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MG MGB Technical - Checking/Setting Front Axle Toe-in
|I thought that this information would be of interest to the club.|
Checking/Setting Front Axle Toe-in
Toe-in can be easily checked on any automobile and with a little effort corrected to the required 1-mm plus or minus1-mm or 1/32nd plus or minus 1/32nd. To complete the task you will need masking tape, a tape measure, preferably metric, a pen and an able helper.
You start by driving the car forward and backward on near level ground for at least 5 tire revolutions in each direction while fiddling with the steering wheel to ensure that the wheels are pointed straight ahead. The last step in this part of the process is to back the car to its starting point and stop the engine. Now, take your hands off of the steering wheel and have your helper push the car forward for about two tire revolutions. If the car tends to roll use the hand brake only to stop and hold the car in place.
The measurement phase consists of placing a piece of masking tape, about 2-3 inches long, at the rear of each tire. The tape should be place vertically near the center of the tire tread. Next, using your pen, place a small horizontal tick mark at the inner edge of each piece of tape. The tick marks should be as high off the ground as possible while still being able to have a clear line of sight from tick mark to tick mark. With the aid of your assistant, CAREFULLY measure the distance between the edges of the each piece of masking tape at their respective tick mark locations. Be sure that the tape measure does not touch any part of the underside of the car. Now, push the car forward, without touching the steering wheel, while carefully observing the masking tape. Stop the car, using the hand brake only, when the tick marks are as high off the ground as possible while having a clear line of sight between the tick marks. Remeasure the distance between the tape edges at their respective tick marks. Now, subtract the two measurements to obtain the toe value. If the distance measured at the rear of the tire is greater than the front, your front axle will be toed-in. It is easier to get an accurate measurement if you use the 100-mm or the 4-inch as the starting point because we are not interested in the actual distance but rather the difference between the front and rear.
This method of checking front axle toe is very accurate because you are measuring between the same two points on each tire while the points are at both the front and rear positions. Tire and wheel run out and tread pattern irregularities are accounted for.
If you find the your toe is out of specification you can make proper adjustments. Start by changing the front distance by one half of the error. Then push the car forward (never backwards) and remeasure the rear distance. Make a small toe adjustment if necessary, and repeat the entire process until you have reached the desired toe-in value. After you think that the toe is correct take the car for a short drive and repeat the measurement process.
Although time consuming, I can assure you that this toe measurement technique is at least as accurate, and more reliable than alignment shops offer. This is a direct measurement that does not rely on equipment calibration to ensure accuracy. If you want caster and camber measured/corrected or a four-wheel alignment on a modern car you must go to an alignment shop. Use the above method to check their work. You will be surprised! Have you ever noticed tire wear after 6-months of driving after an alignment, taken the car back and told that you must of hit a curb or a chuckhole. Sure…
|Nice write up, Frank.|
I've done essentially what you describe several times over the years, but I never thought about how you should always roll forward when making the adjustments. I rolled it back and forth, and that might account for me scratching my head and then finally deciding, "that's good enough".
The only suggestion I might offer for a possible improvement is that if, for example, 4 o'clock is as high as you can measure behind the wheels, then you should use 8 o'clock in front of the wheels even if you could measure it at 9 o'clock with out underbelly interference.
|C R Huff|
|I really don't think you need to roll it 5 and 2 tyre revolutions, most tyre places don't have that much room, let alone home driveways. Any torque on the tyres will vanish in less than one revolution.|
Otherwise this is very similar to the method described in my Toyota Celica Workshop Manual. However the emasurements must be taken behind and in front *in line with the axle*. 'As high as possible' isn't good enough, the further away from the horizontal you get the smaller the apparent toe will be, until at the top and bottom it vanishes altogether. However when you bring camber into the equation it will be camber you are measuring at the top and bottom, and *that* vanishes at the horizontal. But then you also need to take into account castor, which also affects the camber and toe measurements. The upshot is that the measurements should be taken as near to horizontal as possible.
As it isn't usually possible to use a tape measure to get a clear 'line of sight' distance in line with the axle Toyota show using a bar lying on the ground with vertical rods or stiff wires to reach the marks, setting the rods or pointers to line up with the centre of the tyres in line with the axle at the rear, rolling the car forward or back one half revolution, then measuring at the same points on the tyres but at the front this time. The difference between the pointers and the marks is the toe.
Alternatively get a gunsons Trakrite and adjust for no scrub!
|Paul, I agree with your method. A simple home-made jig can be had from a couple of pieces of wood. A board, a bit longer than the wheel tracks and two pieces long enough to reach axle heigth nailed virtically from the edge of the board. Wood shingles are good because they are thin. Slide the jig across under the car then flip it up behind the wheels. Mark the centre of the tire[no need to be accurate] and a corresponding mark on the jig at the top of the verticals. Pull the jig and roll the car forward half a turn of the wheels. Flip the jig up at the front of the wheels.Check the marks on the jig and tires and adjust per your info.|
I cheat doing mine I grease up a couple of bits of cardboard and place them greasy side down on my trailer, then drive the car on the trailer till the front wheels are on the slippery pads then simply measure off the sides of the trailer-- works fine and yes the sides of the trailer are straight and parrallel. Willy
|I know it costs quite a bit, but the Gunsons Trackrite is what I use. With this you simply drive over it and it tells you if your tyre is toeing in or out. You then make adjustments until there is no deflection on the gadget. It works by measuring the sideways scrub on the tyre caused if the wheel is out of line with the car. When there is no sideways scrub, the toe is correct. I love it - it's such a clever idea.|
|The method that I described for measuring toe is an industry accepted method. Yes, it can be improved but if you do exactly as described it works fine and does not require any special equipment. A few comments...|
1. Toe is actually an angular measurement, but because of the difficulty in measuring angles a direct linear measurement can be substituted. To be absolutely correct the linear method is only accurate if the tire's diameter is 28 inches.
2. Yes, it is better to measure the toe at the tire's vertical center(height of the axle. But remember, we are trying to achieve a toe of between zero and 1/32 nd of an inch. If you are off by say 2 times the actual measurement, you are still pretty close.
3. I like using a tape measure because it is easy to get a precise measurement. Using a rod with pointers is less accurate because the points and the reference markings (dots on the masking tape) as well as flexibility in the rod can easily contribute to a 1/32nd inch error.
4. It is critically important to ensure that the wheels are pointed straight ahead and that all forces are relaxed when taking measurements. That is why you must roll the car back and forth as I described in the second paragraph of "Checking/Setting Front Axle Toe-in". If space limitations are a problem, then do the best you can. Most important is to never roll the tires backward as this will create forces that tend to move the tires toward a toe-out condition. The amount depends on the wear and subsequent play that is in axle components, especially the tie rod ends.
This thread was discussed between 16/11/2008 and 21/11/2008
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