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Gear down a TTR125

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I recently purchased a little TTR125 and am getting back into riding after being away several years.  I am interested in gearing it down.  Slowing it down for technical riding (closer to light trials than trails riding).  I already have the engine frame and am working on getting the suspension correctly setup.  Now I want to slow it down.  Even if I jump on the road to run down to the other farm I am not going more than 25mpg so I am not worried about the top speed at all.  Has anyone changed the front sprocket to reduce the gearing, the rear sprocket or both and what were the results?  I would like to gain torque for hoping logs and allow the bike to be ridden slower but never messed with gearing much and was interested in the results.  I was also wondering if I go down a tooth on the front do I need a new chain?  What are the rules on sprockets and chains and changing them around?

Thank you

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Here's a handy online sprocket ratio calculator: http://www.sprocketcalculator.com/

I find the chain 'link' count option gives a general idea what to expect but not always accurate.


I'm not very familiar with TTR125 options but I think your choice of sprockets sizes will be limited,

more experienced members no doubt can chime in with their thoughts.


Ideally you add to the rear rather than go to a lower countershaft tooth count,

the smaller the front sprocket diameter, the more stress it puts on the chain.


If you're into slow trail crawling, also consider a heavy flywheel (if available)

the heaviest available (+12oz) completely transformed my CRF250X,

adding much wanted tractability in low traction situations and resistance to stalling.

Edited by mlatour
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Thanks!!!!!  I will check it out!

Funny you mentioned the fly wheel - the TTR125 has a real heavy flywheel and most guys are out lightening them, I personally love it for what I do!

Appreciate all the info!

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Your original post mentions the interest in slow trail crawling capabilities and wanting to 'slow it down',

unlike Still Bill's (& son) usage which seem more spirited and requires quick throttle response,

lightening the flywheel (less stored energy) will make it more prone to stalling at very low rpms and,

quicker to rev up and spin in low traction situations.

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I ended up dropping one tooth on the front and that is an improvement.  May still end up changing the rear out to slow it a bit more but it was a good start.  next is the air box and a pipe!

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