9th May 2013, 10:03 | #481 | |
Distinguished - BHPian | re: The ABS discussion thread Quote:
Do I understand it correctly (and as I remember reading somewhere) that skinny tyres (on any car) will perform much better in terms of grip/stopping distances in snow than wide treads? Don't quite recall all the physics of it though... | |
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9th May 2013, 10:21 | #482 | |||
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| re: The ABS discussion thread Quote:
* Retardation due to braking is proportional to Mu * Tyre contact area. Skinny tyres would increase the braking distance on *any* surface compared to a fat tyre * Conversely, for efficient torque transfer, one needs larger tyre contact area to achieve greater grip and lesser slip * Fuel efficiency, OTOH, would suffer with a fat tyre. Skinny tyres = lesser rolling resistance = higher fuel efficiency Quote:
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* If a crash into something is imminent, crash evasion is a better option than hoping that the car stops before crashing * Prior knowledge of the braking distance of a car doesn't help in a braking situation, since the driver is unable to measure distance while braking - which could lead to a. triggering the driver to brake at the time that the car would stop within that distance, and b. better confidence in the situation, and lesser panic Since humans intuitively handle situations based on adaptive control (not braking enough - press brake more) rather than absolute forces and distances, crash evasion is then a better option than hope. ABS just prevents one from taking the 'hope' path. One can always *not* do that by getting a car without ABS. End of the day, your previous comment "it is still the individual human brain which works best, when used" makes more sense than absolute values. Though traditionally braking distance has always been mentioned in car reviews (at least used to be), wouldn't it be illogical to compare braking of cars? Wouldn't we be ignoring the basic influence on braking dynamics - car weight dependent Momentum of the car? Controlled 'journalistic' tests (and one never tests cars from different classes even then) always take braking data from specific speeds, but no one projects "Car X which stops in 140' from 60mph, is safer than Car Y which stops in 170'", do they? They actually leave it to the level of paranoia of the reader! Last edited by DerAlte : 9th May 2013 at 11:23. | |||
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9th May 2013, 16:51 | #483 | |||
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| re: The ABS discussion thread Quote:
But here is the interesting explanation, which applies for all slippery surfaces and is very noticeable on compacted snow. A narrower (which usually means slightly taller) tyre will have a contact patch which is more longitudinal than lateral - ie it will lie in line with the car, whereas a wider tyre will have a contact patch which is shorter front to back than side to side. The more longitudinal patch will create more deformation of the sidewall and a given section of tyre tread will be in contact with the ground for longer, moulding itself better to the surface. There will be more grip available for acceleration and braking than a wider tyre with a short front-to-rear contact area. I am taking about sensible tyre dimensions here - if you go to either extreme you induce unwanted characteristics. The other widespread misconception is that a wider tyre means there is more tread on the road - I think DerAlte thinks this is the case. For a vehicle of a given mass with tyre pressures the same, a wider tyre simply alters the shape of the contact patch from the fore-aft one to a side-to-side one. Quote:
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I'm quoting here from the weblink I provided above: "The best way to check these things is to do some testing. High quality testing is done on proving grounds. On November 28th 2002, a part of the Rotterdam (NL) police force goes to the NIV (Nederlands Instituut Verkeersveiligheid or Dutch institue for traffic safety) track in Hellevoetsluis (NL). Here these guys and girls will get advanced driving tuition. All tests and tuition on the day will be done with VW Golf TDI and VW Transporter TDI. Both vehicles are nearly new, have continental tyres with 6-7mm thread on them. One of the guys has brought his 1957 2cv AZU (van), equipped with Michelin 125x15 tyres 4-5mm thread at the front, 2-3mm at the rear, 1981 onwards 2cv braking system with disc brakes at the front, drums on the rear. At first we'll compare braking distances, after a emergency stop from 40km/h on a special slippery surface. As expected the the VW Golf have a shorter stopping distance compared to the VW Transporter. Then 2 Transporters are compared, one had Continental Vanco tyres, the other Coninental Cotrans. The Transporter with Cotrans tyres had a shorter stopping distance. Next the same test on the same surface is done with the 2cv AZU. With locked up wheels and to everybody's supprise the 2cv has a shorter stopping distance compared to the VW Golf and Transporter. This test is repeated but this time special care is taken not to lock up the wheels. The braking distance is shortened by yet another few meters. It is shocking to all participants and especially the instructors that the 2cv can stop so quickly compared to the much more modern ABS equipped VWs! The next excersize is stopping with the left wheel on slippery surface and the right wheels on not slippery tarmac from 40km/h and 70km/h. Here ABS proves it's point and the VWs brake in a good straight line as expected. Up to 60km/h 2cv does very good again, it pulls slightly to one side but is easily corrected. Again the braking distance of the 2cv is much shorter then that of the VWs. At 70km/h the 2cv spins 360 degrees, something that would have happened to every car with no or malfunctioning ABS. Next up, handling test. With orange cones a curve is laid out on the slippery surface of the track. At 30km/h the VWs are out of control and the cones start flying. The 2cv has no control problems at 30km/h, 40km/h and 50km/h. There isn't even a hint of understeer. At 60km/h some understeer is felt but still the cones are left alone as the 2cv goes through the curve. As a last attempt to get the 2cv to spin the throttle is lifted mid corner, hoping to provoke oversteer. This fails, the 2cv still goes through the curve fully controlable without oversteer. The little 2cv van has received respect from the instructors and co-workers for doing such a good job. It's amazing that the little 2cv conquered the expensive VWs with their high tech stuff and many years of computer development." | |||
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9th May 2013, 17:36 | #484 | |
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| re: The ABS discussion thread Quote:
Here is a research paper that puts things together on the tyre patch (forget the fact it is about optical analysis, there is enough stuff there incl. pics). There are a couple of Mech. Engg. professors on the forum, may be they could explain it to ignoramuses like me (before I make a spectacle of myself )? 491723.v2.pdf +1 on Deux Chevaux - someone / some people got it exactly right! | |
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9th May 2013, 21:24 | #485 | ||
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| re: The ABS discussion thread Quote:
I think I should have made my focus clearer. When you say Quote:
Just by itself, it is a (personal) opinion. Perhaps based on (personal) observations. Can be the starting point of a hypothesis. But a hypothesis is not a law. So that statement: What is it? Regards Sutripta | ||
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9th May 2013, 22:26 | #486 | ||
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| re: The ABS discussion thread Quote:
Received wisdom (old wives tales?) supports SS/ FO. But can be another one of those 'everyone knows' which happens to be wrong! Quote:
Regards Sutripta | ||
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9th May 2013, 23:31 | #487 |
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10th May 2013, 00:04 | #488 |
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| re: The ABS discussion thread
Also, doesn't vertical reaction per unit area have an effect? If so, with skinny tyres, in a terrain like snow/gravel, wouldn't it dig into the gripper surface below (tarmac)faster, and hence better braking? |
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10th May 2013, 03:36 | #489 | |||
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| re: The ABS discussion thread Quote:
The point being that most people think a wider tyre puts more rubber on the road, ceteris paribus, and is supposed to be the reason why a car feels to corner better with wider tyres. I am questioning this assumption, suggesting it is a false one. The university paper you have linked to is exceedingly vague, generalised and just an exploratory bit of writing demonstrating what can be done. It concludes very little and is of little direct relevance to this point. As I see it, tyre contact area is directly proportional to mass (weight of car on the wheel) and inversely proportional to pressure (in the tyre). So whether your tyre is wider or narrower (given we are talking here in the real world about tyres which are size-suitable for a given car and have similar rolling diameters ie they are useable), the area of tyre on the ground will remain the same for a given tyre pressure and mass of vehicle. So wider tyres have a similar contact patch area to narrower ones. This sounds really odd, but it would help explain how a car with very skinny tyres out braked and otherwise outperformed a modern one in controlled conditions, as described in my post above. The answer as to why a wider tyre feels to corner better can be answered elsewhere, I'm trying to save words here. Quote:
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Besides, this is a divertion and a minor, insignificant point which is for interest as an aside (and a useless, if interesting fact) rather than cause for pages and pages of disagreement. I also do not want to create the idea in some heads that ABS isn't a good idea - it is one huge safety technology for a motor vehicle. As all scientists do, I have gone out into the field and carried out my own trials, to my own satisfaction, making all my observations. I simply posted them on here, and mentioned that others have found the same. They didn't call in the Guinness Book of Records or some official validation, it was for personal interest. If you don't think my findings would be the case in most situations, then say why you think this is so. It could be that ten years ago, ABS wasn't as good as it is today. It could be that the cars involved were very well engineered, especially the suspension. They could stop more quickly than every other vehicle I could test alongside, by quite a margin. I have noticed that cheaply-engineered cars' ABS tends to kick in much sooner than I would expect, I'm also aware that different ABS systems are set up differently, with regard to longitudinal wheel slip threshold - ie how much wheel slowing they allow before releasing the brake. More means less controlled steering when the ABS is in operation, less means better steering control but slightly less retardation. I have heard some cars have linked their yaw sensors to the ABS operation to try and improve braking performance. Let's debate without letting personal pride get in the way. I mentioned the energy involved in compliance bushing which could tend to work against the pulsing brake, I mentioned the heat build up with a locked tyre, which could generate more grip than a colder tyre. (In a hot country this wouldn't necessarily work in the same way.) And also the fact that the tyre doesn't remain at its optimum point of retardation in the pulsing cycle. Lab tests are by nature clinical and cannot take into account all the variables in the field - what they do is help us understand what is actually happening in the real world. And the real world is rarely black or white. | |||
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10th May 2013, 07:57 | #490 | |
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| re: The ABS discussion thread Quote:
So, do you just go Flat Out while typing, or driving as well? You have documented a lot of the theories which I also have felt, experienced, and learned along the way having spent some considerable time driving in extremes like ice logging roads in Northwest Canada during the winter, and the ensuing slush at spring break. Many things were learnt from getting stuck, spinning out, advice from experienced drivers, etc. Conventional wisdom can seem entirely senseless in extreme conditions. Since you are new to this "informal" forum, you would not have noticed that disagreeable points will be questioned and must be substantiated with satisfactory research findings to which you might get a vague and cryptic reply. That shows immense knowledge of the subject. Hope such distinguished speed-breakers dont stop you from going Flat Out in your sharing of information. Cheers. | |
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10th May 2013, 09:44 | #491 | ||||
Distinguished - BHPian | re: The ABS discussion thread Quote:
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+1 to that. Quote:
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Last edited by SS-Traveller : 10th May 2013 at 09:46. | ||||
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10th May 2013, 11:36 | #492 | ||
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| re: The ABS discussion thread Quote:
* Depends on temperature. If snow would melt (higher pressure, and the water get pumped away by the tread) then yes perhaps this might reach the tarmac. If the temperature is low, and the conditions such that the snow turns to ice (compaction, freezing), then no it won't * Different action in gravel, where it would depend on gravel being physically displaced. I would think it wouldn't happen on gravel since gravel particles would roll but stay generally in place. The tyre is still not thin enough (think knife in gravel) to physically displace gravel particles sideways Quote:
Thank you for creating that new thread. Your comment would be justified if you are looking for direct answers in the paper. You have overlooked that the paper describes mathematically all the relevant parameters in the discussion here, and had illustrations of the tyre-road contact patch. Perhaps the paper had triggered the dormant hatred all of us have for ... text books? | ||
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10th May 2013, 20:56 | #493 |
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| re: The ABS discussion thread
Must have missed it somewhere! I mean, what is that statement:- Opinion, observation, always has to be true statement of fact etc. We can discuss the whys and whereofs after we know what we are discussing. Regards Sutripta |
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11th May 2013, 02:14 | #494 | ||
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| re: The ABS discussion thread Quote:
Thanks gthang, I made your post the first for me to 'thank' - then I saw you have more thanks than posts! You must either be good at flattering other members of this forum or write a huge amount of sense, probably both! I agree, there is nothing like real experience in harsh conditions, whether in a 24 hour race or as you describe. I have been with very well (academically) qualified engineers in such conditions and they so often are at a loss to explain why the reality doesn't in any way fit their models. It is the easiest thing to do, to quote research papers, but like newspapers, just because it is written somewhere doesn't mean it is right, rather that most can be convinced of what they have written. Politicians tend to do a lot of statistical quoting in their arguments, and they are usually wrong! Quote:
If we are going to quote academic research, then we need to find the best there is and explain why it is. Then quote from it to substantiate our claims. There is a list of links to academic research on ABS on this page, http://bit.ly/10gE2tl, search for Robert Farago's reply. I do not necessarily agree with what this website has to say on anti lock brakes. I reiterate that I believe they are a massive safety aid. Mmm, braking and cornering - everything I was taught to avoid, especially in cold and wet (not altogether true!) Britain. It destabilises a car by shifting all sorts of complex forces and asks a lot of the tyres and suspension. We haven't mentioned tyre hysteresis here yet, and I'm trying to keep it out of the discussion, for now at least. My thinking is that there are some conditions where braking through corners will be safer and better with wider tyres, some when it definitely isn't. Did I hear the word antiroll bar? No? Thank goodness. | ||
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11th May 2013, 07:14 | #495 |
Distinguished - BHPian | re: The ABS discussion thread Not sure where this discussion is going. Last edited by Jeroen : 11th May 2013 at 07:22. Reason: punctuation |
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