Lotus Elan and Lotus Elan Plus 2 Forums

The November 2019 issue of Motor Sport has an extensive article on the Porsche 917/30, particularly Chassis No. 002. Not normally of tremendous interest to Elan owners however I could not help but notice that this 917 has metalastic-style doughnuts on at least the inboard ends of the driveshafts, see the photo on pages 74 and 75. Good heavens, 1100 hp in race trim and probably 1500 for qualifying, all going through doughnuts.

GT40s also have them on the inboard ends. Surprised me when I saw them.

Both cars ran much larger donuts than the Elan, had less angular misalignment and were replaced frequently. The 917-30 had 1100-1200 hp in qualifying boost.

For what its worth my understanding of donuts from an engineering perspective

Donuts were originally and still are used for industrial power transmission service. In this service they can handle reliably high torques and power. In this service the angular and axial movements are very small and starting torques low and speeds relatively high compared to a car wheel speed

They were then adopt by race car designers in the 60's for rear drive shaft service before the wide adoption and availability of CV joints. They worked ok again due to limited axial and angular displacements in race car suspensions and frequent maintenance of race cars

Chapman / Lotus pushed the boundaries and adopted them for the Elan. Rootes also did it with the Hillman Imp. It worked OK despite the significant increase in axial and angular displacements given the light weight and relatively lower power / torque being transmitted and the high quality manufacture of the original donuts. These days with cheap replicas the modern donuts struggle to meet an Elans needs with long term reliability.

Donuts are a good solution in the right application, the Elan rear drive shafts was pushing the concept to its limits at the time and today a CV is a better solution especially when comparing to the Donuts available today.

cheers
Rohan

Car manufacturers in the last 30 years have started using donuts again for propeller shaft couplings. In this service the work well as its similar to the original industrial services with the high speeds and lower torques of the prop shaft versus the axles shafts and lower angular and axial displacements. They are also using modern OEM quality manufacture.

For what its worth my understanding of donuts from an engineering perspective

Donuts were originally and still are used for industrial power transmission service. In this service they can handle reliably high torques and power. In this service the angular and axial movements are very small and starting torques low and speeds relatively high compared to a car wheel speed

They were then adopt by race car designers in the 60's for rear drive shaft service before the wide adoption and availability of CV joints. They worked ok again due to limited axial and angular displacements in race car suspensions and frequent maintenance of race cars

Chapman / Lotus pushed the boundaries and adopted them for the Elan. Rootes also did it with the Hillman Imp. It worked OK despite the significant increase in axial and angular displacements given the light weight and relatively lower power / torque being transmitted and the high quality manufacture of the original donuts. These days with cheap replicas the modern donuts struggle to meet an Elans needs with long term reliability.

Donuts are a good solution in the right application, the Elan rear drive shafts was pushing the concept to its limits at the time and today a CV is a better solution especially when comparing to the Donuts available today.

cheers
Rohan

Car manufacturers in the last 30 years have started using donuts again for propeller shaft couplings. In this service the work well as its similar to the original industrial services with the high speeds and lower torques of the prop shaft versus the axles shafts and lower angular and axial displacements. They are also using modern OEM quality manufacture.

Took this photo at the Porsche museum

With donuts you can have slight longitudinal compliance as well as angular displacement. You cant do this with CV joints unless you have splines also. They are also much more forgiving where shaft input and output angles are mismatched. The cyclic variation in shaft speed is absorbed by the rubber compliance and greatly reduces NVH particularly when the shaft installation angles aren't optimum. Greatly reduces the shock loading on drivetrain components too.

PaulH wrote:Took this photo at the Porsche museum

porsche museum.jpg

Those are the largest guibos I have ever seen!

Larger the diameter the less the shear stress in the rubber. That's why it works.

2cams70 wrote:With donuts you can have slight longitudinal compliance as well as angular displacement. You cant do this with CV joints unless you have splines also. They are also much more forgiving where shaft input and output angles are mismatched. The cyclic variation in shaft speed is absorbed by the rubber compliance and greatly reduces NVH particularly when the shaft installation angles aren't optimum. Greatly reduces the shock loading on drivetrain components too.

You have a point with shock loading, but I would challenge your other points - plunging CVs are available and widely used, though with less angular articulation capability than pure Rzeppa joints.

https://www.researchgate.net/figure/Front-wheel-drive-configuration-showing-typical-CV-joints_fig1_245314826

Angular mismatch between input and output shafts only matters with hook type joints, CVs don't suffer the same angular harmonic effects.

A "plunging" CV joint is simply a CV joint attached to a splined shaft - i.e you have to use a CV joint with splines if you want to accommodate longitudinal movement. Yes a CV joint does not cause shaft speed to vary with rotation angle unlike a standard UJ. You can also use a standard universal however with a donut at the other end and avoid the effects shaft speed variation and also not have to worry about splines - probably save some weight too. Yes they do have other drawbacks however!

2cams70 wrote:A "plunging" CV joint is simply a CV joint attached to a splined shaft - i.e you have to use a CV joint with splines if you want to accommodate longitudinal movement. Yes a CV joint does not cause shaft speed to vary with rotation angle unlike a standard UJ. You can also use a standard universal however with a donut at the other end and avoid the effects shaft speed variation and also not have to worry about splines - probably save some weight too. Yes they do have other drawbacks however!

All modern Cv joints accept plunge not on splines but on the rolling CV balls and tracks.

Usng a standard UJ with a donut will result in shaft speed changes with rotation if the single universal joint is at an angle. The Donut may adsorb this rotational vibration or it may not

Ah yes - accept the correction about the plunge joint! Maybe I should stick to what I know. I always enjoy a good argument though. No challenge, no argument = no progress!