Driving long distance with one of four tyres advisory

Thanks. I’m struggling though to understand the nuance, this isn’t a matter of ‘opinion’ where results need interpretation. Like politics, or even to an extent climate change, where result interpretation is important.

In this case the ‘experts’ – independent testers – do some tests that clearly show the deterioration in braking performance as tread depth decreases. It’s that simple – as per link on P1 Robertajobb’s post – tyre depth = new, stopping distance = x; tyre depth = legal limit, stopping distance = 25-40% longer based on the tyres on test.

‘Non experts’ – on the back of no data that I can discern, other than maybe they haven’t crashed and hopefully never do in a situation where better grip or braking would have helped – call those that suggest not running your tyres down to the limit “reflective vest wearing, clipboard carrying bunch of fannies” (composite quote)

I get the need to accept that there is nuance in some situations. But not here.

Clutch down if you’re in a manual. Listen to the motor or engine in an auto and try to keep the revs constant. Foot off the gas and you’ll get engine braking – both wheels aquaplaning and the revs will drop so when you get grip again violent things will happen due sudden engine braking. IME in most aquapaning if you dip the clutch the car just carries on where it’s going, the problems start when one of the wheels has more grip than the other, so if the revs have dropped and you haven’t dipped the clutch the car will pull one way or the other as one wheel finds more grip before the other. Dip the clutch and there’s every chance you’ll maintain control as the car finds grip. All of that is most important in FWD but in AWD and RWD it’s still worth dipping the clutch.

In Short: In aquaplaning you don’t want engine braking so dip the clutch rather than foot off the gas.

Thanks, driving expert! 🙂 Not seen it explained like that before, makes sense.

the data is only available for tyres that were tested. For every mm of wear there will be a decrease in performance. at what point do you replace the tyres?

Wouldn’t that be the nuance?  when the individual decides that they are no longer comfortable with the performance of their tyres, within the boundaries set out by the law.

It’s your only contact point. Replace after every journey

No ones actually discovered how much tread is actually on the tire apart from on the outside edge. And yet the op is condemned  to a death of ditchseeking.

I might be driving a long journey next weekend but my MOT has shown that one of my tyres outer edge (front wheel drive) is 2.7mm. The other front is 3.1mm.

Pictures of the tread would help, because a picture tells a million words.

Think we’ve already got a million words, pictures would be superfluous at this point.

Wouldn’t that be the nuance? when the individual decides that they are no longer comfortable with the performance of their tyres, within the boundaries set out by the law.

Not got a problem with that within reason – I mean I’d like for ditchfinders to not be available, and for a higher legal limit before performance really starts to drop off. I have to share the roads with them as well as sort myself out. But, them’s the rules. The debate was fine for nearly 100 posts before a couple of folks turn up and start slinging insults about. Start an argument, expect to get some back.

For every mm of wear there will be a decrease in performance. at what point do you replace the tyres?

The decline is steady for a large part of the wear, and then drops off quickly past a certain point. Almost certainly tyre dependant but around 3-3.5mm IIRC. But pointing this out, and saying that my choice is to change around that point makes me a doom and gloom fanny apparently

@mert and @multi21 , I think that you’re the ones to ask…

I’ve read in Roadcraft that depressing the clutch may cause problems with an operating/intervening ESP system. I haven’t seen that in a manual (BMW, Kia, Nissan, Skoda, Vauxhall, Volvo) that I can recall, is this a problem?

An interesting survey was carried out in 2016 by Bridgestone who examined tyre debris (i.e. blowouts) on England’s motorways, “…almost three quarters of tyre failure samples analysed by Bridgestone involved poor inflation or debris penetration issues…” https://www.gov.uk/government/news/tyre-related-deaths-and-injuries-preventable-say-highways-england-and-bridgestone

This is an article on certain Michelin tyres that work effectively in the wet at 1.6mm. NB, this isn’t general advice for every tyre out there https://www.michelin.com.hk/en/performance-made-to-last

I’d be very interested to know how deep the water was in those tests. If a road is just surface wet then a semi-slick works just fine, however as water builds up on the surface things get increasingly delicate up to the water depth at which aquaplaning starts and there’s no grip at all. Put a well treaded tyre on and it’s back to driving as if the road were just a bit wet.

@mert and @multi21 , I think that you’re the ones to ask…

I’ve read in Roadcraft that depressing the clutch may cause problems with an operating/intervening ESP system. I haven’t seen that in a manual (BMW, Kia, Nissan, Skoda, Vauxhall, Volvo) that I can recall, is this a problem?

If you’re aquaplaning dipping the clutch won’t make things any worse, think the advice is probably based on when the systems are using the ABS/brakes in skid/slide/yaw scenarios, not having engine torque applied could plausibly confuse the brake controller. I would very much doubt it in a modern system though. Maybe in the days of ABS only being on the front axle?

TC/ESP/DSTC/whatever isn’t much use when aquaplaning anyway as it needs accurate wheelspeed signals from at least one wheel on each axle and a steering wheel angle or it’s just guessing! It will however start preparing for nastiness once it loses confidence in the signals. (There are statistical thresholds between each wheel sensor and the “total system” that i don’t fully understand, but once they go out of range, the system prepares for the worst).

There is also some inertia based stuff that is floating around, you can calculate the yaw angle, steering wheel position, speed, requested torque and so on, so wheel speed becomes just one useful signal amongst many, to encourage the driver to make good decisions. But if they want to slam the brakes on and saw on the steering wheel, you can’t stop them. I’ve not been directly involved as that tech development stuff is (now) outside my area of responsibility.

Early ABS did some things very well but could be beaten by a good driver in terms of stopping distance. The Ford Sierra advert driven by an autotester and rally driver is excellent example of how ABS allows combinations of hard steering and braking. However the advert for another company that showed an ABS equipped car stopping shorter than the normal car required some messing with brake pads to handicap the normal car. Anyhow the honest ad at 47seconds:

I’d be very interested to know how deep the water was in those tests.

The proposal was the current “EU wet braking test, which involves measuring the distance required to decelerate a vehicle from 80 to 20 km/h on a standard road surface with a water depth of 1 mm” https://www.tyrepress.com/2019/07/michelin-wet-grip-testing-at-1-6mm-will-provide-greater-transparency/

I don’t know what was adopted. The article is worth a read

mert, thanks. That helps a lot 🙂

I asked one of the stability guys. It’s the physics of ABS braking on *very* low traction surfaces or *very* high yaw/side slip events that can cause issues. The wheel deceleration and acceleration between each brake pulse is more difficult to calculate because the inertia/torque at the wheel is lower, hence harder to calculate the brake pressure needed (it’s easier to work out and apply/release pressures when the system is heavier). So when you finally get traction the wheel speed *could* be anywhere (even completely stopped). Modern systems almost completely fix the issue as they can still read the wheel speed at very high resolution, even though it’s effectively a gibberish number that’s unrelated to vehicle speed.

I don’t full understand it myself either…

👍

@theotherjonv

But pointing this out, and saying that my choice is to change around that point makes me a doom and gloom fanny apparently

For the record that comment wasn’t aimed at you.

But the comment about getting out wrong side of the bed was.

(even completely stopped

Or turning backwards even if the car is moving forwards if you haven’t dipped the clutch. If one wheel has grip and the other doesn’t and engine speed is below half of road speed the differential will spin the wheel with no grip in reverse. I have no idea how ESP/ABS copes with that. “Clutch down and steer”

Whilst “clutch down and steer” still works one other ‘when all seems to be lost’ method doesn’t work anymore. In pre-ABS cars if you’d lost it but the car was heading in a safe direction you could jump on the footbrake and grab the handbrake thus locking all four wheels. The car would then slide to a stop in whichever direction it was moving regardless of where it was pointing – sometimes you could even let the brakes off if the car ended up pointing where you wanted to go again. Modern cars don’t have a handbrake and jumping on the brake doesn’t lock wheels.

However ESP/traction control is so good you’re much less likely to end up in those situations. The ESP on our little Zoe is probably pretty basic but it requires serious efforts at provocation to unsettle it even on ice and snow. Sure the laws of physics still apply but ESP and Cross Climates mean you’ve got to be making special efforts to get out of line. Early ABS was pretty crap on snow but I no longer feel the need to spend time tinkering under the bonnet to disable it (even if that’s possible) on snowy days now. It’s great.

You know you’ve got grip issues due to the tread depth when it’s pissing down with rain on the uphill stretch of the M4 to the Bath, Tormarton junction in a BMW, and the revs start to go up under gentle acceleration, because the rear wheels are actually spinning in the water! Mainly because the water is running along troughs caused by the weight of trucks in the inside lane. At that point I chose to leave the motorway and use the A-roads.
The Beemer had 1.6mm of tread on the back tyres, the legal limit, and it was deemed safe for me to drive it. But the heavy rain at that point hadn’t been anticipated.
If it had had between 2 and 3mm on the outside edge of one tyre, there wouldn’t have been any problems at all, I certainly wouldn’t have noticed any loss of performance.
Less than a millimetre difference across the width of any given tyre, especially when it’s well outside the legal minimum limit of 1.6mm is completely irrelevant, it’s perfectly safe and legal to drive, and I defy any regular driver to notice any difference to the actual performance of the car and/or tyres.
If, like my rear tyre, it had a foreign object stuck in it, in a position that was irreparable, I’d be changing it before any significant journey, which is exactly what I did, because of the very real likelihood of the tyre going down at the worst possible moment, but a slightly lower tread depth on the outer edge? Please, give me a break! *rolls eyes*.