Alignment - Page 6

LarryIII · #76 04-07-2003, 09:45 AM

Only 2x the posted limit?

#77 04-07-2003, 10:31 AM

If anyone wants to FEEL the change in diameter, come drive my truck with 100% locking differentials.

In the rock world, we run tires with around 10 INCHES of sidewall all the way around. We run as low as 0 PSI (valve cores simply removed) on light trucks with tough bias ply tires. So, there is a lot to play with if you want to.

Where does the extra belt length go? Out the front/back/top. You can see it with 10 inches of rubber to deform.

Does the effective diameter change with pressure? You bet it does!

With the locking diffs you can totally feel it when one tire has more air than the other. Say the left rear is at 35 and the right rear is at 28. EVERY time you hit the gas, she'll go for the ditch. Every time you let off, she'll go for the double-yellow line. Diameter baby.

#78 04-07-2003, 10:32 AM

Mychailo.... Haha, well true.. if you aren't concerned with handling then pressures in the 30s will give you the most "cushy" ride on the stock suspension.

For those of you with upgraded struts (Koni) you will probably want to run higher pressures, as it will allow the suspension to work more fully and eliminate some of the pneumatic "bounce" effect that lower pressures produce on bumps in the road.

ensteele · #79 04-07-2003, 01:00 PM

I learned a lot from this thread too. I just stood back and read the posts, but found out lots of things. Thanks for going the extra mile guys.

lee · #80 04-07-2003, 01:25 PM

Me too, probably more than most (learned stuff that is).

For those starving college students (you know who you are), there's one lesson here - can't afford all 4 tires now - buy 2 so you can drive safely, then adjust air pressure to make them have the same rolling radius as the old ones left on the car. I'm not really recommending this, better to buy 4, but sometimes the budget just can't stand the heat....OK, OK, so maybe this was a dumb idea, now why?

svxistentialist · #81 04-07-2003, 01:47 PM

I agree with Rob in principle here, the rolling diameter is what counts to the driveshaft rotation. If the tire is soft, the hub is nearer the road. The rolling diameter will be twice the lower radius, the distance from hub centre to the road, not hub centre to the top of the wheel, a different value.

One small point of difference though. The fact that the engine up front is heavier is generally compensated for by extra pressure in the front tires, in our case about 4 lbs. This is done to keep ride height level, but will also have an effect for rolling radii in relation to ABS, traction control, and our AWD. So checking and keeping your tire pressures stock is pretty important.

Joe

#82 04-07-2003, 02:02 PM

I think all my questions have been answered.

Thanks guys!

svxistentialist · #83 04-07-2003, 02:04 PM

Sorry guys,

My post above was in reply to Rob at the bottom of page 1. I did not realise you had all gone ballistic on theoreticals for six pages in the middle.

Or, should that be toroidal?

The wooden spoon goes to Beav. Will come in handy for stirring.

Joe

oab_au · #84 04-07-2003, 05:22 PM

OK I can handle that.

So how much air pressure, accounts for the difference in the front to rear weight.

The percentage will vary with tyre type, size and weight difference.
The way I would set it up, Place car on level floor, measure from floor to edge of center cap, adjust air pressure till the height is the same front to rear.

Harvey.

I know you all are trying to piss me off

i'll just go back to the engine room

lee · #85 04-07-2003, 07:55 PM

Quote:

Originally posted by oab_au
OK I can handle that.

So how much air pressure, accounts for the difference in the front to rear weight.

The percentage will vary with tyre type, size and weight difference.
The way I would set it up, Place car on level floor, measure from floor to edge of center cap, adjust air pressure till the height is the same front to rear.

Harvey.
I know you all are trying to piss me off i'll just go back to the engine room

I think your idea would apply even more strongly for anyone running other than stock sizes and pressures in their tires. There's no reason to think the 4 PSI diiference is linear when going from say 29/33 to 35/39.

Ron Mummert · #86 04-07-2003, 08:48 PM

[QUOTE]Originally posted by oab_au
[B]OK I can handle that.

So how much air pressure, accounts for the difference in the front to rear weight.

The percentage will vary with tyre type, size and weight difference.
The way I would set it up, Place car on level floor, measure from floor to edge of center cap, adjust air pressure till the height is the same front to rear.

She: "Honey, it's bedtime.... what're you doing now"?

He: "I'm making little measuring stands for the SVX".

She: "Huh"?

He: "Yes.... you see, I'm gluing these wooden rulers to these wooden blocks, then putting each one next to the car's tire, & drilling a hole where the center of the hub should be when the tires are properly inflated. I then stick a nail through the hole and center the nail EXACTLY on the little mark I made on the hub.
Then in the morning before I go to work, & after I come home I can measure the tire pressure to be sure everything's equal and I....uh...." "Cathy...Cathy?" "Now where'd she go"?

(Sound of engine roaring, gravel flying in the driveway).

He: "Jeez - NOW what'd I do"?

Ron (Yeah, I know. I've done this scenario before, but somehow it deserved a re-run).

Trevor · #87 04-07-2003, 10:41 PM

Mychailo your test is the way to go from the practical point of view and can provide exact accuracy in terms of arriving at the pressures which will reult in all wheels rotating at the same speed. Experiment with pressure ratio front/rear will provide exact figures for each and any individual set up.

Lee has made a suggestion for those on a strictly restricted budget. Odd tyres front and rear could be matched for equal speed of rotation by means of compensating pressures arrived at using Mychailo's method.

Simple observation can provide quite accurate results when a matching set of tyres are fitted. With the car on a reasonably smooth flat surface look down on the tyre side wall and note the degree off deflection (bulge) and judge when all are the same. Used with care and good judgement the eye can be an accurate tool.

svxistentialist · #88 04-08-2003, 04:44 AM

Quote:

Originally posted by Trevor
With the car on a reasonably smooth flat surface look down on the tyre side wall and note the degree off deflection (bulge) and judge when all are the same. Used with care and good judgement the eye can be an accurate tool.

This is a totally reckless suggestion Trevor. If you have ever asked a non-pregnant lady when the baby is due, you will know what I mean.

On another note, when the car is airborne, the rotational speed will change because the pressure is off, so please keep all four wheels on the ground at all times to avoid confusing the differentials!

Joe

CigarJohnny · #89 04-08-2003, 05:40 AM

Quote:

Originally posted by svxistentialist

On another note, when the car is airborne, the rotational speed will change because the pressure is off, so please keep all four wheels on the ground at all times to avoid confusing the differentials!

Hah hah!

mbtoloczko · #90 04-08-2003, 06:07 AM

I went out to that straight piece of road again last night and took more measurements. As before, I measured tire pressure and the difference in revolutions after 0.5 miles, but this time I also measured the loaded radius of the tire. Loaded radius was measured simply by holding a tape measurer up to the wheel. All observations of the difference in the rotations of the front and rear wheels is confirmed by at least two drives down the road. I got some very interesting results.

* run #1 *
front tires:
tire pressure: 37.5 psi
loaded radius: 11.9"

rear tires
tire pressure: 35 psi
loaded radius: 12.15"

After one-half mile of driving, the front tires had rolled about 0.5 more revolutions than the rear. Now if I correctly measured the loaded radius, and the rolling radius equals the loaded radius, then by the loaded radius values I measured, the front tire should have rolled 0.7 more revolutions than the rear. So there's a pretty good match.

After those measurements, I thought I had it figured out. Then I did another run to attempt to get equal revolutions.

* run #2 *
tire pressure: 37.5 psi
loaded radius: 11.9"

rear tires
tire pressure: 30.5 psi
loaded radius: 12.1"

Dropping the pressure in the rear by nearly 5 psi had almost no effect on the loaded radius. For this setup, the front and rear tires rolled the same number of revolutions over 0.5 miles! I remeasured everything several times, especially the loaded radius of the rear tire. So maybe rolling radius doesn't exactly equal loaded radius. One thing that crossed my mind: The rear tires on my car have a negative camber setting of -1 degree while the fronts are at -0.5 degree. This can affect my measurements of loaded radius. Maybe if I had taken the average of the loaded radius at the outside and inside of the tires, the difference in loaded radius between front and rear would have been less. Or maybe rolling radius and loaded radius aren't exactly the same.

I don't intend to keep a 7 psi differential between the front and rear, that seems just too much of a difference. I will probably go with 4-5 psi.

I thought I would also point out that even with a 7 psi difference, the loaded radius of the front tire is still 0.2" less than the rear. Just goes to show that there really is quite a bit more weight on the front wheels. The f/r weight distribution of the car is about 60/40. I wonder if it would take a 60/40 pressure ratio to get the same loaded tire radius.