Guidelines for tire inflation pressures?

[strotter]

I have four 205-55-15 Eagles on my '72. It's got a 327/T5 in the JTR position. I've been fiddling with tire pressures, trying to determine the "right" numbers for everyday driving. It's amazing how much a couple of psi can change the behavior of the car. Are there any guidelines? I know nothing about suspension setup other than "32psi (max pressure) = twitchy". Original factory specification for the 174-14's was 28 psi front & rear. Suggestions?

[Drax240z]

Every tire is a little bit different how it will respond to pressure changes. My suggestion is to find a place where you can do a figure 8 safely, and get a pyrometer. Start at maximum pressure and do some figure 8's, record tire temperatures, and lower them until you get a fairly uniform distribution across 3 points on the tread width. You might find that you have a hotter inside edge, or outside edge, the results of too much or too little negative camber. This can't be corrected by tire pressure changes, however you can make sure you don't have a hot or cold spot in the middle of your tire with this method.

[JMortensen]

I don't think a pyrometer is going to help much in this situation. Like you said Drax, he's gonna do some 8's or something then check tire temps, but this problem occurs when he's on the street, where he's going straight 95% of the time. Of course it might help to know what the problem is too. Twitchy is a little vague. Does it follow ruts in the road? does the front end blow all over the freeway? Does it get sideways when you floor it around a 15 mph turn?

 

I'd say don't underinflate the tires trying to make the car feel better. You have a much shorter sidewall now than the original tires did. The tire should have a max pressure on it, and I would be very surprised if it was only 32 psi. If you underinflate you will probably bend a rim if nothing else, but you can also debead a tire or overheat a tire that way too. Take the car to an alignment shop, and if not twitchy is really what you want have them do an alignment and put it as close to factory spec as possible, assuming there is anything to adjust. If nothing else you could post the numbers here and we can all help you analyze what is going on in your suspension.

 

327 on 205's is gonna be a handful in certain situations anyway, and you may just have to deal with it.

 

Jon

[strotter]

The car was aligned to factory specs recently, after I installed all poly bushings. It hasn't had the bumpsteer mod done, though I'm thinking now it might be a good idea. It has a 1" front swaybar, and no rear.

 

When I say "twitchy", I mean it's prone to steer in a "sudden" manner - sometimes I get more result from a given steering input than I'm expecting. For instance, there's an *excellent* freeway off-ramp near my house. It's an "s" that rises right, peaks, becomes a left and falls, then ends in a reducing-radius right 270 (in case you know Lodi, it's the northbound Turner exit). With 32 psi all around, on decelerating transition from the last left (radius about 400') in the "s" to the right 270 (radius starts about 200', ends about 100'.) I'll experience the expected weight-shift understeer, which will *suddenly* become oversteer when pushed hard. When power off for the final part of the 270, the rear begins to roll around, then tucks itself in, at which point it returns to understeer (again suddenly). It doesn't lose traction - if you're not on it hard - but it's ready to. I haven't pushed it to traction loss (speeds are too high, no verge, no exit).

 

With tires at 28 psi all around, it begins understeer on decel and stays that way throughout. When pushed harder, it'll plow mercilessly. From what I remember (it's been a very long time since I drove a "stock" 240), that's pretty much the behavior of the stock car.

 

I think the ideal pressures are between 28 and 32, but by keeping the pressures the same front and rear I think I'm giving up a smooth transition, which is more-or-less what I'm looking for. I'm ready to start checking out variations on the theme - 28 front, 32 rear, 32 front 28 rear, 30 all around, 30 front 28 rear, 28 front 30 rear, so on and so on. I was just hoping there'd be some shortcut, like "higher rear tire pressures speed the transition to oversteer, higher front pressures speed transition to understeer", or something like that ...

[JMortensen]

More tire pressure equals less traction for the most part, unless you get extreme with it. If you are racing and you have a push, you drop pressure in the front or raise pressure in the rear to fix. Bump steer fix is a good idea.

 

Jon

[Guest LedFoot]

Hi All,

 

With my work at some stage I was lucky to sit down with a tire expert and get some interesting information out of him regarding tire pressures on street tires. This is the info I was given....

 

Normal tires are given a recommended pressure based on performance and ride comfort. That is the manufacturers use the tire flex to give extra suspension for a smoother ride.

 

The problem with this is that once you start to use tire flex for suspension instead of the shock absorbers, which are designed to dampen, then you start to include roll in the tire during cornering, pinching of the tire during braking and excessive heat during higher speed driving. All of these have their own problems;

 

-Excessive roll-

The only way to roll a car is if you hit something that stops the wheels from moving while going sideways. So if you have the back end out, and hit a curb then whump, crunch, crunch and you're up for some health insurance! Another way to supply this is if you have underinflated tires that roll so far during harsh cornering that the rim actually digs into the ground. This supplies your curb and the rest is history! Properly inflated performance tires will slide at this point and not allow this to occur

 

-Pinching-

If your front tires are underinflated during heavy braking then when the load up from the increased downward force on them the tirewill start to flex in the centre. This is because the sidewall bulges out and as the corner area where the sidewall meets the rolling surface of the tire is very rigid it lifts up in the centre of the tire. This reduces the contact patch dramatically and it takes longer to stop. You will probably have seen skid marks on the road where accidents have occured that have to pairs of thin double black lines. These are due to tires pinching and only the outside edges of the tire gripping the road.

 

Pinching also gives you very bad performance in the wet. When you look at a tire the most amount of water moved in the wet goes straight out the back through those nice big channels that go inthe direction of the tires rotation. The ones that go out the side do a little but not as much as you would think. Additonally those other funky patterns are there mainly to reduce road noise and look good. When your tires pinch under load in the wet then these channels actually close up and restrict water from moving out the back. As a consequence you aquaplane and don't get too much grip.

 

Now I've described pinching under braking conditions but the same principle can easily be applied to heavy cornering with under inflated tires as the outside wheels load up with downards force. Same type of pressure but just created differently. Less contact patch on the outside tires touching the ground equals less grip so understeer occurs.

 

-Excessive heat-

The bigest cause of blowouts is underinflated tires due to sidewall weakening (hitting ditches/rocks is also right up there! ). When you drive the heat generated from rolling friction which applies to rolling surface of the tire is easily compensated by the airflow over the tire. What is a problem is that under inflated tires have a bulge at the bottom, and not at the top. If you were to follw a single point on the sidewall as it rotates around then that bulge causes that point to go from being straight at the top to being flexed at the bottom. The faster you go the faster this happens, the faster this happens the more heat generated by metal fatigue in the braid that supports the rubber and finally the braid will fail and the sidewall will blow out.

 

His solution to all this is to increase your tire pressures about 8-12 psi above the recommended pressure. My tires are recommended to 32psi on one of my other cars and he recommended that I put them up to around 40-42 psi.

 

By doing this the following would occur;

1) Reduce the roll as the tire would not flex as much during cornering, therefore your suspension becomes the determining factor for how much the car actuall rolls.

2) Stop the posibbility of the rim digging in and rolling the car.

3) The tires will perform better in the wet

4) Less chance of blow outs.

5) You fuel economy increases as there is less friction to overcome by sidewall flex etc.

 

Now keep in mind this information was for stock type tires and may not be applicable to high performance, racing or specialist tires. As these have a particular purpose the manufacturer is looking more to giving you areal best performance pressure rather than one that incorporates ride comfort.

 

Also I think the best way to figure out the best pressures for your car is probably to go on an open area with a meter that measures g-forces. Go around in a circle and increase your speed gently until you start to get understeer or the tail kicks out. Check out what the max g was for that pressure of the tire. Then change your pressure up or down by about 2psi. Repeat this while plotting the results of g-force versus tire pressure on a piece of graph paper. Where it peaks is the best tire pressure for cornering. You could also just keep the steering on a single angle and plot the speed that you get understeer, but that would be alot more difficult as you would need to get the angle dead on every time.