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Kirkham Motorsports

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  • 2 Post By Grey 65
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  #1 (permalink)  
Old 05-09-2018, 01:08 PM
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Default Tire Pressure Recommendation

I just took delivery of my new BDR cobra. It came with Nitto NT555 tires. Front are 245/40/18 and Back 295/40/18. What tire pressure do you recommend for front and back? Thank you
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Old 05-09-2018, 01:52 PM
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I would try 25psi at all (4) corners as a starting point.

You also might want to set your rake. I would recommend +0.50" rear. Meaning measure the two main rails at the front and the rear of them. The bottom of the rear end of the rail should be about 0.50" higher than the bottom at the front end of the rail. If it is much higher like +1.00" or more this will make your car prone to snap oversteer...
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Old 05-09-2018, 01:59 PM
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Thank you.
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Old 05-10-2018, 08:47 AM
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I concur w/ Grey 65, start w/ 25. I'm running Nitto 555 Gen2 17's w/ VINTAGE WHEELS knock offs and proper tire pressure seems to be more critical with these than my last set up. Over inflation will cause all kinds of vibrations and squirrely steering issues. I think I'm at 26psi now, but I still get some vibration around 40. I need to play with it more, but I barely find time to drive it, much less experiment.
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Old 05-10-2018, 09:11 AM
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I agree... 25 psi all around is a good starting point.

Believe it or not... my owners manual addresses tire pressure... and states 28 psi front and 29 psi rear. I offer that up... not to contradict any advice that's been given thus far... but to merely provide a "baseline" of sorts... i.e., the "book" says...
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Old 05-10-2018, 09:20 AM
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I run 26psi, once it gets lower you can feel it in the harshness of the ride, I wouldnt let it go below 24.
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Old 05-10-2018, 01:39 PM
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Keep an eye on tire wear, if the outsides are worn more than the center, you're too low with psi. If the centers are worn more, the pressure is too high. Make sure the front end is aligned properly.

Also, buy a high quality digital tire psi gauge, or the analog needle type with the liquid in the dial (more stable than dry) not some K-Mart blue light special.
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Old 05-10-2018, 01:50 PM
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Has anyone ever checked tread temps with a probe type thermometer? Not on the track, on the street?
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Old 05-10-2018, 01:57 PM
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Why would you need to know on the street temps ?

Stop to stop light racing ?
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Old 05-10-2018, 07:19 PM
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Tread temps will help you pick the best pressure. I just wondered if anyone had done that, rather than making a very educated guess.
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Old 05-10-2018, 08:07 PM
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I think unless you are swaying side to side, that you won't gain much heat in the tire and air pressure. When we are racing our dirt winged sprint car, people don't know that even on dirt, when we are getting on the gas, we are not "showing off", we are trying to build heat (and test the track) we can gain 2 psi. But, our tires are very thin.

Same goes for drag racing, using water in the water box, to put heat in the tires. Again, we gain about 2 psi.

On the street, it would be interesting to perform a mild burn out then use a temp gun on the tires and check the psi.
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Old 05-11-2018, 03:50 AM
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Rule of the thumb for tire pressure/temp gains... 1 psi for every 10 deg F gained.
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Old 05-11-2018, 04:07 AM
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What is the maximum cold tire pressure stamped on those Nitto tires? Based on that I would pick a starting pressure (3-4#'s) below it as a baseline.
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Old 05-11-2018, 06:01 AM
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Quote:
Originally Posted by CSX 4133 View Post
What is the maximum cold tire pressure stamped on those Nitto tires? Based on that I would pick a starting pressure (3-4#'s) below it as a baseline.
Then you would be very wrong. The Nitto NT555 G2 tire has a max pressure of 50psi. To put 46 psi as a starting point would be a very bad idea.

Generally speaking, a good starting point is 1 psi per 100 pounds of total vehicle weight.

I was just thinking that tread temps would help you help you dial that in. But maybe that's just a bit OCD.
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Old 05-11-2018, 10:24 AM
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Why so, my Corvette with 295/35-18 runs 42 front and 40 rear. Maybe someone should just contact NITTO and ask for their recommendation based on the tire size/vehicle weight. They would be the definitive solution, rather than conjecture.
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Old 05-11-2018, 01:29 PM
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When I use to do Goodyear test on SS cars with Radials and Bias Ply tires....20#-25# is to low for street driven tires.... when we ran SSA and SSGT for any length of time we would set tires at about 40# and go from there.... by chalking the corner of the tire from the tread to the sidewall and then see how much chalk would come off in the particular test we were doing would tell the most info....... most tire pressures are at around 40#-45#psi. to prevent the tire from rolling off of the rim...

So for Daily driving you can back the pressure down 10%-20% for a average ride ... but the minute you start getting on the car.... then it needs to come up quickly and you can't do it with any driving on the street.

So if you use 25# expect the tire to roll off the rim in aggressive driving....
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Old 05-14-2018, 01:19 AM
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When I collected my car from the factory in Durban, the sales person and technicians confirmed that a pressure of 1,5bar (22-23psi) is sufficient due to the lightness of the car. Maybe with a heavier big block 427 the pressure would have to be ramped up to 24-25psi...
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Old 05-14-2018, 09:32 AM
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Quote:
Originally Posted by Morris View Post
When I use to do Goodyear test on SS cars with Radials and Bias Ply tires....20#-25# is to low for street driven tires.... when we ran SSA and SSGT for any length of time we would set tires at about 40# and go from there.... by chalking the corner of the tire from the tread to the sidewall and then see how much chalk would come off in the particular test we were doing would tell the most info....... most tire pressures are at around 40#-45#psi. to prevent the tire from rolling off of the rim... ....
Strange, the recommended pressure on my truck are only 39psi and I think it weighs a good 3,000lbs more than my Cobra.. Oh, we use infrared heat guns these days...

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So for Daily driving you can back the pressure down 10%-20% for a average ride ... but the minute you start getting on the car.... then it needs to come up quickly and you can't do it with any driving on the street.

So if you use 25# expect the tire to roll off the rim in aggressive driving....
That is hilarious, I can guarantee you that there is one other person on Club Cobra who car pulls as many G's as mine does and I run between 19-23psi front and 18-22psi rear on 200 treadwear tires and I've yet to have one come off the rim...

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Old 05-14-2018, 02:26 PM
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Strange, the recommended pressure on my truck are only 39psi and I think it weighs a good 3,000lbs more than my Cobra.. Oh, we use infrared heat guns these days...[/IMG]
And you my friend are not driving aggressive enough to come close to rolling a tire off of a rim.....if you are running pressure that low ....it would take at least 2 laps to get enough pressure to help the tire from rolling over on it's side wall... then maybe the tire will come up to running pressure of 30#lbs....

And using a Infrared temp gauge is the worst tool you can use to measure tire Temps with .... you have to get down into the rubber to know what is going on in the tire.... Our Tire Temps are 200*degrees to 215*degrees across the tire. and thats with a pressure of 30#lbs Hot inflation.
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Old 05-14-2018, 02:42 PM
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That is hilarious, I can guarantee you that there is one other person on Club Cobra who car pulls as many G's as mine does and I run between 19-23psi front and 18-22psi rear on 200 treadwear tires and I've yet to have one come off the rim...

I'm not sure what kind of racing you are doing .... but unless you are on the Real Race tracks like Road America, Road Atlanta or Mid-Ohio you won't last long using those Pressures.... At speeds of 186mph and running the Carousels at those tracks when you are in a turn for a long period of time.... You better have your pressures right or you will drive right off the track at speed..... or be a moving Pylon....

By the way there are some Cobra's out there that are way faster then yours and have many more years of experience

I'd be happy to meet at any track and help you go faster then you ever gone before.

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