Tyres – Part three: Keeping a low profile

Gareth is back with his ongoing look at the challenges and opportunities represented by tyres. This month; tread

Published:  12 March, 2020

By Gareth Banks

In this third tyres instalment, I’m going to write about J sizes and fitting the correct tyre to the compatible wheel, as this caused a little headache for a friend of mine.
Recently he was complaining that his tyres kept losing pressure over the space of a few weeks. A few days passed and when he arrived I took one look at the stretched look tyres on his car, then rolled my eyes. I felt every small stone as I drove over it along the car park and into the workshop. He said all of the tyres were losing pressure, but the two on the nearside were the worst. After taking the nearside front wheel off and examining it a little closer, I could see that this one was leaking between the wheel and tyre, but why?
I took the rear one off the car too for a better inspection, as you can’t check for a rim leak correctly while the wheel is in an upright position. I found that this one too was leaking from around the wheel rim area but also from the inner sidewall.  I explained that these tyres were s-t-r-e-t-c-h-e-d, and they were indeed! The tyre size was a 235/30/20 and the wheels had a 20x9J marking on the inside. The 20 tells you the wheel overall size in inches and the 9J essentially means J- passenger car type bead profile, and 9 meaning 9 inches from inner bead to outer bead -how wide the wheel is on the inside if you like. Yes, the tyres fit on the bead, but once inflated the tyre bead and sidewall of the tyre popped out into their rightful place to withstand the pressure. Obviously, the tread area cannot move in any way therefore giving the stretched look.
To cut a long story short I’d found that it was the front tyre bead that was leaking. It had been damaged when it was fitted and didn’t form a good enough seal when inflated. The tyre that was leaking from the sidewall had actually been driven ever so slightly low on pressure. This tyre had been taking the full brunt of any sort of impacts. As a result, my mate had unknowingly been slowly killing his tyres because of the aftermarket wheel/tyre combination he had chosen or indeed been sold. Even to the trained eye you wouldn’t tell that one of these tyres were low on pressure. After discovering that the tyres and wheels were all damaged due to the nice looking but terrible combination. Standard wheel and tyres have now been re-fitted.

The car may not have the same look as it did before, however it drives a heck of a lot better. The wheels aren’t getting battered on the tiniest of potholes and the tyres aren’t taking a hammering now. The moral of this story is that yes bigger wheels and smaller profile tyres do look good but aren’t very practical in our country with the state of our roads. Bigger isn’t always better. It is a given that these types of wheel and tyre combinations must be properly inspected if they seem to be losing pressure, as the alloy wheel could possibly even be cracked as this is the reason that the two other tyres kept deflating. It is not uncommon to see these types of wheels with a hairline crack on the inner side so please do be careful and inspect the wheel thoroughly before tyres are taken off as you don’t want any nasty surprises, or headaches for that matter.

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