Home / Experts/Tech / How An Alignment Works And Why Your Car Even Needs One In The First Place: Musings From A Suspension Engineer

How An Alignment Works And Why Your Car Even Needs One In The First Place: Musings From A Suspension Engineer

Image (82)

Have you ever wondered why your car seems to be eating tires at an alarming rate or why it doesn’t go straight down the road when the steering wheel is centered or why it wants to head off into the weeds every time you let go of the wheel? It may be that the alignment of your suspension is off and needs to be adjusted. This is very common and can be caused by a number of things. You may have hit a large pothole, or run into a curb. Either of these — or similar events — could have caused some suspension components to bend slightly or for the suspension bolts to slip a bit. 

It doesn’t take much for the alignment, especially the toe setting (which we’ll discuss in a bit), to change. Your suspension alignment should be checked and adjusted every few years as part of normal periodic maintenance to keep your tires from wearing improperly. Poor suspension alignment can also lead to worse fuel economy since your car has to work harder to roll the tires down the road if they aren’t pointing in the right directions. 

[Editor’s Note: For those of you who don’t remember or who are new to the site, this is Huibert Mees, former Ford GT and Tesla Model S suspension engineer. He’s got a column here on the Autopian to write about whatever the heck he wants. -DT]

How A Shop Checks Alignment

Most good tire stores will have a wheel alignment machine and can provide the alignment service on your car. For this work you should expect to pay around $100-150. Not cheap but well worth it.

Many modern machines work by using lasers bounced off of targets hooked to each wheel. The targets are attached using a set of precision clamps that grab the edges of each wheel and can adjust for any size and type of wheel. Lasers housed in the crossbar mounted high up on the machine shine down on these targets, and the reflection is used to very accurately measure the position of each target. This is all done while the car is sitting on a hoist that has low friction plates under each wheel so that the tires can move side to side and forward and backward without any resistance. These plates also allow the car to be steered without scrubbing the tires. At first though, while the car is being loaded onto the hoist and the initial calibration is taking place, the plates are held in place by pins.

Once the car is positioned on the hoist, the technician will attach the targets to each of the four wheels. They will then roll the car backward and forward a small distance. This causes the targets to rotate with the wheels, which are tracked by the machine. The pins that hold the low friction plates in place are then removed and the front wheels are steered left to right about 20 degrees. This is also tracked by the machine. The rolling backward and forward and the steering of the wheel tells the machine exactly where the car is and the tracking of the targets lets the machine calculate what the initial camber, caster and toe measurements are. From there on, it can just track the changes in the position of each target to see how the camber, caster and toe changes as the technician makes his/her adjustments. As you may have guessed, these machines are very complex and precise and also very expensive for the shop to buy. Not only that, the hoist has to be very precisely installed so it is perfectly level. If it weren’t, the car would slide off the hoist as soon as the pins are removed.

How To Read An Alignment Sheet

When the shop does an alignment on your car, the machine will create a printout showing the before and after measurements. Most stores will not necessarily give you a copy of this sheet but you should ALWAYS ask for a copy so you can see how bad your car was to start with and how well the shop adjusted it. Here is an example of a sheet for my 2015 Ford Flex:

Let’s take a look at what this sheet is telling us and what all those terms mean.

The sheet is divided into two main parts: Before Measurements and Current Measurements. The before measurements section shows the alignment of the car before any adjustments are made, i.e. how it arrived at the shop. The current measurements section shows the alignment after all the adjustments have been made and the car is finished. The red and green colors show which measurement is in specification (green) and which are out of specification (red). Ideally, all the measurements should be green in the current measurements section but this might not always be possible depending on the suspension type and design and what happened to it. For example, a MacPherson strut will likely have very limited adjustment for camber so if you’ve run your car into a curb it may not be possible to get the camber alignment into specification without replacing some suspension parts.

So what do each of these numbers mean? Starting from the top we have Camber, Caster, and Toe for the left and right front wheels. After that is Total toe followed by Steer Ahead. Lastly we have Camber and Toe for each rear wheel as well as Rear Total Toe and Thrust Angle. Notice that there is no setting for rear caster. This is because caster is not critical for a rear suspension and is very rarely adjustable.

Let’s talk about each of these terms:

Camber – This is the angle of the wheel when viewed from the front.

Negative camber means the top of the tire is tipped toward the inside of the car while positive camber means the top of the tire is tipped towards the outside of the car.

Caster – This is the angle of the kingpin axis when viewed from the side of the car. The kingpin axis is a line that represents the hinge axis the front suspension rotates around when the wheel is steered. In a double wishbone suspension it is a line from the center of the upper ball joint to the center of the lower ball joint:

Pasted

In a MacPherson strut design it is a line from the center of the spring topmount to the center of the lower ball joint.

The angle between that kingpin axis and vertical is the caster angle:

It is very important for the left and right caster measurement to be as equal as possible. Having different left and right caster values will cause the steering to pull to one side or the other. I know of a few cases where the manufacturer specified different caster values left and right because the car was very sensitive to road crown, which is the curvature of the road that helps water drain to the curbs. Different left and right caster values help the car avoid drifting to the curb which can be helpful if the suspension design turns out to be very crown sensitive. For most cars this is not a problem and you will see the same caster specification for left and right.

If you look up caster in the internet you will sometimes see it defined like this:

This is absolutely incorrect. The caster angle has nothing to do with the angle of the spring. It is simply defined by the upper and lower ball joints or the topmount and the lower ball joint.

Toe – This is the angle of the wheel when viewed from above.

Since on the sheet above we have toe measurements for both left and right wheels, these are referred to as individual toe measurements and represent the angle between each wheel and straight ahead and they are measured with the steering wheel held in the straight ahead position.

Total Toe – This is the angle between the left and right wheels and is the sum of the individual toe measurements.

Steer Ahead – This is a measurement of the angle of the wheel when the car is driving straight ahead and is a combination of the front and rear toe angles. In the case of my Flex it shows that the wheel would not be pointing straight ahead when going down the road.

Camber, Toe, and Total Toe in the rear have the same meanings as in the front

Thrust Angle – This is like Total Toe in the front and is commonly called “dog tracking.” Severe thrust angles mean that the car is driving down the road sideways and is not uncommon in older vehicles with worn out rear suspensions. Here is what happens when the thrust angle is way out of wack:

So now that we know what each of these terms mean, let’s look at the sheet for my Flex and see what is going on here. Notice how each measurement includes a graphic with two steps and two small black lines. As noted before, this graphic will be green if the measurement falls within the specification and red if it does not.

The little black lines show the specification limits set by the vehicle’s manufacturer, in this case the limits span from 0.2 deg to -1.4 deg. The number in the middle (-0.7 deg) is the actual measurement for my car and the black arrow and corresponding thick line show graphically where this falls within the specification limits. While it is okay to be anywhere within the specification limits, it is best to be closer to the middle so that the performance of the car is closer to the ideal set by the manufacturer. This is represented by the central step in the green area which is the target range and is halfway to the specification limits. 

How To Adjust Alignment

So how are the alignment adjustments made anyway? The majority of vehicles these days use something called a “cam bolt” for camber and caster adjustment in the front and rear and in many cases also for toe adjustment in the rear. A cam bolt is a special bolt that includes an off-center washer attached under the head of the bolt and another one that sits under the nut.

The holes that these bolts sit in are actually horizontal slots, and there are two edges on either side of the slot that hold the cams in place. Here is a view of a cam bolt with the nut and cam removed.

And here is an example showing the bolt complete.

To make an adjustment, the nut is loosened and the head of the bolt is rotated with a wrench. The rotation causes the cams to rotate as well. Since the cams are trapped between the two turned up edges, and because the bolt is connected to the cams off-center, the bolt is forced to move back and forth inside the slot. This pushes the control arm bushing back and forth which moves the whole control arm inboard and outboard. This inboard/outboard movement of the control arm changes the angle of the knuckle which changes the camber and/or caster angles. Notice in the image above that there is a cam bolt on each of the two control arm bushings. The technician needs to adjust both bushings in the right combination to make the camber and caster adjustments individually. Once the right camber and caster angles are achieved, the nuts are tightened again and everything is clamped in place. By the way, in the image above, both cam bolts are at the limit of their adjustment range so I hope whoever owns this vehicle was able to get their camber and caster angles in spec because there is nowhere to go from here.

For many rear suspensions, the toe adjustment is made in the same way with a small cam bolt adjusting the position of the rear toe link.

Toe adjustment in the front is commonly done using an adjustable steering tie rod.

Pasted

The technician will first loosen the jam nut and then rotate the inner tie rod with a wrench. This shortens or lengthens the tie rod assembly and pushes the outer tie rod outboard or pulls it inboard, steering the wheel slightly and adjusting the toe setting.

Now let’s get back to my Flex. The before measurements show that most things were within specification except left front toe. Since the left front toe was so far out, it also meant total toe was out and steer ahead was out. The rear actually looked pretty good although thrust angle was starting to go a little too far out. Right front camber was also not great but it wasn’t out of spec.

Looking at the current measurements section we can see that the left front toe was adjusted and is now just about perfect. Both left and right rear toe were also adjusted which brought the thrust angle almost exactly to zero and this in combination with fixing the front left toe fixed the steer ahead measurement. Front camber and caster weren’t touched and are still in spec as was rear camber. In all, everything shows green and I was very happy with this result. If I had my choice though, I would have fixed the front camber and I could have asked the shop to fix it but it’s not too bad. Since it was in spec, I would have had to pay another $150 to have it fixed and it wasn’t worth it.

So next time you have your car aligned, tell them when you drop the car off that you want the alignment sheet and then look it over before you pay your bill. The shop will be totally intimidated by your knowledge and give you extra special service the next time you come in. Of course, I could be wrong on that last bit.

Share on facebook
Facebook
Share on whatsapp
WhatsApp
Share on twitter
Twitter
Share on linkedin
LinkedIn
Share on reddit
Reddit

41 Responses

  1. I could have used this article a few years ago! Fortunately I learned the negative impacts of negaitve caster on a TJ wrangler with 37 in tires without incident (only a few heart stopping moments). I started doing a lot of adjustments going from 35 to 37 inch tires, 4.5 to 3.5 in lift springs, and a few other things. Things really went bad when I went to the 3.5 inch springs…immediately driving home and then to alignment shop I noticed horrible driving characteristics…jeep would dart and veer over every bump. Took it for alignment and Firestone (I bought their lifetime alignment almost 20 years ago for $99!) adjusted toe but said they couldn’t adjust others (caster being at -.5)…so I just assumed it was not important (I’m not always the smartest in the bunch). It drove a bit better and decided it was a jeep and just kept driving it that way. It got worse when I changed jobs and was driving I66 outside of DC which was:is having massive construction – so lots of uneven roads, lane changes, and close concrete barriers. Every time I would get on an uneven portion, the jeep would dart towards one direction or another, often toward a concrete barrier…eventually that frightened me enough to start looking up what was wrong… long story long, I found out I was way out of caster and I learned what caster does! Since I had adjustable upper control arms, it was actually super easy for me to fix myself… I was very fortunate, would recommend not messing with alignment without good understanding! Thanks for the article!

  2. At the shop I used to work at doing alignments, among other stuff, I’d explain that caster is important for steering and road feel. Negative caster makes steering/turning feel easier, but then the steering wheel won’t return to center as it should – think of a wobbly shopping cart front wheel, not something you want to feel trying to drive straight. That is why caster is (almost) always positive. On the other hand, too much will give you a harsh ride and hit hard over every little bump.

  3. I had found that my 3 series BMW has some interesting factory allignment specs. When utilized, steering is tight at higher speeds and feels amazingly steady, but there is the con of premature tire wear. One set of tires will wear faster on the inside and the other will wear faster on the outside. Having two wheel/tire sizes means you can’t rotate them to extend the life.
    Skipping the factory alignment spec for even tire wear results in a loose steering at higher speeds and the vehicle just doesn’t feel dialed in. I always want to understand this more.

      1. They use the spring and strut assembly because that is the caster point on a MacPherson strut.
        Yes, Huibert, it absolutely is.
        Remember that on a MacPherson, the upper control attaches to the strut. Not the chassis. Caster is adjusted by moving the top of the strut front to rear. Which is why virtually all MacPherson equipped cars state caster as a ‘no adjustment.’

        This is how you adjust caster on the F-body, which has MacPherson fronts:
        https://www.alignmentspecs.com/images/adjust/36.gif
        The special tool moves the top of the strut within the tower, which is the only way to achieve caster adjustment. Gross camber is adjusted by lateral adjustment with fine camber adjusted by eccentric.

        1. Roowyrm, Yes, it is true that caster on an old F-body can be adjusted by moving the top of the strut (and the spring) but that doesn’t mean that the caster angle is the same as a line through the center of the spring which is what many of these illustrations would have you believe. It’s the center of the bearing at the top of the strut that determines the kingpin axis. It has nothing to do with where the spring is.

          1. Except it does. Because on MacPherson, the spring is fully located in the strut including perch and bump. So you necessarily change the angle of the spring when you change the angle of the strut. It would be worse to imply that the spring is separate from the strut, because that doesn’t accurately describe the connection points. (And yes I know Hunter’s pictidiot system doesn’t have anything but MacPherson illustrated. I hate the newer software.)

            Plus, these days, seems your average mechanic can’t do an alignment unless you literally draw them a picture. Put them on an older Hunter with a dot matrix and they’ll probably be chasing their tail the whole day. (Hey, maybe it’ll get 311’s to a price I’m comfortable paying.)

            1. Well, I guess we will have to agree to disagree but the next time you are under an F-body, ask a friend to steer the car lock to lock and you will see that the axis of rotation, i.e. the kingpin axis, does not align with the center of the spring. I assure you it will be obvious.

            2. Anyone interested in learning more about why the spring in a MacPherson strut is not located on the kingpin axis should check out “Race Car Vehicle Dynamics” by Milliken and Milliken. It’s the bible of the vehicle dynamics community and it gives a very good explanation on page 632. It also explains why the spring is not concentric with the strut.

        2. This is incorrect. It’s centerpoint of the ball joint to centerpoint of the upper strut mount. This is especially obvious from the front-view when considering KPI which is the front-view of the same kingpin axis, which is why things like the Revoknuckle exist to decrease scrub radius with McPherson struts without increasing KPI. From the front-view, the steering axis is not coaxial with the strut body, and it isn’t in the side-view either.

          https://hips.hearstapps.com/hmg-prod/amv-prod-cad-assets/images/media/51/ford-revoknuckle-and-gm-hiper-strut-explained-article-inline-photo-382164-s-original.jpg?resize=480:*

  4. In regard to a 2015 350 GLK rear wheel drive, should the alignment be checked even though tire wear is even and normal, steering is straight and true, and using a string, the front and back tires seem to be aligned. I believe this vehicle has not been in for an alignment since delivery.
    Best regards,

    1. Well, it’s never a bad idea to check it but if you’re not seeing any odd tire wear or feeling any pulls then it’s most likely OK. Keep your eyes open for any special deals tire stores have where they offer a free alignment check with a set of tires. You should definitely ask for the alignment sheet in that case just to make sure they don’t try to sell you an alignment you don’t need.

    2. By the way, the string method only works if the front and rear track widths and the tire sizes are the same. If they are different, then the string method will lead you down the wrong path because the outer tire sidewalls will not be on the same plane.

    3. Eh…

      If there’s a deal it wouldn’t hurt to get it checked, but MB does have a slightly better alignment machine that hooks into holes in the factory wheels rather than the rim, so if you want it done I would say do it at a dealer.

      But I’ve also never gotten an alignment on my BMW since new and it has 123k on it… Never pulled, always been careful to not hit potholes, and never had any weird wear so I never bothered. As long as the steering is tight and there’s no indication of suspension wear or uneven tire wear you’re probably just fine.

  5. Just had the Transit in for an Alignment after some butchery at my hand. Steering is a lot better than what the factory had dialed in for it.

    BMW 850 is going in for its alignment soon. Replaced every bushing and bearing in the rear.

    It has two of those Cam bolts and two A-Arm per side in the rear. No toe Link. It was amazing how well it still handled with everyone of those links in the rear were just rotten and loose

    1. They actually do scrub, slightly, which is why tires need to be rotated to avoid too much uneven wear. The reason is to help keep the car wanting to to go naturally in a straight line without wandering. In combination with the caster angle, when moving forward the front tires naturally lean in towards each other. If both tires were perfectly straight, when going over bumps and a tire deflects outward, the car would then tend to go that direction, and it would get noticeably worse as parts wear. So it is a fine balance keeping the car straight and avoiding excess tire wear.

      1. A car that is toe-out can have increasing turning feedback in high-speed emergency maneuvers. In other words it can lead to a tank slapper if you start to slide the rear in emergency braking. Ask me how I know.

    2. There are two main reasons why most OEM’s use a small amount of toe in. The first is that it helps with turn-in response. A small amount of toe in means that in a turn the outside tire is already turned in just a tiny bit which helps the tire respond quicker to your steering input. The second is that there is always just a little bit of drag in a tire and in the suspension bearings. This drag pulls backwards on the wheel and causes a tiny amount of toe out. The idea is that the drag offsets the toe in of the alignment and the net result is the tire going down the road more or less straight ahead. Remember we are talking about very small numbers here, on the order of 0.1 degrees or less.

  6. This is a fine article describing what an alignment is and why you need one.

    Now how about a follow up with strategies on how how an aspiring shade tree DIYer can do their own 4 wheel alignment using basic tools without spending $100-150+ and half the day at a shop. Its not THAT hard if you have the tools, space, time and know how and can save you $100-$150 and a few hours of your weekend in the process.

  7. I know it’s been said on every one of your articles already, but I’ll echo it here–loving the engineering content. This has always been some of my favorite work on other sites, and it’s been fantastic seeing it regularly here! You’re doing a great job of getting into specifics while remaining accessible (even if as with all engineering things it takes a couple read-throughs lol).

    Props to the Autopian for virtually instantly becoming my favorite car site. It’s even been interesting to a few non-car friends!

  8. Alignment is a conspiracy of the auto manufacturers proletariat ruled by the Knights Templar. Physics teaches us a object in motion in a particular direction wants to remain in motion and in the same direction. Yet somehow we are supposed to believe that cars defy the laws of physics?
    Nice try now excuse me while I make a new aluminum foil hat.

  9. “I know of a few cases where the manufacturer specified different caster values left and right because the car was very sensitive to road crown, which is the curvature of the road that helps water drain to the curbs.”

    Wouldn’t this cause problems on divided highways, where the left (if you are in the US) lanes are crowned in the opposite direction from usual? Like, if the caster is designed to stop the car from wanting to run off onto the shoulder when the crown is going down and to the right as is usually the case, wouldn’t the car *really* want to pull to the left when the crown is going the other way?

  10. Thank you for this article! I’ve tried to get a basic understanding of it for some time but some things just didn’t come across to my shade tree mechanic mind, but this outlines it so well. Now I know to ask for the data sheet and hopefully that’ll help keep the shops around here honest, as I’ve literally had to take my vehicle after driving around the block.

  11. Being a C10 guy, a common problem, especially in the early trucks, is near zero caster, which leads to the “riding the ball” feeling. Which leads to the “3/4″ mod”, whereby you move the lower a-arm 3/4″ forward on the crossmember. I didn’t want the spring bind, so on my 67, I cut the arm itself, and moved the outer BJ mount forward, then boxed the underside with 3/16″ plate. Then put in Stem top mounts to run stock travel 88-98 c1500 shocks for longer travel and alleviate the bind in the factory design. Tracks like a dream at 100+ mph, as of last weekend.

    1. Unfortunately the school I went to didn’t do FSAE at that time. (yes, I am that old!) We did Mini Baja which was also great fun. I’ve hired many FSAE alums though and I’ve never been disappointed in any of them. It’s an absolutely awesome training ground and I highly recommend it to all engineers

Leave a Reply