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... just out of curious concern, what does the unsprung weight difference measure out to be? I'm speaking of the stock 4.5"/160 combo vs the 5.5"/170 (or even 180) ensemble comparison.

I have to imagine that going from the 4.5/160 to the 5.5/180 increases rotational weight as well, which might have an influence on acceleration rates, along with additional unsprung weight.

When I used to race SCORE off-road/desert events (back in the 1980s ... y'know, before fire was discovered) the general rule of thumb was handling was affected by a ratio of 1:10. Meaning for every one pound of unsprung weight added it "felt" as if ten pounds of sprung weight was added.

Anyone know what those weight differences are (spec'd above)?

Thankies!! :smile2:
 

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haha the fat cati!!

no with a 180 rear you get more tread on the sides when leaning so have more grip ;) my theory anyway haha plus it looks better :p :p :p :p :p
My experience (longtime 900SS owner) is that you don't have anywhere near enough horsepower to break that rear wheel free on that bike unless you are riding on the ragged edge on wet sandy corners. So 180 vs 170 for increased contact patch isn't really a viable reason. With the 170 rear your turn-in is easier, and the tire itself (unsprung mass) is lighter. Win-win-win.

Where you lose, of course, is that you can't find 170 tires anymore. I had an SP so I'm not sure about fit on a CR but I'd guess a 180 would fit... otherwise everyone would be running 170's, which they are not... just about every one I've seen has a 180 tire on the back.

Plus that phat tire does look da' boss.
 

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I dont know about not being able to find 170 tyres any more mate, I got 1 in last week for my 91 900ss, I normally work on BMW's and alot of them use a 170 rear tyre--I have yet to have a problem finding rubber----and as to size of tyre I race on a 130 80 18 rear--far skinnier then a 170 or 180, and I probably can go around that corner faster then you can on your 170 or 180 the contact patch size is basically no different between the sizes.Ride on what you feel comfortable on but the 180 will be slower in the transition then the 170--unless you change the rear shock to one a hair tallier or you can jack up the preload on the rear if you have a preload adjustment-there are lots of things that can be done for handling etc--but mine I leave basically stock and it will handle just fine, unless you are putting the bike on the track or ride hard enough to drag your knee;s thru the twisties all the time--leave it alone, Your bike can probably do more then you are capable of
 

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Fred is right just swap rims .You have everything you need. Sprocket flipped. Speedo will read (10 mph higher with a 180).
 

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Speedo will read (10 mph higher with a 180).
Sorry, maybe I'm completely stupid, but can you explain this to me?

How do you know that it would be 10mph? if you're going 5mph it's going to say 15mph? At 100mph it's going to say 110mph? Also, wouldn't a larger tire reduce the indicated speed as opposed to raise it like you're saying since a larger rolling diameter makes fewer revolutions at a given speed?

I would love to be understand this better since I have a 180 rear on my Supersport.
 

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I’m not sure how a front wheel driven speedometer would be affected by a change in rear tire size. I must have missed class that day.
That was my next question...
 

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The 5.5 wheel is a direct swap for the 4.5 inch on the 900ss/cr. No special parts required, just lift and shift. You can still get 170 tires but the selection seems to be much much better with 180. One odd thing though is the original 16mm bearings on these rear wheels are deprecated with Ducati in lieu of a thinner bearing and matching spacers which are freakishly expensive. But you can get the original spec 16 mm bearings by skf online.

And the speedometer is driven off the front wheel so no change there either ;)

Cheers


Sent from my iPhone using Tapatalk
 

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That was my next question...
The speedo / mph reading would not be changed by using a 180 rear instead of a 170 rear, they are both 17 inch tyres basically only the width has changed & you are not changing the Ft tyre which is where the speedo is driven--If you changed the size of the tyre from a 17 to say a 15 inch ft & rear then yes your speed would not be correct -same as if you went from your 17 inch tyres to a set of 18 inch then your speedo would be off as well
 

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The speedo / mph reading would not be changed by using a 180 rear instead of a 170 rear, they are both 17 inch tyres basically only the width has changed & you are not changing the Ft tyre which is where the speedo is driven--If you changed the size of the tyre from a 17 to say a 15 inch ft & rear then yes your speed would not be correct -same as if you went from your 17 inch tyres to a set of 18 inch then your speedo would be off as well
Just to clear this up, going from a 170 to a 180 will (WILL) change your engine RPM at any given road speed. A 170 and a 180 are NOT the same circumference even though they both use the same sized rim (17 inches).

For instance (for comparison's sake);

** a 175x65x17 tire has a 2069mm (81.46 inches) circumference.
** a 185x65x17 tire has a 2110mm (83.07 inches) circumference.

Note the only difference between those two tires is the width. Aspect ratio and rim diameter are the same. Yet, there is a 41mm (1.61 inches) difference in circumference. So changing tire width alone WILL change the number of times the wheel rotates in a given length of road. That fact will influence engine RPM at a given indicated speed if the rear tire's width is increased or decreased. And it WILL influence your speedometer's calibration if the front tire's width is increased or decreased*.

This also can change just by changing tire brands, or even just tire "models" within the same brand.

There are tire size calculators all over the interwebs. Plug in a tire's dimensions (width, aspect ratio, rim diameter) and plug in the tire dimensions of the proposed new tire. The calculators display all of the dimensions.

*This applies to these Ducatis that have the speedo driver on the front wheel.


Carry on! :smile2:
 

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If I remember reading this post people are talking about MPH on the speedo--NOT R.P.M. --the front wheel is where the speedo drive is--Changing a rear tyre size from a 170 to a 180 --WILL NOT cause your speedo to be off---you could put a 15 in tyre on the rear of the bike the RPM would be way higher but your speedo reading would still be correct --RPM does not have jack to do with speed reading on the speedo---same thing as changing sprockets --your RPM range changes but it does not = speedo being off
 

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Put your Bike on a Dyno--ok-the rear wheel is spinning like crazy your Front tyre is not turning--ask [email protected] 6K RPM (rear tyre turning) --what is your speedo going to read--Just to make this perfectly clear--your Speedo will read Absolutely Nothing----The size of the rear tyre has fuck all to do with MPH reading on your Speedo---does if effect your RPM at a given speed? Yes --But NOT the MPH reading on the speedo
 

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The speedo / mph reading would not be changed by using a 180 rear instead of a 170 rear, they are both 17 inch tyres basically only the width has changed & you are not changing the Ft tyre which is where the speedo is driven--If you changed the size of the tyre from a 17 to say a 15 inch ft & rear then yes your speed would not be correct -same as if you went from your 17 inch tyres to a set of 18 inch then your speedo would be off as well
Now I know you're trying to pull my leg. 170 and 180 tires with the same side profile ratio will of course have different circumferences. That's like the most basic rule of tire sizing and simple math.
 

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rear rim...

I very much doubt that even with stock tires, your speedometer is 100% accurate the day it left the factory. It is just not designed to be that precise....

Who cares if their speedometer/ odometer is 100 per cent accurate?

Fred
 

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Look your ft tyre is where the speedo drive is it reads revolutions of the front tyre--Your rear tyre size has absolutely nothing to do with the revolutions of the front tyre it is still turning at the same number of turns--as with my example of being on a dyno ft trye is not turning hince you will get zero MPH on your speedo--same as if your an idiot doing a wheelie -(not that these bikes have enough H.P. to really do that for long) --if you could hold the ft wheel up off the ground for a long period your MPH reading on your speedo will fall off even tho your RPM number may stay high--again this started with a question by someone asking if changing your rear tyre from a 170 to a 180 would effect your speedo reading--again the answer is NO--If you change your sprockets to a tallier gearing the MPH reading will change at a given RPM, (i am just pulling numbers out of my ass with this example)-lets say with stock gearing @ 100 mph you are turning 5000 RPM--but if you change your gearing to a tallier gearing you may be @ 100 MPH turning 4500 RPM this changes the RPM at a given speed--BUT It does not effect your true MPH on the speedo--Changing a rear tyre size has the same effect your RPM can change slightly--BUT your actual MPH is not changing-If you continue to run the stock front tyre size -that tyre is still turning at the exact same number of revolutions as it was with the original size tyre you were running--Being that the speedo drive unit is mounted to the front wheel-that means your MPH reading is not effected by rear tyre size. --I work normally on BMW's-on the older BMW's the the speedo drive works from the gearbox-Now on those bikes changing the rear drive ratio or tyre size WILL effect your speedo reading, But on these older Ducati's with the speedo drive on the front wheel it will not.-The speedo drive's are calibrated to read X for every Z # of revolutions-
 

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Look at your speedo-at the bottom it has a ratio that works in conjunction with your speedo drive unit--if you went from your stock 17 front rim to a 16 inch front rim your speedo would read fast-because the front tyre is turning at a higher # of revolutions--you would have to change out the drive unit & speedo to a drive ratio for that combo--but again it is not effected by the rear rim size. --keep in mind here we are talking only about MPH reading's on your speedo--NOT RPM.
 

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Think about this---If you change your sprocket sizes does this effect your MPH? -The answer is --NO--Does it effect your RPM?? --YES--rear tyre size / ratio will give you the exact same answer. --Lets give a stupid crazy example--just for round numbers lets say you have a 15 inch front rim and everything is calibrated for that size-& at 100 MPH you are turning 5000 RPM. -now you put on a 30 inch rear rim-now for every revolution of the 30 in. rear your Ft rotates 2 times--does this effect your MPH reading on the speedo--NO -your RPM will drop to say 2500 RPM due to the size of the rear rim--these numbers are not exact just a general example--MPH & RPM are two different things
 
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