Help to calculate sprocket please.

[blurredman]

Hey,

I have currently on my 1992 XL125S a 14 tooth front sprocket and a 49 rear sprocket (came with the bike) and the ratio is far too high, the poor thing screams at 38mph..

I am looking for the proper size rear sprocket. The man at a local shop looked through his books and said the origional was 14 front and 47 rear.

Bearing in mind that the engine currently in the XL125 is a honda CG engine (which is 5 speed and not 6 like an XL) - Don't much care for the OHV/OHC cam issue, arguable performance difference.


The rear wheel including tyre measures about 12.5" from the furthest edge of the tyre to the middle bolt.

I would like to get 50mph but with 5 or so mph extra. Would a 47 rear sprocket allow that? Or would the sprocket have to be smaller?

Help much appreciated!
____________________
CBT: 12/06/10, Theory: 22/09/10, Module 1: 09/11/10, Module 2: 19/01/11
Past: 1991 Honda CG125BR-J, 1992 (1980) Honda XL125S, 1996 Kawasaki GPZ500S, 1979 MZ TS150.
Current: 1973 MZ ES250/2 - 18k, 1979 Suzuki TS185ER - 10k, 1981 Honda CX500B - 91k, 1987 MZ ETZ250 (295cc) - 40k, 1989 MZ ETZ251 - 51k.

[Kickstart]

Hi

2 teeth smaller to 47 on a bike doing 38mph is probably going to get you less than 2mph more.

Not sure on the rear wheel / tyre size but I would be tempted to just go with CG125 gearing.

OHC / OHV difference is also going to be that the OHV engine probably revs higher safer.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[blurredman]

[Kickstart]

Hi

Using a 44 tooth rather than a 49 tooth is only going to raise the gearing by a bit over 10%. Would see that 38mph raised up to 42mph.

That said even stock CG125 gearing would only see it at about 45mph which is far slower than one should go. Tends to suggest either the gearing is dramatically lower for some reason or that at 38mph it isn't really screaming.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[bLiXeY]

I'm a novice, but I was playing around with the gearing on my bike recently.

I found this formula helpful, it won't tell you speeds, just gives you a ratio of torque/top speed to help get the right balance for the particular characteristic you want from your bike.

https://www.ooracing.com/store/index.php?act=viewProd&productId=18

As a suggestion, how about trying a 16 front sprocket (should be cheaper to buy). 2 sizes change on a front sprocket should give you a noticeable change, and if its too much or too little, you can just buy the next one up/down?

Oh, and apologies if I'm stating the obvious, but make sure any new sprockets are the correct chain pitch.

[blurredman]

Thanks both.


This is extremely odd. I believe it to be a standard Jap (pre brasil) 6v honda cg engine. I don't think any gearing has been tampered with..
____________________
CBT: 12/06/10, Theory: 22/09/10, Module 1: 09/11/10, Module 2: 19/01/11
Past: 1991 Honda CG125BR-J, 1992 (1980) Honda XL125S, 1996 Kawasaki GPZ500S, 1979 MZ TS150.
Current: 1973 MZ ES250/2 - 18k, 1979 Suzuki TS185ER - 10k, 1981 Honda CX500B - 91k, 1987 MZ ETZ250 (295cc) - 40k, 1989 MZ ETZ251 - 51k.

[robocog]

Don;t know if you have already been there but a good tool is to be had here >

https://www.gearingcommander.com/

Had a fiddle when I put the GS wheels on the DR and seemed to lay everything I wanted out in a neat and comparible way

Regards
Rob
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Needs a bigger garage...

[robocog]

Cannot see a CG125 in the presets but I do have a manual that is sure to have all the relevent info in it
I will dig it out if you don't have one to hand

Regards
Rob
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Needs a bigger garage...

[blurredman]

[Pete.]

Sounds like the clutch is slipping to me.
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a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[Teflon-Mike]

Let me get this straight; you have the 1992 XL125 which ought to have the six speed gearbox and OHC engine, but you have a CG engine in it instead?

Book says the OHC XL has 14/49 sprokets, and you have 14/47, but are topping out at 38mph?

CG125 would have a 14/42 combo and ought to do 65, but with slightly smaller wheel?

If I have this the right; sounds like your problem is not gearing, but something wrong with the engine.

Gearing is there or therabouts the same as a CG so ought to go there or thereabouts as fast as a CG... but it ent... ergo... problem isn't gearing, problem is something like a diff motor or a dragging brake or a slipping clutch!
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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?'

[blurredman]

Keith, you were right somewhat. 38 isn't screaming it. But 48 is. I could get 48 out of the.
____________________
CBT: 12/06/10, Theory: 22/09/10, Module 1: 09/11/10, Module 2: 19/01/11
Past: 1991 Honda CG125BR-J, 1992 (1980) Honda XL125S, 1996 Kawasaki GPZ500S, 1979 MZ TS150.
Current: 1973 MZ ES250/2 - 18k, 1979 Suzuki TS185ER - 10k, 1981 Honda CX500B - 91k, 1987 MZ ETZ250 (295cc) - 40k, 1989 MZ ETZ251 - 51k.

[blurredman]

Just did a quick look over it,

Spark plug is perfect.

In the join between the back box and the down pipe- there is quite a big blowing coming from it which I didn't notice until recently. I have a feeling back pressure May have something to do with it. But i've ordered a new clutch too to put in there - Just to rule that out.
____________________
CBT: 12/06/10, Theory: 22/09/10, Module 1: 09/11/10, Module 2: 19/01/11
Past: 1991 Honda CG125BR-J, 1992 (1980) Honda XL125S, 1996 Kawasaki GPZ500S, 1979 MZ TS150.
Current: 1973 MZ ES250/2 - 18k, 1979 Suzuki TS185ER - 10k, 1981 Honda CX500B - 91k, 1987 MZ ETZ250 (295cc) - 40k, 1989 MZ ETZ251 - 51k.