Tire pressure general / Benefit of Nitrogen-filling

[TR-Spider]

New started thread to collect the already started discussion.

Please everyone copy/paste his original post.

[TR-Spider]

I have copy/paste here the posts of all other users:


Report this postReply with quote Tyre pressures
by Colin Haggett on Sun Nov 11, 2007 10:03 pm

This why the checking of tyre pressures is so important, Can cause real handling problems on both road and competition vehicles.Darrian T90Colin Haggett

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Report this postReply with quote by Guy Croft on Mon Nov 12, 2007 1:11 pm

Quite so Colin, but of course unless you have manufacturer's data on tyre size and pressure it can be tricky. Change the wheel/tyre type and you're on your own..

It hasn't been mentioned here and a lot of folk won't know how to set pressures 'from scratch'. Would anyone care to talk about the use of a tyre pyrometer to set the pressures for optimum tyre contact patch? The influence of corner-weight settings would be useful too.

GCGuy Croft
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Report this postReply with quote by Mats on Mon Nov 12, 2007 2:37 pm

I think there are probably as many ways to do this as there are drivers/mechanics. I can start with how I do it and maybe someone can pick up the ball and add to the thread.

Car:
Alfa Romeo GTV2000 (Alfetta)
900Kg/130Hp
I mostly race on tight, twisty tracks but they are still quite fast (120-130Km/h average)

Tyres are 195/50-15 or 205/50-15, "R" compound. I.e. Yokohama 032R, 048, Bridgestone 540S, Kumho V70A or similar. Most of these tyres in this size seem to enjoy a hot pressure of around 2.0bar.

Depending on the weather situation you will need to guessimate the cold pressure but it will range from about -0.6 to -0.4bar from hot pressure. In my experience (and this I can't explain) different tyres have different pressure rise for the same hot temp. This is very counter intuitive but as it comes to tyres not everything is working in line with what you would expect.
So if you have a nice warm day and aim for a hot pressure of around 2.0 bar 1.6bar would be a good starting point. Maybe now is a good time to talk about tools. I use:
1 Good quality tyre pressure gauge.
1 Tyre pyrometer (probe type, not IR/surface).
1 Setup sheet to write down the initial setup and weater + changes.
1 Laptimer and/or datalogger. Laptimer is important.
(option)
2 Friends to measure temp and write it down on the setup sheet.

Begin with a few warm up laps to get the car/driver up to operating temp and to put some base temp in the tyres, if this is the first time on a new set you might want to do a quick check in the pits so the pressure looks ok, as I said some tires raise the pressure more then others.
Go out and make sure you do a couple of really fast laps and then make a F1 style entry into the pits and have your gear/mechanics as early in the pitlane as possible. It's important you really drive hard all the way to the measuring site because the track will cool the tires down if you coast around to save the brakes (by the way, stay off the brakes in the pit!).
Record inner, middle and outer temps per tyre and the pressure, if the pressure is too high adjust down to just under what you aim for, say 1.95 in this case.
Leave the pits and repeat this procedure until the temps/pressure have stabilized or there is a big temp variation across the tyres.
Do some cool-down laps to cool the brakes, engine/gearbox/diff and tyres, heat soak can be pretty bad. Next you need to analyze your data.

Load = temp.

If your outer temps are constantly higher then the inner ones you need more negative camber.
If your inner temps are higher you need less negative camber.

If your tyres are hotter in the middle the pressure is too high
Hot on the edges (both) you need to inflate.

This sounds very simple but as you can have both too much negative camber and too much pressure at the same time it can be a bit tricky to catch all the variations.

There are also other issues that can make things a bit difficult, my car for instance almost lifts the inner front wheel in the corner and therefore heats the tyre a bit uneven and makes the tyre wear strange looking. This is very difficult to catch without having someone looking at the car in the corner or in my case have someone take a series of photos during a hard turn. Never underestimate the importance of the double check/reality check. Normally you might dial in less neg. camber to deal with the wear pattern and temps but the laptimer indicated that this was not the right way to go and the pictures proved why.

In the end the laptimes must be what you aim to get as low as possible (for sprint racing) and sometimes that makes you set up the car a bit differently then what the temps tell you.

Since all changes to the setup (toe-in, corner weight, fuel load and so on) will make load the tyres differently you will always have to play around with the pressure and camber to get the most from the tyres, this is why it's extremely important to have a setup sheet record to have all the used setups recorded and what worked best for that particular setup. It will shorten the dial-in time a lot!

This is my experience, the pressures are not the same for othertyre sizes and you need to figure out what works on your car/setup. Don't be afraid to experiment, sometimes going to extremes will open up a whole new world of experience!

Sorry for the long post./Mats StrandbergMats

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Report this postReply with quote by Kev Rooney on Sun Dec 16, 2007 10:54 am

The backyard mechanics version for road cars with occasional track use requires a stick of chalk or light coloured crayon. Mark a single line across the full width of the tread front and rear and drive the car gently for a mile or so. Inspection of how much of the telltale remains will tell you whether the tyre is over or under inflated ie All worn from the middle with edges still remaining =over inflated and vice versa .

Tyres pressures need to be adjusted in conjunction with shock absorber dampening rate if they are adjustable .Also if the spring rate is too high excessive tyre pressure will make this worse and 'launch' the whole wheel at every bump.

The larger the tyre and rim for any set given weight of vehicle the lower you will need to set the initial pressure.Kev Rooney

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Report this postReply with quote Re: RWD car pulling to left-right. LSD to blame?
by cantfindausername on Thu Jan 10, 2008 11:46 am

Great information guys. Made for a very interesting and worth while read!cantfindausername

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Report this postReply with quote Pressure variations for different tires
by mtbr on Tue Mar 04, 2008 10:30 pm

The different rates of pressure rise with temperature, for different tires, Mats mentioned, is usualy due to varying amounts of moisture trapped in the tire. If the regs allow, filling the tires with Nitrogen or "dried" air makes this aspect of set-up easier, as the pressure variations are much reduced.
Sometimes you will find one tire, on the car, that goes way higher than the others; this is frequently the first one of the set fitted.
Fully deflate it and pump it up again, (before you spend too much time looking for something wrong with the car.)

The moral here is purge the airline properly (and maintain the filter drier) before you inflate the tires.

Mark Wilsonmtbr

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Report this postReply with quote Re: RWD car pulling to left-right. LSD to blame?
by cantfindausername on Wed Mar 05, 2008 10:32 am

So would this moisture be a more promenent problem from the bif compressor type of air sources, as opposed to a foot pump say?

I've seen the adverts for nitrogen filled tyres in the Demon Tweeks catalogue, and have often wondered about how goot this was. Does it need refilling/gas changing regularly or anything like that?cantfindausername

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Report this postReply with quote Re: RWD car pulling to left-right. LSD to blame?
by mtbr on Wed Mar 05, 2008 8:56 pm

cantfindausername wrote:
So would this moisture be a more promenent problem from the bif compressor type of air sources, as opposed to a foot pump say?

I've seen the adverts for nitrogen filled tyres in the Demon Tweeks catalogue, and have often wondered about how goot this was. Does it need refilling/gas changing regularly or anything like that?

It is unlikely you will introduce a significant quantity of water into the tire using a foot pump. The problem usually occurs when tires are fitted track side and inflated with a compressor mounted in a van. Such arrangements rarely have adequate filters or traps.
Theoretically, tires fitted on a warm, humid day will have more moisture in them, than tires fitted on a cold dry day. This may acount for the differences in the behaviour of different sets of tires, however the situation where one tire responds to temperature in a significantly different way to the rest, is usually due to condensate, from the airline, being introduced to the tire.

To use Nitrogen effectively, you need access to the equipment whenever you need to add gas to the tires. Having tires filled once and then having to add air in service partially defeats the purpose. The main benefit is that pressure rise with temperature, is more consistent and smaller in magnitude, than with air. This makes adjusting tire pressures far more simple, with the added advantage that the car doesn't need to start the race on underinflated tires, in order to avoid serious overinflation later in the race. Suppliers of Nitrogen filling equipment also claim that the tires don't oxidise, when filled with Nitrogen and therefore stay more flexible. Frankly, this claim is of little importance, given that most race tires will be well passed their best due to heat cycling, long before the air in the tires makes them go hard!

Many club level race series have banned the use of nitrogen (and tire warmers) to help keep costs down. In this situation, the best you can do is make sure the air you fill your tires with is as dry as possible.mtbr

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Report this postReply with quote Re: RWD car pulling to left-right. LSD to blame?
by cantfindausername on Thu Mar 06, 2008 12:56 am

I see your points about it being more a "where every second counts" type of concern. For your average track day driver, there would be very little reason for having this sort of equipment on hand, and chances are I'd only use a foot pump to pump the tyres up anyway.

Great info to have though, always keen to expand my knowledge of stuff like this!

Antcantfindausername

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Edit postReport this postReply with quote Re: RWD car pulling to left-right. LSD to blame?
by TR-Spider on Sat Mar 08, 2008 10:24 pm

I was always wondering about this Nitrogen thing.

The air we breathe is aprox. 78% Nitrogen and 21% Oxygen, where there is no mayor difference in the expansion behaviour between the two gases.
So the main argument remaining is the water-content.
If we fill our tire with ambient air 20°C of 60% humidity, it contains 8g water per kg air.
Aprox. tire air mass 0.150 kg -->so 1.2 g water.
Now, if the tire heats up to 70°C this 1.2g water will change its volume...a bit.
Actually the volume difference (pressure difference) seems to be more in the 1/10 percent range.
The very worst case, pressurized air with 100% humidity at 70°C makes a bold 3% volume difference compared to dry air.

Sounds like another modern myth to me.

ThomasTR-Spider

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Report this postReply with quote Re: RWD car pulling to left-right. LSD to blame?
by mtbr on Mon Mar 10, 2008 10:02 am

The presence of moisture can be significant.
Consider some typical data from a Lotus Elise race car running on Yokohama A048R tires Operating temp for this tire is between 60 and 90 deg C. For the front tires, we found that we achieved the best temperature distribution, across the tread width, with a hot operating pressure of 24-25 psi.

By measuring the tire temps after one lap and then taking further measurements as the tire cools, we are able to estimate the tire temperatures with the car on the grid after the parade lap. These are typically around 35 deg C. Although they can vary a lot, as this phase of the pre race is often subject to delays and disruption.

If the air in the tires were perfectly dry (or nitrogen) and the mass of gas correctly adjusted to achieve 25 psi at a mean operating temp of 75deg C then we would expect the tire pressure on the grid, before the start of the race to be 22psi. (pressure is proportional to temp at constant volume) This would present no particular problems, the car would respond normally, albeit with less grip than when the tires are fully warm and there would be little risk of damaging the tire sidewall due to under inflation.

Now consider the situation where sufficient water has been introduced into one of the tires for a saturated vapour to exist at full operating temp. ( Although this is an extreme example, this situation can occur when condensate is sprayed from the air line into the tire.) The difference between the saturated vapour pressures at 75 deg C and 35 deg C is around 4.5psi (estimated using Bridgeman & Aldrich coefficients for this temp range)

This would mean the car is now waiting to start the race with one tire at 17.5 psi.
This car will require careful handling on the first lap and will probably concede some places on track until the tire comes up to operating temp/pressure. Later in the race, the situation can go then other way. If the tire is overheated, then pressure increases above the other tires, the centre of the tread gets even hotter due to the over inflation and so on.

Moving back from this extreme example, it should be noted that in practice, the pressure change with respect to temperature usually exceed that indicated by proportion alone and that pressure variations due to the presence of moisture are not linear where a saturated vapour can exist.

Whether the use of Nitrogen (or dried air) is justified or not, is a matter for individual teams and series organizers. However, I stand by the practical point in my initial post, the presence of moisture in tires introduces yet another variable into an already complex relationship and if you have one tire exhibiting marked pressure changes relative to the others, then it’s a good idea to check that there is not some water in it, before you go looking for broken dampers, bent wishbones or other problems.

Mark.mtbr

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[mtbr]

Subsequent to my original posts, Alan Meaker posted this on the Lotus on Track forum, I believe it adds to our discussion here.
My name is Alan Meaker, I am the Motorsport Manager for Toyo Tyres (UK) Ltd.

I was at Rockingham on the Saturday 1ST March at the LOT trackday and worked with a couple of cars using our R888 tyre regarding pressures and tread temperatures.

What I would like to do is to offer some advice on tyre temperatures and pressures and how to achieve the best from our tyres when taking part in either trackdays or racing.

The R888 has a semi race construction (very stiff) and a race tread compound. The optimum tread temperature range is between 85C and 95C measured using a probe type pyrometer, and ideally a maximum difference across the tread of 9C. The maximum hot pressure we recommend is 40psi. Camber angles up to 5 degrees are permissible but the final setting will depend on tread temperatures. It is advisable to have as much positive castor as practical as castor induces a beneficial camber change during cornering. I recommend that the tyres be put through 2 heat cycles before hard use.


The pressures you use will initially depend on the weight of the car, too little pressure on a heavy car can lead to over deflection of the tyre and subsequent failure.

Below are some basic settings:

VEHICLE WEIGHT COLD PRESSURE HOT PRESSURE
Very Light < 800kg 17 - 22 psi 22 - 29 psi
Light 800kg - 1000kg 20 - 26 psi 24 - 32 psi
Heavy 1000kg - 1400kg 23 - 27 psi 28 - 40 psi
Very Heavy > 1400kg 27 - 35 psi 37 - 40 psi


As a tyre gets hotter the pressure increases, this is due to the moisture in the air. The cold pressure you set to achieve a desired hot pressure will depend on the conditions on the day i.e. ambient and track temperature, wet or dry. If the day/track is cold you will need to start with a higher cold pressure as the tyre will not get as hot therefore the pressure increase will not be so great.
Hot pressures must be balanced side to side. Once the tyres have cooled you will find that you will have a difference in pressure side to side, if you have been racing on a right hand track you will find the offside pressures will usually be higher than the nearside.


Changing hot inflation pressures by small amounts can be used to fine tune handling.

Reduce Oversteer Reduce rear pressures or increase front pressures
Increase Oversteer Increase rear pressures or reduce front pressures
Reduce Understeer Reduce front pressures or increase rear pressures
Increase Understeer Increase front pressures or reduce rear pressures


Achieving the required tread temperatures will depend again on the conditions on the day i.e. ambient and track temperature, wet or dry.

You often here competitors saying “My tyres started to go of towards the end of the race”, this is usually due to the tread getting to hot.

The tread temperatures are constantly changing through out a race, hotter when cornering and cooler when on the straights and cooling even more when you are slowing to come into the pits. Therefore the temps you record in the pits will be lower than those during the race. So if you record temperatures within the range given above the probability is the temps will be too high during the race.

Increasing your tyre pressures will cause your tread temperatures to increase, more pressure stiffens the tyre’s casing which results in the tread having to do more work resulting in the tread getting hotter. Lowering your pressures will cause them to decrease.


Inevitably changing one thing will affect other things, the whole set up of your car is a compromise between anything that is adjustable.

[TR-Spider]

Hi Mark

I think this part of Alan's statement is wrong:

Changing hot inflation pressures by small amounts can be used to fine tune handling.

Reduce Oversteer Reduce rear pressures or increase front pressures
Increase Oversteer Increase rear pressures or reduce front pressures
Reduce Understeer Reduce front pressures or increase rear pressures
Increase Understeer Increase front pressures or reduce rear pressures

In my opinion (and experience) it is just the opposite way around i.e.
Reduce Oversteer Increase rear pressures or reduce front pressures
Reduce Understeer Increase front pressures or increase rear pressures

I think thats correct as long as the tires are "correctly" pressurized, because then the amount of grip increases with tire-pressure and you need more grip on the axle where you want reduced sliding.
If the tires would be severely over-pressurized ("balooning" so that tire/road contact surface is reduced) then of course tha initial statement is correct.

I would be interessted in an "How To" heat cycle a tire correctly.

Thomas

[Guy Croft]

Thomas, it is actually correct as it stands.

The rule is:

to increase oversteer stiffen the rear
to increase understeer stiffen the front

and vice-versa. The stiffness can be dampers, springs, anti-roll bars, sway braces, tyre pressures etc etc, anything that adds to or reduces chassis or suspension stiffness.

GC

[TR-Spider]

Guy

I agree with this rule, except for the tire.
For springs, swaybars etc it is clear:
if you stiffen one axle, you increase the load on the outside tire of this axle and thus make the outside tire slide earlier.

The tires themself behave like shown on this graph (copied from Fred Puhn's excellent book)

Thomas

[Guy Croft]

I'm no expert on handling!

I just know from exp that if you over-inflate both tyres on a the back axle you get oversteer!

GC

[Julian]

As something of an impromptu expert on getting my old race Punto to handle on DOT tyres I concur with Guy.

Irrespective of the tread behaviour the overwhelming change in tyre pressure is the spring rate of the side wall (from our own findings in this particular instance using either Bridgestone Potenzas - the original list tyre for the car - and Toyo Proxes T1-S).

Increasing the tyre pressure increases the effective spring rate. By making small adjustments (1 to 2 psi) we could fine tune the handling of the car to suit pretty much any circuit without the need for changing springs or dampers (we were not permitted to use adjustable dampers originally). The basic build of the car set the balance of the handling to be direct, light but predominantly neutral tending towards oversteer.

A 2 psi difference between front and rear (soft at the front, hard at the rear) was enough to increase oversteer to a useful level. Making the reverse change had exactly the opposite effect and was generally undesirable. Small side-to-side differences were tried but with the exception of Rockingham were discovered to be utterly ineffective.

There is a caveat - certainly with the Toyo's - we found that running the fronts slightly soft limited the life expectancy of the tyre. After 20 minutes the extra heat generated by running soft resulted in total loss of the tread section of the tyre, thankfully our normal race length was 15 minutes and provided the tyre never overheated we could expect several races from a single set of tyres.

If you refer to your graphs of grip vs. tyre pressure, on DOT-like tyres (as opposed to slicks or cut slicks) running at sub-20 psi inflation pressures would not only decrease grip but would also result in a very short lifespan (as mentioned above). Normal inflation pressures (for us) vary between 24psi and 29psi. This corresponds to slightly sub-optimal performance from the bottom of the three graphs which I can only deduce is a DOT-like tyre given the peak in its grip capability. The other two curves appear to be slick or cut-slick type given the higher level of grip. If I am correct in my assumptions then both sides of the argument are indeed correct.

As a further point I've spent some considerable time tuning tyre pressures for friends at track days and so far I've yet to find an exception to my method and every car tuned in this manner has become both easier to handle around the track and faster for it. Some consideration is needed when dealing with FWD versus RWD versus MR versus AWD - each has its own nuance but ultimately the balance is adjusted in the same way. We use the same method for the road but tend towards higher pressures and a smaller differential in pressure front to rear.