Tire Pressure vs Non-OEM Tires

[91 NSX]

Yes, I have searched the forum!

Yes, I know what the OEM pressures F/R are supposed to be!

Now the question. If most of us are running after market tires, which are larger in diameter and width, does that have a significant effect on what the tire pressure should be? For example, would a larger contact patch necessairly mean you could run a lower pressure because the weight is distributed over a larger area?

I wonder if there is a way to calculate a baseline tire pressure based upon tire/wheel size (to put it simply). I know there are many variables...just wondering.

Any thoughts, forum members?

 

[Peter Mills]

The main function for using air in the tires is to support the side wall of the tire. Wider tires usually have lower side walls, or aspect, unless you want to remove all the paint and liners from your wheel wells from rubbing. Normal 33 PSI front and 40 PSI rear will work just fine. I have 215/40/17 front and 275/35/18 rear and am very satisified with 33/40 psi.

 

[91 NSX]

The main function for using air in the tires is to support the side wall of the tire. Wider tires usually have lower side walls, or aspect, unless you want to remove all the paint and liners from your wheel wells from rubbing. Normal 33 PSI front and 40 PSI rear will work just fine. I have 215/40/17 front and 275/35/18 rear and am very satisified with 33/40 psi.

I understand. I am running 215/40/17;265/35/18. However...while racing motorcycles, I learned that tire pressure is extremely important (especially since you only have two tires = much less margin for error) with regard to traction, wear, etc. Even one or two pounds pressure makes a difference in that application.

I'm just wondering (short of contacting Bridgestone...in this case), if there is a way to attempt to calculate a good "baseline" tire pressure for the NSX versus using the OEM recommendation when using oversize tires that vary from OEM. I've read that several people on this forum run 33/40 and are happy with that.

I'm not trying to make anyone's brain hurt. I just think that using the OEM recommended pressures seems almost too good to be true since (I'm guessing) the tire air volume is different for non-OEM tires...therefore the pressures might have to be adjusted (hopefully slightly) for optimal traction/wear/etc.

 

[nsxtasy]

If most of us are running after market tires, which are larger in diameter and width, does that have a significant effect on what the tire pressure should be?

It might, it might not. The fact that they are different model tires may also have an effect on the optimal tire pressure, quite aside from the difference in size.

For example, would a larger contact patch necessairly mean you could run a lower pressure because the weight is distributed over a larger area?

Wider tires don't have a larger contact patch. As long as the weight of the car is supported by the inflation of the tires, the contact patch is determined by the weight of the car and the pressure in the tires, not by the size of the tires. Inflate all the tires on a 3200-pound NSX to 32 pounds per square inch, and the four contact patches will total 3200 / 32 = 100 square inches, regardless of whether they are skinny 175 mm treadwidth or 315 mm gumballs.

However, if you lower the tire pressure, you will increase the size of the contact patch (again, regardless of size). That's why drag racers use low pressures.

I'm just wondering (short of contacting Bridgestone...in this case), if there is a way to attempt to calculate a good "baseline" tire pressure for the NSX versus using the OEM recommendation when using oversize tires that vary from OEM.

I think your best bet with any tire, in the absence of additional information, is to start with the recommendation of the car's manufacturer (in this case, 33F/40R) as your baseline, use it, and then adjust as needed based on handling, uneven treadwear patterns, etc.

 

[Gerard van Santen]

Tire pressure also effects temperature. The lower the tire pressure the hotter the tire will get while driving. On a wet track (besides the negative effects on tire patterns), you lower tire pressure to get the heat in the tire sooner.

When it is hot (the weather I mean) you want to raise you're tire pressure to get a more stable heat in the tire. When it's cold you do otherwise.

 

[Dave Hardy]

Doing a test session where you measure the cross tread temps in typical driving would be the best way.

Personally, I'd run them up to their max pressures, and then drop one end or the other to adjust handling. Street tires never have enough sidewall stiffness, hence the max pressure recomendation.

 

[NSXDreamer2]

I'm joining the band wagon and will recommend you doing the good old "trial and error" method. As nsxtasy suggested, the tire's different design could also affect the optimum pressure for your application... Also your car's alignment, road/track Temp., driving conditions, habbits, duration (5 mins commute, or 2 hrs drive??) I have Kumho MX as my street tires and when I was following manufacturer's suggested 33/40psi, my tires all wore out excessively in the middle, yet it felt a little too pushy....

I'm happy with 33/36 now... Just to give you an idea.

 

[91 NSX]

Thanks for the info, guys. I'll keep you posted on what I find out!

 

[nsxtasy]

Doing a test session where you measure the cross tread temps in typical driving would be the best way.

That's the best way for track use. I doubt that it works for normal street driving.

Personally, I'd run them up to their max pressures, and then drop one end or the other to adjust handling.

That's a terrible recommendation. Just because a tire can handle higher pressures, doesn't mean you should use it at those pressures.

From the Tire Rack website:

Checking Tire Inflation Pressure

When vehicle manufacturers select a tire size for a vehicle, they evaluate the vehicle's gross axle weights, the anticipated use of the tire, and the tire diameter and width. Adjustments to these factors give the manufacturer a way to improve handling and appearance. This is especially true for performance tire sizes. The size selected is rarely limited to only one capability (i.e. carrying the vehicle's weight). The tire usually needs to have additional load capacity as well. This extra capacity is important because without it all of the tire's performance would be used up just carrying the weight of the vehicle and little would be left for durability at high speeds or responsive handling. For all vehicles produced since 1968, the original tires sizes and inflation pressures (including the spare) are listed on a vehicle placard. This placard can be located on:

- The driver-side door or door jamb (Ford vehicles on the rear passenger door jamb)
- Glove box or counsel door
- Fuel filler door
- The engine compartment

Additionally, some manufacturers also list the original tire pressure in the vehicle's owner's manual.If a car's inflation pressure has varied from that which was recommended by the manufacturer, it's likely that the tire's wear and performance characteristics have also changed.

Underinflation

If your vehicle's tires are underinflated by only 6 psi it could lead to tire failure. Additionally, the tire's tread life could be reduced by as much as 25%. Lower inflation pressure will allow the tire to deflect (bend) more as it rolls. This will build up internal heat, increase rolling resistance and cause a reduction in fuel economy of up to 5%. You would find a significant loss of steering precision and cornering stability. While 6 psi doesn't seem excessively low, remember, it usually represents about 20% of the tire's recommended pressure.

Overinflation

If your tires are overinflated by 6 psi, they could be damaged more easily when running over potholes or debris in the road. Higher inflated tires cannot isolate road irregularities well causing them to ride harsher. However, higher inflation pressures usually provide an improvement in steering response and cornering stability up to a point. This is why participants who use street tires in autocrosses, track events and road races, run higher than normal inflation pressures.

The pressure must be checked with a quality air gauge as the inflation pressure can not be accurately estimated through visual inspection.