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Zycki11 Zycki11
4/1/2018 9:34 AM

I thought this was interesting. PC kawi is running a 13/48 combo, Troy Lee KTM is running 13/48, Star is running 13/47. Those are some big horses them 250f's are cranking out to carry those combos

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mattyhamz2 mattyhamz2
4/1/2018 9:45 AM

Easier when you have factory transmissions also

2017 RMZ450
2005 YZ250-sold :,(
1998 YZ250
2005 KX250F

80% of the time it works every time
IG @2HRacing
Thanks to : Factory Effex, N2Dirt, Acerbis, DT1, Fasthouse, Matix, FMF, ASV, 100% & Mika Metals

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Zycki11 Zycki11
4/1/2018 10:43 AM
mattyhamz2 wrote:

Easier when you have ...more

Of course, although the factory transmission is generally more spread out. But the fact that the Yamaha team is running 13/47 is insane

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walent215 walent215
4/1/2018 11:06 AM
mattyhamz2 wrote:

Easier when you have ...more

Zycki11 wrote:

Of course, although the ...more

That is pretty impressive be curious to know what Geico runs considering they start out with the " weaker" motor

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Jrewing Jrewing
4/1/2018 2:16 PM

I'd say it's to help run one gear longer and shift less, but it surely helps to have the ponies to do it

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level level
4/1/2018 3:05 PM

It’s a supercross setup only I’m sure where you rarely get into higher gears and if I remember right that combo would allow more bottom end in a higher gear. Also you are not running wide open in third gear for very long in supercross so that would allow for it to pull faster in a higher gear. I bet the whole race is second and third gear.

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4/1/2018 3:16 PM

Ktm stock is 14/51. 3.64 so 13/48 3.69. It's actually lower

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Derek Harris Derek Harris
4/1/2018 3:42 PM
Zycki11 wrote:

I thought this was ...more

Let's do some basics here...I swear people should have paid attention in high school physics.

A stock yz250f on my dyno cranks out around 40 hp and 20lb ft.
Same for a kx250f (actually kawis' hate my dyno or like magazine's always usually a bit shy of 40 and around 19.5lb ft)

On my dyno our best effort ktm is in excess of 50 and 24 lbft, yamaha similar torque numbers and a tick off peak hp, kx similar to yamaha etc.

stock ktm gearing 14:51 - ratio is 3.64
stock yz250f gearing is 13:50 ratio is 3.84

Now - this doesn't account for internal gearing - both stock bikes could be IDENTICAL at the tire in ratio depending on internal ratios - which a factory transmission CAN change - but often wont need to. The factory services ktm 250 trans is std ratios...

When making more power you have some options. Keep the gearing you have and get more acceleration - or reduce teeth on the back and trade some of the additional pull you COULD have had - for more width in usable power. AKA less shifting.

So - lets assume the factory yz250f makes 20 percent more torque (the only number that matters for acceleration) That puts it around 24 lb ft - lets be really generous and say its 25 and stock is 20.

You could then re gear the bike 20 percent TALLER and have the SAME pull stock has but HUGE width to power.
What is a 20 percent change?

3.84*.8 = 3.072

Working backwards - 13*3.072 = 39 tooth rear or 39/13 = 3.072.
Going to a 47 from a 50 is a 5 percent change in final drive ratio.
So if the team IS making 20 percent more torque - you would have 15 percent more pull - and 5 percent more width than a stock bike - assuming stock internal ratios. Commonly mis understood - changing sprocket teeth DOESN'T"T change gear drops. When you shift - you will drop an identical amount of RPM using a 50 or 47 rear tooth sprocket.

The funny part I laugh at with privateers and excuses from riders - assuming you have a decent built bike - you shouldn't be more than 10 percent off a factory effort in torque or hp. When you do all the math behind what that means in real world differences of acceleration - it's hardly anything. It's actually an argument against mod engines for amateurs and budget allocation on lower budget efforts.

That 10 percent is best made up with lots of chassis and suspension testing - where most privateers are more likely to be 40-50 percent off a factory effort.

The BIG difference from a factory effort is in RPM. A top team will run well over 15k for sx - I'd guess 16-16.5 on some teams based off what I hear.

We can reliably go to 15 on some builds but past that is playing with a money pit. It takes cubic dollars to chase RPM - and lots of testing. That's not even accounting for making good usable power at that RPM.

So a private effort at 14k - vs a 15.5k motor that pulls properly that high - is night and day in on track performance for a host of additional reasons.

At the ultra high rpms - case integrity becomes a problem - valve train stability and life is a challenge - and piston/rod durability is of serious importance. I personally think timing chains are a massive weak link in performance at those speeds as well.

So im not "all that impressed" by running a 13:47.

on a ktm a 13:48 is nothing far off a 14:51

And to anyone who ever wonders why a stock 250f and stock 450 at a local track in the hands of a good local rider turn similar times - and the 450 isnt just WAY faster feeling - it's all final gear ratios. a stock 250f vs a stock 450 in 3d gear can be 30 percent different in final drive ratio - aka knocking off 30 percent of torque at the tire for acceleration of the 450 - but trading it for 30 percent more width - aka why you don't shift a 450 much.

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es337 es337
4/1/2018 4:39 PM
Derek Harris wrote:

Let's do some basics ...more


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bluestreak04 bluestreak04
4/1/2018 5:20 PM
Zycki11 wrote:

I thought this was ...more

Derek Harris wrote:

Let's do some basics ...more

Listen man, I don't know what that means. But I do know that 4 strokes ruined this sport. CHANGE MY MIND

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Zycki11 Zycki11
4/1/2018 7:47 PM

Isn't the entire point of this thread about the ability to maintain a specific ratio while running a different setup? I still find it amazing that the factory bikes are able to do that. And yes, I understand the engine gap is much tighter from a well built motor guy to factory compared to the chassis. It's the ability to have the power and spread it out that makes the factory motor plants so good. On the flip side, the suspension/chassis setup is a whole new world and one that takes tons and tons of time. Prime example, Cooper Webb's bike to me looks like shit. On the flip side, Andersons bike seems to be planted everywhere.

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Mit12 Mit12
4/1/2018 10:30 PM

The gearing is actually lower geared than stock. I tooth on the counter shaft is equal to almost 4 teeth on the rear. A stock bike would pull this gearing better than stock gearing.

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mxer807 mxer807
4/1/2018 11:03 PM

Those 250f’s probably have Close to stock 450 hp.

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