Lotus Elan

Lightened Flywheel

PostPost by: bcmc33 » Sun Jan 10, 2010 5:14 pm

I am aware of the benefits of taking weight out of the flywheel - but what are the downsides for a quick road car that does the odd track day?

My current flywheel is perfectly standard at 14 lbs - for a half-way house job, how much weight should be removed and from where?
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PostPost by: mikealdren » Sun Jan 10, 2010 6:16 pm

The flywheel is intended to provide rotational inertia to dampen out poor gearchanges!

Mike
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PostPost by: pauljones » Sun Jan 10, 2010 6:35 pm

Not being an expert on this,I was always told never to lighten a cast iron wheel.It takes away to much strength.Also as a side, the whole bottom end should be balanced as an assembly.So therefore just by lightening the flywheel could upset the balance of the entire bottom end and make things worse.I have always been adviced a light weight steel part is the only safe way, and have the bottom end inc clutch ballanced at the time of fitting.

It may cost a bit more but surely saftey on a track has no price..

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PostPost by: alaric » Sun Jan 10, 2010 7:36 pm

Hi. I recalled from 20 years ago that there was a write up of flywheel lightening in the book 'Tuning BL's A-Seried Engine' by David Vizard. The equation he presented gives the effective reduction of weight of the vehicle seen by the engine due to a reduction of the flywheel weight i.e. how many lb lighter the car appears to the engine due to a reduction of 1lb on the flywheel. The expression is (I've not worked out how to do superscripts so the squared terms are written out longhand):

(0.5 x n x n x r x r + R x R) / (R x R)

n is the total gear ratio, which is the gearbox x final drive ratio.
r is the radius of gyration, which is the radius at which the whole mass of the flywheel can be considered to be rotating i.e. is the radius at which we can assume all the mass of the rotating flywheel could be compacted into to give the same effect as it has in its more spread out state.
R is the radius of the wheel, roughly half the outer diameter of the tyre.

So the tricky bit would appear to be working out the radius of gyration of the flywheel. I haven't looked into this yet but from Uni recall an integral using the density, thickness and radius of the object. I bet it's in wikipedia.

He gives an example, which is the main reason I thought it worth posting, as it's interesting to see what impact the lighter flywheel can have, and is the reason why I Lightened the flywheel on my Mini.

The example is of a car weighing 900 lbs. First gear is 2.6:1, final drive is 6:1. It?s a hill climb car. So, n is 15.6, and he takes r as 4 inches, and R is 11 inches. This gives an answer of 17.09 lbs. i.e. every lb off the flywheel is equivalent to 17.09 lbs off the car in total in that particular gear. So, if 10 lb was taken off the flywheel, then that?s 171 lb off the weight of the car seen by the engine.

Regarding strength of the flywheel as I?m sure you know it?s important to take material off the outer edges as this has more effect on the angular momentum, but to watch out for where the clutch bolts in. There?s probably a wealth of experience out there that can advise on tried and tested patterns for lightening.

The only downside that I?m aware of from lightening the flywheel is a rougher idle. I had a very lumpy tickover on the mini, but it flew up the revs when you blipped the throttle.

I would be interested to hear of people's experience of the impact of lightening the flywheel - seems like a must in terms of performance impact per ? spent.

Sean.
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PostPost by: bcmc33 » Sun Jan 10, 2010 8:35 pm

I guess I should have given an explanation for the question.

The crank and flywheel/clutch cover is out of the engine, and as it is a new clutch cover and pressure plate, the whole thing will need to be rebalanced. I have made provisional arrangements to take it to a local specialist who said he could reduce the flywheel weight if I wanted - hence the question.

From what I have read, it seems the average weight of a light flywheel is 8 lbs. for a Twink, but this seems somewhat excessive. I was hoping for some experiences from people who has done such a mod..
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PostPost by: rgh0 » Sun Jan 10, 2010 9:19 pm

The Vizard book on tuning twin cams gives a drawing of how to lighten the standard flywheel. OK if using standard crank and standard rev limits, otherwise need a steel flywheel.

Dont have a copy of the book with me currently but perhaps someone cam post the drawing

cheers
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PostPost by: paddy » Sun Jan 10, 2010 9:27 pm

I have a copy of the book, I can scan and upload the drawing tomorrow.

It does say that the standard flywheel is comparatively light already, and the amount of material you take off is modest compared with many other stock flywheels.

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PostPost by: curly type 26 » Sun Jan 10, 2010 9:44 pm

And use ARP bolts & double dowell :D Curly 1964 26Rr
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PostPost by: paddy » Mon Jan 11, 2010 11:01 am

Hi,

Scanned pictures attached.

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flywheel-vizard.PNG and
flywheel-vizard-detail.PNG and
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PostPost by: Lyn7 » Mon Jan 11, 2010 8:01 pm

From personal experience I have lightened flywheels using the Vizard diagram on a number of occasions.Usually going below 10lbs and found no problem.
Balancing does not seem to be an issue as it is no worse than it was before i.e.if you only have factory tolerances then do not worry as the mass is so much less anyway.
I have regularly used 7500 rpm on a balanced (Xflow sorry) engine with competition starts
no problem.
Also the idle issue is overrated I would definately recommend a lightened flywheel.
From memory (not good) I think the lightest standard Ford flywheel was fitted to the humble 1100cc Escort. :)
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PostPost by: jkolb » Mon Jan 11, 2010 8:26 pm

My flywheel has the Vizzard profile and has been lightened to 8 pounds. My engine builder friend says that when twinks break the crank it happens most often on 4-bolt cranks at the hub that the flywheel bolts to and occurs because of too much inertia with quick changes of RPMs both up and down. This is why racers use smaller diameter clutches and lighten the flywheel at the rim. F-1 clutches are only about 110mm in diameter! Probably cost as much as a twink race engine!

Jerry
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PostPost by: bcmc33 » Mon Jan 11, 2010 9:59 pm

curly type 26 wrote:And use ARP bolts & double dowell :D Curly 1964 26Rr

I shall use ARP bolts and studs in the rebuild

My flywheel is a 6 bolt fixing, but does not have a hole for a dowel even though there is a dowel hole in the crank.

The question is now - should I drill a dowel hole in the flywheel? My feeling is that it would make the clamping area potentially weaker.

A new steel flywheel from Burton will cost ?250 with starter ring and weighs the standard 14 lbs, but would be more suitable for weight reduction. Is this option good value for money?

And Paddy,

Thanks for the Vizerd sketches. The guy doing the crank balancing says he lightens flywheels on a regular basis and knows how to do the Twink. I shall take a copy of the sketches just to compare with his ideas.
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PostPost by: 1964 S1 » Tue Jan 12, 2010 3:43 am

Hi Brian,
Where do you expect to see an improvement in your car's performance once you shave six pounds off its gross vehicle weight?

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PostPost by: bcmc33 » Tue Jan 12, 2010 10:31 am

1964 S1 wrote:Hi Brian,
Where do you expect to see an improvement in your car's performance once you shave six pounds off its gross vehicle weight?

Eric, I have no real idea apart from comments made by people who have done it that the engine becomes more responsive.

It seemed to me, that with the engine in pieces, there is the opportunity to do as many "improvements" as possible.

I shall also balance the conrods as there is no evidence of them having been done by a previous owner during the last rebuild. Again, I'm not sure I will notice any improvement in doing this - but I'll get some personal satisfaction in knowing that the engine is in better shape for doing it.

FWIW, are there any other "improvements" I should consider, apart from new pistons?
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PostPost by: john.p.clegg » Tue Jan 12, 2010 11:24 am

Brian

Balance the pistons ( I managed to within 1gm),conrods end-to end (within 10 gms),have the full assembly (crank,flywheel pressure plate dynamically balanced),cut down the jackshaft if you've an electric fuel pump and remove the lobes,open up the oil drain hole on the head inlet side from 1/4" to 9/32",fit new valve seats (these can be opened-up also before fitting) and blend into the head,new inlet guides (flowed in inlet tract),chamfered on the top after fitting to stop oil pooling,equalise the head volumes,"blueprint" the head to exhaust,etc,etc,etc...

How much time do you have...how much money do you have...

Get a copy of Mr Vizards book "tuning twin-cam fords"

John :wink:
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