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Agreed. String was probably used during construction of the pyramids. Accuracy is probably the same. I like the laser method in that there is less to trip over or knock out of adjustment.



1987 911

Old 02-13-2007, 07:43 AM
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No...the pyramids were built by aliens...so they would have used lasers or maybe something more high tech.

As far as establishing a centerline....

I was thinking for the front...dropping a plumb bob from each side of the control arms (same location at each side). Measure the distance and take the halfway point.

In the rear...drop a plumb bob from each side of the torsion tube cover (again same location each side) and do the same measurement.

Any thoughts?
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Old 02-13-2007, 08:44 AM
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Quote:
Originally posted by Souk
There is absolutely nothing wrong with strings and jack stands....
No, but you will need high jack stands and lots of string if you want to make a "wall" like the laser does

Last but not least... My file for sharing:

Excel spreadsheet
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Old 02-13-2007, 09:35 AM
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Here's how I got a centerline for my track car:

I used the two bolts that hold the reinforcing cross member in the front as references there.

And the two transmission mount bolts in the rear.

Half way between each I hung a piece of masking tape with a vertical line drawn on it.

I used a vertical plane laser from behind the car to line up on these two lines. Then I transferred this center line to other places I had in mind for mounting transverse string holders.

Then I switched to the front and did the same.

Five years later those bits of tape are still under the car!

For grins, I think I checked a couple of other places - between the front suspension front mounts, and between some of the floor pan measuring points. Everything lined up fairly closely, which was nice to know.

I agree with the deficiencies of the jack stand method. After doing it I made my own version of what SmartRacing later sold as smart strings, but without the car to car adjustability and fine workmanship, from stray metal bits in my garage: a front cross piece mounted onto hood pins, and the rear one mounted to the two bottom rear engine mount studs, with things sticking up or down at the ends to get to axle height. This is a race car, so I can remove the rear valance easily.

In both front and rear a guy could make his own, using some fixed point or points to center it. You can insure parallelism by laying the front and rear pieces together, center to center, and marking your end points.

I planned to use a laser on platforms welded to the rear cross piece: line it up with a reference on the front cross piece and read toe. In daylight this was hard to find on the tape measure out from the rims, much less read.

I wasn't thinking about camber, because I have a SmartCamber tool.

So I switched to string. Pink string, no less. Much humor as I'd drive around the paddock to settle the suspension with my string outriggers. I need to switch to fishing line, though - easier to read on the tape measure, and finer resolution also.

Maybe I should try a newer laser though.

On the subject of level, one of the bubble levels - the one that allows me to get a vertical plane laser beam - lost its fluid. I figure I can refill it somehow (hypodermic needle, melt/seal hole with soldering iron). What kind of fluid is used in bubble levels?

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Old 02-13-2007, 11:18 AM
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Using Walt's hard points and double checking other areas sounds like a good idea. I have replaced the front A arms and now I've got the rear suspension disassembled. I'll try this once I've got everything back together.

Dennis: Thanks for the spreadsheet.

Thanks c2rower!

Last edited by vytenis; 02-13-2007 at 02:13 PM..
Old 02-13-2007, 12:04 PM
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I had an accident in the driver's rear of the car, and took the chance to update all the suspension. I also had a track alignment done, with zero toe, and the car now pulls to the right.

Any suggestions as to why? Measurements were double checked.
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Old 02-13-2007, 07:12 PM
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Quote:
Originally posted by Asjac
I had an accident in the driver's rear of the car, and took the chance to update all the suspension. I also had a track alignment done, with zero toe, and the car now pulls to the right.

Any suggestions as to why? Measurements were double checked.
Could need to be corner balanced.
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Old 02-14-2007, 03:40 AM
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Bentley manual states total front toe tolerance as +/-5' (minutes)= +/-.0833 degrees. By my calculation on a 15" rim (using Dennis' 415 mm) this would be .023" tolerance total per wheel . Can you achieve this level of accuracy with a home setup?

What is accuracy of alignment machine for these measurements?

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Old 02-14-2007, 05:23 AM
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I doubt it but I also doubt that local alignment shops are that accurate either.

Added bonus of home DIY is that you get to adjust things without having to put your had in your pocket.
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Old 02-14-2007, 05:27 AM
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I use the laser technique also. I think I can easily achieve that tolerance. The procedure I use is as follows:

1) I use a laser attached to a bar that rests on my rim edges. the bar has a relief to clear the hub. I use a simple bungee to hold it in place.

2) I have a 1x1 piece of wood on stands set at the height of my hub centers.

3) For front toe I simply place the board in front of the car and shoot the laser at the board from both sides marking the position of the laser. Then I place the same board at the rear of the car and repeat the process aligning one of the marks with one side and then measuring how far the two marks are apart on the other side. With a trig calculator you can easily calculate the toe and with the boards 20' apart there is plenty of precision.

4) For the rears I use the same procedure but add one more measurement from the front hubs to the laser light. From this I can calculate and adjust the toe for each rear wheel.
The beauty of the last measurement is once you know what that measurement should be to the front hub you can easily go back and check the measurement with just the laser and a ruler in minutes anywhere.

5) Once you have the board marked for your car and setup going back and making changes is easy quick and accurate.
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Old 02-14-2007, 08:12 AM
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I need a picture Tim I am losing you.
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Old 02-14-2007, 08:24 AM
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Quote:
Originally posted by Elombard
I need a picture Tim I am losing you.
Yes, I think it would help me too.
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Old 02-14-2007, 08:34 AM
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I don't have any pictures of my setup. The trick is attaching the laser level to a bar that is easily attached to you rims. I use a simple laser level from harbor freight and tape it to an 18-inch bar custom fit to my rim. Then use a small bungee through the spokes of my rim to hold it in place so I can mark where the light hits the board placed horizontally in front of the car.
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Old 02-14-2007, 08:44 AM
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Tim - if you could take some time and take some pictures of your procedure, I think we could all benefit.
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Old 02-14-2007, 10:32 AM
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Who has done it the DIY way and then gone to a good pro shop to check on how close the settings are?
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Old 02-14-2007, 10:39 AM
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My experience has shown a "shop" (maybe not a Porsche "pro-shop") to make sime incredible mistakes....rectified only by my DIY methods at home.

Randy...you'd only have two data points..the DIY way and the shop way. Who's to say which is correct unless there is a 3rd data point ??

- Wil
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Old 02-14-2007, 10:50 AM
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Tim if you could put the laser on the bar, hold it up to the wheel with the wood in the back ground maybe we could visualize. Very interested.
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Old 02-14-2007, 11:35 AM
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Unfortunately, the car is parked in a trailer in the middle of a snowstorm.
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Old 02-14-2007, 11:43 AM
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I found jack stands to be a hassle because of resetting. I ended up attaching conduit to the rear bumper and some clip on brackets that I made for the front. Each conduit has a sleeve at each end that is positioned based on the center line axis and hub width, the net result providing two PARALLEL lines, centered.

One thing to remember, whether you use strings or lasers, the farther that reference line is to the rim surface, the bigger your cosine error will be. You are looking for a difference of two measurements. Imagine, say, a 5 degree tilt in measuring rule. If you 5 inches away as opposed to 1/2 away, thats a factor of ten.

I set the strings a little beyond the tire side wall. When I measure, I hold a uniform thickness spacer against the rim and measure the distance from string to spacer with a six inch dial caliper ( usings it depth bar that extends from the base). The spacer allows me to keep the measured distance small.

I also think for total toe measurement, one of the least error prone ways is to scribe a line on the tire as it rotates. Then using a bar with two stable pointers (one fixed, the other adjustable) about 12" long, check the scribed line at the front and rear of the tires. Don't forget to settle the suspension out. There is a good Lotus Seven website that shows this. I'll try to find the link.
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Old 02-14-2007, 11:48 AM
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Wil - if both were the same (for multiple replicates) you would have good reason to think the procedures are correct. If there is divergence you would want to investigate that. If I had to bet on a $29 laser level + amateur vs. a $20,000 machine with a trained operator, I'd make the obvious conclusion.

Your query is one that is ALWAYS TRUE -- one never knows ANY measurement is accurate. Everything is compared to (or traced back to) a reference -- that is usually definitional these days. Scientists simply make sure that the measurement error is small relative to the effect they are trying to detect...

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Old 02-14-2007, 12:03 PM
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