Judging by the size of a 6 mm socket, 3 mm is probably close to 0.12 inches. I'm not good at comparing inches and millimeters, but regardless, you're trying to measure the toe the way they rough it in on race cars. I've tried that before and I can assure you that you won't be close to correct. Toe needs to be set with an alignment computer. You will see the numbers change long before you can detect any wheel movement with your eyes or with a tape measure.
A number of factors complicate the method you're using but it's okay if you're just asking so you can visualize what is taking place. You're right that there would be that 3.0 mm difference, ... Uhm, ... Let me stick with inches so I don't confuse myself and mess up this description. Using 0.12 inches, which is a very common specification for many heavy rear-wheel-drive vehicles, that is for total toe meaning you would be measuring from wheel lip to wheel lip at the front of the wheels and at the back of them. The fronts will be 0.12" closer together than the rear. That is always split equally between both wheels so each wheel would be toed-in 0.06". That further means that starting with a wheel that is perfectly parallel to the centerline of the vehicle, the front of the wheel is turned in 0.03" which turns the rear out 0.03". To say it another way, total toe-in of 0.12" means the fronts of the wheels are 0.06" closer together than when the two wheels were perfectly parallel. We're talking about some really tiny adjustments here that only an alignment computer can detect.
The next problem has to do with why we need a little toe-in in the first place. The goal is for the two wheels to be perfectly parallel to each other when you are driving straight down the road. Due to the flexing of steering and suspension parts, they are going to allow road friction to pull the tires and wheels back a little resulting in toe-out and tire wear. We start with some toe-in to offset those road forces. Start with 0.12" toe-in, allow them to tug back resulting in 0.00" when you're driving, and you'll have the least possible tire wear. The problem comes into play when you take those measurements and make the adjustments. If you jack the truck up for better access, that lets the suspension parts hang down. Toe is designed to change as ride height changes so your measurements are different and meaningless unless the truck is sitting at its proper height.
Related to that ride height is what happens when you set the truck back on the ground. The moving suspension parts will want to push the bottoms of the tires outward but the tires can't move because they're stuck to the ground. All alignments are done with the tires on slip plates which are supported on ball bearings and must be free to move around with absolutely no binding or sticking. You can prove this by watching a mechanic who drove a vehicle onto the plates or set it down onto them, then tried to pull the locking pins. Often there is so much force pushing out on the plates that he needs a pry bar to get those pins out. Once they are removed the plates and bottoms of the tires will squirt out about an inch and a half. THAT is where the tires must be when measurements are taken. If the tires are set down on the ground with no slip plates, or even if the vehicle is simply rolled ahead to where you want to work on it, the tires haven't been allowed to slide out the same as when the alignment specifications were developed by the manufacturer. You can prove that to yourself by having the truck aligned, ending with that proper adjustment of 0.12" total toe, then measure between the wheel lips under various conditions at home. There is likely going to be no way to jack up or lower the truck that will give you the same 0.12".
The last problem you might encounter was one that plagued me many years ago. That was trying to hold the tape measure and get an accurate reading where you have to bend it at the sidewall. There's two ways to overcome that. First I made an adjustable bar that I could hold between the wheel lips. I adjusted it to fit snugly between the fronts of the wheels, marked that spot, then adjusted it to fit snugly at the rear, then measured how much I had extended it. That measurement will hardly change when switching between 14" and 15" wheels. The issue now is how precisely you can read the tape measure. You will need to adjust a tie rod a real lot before you can discern a change in the tape measure reading. That might take as much as half a revolution of the adjuster sleeve. On the alignment computer you will see the numbers change a lot from turning that sleeve less than 1/16th of a revolution.
The way the guys measure toe on a race car at the track is to jack it up and support it solidly on jack stands, then they set a fixture on the ground with a piece of chalk to mark the tire tread. They spin the tire to make a line all the way around so it will be perfectly straight and equal on the front and back. Now they have even reference points on the front and back of both tires to take measurements from. The exact location of the lines around the tires is irrelevant. They're just looking for the difference between the lines in front and in back, not the actual measurements. This method is pretty accurate for their needs but it will not work for your truck. All of the height and other variables come into play just like with your truck, but they have already compensated for that. As an example, lets say they actually wanted 0.12" toe-in. (Race cars call for wildly different settings that are further adjusted according to what each driver prefers and finds makes him go fastest). They will carefully set total toe to that 0.12" on the alignment computer at the shop, then they'll use the chalk fixture method at the race track and see what they have for a measurement that way. It will be drastically different. They may find a reading of 1.50", but if they have to quickly replace a part that got bent in a crash, if they adjust it again for 1.50" they will know that they would find their desired 0.12" again if they had it on the alignment computer. The 1.50" is not accurate or even close to correct because it isn't being measured under driving conditions or on slip plates. It just equates to what they WOULD find if they had it on the computer.
Thursday, July 25th, 2013 AT 7:49 PM