Tire pressure and its effect on handling

I am trying to reduce the oversteer I have with my 70 Charger. I have a 440/493/727/8.75, 1.0 t-bars, 1.25 solid front sway bar and KYB shocks. The rear are MP XHD springs, a 3/4" frame hung sway bar and KYB shocks. 275/40 front and 295/45 rear tires. I removed a 7/8" rear bar last week to make the change to this 3/4" bar, yet I still feel that the car wants to hang the tail end out if I let it.
I have read about how racers will rub chalk on the edges of their tires, then run a lap and recheck the chalk.I am not looking to race the car on the track until next year, but I'd like to get it to only oversteer under throttle. I figure I need more front roll stiffness, but in the meantime, what effect does tire pressure have on all this? I'm aware that higher pressures will make the car more responsive with one drawback being a harsher ride. Should I reduce front pressures to lessen the steering response? I usually run the same 35 lb pressure front and rear.

What tire are you running (model/brand)?

You are just not running enough front TB to justify a rear sway bar. You are probably 55% front end weight which really is not ideal for your current set-up. Tire pressures are not going to help you here.

I would say 1.12+ TB's and some double adjustable shocks, might put you in a place where you could dial it in. I say MIGHT!

I run 1.12, double adjustables, 1 3/8 fr and 3/4" rear, and the car is still very loose. But I also leave it a little on loose side for the big tracks.

I agree! You have a lot of sidewall so lowering tire pressures will likely make it worse. Lose the rear bar and see how it feels...

The tires are Nitto 555s. 275/40/18 front. Not much sidewall there. Rear are 295/45/18.
I disconnected the 7/8" bar when I was using that size, just to test what effect it had. It handled neutral but I didn't like the extra body roll.
I'd add a picture but I don't see how to do that with the new site setup.

adding picture as an attachment;

go below the box where you type your post

click on the words 'file manager'

click on 'browse' to go to the file on your computer for the picture you want

find picture, add it

you can add 2 images per post.

Under the text box click "File Manager." Click "Browse." Choose files and click "Add File." Click "Done adding files." You can do this with up to 2 files.

What are the measurements on the rear sway bars you are using? You can calculate the wheel rate of your rear sway bar and up your front roll rate by a proportionate amount through larger TB's or a larger front sway bar to get to neutral.

To answer your original question, rule of thumb is to lower front tire pressure or raise rear tire pressure to decrease oversteer. Never used it to tune overseer/ understeer myself; and obviously you can go too far. Try 32 front 38 rear. (I don't think just tire pressures will get you where you want to go, though)

Ahhhh no. Lowering your front pressures is not correct. You are carrying a large % of your weight up there. I have concluded after much testing to use opposite of that pressure. We run 38 cold in the front with Super Sports (43 hot) and 33 rear cold (36-37) hot.

I wouldn't lower pressures at all in front it just isn't going to help, I'd try upping the front pressure.

Woooo!

I run very similar to you- 38 cold front, 34 cold rear (thanks Kevin Wesley).

I don't think 32 front 38 rear is 'good', but it's the answer to his question. What to do with tire pressures to get some oversteer out of the car. A band-aid that will take grip away from the front (at least going that low took grip from me on a Skidpad with super sports). Not a viable long term solution.

Once again I haven't done this. Just answering the question based on limited experience playing with tire pressures and the old school rule of thumb based on what to do to adjust handling.

Those tires are marginally better than a Radial TA and will never "stick" when hot. My experience is the hotter they get the worse they get.

Probably tires giving up , keep the sway bar and upgrade the tires to a 200 tread wear.Check rear axle alignment , you want 0 thrust and slightly neg on the camber ,like about .1 or.2 on rear axle.

The change from a 7/8" axle hung bar to a 3/4" frame hung bar is not as radical a change as the size would seem to dictate. This is due to the length of the arms and attaching methods used between the two. IIRC, I think there is only like a 60# difference between applied rate once installed while the actual bars have over a 100# difference due to diameter.

Typically tire pressures are used to maximize the contact patch of the tire to the conditions. You can certainly experiment with them to see if it heads you in the right directions to assist your combo until you make further changes. It costs nothing to try, so why not.

In changing tire pressure you are trying to compromise the tire's grip, so you can do this by over inflating or underinflating to reduce the contact patch. Over-inflation will also compromise acceleration capability, so I'd try the approach of dropping pressure in the rear.

Before tackling the rear, I'd make sure you are maximizing front grip. Try the chalk across the tread face and onto the shoulder, drive and see how you contact patch is and adjust to maximize the contact. Once you've done that, I'd try dropping rear pressure by 5# and see if you notice a difference. You can then tinker with 2-3# changes to try and dial in what you like best.

KYB shocks...start there...
You're chasing your tail with those on there.
I suggest one way adjustable type.
After you solve the shock issue:
Make the rear bar end bushings all rubber - tighten the nut just enough to hold the bushings tight to the sway bar. Be sure the front bar is in plastic bushings and there is the max preload on the bushings. You might be surprised.

Options - Softer rear spring rate. Smaller diameter rear bar. Longer "arm" from the rear sway bar pivot. Lower front tire pressure and raise rear tire pressure at the same time. Wider rear tire/more sticky rear tire.

Great point. When I had the 7/8" bar, I went TO rubber bushings from urethane and did see a slight decrease in oversteer. I'll try that.