Stupidily low gearing

[Mentacycle]

Now that Ive had the CPI Sprint on on the road for a few weeks its apparent that the gearing is way too low: I can hook it into 5th at 15mph, and to get more than 45mph its screaming.

Given that finding parts (even CMPO dont list it) is a real pain, can anyone suggest where I can get a 17 tooth front sprocket? Its has a 38 tooth rear sprocket, but finding sprockets that are non standard sizes has so far escaped me!

Meanwhile :scanning ebay

[Teflon-Mike]

It's chinese; its not one of the more common makes; you will not get very far looking for bits by make/model/year like it was a ten a penny ford mondeo I'm afraid. you will have t get a bit more creative, and prepared to take a chance; going by the chain gauge and flange patern. Which is likely CG/Geeric Small honda... but measure the flange patern up!

As to Suitable sizing? It is likely that its already OVER geared not Under and upping the front cog two teeth is monsterousely going too far the wrong way!

The CB125 Twins, that have a lot more power to pull taller cogs run around 15Front 42Rear.. And I tend to get best speed from them runnng on a smaller 14T front sprocket than a taller 15T as it lets them rev out in top.

The Chinks tend to use a lower final drive ratio, to meet local tax restrictions based on speed capablity; which means few will top out over 55mph.... but f you have a 38T rear already, what's that matched to a 13 or 14T front? If a 15T then the gears have already been raised a heck of a lot! Yu likely do NOT need to raise it any more!

HINT is in your suggestion that you can cog top before 25mph and then it wont go over 45..... yes... that sunds 'normal' and is because you have about 9bhp at 9ooo rpm, which means that you have around 3bhp by 3ooo rpm, and 3bhp is enough to push a motorbike to around 30mph....

But it takes aprox the full 9bhp to go 60, and between 30 and 60 there's not enough 'spare' power to hold speed and accelerate, and to go faster requires more power than the revs delver for it, so you are effectvely topped out, way before the engine has actually achieved max power revs; ad you need lower gears to go faster, not taller.

Simple solution.... Change DOWN use the ruddy gearbox! Thrash the thing to the red line in each cog, and expect to get max speed in 4th, not 5th.. and you will likely find that with the lower gearing you WILL actually go faster...

5th gear on these is a 'over drive'; you can use once you have done your accelerating to top speed in the lower gears, to hold that speed with maybe a few less revs; it is not a gear for acceleration or response; and on such a low powered bike, is seldom much use for anything.

You are 'short shifting'making the motor labour and it just dont have the power to pull even the gearing as tall as its got, let alone gears as tall as you are suggesting giving it.

As rough guide:-
1st for a standng start to about 15mph
2nd round the houses up to aprox 25mph and any trafc situations you need trickle at less than 30, like roundabouts
3rd.. in traffic on 30-40mph limit roads... aught take you to about 40'ish' with plenty of throttle response to react to traffic and changes of speed limit etc.. its the mother of all work gear on a 125 4stoke.
4th... 40mph and up to flat out on higher speed limit roads without the haxards you need respond to.
5th.. MAYBE if you are on a duel carriageway or very open a-road with little chance of needing to change speed for bends or hazards or juntions or anything

Use the revs NOT the gears; you will ride more smoothly, and not get into sch a tangle trying to rush changes either up or down, and you will have far more control, as well as go faster and likely bet better economy for it.
____________________
My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'

[Mentacycle]

Thats a well-thought-out reply, thanks Teflon-Mike.

At the moment the gearing/ speed is this (at 8K rpm):
1st 4mph
2nd 9mph
3rd 22 mph
4th 35 mph
5th 44 mph

The gearing is spot on when I have my son (and his squeaky rubber chicken) as pillion.

Perhaps one way to go is get a spare rear (CG) with a small rear sprocket to swap in for when I'm alone on the bike.

I know what you mean about the Honda CD/CB 125/185 twins, I had a few before/after i passed my test, and they all had 14/15 tooth pinion sprockets.... I mix my old CX.

[P.]

[Mentacycle]

[Rogerborg]

It's likely to be a Honda CG (or CB) or a Suzuki GS (or GN or EN) equivalent.

Measure up the front sprocket and check it against the measurements given at: https://www.jtsprockets.com/catalogue/

Try:
https://www.jtsprockets.com/catalogue/sprocket/JTF259/

or

https://www.jtsprockets.com/catalogue/sprocket/JTF426/ (only goes up to 16 teeth though).
____________________
Biking is 1/20th as dangerous as horse riding.
GONE: HN125-8, LF-250B, GPz 305, GPZ 500S, Burgman 400 // RIDING: F650GS (800 twin), Royal Enfield Bullet Electra 500 AVL, Ninja 250R because racebike

[Mentacycle]

Thanks for the link, thats one site i had not seen yet.