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MG MGB Technical - Front tyre inner edges wearing fast
|I've just noticed the front tyres on my '77 GT are wearing alot quicker on the inner edges. The passenger side tyre is wearing worst.|
What is a likely cause?
The front suspension has been entirely rebuilt with polybushes, lowered springs (CB height), new stiffer front shocks and new track rod ends which is why I'm surprised that they are wearing un-evenly.
|Sounds like too much toe-out. Did you have the front end re-aligned after the rebuild?|
|Not yet, Keep meaning to but never had the time.|
I am planning on getting it done this week after seeing what is happening to the tyres though!
|You MUST always have the tracking checked after replacing something like track-rod ends. Really you should do it if removing and refitting the same ones even if you count the turns, but replacement ones are often a different length so even if you leave the lock-nut in place and screw the new one up to that they could still be way out. Failure to do that has almost certainly knackered your tyres.|
|Try doing it yourself. It will probable take less time than taking it to a tire dealer and the cost is zero. The below method is consistently more accurate that tire alignment machine that are in constant need of calibration.|
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 minus 1-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
|Strictly speaking the marks on the tyres need to be in line with the centre of the axle both in front and behind, which precludes a direct measurement. Easy enough to fabricate a bar to lie on the ground with pointers sticking up and towards the tyre, comparing front with rear rather than measuring.|
I doubt it is more accurate than the laser alignment gauges places in the UK hang on the four wheels, but at least checking is usually free, i.e. when you use them to check your adjustment.
|All done today through the bloke we use at work (Ian at IA motorsport).|
Unsurprisingly the front was toeing out mahoosively and being an MGB nothing else is within tolerance.
It is NOT necessary that the marks be at the center of the tires. Remember, you are using only one mark on each tire and and measuring the distance between these same marks after rotating the mark from the front to the rear of the tires. This method takes into account both tire and wheel lateral run out when measuring toe.
If you use the approx. center of the tire for your measurements you must first jack up the car, spray the tire with a light colored paint, then scribe a true circumferential line using a knife edge (not an easy task). Place a plastic bag that has been folded several times about 2 yards in front of the tires. Then drive the car onto the plastic bags. You can now begin your measurments and adjustments. After each adjustment, grab the tire and move the tire toward the toeout direction using ONLY enough force to take out the slack in the steering system. When you think that you have it right, drive the car forward about two yards and remeasure. If the toe is not correct repeat the above.
Note: You must scribe the circumferential line as discribed above. You cannot use a reference point on the tread surface or the flange of the wheel. The tire and wheel runout will cause a major error. This is especially true with wire wheels.
Trust me, it is easier using the tape method.
PS I am a retired Michelin tire engineer.
|Frank, surely Paul is right? The camber on the wheels means that (with + camber) the tyres are farther apart at the top than the bottom, so, height matters.|
|And I'm not saying the centre of the tyres (although that would be the natural place to put it, and is what you also said originally), I'm saying the one mark on each tyre must be positioned first directly in front of the centre-line of the axle and then directly behind it (or vice-versa) i.e. the wheel must turn 180 degrees. If you take the two measurements with the marks below the centre line the measured toe will be less than the actual. Because you are using only one mark per tyre you are automatically taking any run-out in the wheel or tyre into account, it is the difference between the two measurements that are important, not the absolute measurement. This method is given in Toyota and other manuals.|
Yes, it would be better to measure at the tire's horizontal center-line (e.g the center of the axle)because at that point the length of an imaginary line from the front to the rear of the tire would be longer than if you dropped down so as to be able to get a clear line of sight between tires. You are correct, this will introduce a small measuring error. But if your target is zero toe than the length of this imaginary line is not important. If your target is, say 1/32nd inch than the error would be small, probable less that your ability to read the tape measure. If you want to split hairs, add about 1/64 inch to your target toe setting.
For your information, tire alignment machines are calibrated for a 28 inch diameter tire. Also, caster and camber are not factors in this measurement procedure for toe.
With modern laser alignment tools wouldn't tyre diameter be irrelevant?
|"With modern laser alignment tools wouldn't tyre diameter be irrelevant?"|
I'd say so. Although the gizmos are hung from the top of the tyre they are then adjusted so the lasers and scales are level with the centre-line of the axle.
|Modern cars specify toe in angular measurments rather than linear as was the case with our cars. A alignment machine that measures angles does not consider the tire's diameter. The 28 inch Reference diameter mention above applies to the older(and still in use) alignment machines.|
PS I just finished setting the toe on the rear wheels of my 2001 Toyota Highlander. The toe specs were shown in angles. After a bit of mental gymnastics using trigonometry, I was able to calculate the toe spec in linear values. Toe = tan(of the toe angle)x 14
|Excellent write up Frank. I've added it to my valuable MG information file!|
This thread was discussed between 10/07/2011 and 20/07/2011
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