2015 tyre blanket ban connected to low profile tyre

[Tim.Wright]

Fair enough... but if the pressure raises, the contact patch needs to shrink in area right? (if Fz=inflation_pressure x area is correct). To shrink the contact patch area, the tyre must move up right?

So even if a change in volume is not changing the stiffness, it might change the vertical equilibrium position.

Anyway given the number of other differences between the construction of a 13inch and 18inch wheel, it would be more or less impossible to separate out this effect.

[strad]

if it leads to F1 tyres doing the tyre's job and F1 suspensions doing the suspension's job that's good ?
current F1 tyres are a long way from low profile

Oh God! Real suspensions on F1 cars??? I'd love that.

[DaveW]

There is absolutely zero direct effect from the air pressure on tire spring rate. Physically impossible.

Apologies, JT, but this plot was taken from a rig test of an "historic" (but not ancient) F1 vehicle (consecutive runs, no other changes)....

[Jersey Tom]

There is absolutely zero direct effect from the air pressure on tire spring rate. Physically impossible.

Apologies, JT, but this plot was taken from a rig test of an "historic" (but not ancient) F1 vehicle....

That is completely different. That is inflation pressure affecting ply tension and [mechanical] spring rate as a result.

The air itself has absolutely, positively, zero direct "cushion" effect, as was mentioned in an earlier post.

[DaveW]

That is completely different...

I was quite sure I had misinterpreted your comments, but perhaps others had as well.

[Jersey Tom]

if it leads to F1 tyres doing the tyre's job and F1 suspensions doing the suspension's job that's good ?
current F1 tyres are a long way from low profile

Oh God! Real suspensions on F1 cars??? I'd love that.

What makes a "real" suspension, though?

Let's say they go to slightly lower profile tires on maybe a 15 or 16" wheel. Ultimately.. all other things being equal.. overall ride rate will probably be kept the same. Still have to respect the balance of mechanical grip to aero platform.

So.. maybe the tire stiffness changes for a given inflation pressure, that's not unreasonable to assume. Maybe the tire wants to work at higher or lower inflation pressure so there's that too. Maybe it's ultimately more or less or maybe a wash.

Still, let's say the tire stiffness ends up being different in your final race configuration, and you adjust your ride springs accordingly, so some different proportion of travel is taken up by the chassis vs. the tire.

So what?

For real - what is that really fundamentally changing? And who is it who decrees what ratio of suspension travel to tire compression makes a "real" suspension. What if a F1 car has a "real" suspension and all our consumer sedans and minivans are the compromised crap for soft ride going down the freeway?

[Tim.Wright]

Well, I'd define a real suspension as one which can drive over real roads.

[Jersey Tom]

Well, I'd define a real suspension as one which can drive over real roads.

They get around at Monaco just fine, no?

[flyboy2160]

That is completely different...

I was quite sure I had misinterpreted your comments, but perhaps others had as well.

I think JT is trying to make a point about the air transmitting the force that can be shown in a simple experiment that might help clear some confusion. I vaguely recall this actually coming up in one of my high school physics classes. A gearhead kept arguing the tire air pressure/softness point with the teacher - also a gearhead - until the teacher did this little demo:

Inflate a football or basketball to a reasonable pressure, then push your thumb into it. It resists; it has a certain springiness. If you inflate it a little more and try to push your thumb in, the ball seems 'stiffer.' For the same force, your thumb doesn't go in as far. This can lead you to think that the ball, or tire, would offer more cushioning if it had less air.

Now try the same thing, only put the lower pressure ball on a scale, zero the scale, push your thumb in and note the scale reading - say 10lbs or whatever. Inflate the ball so that it seems stiffer, put it back in scale, zero the scale, and again push in with your thumb until the scale reads the same 10lbs. Huh? With the higher pressure, your thumb didn't sink in as far, but the scale reads the same. I think this is what JT is trying to explain. The air just transmits the force through to the wheel. JT please correct me if I'm all wrong. My excuse will be that I did this without my usual 'Beer Math' beverage.

Flyboy Steve

[DaveW]

They get around at Monaco just fine, no?

Oddly, Monaco is a circuit where even F1 people have always taken care of mechanical set-up.

[Jersey Tom]

To touch on the air pressure thing (even though we're going on a bit of a tangent).. thought about diagramming this up, but in the interest of enjoying a lazy Saturday morning let's see how text goes. First thing to realize is that a tire supporting the weight of a car is not the same as say sitting on a balloon, or on an air spring.

Ultimately the chassis is connected to the suspension, the suspension includes the hub, to which the wheel is bolted. So how are forces transmitted to the wheel to carry the weight of the car? The important thing here is that the wheel is inside the air volume of the tire. Air pressure is uniform inside that volume, and is going to act equally on the wheel in all directions. All the tire pressure acting upward on the bottom of the wheel is exactly countered by the pressure acting downward on the top of the wheel, thus the air pressure cannot support any of the vehicle weight.

What holds the car up then? The tire's two steel bead bundles that seat up to the wheel have the fabric of the plies wrapped around them. If you were to cut a section of a F1 tire you'd find there's hardly anything to it once you move up the sidewall, just a couple thin layers of ply fabric. The plies ultimately carry all the load, they are the load-bearing element of the tire.. Inflating the tire puts all those cords under tension (cords going into compression when loaded = bad news) and allows them to bear load... but there is no "springs in parallel" business of a "sidewall spring" and "air cushion spring."

In a way maybe it's analogous to a wire wheel on a bicycle where the load is carried by the individual spokes. Though in that case the rim has its own inherent stiffness being solid metal, but the spokes are still put in pre-tension regardless. Imagine so many more small spokes that it becomes a fabric-looking mesh, then coat it in rubber.

[Luke]

To touch on the air pressure thing (even though we're going on a bit of a tangent).. thought about diagramming this up, but in the interest of enjoying a lazy Saturday morning let's see how text goes. First thing to realize is that a tire supporting the weight of a car is not the same as say sitting on a balloon, or on an air spring.

Ultimately the chassis is connected to the suspension, the suspension includes the hub, to which the wheel is bolted. So how are forces transmitted to the wheel to carry the weight of the car? The important thing here is that the wheel is inside the air volume of the tire. Air pressure is uniform inside that volume, and is going to act equally on the wheel in all directions. All the tire pressure acting upward on the bottom of the wheel is exactly countered by the pressure acting downward on the top of the wheel, thus the air pressure cannot support any of the vehicle weight.

What holds the car up then? The tire's two steel bead bundles that seat up to the wheel have the fabric of the plies wrapped around them. If you were to cut a section of a F1 tire you'd find there's hardly anything to it once you move up the sidewall, just a couple thin layers of ply fabric. The plies ultimately carry all the load, they are the load-bearing element of the tire.. Inflating the tire puts all those cords under tension (cords going into compression when loaded = bad news) and allows them to bear load... but there is no "springs in parallel" business of a "sidewall spring" and "air cushion spring."

In a way maybe it's analogous to a wire wheel on a bicycle where the load is carried by the individual spokes. Though in that case the rim has its own inherent stiffness being solid metal, but the spokes are still put in pre-tension regardless. Imagine so many more small spokes that it becomes a fabric-looking mesh, then coat it in rubber.

True the air doesn't act separately in parallel to the tyre. But neither does the tyre act independently of the air pressure inside it . The vertical stiffness measured at the contact patch is very much effected by the air pressure inside the tyre which is how I interpreted the original comment in that a tyre with a stiffer sidewall construction, which is usually the case of lower profile tyres are to a (varying) degree less influenced by the inflation pressure in terms of the measured vertical stiffness.

[humble sabot]

In a way maybe it's analogous to a wire wheel on a bicycle where the load is carried by the individual spokes. Though in that case the rim has its own inherent stiffness being solid metal, but the spokes are still put in pre-tension regardless. Imagine so many more small spokes that it becomes a fabric-looking mesh, then coat it in rubber.

Your analogy is basically suggesting that a wheel with low tension is, in a spring-sense, equivalent to a high tension wheel. Not the case in practice. I think you're over-applying Newton's third law, without making sufficient allowance for the relevance of the First and Second Laws. The suspension-relevance of a tyre's inflation pressure is related to its coefficient of restitution (momentum, inertia, you know, that ol stuff) and that changes dramatically with changes in internal pressure.

My counter experiment to you, is to grab two different bike wheels, one with a larger tyre, and one with a smaller, fill them with different air pressures and test their bounciness at each level of inflation. As with suspension design generally, there's an optimum frequency and amplitude for a given riding/driving setup to maximize traction, and minimize rolling resistance.

[strad]

This has been argued many times any I believe that JT is right. What keep popping up and continuing the debate is this thing from some crew chief that adding X amount of pressure is the same as changing to an X pounds stiffer spring.
What that crew chief should have said is that adding a pound of air pressure feels to the driver like you changed to a stiffer spring.
Now JT and the guys can argue about the why all they want but what matters to the driver is how it feels and most times to the crew chief how he can affect a certain change in the handling quickly and without changing the spring or adding a rubber.
He's also right that we're off on a tangent
JT..are we then to see the tire,,,and this may be too far out there,,should we see the tire as a sort of rolling suspension bridge? Or at least a similar correlation between the cord in the tire and the cables of the bridge?

[jz11]

One [spring rate] from the side wall and one from the air or tyre gas cushion.

There is absolutely zero direct effect from the air pressure on tire spring rate. Physically impossible.

what WB may have meant by "tire spring rate" - that "spring" is working not just in vertical (suspension travel) direction - it also works on directional change and power delivery - basically in all possible directions - less pressure - the "spring" is softer - more pressure - the "spring" becomes harder - the analogy with the thumb-pressed-into-the-inflated-ball-on-the-scale - the force lets say is still 100nm (car leans on one side during corner) - but lets add a vibration (wheels go over a curb) with a small amplitude on top of that force and look at the scale in 2 extreme cases - with very little pressure in the ball - that vibration will not show up on the scale at all - it will be absorbed by the soft ball, now inflate the ball to the point it becomes rigid - and you see that vibration on your scale

scale is your chassis - so the higher the profile of the tire - the wider the possible range where you can use that pressure to play with the spring and find suitable equilibrium where unwanted vibration is eaten up by the tire, with low profile tire your window becomes much much smaller - it is nothing that can't be dealt with - helper spring and a bit more complicated shock absorber will fix it - so nothing really changed - it just added a little more complexity to the suspension - which IMO doesn't really go well together with the - lets reduce the racing costs banner