Evening fellas

[Zx7rrducati]

Hugh, hear-say... . That 170 is flat as a pancake on that rim.. 180/55 w a steep profile I believe would turn in much better.. The slight increase in width I would think is negated by the benefits of the profile. Anybody else?

[Duc750]

A 170 is flat eh? Well what dya think a wider flatter profile is going to look like?

I have to admit to not having tried it on the 907 because I've tried on other similar bikes of the same age such as a VFR750 and a cbr1000f with similar purpose and build to the 907 and have found that it slows the handling significantly on them so wouldn't bother on the 907 but its up to you! At the end of the day my Butt dyno supports the physics on this one for me

[Zx7rrducati]

Ok, what I'm saying is that a steeper profile 180 ( remember 180/55) depending on manufactured, they differ in widths for a given size; will out perform the 170 I'm lead to believe with a slight raise in rear ride height, steeper turn in angle, and effectively lessening the rake. Am I that far off?

[Duc750]

Only you can decide whether you're far off or not but here's my logic in detail

Units for this are relatively un importnat for the maths of this so for the purposes of this I'm going to measure in carrots as it avoids any transatlantic metric imperial clashes
a 180/55 tire has a 180 carrot width and a 55% aspect ration to achive its height whihc is 55% of the width
so its height using the aspect ration is 0.55 * 180 = 99 carrots

For the 170/60 tire its height using the aspect ratio is 0.6 *170 = 102 carrots

so the 170 tire is taller by 3 carrots

now the problem is that as far as I know there is no standard wheel definition in automotive (there is in aero) and things like rim channel width and depth can and do have an effect on tire profile but benchmarking two tyres of the same model and brand the 170 will always be taller.

now for my slightly more subjective logic: If I fit a wider tyre then I will increase my contact patch but if my height remains the same I will reduce its radius - i will put more tire on the ground because my lateral measurement of my contact patch will get fractionally bigger...... but i now have a more gradual radius and more tire on the ground so it will require more force to turn. I've also added more rubber and there fore more mass to the rear wheel increasing its propensity to stay in one direction when rotating - I've also added more unsprung weight so given my suspension a harder time.

What you are talking about is fitting a wider and lower tire so you would increase your contact patch and decrease your radius and although the differences are small.

Now take those small differences and start to look at weight distirbution. again a small change but you've lowered your rear ride height and therefore fractionally shifted your weight distribution back on the bike.
Now you can adjuct this potentially by adjusting your rear ride height but keep in mind we have no ride height adjustment on these bikes only pre load.
(Pre load should be used to set sag and that is all, any adjustment after you have that right for the bikes load is just a measure of your capability to screw up your bikes handling - as described to me by an ex formula 1 suspension engineer)

You've also made a smaller change to rake and trail - essentially you have kicked out the forks slightly - not enough to fit ape hangers yet but slightly none the less.

So to my mind fitting a wider tyre means you will require more force to turn the bike, you will have less weight on the front of the bike and you will have made you're steering more "sloppy"
all of which are fractional but combined are quite effective.

WHich leads me back to my view on tyres for this bike - go for the highest profile recommended fit tyres you can get and then ride the hell out of it - I like my bikes to turn quickly and controllably - quickly is a relative turn on a sports tourer as you don't want it to bite you in the butt. From a personal perspective I think Bridgestone are the work of the devil and object strongly to the fact they don't make a 170 rear. But then I've never liked them on any bike.
I still can't get over how horrible Michelin Pilot Roads were when I had them (Pilot Road 1s) and have always found Pirelli/Metzeler, Conti Road attacks and Avon Azaro/Storm tyres work well for me although not always on the same bike.

So thats my reasoning - more than happy to be told I'm wrong but I struggle to find a positive argument for the 180 rear

[Zx7rrducati]

Hmmm logic prevails?
http://www.youtube.com/watch?v=U_eZmEiy ... ata_player

What you are talking about is fitting a wider and lower tire so you would increase your contact patch and decrease your radius and although the differences are small.

What im talking about is fittin a slightly wider 180/55 which will compress slightly to fit said rim and with its increased overall diameter 180/55/17= 24.8x7.1x17 vs 170/55/17=24.4x6.7x17 combine with its now steeper angle and taller sidewall which will in turn, DEcrease rake (steeper), turn in faster, give better mid corner stability, and if your lucky a partridge in a pair tree... Credit be due what your basing your banter on is in fact .3mm
But then again I'm basing mine on a short section of road that's currently under construction..

[Duc750]

Agreed, benchmarking those two tyres you are indeed correct but where the F*** are you finding the 170/55 tyre? 170/60 is the recommended fit and I can't find a single tyre manufacturer making a 170/55!

Also sidewall height is not a truly regulated measure so that will vary between manufacturers.
Only overall height is measured

[Zx7rrducati]

Oh god... I stand corrected. I'm a humble man... I have no idea why I was thinking 55 cross section when I know its a 60.. Dugummit...

[Duc750]

You got me thinking as you can get hold of 170/55 slicks to be cut for supermoto racing so I wondered if one of the manufacturers had used the carcass for a road tyre

[ducinthebay]

FWITW. I've been running a 170/60 and 180/55 on both my ST2 and my Sport for years. Running Pirelli Diablos, the 170 does feel lighter and quicker to turn in than a 180. I try to run the 170, but sometimes you just can't get them in time and have to resort to the 180 from time to time.

I'm bucking the trend and changing from a 5.5" rim to a 4.5" rim and going to a 160/60 next.

Cheers,
Phil