Lotus Elan

Why you need a CV driveshaft.

PostPost by: vincereynard » Fri Jun 17, 2016 1:37 pm

Without wishing to start an argument, this is an interesting video of the limits of Mr Hooke joints.
(Perhaps he should have asked Newton?)

https://www.youtube.com/watch?v=gmV4qwLfOMY
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PostPost by: Chancer » Fri Jun 17, 2016 1:51 pm

The volume does not work for me for some reason but just having read the description I knew what misinformation was going to be put across, there will be no discernible vibration as long as a pair of Hookes joints are correctly in phase, get them 90? out and you will know all about it :D

However propshafts and driveshafts that use one Hooke joint and one elastic doughnut or guibo coupling will transmit vibration and accelerate the failure of the rubber coupling and/or the U/J, it will often manifest itself in the drive bolts loosening, becoming "eaten" together with the holes and/or shearing.
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PostPost by: john.p.clegg » Fri Jun 17, 2016 3:09 pm

I found this out about fifteen years ago, after having problems with my Performance Unlimited (modified) driveshafts ,that the joints need to be in alignment..and have had no problems since,thankfully the input/outputs have the same operating angles by design.

John :wink:

P.S who else as a kid used to put lolly pop sticks in the spokes of their pushbike?
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PostPost by: Fred Talmadge » Fri Jun 17, 2016 5:24 pm

Learned something new "phasing" I've always marked my u-joints but didn't really know why. Now I is smart. :)
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PostPost by: Chancer » Fri Jun 17, 2016 5:40 pm

john.p.clegg wrote:P.S who else as a kid used to put lolly pop sticks in the spokes of their pushbike?


Playing cards for us held on to the fork with a clothes peg!
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PostPost by: Andy8421 » Fri Jun 17, 2016 6:24 pm

The reason you need CVs in an Elan instead of Hooke joints is not the harmonic vibration caused by the Hooke joints, it is because Hooke joints don't plunge.

As the video shows, if the input and output shafts are parallel as they are in an Elan, the harmonic effects cancel out and the input and output shafts run at the same speed irrespective of the angle of the intermediate shaft. The usual fix for the Hooke joint plunge problem is to use splines, but they lock under load, so the better solution is to use plunging CVs.
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PostPost by: vincereynard » Fri Jun 17, 2016 9:15 pm

The way I saw the video is that Hooke joints transmit similar rotational speeds only if both angles are the same.
On an Elan, with its travel length and camber change, this would be very seldom and then only momentary.

OK with a propshaft that is a fair length and has little relative movement. Surely the fact Hooke joints can give large velocity variations is why Constant Velocity joints were developed.

The need for the extra sliding joint is another engineering disadvantage.

If it makes you feel better, CV driveshafts for a Triumph Stag are over ?1000!
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PostPost by: Chancer » Fri Jun 17, 2016 9:28 pm

I still have half a UJ kit left over from my Triumph stag that I had at 21 years old, 36 years ago, it was a special kit to replace sealed for life staked joints and came with a special depth setting and staking tool, it worked for me but they would never be able to sell something like that in todays litigious society.

To further explain British Leyland in their wisdom decided to save a few pennies by not machining the circlip grooves and grease nipple thread and doing away with them both.

The joints lasted maybe 20K miles, apart from the staked joint issue I dont see why you would want CV's on one.
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PostPost by: Chancer » Fri Jun 17, 2016 9:32 pm

By the way, it would be a very magic drive or propshaft that could have different rotational speeds at one end from the other if the input and output drives were not parrallel or at the same angle :D :D

I had a drive like that on my rubber band powered aircraft :D

Editted, I take that back now that I have watched the video with sound.
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PostPost by: Andy8421 » Sat Jun 18, 2016 7:55 am

vincereynard wrote:The way I saw the video is that Hooke joints transmit similar rotational speeds only if both angles are the same.
On an Elan, with its travel length and camber change, this would be very seldom and then only momentary.

OK with a propshaft that is a fair length and has little relative movement. Surely the fact Hooke joints can give large velocity variations is why Constant Velocity joints were developed.

!


The angles of the inner and outer joints on the Elan are approximately equal, and remain equal as the wheel moves up and down (the angles change, but by the same amount). The angle that matters is the relative angle between the diff output shaft and shaft through the wheel bearing. These two shafts are approximately parallel throughout the range of rear suspension travel.

This is not the case on a front wheel drive car where the joint nearest the wheel has to cope with the full range of steering angles, but the joint nearest the engine only has to cope with suspension movement. At full lock the angles of the two joints would be very different.
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PostPost by: jeff jackson » Sat Jun 18, 2016 8:57 am

Is this why the TTR and Kelsport drive shafts have sliding splines?

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Jeff
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PostPost by: Chancer » Sat Jun 18, 2016 9:38 am

I am pretty sure that early minis and perhaps 1100's had one hooke joint as the inner coupling, must have been a CV joint as the outer. All the later cars had doughnuts for the inner coupling.

Or have I been dreaming?
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PostPost by: Andy8421 » Sat Jun 18, 2016 10:08 am

Chancer wrote:I am pretty sure that early minis and perhaps 1100's had one hooke joint as the inner coupling, must have been a CV joint as the outer. All the later cars had doughnuts for the inner coupling.

Or have I been dreaming?


Early minis had CV outer and a nasty maltese cross rubber abomination as the inner coupling. Later minis had a non-plunging CV at the wheel end and a plunging CV at the gearbox end.
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PostPost by: Andy8421 » Sat Jun 18, 2016 10:15 am

jeff jackson wrote:Is this why the TTR and Kelsport drive shafts have sliding splines?

Regards
Jeff

This to cope with 'plunge' - because of the suspension geometry, the shaft length changes as the wheel goes up and down. CVs / Rotflex can cope with plunge, Hooke joints can't so need a sliding spline on the shaft.
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PostPost by: john.p.clegg » Sat Jun 18, 2016 1:22 pm

Andy8421 wrote:
Chancer wrote:I am pretty sure that early minis and perhaps 1100's had one hooke joint as the inner coupling, must have been a CV joint as the outer. All the later cars had doughnuts for the inner coupling.

Or have I been dreaming?


Early minis had CV outer and a nasty maltese cross rubber abomination as the inner coupling. Later minis had a non-plunging CV at the wheel end and a plunging CV at the gearbox end.


I believe the mini cooper inners were splined Hardy Spicer joints ...

John :wink:
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