understanding 3 speed gearbox conversions

[JonE]

  gearbox photo 1.jpeg (Size: 78.75 KB / Downloads: 828) There is a lot of literature on 3 speed conversions but it is difficult to understand exactly what happens from the various diagrams and descriptions. Can anyone eluicidate for the non-engineer, from first principles? This diagram I think is Mike Phelan and has been the most helpful one from the web. Woodrow and the Green book don't actually manage to explain; they preach to the converted.

1. I can't work out how reverse actually works from the diagram. What is the relationship between the 12 and the 16 there? Or is the 16 just a different position of the central lower 16 cog pictured a little to the left?

2. Andes Gears. Mentioned on web from discussion from Vince Leek and Andrew Bird, 18/24 or 20/24 2nd gear pairs. Do these just plug and play into the position of the 20/22?

3. Close ratio conversions otherwise seem to involve the input shaft being machined to replace the 14 (and 28 as a result to maintain the pair)

Would be interested to hear what others' explorations have found. Thanks...

[Damian GT]

As a non engineer to a non engineer..

The first motion shaft has a 'constant mesh' with the layshaft via the 14:28 cluster at the front of the gearbox.

This constant mesh can be changed to make a closer ratio gear box - e.g. 17:25 or whatever. To change it you need to replace the existing first motion shaft with one that has been machined by taking down the 14 existing teeth and shrinking/welding on top another cut down gear with a different tooth count.  To make this work, as you say, you need to replace the 28 tooth gear at the front of the layshaft.  That's why replacements come as pairs on ebay or from Andy Bird, say.

You will notice that you can use an old 16 tooth and an old 26 tooth gear as donors to make the rings for a new constant mesh 16:26 gearbox which is a really nice compromise. I think it's the one they call a 'Rolt' cluster and it's really nice and driveable on the road. A sort of modest close ratio set up.

You can change the second gear cluster too, but that's for the cogniscenti.

The first gear & reverse cluster on the layshaft are made from one lump of metal.  The gear on the third motion shaft slides either forward or back when you move the gear lever either back or forward (respectively).  When the the gear on the third motion shaft is forward it takes drive from the layshaft via the first gear component and off you go.  When it is backward it takes drive from the idler gear which is itself constantly meshed to the layshaft lump.  Because the idler sits to one side and slightly above the layshaft it can accommodate the third motion shaft gear without losing 'distance between centres'.  You can see on your diagram that your 26 tooth gear can never reach the 12 tooth gear on it's own!

You can get a new first gear cluster with a modified (lower) tooth count if you want a really low first gear and again this has to be fitted by removing the layshaft and replacing the existing one.

Generally the second gear cluster takes most abuse in an A7 gearbox and many of these gears are worn.  Generally the constant mesh pair are in good condition whenever they are taken out.

Top gear is simply a matter of joining together the first and third motion shafts without allowing any mesh between lay shaft other than through the well-named constant mesh gears, which are, of course, always engaged. Clearly you can only use top gear if you prevent all other links between lay shaft and third shaft.

Your question 2 - Yes.

I'm waiting for a proper engineer to come and correct some of this sometime soon but there's a start..

Next step is deciding the rear axle ratio for any given combination.

I once bought a gearbox box of bits from Beaulieu for about £20 - someone had taken it apart and soda blasted the casing - had lots of fun putting it back together again - but there are quite a few components that get abused, broken or worn in a three speed box so I was lucky with mine.

[Tony Press]

Quick answer- the numbers on the gears are numbers of teeth on each.

[JonE]

Tony - I'd worked that bit out! :-)
Thanks Damian - very helpful.

I'm intrigued about the 2nd gear cluster "being for cogniscenti" versus the constant mesh generally being sound.
Is it generally that if you disassemble, it's just not worth NOT attending to everything?
From Vince's comments on old forum it looked like the 2nd gear swap was a quick and general running fix.. but of course I don't know if that's related to a limited range of people who strip their gearboxes everyday before breakfast on raceday...

[Parazine]

Hi Jon,

I swapped an "Andes" pair of 2nd speed gears into my Chummy gearbox as one of my first "Lockdown" projects in early April. It's a quick and easy change to make; it took a day to remove the engine and strip the box and a day to replace the gears and rebuild the box. The engine needed some further work, which I was held up on parts for.

   

The "Andes" gearing replaces the (usually) very worn 2nd speed pair and makes the gear nice and quiet. The change in ratios is fairly noticeable, allowing a higher speed in 2nd gear to be obtained. I felt I needed this as 3 speed cars do not mix well with 4 speeders, especially in hilly areas and on "follow my leader" type runs. However, the relative speeds of the gears are different and I'm still having trouble getting my gearchanges right after the change.

I had another iteration of Chummy thirty years ago, which I rebuilt from a VSCC trials car. This had a very worn gearbox with the worst gear being 1st, all the trials sections had knadgered it, which is unusual. At the time, I didn't have any spares so it remained in the car and I learned to live with the noise!

[Damian GT]

Hi 

What I was getting at is that before you know it you can change all the various ratios but you need to know how you intend to use the box before going to the trouble/expense of changing more than one cluster.  The close-ratio-isation of the constant mesh cluster is an easy way to get 'better' driveability (I think 16:26 is fab) and in my opinion anything else one does to a gearbox needs to be thought through with purpose in mind.  I'm not an expert though, and others may think differently.. (!)

[Charles P]

The benefit of the constant mesh change is that it impacts both 1st & 2nd and moves second nearer to top. In any Austin 7 with a standard 3 speed box the precipitous drop in revs from 2nd to 3rd is a moment to dread

[AustinWood]

1) 1st/Reverse:
The 26 tooth gear slides forward to mesh with the 16 tooth gear for 1st gear.
The 26 tooth gear slides backwards to engage with the other 16 tooth gear. Note that 16 tooth gear is behind the main & lay shafts in the diagram.
The reverse gear is permanently meshed with the 12 tooth reverse idler.

2) The Andes gears are a straight replacement for the 20 & 22 tooth gears. The dog clutch on the 22 tooth gear is also modified to ease engagement of top gear by chamfering the side of the dog teeth on the trailing face. This can be seen on Parazine's picture.

3) There is no modification to the shafts for close ratio conversion. It is just the gear wheels.

[Terrytuned]

Chart showing the different ratios in liner graph form.  Terry

[Bob Culver]

I am intrigued by the comment that 3 and 4 speed cars do not mix well on runs, because for stock non sports rpm in 3rd of 4 and 2nd of 3  are about the same. The 3 speed rev lower in top but are mostly much lighter. I dunno to what extent the crash boxes hamper change speed but I usually double declutched downward on the 4 speed syn anyway and lost neglegible road speed when pushed. Upward changes are certainly often a handicap on large old cars as the delay can be very long. The local owner of Sports 20 claims Sevens often do better on hills as his car once committed cannot change up until the road distinctly flattens.
The near doubling of rpm 3 to 2 of small 3 speed cars is not uncommon. Ford Y, Ten etc.