| Motoring Discussion > Bibendum - bienvenu | Accessories and Parts |
| Thread Author: WillDeBeest | Replies: 50 |
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New Sat 9 Nov 13 13:44
Bibendum - bienvenu - WillDeBeest
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Well, OK, since it's quiet in here. The LEC needed tyres, and I've documented my preference for Michelin many times before. The new pair are Primacy 3, a minor-looking update of the HPs I have on the Volvo. Two 225/55R16 95W - as prescribed in the handbook but not actually matching either pair fitted to the car when we bought it - cost me a tad over £270. I bought online through Blackcircles and had them fitted near Heathrow on the way to work on Friday. I did have some choices to make, which - since it's quiet - I'll mention here. Instead of the Primacy 3, I could have had for a similar price the 'MO' version of the HP, which has MB factory approval. Much online digging brought me to the view that this might matter on one of AC's snorting monster MBs but was unlikely to make much difference to my airport taxi edition. On the other hand, the 3's lower noise rating and superior wet grip might well be useful, so that's the way I went. Then - and you knew this would come up, didn't you? - there was the question of where to put them. The LEC is, of course, RWD, which is a new experience for me. The worn-out tyres were the Hankooks on the rear wheels - although these weren't new when we bought the car and have taken 20,000 miles with us to approach 2mm, so it doesn't seem to be a car that's hard on tyres. But I do want rid of the Nexens that have been on its front wheels. Apart from the suspect quality and the stodgy feel they give the steering, they have a nasty Terry-Tryhard palm-frond tread pattern that doesn't look at all nice if I park with the wheels turned. They have about 5pm of tread on them. Michelin still recommends new tyres on the rear, and I agree with their logic. But it could take another 20,000 miles to wear out the Nexens on the front. So I reasoned, with some help from the fitters, that 5mm is plenty, on an ESP-equipped car, to keep the back end in line, so the new Michelins went on the front, leaving the back wheels to wear out the Nexens, which I will replace a little early to give me a uniform set. And I think the car feels better for the change, especially through the steering wheel, although it's a little early to say after 40 miles at very moderate speeds. I do need to pacify the car's run-flat warning system, which seems to have been upset by the change. I've done the reset procedure now, and I hope that'll end the big red 'Check tyres' messages I've been getting. So, what would the panel have done? |
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New Sat 9 Nov 13 14:26
Bibendum - bienvenu - Runfer D'Hills
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>>So, what would the panel have done? Probably the same, although with winter coming I might have put the new ones on the back on a RWD. I can understand your logic though. New Contis on the back for me on Monday. The tyre pressure thingy should be fine now BTW. |
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New Sat 9 Nov 13 15:41
Bibendum - bienvenu - Mike Hannon
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| The same and will be doing in a week or two, for the same reasons. |
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New Sat 9 Nov 13 17:05
Bibendum - bienvenu - Boxsterboy
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Well I needed 4 18" tyres yesterday and bought Nexens which were a bargain at £110 each fully fitted! But they are going on a van and so the finest degree of steering feel is not such a priority. But you are right, the E-class estates are light on their tyres if driven sensibly. If I wanted rid of the tyres in the way that you did, I would have put the old ones on the rear too. |
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New Sat 9 Nov 13 17:26
Bibendum - bienvenu - Runfer D'Hills
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| I suppose if you could be bothered WDB, you could just replace the tyres you don't like too. Possibly sell the ones you take off on some site or other? I seem to remember that you don't do starship mileage? My back tyres have done nearly 40k this time. Those Nexens might be on there a good while yet ! |
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New Sat 9 Nov 13 17:36
Bibendum - bienvenu - Number_Cruncher
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One aspect of tyre management that is particularly important with MBs is tyre rotation. For technical reaons I won't bore you with, the correct tyre pressures tend to produce wear on the edges of front tyres, and in the centre of rear tyres. Bodgers fiddle about with the pressures, but, the best, and correct way to deal with this, which is part of MB's servicing schedule, is to rotate the tyres. The problem comes with the more powerful cars with a staggered tyre set up. Again, the temptation for bodgers is to fiddle with the pressures, but, really it's a case of if you can afford a car with a staggered tyre set up, you should be able to afford the tyres! |
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New Sat 9 Nov 13 18:01
Bibendum - bienvenu - Armel Coussine
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| What are staggered tyres N_C? Never heard of them. |
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New Sat 9 Nov 13 18:04
Bibendum - bienvenu - Runfer D'Hills
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>> What are staggered tyres N_C? Never heard of them. As on my car AC, for example, 265/35/18 on the rear and 235/40/18 on the front. |
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New Sat 9 Nov 13 18:27
Bibendum - bienvenu - Armel Coussine
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Ah, I thought it might mean wider ones on the back. Always seems a nuisance to me, need to carry two spares. I don't know how to put this delicately, but is your jalopy enough of a snorting monster to really need them? There was a DS Citroen - DS 21 or 23 - that had tyres that were not only directional, but bigger on the driven front wheels than the back ones. I think the XM may have been like that too. |
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New Sat 9 Nov 13 18:54
Bibendum - bienvenu - Bill Payer
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>> Always seems a nuisance to me, need to carry two spares. The rolling radius is near as dammit the same so the the same spacesaver works for front or rear. Last edited by: Bill Payer on Sat 9 Nov 13 at 18:54
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New Sat 9 Nov 13 18:56
Bibendum - bienvenu - Armel Coussine
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>> spacesaver Aaaaargh! Get thee behind me Satan! |
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New Sat 9 Nov 13 19:04
Bibendum - bienvenu - Bill Payer
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>> >> spacesaver >> >> Aaaaargh! Get thee behind me Satan! >> I don't think the wheel-well is deep enough in most recent cars to carry a full size spare. Last edited by: Bill Payer on Sat 9 Nov 13 at 19:04
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New Sat 9 Nov 13 19:19
Bibendum - bienvenu - Runfer D'Hills
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>>is your jalopy enough of a snorting monster to really need them? I'm sure it isn't even slightly in need of them but it does handle quite well so maybe there is some benefit. Wouldn't like to say. |
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New Sat 9 Nov 13 19:10
Bibendum - bienvenu - R.P.
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| I'm staggered - My X1 had staggered tyres 255 at the rear and 225s up front - you live and learn. I replaced them as specc'd too. |
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New Sat 9 Nov 13 18:01
Bibendum - bienvenu - NortonES2
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| Is there a "correct" tyre pressure? The wear you suggest occurs, seems to point to under inflation at the front and over inflation at the rear. Is this wear a UK thing, due to greater, deliberate, rugosity of road surfaces here, or is it also a feature in the fatherland? :) |
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New Sat 9 Nov 13 18:27
Bibendum - bienvenu - Number_Cruncher
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>>The wear you suggest occurs, seems to point to under inflation at the front and over inflation at the rear. That's the correction the bodgers make. Without getting into technical detail, MB have deliberately set the recommended pressures like this, and they have good reason to do so. Tyre rotation is the right way to deal with the wear pattern on these cars. |
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New Sat 9 Nov 13 19:44
Bibendum - bienvenu - NortonES2
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NC. Do tell what the technical detail is. A summary would do! A guess: intervening in the interest of taming, or at least limiting, power oversteer on road systems where the foot is to the floor habitually?
Last edited by: NIL on Sat 9 Nov 13 at 19:45
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New Sat 9 Nov 13 18:32
Bibendum - bienvenu - Manatee
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>> Is there a "correct" tyre pressure? The recommended ones presumably. Changing the tyres pressures would alter the limit handling as well as the pattern of tyre wear, so tweaking them to optimise that might be construed a bodge. Of course, markedly different tyres front and rear might render the subtleties of tyre pressure irrelevant. I like 'rugosity' I'll try it out at the first opportunity. EDIT - crossed with NC. Last edited by: Manatee on Sat 9 Nov 13 at 18:33
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New Sat 9 Nov 13 18:51
Bibendum - bienvenu - Bill Payer
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>> .... which is part of MB's servicing schedule, is to rotate the tyres. >> I can't recall seeing that in any MB service schedule going back 10 years. They used to take the wheels off as part of the B ('major') service, but haven't done that for some years. >> The problem comes with the more powerful cars with a staggered tyre set up. >> Not sure how widely fitted they are, but my late 2004 C270 has a staggered set-up with 16" wheels. My impression is that one you get to C Class and above, pretty well every model has a staggered set up. Last edited by: Bill Payer on Sat 9 Nov 13 at 18:52
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New Sat 9 Nov 13 20:23
Bibendum - bienvenu - Number_Cruncher
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Yes BP, I can well imagine that more traditional RWD MBs now have a staggered set up then don't. Sorry, this is going be a long post..... With a RWD MB the engine is usually sufficiently oversized to enable you to break traction in anything other than perfect road conditions, only prevented in the more modern cars by traction control / dynamics control, and in the older cars by the skill and judgement of the driver. So, that's not the reason for the odd tyre pressure recommendation by MB. It's important to consider the fundamental handling bias of a car seperately from a car's ability to break traction on its drive axle. Rather than the case of gross hooligan like skidding by breaking traction, the reason for the front tyres being set a bit low, and the rear tyres a bit high is to move the fundamental handling balance of the car towards understeer. The root of the problem is that these cars tend to have almost exactly 50/50 weight distribution with the driver only in the car. With passengers and/or luggage, the weight distribution moves further back. If with the 50/50 weight distribution the tyres are also the same type, and run near zero camber angle, with the same tyre pressure, and the suspension has roughly the same roll stiffness front and rear, and a few other minor details being set reasonably, the car will steer neutrally. As soon as the weight distribution moves rearwards at all, with passengers or luggage, then the car would, fundamentally move into oversteer. It's not a good idea for a vehicle to oversteer in the fundamental sense, because the passive baseline response of the vehicle should be stable. So, with a 50/50 weight distribution and the same spec of tyre front and rear, some tweak is needed to gently nudge the balance slightly into the understeer regime. One approach would be to fit a very stiff front anti-roll bar, and rely upon overloading the front outside tyre and driving it into a non-linear part of its response, and hence understeer. This would, however, make the car ride much worse, and would not sit well with MB's typical customers for whom ride quality is important. A small tweak could be had by choosing different camber values on the front and rear axles, but, this would result in a tyre wear pattern which would be difficult to correct via a simple rotation. Ditto toe settings So, tweaking the tyre pressures, and adding tyre rotation to the service schedule was MB's preferred approach to the problem. Of all of the possible levers a vehicle designer has at their disposal to affect the car's response, mass distribution is by far the most important, and has the largest effect. By this same argument, the axle which drives isn't the main reason why front wheel drive cars tend to understeer (in a fundamental sense), it's actually the forwards weight bias which means that FWD cars tend to understeer - having front wheels which can also break traction just makes the effect more pronounced. |
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New Sat 9 Nov 13 20:28
Bibendum - bienvenu - MD
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| Well at least that's cleared that up:) |
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New Sat 9 Nov 13 23:39
Bibendum - bienvenu - Dave_
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Wow. Thanks NC, posts like that are why I come here. Before I got my Saab 9-5, my one concern was that the 235/45 R17 tyres would be expensive to replace. It needed a front pair but then so did the car I exchanged... A slow puncture in the rear of the Saab convinced me to replace the whole set at Easter, Falken rubber fulfilling budget requirements as well as meeting the approval of the owners' forums (the consensus was to avoid Avon rubber at all costs). Since then I've had two random punctures in locations 100 miles apart, one in each rear tyre. So that's six £148 tyres in seven months. Ouch. Hopefully i won't pick up any more for the foreseeable. |
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New Sat 9 Nov 13 23:55
Bibendum - bienvenu - Manatee
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So it was to manage the handling balance as I guessed. And having part worn tyres you don't rate on the back and new premium tyres on the front means it's in vain. I would have either put four matching tyres on, or put the new Michelins on the back. The usual reason for putting new tyres on the front at this time of year is for winter traction. In this case that's further reason to put them on the back. If you want a car to have the balance its maker intended it seems sensible to have the same tyres on both ends, in a similar state of wear, whether its a MB or not. I'll continue to ignore people who tell me I shouldn't bother swapping my wheels around. |
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New Sun 10 Nov 13 15:08
Bibendum - bienvenu - borasport
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>> Wow. Thanks NC, posts like that are why I come here. >> +1 Though everytime someone mentions understeer or oversteer, I have to sit down to think which is which. Then again I have the same problem with 'backslash' and 'forward slash' |
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New Sun 10 Nov 13 15:33
Bibendum - bienvenu - Armel Coussine
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>> Wow. Thanks NC, posts like that are why I come here. >> >> +1 Yes, meticulous as usual, a clear explanation of how understeer is engineered into large modern cars when they are driven in a reasonably sane and moderate manner. N_C didn't say so, but surely knows that all cars are set up to understeer, most drivers being poor at controlling or anticipating oversteer. He must be aware too of a phenomenon known as 'roll oversteer', often experienced by imprudent drivers of e.g. American cars of the forties and early fifties despite their nose-heavy weight bias: a sudden loss of adhesion at the rear end when a poorly-damped live back axle either lifts a wheel under roll or is unsettled by a bumpy road surface in the bend. It has sometimes occurred to me that the legendary handling of the front-drive Golf GTI could have had something to do with its tendency to lift a rear wheel at high cornering speeds: the resulting loss of adhesion would move the car from understeer towards more 'neutral' handling. Beneficial roll oversteer so to speak. Haven't driven one enough to know for sure though. Suspension damping is far more crucial than most drivers realise. |
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New Sun 10 Nov 13 01:20
Bibendum - bienvenu - Bill Payer
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>> So, tweaking the tyre pressures, and adding tyre rotation to the service schedule was MB's >> preferred approach to the problem. >> Seems incredibly iffy to rely on owners accurately setting the pressure differential between front and rear tyres. I haven't heard of this on MB's, but apparently later 5 Series, even 'lowly' 520d's are eating rear brake pads due to the amount the ESP intervenes even in normal driving. |
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New Sun 10 Nov 13 10:01
Bibendum - bienvenu - Shiny
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"I haven't heard of this on MB's, but apparently later 5 Series, even 'lowly' 520d's are eating rear brake pads due to the amount the ESP intervenes even in normal driving." Yes, my A8 has eaten the genuine rear pads which are pretty large pads on vented 310mm discs. I read it was due to the rear bias of EBD (electronic brakeforce distribution) when travelling in a straight line at city speeds and affects pretty much all <8 year old cars. In fact on mine at speeds below 7Kph, the rears are used exclusively to come to a halt in normal braking applications. I have never has ESP light in day to day driving, only in snow/ice, does this claim that the ESP is intervening in normal driving still apply or are these people with BMWs and MBs having their wheelspin defeated with their rear brakes (which I don't due to 4WD). |
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New Sun 10 Nov 13 14:54
Bibendum - bienvenu - Bill Payer
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>> In fact on mine at speeds below 7Kph, the rears are used exclusively to come >> to a halt in normal braking applications. >> Hmm..never heard of that before - makes sense in "luxury" car as it would tend to stop more gracefully. >> I have never has ESP light in day to day driving, only in snow/ice, does >> this claim that the ESP is intervening in normal driving still apply or are these >> people with BMWs and MBs having their wheelspin defeated with their rear brakes (which I >> don't due to 4WD). >> I understand the assertion, with BMW specifically, is that the ESP is working behind the scenes even in normal driving, without the driver knowing about it. |
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New Sun 10 Nov 13 17:35
Bibendum - bienvenu - Number_Cruncher
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I'm not sure about iffy, but, it is true to say that there are other effects which will push the handling balance towards understeer, but the tyre pressure differential is among them. As one example of the other effects, consider W124s - their front strut suspensions have a wider spring base than the multi-link rear suspension, so, even if the suspensions have about the same ride frequency with respect to purely bump motion, the front suspension would have greater roll stiffness, and hence the lateral load transfer would be higher than that on the rear, which gives an understeer effect. Yes, AC, having said so rather too many times, I left out that manufacturers set their cars up to understeer. Roll steer is an odd thing to want to build into a car's suspension. Among the last cars which I remember as obviously having it as a deliberate feature was the Morris Marina, where the rear leaf springs were inclined so that the rear axle would steer as the body rolled. The real objection to roll steer is the delay - you have to wait for the body roll process to happen before the steer effect arrives, wheras more direct steering effects only have to wait for the much quicker tyre response lags. Delays in any control system tend to push the system towards instability, and so are undesirable. As for the Golf, the broad idea when a car's mass is concentrated at one end of the car is to make that end's suspension less stiff in roll to minimise lateral load transfer, and so maximise that axle's capability to react sideforce. |
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New Sun 10 Nov 13 18:08
Bibendum - bienvenu - Armel Coussine
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>> Roll steer is an odd thing to want to build into a car's suspension. Roll oversteer, in the sense I was using it, was certainly inadvertent N_C! The cars I'm talking about were engineered not very well for wafting, floating comfort, a smooth if often slightly bouncy ride. The anomaly with those cars is that they are powerful and capable of high cruising speeds, but can't do high-speed cornering for toffee nuts. Driven by a dozy press-on driver they can switch very suddenly from the usual scrubbing understeer to the other thing. Even if the driver knows what to do, the low-geared steering those cars had in the pre-power days (and even later) make it difficult to catch the tail tidily, or even at all. Used to be quite a common cause of single-vehicle crashes. Indeed a friend had just such a crash in the 1947 Buick Special he had bought from some US airmen at Brize Norton, swooping up a twisty hill on the old A40 with a full load and wonderful straight-eight torque. I had had a lovely time spinning it in second gear on an empty car park a day or so earlier, so it wouldn't have happened if I had been driving. But I didn't have a licence and I wasn't there for the crash. No one was hurt apart from the odd bump but the car was wrecked beyond affordable repair, bonnet pushed into the lovely split windscreen splintering it. Hit a tree head on. |
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New Sun 10 Nov 13 18:28
Bibendum - bienvenu - WillDeBeest
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Well, having read all that I thought I'd better check my pressures. The new Michelins were at the prescribed normal-load setting for the front, which is 29 old-money psi; the old Nexens were still at 29, which should have been 33 now they're on the back. I corrected that, of course. Now, I still think I'm missing something. I understand from NC that as the centre of mass moves rearwards, MB still prefers the front wheels to lose grip before the rear. But it seems to achieve this by raising the rear pressure relative to the front, which I would have expected to have the effect of making the front end relatively more sticky and so make the car more likely to oversteer. Looking at the full-load settings - presumably for maximum rearward mass distribution (and what I used for four-up, full boot and 75kg of bikes and carrier on the towbar in France) the front goes up a little to 33, but the back shoots up to 44. Perhaps that's a red herring - I imagine it's a lot to do with maintaining shape under a greater static load (and I certainly didn't drive it in a manner likely to provoke loss of adhesion) - but I'm still struggling to grasp the 'relative stickiness' point under normal loading. What am I missing? And yes, I agree with those, notably Manatee, who would prefer the same all round. That's what I'll be aiming to achieve in the next few months - possibly sooner if I do find myself short of traction this winter. As for selling the Nexens, that would never have occurred to me; I've only ever sold a part-worn tyre if it still had the part-worn car attached. Is this something tyre dealers wil do? Last edited by: WillDeBeest on Sun 10 Nov 13 at 18:31
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New Sun 10 Nov 13 19:04
Bibendum - bienvenu - Number_Cruncher
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>>to lose grip before the rear No, that's not at all what I've been talking about - sorry if I gave that impression. The behaviour I'm talking about is what you might experience during "normal" cornering. Losing grip and skidding is what happens at the limit of a car's handling, and in some cases can be different from the behaviour at lower lateral accelerations. Heavy goods vehicles, as an example, tend to have a benign understeer response at low lateral accelerations but an oversteer response at the limit - this is generally not a problem as only a real idiot would try to push the handling limits of a truck on the public roads. So, instead of the gross skidding "losing grip", think instead of the relationship between the lateral force a tyre produces when related to the slip angle it adopts - in this case, a lower inflation pressure tends to give more slip angle, and more slip angle at the front is an understeering effect. Put another way, the staggered set up on many (most?) modern MBs allows the rear tyre to generate a lower slip angle per unit side force than the front, and hence provide an understeering effect which happens even at low lateral accelerations, long before any suggestion of either axle losing grip in a gross sense. |
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New Sun 10 Nov 13 22:24
Bibendum - bienvenu - WillDeBeest
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Save your apology, NC, the problem is my ignorance, not your explanation. I'd only ever thought of understeer in terms of loss of adhesion. Years ago I remember a downhill right-hander near the tip on the edge of Basingstoke, and being surprised in my then-new Astra that the car felt as if it wanted to go straight on at a speed where my previous, lighter, un-power steered Escort neatly negotiated the turn. I interpreted that as loss of front-wheel grip (and learned to take it slower) but was I actually feeling the slip-angle effect you describe? Today, four-up and before I corrected the rear pressures, I momentarily forgot that a corner near my house is rather tighter as a left than a right and didn't apply the brakes as I approached. The car got round tidily enough (although the lateral forces surprised the family a bit and I apologized for that) with no apparent electronic intervention, so was that neutral handling balance in action? Clearly I have much to learn. |
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New Sun 10 Nov 13 22:49
Bibendum - bienvenu - Kevin
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>Years ago I remember a downhill right-hander near the tip on the edge of Basingstoke,.. Locals know it as the Town Centre. |
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New Sun 10 Nov 13 23:10
Bibendum - bienvenu - Zero
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>> >Years ago I remember a downhill right-hander near the tip on the edge of Basingstoke,.. >> >> >> Locals know it as the Town Centre. Excellent |
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New Mon 11 Nov 13 08:26
Bibendum - bienvenu - PeterS
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>> >Years ago I remember a downhill right-hander near the tip on the edge of Basingstoke,.. >> >> >> Locals know it as the Town Centre. >> My parents moved to just outside Salisbury recently, and were surprised to hear that a number of locals would go to Southampton or Basingstoke to go shopping (their preference being Bath). Anyway, on a wet Wednesday last week they decided to go and see why, having not been for 40 plus years. I was told that they won't be going back!! |
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New Mon 11 Nov 13 09:48
Bibendum - bienvenu - WillDeBeest
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| They should've seen the place 20 years ago, Peter. I left in 1995 but I've been back occasionally in the last few years and it seems quite presentable these days. Relatively speaking, of course; Bath it ain't. |
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New Mon 11 Nov 13 09:56
Bibendum - bienvenu - PeterS
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| I can image that's true WDB - we lived near Reading in the mid nineties and saw how much the town centre was improved (if perhaps at the expense of 'character') through the decade. My mother lived not that far from Basingstoke as a teenager I think, and was also surprised by how much the town had spread out. I guess that's inevitable! |
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New Mon 11 Nov 13 21:57
Bibendum - bienvenu - Kevin
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To be fair to Basingstoke, the Festival Place shopping centre is a huge improvement over what it was in the 90s when we first moved down here. We now have upmarket brands like Poundland, Primark, TK Maxx and BHS! There's even a store that supposedly sells apples but they never seem to have any. They threw me out when I asked if they had something suitable for making sauce for my pork joint. Last edited by: Kevin on Mon 11 Nov 13 at 21:59
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New Mon 11 Nov 13 22:08
Bibendum - bienvenu - Manatee
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| Same with that Orange shop. You get a duck, and end up with no sauce. |
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New Mon 11 Nov 13 22:10
Bibendum - bienvenu - Zero
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>> We now have upmarket brands like Poundland, Primark, TK Maxx and BHS! Don't forget the fine dinning, Greggs has opened up an outlet. |
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New Mon 11 Nov 13 22:14
Bibendum - bienvenu - R.P.
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| I last visited Basingstoke 12 months ago this coming week. I wasn't disappointed...I got a cracking photo of St Marks Church which featured in the civil war ! |
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New Mon 11 Nov 13 22:17
Bibendum - bienvenu - Zero
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| It has surprising country around it, just to the north west is a large roman town with some stunning roman walls. |
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New Sat 16 Nov 13 12:11
Bibendum - bienvenu - Runfer D'Hills
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Well here's a thing, my car ( Merc E estate fitted with different sized front / rear tyre sizes ) has recommended tyre pressures of ( in lbs/sq" ) 33F / 35R unladen and 33F / 46R laden. Inspired by some of the commentary above I've been experimenting this week with a different set up. I've finally settled on 35F / 40R as feeling the best for all round use. Turn in is ( or subjectively, "feels" ) much sharper which surprised me given the minor changes. Guess I'll have to keep a close eye on treadwear in case it has a side effect I don't want. |
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New Sat 16 Nov 13 17:51
Bibendum - bienvenu - Number_Cruncher
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>>Turn in is ( or subjectively, "feels" ) much sharper That's a good way of describing the regime I was trying to describe above - not the extremes of the cornering regime where you're likely to encounter gross skidding, but, rather the handling of the vehicle during "ordinary" cornering. In the case of your car, there's only the 2psi difference in the recommended pressure because the larger rear tyre has already provided some of the required rearwards bias in cornering stiffness. As your revised presure regime has increased that pressure differential, you are certainly on the safe side (good!). By increasing pressures all round, you've effectively stiffened the vehicle up, and hence you feel the response as sharper - so, you might find there's a cost with respect to ride comfort and noise - however, you may notice a small gain in fuel economy. I agree that monitoring tyre wear is a good plan. |
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New Sat 16 Nov 13 18:51
Bibendum - bienvenu - NortonES2
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I always prefer to run a couple of psi higher than the unladen recommendations in my vehicles, but keeping the bias f-r (if any) the same. Was surprised to see many US comments on TDI club where they seem to run at 90% the max figure on the sidewall, IIRC. Commentary by someone who seems to be logical: "So I have been contemplating whether I was going to reply to this thread again. Logic when it comes to anything MPG related seems to be flawed on this site. Anything for that 1 more MPG. Don't even get me started on Hypermiling... So I decided to explain my logic and the science and experience behind it. Explanation: My side of the argument has two simple logic statements. 1) Increase pressure, decrease contact patch, Increase stopping distance. 2) Perceived handling responsiveness, and actual handling performance are not equal. As you increase pressure above recommended the tire deforms, decreasing the contact patch. This is how the increased MPG is gained, because of decreased rolling resistance. Also why skinnier tires typically get better MPG. Here is a diagram to show that. www.tyrepal.co.uk/tyre-pressu...ntact/464.html Regardless of popular opinion, your brakes do not stop your car your tires do. Yes your brakes do stop the wheel from rolling, but the tires are what actually exert the force on the road surface to stop you. The more in contact, the greater the force can be exerted. It is basic friction, and physics. So the reverse is also true, decrease contact patch, and increase stopping distance. Here is a study that goes fairly in depth: article.sapub.org/10.5923.j.i...130202.01.html I will extract the best numbers for the easiest comparison. These have the closest speed for comparison. These numbers are metric. Prescribed pressure: 50.76km/h Stopping distance, 15.2m 20% Increase over Prescribed: 50.84km/h Stopping distance, 18.37m If we use 34 from the label on my car, and 20% of that would be 6.8, this comparison would be 34 vs 40.8. In this case in this study, it was a stopping distance increase of 10ft, and at only 30MPH. Imagine that scaled up to 60MPH. 10ft, yet alone more could mean the difference between hitting another car, or at 30MPH a kid chasing a ball. Now on to the responsiveness and handling. Taking part of the argument from above, how can decreased contact patch equal better handling. In a word, in most cases it can’t. A LOT of new buyers of our cars come on here and complain about their cars following ruts in the road, and not being easy to hold straight on uneven surfaces. This nature to follow ruts/grooves is called tramlining. In most cases we discovered that the dealer had left the tires at their transport pressure of 40. Once the driver lowered the pressure, the car stopped acting that way. I call this type of handling “Twitchy”. Every little input or bump causes a change. This happens with skinny road bike tires vs Mountain Bike tires. I hope that makes sense. Flame Away." Site is tinyurl.com/kmtw3fb FWIW Last edited by: NIL on Sat 16 Nov 13 at 18:53
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New Sat 16 Nov 13 20:44
Bibendum - bienvenu - Zero
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| I run mine at the recomdeded pressure. Don't seem to have had any handling/braking/comfort/wear problems yet. |
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New Sun 17 Nov 13 14:06
Bibendum - bienvenu - WillDeBeest
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Small update from me given that I've had a week to digest NC's explanations of handling in terms of stiffness rather than slipping. I think I'm getting the idea of that now. I mentioned when I had the new tyres fitted that the old Nexens had 5mm left, but closer inspection today reveals that's only in the centre - consistent with having been on the front at lower pressure for at least 20,000 miles. The inner and outer edges are closer to 3 than 4. Given that they're now at the back and inflated to wear the centre more, this may not be entirely a bad thing, but my inclination is now to replace them with another pair of Primacy 3s sooner rather than later - probably much sooner - and effectively start from zero with four fresh tyres. Manatee will be pleased, I hope. |
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New Sun 17 Nov 13 14:37
Bibendum - bienvenu - Manatee
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>>Manatee will be pleased, I hope. He is most relieved. |
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New Sun 17 Nov 13 15:55
Bibendum - bienvenu - Runfer D'Hills
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I'd get them on now WDB unless the contents of your piggy bank have been earmarked for more urgent use. If the new ones last anything like they do on mine you'll get well north of 35k miles out of them anyway. Winter coming etc. |
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New Fri 1 Aug 14 13:03
Bibendum - bienvenu encore une fois - WillDeBeest
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Just re-read this thread because the Nexens finally went yesterday and there are now eight Michelin Primacys on my drive, half of them on the LEC. The Nexens on the rear were still legal, and probably had nearly 4mm in the centre, but the edges were more worn from their time on the front (which drew an advisory at the MoT in March) and with a long holiday trip coming up it made sense to get the job done. No protests from the tyre pressure system this time, so perhaps the fitter knew how to reset it. Some time today - wasn't time yesterday - I'll check the pressures, and see how, and how much, the front pair have worn in 8,000 miles since November. |
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