[freakin_elrod]

Adjustment/selection of the rear swaybar is only critical for turn-in because when the inside wheel lifts off the ground, it's no longer doing anything. This is why the FoST is at its best in long sweepers.

That has been bouncing around inside my head for the past couple years and only yesterday did I recognize that, for STH design purposes(*), I can measure the G-force required for the wheel to lift. All I need to do is mount the GoPro where it can see a rear wheel, drive, and let Solostorm do its thing. Unfortunately, my autox season has ended... I did go through pics from events and matched them up with Solostorm data, but that exercise only told me what I already knew: by 1G, the wheel is already well in the air. So it likely lifts at around 0.9G, maybe 0.8G, which is too big of a range for design purposes. Pity the local parking lots are too small or have too many cameras.

(*) Designing for GS is pointless as tripoding "always happens" well before you've reached peak G-force. A sensitive-butt driver might be able to calibrate their corner entry better, but I'm paying more attention to whether my line is holding or not. For STH, however, knowing the number helps calibrate roll stiffness... @freakin_elrod will probably tell me "just use my spring rates because it will still always tripod" and he wouldn't be wrong, but I'd still like to make a coherent STH suspension design.

LOL My spring rates work though @gemery !! On a serious note, the tripoding isn't as bad in STH trim as it is in GS, given that you don't go with an overly soft front spring rate.

 

[gemery]

2021 PAX is available now, 2021 PAX/RTP Index

GS is now the third slowest class, having leapfrogged ES. GS index has become faster by another +0.002, for a total change of +0.006 since 2019.

 

[bobharly]

2021 PAX is available now, 2021 PAX/RTP Index

GS is now the third slowest class, having leapfrogged ES. GS index has become faster by another +0.002, for a total change of +0.006 since 2019.

What so is this index in seconds? As in these classes are literally separated by less time that in takes to blink the human eye?

 

[gemery]

What so is this index in seconds? As in these classes are literally separated by less time that in takes to blink the human eye?

On a 60-sec course in 2021, GS must run 0.36 sec faster than in 2019.

 

[gemery]

I was already planning on getting some kind of thermometer to measure the tires and brake temps, like one of those hand held IR scanners. However, I keep seeing the probe type mentioned for tires as being more accurate. So I am guessing that keeping tire temps even from outer to middle to inner treads uniform is more important than an exact pressure, as pressure can vary by track temps and ambient temperatures for that day.

I do wish to a question regarding pressure, since I picked my starting pressures by looking at what other online members were using--without consider of tire type or ambient temperatures. Should I go back to basics and start off with 36 PSI all around, check temperatures and then decide to bleed off air?

Moving these questions from the STH thread to the GS thread.

Two things are going on with tire temperatures. The first is that tread surface temperature reflects what the tire compound is seeing at any instant and it can rise & fall very rapidly. This is what the IR scanner will measure. The second is the carcass temperature, which is more stable, and the probe style of tire pyrometer measures carcass temperature. They're both pretty useless for stock vehicles because they'll only tell you what you already knew: the outside edges of your tires are getting abused because you don't have enough negative camber to support the extreme roll angles. You might be able to discern whether a tire is overinflated or underinflated, but with no precision because the inside edge is so much cooler than the outside edge.

Use your hand in place of a tire pyrometer:

• Front tires - if it is very warm, it's time to spray with water. Very warm, depending on thickness of your skin, is about the edge of comfort, but below the feeling you'll get burned. If ambient air temperature is above 70F, then you're probably spraying water after the first run.
• Rear tires - if you can't feel any warmth and the ambient air temperature is below 60-65F, then be prepared for the rear end to snap on your corner entries. Never will you need to spray rear tires with water.

To set air pressure, work with the wear pattern where the tread meets the sidewall. You want to see wear just beyond where the tread falls off a cliff, but not down onto the sidewall. You can use a little chalk or shoe polish to discern the wear if it's not obvious. Some tires have weakish sidewalls and they'll possibly want as much as 43 psi in the front when autocrossing to keep from rolling onto the sidewall (Yokohama A052 and Nexen N FERA SUR4G). For the OEM Goodyears and Bridgestone RE-71R, starting with 35-36 psi on the front is good. Rear tires are pretty forgiving, almost always safe starting 34-35psi.

 

[gemery]

I have found that this car likes to be driven right at the edge of oversteer. On the sweepers, you can feel the rear end slightly drifting out and fronts are slightly slipping too, as the car goes were you point it and carrying the speed through the corner. It's amazing fun and my best times come from runs like this. However, I have to enter the corner about right, both in speed and line. Otherwise I find the rear end jerking in and out as I try to correct with counter steering, and ultimately slowing down to get the rear stable again. If I don't scrub the speed while reeling in the rear I risk spinning, which can happen very quickly. Is this a description of the binding or me still being novice in my core skills?

The stock FoST can hold a sweeper at 1.2G (provided the tire is capable), but the inside rear tire lifts off the ground at about 0.8G, maybe as early as 0.7G, thus the rear swaybar is no longer doing anything. So binding has to happen just before the wheel lifts and possibly depends on how hard you've forced the turn-in. When the binding happens, then you have an instantaneous increase in roll stiffness, so the car naturally oversteers, which you then will countersteer to catch and that tends to start the back-and-forth. Any bumps in the turn naturally exasperate it as the suspension hits bump stops and bounces off of them.

Bump stops also play a role when you're braking into the turn. That's why you mostly use only 70-80% of the braking capacity of the FoST; hitting the bumpstops while braking causes front spring rate to increase dramatically which causes understeer, so you really have to back off the brakes before turn-in.

 

[NimbusST]

Now THIS is a good thread. I can’t wait until my car is finished and I can drive it again. Anyone got a suggestion for how to find a driving coach?

 

[ddfred]

Now THIS is a good thread. I can’t wait until my car is finished and I can drive it again. Anyone got a suggestion for how to find a driving coach?

Contact your nearest sanctioned track and inquire about their High Performance Driver Education Classes.

 

[NimbusST]

Contact your nearest sanctioned track and inquire about their High Performance Driver Education Classes.

LoL, yeah I know...

 

[bobharly]

...
To set air pressure, work with the wear pattern where the tread meets the sidewall. You want to see wear just beyond where the tread falls off a cliff, but not down onto the sidewall. You can use a little chalk or shoe polish to discern the wear if it's not obvious. Some tires have weakish sidewalls and they'll possibly want as much as 43 psi in the front when autocrossing to keep from rolling onto the sidewall (Yokohama A052 and Nexen N FERA SUR4G). For the OEM Goodyears and Bridgestone RE-71R, starting with 35-36 psi on the front is good. Rear tires are pretty forgiving, almost always safe starting 34-35psi.

Thanks for moving this. I hadn't even realized I was off topic.

You make some great points:

1. I had not thought about using my hand to gauge temperature. Clever and simple for the level that I"m at. I'm not on competition tires yet, even with some cheap summer tires.

2. I have used the chalk method up till now. I worked great on cold days, but when I found my self on a warm afternoon, the continually rising air pressure threw a new variable at me. At this point I feel much more prepared to sort out my air pressure in the first 3 runs before the heat affects the internal pressure. The new Falkin have a soft ride on the snowflakes. In fact, softer than BFG Comp AS on 17" rims. So I'll start high and work my way down. I'll do the same for the rear. I"m thinking 42/36. I can bleed down from there as needed.

The stock FoST can hold a sweeper at 1.2G (provided the tire is capable), but the inside rear tire lifts off the ground at about 0.8G, maybe as early as 0.7G, thus the rear swaybar is no longer doing anything. So binding has to happen just before the wheel lifts and possibly depends on how hard you've forced the turn-in. When the binding happens, then you have an instantaneous increase in roll stiffness, so the car naturally oversteers, which you then will countersteer to catch and that tends to start the back-and-forth. Any bumps in the turn naturally exasperate it as the suspension hits bump stops and bounces off of them.

Bump stops also play a role when you're braking into the turn. That's why you mostly use only 70-80% of the braking capacity of the FoST; hitting the bumpstops while braking causes front spring rate to increase dramatically which causes understeer, so you really have to back off the brakes before turn-in.

Now this part, this made my brain cook a bit. So I did some some reading.. and thinking...

In stock form the ST comes with the Eagle F1's, which were rated at 0.94G (car and driver?) and the Pirelli AS at 0.73G (same source I think). Inconsistencies in testing set aside, clearly there is a big real world jump from Summer and AS tires, and so I can infer a similar jump upwards to the TW200 tires. Thus I am concluding that my oversteering and binding are happening at roughly the same time depending on temperature and track inconsistencies. So I maybe I am entering oversteer and then the inside tire finds grip and pulls me back down disrupting my countersteering... maybe?

And bumpstops, I'm sure I"m hitting those on stock suspension given my lack of finesse in both corner entry and on/off braking. I mean I've improved a lot, but my worn out pads offer little modulation after they heat up and I don't get the entry to a corner until 4th or 5th run. Hell, some corners give me grief the whole day.

So driver upgrades are of course a confounding variable, but this all leads me to the next thoughts.

Tire grip determines at what G's oversteer begins. And swaybar bind determines when the inside tire begins to lift. So does that mean an aftermarket swaybar can keep the inside tire on the ground to a higher G load before lifting and the tire grip begins to drift?

 

[gemery]

In stock form the ST comes with the Eagle F1's, which were rated at 0.94G (car and driver?) and the Pirelli AS at 0.73G (same source I think). Inconsistencies in testing set aside, clearly there is a big real world jump from Summer and AS tires, and so I can infer a similar jump upwards to the TW200 tires. Thus I am concluding that my oversteering and binding are happening at roughly the same time depending on temperature and track inconsistencies. So I maybe I am entering oversteer and then the inside tire finds grip and pulls me back down disrupting my countersteering... maybe?

It's not only 200TW tires generating more grip, but also a width increase, from 235 to 255 width. Pavement quality is another variable... smooth asphalt limits at about 1.1G, while a rougher asphalt makes it easy to find 1.2+G.

Tire grip determines at what G's oversteer begins. And swaybar bind determines when the inside tire begins to lift. So does that mean an aftermarket swaybar can keep the inside tire on the ground to a higher G load before lifting and the tire grip begins to drift?

Swaybar bind doesn't cause the tire to lift. Bind causes the abrupt unexpected oversteer (provided it's not a throttle-lift or sudden-braking oversteer) and bends the endlinks. As the rear swaybar gets bigger, the inside tire will lift sooner and possibly more quickly; it's all about relative roll rates.

 

[bobharly]

Swaybar bind doesn't cause the tire to lift. Bind causes the abrupt unexpected oversteer (provided it's not a throttle-lift or sudden-braking oversteer) and bends the endlinks. As the rear swaybar gets bigger, the inside tire will lift sooner and possibly more quickly; it's all about relative roll rates.

Well... clearly I got that backwards.

OK, let's recap. Wider and Sticky tires good. Stock sway bar bad. And novice driver just needs air compressor, chalk and bare hands.

Got it!

 

[buttdyno]

Picking autox back up - first event is in two weeks.

I have a set of 5 y/o RE71R on some 17x8 Modas. I'm debating whether to buy another set of RE71s (255s are on closeout for 135 each, 245s are oddly 146) or get some RT660s, and unsure of size if I go the Falken route.

Would the Falkens be okay as a 255 on an 8" wheel or is the 245 better?

I'm leaning slightly towards the 255s so that I can throw them on the 17x9.5s and run them on the STU car later if I can get the car sorted out, but mostly I just want a brain dead few months of autocross not worrying about setup which is driving me to do 2021 in GS.

Thank you!

 

[gemery]

255/40-17 Falkens should still be okay on 8” rims.

The closeout Bridgestones have been sitting in warehouses for nearly 2 years now, so if you can’t use them up this season, then they’re not a deal imho.

Kuhmo Ecsta V730 are another good choice.

 

[Joe R red14st2]

Picking autox back up - first event is in two weeks.

I have a set of 5 y/o RE71R on some 17x8 Modas. I'm debating whether to buy another set of RE71s (255s are on closeout for 135 each, 245s are oddly 146) or get some RT660s, and unsure of size if I go the Falken route.

Would the Falkens be okay as a 255 on an 8" wheel or is the 245 better?

I'm leaning slightly towards the 255s so that I can throw them on the 17x9.5s and run them on the STU car later if I can get the car sorted out, but mostly I just want a brain dead few months of autocross not worrying about setup which is driving me to do 2021 in GS.

Thank you!

took advantage of the sale (as long as they were not outside of the warehouse @gemery lol)

255 on 17x8 wide(sure would be nice if they let you test out diff. size wheels) comp.18 235/40

 

[buttdyno]

I ended up opting for the 245s just to be on the safe side. I had my first event on them yesterday with WDCR - ended up 2nd/9 in GS, 9th/90 in PAX, though PAX was rain-aided as the 2nd heat was wet-to-drying.

Are the RT660s good to go from the first event or do they need an event or two to get up to speed? (I've seen this behavior on other street tires)

I ended up running about 30-31 psi square. Car felt pretty good and going from the wear triangle that seemed about right, but it was my first event in almost 2 years and I was still making buckets of mistakes every run. Coming from an ST car it's a lot floppier But as an easy button it's great.

 

[buttdyno]

Oh right actual technical question!

When your wheels have TPMS sensors, you can use default mode, sport mode, or totally-off mode.

When your wheels don't have TPMS sensors.. it looks like all you can do is turn electronic stability control off. It doesn't offer the modes quite the same way.

Is that right?

 

[gemery]

I ended up running about 30-31 psi square.

That seems very low front pressures for the RT660 from what I've heard. It's definitely low for the RE71R and the concensus seems to be the front RT660s need 2-3 psi more in the front than the RE71R.

 

[gemery]

When your wheels have TPMS sensors, you can use default mode, sport mode, or totally-off mode.

When your wheels don't have TPMS sensors.. it looks like all you can do is turn electronic stability control off. It doesn't offer the modes quite the same way.

Is that right?

TPMS doesn't matter for modes, at least not on my 2018.

 

[buttdyno]

Thanks! Today's event went a bit better. I was running 38-39 psi up front and 37-38 rear.

I'm slowly knocking the rust off. This was my fast run but there are still a few notable bobbles in here 18, :25, :30)