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Author Topic: Tire Pressure recommendations  (Read 12764 times)
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RebekahsZ
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« on: May 14, 2015, 06:28:28 AM »

Track is ECTA at Wilmington, Ohio-nice clean concrete. Car is 2500 pound 400hp 240z in C/GMS class. Front tires are Mickey Thompson ET Fronts 24.0/4.5/15 (2-yrs old, stored in bags in A/C storage between races) on 3.5" aluminum wheels with sidewall-recommended pressure of 35psi. Car has gone 167, looking to go 174 in June.  I am running brand new MT 275/60/15 Drag radial Pros in back at 35 psi (traction and wear pattern looked great at May meet). Question: what tire pressures do you guys run and recommend?
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ronnieroadster
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« Reply #1 on: May 14, 2015, 02:26:48 PM »

I ran the same size tires at Wilmington on my Lakester front tire presure is 36 the rear is 38 PSI the car weight is just under 2500 pounds. Wilminton speed has been over 170. Our other team car is a roadster the front tire is the same size the rear tire is a drag radial just a lot taller and wider once again same tire preasures roadster has been over 212 at Wilmington.
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RebekahsZ
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« Reply #2 on: May 15, 2015, 04:22:38 PM »

I have a friend, who has records at several tracks from Maine to Utah, telling me to go up to 65-70 psi....says it stiffens the sidewall and makes car wander less.  But, I'm afraid to pump up that high on a tire with such a low manufacturer's recommendation...

My car ran straight but was a little fishy on brake application.  Nothing I can't handle with judicious application of the middle pedal.
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ronnieroadster
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« Reply #3 on: May 15, 2015, 09:26:21 PM »

I have a friend, who has records at several tracks from Maine to Utah, telling me to go up to 65-70 psi....says it stiffens the sidewall and makes car wander less.  But, I'm afraid to pump up that high on a tire with such a low manufacturer's recommendation...

My car ran straight but was a little fishy on brake application.  Nothing I can't handle with judicious application of the middle pedal.


 I'm not sold on raising the tire pressure so high above what the tire is rated for on the sidewall. I know some high speed rated tires for the salt have pressures high like 60 or more but that's what the tire is rated for that pressure. I find at Wilmington my rear tires grow about an inch in height from the speed the roadster tire diameter grows even more that since the speed is greater. The tire diameter increase in size is giving us an added benefit of a taller gear which helps gain more MPH.
 
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Working in the shop I use the 'F' word a lot. No not that word these words Focus and Finish go Fast and Flathead Ford!
 ECTA  XF/BGRMR Record 179.8561
 LTA    XF/BGRMR  Record 186.946
 SCTA  XF/BGRMR Record 192.448
 SCTA  XXF/BGRMR Record 216.131 plus a Red Had
"Life Memeber of the Bonneville 200 MPH Club"
Dynoroom
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« Reply #4 on: May 15, 2015, 11:45:49 PM »

Another thing to keep in mind when playing with tire pressures is knowing WHY you might want to add or delete air pressure.
One of the things to know is what is your rim width? What size tire should be run on this rim? How fast might you go? Etc...
As ronnie posted above, his tire diameter grows at speed. This in turn well pull the tire inward at the bead. If your rim is on the wide side, lets say 7", and the tire co. recommends a rim width between 6" & 7" you MIGHT want to add air pressure to help keep the tire bead seated. This varies greatly depending on tire type, recommended rim widths, tire diameter vs. rim diameter, maximum speed run, vehicle weight, Etc.....  
This is posted as food for thought. I feel know one knows your combination better than you, so you you need to try to understand as best you can WHY you might want to change anything on your vehicle for better performance.

Good Luck on you project   smiley  
« Last Edit: May 15, 2015, 11:47:42 PM by Dynoroom » Logged

Michael LeFevers
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fordboy628
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« Reply #5 on: May 16, 2015, 07:31:04 AM »

Mount a camera in a location to see how the tire deforms at high speeds, and use it on a "test" run.   You might be surprised.   Make changes accordingly.

Higher tire pressures in the front tires can prevent "squirrellyness" under "heavy" braking.   As the driver, you need to be "comfortable" with how the car handles.   Forget what anybody else says about how you should drive it.    When you are constantly "wary" of the handling, it is difficult to extract the last bit of performance.

 cheers
Fordboy
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Peter Jack
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« Reply #6 on: May 16, 2015, 08:55:52 AM »

That sounds like very sound advise from two very experienced "crew guys".  grin grin grin

Pete
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RebekahsZ
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« Reply #7 on: May 17, 2015, 12:51:47 AM »

Ok, I appreciate the coaching, but we are kinda dancing around the subject (and that may be the way it's got to be).  Let me rephrase my question:  what the highest tire pressure you would feel comfortable inflating a front runner to?  Or maybe:  how high do you run your front runners?  Is 65 psi asking for a blown tire?
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rouse
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« Reply #8 on: May 17, 2015, 07:59:57 AM »

Ok, I appreciate the coaching, but we are kinda dancing around the subject (and that may be the way it's got to be).  Let me rephrase my question:  what the highest tire pressure you would feel comfortable inflating a front runner to?  Or maybe:  how high do you run your front runners?  Is 65 psi asking for a blown tire?

Your tires have a speed rating, chances are very good that the manufacturer tested those tires within that rating, so call the manufacture and ask for their recommendations.

As a general rule, higher pressure helps keep the tire beaded and reduces or limits speed growth. The folks that made your tires should be able to give you good information as long as you get it from their engineering and not some sales guru.

Rouse
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gas pumper
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« Reply #9 on: May 22, 2015, 09:16:10 AM »

Ok, I appreciate the coaching, but we are kinda dancing around the subject (and that may be the way it's got to be).  Let me rephrase my question:  what the highest tire pressure you would feel comfortable inflating a front runner to?  Or maybe:  how high do you run your front runners?  Is 65 psi asking for a blown tire?

We run front runners on both ends of a Loring, ME car in XO/GCT.  It's a 57 Chevy ex Stock car racer. We have only been to 116 mph but we run 65 lbs front and rear.  The driver says it's straight as an arrow, no steering necessary. We run a spool and the rears are almost perfect at 1/8 difference in rollout.

Rear tires are new Goodyear 28's and fronts are ancient dry Goodyear 24's.

Front runners are accepted without a speed rating because they are made to go way faster than we can all go in the mile and mileandahalf.

Frank
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Dynoroom
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« Reply #10 on: May 22, 2015, 09:41:47 AM »

Notice the OP said Mickey Thompson front runners.......   rolleyes
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Michael LeFevers
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RebekahsZ
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« Reply #11 on: May 22, 2015, 03:30:25 PM »

Thanks, gas pumper!  That's two guys who are running front runners of some kind or another (even dry-rotted ones) to 65psi, so I'm gonna try increasing to at least 50psi.  If I look at the video of my run, the car only gets fishy (and not terribly so) when I transition from full throttle to braking.  That's either due to needing more toe-in, or the soft bias ply tires in front.  Its a lot easier to add tire pressure than to go get an alignment!  I'm using this site to learn from other guys' experience.  Rule books don't discuss car handling, or car performance-they just state rules.  I called Mickey Thompson before posting and they said: "Our tires are not approved for landspeed racing." (and) "The recommended pressure is whatever it says on the sidewall."  Manufacturers aren't much help when we exceed the design criteria of their equipment.  That's when we either need each other, or just to buck-up and go for it.  Thanks again, fellas!
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Stan Back
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« Reply #12 on: May 22, 2015, 05:41:51 PM »

A toe-in check can be done almost as easy (sometimes) as an air pressure adjustment.  I'd think it might have a bigger effect than a pressure change.
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« Reply #13 on: May 22, 2015, 06:16:15 PM »

Quote
the car only gets fishy (and not terribly so) when I transition from full throttle to braking.

Have you checked the bump steer characteristics?
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manta22
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« Reply #14 on: May 22, 2015, 06:40:20 PM »

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the car only gets fishy (and not terribly so) when I transition from full throttle to braking.

Have you checked the bump steer characteristics?

I agree- bump steer is a definite suspect here.

Regards, Neil  Tucson, AZ
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Regards, Neil  Tucson, AZ
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