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Bill-Sims said:
Different folks have different results and therefore different opinions. It's not in a 115 car, but in an Milano-- but is still the same gearset--that I have lightened 1st and 2nd gears. The car is used strictly for autocross, and I couldn't tell a bit of difference. In fact, my synchros lasted about 6 autocrosses and no highway miles.

For comparison purposes, my OEM synchros with unlightened gears lasted 180K highway miles and about 20 autocrosses before I started getting 2nd gear crunch.

I'll put the transaxle out soon and replace the 2nd gear synchro. Doing it myself this time because seems this is a "consumable" for me!
Hi Bill, next time you have the box apart, grab hold of that big second gear and rotate it. See how much faster the BIG 5th on the cluster has to be spun up? This requires effort from the synchros. Take some serious mass out of the faces of 5th & 4th, this will help. Change the 'economy' bearing under the small 5th for the earlier needle roller type.
Also, make sure the points on the engagement dog teeth are good, otherwise they tend to skip and crunch. I press off slightly blunted ones from 2nd & 3rd before they get too bad and swap them up to 4th/ 5th. I figure this makes most economical use of what you have.
VS
 

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davbert said:
Its time for a gearbox rebuild on my GTV. I too am thinking about a lighten gearset. I had a Formula One gear (Jag-now Redbull) sitting on my office desk for awhile. I notice the straight cut gear was cutback quite a bit but without any lightening holes. I was wonder if the holes in the gear would pick up gear oil negating some of the weight advantage of drilling. More importantly, wouldn't the holes on the lower shaft add significate drag to the gear as it drags through thick gearbox oil?



davbert
Cutting back the faces is the most useful way to remove mass, holes can contribute smaller amounts. I know that holes MUST increase drag, but relative to that of all the rest of the components cutting through the oil, I think it would be imeasurable to us in the real world.
 

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VS,

I printed out your post and will refer to it when I tear down the tranny soon. I never use 4th or 5th gear, so I'm glad to scavenge parts from those gears!

This morning I thought about my post and realized I was really comparing apples and oranges--the transaxle Alfas are spinning so much mass before, during, and after a shift that it's no wonder I couldn't notice the lightening of 1st and 2nd. I do run a racing clutch disk that is lighter, but can't tell that helped shifting any, either. My new engine will have a lightened front flywheel, and maybe that will help a bit.
 

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G’Day All,

This started as a simple post with the following drawing and a description of what the colours represented however terminology and a computer crash (I blame Acrobat and M/S) got in the way.

Terminology:

I have found Main shaft described as both input and output shafts but Alfa use it for the input shaft. I will stick with Input shaft.

Cluster gears are generally described as being on the Lay/Counter shaft but Vince’s “See how much faster the BIG 5th on the cluster has to be spun up?” confuses me as the BIG 5th gear is on the Input shaft but the description of Cluster Gears I find describe them as being on the Lay/Counter shaft. However on some non Alfa gearboxes the gears which are fixed to the shaft are on the Lay/Counter shaft and this may be what confuses me. :confused:

Anyway lets try, the following drawing is of an Alfa Transaxle gearbox and the colours represent:

Yellow- Main/Input shaft Gears
Pink- Lay/Counter shaft Gears (Alfa use Pinion shaft an the Transaxles)
Purple- Dog Gears (these are pressed onto the Lay/Counter shaft Gears)
Brown- Hubs
Blue- Syncro Sleeves
Green- Syncro Rings

Note the drawing shows the gears back-cut, this is wishful thinking and not normal although I have a set of gears from an 80’s Giulietta which has a back-cut (cast actually) 1st Lay/Counter shaft Gear. Note also the back-cut is hand drawn and should not be taken as accurate.
 

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Bill,

If you don't use 4th & 5th gears, have them cut down or ground off so they (the former gears) exist as spacers. It would seem pointless to have the gears in the trans adding to the overworked synchros if you never use them. Be radical!!! You could also lighten up the big reverse gear a lot. Then lighten 1-2-3. A lighter clutch disc ( and or smaller diameter) would help. I worked on a giulietta spider with a triple plate (WAY OVERKILL - one would have been more than enough) carbon fiber setup around 7 inches. I would think a twin plate one would be more than enough for you. Something like a solid center disc with no springs. The rest of the system (F & R flywheels, driveshaft ) shouldn't matter on shifting as they are decoupled during a shift. The clutch disc rides along on the input shaft with the gears.
 

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A little physic's side note

I don't know if this has already been said, but from a physics standpoint, the most effective way to decrease the rotational inertia of a disk such as this is to lighten its mass as close to the outer edge as possible. Inertia is described by:
I=sumof(m(i)*r(i)^2)

Basically, for a uniform disc, its rotational inertia is described by mass/2*(radius)^2 (correct me if im wrong). Lightening towards the center will do absolutely nothing compared to shaving off mass on the edge.

As for the effects of cutting holes in the gear, and thus creating resistance with the moving fluid, the effects would be rather small, and definitely not noticeable by someone driving. However, for maximization, shaving down the outer portion would be much more effective than cutting holes.

Sincerely,
The naive college engineer
 

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Josh,

Thanks for that ... and as you say it was mentioned earlier.

I believe the reason they drill holes where they do is:
  • They cannot reduce the width of the gear and still transfer the same power.
  • Wasting down the gear is harder than drilling holes and requires somebody to work out how thin they can make it ... but this is the better solution.
  • Holes look cool ;) :D .
As many have said, I believe the difference is minimal and the new bearings and synchro rings make by far the biggest difference ... but the lightened gears DO really look cool and if you have the extra funds, why not.

Pete
 

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I have considered getting rid of 4th and 5th, but if I ever decide to put the Milano on the road again--like if I have health problems that preclude autocrossing (have had a mini stroke and do have balance problems from an inner ear condition)--I'd need those gears. But it is something to think about.

I already have a lightweight clutch disk. Aluminum hocky puck type with no springs.
 

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Ok, I've been playing with some calculations (mind you maths is NOT my strong point) in relation to 2nd gear from an 80's Giulietta.

My best guestemate is the following will provide about a 12.6% reduction in inertia in this gear (drilled only not back-cut).

NOTE: This is just playing with ideas, it has not been tested or examined by an engineer. It might be ok, it might not.
 

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Ok, I said maths is my weak point but let's go through my workings.

We have the formula inertia (of a disk) = mass/2 * radius squared

A gear is a disk with a hole in the middle so we need to work out the inertia of the disk and subtract the inertia of the hole.

First we need to find the mass of the gear.

The gear we are playing with is 100mm in diameter with a 42.5mm diameter hole and is 18.5mm wide.

We also need to know the density of the steel in the gear, a search on the web gave a figure of 7900 kg per cubic meter (this is wrong for this gear but that doesn’t matter as I am only after a percentage change).

We also need to take into account the gear teeth (remember inertia increases by the square of the distance from the axis so the mass of the outer rim has a large effect) I chose a nominal 50% mass reduction for the teeth. The teeth are 5mm high so I reduced the nominal outer diameter of the gear by 5mm to 95mm.


Mass1 (Gear ignoring hole) = pi * r^2 * width * 7900 = 3.1415 * 0.0475^2 * 0.0185 * 7900 = 1.036kg
Inertia1 (Gear ignoring hole) = mass/2 * r^2 = 1.036/2 * .095^2 = 0.01168kgm^2

Mass 2 (Hole) = pi * r^2 * width * 7900 = 3.1415 * 0.02125^2 * 0.0185 * 7900 = 0.207kg
Inertia 2 (Hole) = mass/2 * r^2 = 0.207/2 * .0425^2 = 0.00187kgm^2

Inertia of gear = Inertia 1 – Inertia2 = 0.01168 – 0.00187 = 0.00981kgm^2


That was the easy bit.

In the above we calculated the inertia of a disk with an outer radius, an inner radius and a width. Now we have to do the same for the area where the holes are and then subtract the inertia of the holes.

This is where my maths fails me, I don’t know how to calculate the inertia of a cylinder (hole) at a given distance from an axis.

So I faked it :D I calculated the mass of the area of the gear where the holes are, subtracted the mass of the holes and calculated the inertia of the remaining mass.

Actually I went a bit further than that, if you look a circle you will see that the top and bottom have a smaller area than the middle. The top (and bottom) “1/4” has 19% of the area and the middle two “1/4”s have 31% each. So I did four calculations to improve the accuracy (more would have been better but I can be lazy).

Time to get ready for work.
 

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Discussion Starter #54
Impressive Craig! Looks reasonable to me.
Now the question is, whats the inertia of the stuff you cannot change, like the gear shafts. Really, we all believe lightening the gears reduces the inertia of the gear. But one might argue that all the other bits weigh so much it becomes insignificant. Care to make a swag at the main and counter shafts?
C
 

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vintageveloce said:
Impressive Craig! Looks reasonable to me.
Now the question is, whats the inertia of the stuff you cannot change, like the gear shafts. Really, we all believe lightening the gears reduces the inertia of the gear. But one might argue that all the other bits weigh so much it becomes insignificant. Care to make a swag at the main and counter shafts?
C
As others have said, removing weight from the near the centre makes little if no difference. Hence when you lighten a flywheel all you really need to do is take metal of the outer area (easy with an Alfa Nord flywheel cause all the weight is out there ... being effective).

Thus lightening the shafts will make a much less effective difference ... so we are now really get down to the last 1%.

Pete
 

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Discussion Starter #56
Pete, Hmm... I just hesitate to take anything as a given. The shafts do have a diameter greater than 0 and thus must have some inertia. It's a pretty heavy and relatively long shaft...
C
 

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Bill,

Extra gearsets should be pretty easy to come by so you wouldn't really be doing something that can't be undone... It would only be 'work'. I have extra sets around if you want to try it...
 

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vintageveloce said:
Pete, Hmm... I just hesitate to take anything as a given. The shafts do have a diameter greater than 0 and thus must have some inertia. It's a pretty heavy and relatively long shaft...
C
Agree, but you are starting to get into F1 territory ... ie. spending a fortune to make unbelievably small differences :eek: ;)

Heck you could probably make all the shafts and gears out of titanium and reduce the weight and thus inertia considerably ...

You have to set goals/targets so you will stop somewhere :rolleyes: :D
Pete
 

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Discussion Starter #59
Good point Pete,
So here's my opinion:
If the gearbox lightening reduces the rotational inertia by more than 10% it might be worthwhile. More than 25% and you would probably feel it. Less than 10%, not worth the effort, probably won't feel any difference. I have no scientific basis for this opinion, but it's what my $500 bucks of lightining is worth to me ;-)
And I can easily believe all the lighting by holes and backcutting might or might not make a 10% difference. The gears are certainly lightened more than 10%... but there's still alot of other stuff spinning. Think about a steel baseball bat spinning around it's axis at 5000 RPM... I wouldn't just grab that thing...
C
 

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G'Day Carl,

We actually only have to take into account the Input shaft the dog gears and the syncro rings (and bits) as the Lay/Counter shaft rotation is what we're trying to match and the hubs and syncro rings are part of the Lay/Counter shaft.

So basically the syncros are trying to alter the speed of the clutch plate, input shaft and gears ( inc. dog gears and syncro rings).
 
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