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Cup2R Tires Road Atlanta - Managing Pressure at HPDE Track Event

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20
25
Exp. Type
HPDE
Exp. Level
3-5 Years
Athens, Georgia
I was at Road Atlanta with Chin on June 24th and 25th trying out a brand new set of Michelin Cup2R tires. This was my first experience with R compound tires as I had been running PS4S and Bridgestone Potenza Sport tires which are both 300TW. The Cup2R tires are 140TW. Here are my observations:
Cup2R is super sticky and picks up a ton of debris. Thinking now I should trailer the tires to the track and put on in the paddock. Driving these tires on the road they look like a lint brush that has been rolled across a gravel road.
On the track the Cup2R stick! Posted my personal best of 1.39.4 first session out. Probably could have done better with practice but after that one solo session, I was giving rides to friends and family and drove more conservatively the balance of the weekend. Still consistently posted 1.40 to 1.42 times and didn’t feel close to the edge.
Tires give no warning at the edge of grip but recover almost instantly with a slight throttle release and/or steering correction. Felt very safe.
Was able to go much deeper into braking zones with a lot of confidence.
The jury is still out on tire wear. Had an engine cooling issue on Saturday and missed the afternoon sessions. Was back on the track for Sunday but didn’t do the happy hour. So all-in-all only have the equivalent of one full track day on the tires. Wear looks fairly even running -3 deg front and -2 deg rear camber. I’m sure these would give a full weekend of service. But not sure if these will last 2 full weekends or not. I think they probably will. I’m going to give them a try to see. I’ve got a square set-up so definitely would need to rotate.
Managing tire pressure is a challenge. Michelin says these tires should not be run cold below 29PSI and hot should go to 39psi max depending on car weight. I started at 29 psi and ran a few easy laps to build heat and pressure. Once the tires got to 33 psi, I started running fast laps. Probably after 4 or 5 more laps psi would hit 39-40. 37 psi seemed to give the best grip for my car. But the only way to keep the pressure at 37 psi would be to run some laps, Pit, adjust pressure, and go back out. I’m running straight air. Wonder if Nitrogen fill would help? Any one have experience with pressure variation with nitrogen filled R-comp tires?
Bottom line: Expensive tire. Lots of smiles per dollar.
Questions and comments, Please.
 
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Nitrogen would help, so would dehumidified air. You need to find a method of pulling air out of the tire, don't just let the air out and refill it with DA or Nitrogen., You are looking to get the humidity in the tire to below 15%.
The tool to measure that is expensive, but properly purging the tires will get you there, I have never had a 15% or more tire that I have purged. Spoiled nitrogen or dehumidified air is another issue that sometimes crops up, but let's skip that for now, What will happen is the tire, especially the tire that is most loaded, will have run away pressure,
With regards to Michelin's specifications on tires, remember how that number is derived. Some engineer builds and tests the tire, then they add a few psi just to be on the safe side in case you are running too much camber, then legal gets it and adds a few more psi to that, just in case and you get your final psi measurement. NOW DON'T GET ME WRONG, I WOULD NEVER ADVISE GOING AGAINST THE FACTORY RECOMENDATIONS, but then I've never exactly paid a whole lot of attentiont ot hem either.
You are on the right track by using the hot psi measurement to establish the correct cold pressures. I also think that track was just repaved, and Hoosier has already sent out a "heads up" on the MX5 cup guys who are running there. One more item, new track = old tires, old track = new tires. just a general rule. I think your tire wear will go down once the tires are heat cycled or the track gets some age to it.

 
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Bill Pemberton

0ld Ford Automotive Racing Terror
8,522
8,582
Exp. Type
Time Attack
Exp. Level
20+ Years
Blair, Nebraska
I have a set of PSCup2s on a set of Apex 19x11 wheels and they are virtually as bald as me, yet still no cords are showing. They were still working ,to a degree clear down there, though of course not as efficient as prior. I would have to agree with the Tire Mafia Guy and say I am not sure what the Manufacturer suggests , and since I have bifocals and find it hard at my age to stoop way down to see what the tire might say , I just start at 27 lbs. cuz it works for me. They work really well in the 32 - 37 range imho. They do same to take a bit higher hot pressures than some other tires I have run , but hang in there, they are decently quick and I think you will get used to them and find they will talk to you more so you can get closer to their edge.
 
98
129
Exp. Type
HPDE
Exp. Level
20+ Years
Charlotte
Nitrogen would help, so would dehumidified air. You need to find a method of pulling air out of the tire, don't just let the air out and refill it with DA or Nitrogen., You are looking to get the humidity in the tire to below 15%.
The tool to measure that is expensive, but properly purging the tires will get you there, I have never had a 15% or more tire that I have purged. Spoiled nitrogen or dehumidified air is another issue that sometimes crops up, but let's skip that for now, What will happen is the tire, especially the tire that is most loaded, will have run away pressure,
With regards to Michelin's specifications on tires, remember how that number is derived. Some engineer builds and tests the tire, then they add a few psi just to be on the safe side in case you are running too much camber, then legal gets it and adds a few more psi to that, just in case and you get your final psi measurement. NOW DON'T GET ME WRONG, I WOULD NEVER ADVISE GOING AGAINST THE FACTORY RECOMENDATIONS, but then I've never exactly paid a whole lot of attentiont ot hem either.
You are on the right track by using the hot psi measurement to establish the correct cold pressures. I also think that track was just repaved, and Hoosier has already sent out a "heads up" on the MX5 cup guys who are running there. One more item, new track = old tires, old track = new tires. just a general rule. I think your tire wear will go down once the tires are heat cycled or the track gets some age to it.


Let’s say an enthusiast wanted to pursue the idea of purging a tire with the goal of reducing the humidity and refilling with nitrogen. Have you seen affordable methods? I’m on board with the science, but don’t have the budget for fancy equipment.
 
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I use a simple venturi method, you can cob one up with some bits of plumbing from a hardware store..
I would prolly just run air through about 3 water filters and send it. Unless you're hitting over about 175 degree tire temps nitrogen prolly won't be necessary.

 
98
129
Exp. Type
HPDE
Exp. Level
20+ Years
Charlotte
I use a simple venturi method, you can cob one up with some bits of plumbing from a hardware store..
I would prolly just run air through about 3 water filters and send it. Unless you're hitting over about 175 degree tire temps nitrogen prolly won't be necessary.


I’m intrigued by this idea. In my spare time I did some searching and found something I need your take on. Please give this a quick read (it’s about 5 minutes I’d say) and let me know whether you’d be on board with the amount of deformity to the tire, the potential for damage to the tire and whether it’s suitable for DOT radials.


Also, the author advocates for a nitrogen fill after purging the tire, but I agree that starting with filters to dry the air may be a good starting place. However, my mid-session pit lane temps have occasionally been in the 170 range.

You may be the only tire specialist many of us have access to. Your generosity and knowledge sharing are greatly appreciated, sir.
 
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I’m intrigued by this idea. In my spare time I did some searching and found something I need your take on. Please give this a quick read (it’s about 5 minutes I’d say) and let me know whether you’d be on board with the amount of deformity to the tire, the potential for damage to the tire and whether it’s suitable for DOT radials.


Also, the author advocates for a nitrogen fill after purging the tire, but I agree that starting with filters to dry the air may be a good starting place. However, my mid-session pit lane temps have occasionally been in the 170 range.

You may be the only tire specialist many of us have access to. Your generosity and knowledge sharing are greatly appreciated, sir.
Well, the guy is essentially right, only the high roller Indy, F1 and maybe a few WEC teams (that I'm aware of) are doing anything close to what he is talking about. They have multi thousand dollar trick machines that will pull down the tire, and then fill it with nitrogen to the desired psi. That's fine but... there is a whole lot more to it than that, As an example, virtually every tire manufacturer prints warning labels on their tires not to bounce them at temperatures lower than about 45 degrees F. The reason is that they can break cords and you can have a failure. when a Porsche arrives from Europe, the tires are discarded because they have been "frozen" on the plane, on the trip over, so the tire manufacturers are pretty serious about that temperature thing. So now we see this guy pulling a tire down almost to the bead, that just cannot be right and I'm pretty sure that if the manufacturer was aware of that, they would be freaking out,
There are plenty of "octopus" style purging systems out there, and they can be used to purge more than one tire at a time, those are actually pretty common. Some will even allow you to backfill with nitrogen and these are relatively inexpensive compared to the hi zoot stuff.
What we are looking for is 15%, that's the magic number for humidity in the tire, Anything less than that won't give us a problem, anything over 20% is a disaster waiting to happen. Those numbers can be achieved by simply pulling down the tire so there is a "dent" in the tread area, then filling with nitrogen. I have never, nor will I ever, distort a tire to the level shown in those pics, I have an instrument that measures himidity in the tire, using my method, it has never been over 15%. If it should happen some day, I would simply dump the nitrogen, repurge the tire and refill it, If it happened again, I would question my nitrogen source.
I don't mean to throw shade on the guy that did the article, it is a great piece, and it shows a lot of ingenuity, I just don't think that level of purging is necessary and it prolly won't fly with the tire manufacturer either.
I'm still stealing it for Tire Guy Mafia however.
 
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98
129
Exp. Type
HPDE
Exp. Level
20+ Years
Charlotte
Thank you, sir! That confirms some of my suspicions. I’m making turns to construct a Venturi pump and dry air fill system (i.e. filters in series).

By the way I found the Tire Mafia site and registered. If this is the kind of stuff I might find there, I’m excited to join.
 

Dave_W

Cones - not just for ice cream
1,016
1,326
Exp. Type
Autocross
Exp. Level
20+ Years
Connecticut
I think the authorof that article is suffering from a misunderstanding of the cause of the tire pressure increase he was seeing. He seems to think using pure nitrogen is better than using air because of the oxygen in air. "Air" is 78% nitrogen and 21% oxygen, plus about 1% of other stuff (at 0% humidity). If I remember correctly, all gases have the same moles per litre, so nitrogen vs a nitrogen-oxygen mixture doesn't change the pressure increase seen with increased temperature in a fixed volume.

What really makes a difference, as @blacksheep-1 points out, is water vapor. Water vapor does not adhere to the formula of the Ideal Gas Law because it's not a gas. You want as little water vapor in your tire as reasonably possible, but whether you use dry nitrogen (which is easy to find at your nearest welding supply) or nearly dry nitrogen-oxygen mix (which you can make yourself with atmospheric air and a bit of work) doesn't much matter.
 
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I think the authorof that article is suffering from a misunderstanding of the cause of the tire pressure increase he was seeing. He seems to think using pure nitrogen is better than using air because of the oxygen in air. "Air" is 78% nitrogen and 21% oxygen, plus about 1% of other stuff (at 0% humidity). If I remember correctly, all gases have the same moles per litre, so nitrogen vs a nitrogen-oxygen mixture doesn't change the pressure increase seen with increased temperature in a fixed volume.

What really makes a difference, as @blacksheep-1 points out, is water vapor. Water vapor does not adhere to the formula of the Ideal Gas Law because it's not a gas. You want as little water vapor in your tire as reasonably possible, but whether you use dry nitrogen (which is easy to find at your nearest welding supply) or nearly dry nitrogen-oxygen mix (which you can make yourself with atmospheric air and a bit of work) doesn't much matter.
Correct, the rate of rise will be different because there is a difference between air and nitrogen, but if you plot it, you should be kind of close to predicting hot pressures.
I noticed that in the article as well @Bill Pemberton , but I just sort of passed on it. It is basically a decent article, and when you write something technical like that, it always opens one up for criticism.
 
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98
129
Exp. Type
HPDE
Exp. Level
20+ Years
Charlotte
I think the authorof that article is suffering from a misunderstanding of the cause of the tire pressure increase he was seeing. He seems to think using pure nitrogen is better than using air because of the oxygen in air. "Air" is 78% nitrogen and 21% oxygen, plus about 1% of other stuff (at 0% humidity). If I remember correctly, all gases have the same moles per litre, so nitrogen vs a nitrogen-oxygen mixture doesn't change the pressure increase seen with increased temperature in a fixed volume.

What really makes a difference, as @blacksheep-1 points out, is water vapor. Water vapor does not adhere to the formula of the Ideal Gas Law because it's not a gas. You want as little water vapor in your tire as reasonably possible, but whether you use dry nitrogen (which is easy to find at your nearest welding supply) or nearly dry nitrogen-oxygen mix (which you can make yourself with atmospheric air and a bit of work) doesn't much matter.

Agreed. Can you elaborate on how you would approach making dry air from atmospheric air? I’m not quite ready to make the leap to nitrogen if I can create sufficiently dry air.
 

Dave_W

Cones - not just for ice cream
1,016
1,326
Exp. Type
Autocross
Exp. Level
20+ Years
Connecticut
how you would approach making dry air from atmospheric air?
Take a look at info on getting clean, dry air for vehicle paint booths. When painting a car, water in your air supply can create fisheyes in the paint, costing time & money to repair / repaint. A lot of the solutions revolve around 1) condensing the water vapor into water, then 2) removing any entrained water from the compressed air. Pros use things like "aftercoolers" between the compressor and storage tank and refrigerated chillers, but a DIY solution is to run multiple loops of copper tube through a bucket of (ice) water and then a water seperator before the tank. Drain the water from your tank religiously - consider replacing the common radiator-style drain valve with some tubing and a ball valve where it's easy to reach, or step up to an automated timer valve. Use multiple separators / filters (as mentioned above by our in-house tire guru) - at the tank outlet and drop leg or point of use, and maybe a dessicant dryer as the last one. Remember to drain your filters, or get ones with automatic drains.

Note how the main trunk has distance to cool the air and is sloped to a drain leg, and each branch comes off the top of the trunk line and has its own drain leg.


All kinds of filters here

Disposable dessicant dryer you can put right before your air chuck

 
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Agreed. Can you elaborate on how you would approach making dry air from atmospheric air? I’m not quite ready to make the leap to nitrogen if I can create sufficiently dry air.
About 3 water filters in a row, and drain the compressor before you start. I use 1 water filter on my comp, but can add 2 more with a valve. For my auto x stuff, I use air, but when I get serious, it's all nitrogen.

Screenshot_20230722_222611_DuckDuckGo.jpg
 
20
25
Exp. Type
HPDE
Exp. Level
3-5 Years
Athens, Georgia
Thank you for the replies. I just bought two Desiccant filters for about $8 each at Harbor Freight. I’ve got one installed at the output port on my portable compressor. The other is in-line just before my tire chuck. Figured two would work better than one. As a test, I’m going to purge the air from one front tire and fill with the filtered air, set both tires to the exact same cold pressure, then take for a drive with some heavy braking. Will be interesting to see if the tire with filtered air has less pressure increase and how much difference. If I can reduce the rise by a few psi or even a couple, that would be wonderful.
To purge,I think I’ll go for a drive to heat up the tires and air (hot air will hold more moisture), jack up one corner, bleed the air from the tire as far as I can, fill with filtered, go for another drive, and repeat a couple of times.
 
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The desicants are superior, but you can get there both ways @Dave_W pointed out some great ways to run your plumbing from the compressor, things like always coming off the top , running lines uphill, all of this helps the situation. In Fla we have to deal with 100% humidity almost every day, so it's a big deal for us. I'm sort of spoiled because I get to use nitrigen at the races so I don't have to deal with it. Real dehumidified air is fine to use, and as several have stated, it is the moisture, not really the air or nitrogen that is the issue. Get rid of the water, and your problem goes away.
 
98
129
Exp. Type
HPDE
Exp. Level
20+ Years
Charlotte
The desicants are superior, but you can get there both ways @Dave_W pointed out some great ways to run your plumbing from the compressor, things like always coming off the top , running lines uphill, all of this helps the situation. In Fla we have to deal with 100% humidity almost every day, so it's a big deal for us. I'm sort of spoiled because I get to use nitrigen at the races so I don't have to deal with it. Real dehumidified air is fine to use, and as several have stated, it is the moisture, not really the air or nitrogen that is the issue. Get rid of the water, and your problem goes away.

So if I have been seeing pressure gains in the 8 lb range, what should I expect from dehumidified air?
 

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