Lotus Elan

Lightweight flywheel

PostPost by: Midlife » Sun Oct 14, 2007 10:08 am

I need to change the clutch. Whilst changing the clutch I might as well replace the old standard heavy flywheel. Can I put a new lightweight flywheel on without stripping the bottom end down? Does it need dowels? If so can I get it drilled, again without breaking down the bottom end?

Would appreciate any advice on installation. I have plenty to do on the car to improve it but basically the engines working/running well so would rather leave this alone for now.

Cheers

Doug
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PostPost by: rgh0 » Sun Oct 14, 2007 10:32 am

There is a risk of replacing the flywheel without balancing on the crank of getting vibration problems. The risk is small provided the crank and flywheel were originally balanced separately and checked as an assembly.

However its a lot of trouble to pull the engine again if it occurs. I would pull the crank and get the assembly of crank, flywheel and clutch balanced together.

Dowelling the flywheel to the crank is recommended by many experts but I am not sure how critical it is with the six bolt cranks. I personally have never had one come loose before I changed to the datsun cranks and I have never dowelled the flywheel to those either. I am sure if it was really needed Ford would have done it.

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PostPost by: John Larkin » Sun Oct 14, 2007 2:34 pm

Rohan wrote in his reply to this topic: "...before I changed to the datsun cranks... "???

Can Datsun cranks be used in a twin-cam Ford block?

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PostPost by: cabc26b » Sun Oct 14, 2007 4:01 pm

The doweling question will be answered when you remove the old Flywheel.

It will be a combination of bolts and dowels 4-6-12 bolts and one or two dowels ( i have seen different diameters on dowels as well) . Call A reputable mfg like farndon and tell then what you have for bolt and dowel pattern - they should be able to supply a flywheel ( you will have optons on weight , clutch face etc.
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PostPost by: Midlife » Sun Oct 14, 2007 9:28 pm

Take your point on the balancing, had the feeling I should, and chanced the question just in case not(?). I didn?t realise that Farndon did custom flywheels, so it?s probably worth getting it from them and they do the balancing... I was after an ultra light one based on an interesting read I found (if you haven?t seen already). 1kg off the flywheel is equivalent to lightening the car by 39kg in first gear.

http://www.pumaracing.co.uk/FLYWHEEL.htm

Ref the datsun crank, I think this is mentioned in the twin cam rebuild options, tech tips section on this forum?

Cheers

Doug
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PostPost by: types26/36 » Sun Oct 14, 2007 9:44 pm

Midlife wrote:Ref the datsun crank,


http://www.geocities.com/marty7_nz/dats ... index.html
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PostPost by: msd1107 » Thu Oct 18, 2007 3:29 am

To quantify the effects of rotational speed on the effective weight of the car:

A relatively standard flywheel of 16 lbs adds almost 300 lbs at 7000 rpm.

The driveshaft adds 40 lbs at 120 mph.

A lightweight tire/wheel combination (20 lbs tire, 13 lb wheel - yours may vary) adds over 450 (!) lbs at 120 mph.

And the brake discs add a further 17 lbs at 120 mph.

So a curb weight Elan at 1790 lbs grows to over 2500 lbs at 120 mph. Sobering.

David
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PS: For those of a curious nature, I will be posting software shortly that will let you make these same observations, look at all the data, and see what would happen if you made changes. It calculates a simulated acceleration time, so you can see what different changes have on acceleration in different speed ranges.
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PostPost by: John Larkin » Thu Oct 18, 2007 6:44 am

Hi David,

Are you adding the rotational inertia of spinning parts to the car's linear inertia? In other words, are you saying that the total kinetic energy that is required to accelerate or to brake a car is the sum of these two, and that if you lighten rotating parts you can improve performance? I had never thought of this, and I am intrigued by the concept.

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PostPost by: msd1107 » Thu Oct 18, 2007 5:11 pm

Hi John,

Yes, that is the case.

Visualize, if you will, an item of a certain weight. It requires a certain amount of energy to accelerate that from V1 to V2. If the weight is spinning, it has rotational inertia. Accelerating that item from V1 to V2 without changing the rotational speed requires the same energy as before, since the rotational inertia has not changed.

However, if both the linear velocity and rotational velocity change when accelerating from V1 to V2 (as a flywheel would do), then additional energy is required to increase the rotational velocity in addition to the energy to linearly accelerate the item from V1 to V2.

Given a roughly Sprint spec Elan with 3.55 diff, if the stock flywheel weighs 16 lbs, a simulated 10-120 acceleration takes 33.5 sec. Replacing the flywheel with an 8 lb flywheel reduced the simulated time to 31.9 sec. Just reducing the car weight by 8 lbs reduces the simulated time to 33.4 sec, so the rotational inertia is the predominate factor.

The drive shaft and wheels/tires/brakes have an effect also, but they are reduced in relative magnitude because of their slower rotational speed, and are also harder to reduce in weight. Reducing the tire weight from a relatively normal 20 lbs to a lightweight 18 lbs would reduce the simulated acceleration time to 33.2 sec.

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PostPost by: John Larkin » Thu Oct 18, 2007 7:00 pm

Hi David,

I'm looking forward to trying out your software when you post it.

Regards,

John Larkin.
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PostPost by: ppnelan » Thu Oct 18, 2007 7:30 pm

Surely an Elan would not be able to ACCELERATE at 120mph, and even a racing Twin Cam engine would not want to ACCELERATE much more than 7000rpm, so the values (impressive though they are :wink: ) would be better quoted at a more 'realistic' speed... :?:

:arrow: Matthew
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PostPost by: John Larkin » Thu Oct 18, 2007 8:17 pm

In physics the term "acceleration" includes slowing down and changing direction as well as speeding up. Braking would be improved by lighter rotating masses contributing to reduced kinetic energy, and handling improved by reduction in the gyroscopic effects of the rotating masses on changes of direction.

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PostPost by: ppnelan » Thu Oct 18, 2007 8:24 pm

I can feel a 'tongue in cheek moment' approaching... :wink:

:arrow: Matthew
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PostPost by: msd1107 » Fri Oct 19, 2007 12:30 am

John,

Very perceptive observation about braking.

There have been debates before about down shifting during braking versus not down shifting, but these generally have been about brake, clutch, transmission wear.

However, it certainly appears that in theory braking distance would be minimized by dis-engaging the clutch when starting to brake and only re-engaging the clutch at the cessation of braking.

I think the increased braking force would force my tongue firmly in my cheek, though, so be careful.

David
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PostPost by: 264889socal » Fri Oct 19, 2007 6:16 pm

There was an interesting article in I believe Grassroots Motorsports magazine a couple of years ago, where they changed the stock brakes on a Miata/Mx5 to aftermarket units. The new brakes consisted of larger diameter rotors and larger calipers, which required larger diameter wheels and tires. The end result was, the modifications increased the braking distance over the stock components. Some times bigger (heavier) isn't better.

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