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-   -   Incorrect size tire OK? (http://www.horizonsunlimited.com/hubb/bmw-tech/incorrect-size-tire-ok-60807)

goo 19 Dec 2011 18:04

Incorrect size tire OK?
 
I am in Mexico and trying to get a new rear tyre for my F650GS twin. The riders manual says it should be a 140/80 but the dealer I am talking to in Mex City only has either a 130/80 or a 150/80.

Would it be a problem to use one of these sizes or is that a total no-no?

It would be a TKC80 or a Metzler Tourrance and I am leaning towards the TKC80 as I read stories of mud and flooding in Guatemala which is where I am headed next. It´s also what I have fitted at the moment.

Thanks!

garmei 19 Dec 2011 18:16

In a pinch; yes.
The smaller profile will tip into turns better, but the 150 will be a bit more stable and will turn slower.

John Downs 19 Dec 2011 19:06

Feliz Navidad Senor Goo,

Either tire will work fine on your F650. Tire sizes can be a tad confusing. At least for me until I understood the numbers meaning. Thus, 140/80-17 is shorthand for a tire that is 140mm wide, with a height 80 per cent of 140 (132mm) and made for a 17 inch rim.

Clear as mud? Well, fortunately tires are flexible and a slightly wider and taller 150/80 will fit your rim. It might rub slightly on the monoshock or swingarm until it wears in. Especially if you order the blockier TKCs as opposed to the less aggressive tread Tourance. The 130/80 is slightly narrower and lower profile than what your bike calls for.
If you are ordering sight unseen I would get the 130/80 in TKC so you don't have to sand down the outer block tread slightly with a belt sander. BTDT.

The thing is, different tire manufacturers sizing can vary even with the same numbers. I remember a time in Baja when I was riding a rear tire with the cords showing and I was desperate for a tire. I needed a 120/90-18 rear desperately and found a guy out in the middle of the desert who had some 130/80-18 full knobs stashed for the Baja 1000. He sold me one for a hundred bucks and I was ectstatic. The thing is it was an aggressive Dunlop knobby and I was riding a BMW street bike. But I had to make it 1500 miles back to work by monday and it was Saturday afternoon. So I didn't mind the smell of burning rubber as the outer blocks wore down on my shock, or the zzzzzz, zzzzzzz, of the tire bottoming on the rear fender over every large bump in the road for the first few hundred miles.

Others who actually own an F650 and have come across your situation will hopefully chime in with better answers, but I would go for the 130/80 unless someone says different.

Kindest regards,
John Downs

Titbird 19 Dec 2011 20:50

This is a usefull site:Rim to Tyre Application Chart

goo 20 Dec 2011 01:29

Thanks for those replies. I think I will go with the 150/80 as there is a minimum of 20mm clearance with my current TKC80 and a 150/80 equates to an extra 5mm width-wise and 4mm heightwise. Call me boring but I also like the idea of 'more stable' ;)

Also that table kindly provided by Titbird seems to suggest that 150's and 140's are both in range for my 3.5 inch rims. Oddly the table doesn't include a 150/80-17 but I get the gist.

I suppose a slightly bigger tire will reduce torque a little. Prob not enough to worry about I am guessing...

Thanks!

The Raven 20 Dec 2011 16:38

Simple answer in summary of the other responses. BMW tire size is the recommended size, should you find another size that fits without clearance issues and meets your requirements(speed/load) in a tire it WILL work. I have done this on my g/s and KLR with success.

You could ask the same question about oil. The manual on my KLR says 10w40, but I run 5w-30 in the winter and 20w-50 in the summer with no issues. It's the same idea.


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