Tyre Wear Indicators & Minimum Tread Depths

[Bazza79]

Hi,

Novice question here I'm afraid. Ok, so minimum legal tread depth is 1.6mm across 3/4 the width of the tyre.

My E39 M5 is close to needing a new set of tyres. I don't have a tread depth gauge, but the tread is down to within 1-2mm of the tread depth indicator bump. Are these tyres still legal, i.e. do they only need replacing once the tread is level with the bump?

The NSF tyre has heavy wear (pretty much bald) to the outer edge, is this legal?

Thanks in advance for any help!

[EDLT]

If the treads are level with the bump then the tread depth is 1.6mm (or less, because the bump will start wearing out). If there is 1-2mm of tread above the bump they your tread depths will be 2.6-3.6mm, which is legal.

You can get a tread depth gauge from halfords and just about every motorfactors for £5 or less.

EDIT: The outer edges just need visible tread, if you can't see the tread pattern any more then it is illegal

[BOBTEE]

1.6mm is the minimum tread depth required, not the best, or the safest. By the sounds of it, you're at around 3mm. Do you really want to run an M5 (or any car) on fooked tyres?

Time for new tyres from what you've said

[Bazza79]

EDLT thanks for clearing that up.

RE Outer edge - how do you define the outer edge? Is it the outer 1/4 of the tyre?

[Red 4]

Tread wear indicators are not all 1.6mm. Some tyres have them at 2mm. It should say on the tyre sidewall how big the indicator is.

Sorry to be pedantic but in answer to your question if the tread depth is nearing the indictor change the tyre. Simple and not rocket science.

I change my tyres at 3mm. Make sure you get some decent rubber if you are running an M5.

[Bazza79]

Guys, thanks for the feedback.

I know that they need replacing and that at 1.6mm I won't be getting optimum grip. Just wanted to see if they are legal?

Been looking at new tyres and it seems there is mixed opinions on what are the best performance/value balance. Currently it is running on Conti SC3. They seem to be available at a goo price at the mo, so may stick with them. Any opinions?

[Fox-]

Bazza79 said:

Guys, thanks for the feedback.

I know that they need replacing and that at 1.6mm I won't be getting optimum grip. Just wanted to see if they are legal?

Been looking at new tyres and it seems there is mixed opinions on what are the best performance/value balance. Currently it is running on Conti SC3. They seem to be available at a goo price at the mo, so may stick with them. Any opinions?

SC3 of Eagle F1 Assymetric 2. SC5 is out now but Continental are being ridiculous with the rollout and many sizes are simply not available. I can't get them in my sizes so I'm either going to stick with CS3's or move to Assymetric 2's.

[Rabbo]

Sport: On
DSC: Off

Wind the windows down, find a deserted roundabout and laugh your ass off!

[anonymous-user]

I have never understood the people who change their tyres at anything above 1.6mm??


The "legal limit" is just that, it has no bearing on the tyres ultimate performance!

In fact, in anything other than deep standing water the tyre will actually provide maximum performance at shallower tread depth!

If you regularly drive fast in standing water, then fair play, change your tyres more regularly, but if you regularly drive fast in standing water you probably need to change more than your tyres!


(There is also no statistical direct link between accident probability and tyre tread depth)

[EDLT]

Bazza79 said:

EDLT thanks for clearing that up.

RE Outer edge - how do you define the outer edge? Is it the outer 1/4 of the tyre?

The centre 3/4 has to be above 1.6mm, so the outer edge would only be 1/8 of the width of the tyre (and the inner edge would make up the other 1/8) if I remember GCSE maths correctly.

[Fox-]

Max_Torque said:

I have never understood the people who change their tyres at anything above 1.6mm??


The "legal limit" is just that, it has no bearing on the tyres ultimate performance!

What? It has everything do with the tyres ultimate performance!

Max_Torque said:

In fact, in anything other than deep standing water the tyre will actually provide maximum performance at shallower tread depth!

What absolute rubbish.

http://www.youtube.com/watch?v=0jHmZcYACtI

Watch that and stop preaching rubbish.

[timbob]

EDLT said:

EDIT: The outer edges just need visible tread, if you can't see the tread pattern any more then it is illegal

That's incorrect for a normal car - the requirement is just for 1.6mm in a continuous band around the central 3/4 of the tyre - the edges are permitted to be bald. It's only vehicles over 3.5 tonnes who need tread visible on the edges of the tyre.

[anonymous-user]

Fox- said:

Max_Torque said:

I have never understood the people who change their tyres at anything above 1.6mm??


The "legal limit" is just that, it has no bearing on the tyres ultimate performance!

What? It has everything do with the tyres ultimate performance!

Max_Torque said:

In fact, in anything other than deep standing water the tyre will actually provide maximum performance at shallower tread depth!

What absolute rubbish.

http://www.youtube.com/watch?v=0jHmZcYACtI

Watch that and stop preaching rubbish.

right, sorry, you must be right then........


If you actually read my post you might have noticed the bit I wrote about deep standing water. And funnily enough you then linked me to a test in, yup, deep standing water. if you repeat those tests on a dry road, then the results will also be reversed. So, if you plan to crash in the wet, you want lots of tread, if you plan to crash in the dry, you want none.

So unless you weather forcasting abilities are better than mine. in the UK i reckon you will have a 50:50 chance of getting an improvement in stopping distances.

Also, in purely "damp" conditions, the extra water clearing capabilites of the taller tread blocks is not required, so again the shallow tread tyres out perform the tall treaded ones (and why modern high performance tyres have increasingly large area, shallow depth thread forms)

You will note also that the linked video is sponsored by continental. now there is going to be a slight bias there too........

And finally, we have no legal requirement for the "quality" of your tyres. yes they must be E-marked, but the minimum co-efficient of friction is NOT actually a mandated number.

SO, you can quite legally fit a set of fully treaded "yafung death master skid plus" to you car, and no doubt drive around safe in the knowlege that you car can now outstop anything. But in reality the low silica content budget tyre is easily beaten by a bald premium tyre in anything but deep standing water (please note, if you drive, fast in deep standing water you DO need as much tread depth as possible (well actually you need deep tread with lateral water channelling capability. (a deep tread with poor water flow dynamics is just as bad as a shallow tread with poor water dynamics)))


However, of course none of this matters, as long as you drive you car within whatever limits it currently possesses.

You could put you car on solid concrete tyres, and as long as you leave a suitable stopping distance then you will be fine. The value of the absolute stopping distance is in fact irrelevant to the ordinary motorist, most of whom have never been trained to stop a car, and generally don't even know how to best affect an ABS stop for example. When training drivers on our wet handling facility, i generally could stop the test cars something like 25% shorter than our average trainee before they had been coached.


The science and physics behind tyre adheasion is complicated to say the least, but there are 2 direct methods which generate the majority of the lateral and longitudinal load capability of the tyre.

1) Static and dynamic co-efficient of friction between tread surface and road surface. Here the Mu of the interface is critical, The direct mechanical sliding friction generate the force build up. As the tyre slip co-efficient increases the friction moves from the static dominator towards the dynamic one

2) Tread block and edge deformation or effectively the mechanical interlock between the tread (particularly edge features) and road surface. It is best to visualise this as "velcro". it is where the tread form "molds" around the road surface irregularity, in effect forming a lock and key type interface.

Cruical to both these mechanisms is the normal tyre load (vertical force acting downwards through the tyre carcase), the tyres rubber compound ( Modern "high silica" compounds effect high static and dynamic Mu values), and the tyres temperature (effectively the tread softerns as it heats, resulting in grip type 2 increases as the rubbers drapability and conformability increase


With a "wet" tread depth braking tests, those massive elongation in the stopping distances (+44m in your linked video) are as a result of complete separation of the tread and road surface interface by water. Effectively the water pressure under the tyre, driven by the vehicles forward velocity "Lifts" the rubber clean off the road. This is results in a catastrophic reduction in friction, which, until the cars velocity reduces below the point where the waters dynamic pressure falls below that of the normal force, limits Mu by over 80%. This also explains the high increase in stopping distance, as there is a greater reduction in effective Mu at high speed, where the distance/time characteristic is most detrimental.

When your tyres wear towards the legal limit, the largest reduction in performance (outside the reduction in aquaplane resistance previously highlighted) is the effect of passive heating. Every time your tyre revolves, the hysteresis losses in the rubber add heat to the carcase. As the rubber compound warms, it's Mu and particuarily it's conformability increase. So first thing in cold morning, a shallow tread depth tyre WILL take longer to heat, and during this time there WILL be a reduction in maximum load capability. However, as the largest hysteresis is in the side wall and not the tread (on modern "sports" cars tyres, i.e. not trucks/suv's etc) this change in heating is a small effect. (and by the same token, your dampers and brakes also do not perform to there maximum ability whilst cold (nor does your engine for that matter ;-)

(btw, i spent 7 years as a senior principal dynamics engineer for a leading UK automotive consultancy. What do you do? ;-)

[Fox-]

It rains in the UK. The roads often look like they did in that test.

I do hope you didn't give advice as dumb as that in this thread to your clients when you were apparently an engineer. Nobody 'plans to crash'. The people on Friday night on the M5 didnt plan to crash either. You never know when you might need to stop, fast. 1.6mm tyres in the wet are a risk nobody should be willing to take.

[anonymous-user]

I simply recommend that my clients drive their car in a fashion appropriate to the prevailing driving conditions, and that is what we also train them to be able to determine.

(btw, you are clearly all for maximising you cars tyre grip, So can i ask if you fit soft compound lightly treaded tyres (say a R888 trackday tyre for example) everytime you drive your car in the dry? Because they can reduce dry braking distance by 30% ! )


Using the recent terrible M5 crash example, lets say that the people involved had tyres that could deliver a stopping distance reduction of say 20%. What do think the outcome would have been??

Edited by anonymous-user on Tuesday 8th November 21:00

[Garvin]

Max_Torque said:

Stuff

Brilliant, proper stuff no less! Now can you write another tome exploding the myths generally held why wider tyres give more grip than narrower tyres and explaining the real reasons for increased grip as every time I explain it no one believes me!

[RedexR]

I've just campaigned against a fleet policy of not replacing tyres until they are down to the 1.6mm WTI , this is the worst time of year to be running around on marginal rubber , I drive long distances on roads like the A303 , you cannot see deep puddles in the dark or other hazards that can occur from deer to vehicles emerging without due care. If you look at this guide the difference between 4mm and 1.6mm will cost you about 10metres in stopping distances and in view of last weeks Motorway incident I feel more than justified in demanding some new rubber.

http://www.rospa.com/roadsafety/info/tyre_tread_de...

[anonymous-user]

Garvin said:

Max_Torque said:

Stuff

Brilliant, proper stuff no less! Now can you write another tome exploding the myths generally held why wider tyres give more grip than narrower tyres and explaining the real reasons for increased grip as every time I explain it no one believes me!

Simple, tell them about the velcro!


The wider (and actually just as important the larger diameter, remember contact patch increases both with width and length) a tyre is, the more "velcro hooks" are present. (it covers a great surface area of road). With more hooks the ultimate load capable of being carried is higher!

(simple test using velcro, stick 1 strip 10cm long between 2 books, try and pull it apart. Then try with 2 10cm long strips, and it gets a lot more difficult (becuase you have twice as many hooks and links to "break")

Of course, it ain't actually that simple, with the crucial effect being one of a reduction tyre contact pressure (if normal load is constant, a larger contact patch applies a lower surface pressure). Hence, doubling the width of your tyres does NOT double there load capability, because the surface pressure reduces and the "locks" between the rubber and irregular road surface are less well pressed together (similar to only lightly pressing your bits of velcro together, less of the hooks engage)

There can also be a larger difference between the static and dynamic friction of the wider tyre, especially if it uses a compound with a relatively low Mu and relies more on the interlock effect. So as the tyre slip coefficient increases, you might find the grip coefficient reduces at a steeper rate than for a narrower tyre.

[anonymous-user]

RedexR said:

in view of last weeks Motorway incident

Again, please explain to me how last weeks M5 crash was affected by "ultimate" stopping performance??


As a driver you have a responsibility of driving within your cars limits. if you have less grip, drive slower! (again, accident statistics do not support a direct link between a tyres ultimate grip capability and crash occurance, in fact generally the opposite is true. Drivers "feel" they have more grip / control, so they drive faster............. If you look at the recent statistics for the uk's last big snow event (earlier this year) although crashes increased i believe by something like 25% (!!!) deaths actually fell by 8%! (because people drove slower and hence the resulting collisions were more minor)

[Fox-]

Max_Torque said:

As a driver you have a responsibility of driving within your cars limits. if you have less grip, drive slower!

So travel at 50mph on Motorways once your tyre tread goes below 3mm? Is that what you are saying?