[SS-Traveller]

Originally Posted by FlatOut

...very skinny tyres...stop more quickly...
The laws of physics...

Hi,
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...

[DerAlte]

Originally Posted by SS-Traveller

... skinny tyres (on any car) will perform much better in terms of grip/stopping distances in snow than wide treads? ...

Doesn't stand to reason!?

* 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

Originally Posted by FlatOut

... The laws of physics are immutable and quality engineering will always shine through.

Very true, but ...

Originally Posted by FlatOut

... ABS system is not able to hold the tyre at that point - instead it is pulsing between the lock up point and rotating at normal speed, so on average the braking effort isn't quite as high as the potential maximum and in reality this is a little lower than when the tyres are locked. ...

Perhaps it would be better if we were not empirical? The pulsing occurs at 20-50Hz, with a variable duty cycle. When ABS is controlling braking, it is allowing the wheel to lock-up (max. brake force) for some of the time. The balance of a cycle it releases the brake a little (it is never <50%). The picture looks different over 50milliseconds than over 5 seconds. But, focusing on braking distance is taking away the main objective of ABS: creating a situation where the driver can retain steerability of the vehicle.
* 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.

Originally Posted by FlatOut

... Both cars stopped more quickly than a BMW M5, in all circumstances. Similarly a seventy-year old design with very skinny tyres , a Citroen 2cv ...

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!

[FlatOut]

Originally Posted by SS-Traveller

Hi,
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...

This is not a simple answer to explain fully, I will try though. For a start, a wider tyre will have to compress more snow if it isn't already and this needs more energy, to the point where the force to move forwards with really wide tyres would induce wheel spin and you wouldn't go anywhere. So in virgin snow which is more than half and inch deep, the narrower tyre has less resistance to overcome.

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.

Originally Posted by DerAlte

* 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

Yes, grip is as you say but are you assuming, in your next statement that the contact patch size is proportional to tyre width? As I mention above, this isn't the case if the vehicle mass and tyre pressure remain the same.

Originally Posted by DerAlte

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!

I think you are saying there is more to a car's abilities than individual numbers. Yes, that is true, but when there is such an anomoly as the one I mention with a modern VW and an ancient Citroen which was the most basic car, designed in WW2 and intended for peasant farmers, it is worthy of investigation and consideration.

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."

[DerAlte]

Originally Posted by FlatOut

... The other widespread misconception is that a wider tyre means there is more tread on the road ...

I never could get the "forest for the trees" saying right, and I always stay clear! We are getting lost in a forest of words, aren't we?

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!

[Sutripta]

Originally Posted by FlatOut

No definitive explanation, Sutripta, but it's the case as far as I am aware - a friend and myself did back-to-back tests a few years ago on three different cars and there are other similar results to confirm my findings.
....
The laws of physics are immutable and quality engineering will always shine through.

Hi,
I think I should have made my focus clearer.
When you say

A car will stop a little more quickly with all four wheel locked up on dry tarmac than with ABS deployed.

it comes across as a definitive, always true, no exceptions, no qualifiers statement. In which case it should have some fundamental theoretical underpinnings. Which is what I was wanting.

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

[Sutripta]

Originally Posted by DerAlte

Skinny tyres would increase the braking distance on *any* surface compared to a fat tyre

Some figures (preferably published, not internet blogs etc) please. Then we can get into making sense of those figures!
Received wisdom (old wives tales?) supports SS/ FO. But can be another one of those 'everyone knows' which happens to be wrong!

Originally Posted by DerAlte

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).

Could you guide me as to what I'm supposed to concentrate on. Admittedly a cursory reading, but could not find anything relevant to current discussion.

Regards
Sutripta

[DerAlte]

Originally Posted by Sutripta

Some figures (preferably published, not internet blogs etc) ...

Alas, 15 years too late to retrieve them <hanging head dejectedly>.

Originally Posted by Sutripta

... Could you guide me ...

Do be serious, Prof! Show light to the sun? :O

[dhanushs]

Originally Posted by DerAlte

* 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

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?

[FlatOut]

Originally Posted by DerAlte

We are getting lost in a forest of words, aren't we?

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 )?

Well, I have been very gentle and have used as much plain English as possible. The discussion here is quite simple and straightforwards (easily understood by anyone who is interested in driving motor cars), if you don't want to put your thoughts forward, fair enough, but it's unlike you DerAlte! It's quite interesting and not at all complex, except for our own brain's psychology when it comes to challenging long-held presumptions!

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.

Originally Posted by Sutripta

Hi,
I think I should have made my focus clearer.
When you say

it comes across as a definitive, always true, no exceptions, no qualifiers statement. In which case it should have some fundamental theoretical underpinnings. Which is what I was wanting.

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

Originally Posted by Sutripta

Some figures (preferably published, not internet blogs etc) please. Then we can get into making sense of those figures!
Received wisdom (old wives tales?) supports SS/ FO. But can be another one of those 'everyone knows' which happens to be wrong!

Could you guide me as to what I'm supposed to concentrate on. Admittedly a cursory reading, but could not find anything relevant to current discussion.

Regards
Sutripta

I gave you my answer above and answered with candour and example, Sutripta. I am not going to turn this forum into a Masters Degree dissertation throwing contest, with little discussion amongst this community. That would be tedious, boring for most and few would have the time to read it.
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.

[gthang]

Originally Posted by FlatOut

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.

Hi FlatOut,

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.

[SS-Traveller]

Originally Posted by FlatOut

...when it comes to challenging long-held presumptions!

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.
So wider tyres have a similar contact patch area to narrower ones.
The answer as to why a wider tyre feels to corner better can be answered elsewhere, I'm trying to save words here.

Thanks for the other thread too. The interesting thing to consider would be what happens when braking AND cornering - especially, what the front outside tyre does if it's a narrow tread or a wide one. IMO a wider tyre ought to outperform a narrower tyre in this situation with higher levels of grip and without allowing ABS to kick in. Real world scenario being that cornering + braking frequently go together, a wider tyre on tarmac ought to outperform a narrower tyre around corners.

Originally Posted by FlatOut

I am not going to turn this forum into a Masters Degree dissertation throwing contest, with little discussion amongst this community. That would be tedious, boring for most and few would have the time to read it.
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.

Boring for most need not be boring for all. No contests either. At least some of the threads/posts on the Technical Stuff forum are not purely beginners' stuff, and some of us do understand fairly advanced automotive (and other) engineering. So my request would be, don't go on the back foot, FlatOut, and let's have a healthy discussion - as you said...

Originally Posted by FlatOut

Let's debate without letting personal pride get in the way.

+1 to that.

Originally Posted by FlatOut

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.

My current signature (below this and every post of mine) supports your contention.

Absence of evidence is not evidence of absence.

[DerAlte]

Originally Posted by dhanushs

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?

Intuitively your expression sounds right.
* 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

Originally Posted by FlatOut

Well, I have been very gentle and have used as much plain English as possible. ...

So was / did I, though my expression was indirectly about the ABS thread not being the right place to discuss tyre profiles, braking distances and suspension geometry. If there is a ton of discussion on an unrelated topic, it would be difficult for the average reader, who comes here to figure out whether he should buy the model with ABS or not, to know that ABS actually works as stated. Or he doesn't have to wade through a lot of verbiage to discover "it is one huge safety technology for a motor vehicle", as you stated.

Thank you for creating that new thread.

Originally Posted by FlatOut

... The university paper you have linked to is exceedingly vague, generalised and just an exploratory bit of writing demonstrating what can be done ...

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?

[Sutripta]

Originally Posted by Sutripta

So that statement: What is it?

Regards
Sutripta

Originally Posted by FlatOut

I gave you my answer above

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

[FlatOut]

Originally Posted by gthang

Hi FlatOut,

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.

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.

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!

Originally Posted by SS-Traveller

Thanks for the other thread too. The interesting thing to consider would be what happens when braking AND cornering - especially, what the front outside tyre does if it's a narrow tread or a wide one. IMO a wider tyre ought to outperform a narrower tyre in this situation with higher levels of grip and without allowing ABS to kick in. Real world scenario being that cornering + braking frequently go together, a wider tyre on tarmac ought to outperform a narrower tyre around corners.


Boring for most need not be boring for all. No contests either. At least some of the threads/posts on the Technical Stuff forum are not purely beginners' stuff, and some of us do understand fairly advanced automotive (and other) engineering. So my request would be, don't go on the back foot, FlatOut, and let's have a healthy discussion - as you said...

+1 to that.

My current signature (below this and every post of mine) supports your contention.

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.

[Jeroen]

Not sure where this discussion is going.