Knock-off Spinner Tightening Rationale (Yet Again)

[elj221c]

Small hole .......through the thread of the drive peg,

????

No threads on my drive pegs......

Ah, maybe you are referring to four peg 26R hubs with the pegs nutted onto the wheels?......

[JohnDanbyRacing]

Small hole .......through the thread of the drive peg,

????

No threads on my drive pegs......

Ah, maybe you are referring to four peg 26R hubs with the pegs nutted onto the wheels?......

Yes sorry was referring to the 26R hubs. Round one of the wheel spokes also works but can rub the paint over time

[abstamaria]

I have 26R wheels and spinners and did safety wire them when I was still racing. I didn't think of using the pegs, John. I slipped a plastic sleeve over the safety wire where it wrapped around the spoke, so it wouldn?t cut into the magnesium.

Since I and the Elan have retired from racing, I don?t safety wire anymore, but the spinners of course still have the holes I drilled for the safety wire. One is visible in this photo.

Andy

[abstamaria]

To try to understand Chapman's theory -

On the right side of a bicycle, the pedal is threaded so that it tightens clockwise. As the bicycle moves, the crank on that side moves clockwise. However, the pedal attached to the end of the crank rotates counterclockwise on its axle (spindle) as the crank rotates clockwise (look at your bicycle). In theory, the counterclockwise rotation of the pedal should unscrew it. But it doesn?t. The reason is that mechanical precession acts opposite to that counterclockwise rotation and therefore tightens the pedal.

The reverse is true on the left side of a bicycle, where the pedal thread is left-handed and the pedal tightens counterclockwise.

If my understanding is correct (and it probably has major flaws), that process should apply to the Elan spinners, which are threaded the same way as a bicycle.

The question is - why then are spinners on all other cars threaded (and tightened) opposite to Lotus's? Why does the female taper make a difference?

[rgh0]

Hi Andy

Over the years I have also thought "why is it so" and you post finally prompted me to try to understand the forces involved better and go beyond the reel and cap demonstration

I think the key concepts to grasp are as stated in the wikipedia page on the topic in the side box with the rotating demonstration

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

i.e As the wheel rotates 360 degrees the load contact point due to car weight between the nut and the wheel moves through 360 degrees also.

In the Lotus case at the contact point, the nut diameter and thus circumference is a very small fraction smaller than the wheel diameter and thus circumference. This occurs even in a tightened nut due to flex in the components under the load of car weight.

This means that when the wheel has completed a full 360 degree rotation the nut contacts the wheel slight behind where it did the rotation before due to its small circumference. This produces a force that is trying to drag the nut in the direction of rotation. Thus right hand threads on right side

The reverse is true for a Rudge style knock on where the nut diameter is very slightly larger than wheel diameter at point of contact and thus as the wheel spins it produces force trying to drag the nut in the opposite direction to spin. Thus left hand threads on right hand side.

If the wheel and nut assembly is sufficiently tightened as to be rigidly locked up then this mechanical precession will not occur which is why you can drive with the hubs on the wrong side if you keep the wheel nuts sufficiently tight.

If the wheel / nut assembly is slightly loose or sufficiently flexible then mechanical precession will prevent it loosening further with the correct assembly to match the nut style.

In theory if you drove in reverse far enough your wheels would loosen if not sufficiently tightened

cheers
Rohan

[abstamaria]

Many thanks, Rohan. That makes a great deal of sense and is clear. I have not seen the phenomenon explained that way. Thank you.

One question - why would a point on the smaller circle rotate less than a point on the bigger one? If they spun on the same axis in the same direction, wouldn't the contact points remain the same on both circles after a 360 degree turn?

I have been reading your note since you posted it, trying to understand. My brain hurts.

Best regards,

Andy

[rgh0]

The contact points try to stay the same yes. But because the contact circumference of the nut is fractionally smaller than the contact circumference of the wheel as they role through 360 degree the nuts "one turn" is a fractionally less distance than "one turn" of the wheel. This means the wheel drags on it to keep up thus tightening it.

The difference in circumference is very very very small and when the nut is tight only caused by elastic deflection of the nut inwards under the load of the car weight and the wheel itself outwards under the load of the car weight..

cheers
Rohan

[abstamaria]

Rohan, bear with me on this please.

If the spinner and the wheel were spinning on the same axis, their contact points would stay togetherr, as the outer contact point would simply spin faster. I am thinking of two points on a spinning record disk, with one point closer to the outer edge of the disk.

On the right side of the Elan, the contact (load) points on both the spinner and the wheel rotate counterclowise on their respective axes. Their axes are not the same, because of the gap between the spinner and the wheel. They are not concentric. As the outer diameter of the spinner is smaller than that of the mating (inner) surface of the wheel, its contact point will make a 360 degree turn faster or ahead of the wheel contact point, which has to travel al longer distance (larger diameter). The spinner will will want to align with the wheel and so will jerk back in a clockwise motion so that its contact point will again coincide with the original contact point on the wheel.

Is that what you were explaining? I probably have it all wrong.

Andy
So sorry about my being dense on this.

Andy

[rgh0]

Hi Andy
Yes your description is the equivalent of what I am trying to say. There is no real "gap" unless the wheel nut is really loose but due to elastic deformation the axis of the wheel is slightly displaced from the axis of the nut due to the weight of the car.

As the contact point moves around the circumference of the nut with the turning of the wheel the nut tries to slip back as its circumference is very very slightly less than the circumference of the wheel due to this elastic deformation. Note as the wheel turns clockwise with respect to the car, the contact point turns anticlockwise with respect to the wheel and the nut. Visualise putting a mark on both the wheel and the nut and then completing a full turn of the wheel, the mark on the nut ends up behind the mark on the wheel in the anticlockwise direction of the contact point rotation, but ahead in the clockwise rotation of the wheel thus tightening the nut

I think I have it right, as yes, it is very hard to visualise. The principle is correct on why it happens its just hard to describe which is why I am sure Chapman used the cap and reel demonstration!

cheers
Rohan

[abstamaria]

Hello, Ronan.

I am thinking the two contact points don't have to "catch up" with each other. The fact the contact point on the spinner after it has rotated 360 degrees will be ahead of the original contact point on the wheel means the spinner has rotated clockwise relative to the wheel. Or so it seems to me.

The reverse Is true if the "loose" part is the outer circle, as the spinner on on R-W wheels.

I spent time during office hours trying to understand.

[rgh0]

Yes that's the way I understand it.

cheers
Rohan

[StressCraxx]

Gents,

Thank you, that is a very good explanation. No matter which way the K/O spins, the condition of the drive pins and drive pin holes are important. Any forward/aft looseness between the drive pins and the pin holes in the wheels is an opportunity for the K/O and wheel to loosen under braking or acceleration, no matter how much torque on the K/O. The lockwire will not save you.

Regards,
Dan