[Nonfractal]

.. existing ratio should provide a good start point.
..such designs work but if the mismatch is pushed too far, the gears cease to work efficiently.

since we all here, probably run on smallish tracks

I think you are confusing the outside diameter of the gear with the pitch circle diameter

I race on long 6 lane analogue club tracks very long DiSCA  Oxigen (LeMans) tracks.
I'm not confusing the outer and pitch diameters.
Even clever tooth profiling and alteration of the pitch diameter cannot make the gears run smoothly and reliably at the extreme end of the gearing.
Eg Henley LeMans Digital event, teams run 2 to 1 and beyond. It's a long long track. 

Getting gearing to run faultlessly for 24hours non stop is a major  part of the challenge.

[slotloco]

Okay Alan so ONE person here runs on tracks with absurdly long straights.
DISCA is something most unusual, and will only ever be experienced by 80 people a year (16 teams of 5) 

My point was that as NEARLY ALL of us don't, to NOT change the ratio when putting in a much more powerful motor, to run on  a typical small home of club track would generally be a mistake.
So a good start point in my experience has generally been to split the different between the original ratio, and using a ratio which keeps the same top end speed - if the motors have similar torque. Often when messing about with motor upgrades I go a couple of teeth either side of that middle split principle on my spur (or crown for inline), but end up right back there in the middle for the sweet spot.

In regards to pitch diameter versus outer diameter, I was attempting to point out that your wording of
"Instead, the manufacturers stretch and squeeze extra teeth or fewer teeth onto the same diameter gear."  indicates that you were talking in terms of OUTSIDE DIAMETER.

Whereas, as you say in your last post, the contact point along the radii between centre line and outside point of tip changes, so in actual fact, the diameter being talked about is changing based on where the pinion contacts the spur. The pitch diameter is changing.
If you had written

"Instead, the manufacturers stretch and squeeze extra teeth or fewer teeth onto gears with the same outside diameter by changing the contact point along the edge of the tooth which changes the pitch diameter [or working diameter in lay-speak] to keep the effective pitch as close to the base point as possible." 

any possible confusion from the first post would be avoided.

Maybe you think this is semantics, - and I really wasn't trying to be picky or score points or anything; but because the layman so often doesn't understand this principle, they think the D.P. (or modulus) changes in proportion to the tooth numbers. It's just something I have spent ages trying to get people to understand in recent years.

And of course as you say, stretch it too far and you end up with a very inefficient - and dang noisy pair of gears.
- Not that this stops everyone running the biggest tooth number pinion possible to get insanely tall ratios for DISCA. a bit either side 2:1

cheers

mark

[Nonfractal]

Thank you for posting that. 

The hardest thing for me with gears is understanding how to select the correct pairing of gear diameters.

Any simple rules to follow...?

Brian, 
Sorry it's taken a while for me to get to replying to this...
There is no universal and easy to use reference for pair matching.?
If only... but I don't have the inclination to create such a monster. ?

However, here are some clues

1. Inline gearing diameters
The diameter of an inline crown has no effect on the gear mesh.

2. Anglewinder and Sidewinder
Start by referring to the manufacturer web page and make some calculations.
NSR anglewinder: the recommended crown and pinion is 16.8mm & 7.5mm. (16.8+7.5= 24.3mm )
Slot.IT anglewinder :  15mm & 7mm or 16mm & 6mm (= 22mm )
If you want to use a pod from one manufacturer but don't want to use their gears, you have to find a pinion and crown that result in the same total 
( +- 0.1mm )

Different manufacturers use different angles for their angle winder pods. (NSR 15°, Ninco 20° , slot.it 25°)
Match it if you can but it's nowhere near as important as hitting the right diameters.

Be aware that the boss on different manufacturers' crowns can be on opposite sides of the crown so ensure you select a gear with the same as the factory fit.

If you're dealing with a niche manufacturer's motor mount and cannot find the spec...
Good Luck! (You will need it)

I hope that is somewhat helpful ?

AlanW

[BAracer]

Thanks for the reply.

The reason I asked is that I didn't know where to start on a current Slot.it 12/27 anglewinder build, so I simply asked the people selling the stuff, who said...

"...use the 6.75mm 12t pinion with the [16mm] SIGA27pl". 

So looking at your list, it sounds like they like their meshing nice and tight.

[slotloco]

Everything Alan just said - plus for anglewinders, remember that as the pinion moves down the motor shaft, it is getting closer to the axle shaft and therefore the spur teeth, so you have to take that into account, and remember that if you have space, and you start with a pinion/spur combo that is a little loose, you can move the spur out, and pack between the axle bush post and the spur with spacers, until you get a good mesh.

Something I got taught fairly early on, was to stick a ciggie paper between pinion and spur, push them tight together, fix, then rotate to spit out the ciggie paper and the mesh would be about right. Very unscientific, but I often still do that using some really think plastic - like an old single use plastic bag. It certainly gives a reasonable start point.

Alan - I keep meaning to ask - are you the guy from the Rockingham club with the long hair ?

[BAracer]

Do you think you could have another go at "pitch diameter"? 

I think I know what you are talking about, but I don't fully understand the terminology used.

[slotloco]

In simple terms - Alan correct me if there is a better definition
- it is the diameter across a gear where the gear contacts another gear, as opposed to the OD (outside diameter) terms below.

[BAracer]

OK...so now I'm wondering, don't the 'circular pitch' of the two gears need to match, or is there flexibility? 

And is it mis-matched pitches that creates noise?

[Nonfractal]

Thanks for the reply.

The reason I asked is that I didn't know where to start on a current Slot.it 12/27 anglewinder build, so I simply asked the people selling the stuff, who said...

"...use the 6.75mm 12t pinion with the [16mm] SIGA27pl". 

So looking at your list, it sounds like they like their meshing nice and tight.

Jeremy,
Slot it angle winder pod. Long ratios.
It was suggested to you that you use: 6.75 (12T) and Si-Ga27pl 16mm (27T) Ratio 2.25
6.75+16 = 22.75 (sum of the diameters) and is 0.75mm over spec.
Tight? Certainly. Nice? Not at all !  

11T/25T = 2.27 is very close to your desired ratio and can be achieved with stock Slot.IT gears.
6.75mm 11T pinion Pi-6711o
15mm 25T crown GA-1525PL
(21.75mm total. 0.25mm too short diametrically)

21.75mm comes up to 22mm with an 0.5mm axle spacer on the inside of the crown.
(0.25/2 x 3.6 =0.45)
The 15mm crowns are also available in 24 and 26 so that also gives you flexibility ability to lengthen or shorten the gearing a little without having to pull the pinion.
There are other options but I think this will best meet your needs.
Hope this helps

[BAracer]

A couple of days ago I wouldn't have understood any of your answer...now I sort of 'get' gears (if anyone ever does!). So all credit to this thread! 

The 12/27 is the ratio that the original Scaleauto r-series car came with. In practice, the SI gears do not feel tight, so I will take it gently on the throttle, see how it performs and learn something the hard way.