I was idling thinking recently, having been reading about relative braking distances of winter and summer tyres, and the effects of temperature, that there is something that I can't puzzle out.
For any given car with a given tyre size, assuming the ABS doesn't cut in, and assuming the wheels don't skid, then I can't see how braking distances will be affected by whatever tyres are fitted because they will be turning at the same speed and covering the same distance. For each metre the car moves forward, the equivalent distance will be covered by any given point on the circumference of the tyre.
Have I missed something? I can see that the ABS might cut in sooner to extend the stopping distance if the tyre loses grip, but if it doesn't.....?
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Braking distances are governed by the coefficient of friction between the tyre and the road - effectively, you are limited by skidding.
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>> Braking distances are governed by the coefficient of friction between the tyre and the road
>> - effectively, you are limited by skidding.
But that's the point I was making - assuming no skidding, all tyres are equal are they not? So in many circumstances it matters not a jot what the car is shod with. Unfortunately, it's those sharp stops or poor surfaces/road conditions that affect it.
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>> >> Braking distances are governed by the coefficient of friction between the tyre and the
>> road
>> >> - effectively, you are limited by skidding.
>> But that's the point I was making - assuming no skidding, all tyres are equal
>> are they not? So in many circumstances it matters not a jot what the car
>> is shod with. Unfortunately, it's those sharp stops or poor surfaces/road conditions that affect it.
>>
Nope..A hard compound will be different from a soft compound. And tyres with big grooves will be better in rain than those with small ones..
The surface to surface coefficient of friction varies tyre by tyre and surface by surface and with temperature..
Just read any tyre tests..
Just changed some Wanli ditchfinders at front of a Matiz for Uniroyal RainExpert.. Significant impact on feel and grip..
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>>assuming no skidding
That's an assumption you simply can't make when you're talking about braking distances.
As tyre to road friction absolutely dominates the vehicle's braking distance, being on the point of skidding is inevitable if you're aiming for a minimum braking distance.
At speed, ABS should prevent the wheels from locking, and they don't significantly lose grip as such - a well tuned modern ABS controller on a car will keep the tyre quite close to its optimum slip ratio.
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The winter tyres will grip more strongly, so the brakes will be able to generate greater retardation before the tyre begins to slip on the road. A bit like the way walking boots grip to mossy rocks long after trainers would have slipped.
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Interesting point though.
When truck ABS works it's a lot less quick in it's application than a hydraulic system, i assume this is because compressed air pressure can't be dumped and pumped as quickly as fluid under very high pressure.
So a truck can travel some (relative) distance before reapplication occurs**.
This must translate to a much smaller but significant amount for a car with hydraulic brakes, and must be exacerbated by slippery conditions...bear with me i don't explain my thoughts terribly well.
Dry road full power braking, wheel locks only just but ABS unlocks, wheel rolling again instantly as tyre has full grip, extremely short stopping distance, yes?
Wet road braking, wheel will lock quickly so ABS unlocks but wheel takes longer to get rolling again, hence increased braking distance. yes?
Icy road, wheel locks instantly, ABS unlocks instantly but wheel takes a long time to get rolling again etc etc, massive braking distance yes?
You can see why in some cases ABS could work against good braking in the worse conditions, in theory at least would a driver with seat of the pants feel brake better than the machine, not including building up a wedge of snow, just ice braking and not considering steering control at this point.
** i remember early ABS equipped truck trailers would operate ABS at low speeds, if the trailer brakes locked up at say 10mph, they would easily too, you could roll several yards before they would reapply by the time the air had dumped and repressured, different now thank goodness, ABS only works above a certain speed..which i confess i should know but don't.
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And forgot to say, given the above differences in the tyres re-aquiring grip after ABS has locked it, in other words to get rolling again, you can see why a tyre designed for wet or cold will be infinitely superior to the ditchfinder as apart from being less inclined to skid, it will also regain rolling speed again faster, so you get twice the bite.
Without ABS the effect should be felt more i'd have thought.
Last edited by: gordonbennet on Wed 2 Nov 11 at 16:47
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To the OP. Can I suggest you imagine tyres being made of ice?
Rubber can have many different characteristics, from the stuff that pours out of the trees at room temperature (in the tropics), to the stuff they make lorry tyres out of.
Also consider F1 tyre compounds, there's four of them IIRC.
Last edited by: bathtub tom on Wed 2 Nov 11 at 17:48
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Theres an ad for Goodyear tyres running on TV at the moment that claims there is a 28 metre stopping difference between their Summer and Winter tyres in Snow! thats quite impressive!
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>> To the OP. Can I suggest you imagine tyres being made of ice?
>>
>> Rubber can have many different characteristics, from the stuff that pours out of the trees
>> at room temperature (in the tropics), to the stuff they make lorry tyres out of.
I understand that, perhaps I'm not making myself clear. What I am getting at is, that if there is no slippage between the tyre and the surface it's on, then aren't all tyres equal? Or am I missing something?
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>>Or am I missing something?
For tyres to produce any force, there must be some slip - i.e., tyres, when producing force do not work either ar no slip or 100% slip. On a dry road, tyres produce their maximum force at approximately 20% slip. A good ABS controller will keep the tyre near to this optimum slip ratio.
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Can I throw in a question regarding the highway code and it's breaking distances ?
How is the braking distance worked out ?
Most cars have very good breaking distances but then throw in undulating roads, various bodge job road repairs and vehicle unsprung weight and then the breaking distances must be different for different types of cars.
Now on top of this factor in different braking distances for tyre makes and then the whole braking distance recommendation from the highway code goes away.
Now I have confused myself now. he he
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Braking distances in the highway code are worked out using a deceleration of 0.7g
It's not a bad compromise. While there are some tyres which will offer a better coefficient of adhesion, 0.7g is a safe assumption for dry roads.
Commercial vehicle tyres will typically be harder, and may only allow braking up to 0.6g.
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>> tyres re-acquiring grip after ABS has locked it
Don't forget ABS isn't meant to shorten stopping distances, only to maintain directional control during an emergency braking situation...
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Absolutely, and that's what differentiates tyres with good and bad grip - the less grippy the tyre, the more the ABS will release braking pressure, and therefore more distance will be travelled between first pressing the pedal and the vehicle coming to a halt.
Last edited by: Mike H on Wed 2 Nov 11 at 18:51
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I think what Mike H is saying is that if you only drive in a manner such that you never have to brake so sharply that the wheels skid, then all tyres are equal.
But with better tyres you will be able to drive more robustly with a smaller risk that you will reach the point where the tyre fails to grip.
Also you want a tyre to grip sideways so that you can steer, not just allow braking.
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Thanks Cliff, better put than I obviously did in my original post. Entirely agree as well, I always use the best rubber I can afford.
Last edited by: Mike H on Thu 3 Nov 11 at 09:42
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Have a look here for the actual formula as per laws of physics.
www.csgnetwork.com/stopdistcalc.html
Since it has coeff. of friction involved in the formula, the stopping distance will vary for different tyres.
I read somewhere that at microscopic level, the molecules of tyres deform differently on different surfaces. The grip is something when tyre surface deforms and grab irregularities of the road surface (they are not flat if you put them under electron microscope).
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Braking distances - Number_Cruncher
