if that is the "ld50" equivalent, (i.e. 100 or whatever paracetamol tablets will kill 50% of the people who take that overdose)
Take the point though not a good analogy, fact is that Paracetamol is dangerous in quantities only slightly above the recommended dose, adults can take 8 x 500mg in 24 hours though 12 can be harmful and 16 potentially lethal!
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paracetamol is a bad way to go as well, as you will normally regain conciousnous in hospital, but your liver is dead, and you will often come to regret the decison to kill yourself in the week before you die from liver failure, sad cos its soul destroying for the medics
such deaths could be eliminated by the simple addition of the antidote to all paracetomal pills, which does not affect their effectiveness, but would stop the deaths, this is done in many countries - not here cos of the minor cost increase it would cause
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It is not so much the engine torque that is the issue but the braking loads.
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It is not so much the engine torque that is the issue but the braking loads.
Engine torque is a major factor.
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Big MPVs with direct injection diesel engines produce high torque and, presumably, will have high braking loads, but still have tyres with relatively low speed ratings.
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>> It is not so much the engine torque that is the >> issue but the braking loads. >> Engine torque is a major factor.
cheddar,
Very interesting - I have not seen this before. Could you tell me more about it?. How did you find out about this? Have you any pointers to further info - technical references and so on?
Number_Cruncher
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How daft that tyres come in millimetre (metric) widths but fit on to inches (imperial) rims?
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Mattster
Boycott shoddy build and reliability.
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Citreon C8 2.2 HDI with 232 lb ft of torque and top speed of 113 mph - recommended speed rating of tyre is H. Not much to do with torque I think.
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How daft that tyres come in millimetre (metric) widths but fit on to inches (imperial) rims? --
I've no doubt that the tyre industry would prefer to be fully metric but they obviously realise that there are too many imperial diameter wheels in existence to be able to do that. A few millimetres difference in the width of a tyre relative to the width of the wheel is probably neither here nor there, but the correlation between tyre and wheel diameter has to be very close. I assume therefore that, for the time being at least, wheel and tyre diameters will remain imperial. Provided that everyone adopts the same standard there shouldn't ever be a problem.
--
L\'escargot by name, but not by nature.
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>> Engine torque is a major factor. >> cheddar, Very interesting - I have not seen this before. Could you tell me more about it?. How did you find out about this? Have you any pointers to further info - technical references and so on? Number_Cruncher
Hello NC,
It is really kinda common sense, the stresses and strains that a tyre is subject to when a modern 350nm 2.0 ltr TD hits it's torque peak is similar to 4.0 V8 petrol engine producing the same amount of torque although the top speed of the TD might be 30 mph less.
Regards.
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Mind you, all my previous comments in this thread have been ignored, so perhaps I am back to invisible postings again.
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Hi cheddar,
Thanks for the reply. However, do you know of anywhere where this information is published?
The reason I ask is that I understood that the speed rating was based on purely thermal considerations. As the contact patch comes into contact with the road, it deforms (more so if the vehicle is laden). This deformation requires an amount of elastic (potential) energy per unit angle rotation of the tyre. Therefore, as the rotation speed increases, a proportion of the work done (the proportion being due to the hysteretic losses inherent in rubber parts) in elastic deformation is an effective heat input to the tyre, which may be measured in Watts per m/s of forward speed. The temperature of the tyre is then a function of the vehicle speed, and the effective conductance of all of the available heat transfer mechanisms away from the flexing parts of the tyre.
High temperatures ruin tyres very quickly. Running tyres on too low a pressure increses the deflection, and increases heat input into the tyre markedly. Most tread caps that litter the hard shoulders are caused by temperature related disbonds.
Based on this, during design, one can trade some reduction in top speed for an increase in load capacity - I think there is a paragraph mentioning this design trade off in the Bosch Automotive Handbook.
In the higher gears, during higher temperature operation, the tractive effort isn't usually a dominant tyre loading mechanism.
Number_Cruncher
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Absolutely amazing, I give a practical, real world example, and it is totally ignored! I am definitely in invisible mode!
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Calm down, calm down, machika :-).
H tyres are good for 130 mph, so that's 15% more than your notional top speed, so couldn't that account for the torque factor?
Or have I misunderstood your point?
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Calm down, calm down, machika :-). H tyres are good for 130 mph, so that's 15% more than your notional top speed, so couldn't that account for the torque factor? Or have I misunderstood your point?
>>
Personally, I treat a speed rating as just that, safety margins being taken into account too. Big MPVs just have relatively lower top speeds than a saloon with the same engine. The heavier weight is taken into account with the bigger load rating required for tyres fitted on MPVs.
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Hi NC,
You are looking at this much more deeply than any of us and, I think, from a different angle as well, your comments explain clearly and technically factors that effect the rating that a specific tyre might be given by the tyre manufacturer. My point is that given a wide gamut of tyre manufacturers, rim diameters, aspect rations, tread patterns wear rates a car manfacturer will take into account a number of factors when selecting a tyre for a particular model in addition to the regulated load and speed ratings, it is clear to me, despite Machika's example, that high torque cars are generally fitted with tyres that seem to have an excessive speed rating relative to the cars top speed.
I am not sure that Machika's example is typical because the C8 being a large MPV will have tyres with a relatively high load rating, the load rating being the other regulated rating along side speed.
No, I don't have access to any specific technical info on this.
Regards.
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Hi cheddar,
I think you are spot on when you say there are a wide range of factors affecting the final tyre choice for any particular vehicle.
Forgive the awful pun, but tyres are definitely a 'black' art!
However, Machika's point (which I hadn't realised the value of - sorry!) is that a car like an MPV is more likely to be heavily laden. This may be an example of a speed/load trade off.
Number_Cruncher
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I am not sure that Machika's example is typical because the C8 being a large MPV will have tyres with a relatively high load rating, the load rating being the other regulated rating along side speed. No, I don't have access to any specific technical info on this. Regards.
>>
I thought that the factor being discussed was the speed rating. A big MPV will obviously require tyres with a load rating to suit but, despite having diesel engines with a lot of torque, they don't need tyres with a very high speed rating, simply because their top speed is relatively low (usually below 120 mph).
Put a few examples into the tyre selector on the Michelin UK site and see what results you get. The speed rating recommended for the 2.2 16V petrol version of the C8 is also H, despite it having a lot less torque.
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Hi Machika,
I suggest that the reason a big MPV is not typical is that the high load rating of the tyres required by such a vehicle accounts for the torque issue where as on a conventional car the same size and load rating might be required for two models though the one with the higher torque output, albeit not higher top speed, might well have higher speed rated tyres.
For instance the Michelin selector quotes a 1.6 Focus as 84T's though a TDCi Focus as 86H's.
My main point is when selecting new tyres for a car take into account more than size and whether the speed rating accounts for the cars quoted top speed.
Regards.
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Hi Cheddar,
>>for instance the Michelin selector quotes a 1.6 Focus as 84T's though a TDCi Focus as 86H's
Might that not be because of the extra mass of the diesel engine, rather than the extra torque?
Number_Cruncher
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>>for instance the Michelin selector quotes a 1.6 Focus as 84T's though a TDCi Focus as 86H's Might that not be because of the extra mass of the diesel engine, rather than the extra torque?
Yup, could be though it is a fact that cars that produce high torque output are generally fitted with tyres that have an excessive speed rating relative to the cars top speed.
Probably exhausted this subject now.
Regards.
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>> >> >>for instance the Michelin selector quotes a 1.6 Focus as 84T's >> though a TDCi Focus as 86H's >> >> Might that not be because of the extra mass of the >> diesel engine, rather than the extra torque? >> Yup, could be though it is a fact that cars that produce high torque output are generally fitted with tyres that have an excessive speed rating relative to the cars top speed.
The fact that a diesel engine generally has a higher maximum torque than a petrol engine of a similar capacity doesn't necessarily mean that it has a higher torque at the wheels. Diesels usually have a higher gearing because of the lower maximum engine speed and this reduces proportionally the torque at the wheels.
--
L\'escargot by name, but not by nature.
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The fact that a diesel engine generally has a higher maximum torque than a petrol engine of a similar capacity doesn't necessarily mean that it has a higher torque at the wheels. Diesels usually have a higher gearing because of the lower maximum engine speed and this reduces proportionally the torque at the wheels. -- L\'escargot by name, but not by nature.
Sorry l'escargot though that is quite, quite wrong!
Yes diesels are higher geared though the fact that they produce maximum torque at much lower revs more than accounts for this thus the reason why a TD pulls like a train and a petrol needs revs to keep up, 350nm through the front wheels in 2nd gear out of a rutted country lane corner is asking a lot from the tyres.
Regards.
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To clarify, if a TD had no more torque at the wheels it would not pull any more strongly at low revs, it does pull more strongly at low revs because it's extra torque is translated to the wheels despite the higher gearing.
2.0 Petrol = max torque (140lb) at 3500, top gear = 25mph per '000 rpm, 60 mph = 2400, 1100 rpm short of max torque.
2.0 TD = max torque (250lb) at 1800, top gear = 35mph per '000 rpm, 60 mph = 1700 rpm, 100 rpm short of max torque, and a much higher max torque at that!
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Sorry l'escargot though that is quite, quite wrong! Yes diesels are higher geared though the fact that they produce maximum torque at much lower revs more than accounts for this thus the reason why a TD pulls like a train and a petrol needs revs to keep up, 350nm through the front
> wheels in 2nd gear out of a rutted country lane corneris asking a lot from the tyres. Regards.
Cheddar,
L'escargot makes a valid point, particularly in how the gearing affects the torque.
The quasi-static loading that you describe in your country lane example is nothing at all to do with speed rating. This is simply a tyre strength requirement. There is no large, potentially tyre damaging, heat input under these transient conditions - therefore, there is no link with the tyres' speed rating.
I know that you beleive the link between torque and speed rating to be true. However, so far, I am completely unconvinced.
Number_Cruncher
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Do not high performance, small capacity, motorbikes(with no torque to speak of) still have high speed rated tyres?
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L'escargot makes a valid point, particularly in how the gearing affects the torque.
As I said before diesels are higher geared though the fact that they produce maximum torque at much lower revs more than accounts for this difference in gearing. If a TD had no more torque at the wheels it would not pull any more strongly at low revs, it does pull more strongly at low revs because it's extra torque is translated to the wheels despite the higher gearing. I.e. the difference between the gearing is less than the difference between the revs that the max torque is produced at.
Fact.
The quasi-static loading that you describe in your country lane example is nothing at all to do with speed rating. This
is simply a tyre strength requirement. There is no large, potentially tyre damaging, heat input under these transient conditions - therefore, there is no link with the tyres' speed rating.
Tyres with a combination of higher speed rating and load rating will be stronger and built to cope with higher torque input and braking loads because they are designed to be fitted to larger, heavier and/or more powerful vehicles.
I know that you beleive the link between torque and speed rating to be true. However, so far, I am completely unconvinced.
Again as I said before there are only two regulated factors, load and speed rating therefore it is a combination of these two that a car manfacturer will take into account when selecting a tyre for a particular model.
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Cheddar,
As I said before...
Yes, missing the point then, as now.
The speed of the crankshaft when an engine produces max torque has no relevance for the tyres at all.
All you needed to say was the extra torque is more than enough to compensate for any ratio change.
Tyres with a combination of higher speed rating and load rating will be stronger ...
Tyres with both increased speed and load rating will be stronger - yes. Speed rating on its own, however, isn't directly related to the strength of a tyre.
Again as I said before...
Repeating yourself doesn't make what you say correct.
Number_Cruncher
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Yes, missing the point then, as now.
Lets, not get personal NC, it was my point how can I miss it.
The speed of the crankshaft when an engine produces max torque has no relevance for the tyres at all.
I mentioned the speed of the crankshaft in the context of the comment refering to diesels being higher geared and the fact that the lower crank speed accounts for this higher gearing.
All you needed to say was the extra torque is more than enough to compensate for any ratio change.
>>
No, that is not all I need to say! It is not only the fact that the diesel produces more torque, it is the fact that it is produced at lower revs that compensates for the higher gearing.
Tyres with a combination of higher speed rating and load rating will be stronger ... Tyres with both increased speed and load rating will be stronger - yes. Speed rating on its own, however, isn't directly related to the strength of a tyre.
>>
Agreed!!
>> Again as I said before... Repeating yourself doesn't make what you say correct. Number_Cruncher
Practicing in the art of condescension does not make you correct either NC, it just adds an unwelcome edge to what is generally combative though friendly repost.
Is it not a fact that load and speed are the two regulated factors in respect of tyre suitability?, therefore it must be a combination of these two factors that car manfacturers take into account when selecting tyres for a particular model.
Regards.
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>>the fact that the lower crank speed accounts for this higher gearing.
???
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>>the fact that the lower crank speed accounts for this higher gearing. ???
OK. The point here is that although a turbo diesel car is generally higher geared than an equivalent petrol engined car the fact that it produces it's max torque at lower revs (crank speed) more than compensates for this higher gearing hence why a TD has lots of get up and go without cahnging down and being revved and has generally good in-gear acceleration times.
I should point out that I am not critising petrol engines, just comparing charateristics, I am as much of a fan of a crisp free revving petrol engine as anyone.
Regards.
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That may or may not be correct, it may or may not be what you meant, but it was not what you said.
How can lower crank speeds account for higher gearing ?
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Actually, scratch that question.
This discussion has got quite silly enough without me adding to it. And lets stop with the personal comments please.
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Actually, scratch that question.
Sorry, answered above.
This discussion has got quite silly enough without me adding to it.>>
Agree, I mentioned in an earlier post a few days ago that I thought it had gone far enough, more the fool me for carry it on.
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>> Actually, scratch that question. >> Sorry, answered above.
... er below!
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That may or may not be correct, it may or may not be what you meant, but it was not what you said.
>>
It was exactly what I said, in addressin NC's comment I was making it clear that it is not simply the fact that a TD is more torquey, rather it is the fact that it produces it's torque at lower revs that is the point.
How can lower crank speeds account for higher gearing ?
If power and torque are produced at lower revs, lower crank speeds, then gearing can be higher.
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Referring back to Cheddars use of the Michelin tyre selector, he quoted the Focus 1.6 and 1.6 TDCI I think. The fact is that there are different versions of these engines. The lower powered (100 ps) petrol engine has a top speed of 112 mph and could conceivably be fitted with T rated tyres (118 mph limit). However the more powerful 115 ps engine has a top speed of 118 mph and would, I imagine, require H rated tyres. The same would apply to the different TDCI engines, one of which is 90 ps (top speed 111 mph) and the other 109 ps (top speed 117 mph).
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I have just checked with the tyre selector and it is not up to date with the Focus range, as it does not include the 1.6 TDCI.
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I have the 1.6 petrol focus with a top speed of 118mph, my tyres are V rated, best part of 150mph.
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Reckon we are coming back around to the economies-of-scale point discussed right at the start of this thread.
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I think the economies of scale point makes a lot of sense.
Number_Cruncher
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I have the 1.6 petrol focus with a top speed of 118mph, my tyres are V rated, best part of 150mph.
Alloy wheels and low profile tyres, no doubt?
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But, of course, they don't have to be replaced with V rated tyres when you change them.
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But, of course, they don't have to be replaced with V rated tyres when you change them.
Best to tell yout insurance company if you change away from the original size and/or rating.
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Its a company car with standard 15" alloy's, not low profile (65) and always replaced like for like.
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Having thought about it one of the biggest factors (tongue in cheek) is that lower profile tyres tend to be made in higher speed ratings. The car manufacturers tend to fit lower and lower profile tyres (fashion) and hence cars have over-rated tyres.
Anyone know where I can buy a 225/40R 16 rated at just 110mph?
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Having thought about it one of the biggest factors (tongue in cheek) is that lower profile tyres tend to be made in higher speed ratings. The car manufacturers tend to fit lower and lower profile tyres (fashion) and hence cars have over-rated tyres. Anyone know where I can buy a 225/40R 16 rated at just 110mph?
Very good point.
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