Modern Vespa

Hi. I have just purchased a Bajaj scooter from a member of our club, who builds some great prizewinning scooters! It is running a PX125 motor, with a Malossi iron barrel 178 kit, and has a 12" wheel conversion (GTS).

As he built it approximately 18 months ago, and has done many since, he has now not got the clearest memory of what gearbox is in it. He thinks it is a 23 tooth clutch, with a P200 gearbox, and thinks maybe a T5 4th gear.

Only got it on Sunday, and it was apparent from the first tootle around the block, that it was not pulling 4th gear, (approx 36mph up a slight incline!)
So, question is, does anyone have, or know where I can find, gearing information for this set up?

Thanks in advance.

It is better to remove clutch cover and then clutch and then understand what type of clutch do you have. And then you can count teeth on clutch cog and on input shaft. Then it will be easy to decide where to move with your problem.

Thanks for you reply. That is my job for the weekend! I am really hoping that someone has had/done the same conversion, and can advise on the optimun gearing, or at least point me in the direction a a gearing chart. I have scoured the internet, and all the charts that I have seen are only for 8 and 10 inch wheels!

I can only assume that you have 23/65 primary.
If so and you have Cosa 2 clutch then only option is 22 teeth clutch cog

And if you have 7 springs clutch then you have wider range of available cogs - 20T, 21T and 22T.

Thanks again. From my own research, it appears that the 22 tooth clutch is the best option, so will try that and report back!👍

Just a thought, but if you already can't pull 4th, you're going to need a lot more power than you're making to run 12" wheels. The 22t clutch will about even out the gearing difference, so you're going to need to gear down further than that.

You need to figure out where your power band is (min/max RPM's), then you can use that with a gearing calculator to figure out what setup you need. Probably a short 4th, which means splitting the cases. Not a big deal (I split my P200 cases last night in the bike ), but you're never going to be happy until you take that step.

Also, 12"s are good for long, straight high speed runs, but IMO they kind've kill the agility of the bike compared to 10"s.

I have a set of 12"s I ran for a while a couple years ago and ultimately went back to 10"s.

If the goal is to improve ride/handling, you're better of spending that wheel money on new shocks, engine mounts, and rear shock upper mount (get the Clauss Studios rear upper mount).

To add to what Chandler posted.

Would also be good to know what you have for an exhaust, might be able to pick something else more suitable to low rpm torque.

Squish would be nice to know also. Might be able to move barrel a bit for more low end torque to go with low rpm. Spacers under cylinder?

Guy might be good at show bikes, but pretty doesn't get you home. As you've already found out it doesn't work correctly mechanically for 12" tires that are on it.

Thanks 'chandlerman'. Talking to the bike builder, he says he built it for exactly as you say, long, fast, straight road runs, which unfortunately, we do not have too many of, around these parts!😄
Going to try with a 22 tooth clutch first to see if that helps, and then take it from there!

Thanks for your reply. Scoot is running a SIP2 pipe at the moment, with a 24mm carb. I have just bought a 28mm carb and a Pinasco expansion exhaust, so will be try these, which hopefully, will help!
Checked squish yesterday, and it is 1.3mm!

Would it be easy to put it back on 10s? At least then you could see how it runs before you make a load of other changes that could potentially be making it worse.

Otherwise you'll be chasing your tail. Is it the new carb? Is it the new exhaust?
etc etc

Well, at least he built right for his vision, if not your reality. I haven't been to Yorkshire in a few years, but unless they've done some major renovating, I don't recall any long straight runs.

I suspect you're going to need to go all the way down to a 21t clutch *or* split the cases to drop in a short 4th before you're really happy with it. If nothing else, it should definitely give you good first gear wheelies set up like that.

Hi. Don't really want to go down that route if I can avoid it. I havn't got the original parts, so would be more expense! Going to try it with a 22 tooth clutch first, if that is no improvement, then might try a 21! Have got a 28mm carb, and expansion exhaust to try as well!

On a side note, does anyone know where I can get a M9 x 75mm nut amd bolt? Dont seem to make them any longer than the 65mm used in the rear shock!

I was going to say Grainger, but they don't carry M9 hardware.

If by expansion pipe you mean pipe and not box exhaust it'll probably be worse, usually a pipe likes a higher rpm and higher tuned cylinder timings you don't have.

Low rpm doesn't need a big carb, it'll probably be even harder to tune for a low rpm situation. Bigger carburetor makes air velocity slower and reduces suction and Venturi effect in carburetor.

That SIP road 2 is a good exhaust for your use. If anything I'd try a Polini box exhaust it's better at low rpm.

24mm carburetor is a good setup for lower rpm use also.

First off, what is the overall height of the 12" tire? If it's a super low profile tire, then you might get away with just dropping one tooth on the clutch.

But FYI, you're asking a lot for a low torque 2 stoke engine. I'd be looking towards a 20-21 tooth clutch and a 37 tooth 4th. Keep in mind it's not just tire height that is slowing down your scooter, but the bigger tires will have a higher road rolling resistance. That means that you have to gear down an extra bit lower to make up for the extra drag.

Thanks for your input "whodatschrome"! Loads of advice given and options to look at. One thing is clear, it is not going to be a quick, easy fix, but probably a combination of 'adjustments', and a lot of trial and error! 🤔

How about this?...first off measure the height of the rear tire while you're sitting on the scooter. Once you get that measurement then we can make better guesses as to what gearing to use.

)

If it helps, the following is part of the advertisement "12 inch wheel conversion on GTS rims/new tyres, but sits same height as a PX". Guess there must have been a little bit of frame modification!

Or truth modification.

The exact tire size will produce a little variation in circumference, but 1,405mm is a good starting point for calculating gearing.

It's totally possible that that's the case. SIP's 11'' tubless rim with a 110/70-11 tire mounted up is the same diameter at a 3.5"x10 tire. But since the 110/70-11 is just plain heavier and wider it does sap precious torque.

I think that 110/70-11 have bigger diameter than 3.50-10.

Here is 100/90-10 and 110/70-11