Sprocket Changes on a 125

[Bradleyrayner]

On my sinnis i want the top end to be better. So i looked it up and people have said to change ratio on the sprockets. Cant remember the sprocket sizes can someone help me. Do i increase the rear sprocket and decrease the front sprocket or is it the other way around. Any help will be much appreciated.
Thanks

[bojer]

If you hope to get more top speed you need higher gearing - ie a bigger front or a smaller rear sprocket , assuming you have a 15T on the front atm I'd try a 16T and see what you think http://www.ebay.co.uk/itm/Suzuki-GN1...item20bc334ab5

[culcune]

Rear sprockets offer a much more dramatic change; you go with a smaller rear. However, they require much more committment in that you have to take off the rear wheel, and purchase a new chain, as well as resizing the chain by removing links (requires a special tool which usually needs to be done by a motorcycle mechanic).

Bojer's suggestion of a larger front is a better way to go if you want to have to do less work. I may be wrong, but I believe you don't need a new chain (the rear ones require them because re using your old chain tends to destroy the new rear sprocket) when you just change the front sprocket, and don't need to add or subtract links.

[Bradleyrayner]

Hi there thanks for the replies, i thought that you had change the rear sprocket if you have done the front. What MPH will i gain if i change the front?

[spencer2004]

Hi there thanks for the replies, i thought that you had change the rear sprocket if you have done the front. What MPH will i gain if i change the front?

You can change either one and leave the other as it is, some people say that you should replace both sprockets and chain as a set but it probably dosen't matter if your sprockets are fairly new.

I had to get a longer chain when I changed my front sprocket but that was because I went from a 15t to a 17t, an increase in 1 tooth shouldn't require a longer chain.

When you change your front sprocket you need to remove the clutch cover (noting where the bolts go as they are of different lenghts), bend the lock washer back with a hammer and screwdriver, use a 27mm socket to remove the sprocket nut, un-do the chain adjusters as far as they go to create some slack in the chain, remove the sprocket, put the new sprocket on in reverse order. When I changed mine I had great difficulty removing the sprocket nut because the people who built it torqued the nut to about 10,000 nm and the rubber shock absorbers in the rear wheel made it impossible to create any impact force so I wedged a strong piece of metal between one of the spokes in the wheel and the shock absorbers and I used a long breaker bar to get the nut off and even then I had to use all my strength to get the nut off.

The sprocket that bojer linked is the wrong one by the way; you want this one.
http://www.chinesemotorcyclepartsonl...php?i=QM125-2V

As for how much top speed you will gain it is hard to say but I can give you a rough idea. Lets assume you currently have a front sprocket with 15 teeth and a rear sprocket with 45 teeth, this gives you a final drive ratio of 45 / 15 or 3.00 meaning for every 3 times the front sprocket rotates the rear wheel rotates once. If you fit a 16 tooth front sprocket you will then have a final drive ratio of 45 / 16 or 2.8125 meaning that the front sprocket has to rotate only 2.8125 times to turn the rear wheel once. 3 / 2.8125 is 1.0666 so your top speed will in theory be increased by 6.66%.

[Bradleyrayner]

Hi there mate thanks for the reply what is the difference in speed if i put a 17 on front and a 41 rear. Also if i was to change the sprockets whats the best ratio?

[spencer2004]

I can't say because I don't know what sprocket sizes you have already but I think a 17t and 41t would be too much. I have a 17t front and 45t rear (I think) so I've got a ratio of 2.64 which is a bit too over geared as I have to stay in the high RPMs to get any decient acceleration but I have got used to it. If I was you I'd get the 16 tooth sprocket and see how that goes, if you want to try for a bit more top speed then get a slightly smaller rear sprocket. If you want the 17 tooth sprocket you have to order it from China by the way.

[Bradleyrayner]

Ok then mate thanks for all your help.

[culcune]

Do not go 17/41. For my TMEC 200, I decided to have a custom 40 tooth rear sprocket made which now pairs with the stock 17 front. A little too much in that 5th gear is more like an overdrive. However, 4th gear, full throttle, is turning less RPMs than 5th used to be when I still had the stock 56 t rear. Like 'spencer2004' stated, I will probably have to get used to it. As for your 125, using the TMEC 125 as an example soince it is as close to mine for reference, the 17" rims use a Derbi Senda R clone rear sprocket with 56teeth. JTR sprockets manufactures a 48 tooth replacement aftermarket sprocket, the lowest teeth available. I am not sure what front sprocket comes stock on the 125 TMEC, but using my 200cc engine for reference, the 48 is probably the lowest you would want to go since they are relatively easy to get. I am not sure what bike you have, as I don' t remember if you mentioned it, but if I were you, a 46 tooth rear, if they are available for your bike, is what I would seek out. With my 200, if (when) I redo my rear, I think I will have a 44
tooth sprocket made.

[culcune]

OK, you did say Sinnis. If these are the Qingqi, or Pioneer manufactured versions, I believe they use Suzuki DR 125 clone parts. I can't link to it since I am on a tablet, but when I went to the Uk Ebay,and put 'Sinnis sprockets' on the search bar, several came up for Qingqi, Sinnis, etc (there are a few British brands of this bike). They did have a 42 tooth one for £22, but I would be hesitant to get one of those. They had both a 45 and a 46 tooth one, which would be my choice if I was in your situation.