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I had my car aligned yesterday and supplied the shop w/ the specs I wanted, which were in inches - i.e. 1/8" toe out. However, the machine only read in degrees so, frankly, I don't have a clue if they did it right or not. How do you convert inches to degrees?
Answer: "it depends".
Converting angular to linear measurement is wholly dependent on where you're measuring it. While the angles will will stay the same regardless of where you measure it (as long as you're consistent on both sides), the lateral linear measurement at an given angle will increase as you move outwards from the center of the wheel.
For example, say your shop gave you toe out. Imagine looking at your wheels from above. If your linear measurement is at the wheel lip, then that distance will be less compared to the distance at the tire tread. Of course, even when measured at the tire tread, that distance (at the same angle) will be less on a 205/50-13 tire than it will be on a 245/55-17 tire. The lateral distance grows as you move outwards on the wheel/tire.
Since it's quite possible you will be adjusting the alignment of your car to suit your driving style and the track you're on, it's best if you get the tools and learn how to do your own alignments (even if that means two pieces of angle iron and a Stanley tape measure). Significantly more important than the actual numbers, consistency of baseline alignment is important to adjustments, fast times, and driver confidence.
GA
Call the alignment shop and ask them to convert to inches. I know that the Hunter alignment machines have a table available.
Anthony R.
ITA #86 NER
Well, the engineer in me is going to take a crack at this. First, when measuring toe in inches, its exactly like Greg has stated, it depends where on the tire you measure it. If you try to measure toe with a tape measure anywhere except at the extreme front of the tire, the following information will not be right. Let's assume you've spec'd out 1/8" toe out measured at this point. Then the formula is; sin(toe angle)=total toe in inches/diameter of tire. The diameter of your tire is; (section width*aspect ratio*2/25.4)+diameter of your wheel. So lets assume a 225/50R15 tire and you wanted a 1/8" toe out. Tire diameter=23.86", the toe angle should be 0.3 degrees. Hope this helps, but again like Greg stated, get a tape measure and some basic tools and learn how to do this work yourself. Its actually quite easy. I found many shop's equipment and/or personel could not accomodate the "radical" camber I wanted. They typically work with +-1 degree and very little toe and the comments like "you know how badly your tires will wear with those settings?" got to be old. Not to mention, they are never open at 10pm the night before a race http://Forum.ImprovedTouring.com/it/wink.gif Hope this helps.
EDIT: I don't know exactly how the toe angle is reported to you by the alignment shop. This formula assumes they are reporting it to you for each individual tire and both tires are the same. For the total toe in degrees, you need to double the angle you calculated. So for my example, the total toe is 0.6 degrees.
[This message has been edited by John Herman (edited September 17, 2004).]
Longacre and several others make very inexpensive ($50ish) toe plates which I always use, even after having the car on a commercial machine. I know what works for me and if I want that 1/8" toe out, I can set it at home or at the track. I use the commercial machine to get the thrust line correct and set the initial rear toe.
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Chuck Baader
#36 ITA E30 BMW
Alabama Region Divisional Registrar
Converson is .017 per inch is one degree. .170 in ten inches would be one degree. .340 in ten inches would be two degrees. Get a trig book for the exact #.
The distance to angle conversion is tire diameter multiplied by the sin of the angle. Since angular toe is traditionally reported as the total of both sides, the result will be total toe, or half at each side.
The linear equations one may see relating angle to distance make use of the fact that for small angles the sin (x) ~= x, where x is in radians (degrees*3.14/180).
Front toe is easy to measure as described above and is something every racer should be able to do. Setting rear toe is of course a little trickier.
Jeff
I assume because of thrust angle? Toe is just as easy, but thrust angle adds complexity?Quote:
Originally posted by JeffG:
...Front toe is easy to measure as described above and is something every racer should be able to do. Setting rear toe is of course a little trickier.
I got bit when I ignored this on my car. Had a bent lateral link (didn't know it), and one side was "toed out", while the toe angle between the wheels was fine (slight toe in). Being a newby, I had a hard time figuring out why I had sudden unpredictable oversteer, always on left-hand corners.
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Marty Doane
ITS RX-7 #13
CenDiv WMR
I prefer to measure my toe at each wheel relative to the centerline of the car. Steering wheel always stays straight that way, no goofy crabbing either.
Remember, as stated above, the measurement, in inches, that you are multiplying by .017 is the distance between the two points on the tire/wheel that you took the measurements from (i.e. wheel lip, tire shoulder, line scribed on tire) not the OD of the tire unless that is where you measured from.
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Daryl DeArman
The front is easier because the primary issue is only the difference between the two sides. If one side is 0 and the other is 1/4" in, its the same as 1/8" in on both sides, since turning the steering wheel will get you centered. The only other issue with the front is where the wheel is when you are going straight. One usually wants the wheel square, possibly due to hand grip contouring on the wheel.
Rear toe is more complicated only because you need to measure each wheel independently to a reference that is square with the chassis. If you measure only the difference between the two sides on the rear you may end up with both wheels pointing off to one side (say 1/2" toed out on the left and 3/16" toed in on the right, which would not handle well at all!).
A common approach to home 4 wheel alignments is to use fishing line strings on each side, set square with the car. Once these are in place you can measure the distance to a consistent location on the front and rear of each wheel. There are lots of ways to set the strings. Some use the wheel centers, taking in account any difference in track width on the car, however camber adjustments and other things may place each wheel at slightly different positions from the centerline, skewing the strings. You've probably seen adds for a string system that attaches to the bodywork of the car (this would be very quick at the track). I’ve located points on the chassis near each wheel at the centerline height of the wheel to set the strings. I also have two pieces of aluminum angle across the front and rear to ensure the strings are the same distance apart from each other. A good winter job is to level the car on stands and drop some chassis points with a plumb bob to the floor (hopefully level as well). Use these to check for squareness and the centerline location. From these locations you can usually find points on the chassis to set the strings from near the height of the wheel center.
If you really want to get it right,
http://www.truelasertrack.com/
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Gregg Baker, P.E.
Isaac, LLC
http://www.isaacdirect.com
I guess I'm a "do it right" freque http://Forum.ImprovedTouring.com/it/smile.gif I tried the string method and had it right, I thought. I took the car to a shop that had the patience and lazer equipment to check my work and align the car with me in it. I was way out, espically in the rear toe. After alignment, I took the car back home and re-measured everything and now I can feel confident that changes are symmetrical. The major problem I see is finding a baseline point from which to measure. Race cars are succeptable to damage so most any point is subject to rearrangment. The only reliable way I could come up with to set rear toe was from the diff centerline to the wheel rim.
Point is, initaly adjustments need to be done at a reputable shop with excellent equipment with car as it will go on the track. Then changes can be made with confidence.
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Chuck Baader
#36 ITA E30 BMW
Alabama Region Divisional Registrar
No doubt about it that a full shop alignment rig is the best. The problem with strings or spindle-coupled lasers is knowing where the wheels are relative to where they are supposed to be. I've played around some with laser levels that can be placed on the ground up against the tire, but the problem of knowing where the wheel is relative to the chassis still exists on the rear.
IMHO, the best approach is still strings, based off the chassis. An attachment bar across the front and rear sets equal width, then one only needs to ensure the strings are square and set to a known chassis axis. Pick a good surface on the rim, not the outer most lip. I’ve not yet seen any significant errors introduced by the rim whenever I’ve checked rim run out. Inexpensive tools and an investment in time can result in repeatable home and track alignments.
I've got an excel spreadsheet that will do the conversion and you can input the width of the tire you are measuring to compensate for different size wheels or tires. I thought it would be a simple thing, but Excel works in radians not degrees. It gives the degrees in degrees-minutes-seconds which is how most factory manuals are set up. I'll send it if you want it.
Matt
[email protected]
Is there anywhere on the net where I could find pictures or illustrated instructions on the string method? I can kind of understand what you folks are talking about but I think without some visuals I'm a little lost. I would love to be able to do this at home since I'm pretty much broke all the time.
Ben.
You can draw it yourself to see where you need to go.
The strings make two straight parrallel lines. Place the car between the strings so the car centerline is parallel to the strings. Measure the front & back of each wheel to determine toe of each wheel.
Grass-roots magazine had a pretty good article some years back w/ digrams. They were using 4 jackstands which will drive you nuts. Better to use two frames (1 each end) that keep the strings the same distance apart. 2x4's are cheap.
What would be a good way to find the centerline. Driveshaft? Who knows if my bumpers are where they are supposed to be.
I would use either suspension mounting bolts, flanges, structure, which ever looks good, or chassis straightness location points, sometimes shown in a factory shop manual. Get the chassis level on a close to level garage floor and drop plumb bob points to masking tape on the floor. Equal diagonal and edge measurements mean the location points are square. Half way between the two points in the front and two in the rear set the centerline axis.
To find the center of each end of the car, measure to the hubs. Who cares if the body is not on the car quite right, with most cars, the wheels are what touches the ground, not the body.
Wild amounts of camber will throw off measurements to the hub centers, but not by much. Just pick something that you can be sure is the same on both sides, and accessible - ie a flat item on the wheel or hub, not the bashed in bearing cap.
Well, I've put a lot of time in this to get where I am. I have been in contact w/ Hunter Engineering, the co. that makes the most popular alignment machine, and they sent me a conversion table, which I assume is the same as is used by the machine's computer to make the conversion. I also got a neat Excel program from Matt and have been able to reconcile it w/ Hunter's table. Anthony's suggestion: "Call the alignment shop and ask them to convert to inches. I know that the Hunter alignment machines have a table available," has a problem. It turns out that the Hunter table and computer assumes "an industry standard" tire diameter of 28.65", which throws their conversion all off w/ different tire diameters, especially low profile tires. For example, their table shows 1/8" total converts to .25 degrees, (which is exactly what Matt's program calculates when using a tire diam. of 28.65) but Matt's program gives .3 using John's 23.86 diam. I measure the diam. of a mounted, aired Hoosier 225-45-15 at 22.625 and Matt's program converts 1/8" to a tow angle of .633. This is per side so if you wanted a total of 1/8, each side would be .3165, which agrees w/ John's calculation, taking into account the different diam. he used. Does this sound right?
Since I specified 1/16-1/8 total, I should have gotten at least .3165 total but they gave me .07 - quite a bit off.