Tyre Size quandaries

[ST.George]

So I've had the Dragon since Oct'12 and noticed after a ride with Western Australian Glen (WAG) last week (WAG was progressing on a "Right foot in the ocean" tour of the east coast and kindly paid a social visit on the way thru the Gong (ride report next week), anyway, and noticed that my rear tyre was shot and it happened in a very short time.

So am now researching options for the new rear tyre. WAG mentioned that some people are trying out car tyres because of their relative cheapness and longevity. This is hugely contentious and not what I'm on about here. Although I have seen some pretty interesting videos.

The current size of rear tyre I have is 170/60 17 Metzler P6 which is regarded well in the forum.

Thinking tho, that a larger diameter tyre will decrease engine revs required to achieve same speed and will also extend tyre wear. So I looked into other tyre sizes and found that if the width is increased to say 180 the the aspect ratio is reduced to say 150 meaning that the overall tyre circumference is close to the same. So the net gain of a larger tyre is in theory very little and not worth the extra expense.

I met this privateer mc mechanic "Mig" tho and he threw in a different viewpoint. Mig said the newer ST's have a 180 width tyre fitted on a wider rim. However on my ST with a narrower rim some riders are fitting 180 width tyres and the narrower rim is forcing the 180 tyre to elongate into a more elliptical curve and thereby achieving a tyre with a larger diameter that does have the effect of reducing revs and tyre wear.

Has anyone done this and did it make a difference? Mig did warn tho that there must be enough room for the 180 tyre in the wheel space of my ST. However if the tyre is elongating then that should be no problem.

[Brock]

Increasing the diameter will cause the speedo and odometer to under read as well, meanining an indicated 100Ks may mean an actual of 105Ks...

[alans1100]

I think the 170/60 tyre/wheel size has remained as standard on all 1300 models. Don't think any one on here as a wider rear wheel to suit a 180 tyre.

As Brock says you will have error in speed and distance as the 1300 speed sensor is off the rear wheel.

[ST.George]

All my reading so far has indicated that speed is usually measured from the front wheel, but if both Alan and Brock say it is from a sensor on the back wheel of ST's then the speedo reading would be wrong and I will waste no further time considering a larger tyre. Thanx guys.

[Biggles]

The speedo sensor is related to the gearbox, not specifically the rear wheel.  But a change in the circumference of the rear wheel will affect the indicated speed.

It's only the ST1100s that use the front wheel, and then it's by mechanical cable (don't forget to lube it, guys!)

[Tipsy]

  py
I have the 180 on my 1100 and it does NOT affect the speedo reading as mentioned above it is a mechanical drv.
It is fitted to the standard rim and I use PR3's only, sticks like the proverbial ? to the blanket,
whether or not it affects the actual speed I have no idea so long as Ronda goes I am happy.

Tipsy

[BigTed]

...then the speedo reading would be wrong and I will waste no further time considering a larger tyre....

Don't be too hasty... given almost all speedos over-read (ie. actual speed is less than that displayed) then upsizing your rear tyre (1300) may make it more accurate depending on the original error and the profile of the new tyre vs the old. Mine is currently 6% out according to my GPS.

Some simple maths will tell you how accurate it will be (over or under).

[Brock]

The Odometer is accurate, the speedo is designed to over read... Unless of course the bike is white then the speedo is accurate

  

[winston66]

Winston 66,
I have a Kumho 205-50 ZR17 93WXL fitted to my 2003 1300.
A cross reference check with some manufacturers specifications suggested to me that there was bugger all difference in the rolling diameter when compared to the recommended manufacturers tyre size and fitment.
 In use the speedo reads correctly enough for me and at a total cost with delivery of under $120-00 I am very happy.
The reason that I took this supposedly extreme step was mainly financial as apposed to an expenditure of in excess of $270-00 for the Mc tyre and I was intending to pull a trailer around OZ last year but unfortunately my health let me down at the last minute and I had to cancel my plans.
As far as I am concerned this Kumho works extremely well for me and I will be staying on the dark side.
Cheers to all,
Winston66  Northampton WA.

[gaz]

 

[ST.George]

Winston,

That Kumho tyre size 205 50 17 works out in theory to be very close to the diameter of a standard 170 50 17. Having a much narrower rim on your ST than a car wheel, have u noticed any of the effect I mentioned where the tyre is more curved on a narrower rim.
What psi do you inflate it to?
I looked the tyre up and saw that it has much more sidewall tread than most and has more curvature than most. I take it that this tyre is a dark side fave in the US.

[winston66]

Winston 66,
Hi St. George,
Will try to attach a picture of the tyre in question, and yes it appears to be quite a favourite brand (style) and in common use. I am a member of Venture Riders .org and I have a 1996 Yamaha Royal Star XVZ1300 as well and those guys have had a lot of comments on their Darksiders blogs this is where I have garnered a lot oif usefull stuff. The ST. rim did not seem to distort the tyre profile much at all. It was an easy fit to the rim I did it with just some hand tools but had to deflate the tyre when fitting the wheel back into the bike.
I noticed that initially the right hand side of the Kumho just touched the inside of the inner guard but it was so minor there has not been any apparent marking or wear shown on the sidewall I am quite happy with that, and it is quite possible that when underway the tyre does assume a morer oval shape and this would then show some more clearance from the inner wheel guard.
 I have tried various inflation pressures and seem to have finally settled on 35 lbs for normal use one up ( I don't have a passenger to worry about) this seems ok for when towing as well.
 hope this is of interest.
Cheers,
Winston 66

[Biggles]

It's amazing how much more tread depth they seem to be able to achieve on car tyres than bike tyres!
Must be a technological issue.  Or they just pick us for idiots and take advantage.   

I expect 80,000 kays from a car tyre and think it's wonderful when I manage more than 20,000 on a bike tyre.  Another mill or two of tread depth might see me break the 30,000 kays barrier.

[ST.George]

Quote from  Biggles

It's amazing how much more tread depth they seem to be able to achieve on car tyres than bike tyres!
Must be a technological issue.  Or they just pick us for idiots and take advantage.   

I expect 80,000 kays from a car tyre and think it's wonderful

Yes they do "they just pick us for idiots and take advantage."

Whoopeeeeeeee! I'm a FULL MEMBER!  That's 100!  100!! Bloody 100!!!     Think I'll go have a beer and then the member will be truly full!

[Malcolm6112]

The Odometer is accurate, the speedo is designed to over read... Unless of course the bike is white then the speedo is accurate

  

On my 1300, 110 on the clock is 100 real speed. Checked on two GPS devices.
120 on the clock is 110. That's running PR3's, with std size.

 

[basadia]

If you go to big in case size you might end up with the tyre rubbing on the swing arm. I had this issue on my ST1100, I thought after enough K's the rubbing would stop but in my case it did not. It was still rubbing when I replaced it.
I cannot remember the size but it was a Bridgestone and it had a rubber ridge where the sidewall meets the tyre face. This is what rubbed on the swing arm.

[ST.George]

If you go to big in case size you might end up with the tyre rubbing on the swing arm. I had this issue on my ST1100, I thought after enough K's the rubbing would stop but in my case it did not. It was still rubbing when I replaced it.
I cannot remember the size but it was a Bridgestone and it had a rubber ridge where the sidewall meets the tyre face. This is what rubbed on the swing arm.

There appears 2 b adequate clearance on the ST1300, haven't noticed any sign of friction.


BTW Brock, I just noticed I put this thread in the 1100 category rather than the 1300. This is possibly causing misunderstandings so if it is easy can u move it to the 1300 section? If not doesn't matter that much.

[Brock]

I will see what I can do, but in the end it doesnt matter much, as tyres are used on both models

[saaz]

If you want less revs per kph, consider going for a 160/70 rear.  I have used 170/60s on the ST1100 and they give more revs per kph than the standard 160/70.