tire Air pressure differance

[Bruce Williams]

I am somewhat mystified about the air pressure required on my 06 NB tires. Front is 32 PSI and rear is 39 PSI. Seems funny that the air pressure for the front tires is quite a bit less than the rear, especially because of obviously more weight in the front. Is it something to do with the rear suspension? Incidently when I drove it home from the new car dealer, rears were 32 and fronts about 30. I brought them up the required PSI and the car rides somewhat harder, I think the dealer had them reduced to improve the ride, (or never checked them!)

Anybody have a clue?

Bruce in Oregon

[Photo]

The front usually have more pressure in them because of the extra witght. Increasing the pressure will result in a bit firmer ride. You are right on both accounts. I like mine at 35 front and 32 rear.

[Bruce Williams]

I think you misread my post, The owners manual says front should be 32 PSI and rears 39 PSI. That seems weird to me since all the weight is up front.

Any one else got any ideas why?

Thanks, Bruce in Oregon

[Lnzbug]

Ok I never understood why the car manufacturer would tell you what tire pressure should be. Shouldn't that info come from the tire maker?? Sorry to hijack your thread Bruce but I always wondered about this.

[Bruce Williams]

I assume it is the cars weight distribition and suspension charecteristics. But guess that all goes out the window if you change brands and size of tires when it comes time to replace the rubber.

On my NB both the manual and the sticker on the drivers door are in sync with the 32 front, 39 rear. Still confused and waiting for a logical answer.

[InjunDon]

the rear tire pressure should be higher on NB's The dome shape of the car causes a good bit of rear weight transfer on the car when at highway speeds. When going over 45MPH the car's weight distribution is 40/60 F/R > If your changing your suspension components you should even out the front and rear tire pressures since most spring manufacturers (Neuspeed and Eibach compensate for this in their spring rates.

With 18's on stock suspension reccomended tire pressures from Toyo are 35F 42R or 35f 38R if lowered.

[Bruce Williams]

Thanks,

Guess I can drop the plans for a rear spoiler, LOL. Sure wish the manual would have mentioned the reason. I pumped the tires up to the recommended pressures so wide open runs are now attainable. The ride is realy a lot fiormer, but that is good!

Thanks again, Bruce

[Jian]

Originally Posted by InjunDon

the rear tire pressure should be higher on NB's The dome shape of the car causes a good bit of rear weight transfer on the car when at highway speeds

Never heard that one before.

Lower pressures gives the tire a bigger "footprint" so what would that effect? dune runners air way down so the tires run on the samd and not in the sand.

its an interesting question.

I realy don't know why they suggest different pressures f/r I have always run my michelins at 45 f/r to improve mpg.

[mgwerks]

Originally Posted by Jian

Never heard that one before.

Lower pressures gives the tire a bigger "footprint" so what would that effect? dune runners air way down so the tires run on the samd and not in the sand.

its an interesting question.

I realy don't know why they suggest different pressures f/r I have always run my michelins at 45 f/r to improve mpg.

Lessee....

True - off roading often requires low pressures, for 'footprint width' but more for deformaility allowing increased traction on uneven surfaces and in sand.
Tire pressure should be determined on the basis of many things. The pressure on the tire sidewall is the maximum that the tire is rated for - not what it shouldbe run at. Tires can be operated over a range of pressures, depending on the load, the dynamic pressures applied to the tire, what it is used for, it's composition, etc.

Vehicle manufacturers almost always will state a pressure that gves the best RIDE without compromising safety by underinflation. But the forces that act on the tires through accellerationa nd braking, lateral G's in turns, gravity and bumps all come into play. One thing that running the rears high on these cars affects considerably is the oversteer/understeer tendancy of the car. The A4 chassis is by design a seriously understeering car. You can offset this somehat by increasing the rear pressure to make it more neutral. Ido, but if you do it's up to you. I was raised on rear engined oversteering cars and prefer them - look before you leap, YMMV.

You might check to see if your tires are even rated for 45 PSI - I know mine say 44 max. Yes, running them hard may improve mileage, but you also may be overinflating them, lowering the life of the tires as the center wears out first. The best way to run the right pressures is to use a pyrometer, but few of us have them. If you want to go harder, just go up a few PSI from what the manual says, stay away from the extremes, and you should be okay.

Weirded out tire pressure distribution can result in loss of controlled flight.

73,
Mark

[timeless]

Having ask this question a year ago on this forum in addition to asking VW folks, I never did get a definitive answer to the unusual psi recommendations. I do find the weight transfer due to aero configuration interesting ! Anyone out there have any track time in a stock suspension car? Answers from an autocross guru might muddy the issue further. Does anyone know of a tech paper addressing center of pressure to velocity values for this body? Any ideas as to why the rear wing on the Euro cup cars? Wow........

[dunno513]

I honestly wonder if the pressure is lower in the front to add extra give since with more weight = more pressure = more friction = more heat = more psi gain while driving than the rears.

Just a thought.. personally, I keep mine at all the same.. 42 PSI.. I like it rough

[mel72349]

I recall from a previous life auto-crossing a Scirocco, that the tire pressure also has an influence on handling characteristics in turns. We'd fine-tune chassis dynamics by varying the ratio between front and rear. More pressure in the fronts will lessen under-steer (increase over-steer). More in the rears will increase over-steer (lessen under-steer). Along with suspension hardware, we could get just the right handling depending on track conditions.

VW sets the pressures for a favorable balance that is safe in most situations. Unless you really know what you are doing (or trying to achieve), I'd keep the recommended ratio. FYI, I run a couple pounds more both front and rear, to improve fuel economy, and handle the potholes, cracks, ripples and other pavement dangers on our urban commutes.

[dunno513]

Would this be the same "recommended" ratio that Ford used on its Explorers which helped lead to all the deaths and accidents when Firestone's tires were sub-par. Ya, I'll take my chances with my own "recommended" higher pressures and avoid driving like i'm 16 again. "recommended" = for people who are too uninformed to make their own decisions about things. Kinda like that OM where it tells me were the key goes..

[Bruce Williams]

To further confuse things I asked the service guy at the dealer about the difference and he got a baffled look on his face and mumbled something about VW sets the tire pressures for the best ride, hmmm. No answer to why way more in the rear.

Still confused in Oregon!

[Ozz]

I'm confused on this as well and just posted a similar question as the 2007 calls for 33 front and 41 rear tire pressures, which seems odd to us. The dealer had them all roughly the same (34ish). What's right? We have the factory 16" on a NBC.