Lotus Elan

CV drive shaft conversion

PostPost by: 2cams70 » Thu Mar 21, 2024 7:37 am

Andy8421 wrote:
2cams70 wrote:Engineering numbers - you might have just been lucky!

Guys - Please don't infer that I'm saying there's a problem with CV joints. I am however interested if the effect of them on durability. I know it would be able to be determined mathematically by FEA or otherwise but I'm not smart enough to do it..

I'm an ex engineer (electronic), but looking at the problem logically, peak torque in the driveshafts (and corresponding moments on the diff case) will be set by the adhesion of the tyres, irrespective of the drive shaft joints. I could see that there may be a case for considering the rotational inertia of the wheels and tyres, as these will need to be accelerated as well, but I am very suspicious of the shock load 'cushioning' argument about the Rotoflex in limiting peak loads on the diff casing.

As for finite element analysis, its a complex system to model, and the diff casing is a complex shape. I would rather rely on real world testing (Rohan's extensive data set) than a FEA approximation.


Yes your engineering analysis does make sense Andy. The “hitting something with a rubber mallet” analogy is a psychological one only. Rubber deflects and increases the surface area of contact hence less damage to something hit with it. Not applicable to this scenario. Forces are the same regardless all else being equal (masses, etc.). The only thing important is the weight difference (which affects rotational inertia). Whether the torque is transferred via rubber or steel has no effect. Whether the difference in weight is significant or not on durability would require further analysis. “Surging” caused by the doughnuts may in fact be disadvantageous because you are putting more cycles of loading through the system - but then if you are driving the car properly you shouldn’t be causing it to surge!
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PostPost by: alan.barker » Thu Mar 21, 2024 10:02 am

Weight 3.9 kgs original sprint and 5.3 kgs CVDS Elantrikbits.
That is each driveshaft complet with nuts, screws , washers etc ready to fit.
Other ones like MM or USA i don't know.
So 1.4 kgs heavier each driveshaft.
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PostPost by: rgh0 » Thu Mar 21, 2024 11:22 am

We are going down rabbit holes.

CVs work Donut fail full stop period. if you dont like that assessment then keep driving on your donuts (which you dont have as you dont drive an Elan) and keep paying the premium to regularly repalce them and keep spedning time inspecting them and keep running the risk of them failing and ripping up your chassis / subframe. As an educated and experienced mechnical engineer I choose to do otherwise. It is not compulsory to follow what i do, I give my advice freely and you can take it or leave it.

However If you disagree with my advicie then show me some evidence as I have provided plenty around the basis for my advice. I have plenty of evidence on what fails in the Elan drive train when stressed through racing but none of those failures relate to the use of Cvs
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PostPost by: alan.barker » Thu Mar 21, 2024 11:40 am

+1 Rohan,
Well said
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PostPost by: mbell » Thu Mar 21, 2024 1:22 pm

Andy8421 wrote:I'm an ex engineer (electronic), but looking at the problem logically, peak torque in the driveshafts (and corresponding moments on the diff case) will be set by the adhesion of the tyres, irrespective of the drive shaft joints. I could see that there may be a case for considering the rotational inertia of the wheels and tyres, as these will need to be accelerated as well, but I am very suspicious of the shock load 'cushioning' argument about the Rotoflex in limiting peak loads on the diff casing.


I wonder if doughnut actually increase the the peak loads? Under initial loading the doughnut will deform absorbing and storing some energy, it will then try to dissipate that energy and deformation by adding to the existing energy/force being provided by the engine. I suspect on the scale of force here it relatively small and not significant.

(I in the CV shaft camp, my car came with CV shafts and lots of badly damage or badly repaired damage to chassis, wheel carrier and body from a previous doughnut failure.)
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PostPost by: prezoom » Thu Mar 21, 2024 2:22 pm

This post has become a complete waste of bandwidth. Either you are in favor of donuts or of CVs. Both will propel your ELAN/PLUS2. One offers long term reliability over potential failure, while the other provides a cushioned start, but potential catastrophic failure. As far as problems go with with CVs, both my cars are equipped with CVs. Neither has shown any sign of coming apart, and the Plus2 is powered by a 2L Zetec with a 3:55. diff, and the Zetec has significant more torque than your average twin cam. Make your choice and take your chances.
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PostPost by: 2cams70 » Fri Mar 22, 2024 8:44 am

There’s some people on this forum who claim to be all for engineering analysis but when the time comes to actually do it at a meaningful level they seem to get upset. It’s curiosity about things that leads you places otherwise you stay put
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PostPost by: JonB » Fri Mar 22, 2024 9:59 am

Maybe so 2cams70, but Rohan isn't one of them - if that is what you are trying to imply.

The debate over doughnuts vs CVs is an old one, it's been done to death and at the end of the day, an owner makes his/her own mind up as to what to fit to his/her car. I suspect the arguments arise from a need to self validate that decision.

I'm with prezoom on this - the thread has become a waste of time. If I were a moderator I'd lock it.
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PostPost by: englishmaninwales » Fri Mar 22, 2024 10:09 am

JonB wrote:Maybe so 2cams70, but Rohan isn't one of them - if that is what you are trying to imply.

The debate over doughnuts vs CVs is an old one, it's been done to death and at the end of the day, an owner makes his/her own mind up as to what to fit to his/her car. I suspect the arguments arise from a need to self validate that decision.

I'm with prezoom on this - the thread has become a waste of time. If I were a moderator I'd lock it.


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PostPost by: 2cams70 » Fri Mar 22, 2024 11:45 am

You can read into things anything you want to. You are the ones that mentioned names not me.
I do enjoy it however when there are some people on this forum who obviously do from what they say have a lot of knowledge at an engineering level. It's always good to learn something from them. Nobody knows everything. In fact in my experience everyone (myself included) is guilty of talking absolute rubbish at some point in time. For most people the amount of rubbish spoken about falls within the range of 20% to 70% rubbish. Nobody is at 0% rubbish but some people talk more rubbish than others. The key for someone listening is always being able sort out the rubbish from the good stuff and you can only do that by comparing sources of information, asking questions, hearing answers and arriving at your own individual conclusion
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PostPost by: Hawksfield » Fri Mar 22, 2024 12:43 pm

Hi all
I have read this post with interest as I fitted original MM CV's over twenty years ago and around 20k miles running. Iam satisfied with them after having no problems and an experience of a donut parting. Luckily this was pulling away from standstill and able to get the car home driving gently for 10 miles.
Checking underneath I had driven home on an outer donut connected to two bolts only.
My personal view is the cv's are the best option for a road car. my choice!

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PostPost by: 2cams70 » Fri Mar 22, 2024 1:12 pm

And just to conclude from me. I agree that CV joints provided they are properly designed probably wont have any major effect on the durability of the diff mountings. Although the CV joint driveshafts are heavier when you consider that the total rotating mass includes the tyre and wheel, etc. it's would seem to be only a small percentage of the total. Thanks Andy for that engineering logic and the practical experience of others seems to back up the theory. That's the conclusion I've come to after comparing sources, asking questions, sorting out the rubbish from the good stuff and arriving at my own individual conclusion. No offence intended for anybody.
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PostPost by: 2cams70 » Fri Mar 22, 2024 1:47 pm

Oh no!!! - on second thoughts I think Andy is incorrect on the theory. The forces generated are due to mass x acceleration. The dougnut is the same as having a spring in the system. Having a spring in the system reduces the acceleration of the components and hence the force. Theory now says the loadings on the diff mountings are greater if you use CV's. Whether they are greater enough to cause a problem who knows? Practical experience of others says not. That's the new conclusion I've come to by baking all inputs and further thinking in the oven a bit longer.

Andy - your thoughts?

No offence intended for anybody.
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PostPost by: Craven » Fri Mar 22, 2024 2:19 pm

TTR beautiful, no problems
Drive shafts.JPG and
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PostPost by: jcocking » Fri Mar 22, 2024 2:57 pm

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,

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