Section : Wheels and Tyres

Tyre apect ratio

Posted By : Geoffrey Griffiths, 15-Aug-2005, 02:54pm

Can anyone give definitive advice:

I have 13" X 4 1/2 J wire wheels for my Mk1 GT6 all as standard. When the car was rebuilt and new tyres fitted it took me 18 months to realise that the tyre designation for those fitted was 155 80 R 13. I presum that this "80" refers to the aspect ratio of the new tyre and will give the height of the tyre above the rim (which I can measure)
Doing the maths :
Width =155mm
Aspect ratio =80
tyre depth = 155 X 80/100 = 124mm

Measured depth = 110mm (tyres are nearly new)
Difference = 14mm

What am I doing wrong?
Also did early 1970's radial tyres, without the aspect ratio declared, actually have a prefered ratio of width to height?

Geoff

Followups :

Tyre apect ratio

Posted By : Colin Wake, 15-Aug-2005, 03:49pm

Non-identified tyres are 80 aspect ratio.

Have you got the tyre off the rim to measure it, or are you guessing how much is inside the rim.

Cheers

Colin
- Tyre apect ratio
  
  Posted By : Vin Poonwassie, 15-Aug-2005, 03:55pm
  
  14mm could be taken up by the unworn tread and beading hidden by the metal wheel? - you may also find that different tyre manufactures use different rulers!
  - Tyre apect ratio
    
    Posted By : Kevin Rochfort, 15-Aug-2005, 04:17pm
    
    From the Yokohama Website
    
    Aspect Ratio
    Often referred to as the profile or series, the aspect ratio of a tyre is determined by dividing a tyre's section height by its section width when the tyre is inflated to its maximum recommended air pressure, mounted on the approved measuring rim, and under no loading.
    http://www.yokohama.co.nz/understand.htm
    
    Also, from http://www.tyres-online.co.uk/techinfo/aspect_rati...
    
    The beading is not included in measuring the section height
    However, what is not clear is exactly what the "Section Height" is - does it include the actual tread ?
    
    Originally, tyres that didn't quote an aspect ratio were 82, not 80, but this seems to have been lost in time, and would actually make the error worse.
    
    BTW, what size tyres are you running on the Banana now ?
    
    Kevin
    - Tyre apect ratio
      
      Posted By : Vin Poonwassie, 15-Aug-2005, 04:26pm
      
      Sorry off topic, but the banana is having it's latest wheels put on the day before I take it down to the Herts and Beds meet - so they're not on yet - but when they are I'll post a picture - you'll see them Monday Kevin...... (ps. wear your sunglasses)