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Discussion Starter · #1 ·
Just a question... How would i go to gain just a tad bit more speed? I know crap on gearing and both sprockets are stock. I just want that little extra out of my Z.
Thanks in advance
 

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Discussion Starter · #3 ·
Top End Speed ... You could try a 15 tooth sprocket on the front, but most of the stock Z's wont pull it due to the small HP the bike has. Then Engine Modifications is the next step .
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Off the line speed ... you could try a 13 tooth sprocket on the front and a 32 tooth sprocket on the rear, But you will lose top end speed !
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Heres a link for understanding Gearing Choices .
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https://www.z125owners.com/forum/appearance-modifications/13738-sprockets-n-gear-ratio-choices.html
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Current set up is two bros tarmac exhaust and chimera intake with hoedar tuned
 

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You might be able to pull the 15T then ? worth the 15 bucks to try, stock chain length still works
Your particular actual terrain riding conditions will make all the difference .
And small things like Max air pressure , Well lubed chain , Tight clothing .... Etc'
 

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Discussion Starter · #5 ·
You might be able to pull the 15T then ? worth the 15 bucks to try, stock chain length still works
Your particular actual terrain riding conditions will make all the difference .
And small things like Max air pressure , Well lubed chain , Tight clothing .... Etc'
Thanks bud i will try that option
 

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I just ordered a 15t for my stock 2017.
I only run around town, so it should make for less shifting. Maybe get into 4th gear once in a while going over 35. Hoping for good results for my needs.
 

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A 15T is great for around town and riders that are not needing the maximum performance range . It takes a lot off the Jerkiness out while going through the gears and the compression braking is less as well ;)
 

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Careful. A taller rear tire also (a) makes the speedometer UNDERreport the actual speed and (b) steepens the front end geometry and makes the bike notably more twitchy.

Jim G
Sheer ignorance speaking here (I've always just bought OEM recommended tire sizes) but could you offset the geometry concern with a similar increase in the front tire? Obviously clearance is a consideration, but if everything fits the only downside is increased weight right?
 

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Sheer ignorance speaking here (I've always just bought OEM recommended tire sizes) but could you offset the geometry concern with a similar increase in the front tire? Obviously clearance is a consideration, but if everything fits the only downside is increased weight right?
Yes, you can increase the height of the front tire to match the increase at the rear. But as you point out, there is almost certainly going to be an increase in weight, and added to the increase in weight at the REAR tire, it starts to get "big". This has a rather significant effect on the motorcycle's performance because of moment of inertia effects.

Basically, oversimplifying for ease of understanding, moment of inertia is to rotating components what weight is to linearly moving components. The more weight you put in a rotating component, the more resistant it becomes to changing its speed OR orientation in any dimension! i.e. more weight in a rotating component "slows down" handling, rotational acceleration AND linear acceleration! Not a good thing to do, especially on a motorcycle, and especially on a low powered motorcycle. And, to make things worse, the tires, having the largest radius of any rotating component on the bike, have more negative moment of inertia effect than any other part on the bike, including the wheels themselves.

Also, using tires with a higher sidewall than before does degrade the handling as well because it makes the tire less rigid when cornering.

Of course, it depends on how sensitive you are to how your bike feels, and how much it matters to you. A commuter might not notice or care. An enthusiast will notice the degradation in performance.

Another way to compensate at the front end of the bike is to drop the fork tubes lower in the triple, BUT ONLY IF THAT STILL LEAVES ENOUGH FORK TUBE ABOVE THE UPPER TRIPLE TO BE SAFE. I haven't looked at the Z125 front end that closely yet (just got my Z125), but on my Yamaha R3, you cannot lower the front fork tubes more than a tiny fraction of an inch before their tops becomes flush with the top of the top plate of the triple.

Jim G
 

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Good info!

I suppose, everything is going to be more sensitive as your overall vehicle weight and power decrease. I had doubted that a little weight would make a difference but some quick and dirty math proves otherwise.

In my car my butt dyno feels a difference between my summer forged wheels and my winter cast wheels. This is a 0.5% difference in overall vehicle weight (not including differences in summer vs winter tires) and it is noticeable in a car with a 6.80 HP/100lb ratio. By comparison a Z125 is 3.56 HP/100lb (without a rider!) and a 0.5% increase in weight amounts to a measly 1lb 2oz!

Of course the opposite logic is also true, remove 1lb of rotating mass and you should expect to feel it. Interesting. Fun thought exercise, thanks!

(Edit: Now I need to remember to weigh my summer/winter wheel & tire combos to find the real weight difference... for science)
 

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One of the spring kits will raise the front by about an inch, that would offset the 80 series tire change. I'm thinking of doing it just to calm the front end down since we don't have any other way of changing the trail.
I was thinking along the same lines. I like how sensitive this bike is to inputs, but it's my wife's first bike so anything I can do to make it more confidence inspiring feels like money well spent. We say "flickable" they say "twitchy".

Side note: Got my first legitimate "gust" of side-wind on the Z this morning. Hoo boy! What a ride!
 

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Some tire weights that I’ve recorded. Seems there isn’t much difference in the weight of rear tires. All weighed on same scale same day. Handling was greatly improved with tt93/pmts


Tire Weights
<100miles stock tires
IRC 100/90/12 6’ 9 oz
IRC 120/70/12 7’11 oz

~550 hard track miles
Tt93 100/90/12 7’ 9oz
Tt93 120/80/12 7’11 oz

New medium compound slicks
Pmt 100/90/12 5’7 oz
Pmt 120/80/12 7’6 oz
 

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Some tire weights that I’ve recorded. Seems there isn’t much difference in the weight of rear tires. All weighed on same scale same day. Handling was greatly improved with tt93/pmts


Tire Weights
<100miles stock tires
IRC 100/90/12 6’ 9 oz
IRC 120/70/12 7’11 oz

~550 hard track miles
Tt93 100/90/12 7’ 9oz
Tt93 120/80/12 7’11 oz

New medium compound slicks
Pmt 100/90/12 5’7 oz
Pmt 120/80/12 7’6 oz
What are the lightest available STREET tires that fit the Z125?

Jim G
 

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Sorry, just had too
 
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