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Author Topic: Distance from chassis centerline to rim surface
Sphinx Verified Driver
M. Yusuf Mohamed

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I just want to make sure that I didn't screw something up. I'm trying to learn how to do my own alignment on my '99 street car. I set my strings up equidistant from the chassis centerline ('99 street car). (35" in each direction). But when measuring toe, I noticed that the distance from my toe strings to the rim surface was about 1" different side to side (eg: right side ruler would show ~ 2" range front and rear, but the left side would show around ~ 3". (BTW, I was in that range, front and rear. So, that suggested that nothing was tweaked).

Assuming that I didn't screw something up, then I imagine that this is by design to shift the center of gravity to the right of the car's geometric centerline for better balance to counter the driver's weight.

Or is this car perfect symetrical and this is user error?

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Sphinx Racing
Atlanta Region
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Keith in WA Verified Driver Made Donation to Website
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That sounds like a PITA way to set up the string box. The way I was taught to set it up, the hubs are the reference point. Pretty easy way to get a very accurate measurement. Measuring to the center of the car sounds very error prone. Any little sag in the tape measure over 35" will screw the whole setup and rulers that long generally aren't very accurate.

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Keith Novak
(Will work for tires)

Sphinx Verified Driver
M. Yusuf Mohamed

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It is a PITA, but this was the way I was taught by my chassis setup instructor. (Once upon a time, I actually took a college credit class on chassis setups - coolest class EVER).

I assume that for your method you are adjusting for the difference between the front and rear tracks?

http://www.miata.net/garage/alignment/index.html

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Sphinx Racing
Atlanta Region
http://www.sphinxracing.com
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Support our racing efforts by shopping at your favorite online merchants at SphinxRacing.com!

Keith in WA Verified Driver Made Donation to Website
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Different track??? [Wink]

I should have mentioned I use the Iron Canyon kit. Makes it a snap. Hang the rods on the car. Put the strings in the grooves and they're equal distance apart on both ends. Even up the front, even up the rear, tweak a bit, and you have a parallel box that rolls with the car. [thumbsup]

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Keith Novak
(Will work for tires)

Motor City Hamilton
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How did you find the center line on your car to start with?

Sphinx Verified Driver
M. Yusuf Mohamed

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quote:
Originally posted by Motor City Hamilton:
How did you find the center line on your car to start with?

You can really use just about any rigid part under the car. I would prefer to use a part of the unibody that is easy to measure to the "center". But I couldn't find any easily. I ended up using the center of the front cradle and on the rear used the center of the bracing just aft of the diff. http://www.quadesl.com/miata_photos/rear_center.jpg

Here's a writeup that is just about what I did to find the centerline. http://www.quadesl.com/miata_alignment.html

Caution though, I'm not sure how he ends up getting the wheels up on those boards without the the obvious ability to bounce and roll the suspension.

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Sphinx Racing
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http://www.sphinxracing.com
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B Wilson Verified Driver Series Champ
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We measure from the hubs, equal back to back and equal front to front, which does take into account difference in track width better than subtracting 10mm. That is good enough to get a square box assuming a symetric camber setup side to side and the car and suspension is not to bent up.

Question, does it really matter where the chassis sits in relation to the four coners which are aligned relative to each other? i.e. why care where the center line of the vehicle is if it's very close (close enough)?

We align our cars in minutes at the track if needed. We ususally have the scales leveled and everything ready to go, but changing toe/camber can be done extremly quickly using the Iron Canyon bars and just measuring to the hubs. We also use this method at home in the garage.

-bw

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Bruce Wilson
2010 Oregon Region Champ
2010 Monte Shelton Driver of the Year
2010 25 Hours of Thunderhill E3 and Under 2 liter Overall Champion
Oregon Region SM Class Advisor

Motor City Hamilton
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I agree. Measuring to centerline on an OEM street vehicle will be very difficult. On a purpose built race car, maybe, but most teams I have been around still use the string box method.

I too have the Iron Canyon string kit and it works great. Plus, no crawling under the car to take measurements. Way more accurate with the string kit.

You may have found center line of the sub frames, but there are other variables within the suspension that could add or subtract from your overall width. For example, to get the same camber side to side on the rear of my car, the driver's side suspension cam bolts need to be further out than the other side. I'm affraid that working through all of those OEM parts tollerances of all of those components will make it very, very difficult to measure using a center line.

http://www.ironcanyonmotorsports.com/product_info.php?cPath=9&products_id=15

Sphinx Verified Driver
M. Yusuf Mohamed

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quote:
Originally posted by B Wilson:
We measure from the hubs, equal back to back and equal front to front, which does take into account difference in track width better than subtracting 10mm. That is good enough to get a square box assuming a symetric camber setup side to side and the car and suspension is not to bent up.

Question, does it really matter where the chassis sits in relation to the four coners which are aligned relative to each other? i.e. why care where the center line of the vehicle is if it's very close (close enough)?


Can you explain how you measure back to back and front to front? I'm not getting a visual.

As to your other question, of course it does. Let's imagine that your toe settings are so far off that you've got the dog-walking-sideways syndrome, even though toe is zero all around. Car will track. But once you turn, all of a sudden all that cross-weighing you were doing is out the window because the car will move about its center of mass and shift weight in strange ways since the car's attitude is already in one direction due to dog-walking-sidways nature of the alignment. So, in order for the alignment to be perfect, it will need to be in relation to the chassis, not relative to each other.

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Sphinx Racing
Atlanta Region
http://www.sphinxracing.com
http://www.layersdesserts.com
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Steven Holloway Verified Driver Made Donation to Website
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You measure the gap from the string to the hub so that it is equal on both sides in the front, and equal(but a different distance from the front) side to side in the rear. That gives you a square string box, as long as your corners(wheels)are somewhat equal for camber.
It may not be exactly true to the centerline of the car, but is close enough. The car will track straight.
The only way you could get the situation you describe above is if you have something seriously bent, or lots of camber on one side and little on the opposite. That would make your string box a parallelogram instead of a square.
You could always check the squareness of your strings using the 3,4,5, method. But, it seems like a lot of work for little difference to me.

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If you can't fix it with a hammer, it's got electrical problems.

B Wilson Verified Driver Series Champ
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Sphinx, of course you are correct in theory. So the first alignment you do on any car should be carefully done to make sure the car and/or parts are not bent, i.e. look to see where the cam bolts are at a zero toe and equal camber. They won't be exactly in the same place, but hopefully they are aren't rotated 180 degrees. But I've bent and seen enough bent miatas to know that even the slightest difference in geometry (i.e. hubs not about equal distant to centerline with symetric camber) will show up in an alignment either in Toe/camber for the back or camber/caster for the fronts. You just can't align a car that is really bent badly and get the settings that are needed for racing.

-b

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Bruce Wilson
2010 Oregon Region Champ
2010 Monte Shelton Driver of the Year
2010 25 Hours of Thunderhill E3 and Under 2 liter Overall Champion
Oregon Region SM Class Advisor

Motor City Hamilton
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Sphinx - you are an engineer, aren't you?

You're racing a production car, not an F1.

I would bet that 90%+ of the Miata community (maybe even the SCCA community) use the string box method?

Sphinx Verified Driver
M. Yusuf Mohamed

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quote:
Originally posted by Motor City Hamilton:
Sphinx - you are an engineer, aren't you?

[rolling on floor laughin] 2 1/2 years in the engineering program at Ga Tech before I realized that F didn't stand for Fabulous. So, no. [Smile]

quote:
You're racing a production car, not an F1.
That's what my wife says when she hears me in the garage making pretend racecar noises. [Smile]

quote:
I would bet that 90%+ of the Miata community (maybe even the SCCA community) use the string box method?
I'll try it. The one thing I'm not convinced of is that I don't have to correct for the differences b/w the front and rear track. If you were to extend the two strings in front of the car to infinity, then they will meet somewhere way in front of the car. And since toe is being adjusted to a 1/16", then I think the error is going to affect the final measurement.

--------------------
Sphinx Racing
Atlanta Region
http://www.sphinxracing.com
http://www.layersdesserts.com
Support our racing efforts by shopping at your favorite online merchants at SphinxRacing.com!

Motor City Hamilton
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The strings will actually be parallel through infinity. When I measure the strings between my front spindles I will have something like 10.8 cm between the string and spindle on each side. When I measure the rear, I have something like 8cm on each side because the rear track is wider. I actualy measure down to the milimeter on each side, but just can't remember my exact measurements right now. It will take me 15 min or so to set up, measure, measure, measure and measure again until perfect, with the Iron Canyon string kit. After each suspension adjustment it takes me a quick couple of minutes to remeasure and make sure everything is still square.

My old set up was four jack stands and a solid bar across both in front and in back of the car - strings down the side. Same amount of time measuring, but needed to spend 15 minutes remeasuring afer every change to the suspension.

Sphinx Verified Driver
M. Yusuf Mohamed

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Thanks for the tips guys. You have me convinced. The difference in track front to year is 25mm on a '99, which is just about 1" (actually it's 25.4mm=1in.). So, I kept the front string .5 inches further from the hub in the front than in the rear. And it was much easier than what I was doing that might have been technically better, but for sure a bigger PITA.

--------------------
Sphinx Racing
Atlanta Region
http://www.sphinxracing.com
http://www.layersdesserts.com
Support our racing efforts by shopping at your favorite online merchants at SphinxRacing.com!

   

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