A moment of silence...

[MEDMAN808]

Which better PSS or PS3's?

[MrRoboto]

I read on another thread that TireRack is closing out of PSS? Anyone confirm this?

[Pete_vB]

Originally Posted by M3 Adjuster

285 in the rear really makes a big difference. When I put a set on, I realized that I had been spinning my rear tires with 265s in the rear the entire time.

So I'd argue if you didn't realize it before, you were not spinning up. Instead you feeling some longitudinal slip, which is normal. Modern street tires peak in grip at something like 5 to 8% slip, which means the tires are rotating that much faster than road speed. Putting wider tires on reduces this slip percentage so it feels significantly different; doesn't necessarily mean you're accelerating any faster.

Also beware the difference fresh rubber makes. Simply switching from old PSS to fresh PSS would make you swear the tires are completely different.

Originally Posted by MEDMAN808

Which better PSS or PS3's?

PSS

Originally Posted by MrRoboto

I read on another thread that TireRack is closing out of PSS? Anyone confirm this?

Rears in stock sizes, yes. Not clear if that size is being updated or what.

[MDORPHN]

Originally Posted by Pete_vB

So I'd argue if you didn't realize it before, you were not spinning up. Instead you feeling some longitudinal slip, which is normal. Modern street tires peak in grip at something like 5 to 8% slip, which means the tires are rotating that much faster than road speed. Putting wider tires on reduces this slip percentage so it feels significantly different; doesn't necessarily mean you're accelerating any faster.

Also beware the difference fresh rubber makes. Simply switching from old PSS to fresh PSS would make you swear the tires are completely different.


PSS


Rears in stock sizes, yes. Not clear if that size is being updated or what.

I've always believed that the 5-8% is slip ANGLE, referring to lateral slip when cornering. I don't believe that the tires are spinning longitudinally.

Neil

[Pete_vB]

Originally Posted by MDORPHN

I've always believed that the 5-8% is slip ANGLE, referring to lateral slip when cornering. I don't believe that the tires are spinning longitudinally.

Tires also slip longitudinally for the same reasons as they slip laterally, and in fact if there is no slip there is also no friction. It's often referred to as "slip ratio" longitudinally, and can be as high as 20% at peak traction. Some of those great slow motion videos of dragsters taking off show it in action very well.

For some more background read this about traction control:
http://www.motortrend.com/features/p...ction_control/

Or this PDF paper on tire modeling describes it on page 5:
http://www.engr.colostate.edu/pts/Jo...20Modeling.pdf

[MDORPHN]

Originally Posted by Pete_vB

Tires also slip longitudinally for the same reasons as they slip laterally, and in fact if there is no slip there is also no friction. It's often referred to as "slip ratio" longitudinally, and can be as high as 20% at peak traction. Some of those great slow motion videos of dragsters taking off show it in action very well.

For some more background read this about traction control:
http://www.motortrend.com/features/p...ction_control/

Or this PDF paper on tire modeling describes it on page 5:
http://www.engr.colostate.edu/pts/Jo...20Modeling.pdf

For an optimal launch on a dragstrip, some wheel spin is helpful. However, once launched, the only time you have wheel spin is when you're employing power-on oversteer -- great for drifting but generally not the quickest way around a road course.

Neil

[Pete_vB]

Originally Posted by MDORPHN

For an optimal launch on a dragstrip, some wheel spin is helpful. However, once launched, the only time you have wheel spin is when you're employing power-on oversteer -- great for drifting but generally not the quickest way around a road course.

"Spinning" is relative, but this is not just a drag strip thing. The wheel is always turning faster (or slower) than the road if you're putting force through it. In 2nd gear acceleration it's probably about 5%, at top speed it might be 1-2% (to combat the aero drag). The tire moving exactly with the road is an illusion at all times if you're pushing it.

[ozinaldo]

Originally Posted by Pete_vB

...The tire moving exactly with the road is an illusion at all times if you're pushing it.

Don't know if this is absolutely correct or not myself but regardless it sounds to me pistonhead poetry with a philosophical twist, nice phrase

[MDORPHN]

Originally Posted by Pete_vB

"Spinning" is relative, but this is not just a drag strip thing. The wheel is always turning faster (or slower) than the road if you're putting force through it. In 2nd gear acceleration it's probably about 5%, at top speed it might be 1-2% (to combat the aero drag). The tire moving exactly with the road is an illusion at all times if you're pushing it.

This is a very interesting theory.

So, how then does the odometer, speedometer and ABS/DSC compensate for all this?

Neil

[Pete_vB]

Originally Posted by MDORPHN

This is a very interesting theory.

So, how then does the odometer, speedometer and ABS/DSC compensate for all this?

Neil

Not exactly a theory, it's the foundation of every tire model as well as the functioning of both ABS and traction control.

Consider: you're trying to apply a force with rubber, so effectively a spring. And you can't push on something with a spring without deflecting that spring. When a tread block arrives in contact with the ground it's undeflected, but shortly afterwards it's trying to transmit roughly 2000 lbs of forward thrust to the pavement at peak acceleration in a 1M. It deflects, then springs back when it leaves contact with the road. On a micro scale the rubber itself is doing the same thing. Hence even if the rubber on the bottom of the tread doesn't move relative to the ground, the overall tire does.

See the second reference:

"In contrast to slip angle, which is slip in the transverse plane, the slip ratio is the slip in the longitudinal plane. The slip ratio affects acceleration and breaking and therefore bears analogy to the slip angle in the sense that as slip angle is related to lateral force, slip ratio is related to longitudinal force and traction capacity. Generally speaking, the coefficient of friction will change with changing slip ratio. A plot of coefficient of friction vs. percent slip increases nearly linearly up to about 5% slip, peaks near 10% slip and then falls of in a nonlinear fashion (Ref 2).

The definition of slip ratio used here is the equivalent of the SAE definition."

Every tire/ car combo is different, so there is no generic "best" slip percentage. Data from a particular combo looks like this:

Notice there is no flat spot in the middle of this graph.

Now if you're just coasting the tire is going to be transmitting zero force, either longitudinally or laterally, so no slip. But as soon as you ask the tire to transmit forces, either braking, accelerating or cornering, the tire must begin to slip, a small amount at low force, a large amount at large force,

Odometer doesn't compensate and doesn't need to- tire OD plays a bigger factor, etc. Speedometer is driven from the front wheels, so no incorrect reading unless you're braking. It plays a critical roll in ABS and especially DSC function, however.

You know how the 1M comes with a warning that says you need to keep the .5" stagger between front and rear wheel OD? This is why. The DSC is reading the rate that the front and back wheels are turning and calculating the difference. It's targeting a maximum of some percentage difference in ground speed, perhaps 5% in MDM mode. If the rear tires are at 5% difference that's fine, you're near max acceleration. If it's more than 5% difference it shuts the fun down. In normal mode this percentage difference is reduced, maybe it's more like 3% difference.

Again, see the first link I posted above for more of explanation. An excerpt:

"For most road legal tires, the peak mu is usually somewhere between 0.85 and 1.0 and it peaks in the 5-20-percent slip range. Maximum traction is achieved at the peak of the mu-slip curve and the engineers that calibrate ABS/TCS systems generally try to control their systems to keep the tires slipping somewhere around that peak to achieve top performance.

In general, traction control tries to manage the slip on the drive wheels near the peak of the mu-slip curve in order to preserve stability and keep the car from sliding around. It does this by monitoring parameters like engine torque, gear and throttle position, and the driven and undriven wheel speeds to determine when the drive wheels are starting to spin up and break away."

[MDORPHN]

OK, I'm getting persuaded.

Interesting stuff.

Neil

[Yellowceta]

Is it possible that the 'slip' you mention may sometimes present itself as sidewall deformation rather than a relative difference in speed between the wheel and ground? I haven't read either of your links yet, but I am still struggling with believing that a wheel experiencing acceleration or deceleration at the minimal amount is still experiencing 'slip'. I've always thought that the sidewall deformation played a roll as well.

[Pete_vB]

Originally Posted by MDORPHN

Interesting stuff.

That it is...

Originally Posted by Yellowceta

Is it possible that the 'slip' you mention may sometimes present itself as sidewall deformation rather than a relative difference in speed between the wheel and ground? I haven't read either of your links yet, but I am still struggling with believing that a wheel experiencing acceleration or deceleration at the minimal amount is still experiencing 'slip'. I've always thought that the sidewall deformation played a roll as well.

Absolutely the tire sidewall plays a role, though in most performance street tires it's relatively stiff and gets loaded and "wound up" and doesn't contribute to slip as much as the tread as I understand it. Of course it depends on the tire:

If you look at that graph above, if you're close to zero friction, which is what you'd be if you're just pulling away from a stop normally at low acceleration, you're also close to zero slip, so imperceptible. Even a few percent slip is going to be difficult to both feel and see, and that's all you'd experience in many cars at close to max acceleration. The 1M is powerful enough to get to the part of the curve that starts to flatten out in second gear, at which point you can feel it if you're sensitive.

[scotth944]

Does this slippage only apply to when a tire is rotating while generating a horizontal force or does it also come into play with a stationary tire such as when your parked on a hill?

I ask this to understand how the slip ratio chart posted would apply to a rear tire going through a turn with no brake or power being applied and where the traction force is being applied to the tire is not in the direction of travel (low trust directional travel compared to the slip direction travel)?

[Pete_vB]

Originally Posted by scotth944

Does this slippage only apply to when a tire is rotating while generating a horizontal force or does it also come into play with a stationary tire such as when your parked on a hill?

I ask this to understand how the slip ratio chart posted would apply to a rear tire going through a turn with no brake or power being applied and where the traction force is being applied to the tire is not in the direction of travel (low trust directional travel compared to the slip direction travel)?

Rotating only. If you think of each tread block as a little spring, once it's loaded up it will stay that way indefinitely. But every time you pick that spring up and put it down again (which is what you're doing when the tire is rotating) the spring will "walk" slightly.

If you are only cornering, not accelerating or decelerating, you'd be at 0/0 on the longetudinal slip chart, but you'd be somewhere else on the lateral slip chart, which is similar in concept but different in scale and values due to tire shape and construction.

[scotth944]

Understood, thanks for your input.

[Die ///M Rakete]

Wonderful to see how the high caliber of members on this forum managed to elevate what started off as just a silly post

[Artemis]

Recently forum fellow ///Nordschleife mounted PSS on his 19" BBS CH-R rims: 255/35 R19 front | 275/35 R19 rear (instead of 245/35 R19 front | 265/35 R19 rear).

He told me he's very pleased with it. Lots of grip (dry and wet). Significant difference with PS2. Like many of us, also his 1M got the front fender liners upgrade (http://www.1addicts.com/forums/showthread.php?t=682196): only minimal contact/rubbing at full lock with the 255/35 R19 front wheels.

Any others have experiences with a PSS 255/275 setup ?

[///Nordschleife]

http://www.1addicts.com/forums/showt...&highlight=255

http://www.1addicts.com/forums/showt...&highlight=255

http://www.1addicts.com/forums/showthread.php?t=687038

second that !

[DrShimmy]

I am at 64500 km and will go for a tyre change today after work. My rear right tyres has a major personality defect keeps loosing pressure. Been running the same tyres that came with the car since feb 2012. They are down to 1mm from the thread wear indicator. I will go PSS 255/35 front and debating 275/35 or 285/35 rear. Can't wait to feel the new grip. I pack highway miles mostly and some mischief on empty back roads.

[ozinaldo]

Originally Posted by DrShimmy

I am at 64500 km and will go for a tyre change today after work. My rear right tyres has a major personality defect keeps loosing pressure. Been running the same tyres that came with the car since feb 2012. They are down to 1mm from the thread wear indicator. I will go PSS 255/35 front and debating 275/35 or 285/35 rear. Can't wait to feel the new grip. I pack highway miles mostly and some mischief on empty back roads.

Why 285/35 rear? Is it because PSS don't come in 285/30?

If you want a wider alternative to 275/35, PSS comes in 295/30 which would be my choice over 285/35.

[DrShimmy]

Originally Posted by ozinaldo

Why 285/35 rear? Is it because PSS don't come in 285/30?

If you want a wider alternative to 275/35, PSS comes in 295/30 which would be my choice over 285/35.

would it fit nicely? I am so scared to go that wide on stock rims. I will check it out. Having rubber that phat in the back would be