Stringing With Lasers: Building and Calibrating a Better Tool
Once two parallel, vertical planes of light have been established, it is fairly easy to measure the distance from the rims to the light and determine the wheels' angles.
It's been a while since I've posted anything, but I thought I should share this useful information.
In a previous post I mentioned that "stringing" is the classic method for do-it-yourself wheel alignments. By creating two parallel planes of laser light, one on each side of the car, one can easily take measurements from points on wheel rims to those planes. Using those measurements and a bit of trigonometry, one can easily determine how the wheels are pointed and make adjustments.
The first time I mentioned stringing with lasers, I used laser levels with gratings that fan the laser light out across a plane. The fact that these lasers were attached to levels was irrelevant, except that I had a pair of them in my workshop and they had 1/4"x20 threaded inserts that allow them to be mounted on tripods. So, I mounted them on some small tripods and then went through the laborious process of adjusting the light planes to be vertical and parallel to each other. It's a great idea, and better than tripping over strings stretched between jack stands, but the alignment process is still quite a pain!
I mounted my laser levels to a 2x4 in order to ensure that they would remain aligned relative to each other.
Shortly after I wrote that article, I got a crazy idea and bolted both laser levels to a 2x4. This would ensure that they would only require, at most, very small adjustments in the future (due to warping wood, or getting bumped around in the shop). However, this was still a huge improvement in shortening my setup time. Note that the calibration process mentioned below would work for any material that one might choose to use.
Realizing that wood isn't the most stable, sturdy platform in the world, I chose to make a new alignment device from pieces of steel. I would gain more assurance that there would be no changes in the alignment between the lasers from one alignment session to the next. I would still do a quick check, though, to be sure!
Two 4' pieces of 1" square tubing were welded to each other, in order to get an 8' long piece. The angle iron reinforcement at the center was also used to mount the level.
I had two 1" square pieces of steel tubing that were each 4' long. I really wanted an 8' piece of tubing, so I butt-welded them together and reinforced the center where they met with a piece of angle iron. The angle iron wrapped around the square tubing. In addition to reinforcing the steel tubing, the angle iron gave me a convenient place to mount a level. I didn't worry about getting the tubing perfectly straight, or ensuring that the angle iron reinforcement would be perfectly parallel to the tubing. All of this would come out in the wash when the lasers were adjusted and the setup was calibrated. The important thing was that the connections were solid and weren't going to move.
A foot was welded onto one end of the tubing. Also, 1/4" holes were drilled in the tubing on both ends to mount the laser levels with 1/4-20 screws.
On both ends I drilled a 1/4" hole at 1" from the end. These were to mount the laser levels. Later I measured from one level to the other and found out that the actual distance between the lasers would be 93 15/16" I made note of this for calibration purposes later.
Then, on one end I welded on a piece of metal that was cut so that it would touch the ground in two places. This forms two feet of the tripod that supports the device. I angled these two feet so that the laser levels would actually point upwards a bit. The laser fans are somewhat wasted when almost half the light is pointed into the ground. More importantly, they might not be wide enough to take measurements at a car's rear wheels if the device is set on the floor close to the rear of the car.
A third foot was made from all-thread. The pointed end goes toward the ground and it is adjustable to level the device.
On the other end I drilled a 10 mm hole inboard of where the laser level would mount. I cut a piece of all-thread and ground one end to a point. This point would be the third foot in my tripod. By using this piece with a pair of nuts, I can adjust the entire device to be level when it is in use.
I strapped an inexpensive level to the angle iron and used this to level the device.
I bolted the levels onto the ends of the device and started calibrating the system.
First, I pointed the device at my garage door from a distance of 13 feet. This should place two vertical lines on the garage door, but of course I could easily see that some adjustment would be required. First, I adjusted the third foot, to ensure that the level at the center of the device showed it to be level.
The gratings on these laser levels can be adjusted. I wanted to make these light planes perfectly vertical. Note that the laser level is purposely pointed in an upward direction.
The grating on the lasers can be adjusted on these levels to get the desired angle. I placed a long construction level upright on the ground against my garage door. By inserting shims underneath its bottom, I was able to get it to stand perfectly vertical. I adjusted the laser fan on that side such that the line went straight up the edge of the construction level.
Without moving the device, I moved the construction level to the other line on the garage door and did the same. Now I had two vertical planes, but I had to confirm that they were parallel to each other.
Now that the planes of laser light are both vertical, I measure between the two with the device at a distance of 13' from the garage door. Yellow stick-on rulers are stuck to the door in a horizontal line just below the bottom hinge.
When taking my wife to a fabric store, once, I found stick-on rulers that can be used for measuring lengths of fabric on a table. I stuck these to my garage door, starting at zero at the door's center and working outwards horizontally. They come in one-foot lengths, so I used several of these in each direction. I used another ruler to look along each sticker's length to ensure that the measurements weren't drifting from inaccurate placement. I used these for other alignment approaches in the past, but today I could use them to measure the distance between my laser light planes. Earlier, I had measured the center-to-center distance (same as the left-side to left-side distance) between the laser levels at the point where they mount to the square tubing. Now I confirmed that the distance between the planes was the same at the garage door. Obviously, I needed to make some adjustments. Once I had the measurement within 1/16" (at 13', this corresponds to 2/100 of a degree), I tightened the screws used to mount the levels and checked it again.
Then, I got out the construction level and checked that the lines on the garage door were plumb, again.
Here my completed alignment device has been painted, reassembled, and recalibrated. It is shown in position for doing an alignment on "The Silver Standard," my TDI.
Now I have two vertical, parallel planes of laser light that can be used to "string" a car for alignment. I normally place the device behind the vehicle with the laser light planes extending forward. I then align the system with the car by making the planes as parallel to the rear rims as I can get it. Of course, this is a good approach for cars without independent rear suspension. Some commercially-available systems use a third laser mounted in the middle to align the system with the car's center line.
This system has been accurate enough that I've completed alignments and didn't have to re-center the steering wheel when I was done.
One key when using a ruler to take measurements from the rim to the light: Keeping the plane of the ruler vertical, swing the ruler forward and backward a bit, to find the smallest measurement -- this ensures that you are taking measurements that are truly normal to the laser light planes and not succumbing to optical illusions.
--Keep it straight!