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Discussion Starter #1
I purchased a 90/90-21 front for my 900 Custom and a 195/165-15 car tire for the rear. I am getting ready to install the tires, but I just realized that there might be a problem raising the front (with the 90/90) slightly while maintaining the same height at the rear. Any issues (for example, over- or under-steer, or anything else) using what I've got, or do I need a slightly larger CT on the rear? Thanks.
 

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No input on the steering issues but the other person that put on that front tire had to raise the front fender.
 

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I purchased a 90/90-21 front for my 900 Custom and a 195/165-15 car tire for the rear. I am getting ready to install the tires, but I just realized that there might be a problem raising the front (with the 90/90) slightly while maintaining the same height at the rear. Any issues (for example, over- or under-steer, or anything else) using what I've got, or do I need a slightly larger CT on the rear? Thanks.
If you do nothing, the car tire might cause the rear to be lower, while the front might be taller. This will unweight your front tire and cause your steering to be vague, even heavy-feeling as the combination has the same effect as increasing the front end rake angle. You can fix that by lowering the triple tree on the forks so that the top of the forks sticks up over the upper triple clamp slightly. That should level the bike out, back to what it was originally.
 

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Discussion Starter #5
If you do nothing, the car tire might cause the rear to be lower, while the front might be taller. This will unweight your front tire and cause your steering to be vague, even heavy-feeling as the combination has the same effect as increasing the front end rake angle. You can fix that by lowering the triple tree on the forks so that the top of the forks sticks up over the upper triple clamp slightly. That should level the bike out, back to what it was originally.
Thanks! 2 questions - how would I lower the triple tree and/or, if I get the next size up CT would it resolve the issue. If so, what size would you suggest?
 

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Crazyj..............Here's some food for thought...from experience......I have been on the dark side...Choose a narrower tire; a 185 OR 175 and a 70 or 75 ratio.....it will decrease "pitch" or rear offset, that changes the front and rear wheel alignment....that WILL affect stability....I KNOW.....Another consideration...CTs have very thin side walls....To eliminate flex or distortion in turns...and it WILL distort...increase air pressure about 10-15% above 35psi....or to a pressure YOU feel gives you some high speed control..LOW air pressure WILL cause a low frequency "weave or wobble" (between 100-115kph by GPS)......I KNOW!!!!!!!! And finally; IF you get a flat at speed and can't figure out why the damned rear end is all over the place BEFORE the air pressure runs out...well I hope you have comprehensive AND collision...NOW I am NOT dissing the dark side, however there ARE issues that have to be recognized.....and treated with respect.....Believe me, I've been there..and it will get your attention IF you are not attuned to the different "physics"......low frequency "weave or wobble" IS different than a high speed "head shake".....
 

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I also run a darkside tire and agree you need to be aware of differences mentioned. ......but.... the mentioned issues are usually easily accommodated. And I wish to add to the sidewall flex comment.... you want the sidewall to flex a bit... sidewalls can take it.... it is called finding the sweet spot between too little pressure (which does cause the weaving you mention)... and too much pressure which gives you rigid ride but a tiny contact patch. I did videos of this on this forum a few months ago...and too much pressure results in a scary tiny contact patch on a hard turn. A bit of sidewall flex allows a lot more tread to stay in contact without any 'weaving' effect and reduces the 'offset pitch" you mention. The standard advice is to find the sweet spot between these two extremes.


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Crazyj..............Here's some food for thought...from experience......I have been on the dark side...Choose a narrower tire; a 185 OR 175 and a 70 or 75 ratio.....it will decrease "pitch" or rear offset, that changes the front and rear wheel alignment....that WILL affect stability....I KNOW.....Another consideration...CTs have very thin side walls....To eliminate flex or distortion in turns...and it WILL distort...increase air pressure about 10-15% above 35psi....or to a pressure YOU feel gives you some high speed control..LOW air pressure WILL cause a low frequency "weave or wobble"......I KNOW!!!!!!!! And finally; IF you get a flat at speed and can't figure out why the damned rear end is all over the place BEFORE the air pressure runs out...well I hope you have comprehensive AND collision...NOW I am NOT dissing the dark side, however there ARE issues that have to be recognized.....and treated with respect.....Believe me, I've been there..and it will get your attention IF you are not attuned to the different "physics"......low frequency "weave or wobble" IS different than a high speed "head shake".....
 

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Yes you DO need side wall flex...however....you NEED to keep that flex as close to the center line of the bike or you will have problems.....and you need that "sweet spot" found sooner than later...As the tire wears in the side wall becomes softer..I picked up a piece of wire through the side wall last year..and believe me, IF it wasn't for the 30+ years I've been riding and knowing the feel of a rear tire going south..I would not be writing this .....when CT tires go flat......they ARE flat..not like bike tires where there is some rigidity to the side wall....
 

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Discussion Starter #9
Yes you DO need side wall flex...however....you NEED to keep that flex as close to the center line of the bike or you will have problems.....and you need that "sweet spot" found sooner than later...As the tire wears in the side wall becomes softer..I picked up a piece of wire through the side wall last year..and believe me, IF it wasn't for the 30+ years I've been riding and knowing the feel of a rear tire going south..I would not be writing this .....when CT tires go flat......they ARE flat..not like bike tires where there is some rigidity to the side wall....
Thanks, Wazard, Tallanie, I appreciate the input.

Not to be obtuse, but since I don't want to find out the hard way I made a mistake, please allow me to be specific.

On my 08 900 Custom, with a 90/90-21 front (Avon Roadrider), what size would be best/safest if I went darkside, and which size would be best if I stayed MC.

FYI, I currently run 40psi front and rear on my stock Dunlops and plan on keeping it in a similar range unless the feel is very different. Thanks again!
 

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safest

MY preference....A general...HP...maybe 170/70?? Inflate at maybe 42 and work down to where it feels "off" at speed and/or turning...and bump the pressure up 10% or more...The whole exercise is to feel comfortable...There will be more than enough "contact" rubber on the pavement....Look at a M/C tire and see how much contact area you have....It ain't much....You'll notice as a M/C tire wears down it gives a wider tread patch..and it will give a different feel than a new tire.....that's why everybody says how great "new" tires are...The main reason for a car tire is the mileage from it...Highway riding or touring will provide almost three times the amount you will get from the best touring M/C tire....on the other hand....you compromise handling to an extent in tight turns. It'll want to track out of the turn a bit, so you ride a bit slower into and out of a turn..THAT is hotly contested...by some..Doesn't matter what M/C tire you use up front...that's you choice...I've always used Dunlop 491s inflated to Dunlop's specs...It's a good tire and gives great mileage and will probably match the rear tire pretty good...Your MAIN concern is getting a damned flat on that C/T.......If you want to experience "squirrely" ...well bud that's it in spades....Just to compare....run your M/C tire at 5 psi and see how it feels.....Then when you switch...run your new C/T at 5 psi.....get back to me and let me know the results.....I already KNOW ..and so should you...Best of luck with that endeavour.....And as a heads up...Mounting that tire is gonna be a bitch..use LOTS of dish soap...clean the rim like your mother's floors....and DON'T over inflate mounting it..I put about 92 psi in mine and left it for about an hour....You may even have to polish the rim to get it right the first time..

Anyway that's my LONG two cents...enjoy
 

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Discussion Starter #13
Right. Still not sure I'm going to go darkside. What I hadn't realized is the difference in height between a 90/90 in front and a "direct" size match for a car tire on the rear, which I calculated based on the original size MT.

So the question is - for those who have a 90/90 on the front, what MC tire size matches the front to avoid any unfortunate handling issues? Is it a 200/70 or something else?
 

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i run a metz 90/90-21 up front (no mods required) and a metz 180/70-15 out back....
 

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Well a 200/70 will fit IF you use a stock pulley If you have an SW pulley you might have clearance issues....I've hear of 230s installed....but I'd think that there may be issues..???? Belt rubbing and/or tire rubbing the inner fender??

At any rate..darkside is fine IF you realize the limitations and ride accordingly..


http://www.motorcyclephysics.com/...could give you some insight on the wheel placement and ride considerations.....But overall using a C/T is a good experience....It's just a bit of work and patience...
 

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I think the slight difference you will now have with what you bought is going to be insignificant. By the time you have 100 miles on it you wont notice the difference anymore.

Stop worrying now and just mount up what you bought and ride !

As far as air pressure on the rear,start a bit low and take some careful test rides and see if you notice a bit of rear end wobbliness... if so then you need a bit more air....go up in small increments of a couple pounds.

I started at 34# on my goodyear tripletred and noticed a bit of a goosey steering feel.... by the time I got to 40# it was completely gone and it felt like it was on rails thru corners, etc.....but then I took a video and saw that I had almost no tread on the road in higher speed corners...so I let off some air.....back to 37#where I keep it now. It rides fine at that as well.

This is something all darksiders do to find the sweet spot for them...each bike even with the same tire can be different...

good luck and enjoy the mileage you are gonna get with the CT on the back !

Annie
 

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The recommended size is 195/65-15 correct? Also, my reading leads me to believe that the General Altimax HP is the favored tire, correct?

I'm going to make the purchase as soon as I verify I have a shop that will mount it.
 

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Discussion Starter #18

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Discussion Starter #19 (Edited)
OK. One more question. Just to be sure - are tire sizes consistent between CTs and MT? In other words are the dimensions (diameter, width, circumference) of a 185/70-15 CT the same as a 185/70-15 MT?
 

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I think that car tires on motorcycles are only if you ride on straight line or very laaarge curves, in shot curves you loose grip because the radial deformation on car tires is calculated with a lower leaning of tire/wheel against pavement (vertical axe), in the motorcycles the tire deformation is calculated taking care about vertical axe inclination. The side walls of a tire car are gripless

Look what happens in this video

A kinder like graphic :)
 

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