction of his spinners.

Now, I have to understand why RW spinners are threaded the opposite way, simply because they are female nuts.

Perhaps someone smarter than I am can explain.

Best,

Andy
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PostPost by: rcraven » Sun Jan 11, 2015 10:53 am

It's because they have a female taper not because they have a female thread.
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