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Thought I might as well consult the group on this.

Looking at building a new gearbox for a standard PX engine. Don't have any existing parts for this engine, thought it would be a good opportunity to custom pick gears…

End goal is a street racer with some explosive acceleration… not a touring setup…

Any thoughts on what gear cluster it should be? Away from the standard 57 - 42 - 38 - 35?

Or just modifying the primary?
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If cost is not an issue

i would look to an aftermarket "christmas tree" which shortens the gap between 3 and 4 gears. then add to that something like a DRT 37 tooth 4th gear.

One should build the gearbox around the rest of the components you are considering. which kit exhaust etc are you looking at? to recomend gearing without the details of the intended kit / exhaust / rear wheel ratio/size. is a lottery and you could be sent on a wild goose chase and spend money when you had no need to (there would be no discernable improvement....

are you looking at a decent top end speed? or looking for acceleration and grin factor when you are at the lights.... or are you looking to build a track oriented machine
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I'm pretty certain this endeavour won't be low budget, but I'm not looking for a BFA engine…

Will be:
Alu reinforced stock crankcases
Malossi 210 MHR
Nordspeed mustang 2 (but I actually want something quieter :/)
Reed valve (probably vforce4)
60mm worb5 flowed crank
PWK 35 airstriker or TMX 30 (collected both over the years in boxes now)
100/80-10 tires

Like you said, looking for grin factor… the engine i have on the bike now is a pretty safe setup so looking for can't keep the wheel down fast…
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That will wheelie in first and make the front wheel iight in first and second as it is, with a standard box... however, if you want to improve on that and make it even more responsive and lift the front wheel in other gears.... i would look at something like this

https://www.sip-scootershop.com/en/product/gearbox-kit-serie-pro-px-lusso-sport_88108000

couple that with a 21 or 22 tooth clutch gear and and a 67/68 layshaft gear, that will lift in pretty much all gears.........

however and this is a big however, you absolutely must have a clutch capable of handling that much rotational force..... if you are sticking under 25bhp than a standard banded cosa with improved friction plates is a must....... if you are going over 25bhp, then you are into the realms of a more exotic clutch. dont do what everyone else makes the mistake of doing........ there is no need to put the stiffest springs you can lay your hands on in the clutch. too strong a spring on the clutch will wear your engine bearings out and you will be replacing them on a regular basis... strong enough, but not too strong.
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I have a left over primary from an old build… can't remember what teeth it has… must double check, but most likely will be using that…

Edit: it's the usual 12 13 17 21 primary

Yeah with the clutch, it'll be a no brainier to get a CNC clutch of some type…

I doubt a banded clutch would be able to hold up with the rotational forces… my first cosa clutch flowered outwards after about a year and that build wasn't even that powerful at that point

68-21… seems like a fairly high ratio… sounds like it could be fun
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I get frustrated by the amount of times some questions are asked on this forum.... Have you used the "Search" function or done research?

Theres a bit on gearbox setup here... modify VBC1. 2-port - Fuel... Gimme MORE FUEL ! (pg 2&3?)
Basically: use the gearing calculator
& take into consideration that wind resistance kicks in at about 120km/h (80mph)l so there is no point designing a box that does 140km+ as it struggles.
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I can't remember exacty which gears i have in my T5 track bike, but it has a complete DRT close ratio transmission, primary, and clutch. You'll want a tall 1st and 2nd gear, a standard-ish 3rd, and then a 37 tooth 4th. I have a 68 tooth primary in my T5. I'll swap back and forth between a 22T and 23T, depending upon what track i'm racing on (HINT: the 22T is more fun). FYI, 2nd gear (with my engine/ pipe combo) with the 68/22 puts it right about 60mph, but with that short 37 tooth 4th, my top speed isn't too astronomical. Pretty much all you need to go fast is lots of money, but then again you already knew that.
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SubEtherBASS wrote:
I get frustrated by the amount of times some questions are asked on this forum.... Have you used the "Search" function or done research?

Theres a bit on gearbox setup here... modify VBC1. 2-port - Fuel... Gimme MORE FUEL ! (pg 2&3?)
Basically: use the gearing calculator
& take into consideration that wind resistance kicks in at about 120km/h (80mph)l so there is no point designing a box that does 140km+ as it struggles.
FINAL Gearing:
Primary Drive: 24/63 Malossi (as they were the only ones that did it!)
PX xmas, with following: 12/57 (12.4), 13/40 (8.1), 17/37 (5.73), 21/37 (4.63)
1st is standard, 2nd is exceptionally long, 3rd is standard, 4th is exceptionally short
There is about 1800-2000rpm drop between 2nd & 3rd, & only 1200-1500rpm drop between 3rd & 4th
Is was hard to work out which gear I was in at times... but ohh soo much power from about 45km/h to 120km/h!

P.S: Speedy had 3.50-8" tyres, so adjust your primary to suit 10"... however, it will work ....
P.P.S: if i did it again I would take 1 more tooth OFF 4th (make it even shorter) & have a LONGER Primary!
P.P.P.S: there is some more blah blah review somewhere else in that post or possibly here; Ode to Speedy - damn these expensive shopping trolleys!
Gear calc is helpful & fun to play with
Gear calc is helpful & fun to play with
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whodatschrome wrote:
I can't remember exacty which gears i have in my T5 track bike, but it has a complete DRT close ratio transmission, primary, and clutch. You'll want a tall 1st and 2nd gear, a standard-ish 3rd, and then a 37 tooth 4th. I have a 68 tooth primary in my T5. I'll swap back and forth between a 22T and 23T, depending upon what track i'm racing on (HINT: the 22T is more fun). FYI, 2nd gear (with my engine/ pipe combo) with the 68/22 puts it right about 60mph, but with that short 37 tooth 4th, my top speed isn't too astronomical. Pretty much all you need to go fast is lots of money, but then again you already knew that.
Here is a DRT kit, not sure if it's the one you have, but DRT make some cool stuff, & stuff I don't seem to break!
Ode to Speedy - damn these expensive shopping trolleys!
12/57, 13/42, 17/38, 20/35 with 64/24
I like the idea of 20 tooth for 4th, but my budget was not enough, PLUS it only seems that the 20 tooth 4th is the "only" difference from a standard box
6000rpm
6000rpm
7000rpm
7000rpm
7500rpm
7500rpm
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I'm looking at my notes here and i think my loose gears are? I don't want to split the cases to see exactly what i have for toofuses on all of my gears!
1st- 55T
2nd- 40T (or possibly a 41T)
3rd- 39T
4th- 37T
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108,
The tool that SEB posted is super helpful.
Its a pain to re-gear - 'cause you don't want to get it wrong...

I had good luck doing a couple things.
1. Use your current experience to estimate where you want to be. IF you know your current numbers - put them in that calculator and look at them. (The visual representation of the lines is helpful. Yeah, yeah, I've noticed I have too big a jump to 4th!)
2. Are any of your items fixed - meaning - you own them. So for example - I had a 22 tooth clutch. I worked around that. Wheels were also fixed.
3. I like to keep the gaps between the lines about equal. The tool shows you that.
4. Is there anything you don't like about what you currently ride? apply that. In my case, 4th gear at 55 was just under my power band. I changed that using the calculator to predict - and moved it up 500RPM at 55MPH. That meant I was right in the start of my power band at 55 - which I like.

Hope helpful.
-CM
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Thanks for the replies gents!

Just having time to reply.
charlieman22 wrote:
I had good luck doing a couple things.
1. Use your current experience to estimate where you want to be. IF you know your current numbers - put them in that calculator and look at them. (The visual representation of the lines is helpful. Yeah, yeah, I've noticed I have too big a jump to 4th!)
2. Are any of your items fixed - meaning - you own them. So for example - I had a 22 tooth clutch. I worked around that. Wheels were also fixed.
3. I like to keep the gaps between the lines about equal. The tool shows you that.
4. Is there anything you don't like about what you currently ride? apply that. In my case, 4th gear at 55 was just under my power band. I changed that using the calculator to predict - and moved it up 500RPM at 55MPH. That meant I was right in the start of my power band at 55 - which I like.
CM22, Food for though, this is why I was asking... good to get some thoughts on it. Definitely want to be buying the right gears, I dont want to be playing around with the output shaft too often.

Need to have a think about what i dont like. I'd probably address 1st and 2nd to pull harder... but will try inputting that into the calculator.

The list i mentioned before is mostly owned besides the MHR cylinder and Reeds... Hopefully i wont need to swap any more parts, the output shaft will be costly enough... but its a project im not in a hurry to finish... the engine i'm riding on now, took 3 years to piece together, the vforce4 was sat around for 2 years...

But looking at the welded cases (which i just finished grinding down to a port-able stage)... got the itch to look at how i want this engine to be. Started piecing things together and started relooking at worn parts...
whodatschrome wrote:
I'm looking at my notes here and i think my loose gears are? I don't want to split the cases to see exactly what i have for toofuses on all of my gears!
1st- 55T
2nd- 40T (or possibly a 41T)
3rd- 39T
4th- 37T
WDC, Good to compare notes... which setup is it?
SubEtherBASS wrote:
P.P.S: if i did it again I would take 1 more tooth OFF 4th (make it even shorter) & have a LONGER Primary!
SEB, That would give you more touring top speed right? I thought the Malossi was already pretty long...
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Nope, it's nothing to do with touring top speed... that's generally not as "real" as you may think, as a LOOOOOOONG 4th is not much help, as wind resistance kicks in before you get on the pipe, so you barely ever get crazy fast speed & constantly have to go back to 3rd if there is a headwind or hill/rise & you will end up being frustrated (been there, done that).

The "cost effective option " is add teeth to 2nd, knock as many teeth off 4th as you can & "adjust" your 6500rpm "cruising speed " by changing the primary.
That will mean: 2nd, 3rd, 4th are VERY CLOSE RATIO, & therefore you can stay on the pipe ALL THE TIME. It means a different "style" of riding & gear changes, basically if you're going off the pipe, then change gear.
Have a look at the Dyno results for Speedy to get an idea.
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SubEtherBASS wrote:
Nope, it's nothing to do with touring top speed... that's generally not as "real" as you may think, as a LOOOOOOONG 4th is not much help, as wind resistance kicks in before you get on the pipe, so you barely ever get crazy fast speed & constantly have to go back to 3rd if there is a headwind or hill/rise & you will end up being frustrated (been there, done that).

The "cost effective option " is add teeth to 2nd, knock as many teeth off 4th as you can & "adjust" your 6500rpm "cruising speed " by changing the primary.
That will mean: 2nd, 3rd, 4th are VERY CLOSE RATIO, & therefore you can stay on the pipe ALL THE TIME. It means a different "style" of riding & gear changes, basically if you're going off the pipe, then change gear.
Have a look at the Dyno results for Speedy to get an idea.
Im relation to this specific question "I thought the Malossi was already pretty long."...
i think you're almost missing the point of what I'm trying to say... yeah, the current Malossi MAY BE quite long BUT its "designed" (at a guess) for use with the T5 4th (1 tooth less than standard)...
it's not very long at all once you knock 2 or 3 teeth off the standard 4th.... you want fast fun, then you want 1200-1500 rpm rev-loss when you change gears, then you can be on the sweet spot of the pipe in every gear, all the time (hence why I said you will need to "change/alter" your gear change style.
Have you ever driven a car with close ratio straight cut gearbox? If you have, you will understand what can be achieved

Have a read here.
modify VBC1. 2-port - Fuel... Gimme MORE FUEL !
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108 wrote:
WDC, Good to compare notes... which setup is it?




It was one of the DRT christmas trees that i used. It was many years ago that i bought all the parts, so i don't remember exactly which bits i bought. I DRT called it the spitfire back then. Looking at SIP's website, it shows that they sell https://www.sip-scootershop.com/en/product/input-shaft-assembly-12-13-17-20-teeth-drt-sip-spitfire-road-2015-short-4th-gear_40436316 which DRT calls their "road" version. I think mine might be their "race" version, and if that's the case, i might have a 19 instead of 20 teeth.

And more options... https://www.sip-scootershop.com/en/category/klassik-vespa_2/spares_1751/vespa-px-t5-cosa_T5/engine-vespa-px-t5-cosa_1777/gear-box-input-shaft_1339?explo=13


With my super close ratio, my T5 can always be in its powerband with a flick of a wrist. Having that tall 1st and 2nd is a game changer for off the line acceleration. The T5 isn't much of a highway cruiser (it wasn't built for that), but it can easily pass traffic!
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