Flywheel/clutch weight issues

[Rink]

Hi all,

(Please bear with me, there is a question at the end of this

I own a 1958 German Sports car that is equpped with a 4-cyliinder, opposed, air-cooled engine of 1.76 liters. It makes about 85 HP and has a redline under 6,000 RPM.

In leu of spending $$,$$$ on engine upgrades I have chosen to lighten the car (stock ~2000 lbs), now down to 1430 lbs. The car is not registered or street-driven but it is a fun autocrosser.

I'm considering lightening the flywheel and clutch assembly to improve the acceleration. The stock FW is 19 lbs and many are lightened to 10-10.5 lbs, perhaps some lighter (?). The clutch assembly weights about 9 lbs with disk. I have discovered a modern flywheel/clutch/disk assembly that fits and works well, that weights 9.9 lbs total. That might help the acceleration!

Question(s) regarding a lightweight flywheel/clutch assembly-
Does this affect the longevity of the crank?
Does this affect the reliabillity of the transaxle?
Are there other considerations (other than cost).

TIA,

Rink

 

[patprimmer]

It will be more sensitive to clutch action when launching as there will be less inertia to carry it through the engagement load.

There will be less heat sink to dissipate heat generated by clutch action.

Diameter as well as total mass will have an impact on inertia in the drive line due to the clutch.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[evelrod]

Also, deceleration will be dramaticlly quicker when you lift the throttle---much improving overall breaking.
A couple examples I'm currently familiar with---My Mini is 1380cc/130hp at 1389 lbs. with an eight lb. steel flywheel. Our Lotus Cortina 1594cc/185hp has a ~6 lb. aluminium flywheel, a 12 lb. clutch...in a ~1800 lb. car. It is a modification well worth the effort in my opinion.

Rod

 

[samplesecond]

as evelrod had mentioned, ENGINE acceleration/deceleration will quicken-in actuality, VEHICLE acceleration will be marginally measurable (if that..)...but thats not to say that its not worth it..one of the major benefits will be smoother/quicker up/downshifts and revmatching (which is critical for autox)

With respect to longevity of associated componentry..you dont really have anything to worry about..the one aspect that might potentially cause troubles is if you move to a very heavily sprung pressure plate..which will place an axial load on the crankshaft, and hence thrust washers. But with a whopping 85hp..you wont have to worry about that =D

 

[fishracer]

Is the flywheel used as an additional harmonic dampener on this engine? If so, you might develop engine-problems related to vibration.

 

[GregLocock]

No, it isn't. The flywheel is considered to be near-nodal for the first few modes of the crank and so there is no energy there to absorb.

Some cars use dual mass flywheels, which imply that there is some energy there worth absorbing - in my experience they are there to damp high frequency rattles as much as anything.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[WGJ]

I was going to mention the value of flywheel inertia and the use of dual-mass flywheels earlier on, but in checking the original post I see that the vehicle is used for autocross so didn't bother to do so.

What I was going to say is reflected in fishracer and Greg's posts: Doesn't flywheel inertia play a part in transmission noise and transmission durability? (I assumed an autocrosser wouldn't be interested in these).

The dual-mass seems to have been adopted on some cars to control first-motion shaft noise at idle in addition to clutch action/take-up.
First motion shaft torsional vibrations will also be affected by flywheel inertia, won't they?


Bill

 

[Lou Scannon]

There are straighforward calculations you can do to determine the percent of engine torque that is going into accelerating the flywheel, provided you have know or can estimate vehicle mass, tire diameter, rear wheel/axle rotational inertia, final drive ratio, driveline rotational inertia, transmission gear ratio, flywheel rotational inertia, and rotational inertia of the remainder of the engine. SAE J1263 may have some guidelines for estimating some of the above factors.
Obviously, the percent torque lost to accelerating the flywheel depends heavily on the gear ratio, and if there is a grade that will play a role also. Knowing how much it is costing to spin up the flywheel will allow you to calculate the potential gain from reducing its rotational inertia.

I did this for my vehicle a while ago, and as I recall, I calculated that the flywheel accounted for 24% of the total kinetic energy in first gear, i.e. absorbs 24% of the torque used to accelerate.

 

[rob768]

The purpose of the flywheel is to isolate someof the engine's torsional vibrations from the rest of the installation. Lightening the flywheel means less isolation. This will decrease vibrational stress in the crankshaft, and increase torsional vibrations in the rest of the installation. This may be a problem fro gears (gear hammering) or drive shafts. What it does with regards to resonance frequencies is difficult to estimate without in depth knowledge of the installaton, but they will increase in frequency due to the lighter total inertia.
I'd think twice without knowing what the consequences are.
Any changes of modifying the shape of the flywheel? displacing the weight to the edge of the wheel may increase the inertia and decrease the weight.

 

[patprimmer]

In my experience replacing a noticeably heavy flywheel with a very light one, in a light car, gives a performance gain in the lower gears, that is not only measurable, but can be felt in the seat of the pants.

Downside is loss of idle stability in some cases and tendency to stall very quickly if the clutch is engaged to quickly with not enough throttle and rpm. The clutch throttle coordination takes considerably more skill, but the performance gain is real.

It is a very common modification and has no noticeable impact on durability.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[rob768]

My experience is from marine (mainly diesel) engines only, so i think with cars engines it is a different story altogether.

 

[evelrod]

I agree that the only downside to a light flywheel in my experience is a tendency to be sensitive on (usually very heavily sprung) clutch takeup, especially on cars with low ratio overall in 1st. as is the case in most 'rolling start' road racers (it often can approach 'on, off' switch with NO slippage).
I can, however, make somewhat of a case for additional wear and tear on the total drive train from such installations---again, from my experience. Typically, you will not find light flywheels in 'high mileage' use on the street. Street roadsters, hot rods, wannabe racers, etc. do not usually accumulate high mileage without something else going wrong or additional mods. Certainly, on circuit race cars, such as mine, see lots of maintenance and TLC...not going to show wear problems associated with the flywheel, again, IMO. (Let us pray)!!!

Rod