Gear Sprocket ratios

[85YZRacer]

I have an 85 yz125 and im able to fly through the gears really really fast. first,second,and third gear are very low and i dunno why yamaha made them that way but whatever. My question is that i heard you can change the top speed and acceleration of your bike by changing the number of teeth on the small and larg gear sprockets. Im not an expert so what would be a good combonation. How does all that work? My larg sprocket uses 52 teeth and the small sprocket uses 12. both are stock.

 

[FruDaddy]

So, you want a little more space between shifts, and a bit more top speed? If so step up to a 13 tooth front sprocket or a 49 tooth rear. I you want to go the other way, try to find a 55 rear. The geasring you speak of doesn't sound like any thing that the factory would have dont, but I am not really knowledgeable in that arena. But if your don't like the way it's geared, changing it is easy. Remember, once there is a little wear in the final driveline, it is best to change it all at once.

 

[Mavrick70004]

ya i agree that dosnt sound like something yamaha would put stock on a bike. Did you get the bike new? 52 teeth on the rear and 12 on the front will def make u rev higher but go slower. My yz250 has 14 on the front and 50 on the rear. when ever i tell someone about sprockets i always talk about a 21 speed bike, the front has a small medium and large and the back has pretty much the same thing. when you have the front on the small gear and the back on the big gear you can go up hills alot easier but can only go so fast but when you switch the bike to the big gear up front and the small gear on the rear you can go really fast but good luck trying to pedal up a hill. Its the same thing on a dirtbike. Right now it sounds like your sprockets are set up for some hill riding which makes it easier for the engine to stay in the powerband. what kind of riding do u mostly do?

 

[Mavrick70004]

After looking it up it appears that for some strange reason yamaha did put those sprockets on that bike stock :whoa:
Just for your Info The 2006 yz125 has a 13 tooth front and a 48 tooth rear seems like a good all around sprocket sizes.

 

[2-Strokes 4-ever]

One LESS tooth in front will equal adding aprox. 3.5 teeth in rear. More teeth rear........more low-speed, quicker acceleration. And vice-versa. Changing 2-3 teeth makes a quite noticable difference.

 

[85YZRacer]

I mostly do alot of trail riding. I dont race at all. I bought the bike used in the early 90's but the gearing is stock. I do notice that when i'm climbing up hills, i can get up them fairly easy. But when i hit strait road, i can get into 6th gear pretty fast. So i guess what i want is to still go up hills but with a little more time in between shifts and hopefully a little more top speed. And from what i'm reading, a 13 tooth in the front and a 48 or 49 tooth in the rear is a good all around gear set-up for that. Am I wrong?

 

[tony91]

Right now your final drive is 4.33 (52/12=4.33).

52/12= 4.33
51/12= 4.25
50/12= 4.17

49/13= 3.77
48/13= 3.69

I'd go with the 51/12 combo first. You'd be surprised how much one tooth changes things.

 

[BSWIFT]

52/12 is mountain goat gears!
49/13 would be closer to stock. The 1st, 2nd, and 3rd ratios will be the same but he overall ratios will be a bit wider. These bikes are designed to have a close ratio gear box for MX.
Ignor this post. My assumptions were incorrect, see later post for links to the correct data.

 

[FruDaddy]

52/12 is mountain goat gears!
49/13 would be closer to stock.

From all of the data that has been provided, 52/12 is stock on that bike. My opinion: 49 or 50/12 is the way to go. 49/13 is a huge jump. 48/12 and 52/13 will each result in an even 4:1 ratio, and even ratio like that are a no-no in the automotive world (for reasons that I don't care to research now), so I would not recommend using them.
For reference, the 07 Yz125 is leaving the factory with 48/13 gearing. It seems that that have gone from one extreme to the other.
52/12= 4.33
51/12= 4.25
50/12= 4.17
49/12= 4.08
48/12= 4.00
52/13= 4.00
51/13= 3.92
50/13= 3.85
49/13= 3.77
48/13= 3.69

 

[BSWIFT]

I don't have exact data, but I don't thing that the 12T is a stock counter sproket. A 13T or 14T are common with the 13T being most common. I'll look into it.

 

[BSWIFT]

http://www.yamaha-motor.com/sport/parts/home.aspx
http://www.yamaha-motor.com/sport/parts/home.aspx
I stand(sit) corrected. The first link is for the parts fiche that includes the counter sprocket, stock is 12T.
The second link is for the parts fiche that includes the rear sproket, stock 52T.
With the correct data, I would now go with the 51/12 or 50/12 depending on your riding style and weight.

 

[FruDaddy]

85YZRacer says it's stock and Maverick70004 said that he did look it up, so I didn't bother. It definitely looks steep to me, but who knows what they were think 20 years ago. Either way, it has little bearing on what he should go with now.

 

[jason33]

12 is stock!!
you can get a 12, 13,or a 14 for the front

 

[Mavrick70004]

Just for Info

Just to let everyone know http://www.rockymountainatv.com/
has specs for pretty much every bike tire sizes, spark plug, sprockets etc. on the left hand side under Terms click Machine specs.