Gear ratios knowledge required

[bigswifty]

This relates to CG125 (move if this isn't appropriate)

I would really apreciate knowledge/understanding of gear ratios and implications

Further to the renovation of my CG125B, I have fitted an 18 inch rear wheel (standard in 17). It fits and sits fine, no issues with suspension.
I hadn't however considered gear ratios and the effect of any alterations. Previously on me Cub I've changed the front sprocket to 16, which stopped the little engine screaming for mercy Honestly, I don't understand how the one tooth makes such a difference (well I kind of do, though the CG issue is more complex.
With the CG I am lost in terms of implication of changing things ...

The CG 125 K1, B & C has the following: (source, big yellow Honda parts book cross referenced with Haynes manual 433 and CSML fiche)

PD 18/73 Primary drive gear 18 to clutch basket 73
1st 13/36 (mainshaft/layshaft)
2nd 17/32
3rd 20/29
4th 23/27
5th 25/25
Final drive 15/34 (front/rear sprocket) 17inch rear wheel.

CG 125 BR E/F/J onwards has the following:

PD 21/70 Primary drive gear 21 to clutch basket 70
1st 13/36
2nd 17/32
3rd 20/28
4th 23/26
5th 25/24
Final drive 14/41 (18inch rear wheel and slightly longer wheel base).

Of note,
The engine output is the same for all models
The wheel base is an inch or two longer on later models
Early CGs 17inch rear wheel
Later CGs 18inch rear wheel
1st & 2nd gears are identical throughout
All mainshaft gears are identical
Layshaft 3rd 4th & 5th are one tooth less on later models
Final drive appear significantly different at 14/41.

Any info or thread conversation would be welcome and appreciated

bigswifty

[Diesel Dave]

Well that's a lot of maths.....

There is a 8.9% difference in the gearing between the two. Closest you can get to a correction is a 14x38 rear sprocket swap which will correct by 7.8%

Someone better check independently though.

[bigswifty]

Aw Dave, how does your mind work ...

I think I have the option to change the layshaft gears (3rd, 4th & 5th)(would this be advantageous?), though not the primary set up (changed all of that for new and I still have the noise when engaging drive; next step is to change the shaft bearings).
I like your solution of 14/38 and sounds like that may rectify/address the difference which seemed significant (in performance) in comparison with Alan's standard CG.

[bigswifty]

Hi Dave,
Just to share some good news
Alan and I stripped the CG engine yesterday, changed the bearings in the gearbox and three gears. Used the training engine for reference and to cherry pick. Got the engine back together last night, back in today with your suggested final drive ratios and ... it's perfect like a new bike.
Going to try the original carb now it's been cleaned. The Chinese carb works ok though is a bit sucky.
Thanks again for your support Dave, mucho apppreciated

[bigswifty]

Works well, good maths Dave

[Diesel Dave]

It was over 40 years ago when I did ratio's in Mr Athertons maths class.

He gave me the slipper once for 'not trying hard enough'.

I let the tyres down on his Mk3 Cortina for that one.

[bigswifty]

Had a bidova scare thisaft, got VERY hot, timing was out slightly and was a bit eager with the throttle first time out. All good though now. I'll fit the temp gauge will running in. Will 190c be the upper limit as on the Cub?

[Diesel Dave]

To be safe yes!

At 205 chinkie piston melt - trust me on this one!

[bigswifty]

Investing in the temp gauge saved my Cub on its run in, good advice Dave particularly on the first 200 ... Instant schorchio, surprising