Stringing for alignment with lasers

Laser String Alignment

This weekend I used lasers to “string” my wife’s Liberty CRD and adjust its alignment.

I was recently looking at tools to improve the accuracy and the amount of time I spend doing my own wheel alignments in my workshop.  Like many other things, I believe that I can use some intelligence, ingenuity, and basic tools to do a better job than I would pay somebody else for.  Yes, it will take time, but I will know that the job is done right.

While looking at tools I stumbled across a great article on that discusses how to align vehicles the “old fashioned” way using strings stretched between jackstands.  I thought this was an instructive read, but I’d certainly trip over one of those carefully-aligned strings while making adjustments and have to start over.  Toward the end of the article, the author (Jeff Honeycutt) said that they rarely use strings anymore, as inexpensive lasers are actually easier to use.  Immediately this got my attention!

I knew that I had a pair of laser levels I used for my previous alignment efforts, and realized that they could be used this way, by lowering the included diffraction grating over the laser’s lens to spread the light out across a plane.  If this process worked well, I could save a bunch of money while doing a better job aligning my vehicles.

My previous alignment process used these same laser levels to check toe-in by attaching them to appropriate lengths of aluminum angle, laying them across the rims, and measuring the distance between the dots on my garage door with fabric store stick-on rulers that I’ve installed for this purpose.  By doing this at two measured distances from the garage door, I can determine the toe-in with some trigonometry.  I’ve set up a spreadsheet in my shop’s linux machine to do the math for me.  The problem with this approach is that I have to move the vehicle back and forth, hanging the laser levels and removing them again several times.  After I get the toe-in just right, I then have to adjust to center the steering wheel by trial-and-error.  If I’m stringing the vehicle, I’ll know when my wheels are pointed straight ahead (+/- some toe angle) and I can center the steering wheel without moving the vehicle.

So, I used this method with my wife’s Liberty CRD (yeah, it’s another diesel in the family!).  Pardon the crud on the vehicle, as winter refuses to release its grasp as of this past weekend.

First, I already know that the floor in part of my workshop happens to be almost perfectly level.  If the floor is not level side-to-side, you should look at using sheets of thin plywood, plexiglass (yes, I’ve used it that way before), or whatever you have laying around to get the vehicle level.  You can use a long length of clear tubing with water in it as a type of level to compare the positions of the hubs or the bottom or the top of the rim on both sides of the vehicle.

Checking Camber

I used this simple, inexpensive camber measurement device to ensure the camber was OK. Caster doesn’t tend to change much on its own.

Knowing that the vehicle was level, I pulled out a camber gauge that I purchased years ago from a forgotten source.  Racer Parts Warehouse sells a very similar one for $40, but I’m still not absolutely sure this is where I got it.  On this simple device, the standoffs are adjusted for the rim diameter and placed directly against the rim.  The knob, marked in 1/8 turn increments, is turned in order to bring the level’s bubble to the center.  A full turn is one degree, so the markings each represent 1/8 of a degree.  I found out that the Liberty’s camber was within spec, but that there was a little bit of asymmetry that I removed.  I don’t buy the arguments for asymmetry to counter crowned roads today, because we drive on such a wide variety of surfaces.  The vehicle should be set to drive straight on a more common uncrowned road.

Squaring the Plane of the Laser

The laser’s diffracted plane was adjusted to be vertical using a carpenter’s square.

Before starting on adjusting toe-in, I also checked to ensure that the steering wheel was in the straight, level position.  I would correct the front wheels’ position relative to the steering wheel, so that I wouldn’t have to mess with centering the wheel later.

Next, I put the laser levels onto some inexpensive tripods I picked up.  I might opt for some larger, heavier, more stable tripods later, but these worked well enough for now.  I set one up on the vehicle’s left side and used a carpenter’s square to check that the laser’s plane was at a right angle to the ground (yeah, I know the photo shows me doing this on the vehicle’s right side, which comes later).  I then rotated the tripod left and right with repeated measurements taken on the left rear wheel to ensure the plane was parallel to the wheel.  This is done by holding a ruler up at a right angle to the rim and seeing where the laser marks it.  By comparing the front and the rear of the rim at hub level, I was able to get the laser positioned properly.

Then, I set up the laser on a tripod on the other side of the vehicle and used the same carpenter’s square to adjust it to be vertical.  I measured the distance between the lines on the ground at the rear of the vehicle and at the front to see if they were parallel.  With some time, I was able to get them exactly parallel to eachother.  I then double-checked them with the carpenter’s square and knew that I could take good measurements.

Two Lasers Used for Alignment

Two lasers with diffraction gratings installed were used to make a pair of parallel planes that were at a right angle to the rear axle. Measurements for alignment were made from these two planes.

The Jeep Liberty takes very near zero toe (0.10 degree total toe), and that was what I targeted, with just a hair of toe-in.  Measurements are taken at hub level from the front and rear of the rim to the laser plane on both sides.  When you know the total toe spec, the difference between these measurements will be:

Delta = (distance across rim)*sin[(total toe degrees)*pi/180]/2

The pi/180 bit is there for spreadsheets that default to radians for trig functions.  The /2 bit allows for the fact that each side should cover half of the toe-in.  My calculation shows that the difference should only be some 1/100th of an inch, and so zero toe is a good target.  For applications where measurable toe-in is required, remember that the longer measurement should be at the front of the rim.

When I tweaked the adjustments I took the Liberty for a drive.  It tracked perfectly straight and the wheel was straight.  It wasn’t quite right before, but I was quite pleased with the results.

–Do it yourself, if you want it done right!




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