Tread a mile in your shoes?
In an appropriate conclusion considering the number of wheels on most cars, Gareth Banks brings his four-part look at tyres to a close with their relation to the MOT
Published: 02 June, 2020
We have arrived at the fourth final part on the subject of tyres. This month, the focus is on the MOT, regarding tyres and tread depths. I’m hoping the manual hasn’t changed by the time this is published, (the MOT manual is, after all, changing all the time). As I write this, it states that the legal limit for a passenger car with no more than eight seats excluding the drivers seat used on or after 3 January 1933 is 1.6mm around the whole circumference of the tyre across the central three-quarters of the tread area.
Therefore, if a car was to come to me for a MOT with one or more of its tyres measuring 1.6mm, I would be forced to pass the car with just advisories on the tyres in question. This is a shame as you wouldn’t want to send a car out of your depot with a tyre on the legal limit and 100% worn, would you? The owner could quite easily travel 100 miles or so in the next few hours and their tyres may very well be then deemed illegal If stopped by the police.
“But officer! My car passed its MOT earlier today" may well be their perfidious reply. I suppose all that matters is how the car was presented at the time of test. From a tester’s point of view though, you cannot try and pre-empt what may or may not happen after the test. The car maybe stored in a garage for months at a time for all we know. It may only do 200 miles a year. Regardless of what we think may happen, by following the guidelines and rules set out in the manual you can’t go wrong. As long as the customer in question is made fully aware of the situation and you’ve made the best judgement call you can, then you have done your part and can you sleep at night.
Measurements
Here are some measurements to get your head around regarding tread wear: 8mm-0% worn; 7mm-16% worn; 6mm-31% worn; 5mm-47% worn; 4mm-63% worn; 3mm- 78% worn; 2mm-94% worn. Obviously 1.6mm would be 100% worn. However, as I mentioned, it still passes an MOT as an advisory. A tyre on 2mm also passes but certainly with advisory notice. Me, I give an advisory notice for a 3mm tyre. To me when something is 78% worn, it’s almost had it. If you were to wear a pair of shoes that were 78% worn, I would think that somewhere along the line someone may advise you to replace them too, as they’d look pretty shabby.
The garage I work in receives tyre bookings all the time from cars that have been for an MOT elsewhere, a lot of the time, at a main dealer. For some reason they have advised tyres on 4-5mm tread readings. I’m not sure why, as a 4mm tyre still has some life left in it, and a 5mm tyre certainly shouldn’t be classed as worn. Maybe the testers are overzealous. They should take their tread depth gauge and make sure its calibrated. There was me thinking I was being cautious advising on 3mm! I like to think I’m firm but fair when it comes to testing. I’d like to think others are too. Lastly, don’t forget, the next time you’re out walking in bad conditions and keep falling over, check the tread depth on your shoes, it maybe time to replace them.
- The shoe’s on the other foot
After my recent articles on tyres and TPMS systems, this month’s topic is wheel alignment, and the massive impact excessive toe-in or toe-out could have come MOT time.
A lot of us can spot alignment issues a mile off. I would worry if after 17 years in the trade, that a technician such as myself couldn’t tell the difference between a tyre that had been toeing-in and a tyre that had been toeing-out most of its life. If you look at a front tyre on a car and only the outer edge is bald (and the tyre has plenty of pressure in) then you walk around to the other side and discover the same issue with that tyre, it would be plain enough to see that the wheel geometry isn’t correct.
Compensate
A car with tyres like this and wheel alignment so far out must handle like it has a mind of its own - like Herbie the Beetle. However, people get used to a car in this state and will compensate to a surprising extent. When we take the car for a test drive, we may come back to the customer and say things like; “there maybe an issue with the tracking” or “the car is pulling rather a lot to one side.”
The customer then looks at us in amazement as if we are saying things just get their hard-earned money from them. We then offer a FREE wheel alignment check, and show them the virtual view on the screen. By Jove, the customer can then see with their own eyes exactly why their tyres have worn the way they have.
I first mentioned toe in and toe out. This is the way the wheels ‘point’ in relation to the forward motion of the car. Look down at your feet. Now keep your heels in position, and point your toes inwards so they point towards each other. If you try walking like this you will wear off the outside of your shoes first.
Do the same again with your heels, but point your toes outwards. This is toe-out and walking with excessive toe-out will wear the inner part of your nice new shoes and that’s the last thing you want. I’m sure you’d like it if your shoes wear evenly. The majority of track cars/race cars will be running with toe-out and negative camber. This will be topic of conversation next time as wheel geometry is a large subject.
Minutes and degrees
I also mentioned minutes and degrees earlier too. A while ago I tried explaining this to another tech and he struggled to grasp the concept of the theory behind alignment/ geometry, but he knew how to set the wheel alignment up on 99% of cars.
Getting your head around the actual maths is a different story. Think of it as a clock; there are 60 seconds in one minute; there are 60 minutes in one degree; and there are 360 degrees in a full turn. Our alignment set-up only works in minutes and degrees as it is really sensitive so no need for seconds-it would be worthless. The wind could blow and move the car slightly and that’s enough to make you panic and pick up your spanners again.
Once the alignment is set-up correctly and you’re happy with the end result, your customer will in turn be very happy. Remember, in the end they are the ones paying our wages, not just your boss. It does help if you have a happy boss too of course.
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