Some Hypermiling techniques denounced by AAA

[MazdaMatt]

Just reading through this post I need to correct two things. These are not anecdotal evidence or some lost study, this is from personal experience and knowledge as a racer (sports-car, not drag or circle).

Never EVER EVER EVER inflate your tires to the maximum sidewall pressure when they are cold. In any emergency when your tires experience ANY additional stress you will blow out a sidewall. If you get pushed off the shoulder of the highway, hit a hard pothole while turning, slide sideways into a ditch, whatever. That can lead to flipping your car as the rim digs sideways into the dirt. BAD idea.

Contact patch is absolutely determined by vehicle weight and tire psi. To the poster suggesting otherwise, do the following test: let all the air out of tires and watch how the tire goes FLAT (ie, flattens out on your driveway). psi is pounds per square inch. That is to say, every square inch of contact patch will hold 1 pound of car per PSI of tire pressure. Cut your psi in half and you will double your contact patch. This is not PERFECT, however as there are sidewall stiffness and tread stiffness to factor in.

[dcb]

I hate it when people use scare tactics. All the data points to underinflation being the largest cause of sidewall seperation from the rim. Think about it, you can drive on two wheels with enough pressure holding the tire to the rim, but without enough it will come off a LOT easier (and probably result in crash and fire and death and sterility and poverty etc. etc.)

[MazdaMatt]

This is not scare tactics. Of course "the data" points to underinflation being the more common cause of sidewall seperation. Probably because underinflation is far more common than overinflation. "the data" says so.

What I said is not to scare people, it is to keep them alive in "extreme" situations. Do baloons pop from overinflation, or underinflation? Also, i did not say "do not overinflate your tires". I said to not inflate them to the maximum sidewall pressure while cold. On the race track I inflate to 36psi cold up front and when I come off the track and check them they are at 42-44 hot. If you inflate to 51psi, you will be at 55-60psi depending on the tire when you are on a highway. If you come off that highway sideways you WILL blow a sidewall (not have it fall off the bead, but tear it open) and you may hook a rim in the dirt.

[PaleMelanesian]

If you come off the highway sideways at speed, you'll blow a sidewall no matter what the inflation.

Also, the sidewall of my tires clearly says "44 psi COLD". They're designed for some expansion from warming up.

[MazdaMatt]

Originally Posted by PaleMelanesian

If you come off the highway sideways at speed, you'll blow a sidewall no matter what the inflation.

Also, the sidewall of my tires clearly says "44 psi COLD". They're designed for some expansion from warming up.

If you set your pressure to 44, you should be safe from dissaster, then. Feel free. My tires say 51psi. If i set them to 51 cold, i would be at major risk. You are drastically reducing your stopping and turning power (the whole point of inflating is to reduce friction) so good luck in your next emergency situation.

You are wrong about sidewalls always blowing out when coming off at highway speeds. Many of my friends have gone off the race track at MUCH higher than highways speeds with no issues. My fastest off-track was at 90km/h and I went sideways with no issues. I've watched people exit at 150km/h with their tires intact. None of these people would risk overinflating their tires to MAXIMUM rated sidewall pressure.

[PaleMelanesian]

Why would my tires be any safer at the specified maximum pressure (44) than yours at their specified maximum of 51?

[MazdaMatt]

because mine is not a cold maximum, i belive. If i set mine to 51, they'd be mid-high 50's on the highway and my sidewalls would have very little tolerance to shocking forces such as skidding sideways through a ditch. You are DEFINATELY NOT at the same level of safety as if you inflated to 40psi, though.

Do some searching for racing research for a chart of psi vs. grip. You'll see a major decline in grip after the "peak" level. If you live in a little boon-town and you never see any risky driving, sure, use the maximum rating. If you ever use highways near a city, I wouldn't risk it.

[PaleMelanesian]

So you're saying the 51psi tire isn't built any differently than the 44psi one? I doubt that.

A quick search brought up pages on Goodyear, Michelin, and Firestone's sites, all stating to set the pressure COLD. I know they make tires with both 44 and 51 psi ratings, and yet they don't make a separate statement for those.

[MazdaMatt]

Ok, i must be mistaken about the cold max vs. max part. I assumed that setting cold pressure to the max was resulting in over-rated pressure when warm. Either way, for the mpg vs. risk of death, i will never inflate my tires to that pressure. The reduction in grip from such a drastic psi difference is just too much. Racing a purely stock car means that the only thing i can adjust is my tire pressure. I have gone up 2psi and lost major time. I'd never go up 15psi because when I need that grip (risk of death), it won't be there.

[PaleMelanesian]

I admit, I know nothing about racing. What I do know is I'm driving in such a way as to minimize, to the extreme, anything that brings my tires near the limits of traction. Driving as if you have no brakes - you anticipate far ahead. If you're not even using your brakes, traction is not going to be an issue. This is a major point of hypermiling - situational awareness and leaving yourself plenty of time to react.