TIRE SIZE IMPACTS ON RPM

[Paxton]

Greetings...

I am plunging into the "Dark Side." Changing the rear tire on the Valkyrie in favor of a car tire.
The OEM stock size is 180/70/16. The car tire to go on the Valk is 205/65/16... 

Using 3000 RPM as baseline, here are the RPM differences. 

TIRE SIZE - 205/60/16 - http://www.dakota-truck.net/CGI-BIN/TireCalc.cgi

Replacing your 180/70 R 16 (26 x 7.09 x 16) tire with 205/60 R 16 (26 x 8.07 x 16)
would raise your finish line RPM by 9.  ???

TIRE SIZE – 205/65/16 - http://www.dakota-truck.net/CGI-BIN/TireCalc.cgi

Replacing your 180/70 R 16 (26 x 7.09 x 16) tire with 205/65 R 16 (26 x 8.07 x 16)
would lower your finish line RPM by 22. 

[0leman]

Not trying to start an argument here, but when I put my "65" series CT on it dropped my RPM by 200 RPM's or more.  Not 22 RPM's.   Maybe my I/S is different from others, but I did notice this drop.

[Mr.BubblesVRCCDS0008]

Seems to me something doesn't add up. All three tires are showing the same height so all rpms should be the same. The cir. of the tires is the same so distance travelled per rotation would be unchanged so rpm and speed will be the same. I maybe too simple to figure this out, but that is the way it looks to me.

[Valkpilot]

Here's the way it works out mathmatically:



Your actual experience may differ.

Stated tire diminsions are 'nominal' and known to vary from manufacturer to manufacturer, and sometimes from tire model to tire model for a given manufacturer.  Added variability comes from the difficulty of directly comparing MC tire dimensions to DS tire dimensions.

Comments regarding nut cage mod are my assessment gleaned from lots of reading on this and other forums.

[PhredValk]

The term 'finish line RPM' denotes racing, and I think it means wheel revolutions per mile.
Fred.

[2qmedic]

Correct, in this instance RPM is revolutions per mile, not motor rpm's

[Tropic traveler]

What a lot of people forget when making these tire size to RPM observations is the variance of tolerences. One brand of 205/65/16 may be larger or smaller than another. It is not an exact science. When you watch Nascar notice how much effort is put into tire circumference measurements.
The Michelin HydroEdge 205/65/16 I am taking off the '99 RPMs out slightly higher than my Avon 180/70/16 MC tire that my '97 has.  ???  I guess the Mich is on the short side of the 205/65 lot. 
Air pressure can make a difference as well. 

[da prez]

I ALSO PUT ON A 205 /65/16 AND BEFORE DISMOUNT I MEASURED THE CIRCUMFRENCE OF THE STOCK TIRE WITH 4700 MILES ON IT AND THE NEW TIRE AFTER MOUNTING. THEY WERE THE SAME.
IN MY CASE THERE SHOULD BE NO DIFFERENCE IN RPM.
                       da prez

[Paxton]

Well,

It sounds like my apprehension about my CT tire size on my lowered bike might be premature. 

Thank you folks.

[Ricky-D]

Well,

It sounds like my apprehension about my CT tire size on my lowered bike might be premature. 

Thank you folks.

The only real problem you have to contend with is the sidewall of the tire hitting the nut cages inside the fender.

The nut cages are easy enough to remove however!

***

[Paxton]

I just rode back from the shop the looong way!  Had the 205/65/16 tire size installed.   
They cut the tabs without removing the fender while the wheel was off. 

COSTS: Parts (new tire included) plus tire-balancing and installation-labor totaled $190 out the door.

The job took 'bout 1-3/4 hrs. I am glad THEY did it! Well worth the expense. 
No free coffee, pastries or beer at the shop... That part sucked! (just kiddin'). 

If one does the math, the total costs are close to the price of a new, good-quality M/C tire. 

NOTE: Because the 205/65 series tire is taller and my Valk is 1-1/2" lower, the right side bolt and nut had to be cut.
The nut was sliced in half (no pun intended)  and the bolt was shortened about 1/4 inch.

SPEED / RPM: I forgot to check the RPM with the stock M/C tire.  However, with the car tire it reads 3200 RPM @ 75 MPH.
Is that different than stock? 

Yes, it will take some getting used to the different "feel," but the new "behavior" is not as challenging as I had anticipated. 

The taller-fatter-tire looks very Kool.    I'll post some photos l8r. 

  "Never over-estimate the wisdom of a Traffic Cop." 

[B]

All that thought and calculating makes my head hurt.   ???
All I know is when I twist the grip, rpms increase   

[gordonv]

SPEED / RPM: I forgot to check the RPM with the stock M/C tire.  However, with the car tire it reads 3200 RPM @ 75 MPH.
Is that different than stock? 

At 3200 RPM on my IS, I would get 70 MPH.

[PhredValk]

I rode out to the lake Sat, and home today (in the rain, even wet paint and tar snakes seemed not to have any effect on the CT). Falken Ziex ZE-512 205/65R-16. 3000RPM speedo said 69MPH; 70MPH indicated tach just a hair over 3000.
Fred.

[Paxton]

Here are some before/after "Darkside" photos. 

https://picasaweb.google.com/103917731019176896112/ValkyrieOnTheDarkside#

[Black Dog]

There are probably many other sites like this on the www, but this one has served me well for the past few years...

Just plug in yer tire specs, calculate, and ya get everything you need to know - Speedometer error +/-, RPM's additional/reduced miles over a number of miles...

http://www.dakota-truck.net/TIRECALC/tirecalc.html

Black Dog

[Rocketman]

Correct, in this instance RPM is revolutions per mile, not motor rpm's

The effect is the same.  You're simply removing the time factor.  Instead of mph compared to rpm(inute), you're reducing a ratio of two factors down to one factor, rpm(ile).  In mathematical terms, rpm(inute) divided by mph, divided by 60 (to change hours to minutes) equals rpm(ile).

[sailed2japan]

another thing to think about is that when your are riding it is not the actual diameter of the tire, bu the loaded radius that counts. It would make sense to me that the CT would not conpress as much under the load of a motorcycle as a MT would. Again, I hav no hard facts on this, but it seems logical.

[X Ring]

another thing to think about is that when your are riding it is not the actual diameter of the tire, bu the loaded radius that counts. It would make sense to me that the CT would not conpress as much under the load of a motorcycle as a MT would. Again, I hav no hard facts on this, but it seems logical.

And I would disagree with you.  With the wider width and contact patch the DS tire requires less air pressure than a MT so it sidewalls can flex.  So the MT requires higher pressures so the sidewall doesn't flex; therefore, the DS tire compresses more than the MT does.

Marty

[sailed2japan]

Even so, CT's are stiffer because they are designed to carry a heavier load. ie: max load on a Goodyear Eagle 205/55R16 is 1,356 lbs. @ 51psi, for a Dunlop 180/70R16 it's 992 lbs @ 41psi. The max psi a tire can handle is all in it's construction. I'd run 70-80 psi in my 3/4 ton truck with it fully loaded on Load range E tires. I'd never even entertain the thought of running over 44 psi in the tires on my wife's Canyon, even with the bed full of dirt.

[2qmedic]

Area x psi = force
Motorcycle tires have stiffer sidewalls, designed not to flex as part of their job duty in handling.
Car tires have much thinner sidewalls, designed to flex.
The MC tire must have more air psi because it has a smaller area to support the weight.
The car tire does not have to have the same psi because it has a considerably larger area.
Simple explanation: fire rescue air bags for lifting vehicles ect. are different sizes. A 18 x 18 " bag only requires 10 psi to lift 3,240 pounds.
A 24 x 24" bag only requires 5.625 psi to lift the same 3,240 pounds.