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Flywheel Ring Gear

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A friend of mine has fitted a Moss lightweight flywheel and new ring gear to his 3A. The TR is also fitted with a hi-torque starter motor.

I attach a photo. of the set-up. Has he fitted the ring gear the right way round?

Tom.

IMG_0757 - Copy.JPG

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Hi Tom,

are you concerned about the beveled edge to the teeth?

The original starter motor pulled in from the rear so the above looks OK.

The HT motors push in from the front. Si the bevel would not be used. However many people fit the HT SMotor with the bevel fitted as original with no problem

 

Roger

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Hi Roger ~

Thank you for that.  You're on the ball, as usual!!   My friend said he replaced the ring gear exactly the same way as the original.

Best regards ~

Tom.

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I believe that it sepends on the age of the car.  My TR3a was fitted with a starter with a bendix gear that engaged from the front but the older cars pulled in from the rear.  At least that's I was told when I asked the question a while back when I fitted a lightened flywheel that I got from Ian Cornish.

Rgds Ian

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I was told it all depended on whether the bendix gear is rotating as it engages with the ring gear. 

On a pre-engage starter the gear is not rotating and therefore does not need the tooth taper lead. The gear starts to rotate once engagement has happened. The pull in solenoid connects motor drive fields to make the gear rotate after gear has been moved into mesh with ring gear.

On an inertia starter the gear is rotating as it engages and does need the help of the tapered tooth lead.  The motor is rotating so inertia pushes the gear into the ring gear.

This could all be rubbish of course.....

Cheers

Peter W

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If you reverse the ring gear you have two problems one, if you want to go back to the original starter it will have problems engaging two,  the angled tooth face will be on the wrong side of the tooth to assist the pinion to engage on the hi-torque starter. I had this problem with my car, the PO had done just that and the hi-torque starter pinion would catch on the face of the tooth and make a nasty grinding noise, very embarrassing. Fixed it when i put it back the right way so your man has got it where it should be.

 

Graham

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I have a high torque starter motor with a std flywheel on TuRK and its worked perfectly well for over 10 years, but I think a wider question is what value is a lightened flywheel without a lightened and balanced crank?

Edit: opinions will vary of course.

Cheers, Andrew

Edited by Andrew Smith

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I was taught that any reduction in weight of the rotating assembly, translates to more power to the ground. If you feel it in your right foot is a different story. Only counts if it shows as a reduction in your lap times. 

 

Dean T.

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OK., here is what makes sense..

P1330164as.jpg.9af8822961712e33060fda173fbbe59a.jpg

^ This is a bendix type starter motor used on the later 4-cyl. TR engines.

 

The point which is so obvious that I was missing it ..is that the chamfer / bevels on the gear-ring are there to both help engage and disengage. 

As you can see in the above photo, the starter's gear pinion also has bevels on its teeth to help engage it  ..so even if the flywheel's gear-ring was on back to front it would generally work.  But the very important function of those bevels is to ensure that the starter motor disengages when the engine starts.  

Consider for a moment, the ratio between the starter's gear and the flywheel's gear ring is (..lets say) 1 :10.  So the starter motor turns at 1000rpm,  and because of that gear ratio - turns the engine over at 100 rpm to start it.  That may not seem a lot but I can assure you it's very much more than a motorcycle kick-starter ..however many cylinders or powerful the engine is.  

Now, what happens when you've blipped the throttle and the engine starts ?   The engine fires, starts, and almost immediately revs to spin up to 2000rpm.  So, if the starter motor is still engaged - that would be spinning at ;  2000 rpm x gear-ratio of 10:1  = 20,000rpm !     Those ring-gear chamfers / bevels are there to protect the starter motor by encouraging the gears to disengage as soon as the engine kicks into life. 

Clearly that's a pretty important thing to happen, so the chamfers/ bevels need to face the bevels on the starter motor's pinion.   

And as the early cars and the later ones had a different starter motor, and the hi-torque may come in either configuration, and then there's probably some which have been mix n' matched or put on backwards by a P.O.  - then the sure-fire answer is to look at your starter motor to see which side of it's gear pinon the lightweight spring is.  (see illustration above).

Pete.

Edited by Bfg

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4 hours ago, Andrew Smith said:

I have a high torque starter motor with a std flywheel on TuRK and its worked perfectly well for over 10 years, but I think a wider question is what value is a lightened flywheel without a lightened and balanced crank?

This is off topic, but if Tom might excuse this little diversion I'll attempt to quickly answer the query..

Given that the stroke of the 4-cylinder TR engine is 92mm ..then it will be realised that half of this amount (46mm) is the crank throw. The mass of metal outside of that dimension is in the outside radius of the con-rod's big end caps.  And that is quite similar to the outside radius of the bob weights.  I don't have my crank or con-rods here but let's say the caps have a 3" outside diameter, so their outside radius is about 38mm (1-1/2"). 

Then, we have the mass of the crankshaft spinning around, and it's greatest radius is 46 + 38 = 84mm.  In fact, due to the shape of the crankshaft, and all the air gaps inbetween the big-ends - the moment of inertia of the crank might be considered to be very much closer to that 46mm (from its rotating axis) dimension.

Conversely, the TR3 flywheel is 288mm in diameter + gear-ring teeth,  (8mm less with the diaphragm-clutch).  So we have 140+ mm radius.  And much of the weight of a flywheel is specifically designed to be close to the outside perimeter.

I'm talking of "radius"  here because Torque = Force x Distance.  Radius being the distance , which is the "moment" of inertia.  And of course 'Force' is a direct function of the engine's power.  So the radius (distance) from axis of rotation is important (in direct proportion) to how much torque is needed to accelerate that mass quickly and to much lesser extent to maintain that rate of rotation.  If the distance is half, then similarly - the amount of engine force needed will be half.  

Bottom line ;  Altering the amount of weight and its concentration within a flywheel has (because of : moment-of-inertia)  a far greater effect than taking the same amount of weight out of the crankshaft and con-rods (.. if it were at all possible while retaining their strength and rigidity).      

Hope this (simplification) helps.

Pete.

 

Edited by Bfg
typo

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38 minutes ago, Bfg said:

This is off topic, but if Tom might excuse this little diversion I'll attempt to quickly answer the query..

Given that the stroke of the 4-cylinder TR engine is 92mm ..then it will be realised that half of this amount (46mm) is the crank throw. The mass of metal outside of that dimension is in the outside radius of the con-rod's big end caps.  And that is quite similar to the outside radius of the bob weights.  I don't have my crank or con-rods here but let's say the caps have a 3" outside diameter, so their outside radius is about 38mm (1-1/2"). 

Then, we have the mass of the crankshaft spinning around, and it's greatest radius is 46 + 38 = 84mm.  In fact, due to the shape of the crankshaft, and all the air gaps inbetween the big-ends - the moment of inertia of the crank might be considered to be very much closer to that 46mm (from its rotating axis) dimension.

Conversely, the TR3 flywheel is 288mm in diameter + gear-ring teeth,  (8mm less with the diaphragm-clutch).  So we have 140+ mm radius.  And much of the weight of a flywheel is specifically designed to be close to the outside perimeter.

I'm talking of "radius"  here because Torque = Force x Distance.  Radius being the distance , which is the "moment" of inertia.  And of course 'Force' is a direct function of the engine's power.  So the radius (distance) from axis of rotation is important (in direct proportion) to how much torque is needed to accelerate that mass quickly and to much lesser extent to maintain that rate of rotation.  If the distance is half, then similarly - the amount of engine force needed will be half.  

Bottom line ;  Altering the amount of weight and its concentration within a flywheel has (because of : moment-of-inertia)  a far greater effect than taking the same amount of weight out of the crankshaft and con-rods (.. if it were at all possible while retaining their strength and rigidity).      

Hope this (simplification) helps.

Pete.

 

.....and in street use I have found a lighter flywheel means more revs needed to pull away or the engine may stall.  
Not the old TR ‘sidestep the clutch at tickover’ technique of the previous 30 years

 

Peter W

Edited by BlueTR3A-5EKT

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Hi Peter W ~

In the 60's I had an 'Aquaplane' aluminium flywheel fitted to my Mini 850 and I never experienced any problems with driving.

I've still got the flywheel somewhere in my shed!

Tom.

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I fitted an aluminium flywheel during my recent engine build and came up against the "ring gear bevel" issue. I was using what was actually a front wheel drive engine and turning it through 90 degrees into RWD format. The RWD starter engages from the opposite side of the flywheel, so no bevel. As Graham H has already stated, you can't just turn the ring gear round so I hand ground a new set of bevels in the correct place. Not difficult at all, especially when you use a potter's wheel as a handy rotatable work surface! The amount of metal actually removed is miniscule.

The benefits of the light flywheel are greatest in first gear and diminish progressively from there. I had discussed the potential benefits with Peter Burgess and he said it would be really worthwhile. He said it shows up on the dyno.

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