Doctor Chris>> I obviously replace tyres before they are illegal but have to admit I wait untilcornering in the wet deteriorates.
How PFDing fast do you drive? I have only once in my life had my wheels lose contact with the tarmac (on ice, aged 17, car a write off). Do you really push your car that close to its limits?
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You don't have to be going that fast to get the front end to slide a bit on a wet roundabout when it's a bit greasy, or to lose traction pulling out of a tight side road in the wet. I certainly use that as my benchmark on grip.
Indeed, I'm currently sitting in waiting for a courier to turn up with two new Pilot Primacy 2's - previous front tyres had got down to about 2 mm, which is lower than I had intended them to get.
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Tyres just arrived - blimey they're big when you bring them indoors! Am quite pleased, my local indie can fit them this afternoon not Friday as arranged - result. HJ Good Garage guide is going well so far..
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I change them at 2.5-3mm.
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The age of tyres matters too. I think HJ said somewhere that he wouldn't use tyres more than five years old. I was tempted at first to carrying on using the old tyres on my now-restored Capri, which had been in dark dry storage for 13 years, but on checking around for opinions and advice decided it was not worth the risk. Rubber does deteriorate, not just through physical wear but also simple exposure to light.
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Cycle tyres are much better when they are old the rubber matures with age when they are kept in a dark cellar.
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Being brutally honest I rarely change my tyres before they are close to legal limit, say 2mm or so. It much depends on time of year and type of journeys. If tread was getting thin and I was doing a driving holiday, or we were coming into deepest winter, then I might change earlier.
I stand corrected but 90% of my journeys are urban commutes to work, stop start traffic, traffic lights etc and I don't really think that thread depth is a massive factor in this case.
On a slight tangent, if my basic knowledge of ABS is correct, then this has sensors which release the wheel if it senses it locking up. So if you have good tread that instantly locks up, will this not be detrimental to safe stopping?
And on a further slight tangent, do all new tyres need to have the same tread depth? Must admit I always shop around for prices when changing but I have never thought that maybe Tyre A has 8mm tread and Tyre B 6mm tread which could have a major impact on life of product?
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ABS (...) has sensors which release the wheel if it senses it locking up. So if you have good tread that instantly locks up will this not be detrimental to safe stopping?
No, if you are on a slippery surface (or your tyres have been coated in grease, for instance) the braking force will be far less before the ABS is activated.
And on a further slight tangent do all new tyres need to have the same tread depth?
I don't think so.
maybe Tyre A has 8mm tread and Tyre B 6mm tread which could have a major impact on life of product?
Yes, as can the tread pattern, in all probability.
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Very few fleet cars will have their tyres changed before 2mm
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Tyre A has 8mm tread and Tyre B 6mm tread which could have a major impact on life of product?
Ideally, both front and both rear tyres should have same tread depth.
If one front is better than other front, you may be uneven tyre wear, which may affect steering.
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If your car is working properly, the less tread there is on your tyres the better, in dry weather. Tread only gives an advantage in the wet, by clearing surface water away to prevent aquaplaning. In the dry it is a disadvantage, putting less rubber on the road and increasing the amount of distortion and squirm in the tyre under cornering forces.
Racers in classes that specify treaded tyres used routinely to scrub or machine tread down to the minimum allowable depth before important races to improve grip and handling precision.
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Where we live the roads - especially the country ones are either:
dry, wet, very wet, 4cms deep of water : and that is the summer.
In autumn and spring the same plus add frost and mud.
In winter add snow and ice - occasionally.
Under 3mm = risk. So I change then..
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Very few fleet cars will have their tyres changed before 2mm
Yep. Had many a row with fast-fit place. It was one of the factors in me opting out of a company car.
I have been in with the tyres worn flush to tread-wear indicators and yet they've measured them at 3mm and the leasing company won't agree to change them. They also rarely fit the same tyres as are being taken off, so the car can feel quite different.
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If you've got a health and safety man in your business unit, it may be worth mentioning to him that the risk increases when the tyre gets below 3mm - There are a lot of fleets now making 3mm the limit. Personally I don't mind any of my drivers changing at 3mm, indeed I've had to force one driver to change his tyres (Vectra 150 CDTI) before he had no grip at all.
With the recent changes in health & safety legislation, it is becoming more important not to rely on the 1.6mm legal limit for companies.
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It surprises me that for MoT the outer 1/8 of tread width can legally be bald (an inch on a 205 tyre), the 1.6mm minimum applies only to the middle 3/4 of the tread.
If the tyre is trying to channel water outwards where does the water go when it reaches the bald strip?
www.motuk.co.uk/manual_410.htm
My car wears its front tyres unevenly but I still work on approx 2mm minimum on the outer edges of my tyres, which can mean 3mm over most of the tread. If winter is approaching will tend to change them earlier so I've got 'good tread' for any snow (dont see much snow these days but used to get quite a bit every year in Derbyshire).
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It surprises me that for MoT...
As ever, the seemingly simple requirements of the MOT are underpinned by some good thinking.
As the tyre (and hence contact patch) becomes wider, then clearing water in the fore/aft direction becomes more and more important than clearing it to the side, and so, specifying a region of tread which can fall below the usual limit in a manner proprtional to the width of the tyre is a very reasonable way to do it.
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Around 2mm.
I find that quite often one front tyre is worn more than the other, and I change them both at the same time - so some might be changed when at 2.5 or even 3.0
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It surprises me that for MoT the outer 1/8 of tread width can legally be bald
To my understanding this is not the case - the tread pattern should still be visible across the whole tyre.
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>>To my understanding this is not the case - the tread pattern should still be visible across the whole tyre.
No, it can be completely bald - of course, there should be none of the re-inforcing plies or cords visible.
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To my understanding (...) the tread pattern should still be visible across the whole tyre.
That's from the time before (or the time before that). The central 3/4 must have legal depth tread all around. The outsides can be as bald as the proverbial.
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I replace my tyres just before they reach the legal minimum - probably around 2mm.
After reading some of the posts on this thread I think there is a certain element of urban myth around tyre tread depth.
The reason for having tread patterns is to prevent aquaplaning on a wet road. This is when water lays on the road and when driving at speed a 'wedge' of water builds up in front of the tyre and can reduce grip.
For dry and damp roads a bald tyre would in fact give better grip. I find it surprising to hear people talk about better grip on roundabouts in the wet with new tyres. My experience is that new tyres decrease grip and handling until the tread blocks have had a chance to wear down.
New tyres will of course give better grip if they are of a different (better) brand or softer compound.
Note that many of these charts are produced by tyre manufacturers and do not state the conditions under which the test was done.
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Should have also added that a new tyre may grip better not so much due to the increased tread but due to the old tyre having aged. I understand that the aging is partly due to the number of heat cycles that the tyre goes through which continues the vulcanisation process, making the rubber harder over time.
For dry roads the ultimate grip would be from new tyres with the tread shaved down to around 4mm?
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It surprises me that for MoT the outer 1/8 of tread width can legally be bald (an inch on a 205 tyre) the 1.6mm minimum applies only to the middle 3/4 of the tread.
The outer edges of both fronts on my Merc are bald, while there's 4-5mm in the centre. Merc's, especially C Class, tend to do this, apparently. I don't feel too bad about it as they've done 36K.
I got an advisory at the last MOT and will change them for the next one, especially as winter approaches, but they still have plenty of tread.
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The outer edges of both fronts on my Merc are bald while there's 4-5mm in the centre.
Are they regularly pumped up?
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Are they regularly pumped up?
No. The pressures never vary.
I do run them slightly overinflated (3lbs above the min figure) to try to minimise the edge wear. Merc's wear the rear's in centre (common on RWD cars) so I run them 2lbs under, which has maybe helped a bit compared to how the first set wore.
Not sure 2-3lbs has any effect - I use a dial type and an old pen gauge which both read the same, but I still don't know if they're accurate and I doubt most garage gauges would be accurate to 2-3lbs anyway.
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Running the fronts hard, and the rears soft in comparison with the spec is "A bad thing" to do. Effectively you are re-tuning the car towards oversteer. I know that many people with MBs do exactly this, but I wouldn't, and I wouldn't advise that anyone does it either.
I think there are offset bolts available which can be used to reduce the front castor angle if it is out of spec - apparently quite a number are found to be out of spec.
With standard pressures, how many miles do you get out of a set of tyres?
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With standard pressures how many miles do you get out of a set of tyres?
Thanks for other comments.
Fronts are original and have done 36K. First set of rears lasted 18K. All were Bridgestone Turanza's.
It was at 18K that I started adjusting the pressures. Standard is 30F33R and I run 33F30R.
Replaced the rears with Michelin Primacy HP so they've done 18K and perhaps might go to 30K.
I really only use the car for long steady motorway runs, so it doesn't do corners! I drive it extremely gently so it's most unlikely I'd be anywhere near being bothered by oversteer and anyway the car has a lot of electronics to deal with that apparently faster than I could ever do!
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>>Merc's, especially C Class, tend to do this, apparently.
Yes, it's related to the larger than normal castor angles which MB use making the wheels camber over more in corners. MB typically use about twice the castor angle of other cars.
Edited by Number_Cruncher on 14/08/2008 at 13:24
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>>Merc's especially C Class tend to do this apparently. it's related to the larger than normal castor angles
Oooh arr. So. No special tyres for these cars?
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>>So. No special tyres for these cars?
Sorry, I don't follow your question FT.
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N_C, can you explain the oversteer, please? I must be misunderstanding it, but would have thought that overinflated fronts would exaggerate understeer, as would underinflated rears.
I agree with you about sticking to pressures recommended by the manufacturer.
Edited by Roger Jones on 14/08/2008 at 13:50
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Hi Roger,
An over-inflated tyre is stiffer in cornering. i.e., for a given side load it will run at a lower slip angle.
Oversteer is where the slip angle at the front is less than the slip angle at the rear. (There's a much more involved technical definition as defined by the SAE, but, front and rear slip angles is what it all boils down to)
In this respect, all modern vehicles understeer in a neutral corner - they need to be provoked by harsh acceleration or braking into gross slip at the rear axle.
Increasing the front pressure, reducing the front slip angle, while reducing the rear pressure, increasing the rear slip angle are **both** changes that drive the vehicle towards oversteer.
In road vehicle dynamics design, a margin against oversteer is usually aimed for, which allows for small deviations before the vehicle actually begins to oversteer. Changing the tyre pressures in the way described at least eats into this safety margin if not actually making the vehicle oversteer.
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Nice post NC.
So a simple take on this would be that underinflation would improve grip in a straight line, but as soon as you try to corner it has the opposite effect and reduces grip?. Those two combined sound quite scary, fast into a corner and then...
I would have thought the same as roger until reading your post. This forum is at its best when we learn something new.
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I suppose for me it is as simple as this;
The manufacturer's tyre pressure recomendations should be followed.
Increasing rear pressures in line with manufacturer's guidance for increased load is also required for both controlling heat input, and for understeer margin reasons. A heavy load on the rear will in a given cornering scenario increase the rear slip angle, and so increasing the rear tyre pressure counters this effect.
For a very readable introduction to the subject which doesn't skimp on the necessary technical detail, see Fundamentals of Vehicle Dynamics by Gillespie (probably one to order via your library, as it isn't a cheap book!)
Edited by Number_Cruncher on 14/08/2008 at 14:37
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