CV drive shaft conversion

[2cams70]

It is time to stop when we shift to talking about people instead of the topic.

I am asking everyone to step back and take a breath.

I believe we can all agree on the following:

1. The Elan is a great car.
2. Anyone who owns an Elan is an intelligent person. (maybe a little weird, but intelligent)
3. We respect intelligent people.
4. Intelligent people will come to different conclusions. This does not make them wrong; it just means they came to a different conclusion than mine.

There are so few Elans left in the world. We should be supporting one another.

Thank you,

jeff

But let’s not make this forum too dry and boring Jeff. I’ll leave if it is. I don’t own an Elan. I’m only here because I have cars with Lotus engines and because I find the discussion and the people on this forum interesting. There used to be some really eccentric interesting people on this forum whom it seems have unfortunately left.

[AshleyPark]

I don’t own an Elan. I’m only here because I have cars with Lotus engines and because I find the discussion and the people on this forum interesting. There used to be some really eccentric interesting people on this forum whom it seems have unfortunately left.

A forum troll in other other words.

[The Veg]

One offers long term reliability over potential failure, while the other provides a cushioned start

Just my own anecdotal observation- my Plus 2 was converted to CVs by a previous owner so I've never driven on donuts, but I've never noticed anything that would indicate that setting the car into motion needs any cushioning. Even when I'm less than kind with the clutch there's no noticeably unpleasant NVH.

As for the weirdness of Elan owners, that's one of the reasons I love these cars and have always felt right at home among my fellow enthusiasts!

[2cams70]

I don’t own an Elan. I’m only here because I have cars with Lotus engines and because I find the discussion and the people on this forum interesting. There used to be some really eccentric interesting people on this forum whom it seems have unfortunately left.

A forum troll in other other words.

Yes Ashley I’m a forum troll. I don’t own an Elan but I’d like to think (others may disagree) that I can speak with some authority on at least some of the Ford parts that make probably around 50% of a Lotus Elan. I do have two Lotus Ford Escorts and other assorted vehicles of interest to me here and there. I am a qualified professional mechanical engineer although admittedly I’ve never had to work at a detail design level and couldn’t navigate a CAD machine for nuts in my career. Unlike most on this forum I’ve actually worked in companies that make vehicles so I have a bit of an idea of how it is all done from the inside. I have over time contributed somewhat more than you have if the number of posts is an indicator. It saddens me greatly and it’s not just to do with this forum but with the world as it is today more generally - that is that people who ask questions or challenge are hounded down relentlessly. If you had bothered to read carefully what I’ve said through this topic I’ve never actually said that CVs are a bad thing or that doughnuts aren’t dangerous. All I’ve said is that there may be a disadvantage associated with CV’s in that the diff mountings might not last as long as they would otherwise and I think on the face of it the engineering analysis would confirm that. Whether it’s significant or not on a low mileage classic is another question. Probably it isn’t and the pro’s outweigh the cons.

I’ve done it before and left this forum but came back again because I still found it interesting and thought I had a bit of know how and experience that was worth sharing with people to help them overcome their issues. If you think what I say is crap and a waste of space I’ll leave again and probably won’t come back again. People on the whole seem to be whole lot nastier now than how it used to be back in the day. I think the internet is to blame for that.

[rgh0]

Having been somewhat amused by the posts I decide to dive down this rabbit hole one more time to see if some simple analysis can answer the question "Do Donuts contribute in any signficant way to drive train cushioning".

Another way to put it is if you consider the drive train to be a torsional spring then does changing the spring rate to make it stiffer by replacing the donuts with Cvs increase the maximum torsional load on that spring.

The maximum load on the spring is determined by the short term impact load over a few seconds generated by events such as drop the clutch starts with a high HP engine and sticky tyres. This impact load can be represented by a mass times the mass velocity change imposing a torsionai load on the drive train over this period . We dont need to know what this load is, just need to assess how the loads and stresses change with increasing torsional stiffness

The effect of making the torsional spring stiffer is to reduce the torsional defection of the drive train overall. (Hence the elimination of the surging experienced when replacing donuts with Cvs.) However the peak torsional load and stress on the drive train is the same given the same impact load regardless of system torsional spring stiffness within the limited range of deflection change allowed by donuts verus Cvs in the overall drive train system. i.e the spring just defects less until it matches the imposed impact load and the maximum load does not change by making the spring stiffer.

The impact load is generated by the engines torque combined to some degree by any additional load generated by the engine and flywheel slowing as the clutch is dropped. In a racing drop the clutch start I hold the engine at around 4500 rpm and it may drop to 4000 rpm as the car moves off without wheel spin and I open the throttle to hold the revs, this all happens over a couple of seconds until the clutch stops slipping. With donuts they will wind up the few degrees of movement they allow in much less than this time so I dont believe the imposed maximum torsional load will change signficantly between Cv's and donuts

So in conclusion maximum torsional load and stress is not affected by drive train torsional flexibility in a first order of magnitude analysis. I am sure there may be other more subtle effects with a more detailed analysis but this explains why the practical experience shows no detrimental effects of fitting Cvs. Drive train cushioning may impact things like NVH but it does not reduce maximum torsional stress on drive train components.

cheers
Rohan

[2cams70]

Sorry Rohan - and by the way I do as always appreciate your thoughts. At a basic level we have force being a product of mass times acceleration (F = ma). That’s one of the few things I remember from a long time ago! Doughnuts are like having springs in the system. During the transfer of torque they deflect and limit the maximum acceleration the other half of the coupling experiences when the load is being transferred from one side of the coupling to the other. Acceleration is less and mass is less with doughnuts as compared to CV driveshafts. Therefore peak loads are less, maximum deflection on the diff ears is less, fatigue is less and things last longer. When things are in a steady state there is no difference but during torque transfer from one side of the coupling to the other there is a difference.

To draw another analogy. If you threw out the cushioning springs in your clutch plate and replaced them with solid metal blocks I don’t think your drivetrain will last as long as it would have otherwise.

The most important thing though is that your life outlasts the life of your diff ears. For some that might be something to consider in the decision but for a true Lotus fan that shouldn’t enter the argument because a Lotus driver is supposed to accept living life on the edge

And one other thing. I’m very pleased to hear that you are amused. I think a bit of friendly banter and humor is important. Otherwise things could become very boring like the forum becoming mainly about very politely discussing so as not to upset - different brands of car polish

[h20hamelan]

https://www.youtube.com/watch?v=xVhRm0eJ4A4

[Andy8421]

Doughnuts are like having springs in the system. During the transfer of torque they deflect and limit the maximum acceleration the other half of the coupling experiences when the load is being transferred from one side of the coupling to the other. Acceleration is less and mass is less with doughnuts as compared to CV driveshafts. Therefore peak loads are less, maximum deflection on the diff ears is less, fatigue is less and things last longer. When things are in a steady state there is no difference but during torque transfer from one side of the coupling to the other there is a difference.

To draw another analogy. If you threw out the cushioning springs in your clutch plate and replaced them with solid metal blocks I don’t think your drivetrain will last as long as it would have otherwise.

And one other thing. I’m very pleased to hear that you are amused. I think a bit of friendly banter and humor is important. Otherwise things could become very boring like the forum becoming mainly about very politely discussing so as not to upset - different brands of car polish

Firstly, I find the discussions on here always to be very positive, just because members disagree is no reason to close a thread. I have always found this forum to be respectful and polite. Having a point of view challenged is one of the reasons I post on here, I have learnt a lot from other members views. No one is forcing you to read a discussion if you don't want to.

In answer to 2cams' points:

I think you are confusing the 'rubber mallet' analogy when thinking about peak torque. While the rubber doughnut will absorb a shock load, clearly it can only deflect so far, at which point it will transmit the full torque. It is not a torque limiting device. The torque limiting device on the Elan is the adhesion point of the rear wheels.

In respect of the little springs in the middle of a clutch driven plate, my understanding is that they are there to limit torsional vibrations and reduce NVH. I also understand that the springs bottom out during pull-away. My multi plate racing clutch has no springs, the driven plates are all solid.

...and just to be clear, I have never posted about car polish.

Andy.

[rgh0]

Yes F=MA is true but does not apply for the majority of the torsional load on a drive train. The maximum torsional load will not signficantly change as there is no significant deceleration of the engine and having donuts does not reduce any deceleration that may occur in any signficant way. The torque load on the drive train comes almost fully from the engines output not from the engine slowing and any engine inertia imposing a heavy load due to this slowing.

cheers
Rohan

[2cams70]

Sorry Rohan. We are going to have to agree to disagree on this one. The peak loading will be greater with cv joints but probably not a major issue.

One thing we can agree on though is that it’s probably better to live longer than your diff ears!

[Rob1n]

This is what I love about this forum, there is more information, ideas and help than you could ever wish for.
I for one do my own work as much as my experience will let me. Use the information provided by some would seem, very qualified people and make your own mind up. I have installed Tony Thompson half shafts is that a good decision against others on the market, I hope I have made a good decision but who knows. What I do know is they have got to be a big step forward as apposed to rubber donuts, when the donuts fail it is not pretty and generally cause a lot of damage. If that happens to you that will make your mind up as I can attest to.
Let’s remember there is a human on the other side and be civil to each other.
I am hoping to start the engine on my Elan +2 over this weekend for the first time in nearly 35 years so hopefully all goes well, as I have rebuilt the engine and everything else except the gearbox and diff.
Enjoy your weekend and thanks to everyone who chips in with the knowledge that is so inspiring!!!!
Robin