The only characteristic of a tire that would change after "scrubbing" is maybe the tread rubber compound's durometer. The internal fabric plies and bead would not be affected. So the tensile structural properties, which are mostly dependent upon the fabric properties, would not change much.
"Q: How do you make a small fortune in racing?
A: Start with a large one!"
Indeed riff, but the durometer (shore), or hardness of the rubber compound is most of all temperature-dependent,
why I wonder about the true influence of said "scrubbing" in that respect as well?
"I spent most of my money on wine and women...I wasted the rest"
That was a positive definite statement, riff-raff, especially when you are talking about tyres...
Actually, I find that both stiffness & damping coefficients of new ("sticker") tyres generally reduce slowly over the course of a rig test. "Scrubbed" tyres typically have lower initial values and are more consistent over time. The effect isn't massive, & doesn't affect "optimum" suspension set-ups, but it does affect set-up "Performance Index" values.
I like to think the effect is consistent with manufacturer's advice that new road car tyres should be "run in". It might also explain, in part, the higher "first heat cycle" performance of race tyres, although I am happy to accept that chemical changes are significant in the latter case.
riff_raff wrote:The only characteristic of a tire that would change after "scrubbing" is maybe the tread rubber compound's durometer. The internal fabric plies and bead would not be affected. So the tensile structural properties, which are mostly dependent upon the fabric properties, would not change much.
This is not the case.
Grip is a four letter word. All opinions are my own and not those of current or previous employers.
You know what JT, I think you would earn a lot more credibility with this forum if you would care to elaborate a little on your xperiences once in a while, rather than just produce one-liners like the one above?
"I spent most of my money on wine and women...I wasted the rest"
My credibility here is something I couldn't care less about Tires are what I do for a job. I can't elaborate on much past basics and "common" knowledge because it's professional information I don't care to share.
Suffice to say, a tire definitely changes from when it's brand new to when it's broken in to when it's got some miles on it. More than just the modulus or hardness of the tread. Pretty easy to see in some really basic tests... even some you can do at the track or in your own garage! Passenger tires are fundamentally no different than race tires. Couple beads, couple body plies, maybe some breaker plies, with a few extra odds and ends here and there.
If you don't believe me, take some tires to DaveW and do some springrate work. What's the tire rate when it's new? With a few lap scrub? After 40 laps?
Grip is a four letter word. All opinions are my own and not those of current or previous employers.
Jersey Tom wrote:My credibility here is something I couldn't care less about Tires are what I do for a job. I can't elaborate on much past basics and "common" knowledge because it's professional information I don't care to share.
Suffice to say, a tire definitely changes from when it's brand new to when it's broken in to when it's got some miles on it. More than just the modulus or hardness of the tread.
....
If you don't believe me, take some tires to DaveW and do some springrate work. What's the tire rate when it's new? With a few lap scrub? After 40 laps?
Well, if you don't care about your credibility and simply xpect members to take your statements for granted, however un-engineerish they come across, just because you claim to be working with tyres, I guess we have a problem then?
Why the tyre's fabric would change it's stiffness over time is far beyond me, but then again, I don't work with tyres.
"I spent most of my money on wine and women...I wasted the rest"
Nah, not a problem. I don't care what my credibility is here, nor do I care if people believe what I say. Just offering bits of knowledge. Take it or leave it, makes no difference to me.
Regardless of if it makes sense or not, a tire's mechanical properties - certainly spring rate - change significantly as it gets run. Why? Hell probably a variety of reasons.
What is a tire carcass? It's a laminate composite. A composite's mechanical properties are a function of the properties of both the matrix and the reinforcement. In this case one is rubber, one is fabric.
Rubber's mechanical properties change with strain and heat histories. Don't think anyone's going to deny this. The reinforcement is interesting as tires use so many different types of fabrics and steel. Anyone who has had a shirt shrink in the drier or stretch over time can appreciate the fact that some fabrics DO change with use. Some are susceptible to creep at elevated temperature. Some are relatively insensitive to temperature.
Point is, testing tires in a perfectly controlled manner is impossible. Sorting out all the highly nonlinear effects of load, inflation, camber, speed, temperature, time, strain, wear... good luck.
Grip is a four letter word. All opinions are my own and not those of current or previous employers.
Have to agree with JT..As the sidewall flexes over time it's bound to lose some of it's stiffness...But overall the stiffness of an F1 tire is mostly determined by the pressure not the sidewall. As I told youi before...You can take a modern unmounted F1 tire and wad it up using your hands,,They showed it on Speed,,It without inflation has alnmost no stiffness.
To achieve anything, you must be prepared to dabble on the boundary of disaster.”
Sir Stirling Moss
Air does not carry load, so saying that the inflation pressure gives the tire stiffness and not the carcass is not true.
The way inflation pressure gives the tire "stiffness" is that it gives the carcass cords preload and puts them under tension. When cords are in tension they can carry load. This is also evidenced by the fact that when you add air to a tire like that it doesn't just balloon out. If anything, the "dishrag" tires get ALL of their stiffness from the cords and their geometry... as opposed to some tires with very short sidewalls that are basically "bricks" of solid material which impart mechanical stiffness.
Grip is a four letter word. All opinions are my own and not those of current or previous employers.
Let's try a simple static analysis of what happens when a vertical load is acting on the wheel Pup:
- Acting on the entire wheel is tyre-wall stiffness plus the tyre-pressure times contact-patch area.
- Contact patch area is defined as vertical-load minus load supported by sidewalls, this over tyre-pressure.
- Contact patch area is tyre-width times contact length.
Xample, load supported by sidewalls disregarded here:
- A vertical load of 6 kN and a pressure of 130 kPa (19 psi), would indicate a contact-patch of 45 000 mm^2.
- An area of 45 000 and a tyre-width of 300 mm, means a contact length of 150 mm.
- A contact length of 150 and a tyre-radius of 330, gives a geometrical deflection of about 9 mm.
- Conclusively; A lower inflation pressure means a larger contact patch and higher geometrical deflection.
Acting on the rim however, is only forces from the sidewalls, as air-pressure is evenly distributed all around.
"I spent most of my money on wine and women...I wasted the rest"
I guess you'll then have to explain to me how increasing the tire pressure by a pound or two is the equivalent of increasing the spring rate of the vehicle..if it doesn't make it stiffer.
Then explain how say a soccer ball is not harder,,therefore stiffer when you add pressure.
I think you are wrong...unless you can find a reasonable way to explain those things.
To achieve anything, you must be prepared to dabble on the boundary of disaster.”
Sir Stirling Moss
Don't know if you're talking to me strad, I don't see anyone else around? Talkin' to me?
If you are taking to me, and I mean me, the spring rate of the vehicle is defined as the stiffness of the entire vehicle, including wheels, not only the suspension and sidewall stiffness. The tire pressure plays its part as I humbly tried to model above.
"I spent most of my money on wine and women...I wasted the rest"
Sorry...Actually I was talkin to JT.
Stiffness in general is considered to be somethings resistance to compression or flexing and indeed when you up the air pressure in the tire you make it stiffer...AND, IF you change nothing else but the air pressure it will be similar to increasing the spring rate.
There are people on this board that well fit the other definition of "stiff"
To achieve anything, you must be prepared to dabble on the boundary of disaster.”
Sir Stirling Moss
Tires are what I do for a job. I can't elaborate on much past basics and "common" knowledge because it's professional information I don't care to share.
