Topic: Why 29 psi?

[Treeman]

So, I have been playing with this a bit more and doing more digging on the web.  First of all, Tire Rack's web site confirms the max pressure listed on the sidewall is a cold inflation pressure, meaning it will go higher when driving.  Folks did not have in car tire pressure readings until recently.  Second, digging through the tire posts on the big board, I learned a few things from self-professed tire engineers but was surprised that, despite experimenting with just about every other component of the car, folks don't seem to do much fiddlin' with tire pressures.  Well, I'm a scientist and I like to experiment so here goes.  If this bores you, go on to the next post. 

Before I get into the results, I learned that the tire and car manufacturers establish the pressure setting based mainly on load - they are trying to achieve the maximum contact patch.  Mo grip is good and is safe if you are worried about liability.  Comfort and handling come after that.  There was a thread talking bout changing tires - do you need to change pressures.  There is a chart that Tire Rack and others have that looks at the load rating of the standard tire and the replacement that they use to recommend tire pressures for the replacement tires.  So, Tire Rack recommended 33 psi, not 29, for one guys replacement.  If you did not ask the folks who sold you a replacement tire what the adjustment should be for tire pressures, you might want to do that.  With the correct tire pressure, the contact patch is rectangular.  Increasing tire pressures above optimum for the contact patch makes the tire rounder (even radials) and makes the contact patch more oval.  Underinflation distorts in the opposite way.

We all know that tires do several things.  First, they grip the road.  Second, as someone pointed out earlier, they are springs (think of a basketball).  Third, they transmit directional changes - a front to back twisting force.  Fourth, they are communications devices, telling how happy they are with the road.  At 29 psi (cold), my tires (GS2s) were at their optimal contact patch (grip is good), the springs were soft and absorbed bumps (smooth ride), were slow to transmit directional changes, and told me nothing about the road until they screamed.

As a recap, I drove the car around after the autocross a few days at 40 psi (cold).  I did not check the contact patch at that pressure but, based on what I saw at lower pressures (below), I am sure it was quite a bit less than optimal.  Less grip - yuck.  They bounced a bit too much like a basketball which is why I abandoned 40 psi (less of an issue on a smooth parking lot for autocross).  The directional changes were outstanding and they told me everything about how they and the road were doing.

Following the previous posts, I lowered them to 35 psi.  This time, I did watch the contact patch, as followed by the rough guage of dirt and gravel on the treads.  The dirty area of the tire barely extended into the outer blocks on each edge of the tire.  Less than optimal contact patch, less grip, more wear in the center (since these tires are dogs, I was less worried about wear).  Spring rate - fine.  I felt bumps but not harsh and the tires were not bouncing.  Directional changes were still outstanding.  I have traffic circles near my house - they are negative camber slow folks down.  For me, makes a wonderful chicane.  At 29psi, the car would slide down the hill of the circle.  At 35psi, I was on rails.  I went faster even with less of a contact patch.  Finally, I still could feel what the tires were telling me about the road.  I knew when they were reaching their limits before they did so.

I tried lowering to 32psi - halfway between 29 and 35.  The dirt on the tire now extends 3/4 of the way through the outer blocks.  I have most of the contact patch back.  Directional changes are suffering a little.    But I feel like someone unplugged me from the road.  I have no idea what the tires are doing until the car starts sliding or rotating.  Back is the sort of slide - catch - slide routine of the front tires in my chicane - not as bad as at 29 psi but starting up again. 

So, what am I taking away from this.  First, this is only for these tires.  Second, winter is coming and when it gets cold, grip is everything and I am not rocketing around - 29psi.  But, the rest of the year I am going back to something between 33-35 psi - have to fine tune it.  Yes, grip is good but if I am uncomfortable using the extra grip because I don't know when it ends, I will be slower.  I have no doubt I was faster around my traffic circle chicanes at 35 psi because the tires reacted to the wheel, not the slope, and I knew for sure what they were doing.  A better tire will also do that - in time I will get them.  And then I will probably do the same experiment.

So, I challenge you folks who are trying to get the best out of your suspensions - play with the tire pressures a bit.  Contact patch is just one part of the equation.

[Uranium-238]

If you did not ask the folks who sold you a replacement tire what the adjustment should be for tire pressures, you might want to do that.

Forget that, I want that chart!

[2kwk4u]

U-238.  I'm a visual guy, too.  Here's your chart:

(Treeman, I hope you don't mind.  feel free to correct this if I've misrepresented any of your impressions - thanks for posting all of that, by the way!)

Pressure            Contact Patch          Perceived Grip         Harshness            Responsiveness       Feedback
29 PSI                  Optimal                   "sloppy"                 comfy                   lazy                     dead
32 PSI                damn good                 "slippy"               not reported             slow                    numb
35 PSI                 suffering                    good                   acceptable            crisp                     good
40 PSI                not observed              excellent              painful                   excellent              superb

*Based on GS2's on an '09 GXP with backbone brace.  Your mileage may vary.  Get out there and do your own testing!

[Ben L]

This is GREAT STUFF guys!!  I love you hard science types!   

[Critterman]

Scott, I think what U238 is looking for is the tirerack chart on replacement tires.

[Critterman]

Okay so I went to tirerack and got the specs on my standard tires:

RSA -245/45x18 96V SL 260 A A
96v sl means they can carry 1565 lbs each up to 149 mph

HTR ZII - 255/45x18 103Y XL 300 AA A
103y xl means that can carry 1929 lbs each up to 186 mph

If the stock tires should be at 29 lbs, where is the chart that tells me where the HTR's should be,  I'm guessing way lower than the 32 I currently run, probably around 25 lbs or so.

[Uranium-238]

Scott, I think what U238 is looking for is the tirerack chart on replacement tires.

[Ben L]

But I like Scott's/Critter's chart, too!

[lil goat]

Don't forget guys the front an back tires do NOT have to be inflated the same, something else to play with. I sat at Road Atlanta with the RMR guys while Rhys dialed in the Solstice (he won that day) he end up with 35 in the rear and 25 up front, he told me he just wasn't getting the grip he wanted up front until he dropped the pressure. He was running a slightly smaller tire a 225, and 265's on the back, he was running the Bridgestone Re-01's (same as me) they have a super stiff sidewall. Just gives you an idea that you can play with the front to rear balance as well. I had heard that many times for autoX the rears on the Solstice are set up as high as 42, and the fronts are in the low 30's. That is on the stock sizes. I know my tires work much better up over 30. I am at 31 now and may try a bit higher this weekend.

[Treeman]

Quote from: Critterman on Today at 09:15:08 AM
Scott, I think what U238 is looking for is the tirerack chart on replacement tires.

Yeah, got that.  I don't think they post the chart.  The posts I saw about this on the BB talked about calling Tire Rack and they would check the chart for you.  I would bet any tire store has this chart.

U-238, after I posted this last night, I thought of your harshness comments about your new tires and was wondering if you needed to drop tire pressures a little bit.

[Treeman]

Thanks 2KWK4U.  I meant to do that and forgot.  Got tired of typing.  You are close but the corrected version (according to my observations) is below.

Pressure            Contact Patch          Perceived Grip         Harshness            Responsiveness       Feedback
29 PSI                  Optimal                   "sloppy"                 comfy                   lazy                     dead
32 PSI                damn good           "a little slippy"             comfy                   decent                 dead
35 PSI                 suffering                   excellent                acceptable            crisp                     good
40 PSI                not observed              excellent                     harsh                   excellent              superb

*Based on GS2's on an '09 GXP with backbone brace.  Your mileage may vary.  Get out there and do your own testing!

[KWhale]

I did a similar test when I first got Raggy.  I do this with all my cars, or when I replace a set of tires.  On the GXP, with stock tires, I did 1lb increases from 29 to 35.  I quit at 35lbs because the car started to feel every bump and crack because most of our city roads here are absolute crap due to the winters and lack of repaving.  I slowly backed down on the pressures again 1lb at a time until I got back to 32psi.  For the roads in my area, and the average temps we have in the summer, 32psi was the best ride/handling/noise level for me.  For what it's worth, I also rotate the tires every 10,ooo kms (6,000 miles roughly), not sure if that helps or not.

[elff]

There is also the Chalk test
You run a horizontal line of chalk across the tire
You then mess with the tire pressure to see how much of the chalk is worn away by driving.
This lets you know what the contact patch looks like

[RufusMacblorf]

This is a really interesting thread!  I just ordered a set of Direzza Star Specs.  Does anyone have psi observations on those tires?  I don't autoX.  My car is mainly for hard driving on twisty mountain roads.  I'd really appreciate input from the hardcore drivers.

[Ben L]

I am not sure I am a hardcore driver, but my car setup certainly is. 

The Dunlops have pretty stiff sidewalls.  I run them at 28-30 cold rear and 30-32 cold in the front.  That gives me a good contact patch across the tread, and fine grip, with no sidewall bulge. Max rated pressure is like 51 or something.

I have run the snot out of them on Virginia 211 and through the Catoctin mountain roads, and they deliver remarkable grip, even on rougher pavement.  I have 275s in the rear and 245s in the front, so I can get away with a little tail happiness.

They can be squirrely at ambient temps below 38 F. 

So you just have to brake in a straight line, and pay very close attention to your weight transfers and lateral Gs in those conditions. 

If its wet and cold, baby them!

[RufusMacblorf]

I am not sure I am a hardcore driver, but my car setup certainly is. 

The Dunlops have pretty stiff sidewalls.  I run them at 28-30 cold rear and 30-32 cold in the front.  That gives me a good contact patch across the tread, and fine grip, with no sidewall bulge. Max rated pressure is like 51 or something.

I have run the snot out of them on Virginia 211 and through the Catoctin mountain roads, and they deliver remarkable grip, even on rougher pavement.  I have 275s in the rear and 245s in the front, so I can get away with a little tail happiness.

They can be squirrely at ambient temps below 38 F. 

So you just have to brake in a straight line, and pay very close attention to your weight transfers and lateral Gs in those conditions. 

If its wet and cold, baby them!

Thanks for the advice!  I stuck with stock size (245/45) on both ends, so I'll probably start with 30 front and back and experiment from there.

[lil goat]

I was at a little event with Ben last weekend, I had set the tires at 31 cold all the way around, checked them hot after a brisk drive to the track and they were 35 hot, they worked very well, back end stepped out nicely and behaved well, front end didn't seem to plow to much. The drive home through the mountains they worked extremely well. I could maybe play with the fronts a little bit but the backs for me are spot on at 31 cold. Bridgestone Potenza RE-01's, very stiff sidewall. From reading the article below I may need to increase the front tire pressure like Ben did, as we have a lot more weight over our front tires they should most likely have more air in them.

Tire Pressures in Autocross
or
Won't Somebody Please Think of the Carcasses?

In my 13 years of autocrossing, I've found a lot of misunderstanding about tire pressure and pressure's effects on handling. This paper is an attempt to dispel some myths and give you a guide to tuning tire pressures. It is not a scientific treatise, so I will gloss over a lot of technical details and make a few gross assumptions. I am not a tire engineer, but I do have a mechanical engineering degree, and I have studied tire construction techniques.

This paper is written for the autocrosser with a little experience. It is also slanted towards people running DOT tires on production based automobiles. It is not geared towards road racing, slicks, pure racecars or street driving, but some of the information will cross over to each of these disciplines.

To put it simply, your goal when setting the tire pressures on your car for autocrossing should be to maximize tire grip. There is a tire pressure that will give you peak grip. If you have your tires set above or below that pressure you will have less than optimum grip.

This brings up the first gotcha and your first car setup lesson for today. You can find the optimum grip pressure at each end of your car and still not be quick. If your car greatly understeers or oversteers by nature, finding the optimum grip at each end may not change that nature. Most competitors would acknowledge that the quickest car is one where the handling balance is neutral (neither understeer or oversteer) to slightly oversteering. You can use tire pressures to obtain this balance. You want the end that has to work the hardest in cornering, usually the engine end, to be at the optimum pressure for optimum grip so you can maximize the cornering force from that end of the car. Then tune the opposite end of the car to a balance that you like. This is the first myth, that there is one front to rear pressure difference that is best for each car. There is not. You need to tune the front to rear difference to suit your driving skills and tastes.

Let's look more closely at the first problem, how do you find the pressure that yields optimum grip for your car. To answer that question thoroughly, we need to get a little technical. The second myth we encounter is that the air in your tire holds up your car. Not since the days of "balloon" tires and wooden spokes have car wheels been held off the ground by air. It is the carcass of your tire that holds your car up. Air pressure in the tire is just a stiffener for the carcass. I underlined this since it is the crux of this whole document. We are altering the stiffness of the tire's carcass when we alter the air pressure in our tire.

What is the carcass (or casing) of the tire? It is an assembly of materials such as Rayon, Nylon, Arimid, fiberglass, steel and other materials braided, woven and formed into an open-sided doughnut. Think of this doughnut as the frame of the tire. This doughnut is what gives the tire most of its handling character. Surprised? Myth number three is the rubber in the tread or in the sidewalls gives a tire its handling characteristics. The rubber we see is bonded to the carcass and its primary contribution to autocross handling is the friction it can develop on the pavement and the characteristics with which it loses that grip with the ground. Rubber itself provides very little stiffness for the tire. When we describe a tire as floppy, sloppy, harsh, stiff, numb, imprecise, or slow to respond, we are describing the characteristics of the carcass, not the rubber.

Now, how do we use that knowledge to find the optimum grip? Let's write a little scientific statement: To achieve optimum grip, the carcass of the tire must present the rubber to the ground in a way that maximizes the friction force. To maximize the friction force, you want to maximize the amount of rubber in contact with the ground. That sounds like a simplistic statement, but in practice it is quite complex.

The carcass is very flexible. Every little force that acts upon it cause it to change shape. If that shape change pulls part of the carcass away from the ground, you now have less rubber in contact. Therefore, we need to set the tire pressure so that the carcass retains a shape that maximizes rubber in contact with the road.

Let's use the example of a front-engined, rear-wheel-drive car going around a constant radius corner in the hands of a skilled driver who is holding a speed and attitude such that the car is understeering and just about ready to slide out. This means the front tires have reached the maximum cornering force they can deliver.

The cross-sectional shape of the carcass of the tire at that moment will fall into one of 4 general categories.

A. Under-inflated
B. Rollover
C. Level
D. Over-inflated


A. Under-inflation causes less pressure to be applied to the center of the tread meaning there is less rubber being pressed into the pavement. It can also lead to B.

B. Rollover is where the sidewall of the tire now contacts the ground during cornering.

C. The level cross section shows equal pressure across the tread.

D. Over-inflation causes the center of the tread to balloon out so less than the full tread is in contact with the ground.

Most drivers won't know from the driver's seat which shape the tire is taking, with the possible exception of B. All they will know is that the front end is ready to slide away. What follows are some methods of determining what the tire is doing?

* You can use chalk or shoe polish on the shoulders of the tires and observe where it is worn away.

* The first time you use a tire, you can observe the fresh wear on the tread.

* You can drive the tire at a single pressure for many events and observe the long term wear with a tread depth gauge.

* You can have an outside observer watch your tires to see if there is daylight between the tread and the pavement (tough).

* You can have a skilled driver drive the car and tell you if he thinks the tire feels like it is doing.

* In my opinion, the best way is to use a piercing type pyrometer and take temperature readings just after completing a skidpad run. Interpretation of pyrometer readings is beyond the scope of this paper. You can read any of the "how to make your car handle" books for more info.

The next question to be answered is what carcass shape will yield peak grip? If you graph tire grip vs. pressure, you get a curve that looks something like this.

(Illus. resembling a mountain with a rounded peak.)

If you are currently running below the optimum pressure you will increase grip as you raise pressure. Once you've passed the optimum point, grip will decrease as you raise pressure. This happens because at much less than optimum pressure, you are allowing the sidewall to come in contact with the pavement. Generally the rubber of the sidewall is not compounded for grip, it is compounded for durability and UV resistance. Also, a carcass that is rolling over will tend to pull the opposite side of the tread off the pavement. If you over inflate past the peak, you begin to balloon out the center of the tread, and it will remain ballooned out even as the carcass rolls under cornering.

Most autocrossers shoot for just enough rollover that the contact patch reaches the edge of the tread and not beyond. In general, this is the carcass shape that maximizes rubber in contact with the road. An important warning is needed here. Dedicated racing tires may deviate from this generalization due to their specialized carcass construction. Myth number four is that changing tire air pressure up or down will always have a predictable effect on handling. The final arbiter must be the clock. If you have access to a test site and a skilled driver, you could ignore all this info and just vary pressures until you came up with the lowest times.

Back to our example. Once you are convinced that the front tires are at their optimum grip pressure, you can start tuning the rear tires to your driving preference. We're at our optimum pressure for our example car's front tires and it still plows. I prefer a neutral car so I would need to reduce the grip on my rear tires to change the handling to neutral. Remember, understeer is the front losing grip first, the corollary then is that the rear has too much grip. I can reduce grip by putting my rear tire pressures down the slope on either side of the mountain peak of optimum grip pressure. The downside of going to the high-pressure side of the peak, is that I reduce my traction for acceleration by my drive wheels due to the tread ballooning. Therefore, I would choose to run my rear tires at a less than optimum pressure to balance the car's handling and improve traction on acceleration. Mix and match as your car demands.

Be careful about borrowing pressure setups from your fellow competitors. Ask yourself, "Is his suspension the same? Is his alignment the same? Is his driving style the same? Is his tire gauge the same?" You might be surprised how widely tire gauges can vary calibration. This is myth number five. "I can borrow pressures from the fast guy and go faster."

One last suggestion: Write down the pressures that you are happy with. It is amazing how the mind can forget the most important data.

Warnings!
* If you are not truly driving at the limits of the tire's grip, all this information can get you upside down in a hurry. Tune up the driver first, and then worry about what the car's setup is.

* Every brand of tire has a different optimum grip pressure. Every brand of tire also has a different shape to the mountain peak. So a 6-psi Front/Rear differential on Kumhos might work as well as a 10-psi difference in Hoosiers.

* Some tires have a very large plateau on top of their mountain. You can move the pressures around quite a bit and not notice a significant change in grip. Some tires have a very sharp peak, which can be frustrating to tune.

* Optimum grip pressure is almost assuredly going to be different from the front to the rear, due to the differing weight each end has to carry and the different suspension geometry on each end.

* Optimum grip pressure may change from event site to event site.

* Optimum grip pressure may change from hot days to cold days.

* Tire temperature has an effect on how much grip you can extract. Overheated tires will lose grip no matter what.

* The optimum pressure is the one you are running at when you are on course. Autocrosses are not won in the paddock, so I do not really care what the cold pressure is, other than as a starting point. Take your pressure readings from hot tires as you come off the course, and tune to that pressure.

* Optimum grip pressure can change from dry to wet surfaces.

* Optimum grip pressure can change with each suspension change. You need to revisit pressures each time you upgrade. Even shock settings and alignment changes can cause different pressure needs.

* Even if you do not change the suspension and run on the same lot all of the time, as you improve as a driver you may be able to go faster with some tire pressure changes. You should not be afraid to experiment routinely.

* As I said above, the clock is still the final arbiter of the changes you make. What feels good may also be slow.

Gross assumptions:
* Tire pressure changes at one end of the car don't affect what happens at the other end.
* Car suspension is in good health and operates predictably with no gross errors in alignment.
* Car is not being wildly overdriven.

[Gentleman Jack]

lil'goat,

That was fantastic!  I have never AutoX'd, but I think I am going out tonight to find a race!  
Very well written, I am smarter now for having read it.  and I have no idea what PSI my tires are right now.  

Gentleman Jack (formerly Tierra del Sol)

[lil goat]

Thanks Jack, I stumbled on it and it explained a lot to me. It helped me understand the sway bar thing as well. I was always told that the rear sway bar effects the front of the car and vice versa. Now I get it. To improve traction and grip on the front you reduce traction on the back, it actually makes sense. The one thing I did right at the auto X with the instructor was get the back end loose that is when you know you are right where you want to be at the limit. I have learned so much in the few auto X events I have been to, I highly recommend getting an instructor to drive your car with you in it, it will open your eyes.