birrclan

just got my new S with 20" wheels riding on steel springs. factory settings call for 44 rear and 38 in the front but the ride is just too harsh at that pressure. any suggestions as to what benefit there would be to lowering the pressure to 34-36 all the way around. also, since i have the tire pressure monitor, i've been told the pressure settings are set and unchangeable. is that the case? if so, any problem in turning off the monitor.
thanks

Elton in SoCal

Lowering the tire pressure will increase the friction, and increased friction will mean more heat. Possibly more heat than the tire is engineered for.

In the case of the Ford Exploder, more than a few cases were caused by underinflated tires. The increase heat caused delamination blowouts. Of course Exploder tires are not high performance tires and therefore not engineered for high speed (read high temperature).

The 20s are beautiful and my test drive was in a S with 20s. To me the ride was a bit rough for a SUV, which is why I will order our S with 18s, possibly 19s. I need a thrill I will take my RoW 030 986S for a spin.

Don't take this the wrong way, but shouldn't you have expected this problem? It's kind of like the JD Power surveys of H2 owners who were complaining about gas mileage!

JeffES

Hate to say "your stuck with it", but you're pretty much stuck with it. That's the trade off of larger wheels - there's less tire to absorb road surface irregularities. You may reduce the pressure a bit without too much downside, but Elton is correct - if you reduce the pressure, the tire sidewalls will experience more flex (duration and freqency), thus pretty dramatically raising the tire's temperature.
I'm planning on getting the 20's on my upcoming S, and I'll run pressures on the low side (DC streets are nasty), and then bump the pressure on any longer/high speed trip(s). It's a bit of extra effort, but worth it to me.
J
PS - be sure to maintain the ratio of air pressures - more in the rear than in the fronts.

Jack

I believe that's the factory recommended pressures for the fully loaded vehicle. Try the light load pressures -- 38/40? -- as a starting point and see how you like the ride. I wouldn't worry about dropping the pressures a bit and doing some experimentation. Unless you're doing sustained high speed runs, I won't be concerned about over heating the tires. I don't run the factory recommended pressures in my other Porsche, since I've convinced myself (after significant experimentation) that the car handles and rides better at lower pressures. However, this question of whether to run the factory recommended tire pressures can result in some "heated" discussions.

Just my 2 cents--YMMV.

Torags

Discussions here are for paved road use. This is an SUV and if you're going over fire roads fast...... I don't think you want lowered pressures, sidewall damage may occur.

rijk365gtb4

I'm not so sure that the tires will overheat with lower pressure. What I would be more afraid of is rim damage if you hit a curb. The higher pressure in the low ratio tires avoids the sidewall to be pushed in locally

NMoore

You can change the tire monitor settings, but you should reconsider running low pressures beyond a couple pounds. That is a 5200lb beast, it's not analogous to the fairly large, very lightly loaded tires on 911s and Boxters.

Juan

Go to tirerack and buy a nice looking set of 17 or 18" wheels/tires.

IMMHO Ford should have been put of business for killing/murdering all those Explorer owners/passengers who didn't know the truth about lowered tire pressures, high OAT's and sustained high speed (70MPH+) driving.

JeffES

Juan,
Great point @ tire pressures & speed. Some years back I was able to be in on tire cert. testing for a new model car (3500lb sedan). Amazing to me, but steady state high speed caused little temp rise, but compare low vs. high tire pressures using the same high speed (75 +) turns, and the sidewall temp difference was over 90 degrees F! Dropping another 10% below the recc's, and the temp change jumped another 15 degrees! And that was from cold! By the time the tires were warm, the same tests resulted in temps that were at the maker's outside limits. Very scary how quickly an underinflated tire heats up, and it's even scarier how much pull the marketing guys have over the engineers in setting pressures/limits.
J

M758

That is why smaller wheels with larger sidewalls are better. Going over dirt roads. The larger sidewall allows a lower pressure to carry the same load (greater volume of air for same psi) and allows more flex for the same sidewall stiffness. If you run low profile tires off road even on smooth stuff you have a greater potential for tire failure than for larger profile tires.


34 psi seems way too low for such a heavy vehicle. Your best solution is to attempt to trade the 20's for 18 or 19 inch wheels.

Also the stiffness of the sidewall of the tire is big factor as well. Performance tires tend to have stiff sidewalls while comfort or off-road tires have softer sidewalls. In the case of off-road tires these sidewalls are not more prone to puncture (in fact less prone) as compared to stiff performance sidewalls. The flexing of the sidewall is actually one of the things that prevents a tire from puncturing as the tire can conform to point object more easily and this reduce the load at the pointy end. Stiffeness due to air pressure tends to focus more of vehicle weight on smaller point thus making tires more prone to puncture.

Typically the only issue off-road with too low a pressure is tires "losing the bead" and popping off the rim. I would not worry about this how ever unless you under 10 psi and I would not recommend 10 psi for anything over 10-15 mph or so.