Tire pressure and its effect on handling
#1797519
04/07/15 04:04 PM
04/07/15 04:04 PM
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Joined: Feb 2010
Posts: 18,493 Granite Bay CA
Kern Dog
OP
Striving for excellence
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OP
Striving for excellence
Joined: Feb 2010
Posts: 18,493
Granite Bay CA
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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.
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Re: Tire pressure and its effect on handling
[Re: Kern Dog]
#1797746
04/07/15 07:33 PM
04/07/15 07:33 PM
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Joined: May 2003
Posts: 96,664 On The Boat, On The Lake, Wa. ...
amxautox
Still Retired. Still Posting on Moparts. A Lot.
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Still Retired. Still Posting on Moparts. A Lot.
Joined: May 2003
Posts: 96,664
On The Boat, On The Lake, Wa. ...
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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.
Last edited by amxautox; 04/07/15 07:33 PM.
Tom
"Everyone should believe in something; I believe I'll go fishing."
-Henry David Thoreau
Men and fish are alike. They both get into trouble when they open their mouths
author unknown
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Re: Tire pressure and its effect on handling
[Re: Kern Dog]
#1797748
04/07/15 07:34 PM
04/07/15 07:34 PM
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Joined: Dec 2010
Posts: 12,375 SoCal
MuuMuu101
I got lucky at Woodward!
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I got lucky at Woodward!
Joined: Dec 2010
Posts: 12,375
SoCal
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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. 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.
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Re: Tire pressure and its effect on handling
[Re: Kern Dog]
#1797769
04/07/15 08:00 PM
04/07/15 08:00 PM
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Joined: May 2005
Posts: 723 Houston Tx
Uhcoog1
super stock
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super stock
Joined: May 2005
Posts: 723
Houston Tx
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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)
-'02 Dodge Viper Ex-World Challenge racecar -'73 Duster, 6.1 based 392 hilborn hemi, tko600, full floater rear 9", Hellwig custom bars, viper brakes, built for road course
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Re: Tire pressure and its effect on handling
[Re: Uhcoog1]
#1797776
04/07/15 08:04 PM
04/07/15 08:04 PM
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Joined: Jan 2007
Posts: 780 Woodinville, WA
Viol8r
super stock
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super stock
Joined: Jan 2007
Posts: 780
Woodinville, WA
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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.
Last edited by Viol8r; 04/07/15 08:51 PM.
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Re: Tire pressure and its effect on handling
[Re: Kern Dog]
#1797778
04/07/15 08:17 PM
04/07/15 08:17 PM
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Joined: Oct 2013
Posts: 380 Escondido CA USA
Tomswheels
enthusiast
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enthusiast
Joined: Oct 2013
Posts: 380
Escondido CA USA
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I wouldn't lower pressures at all in front it just isn't going to help, I'd try upping the front pressure.
Last edited by Tomswheels; 04/07/15 08:18 PM.
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Re: Tire pressure and its effect on handling
[Re: Viol8r]
#1797868
04/07/15 09:35 PM
04/07/15 09:35 PM
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Joined: May 2005
Posts: 723 Houston Tx
Uhcoog1
super stock
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super stock
Joined: May 2005
Posts: 723
Houston Tx
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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 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.
-'02 Dodge Viper Ex-World Challenge racecar -'73 Duster, 6.1 based 392 hilborn hemi, tko600, full floater rear 9", Hellwig custom bars, viper brakes, built for road course
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Re: Tire pressure and its effect on handling
[Re: Kern Dog]
#1798074
04/07/15 11:56 PM
04/07/15 11:56 PM
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Joined: Feb 2011
Posts: 4,302 Nebraska
72Swinger
master
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master
Joined: Feb 2011
Posts: 4,302
Nebraska
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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.
Mopar to the bone!!!
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Re: Tire pressure and its effect on handling
[Re: Kern Dog]
#1798452
04/08/15 01:12 PM
04/08/15 01:12 PM
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Joined: Jan 2003
Posts: 5,394 Pikes Peak Country
TC@HP2
master
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master
Joined: Jan 2003
Posts: 5,394
Pikes Peak Country
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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. 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.
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Re: Tire pressure and its effect on handling
[Re: Kern Dog]
#1799722
04/09/15 08:51 PM
04/09/15 08:51 PM
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Joined: Aug 2003
Posts: 2,468 Answering the call of the wild
ThermoQuad
top fuel
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top fuel
Joined: Aug 2003
Posts: 2,468
Answering the call of the wild
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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.
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Re: Tire pressure and its effect on handling
[Re: ThermoQuad]
#1804712
04/16/15 02:16 AM
04/16/15 02:16 AM
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Joined: Feb 2010
Posts: 18,493 Granite Bay CA
Kern Dog
OP
Striving for excellence
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OP
Striving for excellence
Joined: Feb 2010
Posts: 18,493
Granite Bay CA
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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. 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.
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