Alignment Technical Questions (2)

Originally started this thread in the e46 forum, but I thought I'd cross post it here to appeal to a broader audience -
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Been doing a lot of research into alignments and while I've learned much, but I haven't been able to come up with "definitive" answers for some fundamental technical questions:

* Why does BMW require the car to be ballasted for alignment?

* What percentage error is introduced if the car is aligned without ballast?

* What percentage error does a typical alignment machine inherently have?

* What percentage error does a typical alignment machine usually produce after accounting for errors induced by maintaince and/or training issues?

* Where in the tolerance range of the alignments were the cars typically set at the factory?

Anybody care to have a go?
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One other question has since come up:

* How much do the toe and camber angles change across the operating range of the suspension? That is, how what would be the difference between the measurements for a car in the following conditions: at rest, driving normally, hard cornering, and full compression?

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Here's a link to the original thread -
http://www.bimmerfest.com/forums/sho...d.php?t=111829

Reply » Alignment Technical Questions (2)

Because it sets a known loaded condition. Also things like camber may be set for a given load to simulate some amount of body roll during cornering.

A good bit.

Very little. Even using Smart Strings are pretty accurate.

Modern machines are pretty fool proof. They even take into account rim tracking accuracy.

The same as the tolerance specified for each car. ie, tech manual says 1 degree =/- 0.1 degress, the car is done to that standard at the factory.

Depends on the car. Camber a lot, because one job of the suspension is to provide additional negative camber as teh suspension compresses to keep the tread flat during cornering body roll. Toe should not change at all, toe change over suspension travel is called bump steer and goes from annoying to dangerous. Race teams spend LOTS of time and effort to eliminate bump steer. Caster should not change through suspension movement.

Reply » Alignment Technical Questions (2)

* Why does BMW require the car to be ballasted for alignment?
Quote:

Because it sets a known loaded condition. Also things like camber may be set for a given load to simulate some amount of body roll during cornering.

That's the same conclusion I've pretty much come to - nice to know I'm not nuts!

* What percentage error is introduced if the car is aligned without ballast?
Quote:

Originally Posted by Pinecone A good bit.

Hmm - the main reason I asked this question is it seems like aligning the car without ballast is the normal way it gets done even if it is not the proper way. Knowing how to offset the alignment setting to account for that would be handy.

* What percentage error does a typical alignment machine inherently have?
Quote:

Originally Posted by Pinecone Very little. Even using Smart Strings are pretty accurate.

Makes sense - in most cases I would guess the limiting factor was the operator not the equipment.

* What percentage error does a typical alignment machine usually produce after accounting for errors induced by maintaince and/or training issues?
Quote:

Originally Posted by Pinecone Modern machines are pretty fool proof. They even take into account rim tracking accuracy.

in general I'd agree, but from reading more than a few alignment horror stories out there, it seems like there are a lot of clever fools operating these things. The downside of making these machines easy to operate seems to be that a lot of people can operate them without really understanding what they are doing. Caveat emptor.

* Where in the tolerance range of the alignments were the cars typically set at the factory?
Quote:

Originally Posted by Pinecone The same as the tolerance specified for each car. ie, tech manual says 1 degree =/- 0.1 degress, the car is done to that standard at the factory.

This one seems off to me. For example, on the E46, the bentley manual setting for the standard suspension front camber is 20minutes +/- 20 minutes. I can't imagine cars would show up from the factory with a front camber setting possibly varying from 0 to 40minutes

* How much do the toe and camber angles change across the operating range of the suspension? That is, how what would be the difference between the measurements for a car in the following conditions: at rest, driving normally, hard cornering, and full compression?
Quote:

Originally Posted by Pinecone Toe should not change at all, toe change over suspension travel is called bump steer and goes from annoying to dangerous. Race teams spend LOTS of time and effort to eliminate bump steer. Caster should not change through suspension movement.

In general yes, but I believe the toe does change to some degree as camber changes. It seems like it would have to unless the tie rod is connected aalong the exact horizontal centerline of the wheel. It also seems to be the reason behind the guidance to always set camber first, then toe. If there was no relationship, it wouldn't matter. I don't necessarily think it is a dramatic change, but the toe settings are generally cited in hundredths of a degree, which is a pretty fine measurement. So it seems like even a relatively small change, i.e. the one resulting from the camber change froman unballasted vs. ballasted alignment would be noticable.

Reply » Alignment Technical Questions (2)

I can describe to you from my Dinan Suspension, and you can get a feel for it.

Dinan used to give their specifications with weights in the car. Lately they changed it, and now their instructions are a full gas tank and no weights in the car. When I called them to clarify, they explained that they adjusted the alignments number to compensate for the lack of weights. It should not be difficult to figure out: Do an alignment with weights, remove weights, measure angles, and from now on, use these numbers for an alignment without weights.

The alignments machines has the instructions to tell the technician if weights are needed. Most new machines compensate for no weights.

The resone for weights in the car is to simulate the down forces that press the car down at a certain speed. IIRC, the weights simulate the down force at a speed of 80mph. Guess the alinment is optimized to that speed.

However, I would assume that as shocks and springs get worn out and weakened, the same down force will lower the car deeper, and therefore alignment with zero weights will start to get a little off over time.

Just my $4.95 + tax.

~

Reply » Alignment Technical Questions (2)

The 1 degree was a number used to illustrate, not a real one.

But 40 minutes is 2/3 of a degreee, so 0 - 2/3 degrees is a reasonable range for camber. Remember camber goes up a good bit as the suspension compresses.

Toe may change with camber, but should NOT do so. Depending on the susepnsion design you may get toe chamge with static camber change, but not with dynamic (due to loads and suspension compression). Toe changes with dynmaic camber changes leads to the car steering itself when entering and exiting corners.

The concept that suspension compression will change with age of hte car is probably a good one.

Reply » Alignment Technical Questions (2)

Quote:

Dinan used to give their specifications with weights in the car. Lately they changed it, and now their instructions are a full gas tank and no weights in the car. When I called them to clarify, they explained that they adjusted the alignments number to compensate for the lack of weights.

Thank you - I've been kicking this issue around for months now and that is the first bona-fide actual confirmation of what I suspected - the alignment specs COULD be written for any loaded condtion so long as it is specified. I don't suppose you know what those two different specs are?

Quote:

Originally Posted by MatWiz It should not be difficult to figure out: Do an alignment with weights, remove weights, measure angles, and from now on, use these numbers for an alignment without weights.

Difficult no, expensive yes. My original hope was to find this info already published somewhere so that I wouldn't have to pay for two alignments.

Quote:

Originally Posted by MatWiz The alignments machines has the instructions to tell the technician if weights are needed. Most new machines compensate for no weights~

That's seems entirely reasonable, but from various alignment report printouts I've seen posted it appears as if the numbers used for "spec range" are the ballasted numbers. Perhaps the actual readings are converted "ballasted equivalent" by the computer prior to being printed.

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