Author Topic: Tyre sizes imperial to metric  (Read 561 times)

Offline SteveW

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Tyre sizes imperial to metric
« on: September 15, 2021, 07:53:46 PM »
My 650 needs tyres. Currently 3.5 x 19 and 4.5 x 17 imperial.

I realize 3.5 is going to be approx 90 mm width and 4.5 is going to be 110 /120 mm.

When looking on the web for replacements they have the second number which is guess is the profile as a percentage of the width (the same is a car tyre)

My question is what profile percentage would be closest to the original imperial tyres, most of the ones seem to have listed 70,80 and 90.

Hope this makes sense.
1974 CB550 K0
1980 CB650
2000 CBR929RR Fireblade
1966 Lambretta LI150 Series 3
1981 RD350LC
1972 Raleigh Chopper
1974 Raleigh Tomahawk
2011 Henry Hoover

Offline McCabe-Thiele (Ted)

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Re: Tyre sizes imperial to metric
« Reply #1 on: September 15, 2021, 08:39:47 PM »
My 650 needs tyres. Currently 3.5 x 19 and 4.5 x 17 imperial.

I realize 3.5 is going to be approx 90 mm width and 4.5 is going to be 110 /120 mm.

When looking on the web for replacements they have the second number which is guess is the profile as a percentage of the width (the same is a car tyre)

My question is what profile percentage would be closest to the original imperial tyres, most of the ones seem to have listed 70,80 and 90.

Hope this makes sense.

It's my view that most old tyre sizes are 100% profile (think veteran cars) - you can of course fit a lower aspect ratio you then end up with an inaccurate speedometer due to the lower rolling radius and effectively lower final drive. Aspect ration is Height/Width as a percentage.

As a bonus you could convince yourself the bike is suddenly more economical.lol
« Last Edit: September 15, 2021, 10:55:16 PM by Macabethiele (Ted) »
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Offline Bryanj

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Re: Tyre sizes imperial to metric
« Reply #2 on: September 15, 2021, 10:18:42 PM »
There is a "standard" aspect ratio for the imperial tyres but cant remember what it is

Offline McCabe-Thiele (Ted)

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Re: Tyre sizes imperial to metric
« Reply #3 on: September 15, 2021, 10:51:05 PM »
There is a "standard" aspect ratio for the imperial tyres but cant remember what it is

I believe it's 90% so it's not marked unless it's below that but some show the 90 profile on scooters etc - on modern vehicles not counting baloon tyres for off roaders I think they generally start marking the aspect ratio from 80 and below
« Last Edit: September 15, 2021, 11:10:00 PM by Macabethiele (Ted) »
Honda CB500 K1 (new pit dug out ready)
Honda CB400 four super sport (first money pit)
Link to my full restoration http://www.sohc.co.uk/index.php/topic,23291.0.html

Offline sye

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Offline SteveW

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Re: Tyre sizes imperial to metric
« Reply #5 on: September 16, 2021, 07:59:48 PM »
Thanks chaps.
1974 CB550 K0
1980 CB650
2000 CBR929RR Fireblade
1966 Lambretta LI150 Series 3
1981 RD350LC
1972 Raleigh Chopper
1974 Raleigh Tomahawk
2011 Henry Hoover

 

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