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jim01

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Jul 9, 2002
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Hello my fellow 4 stroke riders,

My buddy on his (01)DRZ400 is comming out this weekend to race my (99)YZ400F. I have all stock gearing and so does he(im pretty sure). Who will win on Top speed? What would a YZ400F top out at?
Thanks,
Jim :aj:
 

OldMaiconut

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I asked this same question a while back regarding a 2000 YZ426. Got a long winded reply, but the bottom line, with stock gearing was 87 mph.
 

Rich Rohrich

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Originally posted by jim01
Thanks, but isnt a yz426 more powerfull?

The available power isn't necessarily the limiting factor. Here's why:

Gearing is really pretty simple if you take a step back and look at what is actually happening. Basically gearing just provides a way to reduce the rear wheel speed in relationship to the crankshaft speed. By reducing the rear wheel speed the gearing provides a mechanical advantage. The greater the speed reduction the greater the mechanical advantage. The total speed reduction comes from three separate speed reductions. You have the Primary reduction which comes from the connection between the crankshaft and the clutch outer hub. If you look in the engine on most modern bikes you’ll see they are connected directly together with straight or helical cut gears using a small gear on the crank and a large gear on the clutch. Some older designs used a small primary chain between these two gears. Vintage racers can no doubt tell you lots of horror stories about primary chains that have failed. On the 1998 YZ400 the crank gear has 21 teeth and the clutch gear has 62 teeth , for a primary reduction of 2.952 :1 (62 / 21 = 2.952) . Other dirt bikes will have similar ratios. The 1972 Hodaka Super Rat 100 uses a primary ratio of 3.71:1, while the dual sport Wombat of the same year uses a ratio of 2.75:1 . Street bikes tend to use much lower primary ratios (below 2.0 :1) to provide a higher top speed . Because the crank sprocket (drive sprocket) is smaller than the clutch sprocket the crank will turn more rpm than the clutch when they are connected together. So in the case of the YZ400 the crank will turn 2.952 rpm for each complete revolution of the clutch. Once the clutch is turning it will transfer it’s rotation to the counter shaft. There are two shafts in the transmission , the main shaft that is connected to the clutch inner basket. This is our second speed reduction. This transmission (or internal) speed reduction is variable depending on the gear that is selected. In this case the main shaft is the smaller or “drive” gear while the counter shaft has the larger or “driven” gear . The YZ400’s first gear has a 14 tooth gear on the main shaft and a 27 tooth gear on the counter shaft for a 1.929 :1 ratio (27 / 14 = 1.929) . So now the main shaft is turning 1.929 rpm for each full rotation of the counter shaft. We have the counter shaft turning now, so if we hang a small sprocket on the end of it, loop a length of chain around it and connect it to a large sprocket on our rear wheel we can provide one more speed reduction that we’ll call the secondary or Final reduction. If we use the YZ one more time we end up with a 14 tooth counter shaft sprocket or “drive” sprocket, working with a 49 tooth rear or “driven” sprocket, for a final reduction of 3.500 : 1 (49/14 = 3.500 ) . Now that we know all the individual speed reductions between the crank and the rear wheel we can figure out the Total reduction with some simple math .

Primary ratio * Internal ratio * Final ratio = Total reduction (ratio)

.

Some simple math
For the 98 YZ400 with stock sprockets in first gear that works out to :

2.952 * 1.929 * 3.500 = 19.930 ( Total reduction)

Which means the crank will turn 19.930 rpm for each rotation of the rear wheel when the bike is in first gear. Now if we plug in your WR & YZ426 numbers for first gear we can see what changes we need to make. If we assume the WR and the 426 has the same Primary reduction as the 98 YZ400 (not sure if this is the case) the numbers look like this

For the WR400 with 14/49 sprockets and a first gear ratio of 2.4126 we get :

2.952 * 2.416 * 3.500 = 24.962 ( Total reduction)

For the YZ426 with 14/49 sprockets and a first gear ratio of 1.846 we get :

2.952 * 1.846 * 3.500 = 19.072 ( Total reduction)

So the first two speed reductions on our virtual WR400 are 23.6 % lower than the YZ426. Now we know that we can’t easily change the primary or the internal reductions, all we have left is the final drive ratio.

If we use the first two ratios as a constant for each bike, figuring the gearing is just a matter of juggling the sprocket numbers:

WR400 first gear constant : 2.952 * 2.416 = 7.132

YZ426 first gear constant : 2.952 * 1.846 = 5.449

Now we can use these constants to determine what final drive ratio we need.

WR400 Total reduction / YZ426 constant = Final drive reduction we need

24.962 / 5.449 = 4.581

Now all we have to do is find a sprocket combination that provides close to 4.581 : 1 ratio . 55/12 gives us a ratio of 4.583, and 55/13 is 4.23 :1 . These are pretty low ratios, and I don’t know if a 55 would even fit on the YZ.

So to get as close as possible to the WR you’ll need to verify the primary ratio on the WR400 and the YZ426 and plug the numbers in to the original equation:

Primary reduction (ratio) * Internal reduction (ratio) * Final reduction (ratio) = Total reduction (ratio)

You can use this simple formula to determine the top speed of a bike as well.

If we plug the fifth gear ratio (0.952) into the equation we get

2.952 * 0.952 * 3.500 = 9.836 (Total reduction) .

If the YZ400 peaks at 11,000 rpm then we are reducing the rear wheel speed to :

peak rpm / total reduction = rear wheel rpm

or

11000 / 9.836 = 1118 rpm

Now we can figure out how far the bike went in one minute

Using the equation rear tire circumference (inches) * rear wheel rpm = inches traveled / min we come up with

84 * 1118 = 93912 inches traveled in one minute.

If we convert that to feet we get : 93912 / 12 = 7826 feet traveled in one minute.

Then converting that to miles/min gives us - 7826 / 5280 = 1.482 miles / min

If the YZF has sufficient torque to turn 11,000 in fifth gear and we do the final conversion to miles per hour we get 1.482 * 60 or 88.93 mph.




Further Reading
The Racing Motorcycle : A technical guide for constructors - John Bradley
This is one of the best motorcycle books ever written. It's distributed through Michael Moore's website www.eurospares.com . Even if you don't buy the book, go check out Michael's website. It's one of the coolest sites you'll ever see. Michael is a truly hard core motorcycle freak.
 

kciH

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Jan 28, 2002
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Yeah, 88 sounds about right to me. Give or take .93 mph. :) Rich, what if your rear tire is slightly worn??
 

DC3PIC

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Jul 4, 2001
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Isn't 88 mph the speed at which the flux-capacitor becomes active, allowing time travel to become possible?
 

jim01

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Jul 9, 2002
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Thanks everyone, Thanks a lot rich. Ive learned a lot. That gives me an idea DC3... ;)
 
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Humai

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Feb 6, 2000
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Jim, the bottom line is this: Don't try a top end contest against a DRZ400 with stock gearing, because he'll blow you away.

A stock YZ400 will do nothing like 87 mph - mine with stock gearing did more like 60-65 mph at revs high enough to be uncomfortable for anyone with a modicum of mechanical sympathy. With stock gearing, a DRZ will be easily able to manage 60-65... in fourth gear!

You have two choices: avoid top end contests with your buddy or gear your bike up considerably - ideally so it tops out at around 9,000rpm.

I can tell you from personal experience in side by side lineups that a 98 YZ400 is MUCH more powerful than a DRZ400 stock to stock - your gearing will be your limiting factor.
 

bigred455

"LET'S JUST RIDE"
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Sep 12, 2000
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I installed a space modulator,x19 in my tranny,with a p-54 gear resistor.You don't even wan't to know what my bike does on the tall side. :p
 

LJW

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Dec 3, 2001
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Download a demo copy of GEAR CALC from www.ftwo.com and you will never again have to deal with the gut wrenching nightmare of 7th grade mathematics.
 
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