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09-19-2019, 08:56 PM | #15 |
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They are both important tuning tools. It must be mentioned that track conditions change too, so a stopwatch one day is not the same as a stopwatch the other day.
Also worth noting it's 2019, by stopwatch I assume you mean super high refresh rate GPS datalogger, not an actual stopwatch haha
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09-19-2019, 09:46 PM | #16 |
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Thanks all! I knew I could count on the Twins brotherhood. Gave me a lot to chew on. Obviously I have entered a brave new world with these tires and must "unlearn what I have learned."
I'll take it from here but these are the answers to your questions:
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09-19-2019, 10:50 PM | #17 |
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Front camber is your friend, you need more of it... Camber bolts is the easiest/cheapest way. Not quite enough, but a lot better than stock which is disturbingly close to zero...
Also, zero toe all around for alignment is a good place to be for handling and tire life. Never just get a generic alignment, specify exactly what you want or you'll just get "in the green" which is a quite wide range of toe settings... The outside edge of the middle rib in the RE71R wears down *fast*, that's just the way it is. When you have half tread left overall, the middle rib will be radically beveled with the outer edge worn almost to the bottom of the tread. Last edited by ZDan; 09-19-2019 at 11:01 PM. |
09-19-2019, 11:31 PM | #18 |
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Really interesting seeing everyone here running entirely different pressures than what I run, lol. Maybe I'll try a run at these mid-30s pressures and see what happens.
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09-19-2019, 11:53 PM | #19 | |
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DS/CS legal camber bolts are the OE 14mm bolt, Google will point you to the right factory part, stick that in the top hole, push in, tighten, take to high quality alignment shop and hope you can hit -1.3 on both sides, don't bother going back to the alignment shop without at least that. Other camber options bump you to STX, imho burn up these tires before jumping into that pool.
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When I said 29 that's usually my target and I'll bleed a pound out of whatever end needs more grip so really 28 hot is my 'optimum' as above lots of factors, maybe you've got a gauge that reads different to mine,a longer haul between course and grid, etc. 27 psi is close enough to what I run that I doubt you're leaving much on the table. (For OP this is a 245/40/17 on 17x9, I'll be surprised if you can get away with much lower than 30psi but could be wrong you're not pinching like most do) |
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09-20-2019, 07:14 AM | #20 | ||
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My experience is at road courses, no autoX. To get to the quite low hot pressures strat is talking about, 27-28psi hot, I'd have to go out ~18psi cold. I seriously doubt that would give any more grip, it's likely well into a range where increased rolling resistance = slower, and likely getting into the realm of *dangerously* low...
I don't agree with the notion that less pressure => more grip is an absolute truth. Not for a radial tire at autoX/road course on appropriate radial tires anyway. As opposed to a dragstrip with a very tacky launch pad and drag tires, which is a very different scenario... When I get to work later today I'll look at lap times over a session from Watkins Glen (in Cayman, not BRZ) last week, with cold pressures 24/26psi and hot up to 36/37psi (outside rear saw 40 one session early in the day) and see how my best lap times compare early to mid-session. Very hard to get reliable data, though, as there's traffic to contend with. Also later in the session, 200tw street tires can be overheating so fall-off in laptimes can be due to that rather than "too much pressure". Quote:
I seem to be able hit lap times within a few tenths over a fairly broad range of pressures. But it's impossible for me to test lap times vs. pressure in isolation, without a ton of other variables coming into play, which makes drawing hard conclusions impossible. But from what I've experienced I don't think there's a whole lot of difference in lap time potential for me from 30psi to 36psi. Quote:
For OP, with limited camber and 215s on 7" wheels, I doubt that running 28psi hot vs. 34-36psi hot is going to be worth anything. But again, I don't autoX... You're likely totally fine at the 32/33psi hot pressures for autoX. |
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09-20-2019, 07:41 AM | #21 |
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I agree with you, apples to oranges
Doing scientific tests is hard, test and tune days exist with shorter simple courses and skidpads to reduce variables, 28 feels good to me and is reflected in consistent times that I'm happy with. Lower and I find the tire less predictable causing errors, higher and I feel that I'm losing lateral grip, unfortunately I did not have good Accel data to support that only the stopwatch. I've also spoken with people I respect with similar setups and we've arrived at similar results. Edit: approx -4 and -2.3 camber here iirc, been awhile since I've been on a rack. |
09-20-2019, 08:42 AM | #22 |
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If overheating is brought up, low pressures = more flex = more heat into tires.
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09-20-2019, 10:36 AM | #23 | |
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For my last few set of tires, all of this on 17x9 +35 wheels, -4.7F/-3.0R camber and 0F/0.05R toe (toe-in rear) and on teh same day (70F ambient temp, dry and humid):
Now do with this information what you want, flush it down the toilet for all I care, but I just want to point out that the "as low as you can get it" statement from me came from my own personal data gathering. I have been bitten on the ass many times already by trusting what "experts" or "experienced" people say. |
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09-20-2019, 05:01 PM | #24 | |||||
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Note to OP et al, this is regarding track pressures, not autoX!
Icecreamtruk, what are your cold pressures? I'm getting hot pressures up to 36ish going out with cold pressures of 26ish. I would have to start with cold pressures of about 20psi to get 30psi hot pressures. And I'm not going out with 20psi... Quote:
I don't mind learning new things, but I have never had anything like the resources required to truly hone in on "optimal" pressures, which would pretty much take an open track mostly to myself and multiple sets of same make/model tires from the same batch on the same make/model wheels. Long/short it ain't happenin... For 36psi hot, which feels fine to me on my streetish setup cars, I'm already going out with cold pressures as low as I feel comfortable with. Quote:
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I.e. I don't think pressure vs. lap time should be a step function, where 1psi below "optimal" is a handling disaster. And if that *were* the case, it would again argue for ensuring your tire pressure is comfortably above the step... Quote:
Also, a proper test for "optimal" pressures? Ideally with pressure the *only* variable, with enough runs at different pressures to remove effects from other variables including heat in the tire, number of laps on the tire, time of day, driver being "cold" vs. "hot", etc. etc. Ideally testing would also be *blind*, i.e. driver doesn't know what pressures he's driving on. Not easy to arrange... Quote:
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The Following User Says Thank You to ZDan For This Useful Post: | Icecreamtruk (09-22-2019) |
09-22-2019, 11:47 PM | #25 |
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@ZDan Overall I agree with you on the argument of erroring on the high side and it all being safe. I dont want to argue or anything, it seems like you want some honest answers to some of the questions you raised after my post, so here they are.
For cold pressure, depending on day or track I go out at 22 to 24psi. I dont drive like I stole it on my first session, Im aware of my tires being underflated, and even if somehow I wasnt, I can feel the tire distorting under braking and turning. I rarely get the pressure I want in one session, it takes 2 or 3 sessions doing small adjustments to get it right. I also dont set equal presures on all corners, more on that in a second. Lower pressure = more heat. I've heard this a number of times, but I wonder if its actually true, because the pyro says completely the opposite. Higher pressures = more heat, lower pressures end up with less heat on the tire. I imagine it has to do with people thinking the rubber moving more with lower pressures or the bigger contact patch makes it heat up more, but thats not what the pyro says to me. Im not running 2-3 laps at a time, I run 20 minutes sessions. But at the same time, I run time trials, so only the fastest lap count, the rest I use it as practice if I know I have tires to spare and that Im not destroying them by overheating them. On a street tire, the fastest lap is almost always the first lap out of the pits (first full lap after the outlap). When I say it takes 2-3 laps to get the ideal pressure, what I mean is that 2-3 laps in will be my fastest lap on that tire, meaning usually that pressures were too low and I was having to deal with the tire rolling over, or not heated up enough yet (im in canada, track days can be cold sometimes). Regarding pressure changes, 1-2 PSI dont change my car into a mustang, but might be enough to gain or drop half a second. Half a second can be the difference between first and sixth place in a time trial, so to me, it is drastic. If you go under the point where the tire starts rolling over, it will handle quite different, taking more time to settle up during turns, and getting easily upset by bumps and curbs (and times suffer much more). Thats all I got, more or less, if you have more questions, shoot away. I know this all sounds very subjective and non scientific way but the stopwatch doesnt lie, even if a driver isnt Senna, as long as he can drive to his limits every time out, the stopwatch will tell you if the change was good, or bad. Its then up to the driver/team to either record information on what changed and the effect it had, or chuck it all up to external factors and decide it did nothing or little. Edit: forgot to add, this all works very well for a car that is track ready, at least suspension and alignment wise. Trying to optimise pressures too much when you have too little camber is just going to end up killing the tires. I imagine that somebody looking to extract the absolute maximum of their tires will have taken the time to get an alignment dialed up to maximise tire performance, otherwise, yeah, "36 hot and your good" is as good as anything else. |
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09-24-2019, 03:12 PM | #26 | |||||||||
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I go out with ~27psi first session (sometimes as low as 24 on the corner that gets worked hardest), and I drive it hard from lap 1. Even when I didn't plan to! Bleed pressures down to 36 all around after 1st session, repeat as required throughout the day. Quote:
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I think that what happens is that the tread does get up to temp very quickly, but that pressure, which is related to *carcass* temperature and wheel temperature, increases at a much slower rate. It takes some time for the mass of air in the tire to heat up. Worth noting that pressures can continue to rise on the cooldown lap and even in the pits for short time as heat from the hubs and brakes continues to go into the wheels and the air in the tire, unlike tread temperature which falls off quickly. Quote:
But given that track and weather conditions are changing throughout the day, and repeatability variations, how do you *know* that half second is due to pressure and nothing else? Quote:
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But based on my observations at the Glen I may revise my approach to cold pressures for the 3-lap time-trial... |
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The Following User Says Thank You to ZDan For This Useful Post: | Icecreamtruk (09-24-2019) |
09-24-2019, 04:54 PM | #27 |
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@ZDan Yeah you are pretty much spot on on most things. I just wanna come back into the temperature and pressure relation topic, since I find it interesting.
Whether or not the extra movement on the carcass is generating heat is of course not a question, it is generating more heat. But what I wonder is, if that heat is actually more than the heat generated from the extra pressure on the tire. I measure temps with a needle pyro so it should not be surface temps that I get but rather the actual core temps (or closer to it at least). On the same day, with track temps not varying much, dropping about 4PSI on the tire made the temps drop by about 15F after a 4 +2 laps (out lap, 4 hotlaps, cooldown lap). Where is the extra temp coming from at higher pressures? I dont think it is from more tire slipping or extra slip angle, I wasnt getting much more of that anyways, I think its the interaction of pressure and temperature. Disclaimer: its about to get a bit nerdy. We arent in a lab, but the ideal gas law should still apply to air to some degree, and PV = nRT gives us a simple way to look at it. Tire volume (V) does not change, only the shape of the contact patch does, R is a constant, n changes (goes up) a bit, but a very minuscule amount, so small that its ok to assume it a constant. So if the product PV goes up (pressure increases), so must T (temperature) if the law is to be respected. My readings at the track seem to indicate, altho not linear, that this is what affects temps the most, as higher pressures have always yielded higher temperature, not once has it been the other way around since I started monitoring it. Before recording this, I was convinced it was the other way around, and was setting my pressures accordingly (lower pressures for more temp and vice-versa) and I could not understand the results I was getting. Every time I made an adjustment, it got worse so at some point I stopped caring and started doing the "34-36psi hot and forget it" method. I then got the pyro to check my alignment settings, not for tyre pressure adjustment, and it was then that I started noting this on the recorded that. Im actually able to get better times after adjusting pressures now, most of the times at least. |
09-24-2019, 08:49 PM | #28 | ||||||||||
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Even if there was one, it would equalize back to ambient over time. Assuming we both arrived at the track with our desired starting pressures, you at (say) 22psi and myself at 26psi, we are starting out at the exact same tire temps, I don't have higher temps in my tires just because the pressure is higher. that's not how PV = nRT works. (my college roomate's mnemonic for this: Per-Vert equals neRT) *If* you're talking about pressure gained by being out on track, that is due to the temperature change of the air inside the tire. Pressure goes up because temp goes up, not the other way around! But the important temperature here is the temp of the *interior* of the tire, and the wheel. Quote:
Going back a few years, but back when Garry McCoy was throwing his 2-stroke MotoGP bike around, sliding the rear all over the place, people were *amazed* that he wasn't quickly overheating the rear tire, as that was a huge problem for most bikes/riders at the time. A theory was that he was *working* the tire less by lighting up the rear and sliding it around, so while outer tread surface temps may have been locally higher, the internal tire temps were lower due to less deformation under loads! Like I say, this was a "theory"... Quote:
For sure if I don't want to go out on less than 26psi in general, 24 psi absolute minimum on the tire that sees most pressure change, I'm going to see hot pressures of 36psi by end of session, so just based on that I don't see myself trying to run less. |
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