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Should A Car Pull To The Side...


RidingOnRailz
11-21-2020, 08:44 PM
With lower tire pressures?

Assuming a vehicle with suspension in good shape, and, IE., up to 5psi lower tire pressures in the tires on one side, than on the other.

My theory is: NO.

#1. The tires with lower pressure, will have a shorter rolling radius, and will, up to a point, roll faster(higher RPM) than the higher pressure tires on the other side. The vehicle might even pull toward the side with higher pressures, IE: 1-3 higher. But, beyond a certain point - more than a 5psi difference side to side, the drag on the lower pressure side will overcome rolling radius, and pull the vehicle to that side.

Thoughts?

CapriRacer
11-22-2020, 08:40 AM
If you lower the inflation pressure of a tire, you get more drag. The fact that there is a smaller rolling radius, doesn't change that fact.

I could try building a case that the smaller rolling radius produces less torque (on a drive wheel!), but I think the lower rolling radius (less torque) would also result in a pull - but I don't think that's the way it works unless you have a locked differential.

But on a freewheeling corner (as in a steer tire on a RWD), torque doesn't play a role here.

Since one of the items to check if you have a pull is the inflation pressure. it would seem that practical experience confirms this.

But for some reason, the issue of pulls seems to only apply to the front (steering) axle. I wonder if a rear steer car (and by that I mean the steering mechanism is on the rear axle, not that the steering rack is mounted behind the centerline of the front axle) would pull in the same way. I think not.

RidingOnRailz
11-22-2020, 09:40 AM
If you lower the inflation pressure of a tire,
you get more drag. The fact that there is a
smaller rolling radius, doesn't change that fact.

I could try building a case that the smaller rolling
radius produces less torque (on a drive wheel!), but
I think the lower rolling radius (less torque) would
also result in a pull - but I don't think that's the way
it works unless you have a locked differential.

But on a freewheeling corner (as in a steer tire on a
RWD), torque doesn't play a role here.

Since one of the items to check if you have a pull is
the inflation pressure. it would seem that practical
experience confirms this.

But for some reason, the issue of pulls seems to only
apply to the front (steering) axle. I wonder if a rear
steer car (and by that I mean the steering mechanism
is on the rear axle, not that the steering rack is mounted
behind the centerline of the front axle) would pull in the
same way. I think not.


My own experience with my 2010 Honda Accord EX, 76,000mi, suggests otherwise.

Even after two alignments, plus a negative thrust angle at my request, I still have leftward highway drift. The car's front wheels both lean to the left - positive front left Camber, negative front right Camber, both still within spec, but neaarly half a degree Cross-Camber resultingly. Car tracks mostly straight on local streets, which have visibly more crown.

I keep my tires at 33psi cold(1psi above Honda recommended for this trim level).

I attempted to compensate for the highway left drift by reducing the rights to 32.5psi, felt no difference from when they were all equal. When I lowered both rights to 32.0psi, the highway left drift actually became more of a left PULL, so I aired the rights back up to match the lefts(aforementioned 33.0), and keep them that way.

CapriRacer
11-23-2020, 08:55 AM
First, my experience says that the factory alignment tolerances are too wide - by half - that the alignment has to be within the inner half of the spec.

Second, I don't think 1 psi is enough to cause a pull to appear - unless there is some other problem.

Quite some time ago, I sat down and went through everything I could find on the subject of alignment and came up with this: http://barrystiretech.com/alignmentrecommendation.html

It's been so long ago I can't remember everything I thought about - BUT - I list the cross camber as needing to be 0.2. That's quite different than the half a degree you quoted - and in the wrong direction. The fact that on high crowned streets it tracks straight says that it ought to track left on flatter pavement.

If you want to know why I think factory alignment tolerances are specified too wide, I can - but it is a bit of a long story.

RidingOnRailz
11-23-2020, 09:40 PM
First, my experience says that the factory alignment tolerances are too wide - by half - that the alignment has to be within the inner half of the spec.

Second, I don't think 1 psi is enough to cause a pull to appear - unless there is some other problem.

Quite some time ago, I sat down and went through everything I could find on the subject of alignment and came up with this: http://barrystiretech.com/alignmentrecommendation.html

It's been so long ago I can't remember everything I thought about - BUT - I list the cross camber as needing to be 0.2. That's quite different than the half a degree you quoted - and in the wrong direction. The fact that on high crowned streets it tracks straight says that it ought to track left on flatter pavement.

If you want to know why I think factory alignment tolerances are specified too wide, I can - but it is a bit of a long story.

"The fact that on high crowned streets it tracks straight
says that it ought to track left on flatter pavement."

And on interstate highways! And the cross-camber is 0.4.

My aligner suggests that, instead of fooling with uneven tire pressures side to side, or deliberately requesting a non-zero(negative) thrust angle, I should let him shift the forward subframe, which would result in a hair positive(0.1) camber angle for both front wheels, vs the positive 0.2 camber on left and negative 0.1 on the right. My research on front camber seems to indicate that slightly POSITIVE front camber forces the wheels to want to go straight, vs. negative, which causes them to "hunt" to the sides more.

Honda specified 0 camber angle for the front wheels on all 2008-'10 8th Generation Accords, with up to +0.5 to -0.8 tolerance, so I don't know why both of my front wheels lean to the left...!


Keeping the cold tire pressures as close to B-pillar spec(32psi) as possible seems to minimize the highway incursions into lanes to my left(or center guard rails!). 33psi in summer, 34 in winter, NO higher, or else the left pull really becomes noticeable.

CapriRacer
11-24-2020, 08:45 AM
Before my previous post, I wondered about how square the chassis is. So there's another person who thinks it isn't - and he is much, much closer to the problem.

And No!, negative camber doesn't cause the car to "hunt". Negative camber is what all racecars use. Many street cars use negative camber to help in cornering, but I feel too much negative camber (over 1) hurts tire wear.

However too much toe out does cause the car to "hunt".

RidingOnRailz
11-24-2020, 10:27 AM
Before my previous post, I wondered about how square the chassis is.
So there's another person who thinks it isn't - and he is much,
much closer to the problem.

And No!, negative camber doesn't cause the car to "hunt". Negative camber
is what all racecars use. Many street cars use negative camber to help in
cornering, but I feel too much negative camber (over 1) hurts tire wear.

However too much toe out does cause the car to "hunt".

That's the problem: There's as many opinions out there as there are facts about alignment angles and their effects on handling and stability.

The site, I don't remember which, suggested that positive front camber contributed more to stability than did negative front camber.

My mechanic/aligner inspected the underneath on this Accord back when I bought it just before Coronavirus arrived, and reported no indications of heavy accident impacts, nor signs of any parts replaced. The car had 71,000 miles on it at the time I took the helm and shortly thereafter brought it to my aligner. Currently 77,000 and change.

This front end positive left, negative right situation sounds like the previous driver might have somehow kissed a curb or raised center divider rather hard with that front left wheel, to have caused it. The front left SAI(axis inclination) is also about 0.4 less than that on the right side. All points to some sort of harder than avg. impact on that front left corner. We'll never know definitively.

But my aligner's method in such cases is a slight subframe shift to at least equal out that front camber, and eliminate some of the left highway drift.

CapriRacer
11-25-2020, 08:46 AM
...... The site, I don't remember which, suggested that positive front camber contributed more to stability than did negative front camber..........

Are you sure that wasn't caster?

......My mechanic/aligner inspected the underneath on this Accord back when I bought it just before Coronavirus arrived, and reported no indications of heavy accident impacts, nor signs of any parts replaced. The car had 71,000 miles on it at the time I took the helm and shortly thereafter brought it to my aligner. Currently 77,000 and change.

This front end positive left, negative right situation sounds like the previous driver might have somehow kissed a curb or raised center divider rather hard with that front left wheel, to have caused it. The front left SAI(axis inclination) is also about 0.4 less than that on the right side. All points to some sort of harder than avg. impact on that front left corner. We'll never know definitively.

But my aligner's method in such cases is a slight subframe shift to at least equal out that front camber, and eliminate some of the left highway drift.

I wouldn't discount the body warping. While I don't have much experience in what happens to body integrity over time, I can easily imagine that the structure could get warped with only minor impacts.

RidingOnRailz
11-25-2020, 10:14 AM
1. Are you sure that wasn't caster?



2. I wouldn't discount the body warping. While I don't have much
experience in what happens to body integrity over time, I can easily
imagine that the structure could get warped with only minor
impacts.

1. Read specifically the section "What does Camber mean?", second paragraph: https://blog.firestonecompleteautocare.com/alignment/alignment-settings-wheel-camber-and-caster/amp/

2. Possibly. But everything else, aside from that SAI, is not only within tolerances, but close to the middle of those ranges. And again, it's mainly highways where I feel the pull, and only if I exceed door pillar tire pressure numbers(32psi cold) by at least 5psi.

CapriRacer
11-26-2020, 06:35 AM
1. Read specifically the section "What does Camber mean?", second paragraph: https://blog.firestonecompleteautocare.com/alignment/alignment-settings-wheel-camber-and-caster/amp/ .......

Not only does it say that positive camber is for stability (I disagree!), it says negative camber is for better cornering.

Why do I disagree? If the center of the footprint is outside of the SAI, then the tire is going to be pulled outwards, making it more toed out - and that is the direction for instability.

.......
2. Possibly. But everything else, aside from that SAI, is not only within tolerances, but close to the middle of those ranges. And again, it's mainly highways where I feel the pull, and only if I exceed door pillar tire pressure numbers(32psi cold) by at least 5psi.

The more I think on it, the more I like the idea of shifting the engine cradle.

OTOH, it isn't too bad now, so why not live with it?

RidingOnRailz
11-26-2020, 07:44 AM
Not only does it say that positive camber is for stability (I disagree!), it
says negative camber is for better cornering.

Why do I disagree? If the center of the footprint is outside of the SAI,
then the tire is going to be pulled outwards, making it more toed out -
and that is the direction for instability.



The more I think on it, the more I like the idea of shifting the engine
cradle.

OTOH, it isn't too bad now, so why not live with it?

It's a matter of safety - best to be drifting away from opposing traffic, and of physics: In North America we drive on the right hand side of the road, so some rightward drift should naturally be expected. The last car I had that drifted naturally right under all circumstances was my college-era A-body 1981 Buick Century. Holding the steering wheel just between 11 & 12 o'clock countered that drift, with next to no constant correcting - or periodic nudges to the right, as with my current 2010 Accord. That Buick had nice skinny 75-series tires on 14" rims, tracked as true as could be. Miss it!

Experimenting with side-to-side tire pressure differences on this 2010 Accord has proven moot, for reasons I explained earlier on, so I will probably go the sub-frame shift route, when finances allow..!

Happy and safe Thanksgiving Day to you and anyone else celebrating!

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