Autocross- SCCA EV-X Class vs. SS

[dsgerbc]

I have those whiteline bushings. Not sure I feel much of a change compared to rotated OEM ones with MPP inserts in them I had on my prior setup. Extra bit of caster is nice tho, my tires no longer rub on the fender liner, at least until I bother to dial more caster via camber arm adjustment.

A tire question to you all:
Looks like this weekend's nationals style event on concrete will feature rain showers and 70F-ish temps. So, there will probably be wet periods and periods of rapidly drying track. Would you guys bother with (half-tread) RT660s or run 255-wide PS4S?

 

[superflyrolla]

RT660. One partially dry run and they're going to be faster IMO.

 

[Lindenwood]

Out of curiosity, has anyone done reasonably close comparisons of tire width against relative times? My understanding was increasing total tire surface area is more about mitigating heat (I.e. higher power and weight), given it doesn’t actually increase the overall contact patch at nominal pressures.

 

[The Other Dan]

I'd talk to some ES miata driver. They've been doing the large tire / small rim for decades do to rim width restrictions. If it wasn't faster for them, they would have seen that and stopped doing it long ago.

 

[gearchruncher]

How the tire behaves when wider is as much about the wheel width as anything as well. Just going a wider tire does not for sure make you faster. Tire Rack just did a whole thing on this and making the tire wider without a wider wheel can slow down the car.

https://www.tirerack.com/tires/tests/testDisplay.jsp?ttid=296

 

[dsgerbc]

given it doesn’t actually increase the overall contact patch at nominal pressures.

What makes you think that it doesn't? I usually use this calculator based on one popular tire model to guage these things.

 

[gearchruncher]

What makes you think that it doesn't? I usually use this calculator based on one popular tire model to guage these things.

There was a whole discussion here recently. Physics says that a wider tire with the same pressure in it has the same contact patch total area no matter how wide the tire is. But the shape changes. Just like the amount of boat under the waterline is not based on the size of the boat, but the weight.

Effect of wider tires on range

Sure, deforming a thin flexible sidewall is less of a loss than the deforming of a thick stiff tread, but both a wide and a thin tire deform the tread and a thin tire would be deforming less total tread width but at a higher angle which is worse because the tire is losing more of it's shape...

teslamotorsclub.com

 

[dsgerbc]

There was a whole discussion here recently. Physics says that a wider tire with the same pressure in it has the same contact patch total area no matter how wide the tire is. But the shape changes. Just like the amount of boat under the waterline is not based on the size of the boat, but the weight.

Effect of wider tires on range

Sure, deforming a thin flexible sidewall is less of a loss than the deforming of a thick stiff tread, but both a wide and a thin tire deform the tread and a thin tire would be deforming less total tread width but at a higher angle which is worse because the tire is losing more of it's shape...

teslamotorsclub.com

That's really simplistic physics that applies only to loose flexible tire tubes. Pneumatic car tires cannot be modeled that way. In reality (captured by more complex physics models, like the one I linked to), wider tire = larger contact patch.

 

[Lindenwood]

That's really simplistic physics that applies only to loose flexible tire tubes. Pneumatic car tires cannot be modeled that way. In reality (captured by more complex physics models, like the one I linked to), wider tire = larger contact patch.

Yes, differences in tread and sidewall stiffness can modestly affect this (again, at nominal pressures), but most of the difference in raw grip come from compound and tire pressure. Indeed, though, lots of factors affect “feel,” how quickly a tire transitions through limits, etc.


Frankly I am not really sure we have enough data about tire construction to reliably predict tire performance beyond that.

 

[gearchruncher]

That's really simplistic physics that applies only to loose flexible tire tubes. Pneumatic car tires cannot be modeled that way. In reality (captured by more complex physics models, like the one I linked to), wider tire = larger contact patch.

The one you linked to is not a more complex physics model. It's a calculator with zero transparency on how it works.

Jason Camissa and many other sources disagree that it's more complex:

Point(s) of Contact: Wider isn't necessarily better

Wider tires put more rubber on the road, right? Wrong. Jason Cammisa explains why upsizing your tires may not be the path to improved performance.

www.roadandtrack.com

Physics

Site Maintenance | Discount Tire

If the sidewall and tire really was that stiff, then the tire wouldn't collapse when it didn't have air in it. Yet it's clear to anyone that has worked with tires at all that the air pressure in the tire is by far the dominant element defining how much of the tire is touching the road.

But we should really discuss this in the other thread.

 

[dsgerbc]

The calculator I linked to belongs to a respected tech modelling shop used by many OEMs. It was recommended to me by an Avon tire engineer. Most tire data/models are proprietary and cost money. This one is a rare exception.
Automotive journos using high-school physics models are not a good source answers to empirical questions. Those simple models are a good first qualitative approximation, but usually give wrong answers to quantitative questions.
This tread seems as good as that other one to discuss effects of tire width on performance.

 

[SK360]

Yesterday was points event 3... I historically don't do great on this course and I apparently woke up forgetting how to drive.. 79SSR was def not in his S2000, he was driving 9SSR's GT3. Poured most of the day... Only first heat and very end of last heat got dry runs, Coned away a 30.9. We also had multiple instances of cones being out of place for many runs due to the rain and course workers not paying attention to where the marks were. Good example is compare 0:30 vs 1:19.

I also struggle keeping heat in the battery and I notice my peak power output suffers because of it. I don't think I saw much over 33C yesterday.

 

[dsgerbc]

I also struggle keeping heat in the battery

Maybe we need to be covering the bottom of the battery with a blanket between runs....

 

[SK360]

Maybe we need to be covering the bottom of the battery with a blanket between runs....

I did a lot of logging a few weeks ago... I left my house at 7:00 at 27.5c after charging to 100% from 70%. I stop at a Sheetz that happens to have a supercharger for breakfast so I plug in and let it build some heat. Left the supercharger at 7:45 @ 52.5c. Was 27.5c after the drivers meeting at 11:11 and 43.0c after I sat in it and navigated to a supercharger the whole time before my first run at 11:48. It's a pain to build heat by navigating to a supercharger, even if you plug the seatbelt in I think the car stops after 10 minutes if you're not in it.

 

[SK360]

Decided to do data overlays of my last event, wasn't going to but was curious. I think I may go 0 regen for a bit, it seems I've fallen into a habit of riding regen too much and not using friction brakes hard enough.

 

[SK360]

MPP rear arms on as well as the Whiteline poly offset compliance arm bushings set to full offset. Hopefully this alignment keeps my tires happier

 

[Lindenwood]

MPP rear arms on as well as the Whiteline poly offset compliance arm bushings set to full offset. Hopefully this alignment keeps my tires happier

View attachment 936756

Do you have aftermarket UCAs? When you describe the compression rod bushings at full offset, do you mean for maximum camber or maximum caster (or a mix of the two?)

Also, have you tried running 0 toe f/r if tire wear has been an issue?

 

[gearchruncher]

Also, have you tried running 0 toe f/r if tire wear has been an issue?

He's getting inner tire wear- toe in should help this.

 

[buckets0fun]

He's getting inner tire wear- toe causes this.

*fix0r’d.

Depending on a bunch of other factors, including but not limited to tire temps.

 

[SK360]

Do you have aftermarket UCAs? When you describe the compression rod bushings at full offset, do you mean for maximum camber or maximum caster (or a mix of the two?)

Also, have you tried running 0 toe f/r if tire wear has been an issue?

I’ve been aligned to 0 total toe F/R 3 times and each time I’ve killed street tires inner edges in 7500-10k miles. Car toes out on compression so hoping slight toe in helps.

*fix0r’d.

Depending on a bunch of other factors, including but not limited to tire temps.

I haven’t ever temped my street tires