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@crazycarl avatar
UTC

Ossessionato
2007 250 GTS, 1980 P200E, 2010 ThunderFly 190 (SOLD) 2015 Yamaha SMax (SOLD)
Joined: UTC
Posts: 3491
Location: Springboro, OH
 
Ossessionato
@crazycarl avatar
2007 250 GTS, 1980 P200E, 2010 ThunderFly 190 (SOLD) 2015 Yamaha SMax (SOLD)
Joined: UTC
Posts: 3491
Location: Springboro, OH
UTC quote
So I've just bought 2 new Pirelli Scooter Angels for my GTS. The front is std size 120/70-12, but the rear I bought is 140/70-12 and I'm trying to derive the circumference of the std 130/70-12 to determine the ratio change (i'm just being overly curious)

I took a cloth tape and measured the circumference of each. 120/70-12 came out as 1415 mm and the 140/70-12 is 1535 mm. If I take the two and average them to split the difference, that comes to 1475 mm for the 130/70-12. With those assumptions, I can tell you that the circumference of the 140 is 4.06% greater than the 130.

But I'm fairly certain it's not that simple. Mathemetically, you can take the diameter of a circle and multiply by 3.14 to get the circumference. But I don't have a way to measure the 130/70-12 tire

Ultimately, I'm trying to determine improvement in mileage and top speed by moving from the 130 to the 140 in the rear.

I'm certain there is someone who can help me out here.

Thanks, Carl
@wheelman-111 avatar
UTC

Addicted
2010 S50 Modified "Punkin"; 2003 ET4 Malossi 187 "ISO"
Joined: UTC
Posts: 778
Location: Deepinnaharta, Texas
 
Addicted
@wheelman-111 avatar
2010 S50 Modified "Punkin"; 2003 ET4 Malossi 187 "ISO"
Joined: UTC
Posts: 778
Location: Deepinnaharta, Texas
UTC quote
Diamter
Greeetings:

On a 130/70, the height of the tire is 70% of 130mm. Taken twice, plus the 12" diamter of the rim, should approximate the diameter. You know the rest. Can you not measure the 130 directly, or is it already dismounted?
@jimc avatar
UTC

Moderaptor
The Hornet (GT200, aka Love Bug) and 'Dimples' - a GTS 300
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Posts: 44238
Location: Pleasant Hill, CA
 
Moderaptor
@jimc avatar
The Hornet (GT200, aka Love Bug) and 'Dimples' - a GTS 300
Joined: UTC
Posts: 44238
Location: Pleasant Hill, CA
UTC quote
The circumference of a 3/4 worn Michelin City Grip 130/170-12 is 1502mm - just measured my working spare.
@fudmucker avatar
UTC

Ossessionato
2021 GTS 300 HPE +2013 GTS 294 Polini
Joined: UTC
Posts: 2104
Location: Pretoria, South Africa
 
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@fudmucker avatar
2021 GTS 300 HPE +2013 GTS 294 Polini
Joined: UTC
Posts: 2104
Location: Pretoria, South Africa
UTC quote
The only way to accurately determine circumference is to measure it directly when inflated.

Different tire manufacturers make different tire sizes for the same "standard".
This is particularly true on the tire WIDTH. On my 1981 BMW R80G/S I sometimes have to cut a little off the RH edge of a more knobbly tire to get clearance and prevent rubbing against the swing arm housing.
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