Adjusting tire pressures

[rbpeirce]

Somewhere on this forum a few months back it was suggested that tire presues be checked after the first run at an event and adjusted back toward cold pressure. I did this in June and found the track seemed to be very greasy. This past weekend, I just set the cold pressures to my customary levels of 33# front and 36# rear and left them there all week-end. I got terrific grip. I am running Sumitomo HTRZ II tires, so your experience may vary.

 

[L8apex32]

Yes, tire temp and pressures do vary - with track, tire model, etc.

I started with 35psi all around this weekend and the rears shot up to 42 and the fronts to 39 after one session. You likely had that pressure or more during the hottest of your sessions in mid-afternoon.

I can imagine some street tires doing ok at that pressure. Some have weak sidewalls that high pressure will help.

On the other hand, high pressure like that is too much for many tires. YMWDV (your mileage will definitely vary).

 

[john@johnhudakracing]

Somewhere on this forum a few months back it was suggested that tire presues be checked after the first run at an event and adjusted back toward cold pressure.

Hi Bob,

Not sure which passage you are referencing. The idea is to adjust/tweak toward the tire's "optimal operating" or "hot" pressure, not back to the starting low cold pressure.

This past weekend, I just set the cold pressures to my customary levels of 33# front and 36# rear and left them there all week-end. I got terrific grip. I am running Sumitomo HTRZ II tires, so your experience may vary.

So much depends on driver skill/usage- specifically how hard the tire is getting continuously loaded versus its optimal slip angle... with tire model, track layout, traffic, conditions, etc... all checking in right there after..

Thus, honestly, throwing around pressure tends to be a canned qualification.

As a supporting data point, in an HPDE context- in the dry, those higher "cold starting pressures" or "street pressures" on street rubber could well seem "just great" for novice driver 1 in group C even after a whole day of full 20 minute sessions; while advanced driver 2 in group A [whom probably can post a faster lap-time on their warm-up lap]... well, would pit on about the fourth lap pulling in with high pressures and measurably higher tire temps blistering/over heating absolutely needing to bleed down.

Further, a little more technical but unless you are using tire warmers- "cold starting pressure" really means "cold". If you look at a tire temp vs. pressure chart after a hot dismount- there is a steep fall off, which as we all know is why it is absolutely imperative to check pressure, temps, etc... in the hot pits asap in order to obtain meaningful and relevant data. However, there is also a very long tapered fall-off there-after. Depending on changing conditions (e.g. cold desert morning, followed by a blistering hot afternoon sun heating up the asphalt) they very well might never see the exact same plot point that day even letting it sit there between sessions and these subtle changes do matter.


Here is a quick and easy article on setting pressures geared toward weekend warriors you might find useful:
http://www.turnfast.com/tech_handling/handling_pressure.shtml


However, if you haven't yet done so and are looking for full depth, I'd really encourage you to read Paul Haney's book. It really helped me understand the core fundamental concepts, and will quickly make you realize just how canned some of the perceptions are surrounding tire tech.

http://insideracingtechnology.com/booktiredescrptn.htm

Hope that helps.

 

[rbpeirce]

Yes, tire temp and pressures do vary - with track, tire model, etc.

I started with 35psi all around this weekend and the rears shot up to 42 and the fronts to 39 after one session. You likely had that pressure or more during the hottest of your sessions in mid-afternoon.

I can imagine some street tires doing ok at that pressure. Some have weak sidewalls that high pressure will help.

On the other hand, high pressure like that is too much for many tires. YMWDV (your mileage will definitely vary).

I get closer to a 7-8# increase, so I end up with around 41# and 44#, give or take. The Sumitomo tires seem to be able to handle that with no problem, and they have much better grip than occurs at even slightly lower pressures. It could be the tire, track or me, but it works.

 

[rbpeirce]

John@Microsoft has gone into far more depth than I need!!

I run in the A groups but I am too old to worry about lap records. Besides, the NSX ain't what it used to be!

I run a totally stock car on street tires and my goal is to have a lot of fairly safe fun. At BeavRun, I am usually a second or two a lap slower than the hot shoes. I am 4-6 seconds slower than the guys in race prepared cars running on slicks.

For comparison, the lap record at BeaveRun (1.6 miles) was set by an open wheel Mazda race car at about 50 seconds. Race cars derived from street cars run bout 65-68 seconds. My best time was a hair over 71 seconds and I usually run around 72. I have a lot of fun but I point-by a lot of cars.

When I was at Mid-Ohio with NASA in April, I lapped in the 1:55-1:56 range. There are a few more seconds available there for a more enthusiastic driver.

I am not trying for the ideal tire pressures. I just want them to be close. For me, letting the cold presures build up as the tire heats gives me better grip than trying to bleed them down. Although I am sure that a couple pounds one way or the other could easily mean a few tenths in lap times, I am usually working in a one to two second range and probably wouldn't even notice it!

 

[john@johnhudakracing]

John@Microsoft has gone into far more depth than I need!!

I run in the A groups but I am too old to worry about lap records. Besides, the NSX ain't what it used to be!

I run a totally stock car on street tires and my goal is to have a lot of fairly safe fun. At BeavRun, I am usually a second or two a lap slower than the hot shoes. I am 4-6 seconds slower than the guys in race prepared cars running on slicks.

No worries there, all other things being equal; it is rather unrealistic to expect to hold out against others on slicks, particularly running on a 220 tread wear street tire.

I am not trying for the ideal tire pressures. I just want them to be close. For me, letting the cold pressures build up as the tire heats gives me better grip than trying to bleed them down. Although I am sure that a couple pounds one way or the other could easily mean a few tenths in lap times, I am usually working in a one to two second range and probably wouldn't even notice it!

I see you your angle, but still no need to give-up time unless you have to.

Given your last post. As previously mentioned, it could well be the softer sidewall on that specific tire. Max inflation pressure is 51psi, and if you google around you'll find several reports of owners running on the way high threshold of that figure- both on the street as well as the track.

It could well have been the flexing of the sidewall that your butt sensor was nabbing at you with that uneasy, greasy feeling.

It definitely doesn't hurt to experiment. Maybe try an effective 10% f/r increase next time on a hot lap and see what you get. Keeping a good tire log is an invaluable tool.

 

[nsxnut]

Hi Bob,


However, if you haven't yet done so and are looking for full depth, I'd really encourage you to read Paul Haney's book. It really helped me understand the core fundamental concepts, and will quickly make you realize just how canned some of the perceptions are surrounding tire tech.

http://insideracingtechnology.com/booktiredescrptn.htm

Hope that helps.

Thanks for the pointer... I just ordered this book.

Later,
Don

 

[john@johnhudakracing]

Thanks for the pointer... I just ordered this book.

Later,
Don

You'll probably very much enjoy it. It's not a bad read since it's not overly thick on the technical stuff (more than it has to be), being that he sticks to what we all care about- going fast.

He recently did a presentation at the shop not long ago.

 

[NSXDreamer2]

my past experience is that 40psi is too hot for the car, you will better off keeping it lower than 38psi... hot temp.

adjusting tire pressure is the very first step of suspension setting, doesn't matter what kind of track driving you doing, this is the least thing you need to do along with checking brake fluid level, oil level and torquing nutlugs...

Happy Tracking!!!!

 

[rbpeirce]

my past experience is that 40psi is too hot for the car, you will better off keeping it lower than 38psi... hot temp.

adjusting tire pressure is the very first step of suspension setting, doesn't matter what kind of track driving you doing, this is the least thing you need to do along with checking brake fluid level, oil level and torquing nutlugs...

Happy Tracking!!!!

I think this must depend on the tires. When I bled the Sumitomos down to this level they were very greasy. I normally run 33/36 cold, so this would be about 35/38 hot. The tires don't work well at these pressures but they do above 40.

 

[nsxnut]

I think this must depend on the tires. When I bled the Sumitomos down to this level they were very greasy. I normally run 33/36 cold, so this would be about 35/38 hot. The tires don't work well at these pressures but they do above 40.

Not likely unless you are not going very fast. If you are pushing the car
I suspect the real hot pressure is north of 40psi. More like 45.
I start off with ~23 to get to 34 yep 11psi hot to cold difference.

Look at your tires. How much are you using the outside edge? As your tire
pressures get higher your contact patch start to get smaller. You will only use the center of the tire and not the edges. Letting air out will reduce the ballooning effect.

Try this go out and warm things up. come in 1 lap before the Checker. Don't do a cool down lap. Have your tire gaguge in the glove box and pull into the hot pits and check... if more than say 35 Let out 2 psi. try another session.
Do the same come in early. Do this again..

If the car feels good in the beginning of the session and get's greasy after a few laps your pressures are to high. make a 2psi adjustment and see if this gets better.

The faster you get you will find what works now will need to be ajusted. As you get faster you will find you will have to let more air out.

Later,
Don

 

[stuntman]

For true tire pressures for the tires/setup you are using. You need a pyrometer.

 

[nsxnut]

For true tire pressures for the tires/setup you are using. You need a pyrometer.

I agree. I have a probe type specific for tires..
He is complaining the tires are getting greasy after a few laps.

A pyrometer will confirm that the centers are to hot and doing all the work.
Most people won't spend the money on a probe pyrometer.
A laser non contact one will not work.

A pyrometer will help fine tune, I suspect he is not at that point yet.

 

[rbpeirce]

I agree. I have a probe type specific for tires..
He is complaining the tires are getting greasy after a few laps.

A pyrometer will confirm that the centers are to hot and doing all the work.

My complaint was the tires were greasy after reducing pressure. I guess the outside edges were too hot.

 

[nsxnut]

My complaint was the tires were greasy after reducing pressure. I guess the outside edges were too hot.

If your tires are good at the beginning of the session and go away after a few laps than then you pressures are to high.

If they are bad at the beginning and get better as they get hot then they are to low at the start. As they heat up the pressure will come up and they will get better.

As pointed out... A pyrometer is all telling. But you need a probe type.
A prob type will measure the core temp. Surface temp will change very quickly. You need to check the core temp in the hot pits.


But all said go have fun.
The nsx is very nice on the track and so few are tracked.



Later,
Don

 

[rbpeirce]

We seem to be getting far afield of my original comment which was simply that I had tried reducing pressures after the first session and I had tried leaving them alone after the first session and leaving them alone worked better for me than reducing them. I was not trying to imply that either pressure was optimum or even nearly optimum, just that one approch worked better for me than the other with the tires I used on the track I ran on.

 

[NSXLuvr]

I saw this TPMS on another site. I think this is cool solution to monitoring tires pressures. However i question if the monitors can stay on the valve stems and also wonder how accurate it is.

http://www.vividracing.com/catalog/product_info.php/products_id/45048

 

[rbpeirce]

That is very cool for other than racing apps as well. I check my tire pressures each month on all my cars and a device that screws on and displays results would be great. I know this because my RL has tire pressure sensors and once I confirmed their accuracy, I use them instead of a pressure gauge. The problem with the presure gauge is it can bleed pressure if you don't press it on exactly right.

It would also be great for track events since you could check your pressures coming off the track. It appears it would lock on okay for this app.

 

[nsxnut]

I saw this TPMS on another site. I think this is cool solution to monitoring tires pressures. However i question if the monitors can stay on the valve stems and also wonder how accurate it is.

http://www.vividracing.com/catalog/product_info.php/products_id/45048

For the track you may want these instead?
http://www.pegasusautoracing.com/productselection.asp?Product=3336

I won't use ether. I like to keep things simple on the track.
If they fail when I am out, it could spoil my day!

I was helping a friend at the track. There were 3 of us that day.
One asked the other what pressure to use. He set his tires with his gauge.
He ran a session and complained that was bad... We checked with the other guy's gauge. It turns out there was a 5 psi difference between the two gauges.




Later,
Don

 

[rbpeirce]

For the track you may want these instead?
http://www.pegasusautoracing.com/productselection.asp?Product=3336

I like to keep things simple on the track.
If they fail when I am out, it could spoil my day!

I was helping a friend at the track. There were 3 of us that day.
One asked the other what pressure to use. He set his tires with his gauge.
He ran a session and complained that was bad... We checked with the other guy's gauge. It turns out there was a 5 psi difference between the two gauges.

I would carry a calibrated pressure gauge regardless.

I have told the shops that work on my cars NEVER to set pressures. All their gauges seem to be off by anywhere from 1-5#. Carry your own gauge. It may be off but you should know by how much and it will be consistent.

 

[stuntman]

Somewhere on this forum a few months back it was suggested that tire presues be checked after the first run at an event and adjusted back toward cold pressure. I did this in June and found the track seemed to be very greasy. This past weekend, I just set the cold pressures to my customary levels of 33# front and 36# rear and left them there all week-end. I got terrific grip. I am running Sumitomo HTRZ II tires, so your experience may vary.

WHY are you putting Sumitomos on your NSX?

You have a Supercar, why put Tercel tires on your car????

NEVER skimp on tires or brake systems, especially if you track your car!

Sumitomos are cheap tires, probably have a really soft side-wall, which will result in a 'greasy' feeling if their's too little tire pressure in them. Softer side-walled tires (especially cheap ones) need higher tire pressure to retain the structure/shape of the tire carcass under hard cornering loads.

I think that relatively higher tire pressures (high 30s to low 40s hot) should help your sumitomos. but
My biggest reccomendation would be to get better tires ASAP!

 

[docjohn]

I would agree that the sumitomos would not be my choice for track tires,but the op seeems to be happy with his on track experiences.I would like to mention though that the htrz tires in question have had pretty favorable reviews on prime esp by MJ of Dali fame as a cheap pseudo track tire alternative.I would also suggest a tire change to goodyear,oem,faulkan(sp) or any of tire racks top tier performance tires when Robs' tires are ready for replacement.

 

[rbpeirce]

WHY are you putting Sumitomos on your NSX?

You have a Supercar, why put Tercel tires on your car????

NEVER skimp on tires or brake systems, especially if you track your car!

Sumitomos are cheap tires, probably have a really soft side-wall, which will result in a 'greasy' feeling if their's too little tire pressure in them. Softer side-walled tires (especially cheap ones) need higher tire pressure to retain the structure/shape of the tire carcass under hard cornering loads.

I think that relatively higher tire pressures (high 30s to low 40s hot) should help your sumitomos. but
My biggest reccomendation would be to get better tires ASAP!

Ordinarily, I would agree, but for some reason these tires seem to work well with the NSX. I heard about them from several owners out west who have been happy with their track performance. I doubt they are the best available tires, although I could be wrong, but they are hard to beat for price versus performance.

OTOH, you may have put your finger on the reson they sem to work beter at high presures.

 

[rbpeirce]

I would agree that the sumitomos would not be my choice for track tires,but the op seeems to be happy with his on track experiences.I would like to mention though that the htrz tires in question have had pretty favorable reviews on prime esp by MJ of Dali fame as a cheap pseudo track tire alternative.I would also suggest a tire change to goodyear,oem,faulkan(sp) or any of tire racks top tier performance tires when Robs' tires are ready for replacement.

Mark was one source, but I don't use these as track tires. I use them as street tires I can drive on the track. There is a difference. There are also track tires you can drive on the street; it works both ways.

I have tried OEM tires from Bridgestone and Yokohoma, Toyos and the Sumitomos. The Sumitomos may not be the last work in ultimate performance but they are 1/2 to 1/3 the price of only slightly better tires. I like the trade-off, especially when I have to replace my rear tires every 5000 miles. If they cost me a second or a few per lap, I still have a lot of fun.

Here's the bottom line for me. Up until a few years ago we had one of the hottest cars on the track and extra money for better tires was money well spent. Today we get blown off by just about any performance car manufactured in the last couple of years, so why worry about that extra second? It really isn't worth the money. In spite of that, the NSX is still one of the best balanced cars available, so I use it to have fun, not to set lap records.

 

[nsxtasy]

Today we get blown off by just about any performance car manufactured in the last couple of years

That logic is nonsense. On the track, the big variable is driver skill, much MUCH more so than differences between cars' capabilities. Very few of us are using such a high percentage of their capabilities that the difference in cars matters.