Tyre pressure tuning

[Petrus]

When tuning oversteer/understeer, tyre pressure is one aspect that can be used.

Traditionally the wisdom is that reducing understeer can be achieved by increasing front pressure a bit and vv. Same thing at the back.

However!
Read the info on the Nankang site for their track rubber:

If you are having handling issues, please see the table below which may resolve your issue, remember to do this in small increments at a time

To reduce Over steer
Reduce rear pressures or increase front pressures
To increase Over steer
Increase rear pressures or reduce front pressures
To reduce Under steer
Reduce front pressures or increase rear pressures
To increase Under steer
Increase front pressures or reduce rear pressures

Wow! 

The thing is that there are two different drift angles aspects adressed here:
- On road tyres the drift angle is caused by creep due to deformation. Add air and you get less.
- Track tyres have stiff enough sidewalls so a bit less pressure hardly increases creep but it does raise the rubber temp, adding stickynes, reducing drift angle. Will, till you overdo it and cook the rubber.

With the AD08R I am in the doldrums. It bóth applies depending on use/condions so I have gone back to OEM values compensated for weight and size *.

I have gone from 185 SL82 to 195 SL85 meaning going from 26 to 24 psi to maintain status quo.
Here is a handy on-line calculator: https://tiresize.com/pressure-calculator/

To compensate for the about 15% lower weight thus less load I have dropped the pressure 2 psi front and 3 rear so to 22 and 29.  Also the AD08R has a bit stiffer wall than the OEM fitted ones so though the pressures séém lowish, they are not.

[Zxrob]

All very interesting

Tyre pressures are a bit like tyre choice imo, you get such a variation of comments regarding them, coming from a Motorcycle background I can ASSURE you that the pressures the manufacturers give for "road tyres" are generally way off the mark for any kind of spirited riding, I always drop the pressures to suit the bike set up and tyre wear pattern

I am no expert on car tyre pressures but one ting I have found on the MR2 is a little similar, lots of posts on run 32 rear 26 front, its what the MR2 likes, must admit, I dropped my rear to 28 and the front to 24 and imo the car feels much more planted and thats with TR1 which are seam quite soft walled to me

Rob

[JB21]

I use tyre pressures to fine tune tyre temps. The aim being to have consistent temps across the whole tyre.

[Zxrob]

I use tyre pressures to fine tune tyre temps. The aim being to have consistent temps across the whole tyre.

I know you do track days so are pushing the boundaries far more than most who are probably just using their car for the odd spirited blast, there is a massive difference however and as you say, you will tune your tyres to the days and the conditions, I use tyre warmers on track, (bike) this helps gives you that

People should not be afraid to experiment with tyre pressures

Rob

[1979scotte]

Surprised you've lowered them.
I've found my various 2s to be very dicey with low pressures.
Obviously plenty of people up the pressure although I've not felt the need on anything other than T1Rs.
The newer TR1s do seem better from what people say.
Although I'm very much into AD08Rs.

[Petrus]

Surprised you've lowered them.
I've found my various 2s to be very dicey with low pressures.

Can you expand on ´dicey´?

I have btw basically maintaíned the OEM spec pressure; compensated for size and weight.

Yes, AD08R it is. Shame they messed with it for eco sake. Fingers crossed they sort it with an AD08RS2 by the time I need a new set.

[1979scotte]

Not really just felt very unsafe.
Dont think i was referring to you anyway mate it was whoever was running toyos

[Petrus]

The ´issue´ I have is that I lóath understeer. Not any slight wandering but the front starting to slide before than the rear. There is nothing that undermines my confidence as a front end sliding first.
The AD08Rs are as good as it gets on the road, so all that is left is negative camber and the tyre pressure.

[AdamR28]

all that is left is negative camber and the tyre pressure.

You can also increase roll stiffness or spring rate at the rear, or decrease it at the front. Toe adjustments also make a huge difference to the attitude of a car. Consider changing from toe out to toe in at the front end, for example, what effect that has at initial turn in, and through the middle of the corner.


Tyre pressure changes are a 'crutch' for other handling problems. The increase or decrease in pressure mainly changes how quickly the slip angle builds, so you only get a change in handling tendency in transitions. It is a valid tool, but generally better used as a tuning tweak when 'in the field'.

It's also worth considering the tyre is basically a spring - an air spring. More pressure makes the tyre stiffer. For tyres with soft sidewalls, more pressure can be beneficial to support them, especially if you are adding more load to the car (track use, 'spirited' driving). More pressure can be very useful here too.

When racing in the 24hr at Spa last year, we used 55 front and 70 rear in our C1. I've run race-winning MX-5s starting at 45psi cold.

Personally I don't enjoy a 'sloppy' feeling car, so always go up from recommended pressures in a 'performance' car.

Manufacturer stated pressures are a guideline and not a one size fits all, as @Zxrob says we should not be scared of experimenting with this stuff, it is absolutely safe to do so.

[Petrus]

You can also increase roll stiffness or spring rate at the rear, or decrease it at the front. Toe adjustments also make a huge difference to the attitude of a car. Consider changing from toe out to toe in at the front end, for example, what effect that has at initial turn in, and through the middle of the corner.

Spring rate is something which will affect the comfort (and ride hight) too. Also would need matching damping. Not going there yet.

Yes, increase toe in a mite is one I have on the list. At OEM now.

The tyre being air suspension I am véry conscious about. It also affects tyre conformation = traction. Not, or not much of, an issue on a smooth racing track but on real world roads it makes a massive difference.

Thank you for sharing btw!!

[AdamR28]

No problem, always good to discuss this sort of thing!

Technically yes you would need to change the damping - however only a small (say, 5%) change in wheel rate (through either ARB or spring) will make a noticeable effect on under / oversteer balance. The damper will be absolutely capable of dealing with this - and there is no reason that you 'have' to increase spring rate, it can always go down... And ride height doesn't need to be changed - one of the benefits of swapping the ARB, it's a much easier job and doesn't affect ride height or two wheel rolling bumps at all.

Try some front toe out, around 15 minutes / 2mm. It will make a huge difference from the stock setting of zero toe.

Regarding tyre conformation and traction... I do agree that a large increase in tyre pressure will have an adverse effect on a bumpy road all other aspects being equal, especially with the basic standard dampers. But 5 or 10 psi isn't all that much, and worth a try.

The spring rate of a tyre (approx 20kg/mm) is a LOT higher than that of the suspension components (approx 2kg/mm OE spring rate at the rear on an MR2) given the sort of tyres and pressures we use. My point here is that adding 5psi to the tyre will increase its spring rate a little, perhaps another 5kg/mm, but the overall effect on the car isn't that much when the tyre is already so much 'harder' than the suspension.

In terms of traction, if you reduce the surface area (footprint) of the tyre by increasing tyre pressure, you are then increasing the pressure upon said area (the weight of the car hasn't changed!), and the overall friction amount remains almost unaffected.

[JB21]

No problem, always good to discuss this sort of thing!

Technically yes you would need to change the damping - however only a small (say, 5%) change in wheel rate (through either ARB or spring) will make a noticeable effect on under / oversteer balance. The damper will be absolutely capable of dealing with this - and there is no reason that you 'have' to increase spring rate, it can always go down... And ride height doesn't need to be changed - one of the benefits of swapping the ARB, it's a much easier job and doesn't affect ride height or two wheel rolling bumps at all.

Try some front toe out, around 15 minutes / 2mm. It will make a huge difference from the stock setting of zero toe.

Regarding tyre conformation and traction... I do agree that a large increase in tyre pressure will have an adverse effect on a bumpy road all other aspects being equal, especially with the basic standard dampers. But 5 or 10 psi isn't all that much, and worth a try.

The spring rate of a tyre (approx 20kg/mm) is a LOT higher than that of the suspension components (approx 2kg/mm OE spring rate at the rear on an MR2) given the sort of tyres and pressures we use. My point here is that adding 5psi to the tyre will increase its spring rate a little, perhaps another 5kg/mm, but the overall effect on the car isn't that much when the tyre is already so much 'harder' than the suspension.

In terms of traction, if you reduce the surface area (footprint) of the tyre by increasing tyre pressure, you are then increasing the pressure upon said area (the weight of the car hasn't changed!), and the overall friction amount remains almost unaffected.

Toe out at the front helps massively with turn in and understeer, toe in on the rear helps oversteer. I currently run (total) 4mm out on the front and 4mm in on the rear. Front ARB full stiff (3 settings) rear ARB medium setting (4 settings). Not a hint of understeer, even in the wet but with just enough oversteer to rotate into corners, but does get a bit hairy in the wet at the rear.

Also running -3 camber front with -3.5 rear. Nice even temps across all 4 tyres.

Only thing I really need to adjust on a track days is damping and tyre pressures.

[Petrus]

In terms of traction, if you reduce the surface area (footprint) of the tyre by increasing tyre pressure, you are then increasing the pressure upon said area (the weight of the car hasn't changed!), and the overall friction amount remains almost unaffected.

Well, yes and no.
Mathematically yes but this steps over the difference in conformation; a softer tyre will adjust better to unevenness.
That observed, the footprint will hárdly change as we are dealing with relatively stiff (radial) tread surface.

Thanks for the toe suggetstion. Will have a go next time I have it in the garage.
I have refrained from it because of the slight tendency to occasionally ´tram rail´.  A bit more for less understeer would be worthwhile trade off though.

I already have just under -2 camber front and over - 2.5 rear.

[Dev]

all that is left is negative camber and the tyre pressure.

Tyre pressure changes are a 'crutch' for other handling problems. The increase or decrease in pressure mainly changes how quickly the slip angle builds, so you only get a change in handling tendency in transitions. It is a valid tool, but generally better used as a tuning tweak when 'in the field'.

I couldn't agree more and as I had this discussion multiple times in the past. If you have to tune by air pressures it shows the weakness of your set up. Air pressure tuning is for coarse correction of a bad handling car in the moment and usually a last resort  not meant for a street driven car.

Factory tire pressures are mainly for the OEM tires based on the weight of the vehicle and the tire load rating. When you change the tires you need to take note of the load rating differences written on the sidewall and compare them to the OEM tires and make the adjustment based off a chart.  If you plus size your tires and have to use reinforced tires you might have to run more tire pressure otherwise you will damage them.  It is the air that holds most of the weight not the tires construction.   
 If you have to tune by tire pressure it is better to instead figure out what needs to be changed with the suspension and alignment first.  The tire should be optimized for its contact patch not weakened.

 Often times people chose the wrong components and then have to resort to extreme measures of changing tire pressures which is counterintuitive making the car handle well in one situation and worse in another.     

[Petrus]

Interesting how one ´faction´ sticks to OEM pressure whereas another sees it as a good point of departure.

I think OEM is in this case too the by far best researched compromise for the OEM car and intended use.
As are the suspension compenent and suspension set up spec.
The aforementioned toe in/out is a perfect example. The 0 degrees is a compromise to get a car with initially slight understeer and minimal tram-lining. With the FL Toyota went wider rubber at the rear to get more bland, oops predictable, oversteer, a bit more understeer. Tuning by tyre width illustrated ;-)

Imo the tyre pressure is just as ´unholy´ as any of the mentioned components and to some extend under investigation when the suspension or it´s set up are changed.

Indeed Dev, compensating the tyre pressure for tyre/load changes is still OEM yet álso tuning for the changed conditions, just back to OEM ;-)
It applies to a héavier load too. On the MR2 Spyder not an issue but a fully loaded SUV with a high max. load it probably is. Our previous family 4x4 had a max payload of 1 tonne with a curb weight of 1600. The instruction manual had simple table for matching tyre pressures. 
By and large a forgotten adjustment to circumstances as who reads the effing manual??!!


 

[Dev]

Im not saying to run OEM tire pressure. OEM tire pressure is for OEM tires. As a guide you should base your tires pressures first on the load rating of the tires and the weight of the car. After that you should tune your tires by pyrometry if you want to get it to be optimum to have the right contact patch or you can use the chalk method. 
Once that is done then you can make adjustments to the suspension and alignment.  Many of these issues unfortunately are based on poor tire size choices and alignment.
Usually if you have a problem with oversteer or understeer the best course correction is to adjust the sway bar in the rear and that is why a four position adjustable is a good way but its still not ideal as addressing the foundation of the suspension system particularly the damping, springs and alignment.

 This is why if you throw aftermarket suspension on the car you have now introduced a situation where the car is more likely to be unbalanced, unpredictable and most of all no longer confidence inspiring. Good suspension starts with good planing and researched components having adjustability to align the car for great handling as a complete system.  This is not to say adjusting tire pressures doesn't have its place but it should be the last refuge of adjusting for handling behavior.   

[Petrus]

Good suspension starts with good planing and researched components having adjustability to align the car for great handling as a complete system.

Quite.

This is not to say adjusting tire pressures doesn't have its place but it should be the last refuge of adjusting for handling behavior.   

Hence I have basically stuck to OEM, compensating for pr and load.

Next a whiff toe out.

[Dev]

Good suspension starts with good planing and researched components having adjustability to align the car for great handling as a complete system.

Quite.

This is not to say adjusting tire pressures doesn't have its place but it should be the last refuge of adjusting for handling behavior.   

Hence I have basically stuck to OEM, compensating for pr and load.

Next a whiff toe out.

 I do not run OEM air pressures for my tires. I have instead calculated what tires pressures I need.
   The link you posted is what I have used along with a chart after I bought new tires in April  and they work perfectly.
   https://tiresize.com/pressure-calculator/   
 
 Simply adjusting alignment to tune the behavior of the car by introducing toe is not a one size fits all. It is a compromise between high speed stability vs low speed cornering.  If you get it wrong you can compromise stability and easily lose control at higher speeds especially in the rain. Radical alignment is best for the track and not for the street. Just changing one variable will mess up another and that is why the entire system has to be tuned together conservatively if you go outside the scope of factory suspension. When we modify our suspensions there is a lot less forgiveness even though most feel like they can drive at higher speeds but for the demands of the road it's much easier to lose control.     

[Ardent]

@Petrus @Dev

Just re-visiting this thread.
In particular the tire pressure calculator.

Having failed miserably to find an alternative, are you aware of any.

Only reason I ask, the load rating I want to enter is not provided.

[Petrus]

@Petrus @Dev

Just re-visiting this thread.
In particular the tire pressure calculator.

Having failed miserably to find an alternative, are you aware of any.

Only reason I ask, the load rating I want to enter is not provided.

If you cannot find the rating Jason, it is not that complicated.
You want to compensate cahnge in tyre rigidity with air pressure
Starting from OEM a neat rule of thumb is 0.1 bar more/less for each rating down/up.

[Dev]

@Petrus @Dev

Just re-visiting this thread.
In particular the tire pressure calculator.

Having failed miserably to find an alternative, are you aware of any.

Only reason I ask, the load rating I want to enter is not provided.

We can use a chart.  What is your the load ratings of the tires?

[Dev]

I did a write up on tire pressure and load index with the charts.
 It explains in detail how to use the chart and the rational of why you have to match the load index.
 
  https://www.mr2-spyder.com/community/diy-how-to/how-to-set-your-air-pressure-using-the-tire-load-rating-chart/
 

[Ardent]

Oem rear 86.
Fitted 90 xl.

[Ardent]

Cheers for replies both.

Will have a read up after dinner.

[Joesson]

@Dev refers to a "Chalk test" in the link.
The only chalk / tyre combination I can remember is the numbered marks a coach driver would put on the front tyre of the coach, iirc the passengers put a small coin ( before decimalisation) into a hat, when the coach stopped at the " Half Way House" the mark corresponding to 12 o'clock indicated the winning seat number.
My guess for Dev's version would be a chalk mark across the width of the tread to indicate over inflation/ chalk missing from middle of tyre, under inflation / chalk missing from each side of tyre and chalk evenly worn would be just right inflation.
The same way as gauging correct tyre inflation  with tread wear but a lot quicker..