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An
owner of a Mitsubishi Lancer car came to our workshop complaining that
his tyres, which had covered approximately 18,000km after about 8 months
of service, were defective. His complaint was that though the tyres were
tubeless, he had to put in tubes as they suffered numerous punctures,
wobbled due to sidewall bulges and the ride quality was atrocious.
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| Directional
Tyres Sizes 205/50-ZR-15 Tyres fitted in lieu of Original Size
175/80-R-13. The tyre was improperly fitted adding to the misery
of the customer. |
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On
inspection of the tyres, we found that actually there were only two tyres
with a 'bulge' due to cuts on the sidewall, and leaked air since the tyres
were tubeless. It had been repaired using a tube patch and was also fitted
them with tubes. The other two tyres were in a fairly good condition though
they showed uneven wear. Why did these tyres get damaged or show uneven
wear at such an early stage?
It was due to the wrong profile chosen by the owner. The Lancer comes
with the size 175/80-R-13 as originals, while the customer put in oversize
tyres of 205/50-R-15. Though the OD of the tyres remained nearly the same
(175s- 619mm, 205s-620mm) the profile chosen and tuned by the car manufacturers
is 80 per cent dropping down to max 65 per cent. Therefore the suspension
has been tuned accordingly. Going down to the 50 per cent aspect ratio
changed the general geometry of the alignment angles, producing a harsh
ride for the driver. Secondly the tyres being a bit too wide, every road
divider or edge of a pothole creates a vulnerable area at the shoulder
of the tyres to take an impact. Since tyre manufacturers reinforce this
area, the point of damage is the sidewall, which takes the whole impact
and has a cut. Another factor is that no tube is made for lower than 60
aspect ratio, so even the tube supplied were wrong adding to the mess.
His problem was solved by repairing the cuts and reconverting the tyres
into tubeless, recorrection of the thrust angles and the alignment, increasing
the air pressure of the tyres, after explaining that such low-profiles
do give harsh ride, but reducing the pressure meant a greater problems
for tyres.
My sincere advise to those wishing to up-size your tyres is, seek advice
from either the car manufacturers or the tyre manufacturers or a dealer
who has an adequate knowledge regarding up sizing.
I wish all readers a Happy and a 'tyred-free' Year for 2002. |
February 2002