Tyre pressure and contact patch size
07 July 1999, 12:55 AM
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(This is a continuation of the discussion of that has developed in the "Pitfalls of trusting AWD" thread in General Questions...)
Mike:
First things first, this action/reaction business. If you put a 1lb object on a table top, it obviously presses down on the table. The force is the same however much of the object is touching the table top (e.g., whether its laid down flat or stood on end) and is also unaffected by the consistency of the material the object is made of. If you stand on weighing scales with one foot, the reading doesn't change (the *pressure* - force per unit area - exerted on the scale platform would be doubled, however). Similarly it doesn't matter whether you're wearing Nike Airs or hobnail boots.
Looking at the table now, it has a heavy object lying in the middle of it. To make it easier to imagine, lets say this table has a thick table cloth on it. When you put the object down, you'll see the table cloth "give" a bit. The heavier the object, the more it gives. Whenever you put an object on a surface, there is always some deformation of the surface, even if it is miniscule as in the case of something resting on hard tarmac. All hard surfaces are to some degree elastic, meaning that they will try to spring back to their original shape when the weight is removed from them. It is this tendency that provides the reaction force. Its like if you compress a spring by hand and hold it, you're pushing on the spring, the spring is pushing against you with an equal force, and nothing is moving.
This is what I meant when I said "In a static system any force applied by one object on another must be balanced by an equal and opposite force applied by the second object on the first one. Either this condition is held or things start moving and you don't have a static system any more."
Does that make sense?
Moving on to some of the specific points...
> O.K. For the sake of argument if the width is increased by 1 cm, you lose 2cm square of contact area?
No, you don't lose any contact area, but you don't gain any either. As you increase the tyre width, the width of the contact area increases but the length decreases proportionally. If you had an infinitely wide tyre, you would indeed have a zero length contact area, but that's not a real world case. If you have a 5in wide tyre with a 4in long patch, you have a 20 sq.in contact area (5 x 4). If you increase the tyre width to 8in and keep the same inflation pressure, all that happens is you end up with a 2.5in long patch which still has an area of 20 sq.in (8 x 2.5)
When it comes down to it, tyre pressure is the first thing you should decide upon in your setup. You made a comment about load distribution, which is actually the main parameter you are playing with by varying the tyre pressure. Track cars run their tyres at low pressures because on smooth dry tarmac you get the most friction by distributing the load "gently" over a large contact area. If there are any small surface irregularities or stones somewhere under the contact patch, there is still plenty of rubber touching elsewhere to do the work. That isn't true for loose surfaces, where most of the time your tyre will be evenly surrounded by gravel or whatever. In that case, you increase the tyre pressures so the smaller contact patch is pressed harder against the surface, tending to compact the surface particles and making them dig into the rubber good and hard (c.f. the discussion above about standing on one foot).
These are all rules of thumb. The tyre manufacturers have jealously guarded graphs showing how much friction a particular tyre generates on different surfaces with different pressures and amounts of loading. These might contain a few surprises - e.g., you might find that while you get the best grip at a lowish pressure on a dry road, you need a higher pressure to get the best grip if the same road is wet.
It all comes down to compromise, especially when you consider that the tyre width and pressure are going to affect the handling feel of the car one hell of a lot.
In any event, what you generally want to do is run the *least* width of rubber you can get away with for your chosen pressure. That way, you get the lowest unsprung mass (= more grip on rough surfaces), more steering feel, less tramlining and a more progressive loss of adhesion when it happens. Wide rubber is a necessary evil that you have to put up with on track cars and the like, because if they ran narrower rubber at low pressures, the contact patch would be so long that the tyre would be severely deformed and it would overheat (agreeing with Russ here).
> If you follow that arguement it would be possible to prove that the car will grip better with 5psi pressure due to the increased size of the contact patch?
Each type of tyre is designed to adopt the correct shape within the desired pressure range. Dragster tyres will run quite happily at 5psi, but don't try it on your 205/50 Bridgestones!
>Tried [getting advice from the manufacturers] when this all started, E Mailed 4 of the big ones, never got a single reply, even after complaining about receiving no replies.
Ah. Oh well.
Sorry for the massive post, but as you can see this is not a trivial subject. I'm far from being an expert, but I've understood enough to put me off fitting bigger tyres to my Scooby. In the end, it was worth it just for the cash I've saved
Best regards,
-= mike =-
Mike:
First things first, this action/reaction business. If you put a 1lb object on a table top, it obviously presses down on the table. The force is the same however much of the object is touching the table top (e.g., whether its laid down flat or stood on end) and is also unaffected by the consistency of the material the object is made of. If you stand on weighing scales with one foot, the reading doesn't change (the *pressure* - force per unit area - exerted on the scale platform would be doubled, however). Similarly it doesn't matter whether you're wearing Nike Airs or hobnail boots.
Looking at the table now, it has a heavy object lying in the middle of it. To make it easier to imagine, lets say this table has a thick table cloth on it. When you put the object down, you'll see the table cloth "give" a bit. The heavier the object, the more it gives. Whenever you put an object on a surface, there is always some deformation of the surface, even if it is miniscule as in the case of something resting on hard tarmac. All hard surfaces are to some degree elastic, meaning that they will try to spring back to their original shape when the weight is removed from them. It is this tendency that provides the reaction force. Its like if you compress a spring by hand and hold it, you're pushing on the spring, the spring is pushing against you with an equal force, and nothing is moving.
This is what I meant when I said "In a static system any force applied by one object on another must be balanced by an equal and opposite force applied by the second object on the first one. Either this condition is held or things start moving and you don't have a static system any more."
Does that make sense?
Moving on to some of the specific points...
> O.K. For the sake of argument if the width is increased by 1 cm, you lose 2cm square of contact area?
No, you don't lose any contact area, but you don't gain any either. As you increase the tyre width, the width of the contact area increases but the length decreases proportionally. If you had an infinitely wide tyre, you would indeed have a zero length contact area, but that's not a real world case. If you have a 5in wide tyre with a 4in long patch, you have a 20 sq.in contact area (5 x 4). If you increase the tyre width to 8in and keep the same inflation pressure, all that happens is you end up with a 2.5in long patch which still has an area of 20 sq.in (8 x 2.5)
When it comes down to it, tyre pressure is the first thing you should decide upon in your setup. You made a comment about load distribution, which is actually the main parameter you are playing with by varying the tyre pressure. Track cars run their tyres at low pressures because on smooth dry tarmac you get the most friction by distributing the load "gently" over a large contact area. If there are any small surface irregularities or stones somewhere under the contact patch, there is still plenty of rubber touching elsewhere to do the work. That isn't true for loose surfaces, where most of the time your tyre will be evenly surrounded by gravel or whatever. In that case, you increase the tyre pressures so the smaller contact patch is pressed harder against the surface, tending to compact the surface particles and making them dig into the rubber good and hard (c.f. the discussion above about standing on one foot).
These are all rules of thumb. The tyre manufacturers have jealously guarded graphs showing how much friction a particular tyre generates on different surfaces with different pressures and amounts of loading. These might contain a few surprises - e.g., you might find that while you get the best grip at a lowish pressure on a dry road, you need a higher pressure to get the best grip if the same road is wet.
It all comes down to compromise, especially when you consider that the tyre width and pressure are going to affect the handling feel of the car one hell of a lot.
In any event, what you generally want to do is run the *least* width of rubber you can get away with for your chosen pressure. That way, you get the lowest unsprung mass (= more grip on rough surfaces), more steering feel, less tramlining and a more progressive loss of adhesion when it happens. Wide rubber is a necessary evil that you have to put up with on track cars and the like, because if they ran narrower rubber at low pressures, the contact patch would be so long that the tyre would be severely deformed and it would overheat (agreeing with Russ here).
> If you follow that arguement it would be possible to prove that the car will grip better with 5psi pressure due to the increased size of the contact patch?
Each type of tyre is designed to adopt the correct shape within the desired pressure range. Dragster tyres will run quite happily at 5psi, but don't try it on your 205/50 Bridgestones!
>Tried [getting advice from the manufacturers] when this all started, E Mailed 4 of the big ones, never got a single reply, even after complaining about receiving no replies.
Ah. Oh well.
Sorry for the massive post, but as you can see this is not a trivial subject. I'm far from being an expert, but I've understood enough to put me off fitting bigger tyres to my Scooby. In the end, it was worth it just for the cash I've saved
Best regards,
-= mike =-
07 July 1999, 05:51 PM
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Mike Nunan, & Russ.
Mike first. From your description above, (action/reaction). Yes, I understand. (EUREKA)
>No, you don't lose any contact area, but you don't gain any either. As you increase the tyre width, the width of the contact area increases but the length decreases proportionally.
Russ Haley writes, so the solution is a wider tyre at a lower pressure which maintains a workable patch length but increases its area through width.
Is not this the important point of what we are discussing? Dynamically, the option of wider tyres, at the limit extends more rubber on the road, more usable rubber as it has not overheated and exceeded its ability to grip.
Am I interpreting Russ' comment correctly? (increases its area through width.
Reading further on I suspect not?
Would it be better or more appropriate to say, a wider tyre can achieve the same area (as an under inflated tyre) by running within its design/operating parameters. Hence effectively with suitable tyre pressures, putting more rubber (larger contact patch?) on the road, extending more grip.
If so, this surely brings us back around to the original point, that wider tyres extend more grip than narrower ones (I concede that they bring a myriad of other side effects to the performance table).
In dry conditions, wider tyres extend more grip.
Bigdram, made the relevant point , F1 teams had narrower tyres forced upon them, the consequence of which was that their levels of grip were reduced.
This general argument implies that it wouldn't. Back to the Bee thing again, shouldn't give more grip with a wider tyre, but we know it does.
It would appear that the overwhelming evidence and facts to support it are compelling (if not entirely understood), in favour of the conclusion that the footprint SIZE does not increase with additional width (without increases in weight also).
How do you argue with laws written by a man who has been dead over 272 years?
However it would appear if I am interpreting Russ' comments correctly (it is possible I am not) that as the saying goes, Never mind the length... feel the width!
Please tell me if I have mis-interpreted anything in the above post, I'm nearly there.
Mike.
Mike first. From your description above, (action/reaction). Yes, I understand. (EUREKA)
>No, you don't lose any contact area, but you don't gain any either. As you increase the tyre width, the width of the contact area increases but the length decreases proportionally.
Russ Haley writes, so the solution is a wider tyre at a lower pressure which maintains a workable patch length but increases its area through width.
Is not this the important point of what we are discussing? Dynamically, the option of wider tyres, at the limit extends more rubber on the road, more usable rubber as it has not overheated and exceeded its ability to grip.
Am I interpreting Russ' comment correctly? (increases its area through width.
Reading further on I suspect not?
Would it be better or more appropriate to say, a wider tyre can achieve the same area (as an under inflated tyre) by running within its design/operating parameters. Hence effectively with suitable tyre pressures, putting more rubber (larger contact patch?) on the road, extending more grip.
If so, this surely brings us back around to the original point, that wider tyres extend more grip than narrower ones (I concede that they bring a myriad of other side effects to the performance table).
In dry conditions, wider tyres extend more grip.
Bigdram, made the relevant point , F1 teams had narrower tyres forced upon them, the consequence of which was that their levels of grip were reduced.
This general argument implies that it wouldn't. Back to the Bee thing again, shouldn't give more grip with a wider tyre, but we know it does.
It would appear that the overwhelming evidence and facts to support it are compelling (if not entirely understood), in favour of the conclusion that the footprint SIZE does not increase with additional width (without increases in weight also).
How do you argue with laws written by a man who has been dead over 272 years?
However it would appear if I am interpreting Russ' comments correctly (it is possible I am not) that as the saying goes, Never mind the length... feel the width!
Please tell me if I have mis-interpreted anything in the above post, I'm nearly there.
Mike.
08 July 1999, 08:17 AM
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Mike
Your interpretation of my ramblings is pretty much there - so long as the underlying pressure=force / area is not forgotten.
If you fit wider tyres, then by maintaining the same tyre pressure you maintain exactly the same area of rubber on the road.
By decreasing the tyre pressure for any width, you increase the area in contact with the road, hence increasing frictional area and grip (up to a point).
However, as Mike Nunan says, the tyre will only work well within a defined design pressure range, so deflating too much can only go so far.
So to get a given area without having a footprint that looks like Bugs Bunny's, you need a wider tyre at a lower pressure.
The contact patch's shape generally works best if it is not vastly longer than it is wide anyway , as too long a patch interferes with the self-restoring effects of the steering castor angle.
Thing about suspension is that the whole system of tyres, geometry etc. has so many feedback loops and secondary effects that it is as much an art form when it comes to tweaking it as anything else (see what Lotus can do that nobody else can ).
Russ
Your interpretation of my ramblings is pretty much there - so long as the underlying pressure=force / area is not forgotten.
If you fit wider tyres, then by maintaining the same tyre pressure you maintain exactly the same area of rubber on the road.
By decreasing the tyre pressure for any width, you increase the area in contact with the road, hence increasing frictional area and grip (up to a point).
However, as Mike Nunan says, the tyre will only work well within a defined design pressure range, so deflating too much can only go so far.
So to get a given area without having a footprint that looks like Bugs Bunny's, you need a wider tyre at a lower pressure.
The contact patch's shape generally works best if it is not vastly longer than it is wide anyway , as too long a patch interferes with the self-restoring effects of the steering castor angle.
Thing about suspension is that the whole system of tyres, geometry etc. has so many feedback loops and secondary effects that it is as much an art form when it comes to tweaking it as anything else (see what Lotus can do that nobody else can ).
Russ
08 July 1999, 11:09 AM
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Mike,
I think we're in agreement on all points now (cool!)
You *can* get more grip by running at lower pressure with a wider tyre ("feel the width", as you say). I suppose the reason for my hobby-horsing is that many people run their 215s at the same pressure as their 205s thus gaining nothing in grip while losing various other positive attributes such as steering feedback.
However, most people tend to run their pressures a bit higher than the recommended settings, so perhaps the moral of this story is to try knocking off a pound or two at each end. If you're running 215 width tyres at 34/32psi front/rear, then try 32/30. This certainly isn't outside the working range of the tyres and will give you a bit more grip (albeit at the cost of a bit of turn-in, but it'll still be as good as with 205s running at 34/32).
Wider tyres don't produce more grip on a dry road (at the same pressure as narrower ones, that is) but they *feel* more confidence-inspiring, because they save all their warning signs for the last moment before breakaway. If you actually did a comparison test driving around a marked circle, you would find that the level of ultimate grip was the same. Look at it this way and this is just another disadvantage of wider tyres -- they imbue the driver with a false sense of security.
-= mike =-
[This message has been edited by mike_nunan (edited 08-07-99).]
I think we're in agreement on all points now (cool!)
You *can* get more grip by running at lower pressure with a wider tyre ("feel the width", as you say
). I suppose the reason for my hobby-horsing is that many people run their 215s at the same pressure as their 205s thus gaining nothing in grip while losing various other positive attributes such as steering feedback.However, most people tend to run their pressures a bit higher than the recommended settings, so perhaps the moral of this story is to try knocking off a pound or two at each end. If you're running 215 width tyres at 34/32psi front/rear, then try 32/30. This certainly isn't outside the working range of the tyres and will give you a bit more grip (albeit at the cost of a bit of turn-in, but it'll still be as good as with 205s running at 34/32).
Wider tyres don't produce more grip on a dry road (at the same pressure as narrower ones, that is) but they *feel* more confidence-inspiring, because they save all their warning signs for the last moment before breakaway. If you actually did a comparison test driving around a marked circle, you would find that the level of ultimate grip was the same. Look at it this way and this is just another disadvantage of wider tyres -- they imbue the driver with a false sense of security.
-= mike =-
[This message has been edited by mike_nunan (edited 08-07-99).]
03 January 2014, 04:32 AM
#6
God, look at this absolute gem of a post from 15-odd years ago!! You cant say I didn't use the "search" facility...
So leading on from what these old duffers were banging on about (could resist it
) what would be the optimal tyre width for performance? My car is a classic wagon and I will be fitting wide arches (a few of the lads are running 255s with similar bits of bolted on plastic). In addition I will try some Federal RSR track tyres too and the wheels I choose will be 17". My shocks and springs are off a P1 with Paranoid Fabrication saggy *** spacers for the rear. 22mm ARBs F&R, maybe go to 24mm rear as the relative delta between the spring rates front to rear is slightly higher for the P1 setup compared with standard WRX wagon.
Would 255 (at a suitably low pressure) and semi slick tyres be too grippy?? i.e. "huge" grip which suddenly falls off a cliff. Would 235 width tyres with something like 15 to 30mm spacers be a kinder option in terms of transition from grip to slip? They would also be lighter and perhaps be a little cleaner in terms of handling and feedback?
The car is not a daily driver as i'm never bloody well home and will be used for track days when I can, big skids in a forest if i'm feeling foolish but mainly proper fast road (think A93 through Glen Shee hence P1 suspension rather then stiff coilovers).
Thnaks for reading, if nothing else at least I resurrected a good geeky thread!
So leading on from what these old duffers were banging on about (could resist it
) what would be the optimal tyre width for performance? My car is a classic wagon and I will be fitting wide arches (a few of the lads are running 255s with similar bits of bolted on plastic). In addition I will try some Federal RSR track tyres too and the wheels I choose will be 17". My shocks and springs are off a P1 with Paranoid Fabrication saggy *** spacers for the rear. 22mm ARBs F&R, maybe go to 24mm rear as the relative delta between the spring rates front to rear is slightly higher for the P1 setup compared with standard WRX wagon.Would 255 (at a suitably low pressure) and semi slick tyres be too grippy?? i.e. "huge" grip which suddenly falls off a cliff. Would 235 width tyres with something like 15 to 30mm spacers be a kinder option in terms of transition from grip to slip? They would also be lighter and perhaps be a little cleaner in terms of handling and feedback?
The car is not a daily driver as i'm never bloody well home and will be used for track days when I can, big skids in a forest if i'm feeling foolish but mainly proper fast road (think A93 through Glen Shee hence P1 suspension rather then stiff coilovers).
Thnaks for reading, if nothing else at least I resurrected a good geeky thread!
12 January 2014, 12:36 AM
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Great read and very informative.
I read through from the beginning and hadn't noticed they were dated 1999
until I read your post
I doubt the laws of physics have changed much since then lol
Thanks for finding it
I read through from the beginning and hadn't noticed they were dated 1999
until I read your post
I doubt the laws of physics have changed much since then lol
Thanks for finding it
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