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Discussion Starter #1 (Edited)
MPH vs. RPM

Since there has been much talk about “Pulley” upgrades, “Tire sizes”, I thought I’d throw down some general notes to help out with the outcome of certain modifications and their relative impact compared to others. Since the VN900LT doesn’t come with a Tachometer, it’s nice to know where the RPM’s are for a given speed.

Simplified equation(s):

1) Primary Ratio = Clutch Tooth count / Camshaft Tooth count
2) Secondary Ratio = Rear Drive Pulley Tooth Count / Transmission Pulley Tooth count
3) Overall Ratio (Final) = Primary Ratio x Secondary Ratio x Gear Ratio
4) Rear Wheel Height = [Radius of your rim + (Rear wheel width x Aspect Ratio / 25.4)] (Inches) (You can measure it as well if you like).

e.g. – 180/70 – 15; RWH = [15”/2 + (180mm x .7 / 25.4] = 12.46”

5) “168” – This is used to adjust for minutes, inches and miles per hour, and is a constant in the final formula.

Therefore:

6) MPH = (RPM x Rear Wheel Height) / (Overall Ratio x 168)

7) RPM = (MPH x Overall Ratio x 168) / Rear Wheel Height


Example: (MPH)

From the Kawasaki spec sheets for the VN900 Classic LT (2008)

1) Maximum Torque = 58.2 lb.-ft @ 3500 rpm
2) Overall Ratio = 4.338 (@ Top Gear)
3) Stock tire: 180/70 - 15 - As in step “4” Rear Wheel Height = 12.46”

Therefore;

MPH = (3500rpm x 12.46”) / (4.338 x 168) = 60 mph.

Hence one can “hear” why the VN900 starts to “rev” at that speed!

Check out the Kawasaki website for General / Specific Specification sheets.

Have fun mixing things up for different parameters. Hope this helps when trying to see the impact of changing one item over another, especially contemplating upgrades.

You can also visit www.bakerdrivetrain.com . Select “About Gear Ratio’s” and Automate some of the above.

Denis
 

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Hey thx for the tech info, someone posted a chart where you key in the variables and it pops out the rpm's vs speed a few months ago. I will say that to me, (and i dont have a tach) i tend to use the ear method, the 900 seemed to rev a litte higher than i wanted (to the ear) at cruising speeds (65mph), hence i went to the 200 rear tire.
 

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Discussion Starter #5
That's where the simplified equations are nice. You can quick & easily check out the benifits/results of certain modification over others. Doesn't take long to come to some conclusions.

Denis
 

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Can someone please explain the difference

I thought the stock rear tire on the 900 is a 180/70/16, Correct me if I am wrong, but does the 180 not refer to the width? How is buying a tire that is wider affect the height which (I thought) was the middle number 70? Is it because the wider tire is being squeezed onto the same with rim that it pushes in on the sides and thus stands taller? Someone please assist me, as I need a new rear tire in the next 8 weeks, and need time to research the brands.
 

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Section Width
"Directly after the letter on a tire size, there is a three-digit number. Or if it's a Euro-metric tire, the three-digit number will be at the beginning. This number tells you the section width of the tire in millimeters. The section width is measured from the inner sidewall to the outer sidewall. So if your tire is a P205/45R17, the section width is 205 mm.
Sidewall Aspect Ratio
The next part of the tire size is the sidewall aspect ratio. The sidewall aspect ratio is a two-digit number that directly follows the section width. The sidewall aspect ratio represents a percentage of the section width. So if your tire is a P205/45R17, the sidewall from tread to rim is 45 percent of the section width. The higher the number, the taller the tire."

So a "200" tire is definitely going to be wider than the stock 180/70, but it may or may not be taller. That will depend on the second number, the sidewall aspect ratio. Your stock tire is 180 mm wide, or 7.09", and it's height is 70% of that, or 4.96". Mr. Clean posted this chart on another thread, of the 3 most popular rear tire sizes on the 900.

Quote:
Originally Posted by MrClean
Tire____________170/80-15___180/70-15___200/70-15
Sidewall Height____5.35________4.96________5.51
Section Width_____6.69________7.09________7.87
Overall Diameter___25.71______24.92_______26.02
Circumference_____80.77______78.29_______81.76
Revs per mile_____784.49_____809.27_____774.99
Tire Comparator
 

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So my understanding is from the factory at 60 mph your reving 3500. Changing only the front pulley will change the final drive ratio from 2.0625:1 to 1.941:1. This yields a 5.9% reduction in final drive ratio. I hate to ask but, could someone smarter than me please tell me what you would be reving at 60 after making the change. Thanks
 

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So my understanding is from the factory at 60 mph your reving 3500. Changing only the front pulley will change the final drive ratio from 2.0625:1 to 1.941:1. This yields a 5.9% reduction in final drive ratio. I hate to ask but, could someone smarter than me please tell me what you would be reving at 60 after making the change. Thanks
The answers you seek are here...

Great post, good to have someone who can do the math. Me I am lazy and just click on this :cool:

http://www.gearingcommander.com/

Then enter the selection of bike and play with 'what if' till you find what you want.
 

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Whiting57, you mention having the larger rear tire on your machine. I was just reading about the Scootworks front pulley option as well. Was there a particular reason you went with the tire change as opposed to a pulley (or tire & pulley) change?
I don't generally like to go too fast anyway, but it would be nice to have the option for those flat open road occasions. BTW, was there an obvious choice for the brand/model of the tire you bought?
thx
 

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Whiting57, you mention having the larger rear tire on your machine. I was just reading about the Scootworks front pulley option as well. Was there a particular reason you went with the tire change as opposed to a pulley (or tire & pulley) change?
I don't generally like to go too fast anyway, but it would be nice to have the option for those flat open road occasions. BTW, was there an obvious choice for the brand/model of the tire you bought?
thx
The choice of tire was based on a long term road evaluation by a bike magazine that i suscribe to, i was impressed with the milage that they were getting, the overall review it got in all types of weather and riding conditions.

I have 2 buddies using avons, getting great milage and they rave about them. I have used metzler may times over the years and cant complain.


As far as the pulley, i wanted a greater reduction that what the front pulley offered, i just did not want to pay the 3 to 4 hundred for the rear pulley, and you cant use the metz 880 and the scootworks front pulley ....just not enough room. You can use the front if you also use the rear pulley with a 880,the smaller rear pulley pulls the rear tire back from the inner splash guard.

Remember i am pulling the trailer, i wanted a good reduction, i just did not want to go to far, Its supposed to give a 9.5% reduction ... probably with a proper fitting rim, using our rims, i suspect its even more. I have only put around 60 miles so far on the new rear before winter, cant say its the best thing since sliced bread was invented ... but i did like what i have seen so far..

Changes were not done for speed issues ...i tour at 65mph, ... should give slightly better fuel milage, fixes the speedometer ....less rpm's and a nice relaxing ride. .... RPM's to low , loaded 2 up with the trailer going up a big hill..... I'LL just down shift.


Cheers Maurice
 

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It sounds like you did your homework. I didn't realize that the tolerance was so tight on the back wheel. It is my first belt drive bike. I guess no stretching chain allows the mfg to limit the amount of room back there!

I am primarily looking for the same thing as you I suspect. I like the 60-65 mph (100-110kmh) range and just want the engine to enjoy it as much as I do.
I appreciate the well thought explanation.
 

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I didn't realize that the tolerance was so tight on the back wheel. It is my first belt drive bike.


I am primarily looking for the same thing as you I suspect. I like the 60-65 mph (100-110kmh) range and just want the engine to enjoy it as much as I do.
Because the rim is narrower than the standard rim for this size tire, when the bead attaches to the tire and pulls it in a bit, increasing the "sidewall" with inflation. There is not a lot of room left in front of the tire and the splash guard. A scootworks rear pulley is down 4 teeth (from memory) allowing the rear to slide back a bit. 15% plus is a big reduction, depends on what weight you will be hauling around.

The bike is designed to rev, many ride at 80 mph ... no changes and are happy. I personally prefer the lower rpm's.

As far as tires are concerned, its a personal thing, a wooden wheel would be better than the stock tires, there is a good range for the aftermarket stuff.

I wanted a good all round tire that does well in all areas, milage, rain, corners etc, this seems to fit the bill, my front is still the stocker, has about another 8000 kms left on it, i will change that to the metz front when its done.

My buddies with the Avons are on a ST1300 and a BMW k1200s and really like the milage and cornering abilities, my cruiser does not go over as far as their bikes do .... now my sport bike, well .....:D

My suggestion is .. try the 200 / 70 / 15, see what you think, if it doesnt work for you, when you replace it make the change to the front or rear pulley. I will say, in the short ride that i had on it, it is like riding a different bike.

Cheers Maurice
 
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