Modern Vespa

So, i'm thinking about gearing ...particularly my SS180 4th gear, which i might swap out for something that lets me cruise at a higher gear rather than topping out so quickly.

Whenever you read gear specs, they are expressed as ratios thusly:

GS160:

1st gear- 14.72:1
2nd gear- 10.28: 1
3rd gear- 7.61: 1
4th gear- 5.84: 1


SS180:

1st gear- 14.466:1
2nd gear- 10.088: 1
3rd gear- 7.461: 1
4th gear- 5.710: 1


So, how can i get tooth counts out of those numbers? Obviously that's an expression of the rear wheel...how many times a gear rotates per 1 revolution of the rear wheel, right? Is there a way i can derive tooth counts from that? Or does anyone know what the tooth counts on those gear stacks are?

You gotta know the number of teeth on one gear to figure the number on the other for certain.

However, if you can factor out the fractions (lowest common denominator and all that), then you can arrive at the smallest pair of whole numbers that could give you that ratio, and that will probably be it, since the tooth counts of the small gears is pretty small, if I'm not mistaken.

- Eric

Well, the tooth count on the smallest gear on the gearstack was 47 teeth.

I can count the rest... but that's doing the work i was trying to avoid using math.

Found this resource.

Apparently a lot of folks will put a P200 clutch gear on the back of a SS / GS clutch ( if it's at least 180cc ) and run that for more of an "overdrive" setup.

http://www.5thgear.orconhosting.net.nz/tech/upgear/index.htm



Also, for helping out other folks :

Transmission gear ratio
Ratio between the tooth count of the layshaft and drive shaft gears
ie.: V90 (has 3 gears) Layshaft, 10, 16, 22 Drive shaft 60, 54, 47
1st gear: TR = 60/10 = 6
2nd gear: TR = 54/16 = 3.375
2rd gear: TR = 47/22 = 2.136

you can back into some of the numbers, I did it a while back on the BBS. You have to know the primary drive (clutch) and cush gear tooth counts.

I have a spreadsheet where I can plug in the tooth counts to get the gear ratio. If you know some of the tooth counts then you can sort out the others by trial and error.

One other hint is that if the tooth pitch and the distance between centers is constant (as it is with the stack 1st - 4th) then the sum of teeth of each set will tend to be the same.

smallframe example:
1st gear layshaft has 10 teeth, while the gear on the final drive has 58 for a total of 68. 2nd gear is 14 and 54, again the total is 68...

This does not always hold (T5 4th or ziri short 4th) but you can't move too far from this without things going wrong.

So this cuts your search down, as you bump the layshaft one direction you adjust the final drive the other till you get the right ratio.

If you can get me the tooth counts you know I'll see what I can do. Also search the BBS as I think I posted them there the last time I worked it out. I really should start a table that just has tooth counts.

Yeah I did the opposite with the racer - put a P125 clutch gear (21 t if I recall) to give me the 3rd to 4th shift and ran a P200 4th (instead of the T5 4th) to give me a good upper speed.

Well, ideally i want to keep the quick acceleration of the lower couple gears, and just have a really long 4th gear. Maybe that means i don't tweak with the whole clutch / layshaft gear thing, and just tweak the 4th. I was under the impression that you can't be off by more than a tooth or two, though.

got the numbers to work for the GS but not for the SS and also not with the 47 tooth gear :(

primary 22/67

1st 12/58
2nd 16/54
3rd 20/50
4th 24/46

With that it matches the ratios published, don't know if it matches reality.

Super old thread, but thought I would list the tooth counts for the SS180 gears.

1st 57
2nd 53
3rd 49
4th 45

I am not an expert here, but I believe the following to be true, I have heard it form multiple sources...SS and GS gearboxes are interchangeable and the only difference is the tooth count on the gears all being -1 for the SS, the layshafts/trees are the same.