Go Back   Jeep Garage - Jeep Forum > Jeep Platform Discussion > Grand Cherokee - WK > Suspension/Brakes/Wheels/Tires

Join Jeep Garage Today
Reply
 
Thread Tools Display Modes
 
  #25  
Old 12-14-2012, 12:46 AM
bobharly's Avatar
Member
My Jeep: 2008 5.7L WK
 
Join Date: May 2012
Posts: 321
Thanks: 0
Thanked 3 Times in 3 Posts
Rep Power: 1210
bobharly is on a distinguished road
Re: Overinflating tires..good Idea or bad Idea?

Quote:
Originally Posted by dcmski View Post
In theory what you say and what the math guy said is true about contact patch. In reality something else happens when you overinflate a tire. The side wall stiffens, and that will trump a small contact patch decrease in the handling dept. I added 5-10 psi (depending on ambient air temp) to a tire when I took my sports car to a track day. (to improve handling)
Granted this sidewall phenomenon effects a low profile tire more than a big side walled All Terrain, but many of the tires on JGC's these days are highway tires with lower profiles. Especially if you're sportin 20's
Over inflating the tire does stiffen the sidewall, but without that sidewall flex you lose that Old Skool feed back called "rolling the tire" that tells you that you are about to exceed your grip. In a sports car and a controlled track environment I would most certainly give this a go. It sounds like fun actually. Should be easy to slide around when you want to. But in a 4500 to 5000 lb SUV on regular roads this is a recipe for oversteer, understeer or even a roll over in loss of grip situation. Outside of the track, I'd rather upgrade my tire options and invest in my suspension before I over inflate my tires. Also, under inflation gives the sidewalls too much flex and heat build up, reducing tracking/braking even though the mathematically calculated foot print of the rubber is bigger. Underinflation only works off road at low speeds, but not for road use. Think ford explorers and roll overs, which is why we now have to have freakin TPMS!

Use the recommended tire pressure because it is the sweet spot for maximum efficiency and safety. Those engineers get paid to figure this stuff out. Now if you are towing stuff or have a heavy load, increasing the tire pressure might have a practical application related to load capacity...

The original question was in regards to being 2 lbs over in snow/icy conditions. I've seen the tire pressure move up or down 5 lbs with outside temperature changes and driving (remember your tire pressure increases as the tire heats up rolling down the road). 2 lbs is less critical than maybe slowing down in the corners if you know there is snow and ice. Also, how about some snow rated tires if you live in an area that snows? When I was younger, wide tires had better grip in dry condition (the spoon for grip) and narrow tires were better for resisting hydroplaning (the old knife through butter). Tires have come a long way since the 80's or before. The directional tires that I run resist hydroplaning and give good dry and wet grip, so I can have my wide tire and drive it too. I don't get much snow, but I do think there is a difference between running the right tire at the right pressure, vs running the wrong tire at the wrong pressure because of an old wives tale.

Not that I have ever actually meet an old wife that gave advice on tire pressure.
__________________

__________________
Reply With Quote
  #26  
Old 12-14-2012, 07:36 PM
Member
My Jeep: 2006 5.7L WK
 
Join Date: Dec 2011
Location: Hilliard, OH
Posts: 35
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 1066
YJ John is on a distinguished road
Re: Overinflating tires..good Idea or bad Idea?

I disagree in that Jeep wants to keep the ride soft and comfortable and so will give a recommended number with that as a primary focus, with a measured pressure only a few PSI higher I believe you will get higher performance, better MPG and "feel". Just don't stray far from recommended.
__________________

__________________
Reply With Quote
  #27  
Old 12-16-2012, 11:55 AM
Member
 
Join Date: Feb 2010
Posts: 860
Thanks: 0
Thanked 6 Times in 5 Posts
Rep Power: 7980
dmsfun has a reputation beyond reputedmsfun has a reputation beyond reputedmsfun has a reputation beyond reputedmsfun has a reputation beyond reputedmsfun has a reputation beyond reputedmsfun has a reputation beyond reputedmsfun has a reputation beyond reputedmsfun has a reputation beyond reputedmsfun has a reputation beyond reputedmsfun has a reputation beyond reputedmsfun has a reputation beyond repute
Re: Overinflating tires..good Idea or bad Idea?

All MFR are also using the inflation figure for the OEM tires. Some other times work better with a little more or less. With GY Fortera triplecraps, they were rough at 32, and so were the Destination AT tires, so I ran both at 30 since the ride was too harsh and bouncy any higher. With the Cooper AT3, I run 33, and it is far smoother than either of the others at 30.
__________________
2006 GC Overland QD II
5.7 Hemi, Modern Muscle 85 mm CNC ported TB, Apexi super voltage stabilizer, Billet Tech. Catch Can, Koni FSD shocks/struts, SRT8 sway bars, Cooper AT3's, Amsoil fluids, Diablosport Intune CMR tuned by Hemifever tuning.
Reply With Quote
  #28  
Old 12-20-2012, 01:54 PM
05wkguy's Avatar
Member
My Jeep: 2006 6.1L SRT8
 
Join Date: Jun 2009
Location: Regina,Sask,Canada
Posts: 806
Thanks: 1
Thanked 8 Times in 8 Posts
Rep Power: 9335
05wkguy has a reputation beyond repute05wkguy has a reputation beyond repute05wkguy has a reputation beyond repute05wkguy has a reputation beyond repute05wkguy has a reputation beyond repute05wkguy has a reputation beyond repute05wkguy has a reputation beyond repute05wkguy has a reputation beyond repute05wkguy has a reputation beyond repute05wkguy has a reputation beyond repute05wkguy has a reputation beyond repute
Re: Overinflating tires..good Idea or bad Idea?

Not trying to be rude, but I actually have to disagree with bobharly, although his math and theory are absolutely sound, they apply to objects that are solid, not made of pliable rubber. Now I'm not an engineer or anything but i have been autocross and road racing for a while, and used to be a drag racer before that.

Tires are contructed in a way with different "stiffness" in different areas. Steel belted radials (all street tires now) are designed to be softer in the sidewall than in the tread areas. This is why when you add a few pounds of air pressure,you can see the tire get taller, however, if you were to measure the contact area between a tire at 33psi and then the same one at 35psi, you would need some very high quality equipment to measure the difference, less than 2%. What happens in the real world is that when a tire's sidewall rolls over, you actually have an effect of "jacking up" the inside of the tire. What I mean is that because the tire is designed to try and keep itself flat across the tread(from inside to outside),when the sidewall rolls it is lifting the inside of the tread up, actually removing up to 1/3 of the contact patch from the road. This is why we always raise tire pressures up to around 38-40 psi for racing. So you lose a tiny bit of contact front to back but gain so much in the left to right, when cornoring. Keep in mind that at these pressures, we are also driving the cars quite hard and will add alot of heat, still blowouts are almost unheard of.
Next something different happens when you accelerate or brake;there is weight transfer to the front or rear of the vehicle. When you brake, you are increasing the pressure on the front tires by 2-3 times over that of static. Extra weight causes the tires to squish down and you gain your contact patch back,plus more. Same thing on the rear tires when you accelerate, and this still happens,less drastically,at light brake and throttle applications.
So yes, you technically should lose grip when adding pressure,but in the real world, things happen differently than on paper.

Some other things to remember: we are talking about differences in pressure of 2-4 psi (for street use), not the 20-25psi difference like in racing or off-road environments. Your typical racing slick for drag racing is NOT a radial,it is a much softer bias-ply, designed that way for absolute maximum traction in a straight line, and run at about 15psi. Off-road tires are designed to be aired down to around 6-9psi, from the typical 30-35psi they would use on the street. If you were to run 15 psi or less in your street tire, your chances of doing damage to the tire from overheating go up drastically. You can't compare the two for the prupose of saying that you should or shouldn't do. The reason you CAN apply that to autocross situations is because we use street radial tires all the time.
Also, snow is different,trust me, i live where snow on the ground is a reality for 5-6 months of the year. Narrow tires to cut through the snow is better,because any contact patch on the ground is better than a big contact patch on snow.

(almost done guys, lol)

So adding 2-3 psi will not put you in danger of blowing out a tire, it generally offers a little better gas mileage, but does affect the ride quality. I would never recommend running a tire at 40+ psi all the time, unless you are a glutton for punishment, lol. You put yourself in more danger by running a tire underinflated than overinflated.
__________________
Borla S catback.Stillens. BT catch can. Hemifever tune.
Reply With Quote
  #29  
Old 12-21-2012, 01:44 AM
bobharly's Avatar
Member
My Jeep: 2008 5.7L WK
 
Join Date: May 2012
Posts: 321
Thanks: 0
Thanked 3 Times in 3 Posts
Rep Power: 1210
bobharly is on a distinguished road
Re: Overinflating tires..good Idea or bad Idea?

No offense, 05wkguy. I like thoughtful answers. I was just under the truck recently and while down there I could see the tire wear, or lack of wear on the inside edge, which would support the idea that when the tire is "rolling" on it's (out-)side edge, it is losing it's contact patch on the "jacked up" inside edge. I also like the observation of weight transfer, as the air in the tire does compress, even when pressurized. I should thank dcmski for resurrecting this old thread. I just learned something.
__________________
Reply With Quote
  #30  
Old 12-21-2012, 10:25 AM
05wkguy's Avatar
Member
My Jeep: 2006 6.1L SRT8
 
Join Date: Jun 2009
Location: Regina,Sask,Canada
Posts: 806
Thanks: 1
Thanked 8 Times in 8 Posts
Rep Power: 9335
05wkguy has a reputation beyond repute05wkguy has a reputation beyond repute05wkguy has a reputation beyond repute05wkguy has a reputation beyond repute05wkguy has a reputation beyond repute05wkguy has a reputation beyond repute05wkguy has a reputation beyond repute05wkguy has a reputation beyond repute05wkguy has a reputation beyond repute05wkguy has a reputation beyond repute05wkguy has a reputation beyond repute
Re: Overinflating tires..good Idea or bad Idea?

Thanks! Was hoping i didn't come across as a jerk, just trying to throw in some ideas about what i know.
__________________

__________________
Borla S catback.Stillens. BT catch can. Hemifever tune.
Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Any idea what I need to do here? jeff05wk Detailing 19 08-28-2011 05:06 PM
So...HID high beams are a bad idea....alternatives? TheAngryMinnow Grand Cherokee - WK 11 11-01-2010 09:29 AM

Powered by vBadvanced CMPS v3.2.3

All times are GMT -5. The time now is 11:34 PM.


Powered by vBulletin® Version 3.8.8 Beta 4
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
Copyright 2012 - JeepGarage.Org
The Jeep Grand Cherokee Owners Community

JeepGarage.org is in no way associated with or endorsed by Chrysler Group LLC.