Anti Sway Bars 101 (LXi and LX)

[davidbrady]

Hi Ross,

For grins, the ideal formula is P1T2=P2T1, temperatures in kelvin.

For example:

In the morning when it's 80 deg F (300 deg K) you set your 315 pressures to 130 psi and you get on the road, the day warms to 100 deg F (310 deg K) and the work done by the tire warms the tire an additional 50 deg, so your tires are now at 150 deg F ( 340 deg K).

Solving for P2 your runtime hot 315 tire pressure will be (340/300)*130 = 147 psi,

and with the 365's the runtime hot pressure is (340/300)*110 = 125 psi

Add in some sun baking, stops and brake heat, turns and hub bearing friction and we're pretty close to what you are seeing. Looks normal.

[dentmac]

Neat formula. Thank you.

So if you start off with 130 psi on a cool morning 4.5 C.(Canadian Eh!) 40F =277 K


Head south a bit..80 F day = 300K plus 50F for friction etc(tires now at 130F = 328K )

so 328/277 x 130 = 154.. Bloody hard !

And if owners don't check tires.......and continue south... to your 100F day

then:
340/277 x 130 = 160 psi That is simply crazy and unsafe.

Fortunately, NHTSA finally demanded that Bluebird replace the fronts to the 365' tire. Now with the 365.... starting off at 110:

340/277 x 110 = 135 which is hard but OK.

[davidbrady]

True Ross, crazy hot pressures. I've been known to bleed air out of hot tires if I know there's no way I'd be running those pressures starting from cold at that location, and I know it's only going to get warmer the further I go. I don't recommend anyone do it, but once you know your load, your current ambient temp, and what your tires typically run at when warmed up, and you build up that experience, then it can be done.

As we all know, never, ever, ever, ever add air to a hot tire. For the newbies out there, this is almost a sure way of exploding a tire. So if I'm bleeding air from a hot tire after driving the route you described, I know that I need to do it very carefully and slowly because I can't put it back in.

BTW, why 115 psi on your 365's? That's good for a load of 19,000 lbs to 19600 lbs and you're nowhere near that.

[dentmac]

Hi,
I go with 105-110 psi cold at lower temperatures. By the load table it is true that 100 will carry 17560 load but the power steering capacity requires at least 110 psi for static steering capabilities. ie can't turn the wheel below 110 psi on pavement. It turns easier with the higher pressures. Perhaps less strain?

[al perna]

well , thank you for the Never,never,never post here I also have let air out to maintain lower temps and pressure but I did not know, that I could not add air to a hot tire. Thank you for that info David