Pete Gossett
Pete Gossett MegaDork
6/17/18 4:44 p.m.

Please check my thought process here:

I’m stringing up the Miata for an alignment. However, since the eccentric bolts cause some inward/outward movement of the hub face during adjustment, which affects track width, I can’t just square up the string to the hubs. Instead I need to square them to the chassis. Am I on the right track so far?

The problem is the Miata curves inward slightly between the front & rear wheels. The doors appear straight, but I’m not sure they’re parallel front-to-rear. I thought about using the pinch welds, but like most old Miatas mine has lived a hard life & I’m not sure how straight they are either. I thought about using the line where the black rockers transition to the white above it, but I have no idea if it’s even side-to-side. 

Any ideas or suggestions?

freetors
freetors Reader
6/17/18 5:21 p.m.

You have some very astute observations about this that most people never realize, especially the part about adjustments causing track width changes. Keep thinking about every aspect of this critically and you will be on the right track.

I personally don't attach my strings to the car, not saying you can't, I just have always done it off the car. I think you will find that you can never trust what you think your track measurements are. I always start with an educated guess on how far the strings should be from the wheel center caps, front and rear distances will be different obviously. Then I physically measure how far the srtings are away from each other at each end. Hint: the lengths of your strings are unlikely to be exactly the same, therefore your conduits are also unlikely to be parallel, which ultimately means you can't measure your strings from the conduit. I have a method for doing that I can explain if you need it.

This begins the iterative process of slowly modifying your string to hub distances until your strings are parallel and equidistant to the hub. Once you know those distances, WRITE THEM DOWN! From this point on you will operate on the assumption that your strings are parallel as long as you don't bump them. As you make adjustments you will find, like you stated, that the track widths will change. You will have to keep moving the car around slightly between adjustments to recenter the hubs between the strings (this will probably a little different for a car mounted setup, I've never really thought about it). When you're done record those new hub distances and you'll have a very good starting point for the next alignment you do. 

I also have a spreadsheet that I put together that does all the alignment math for you, even caster the hard way, that I can post if you're interested.

I would also advise having a helper that has really good eyes if they're reading the scale or is very good at following instructions. The rear alignment adjusters on the miata are a total PITA because they have equal amounts of crosstalk while adjusting camber and toe.

Hope this helps.

Pete Gossett
Pete Gossett MegaDork
6/17/18 5:36 p.m.

In reply to freetors :

Thanks, that definitely helps! And gives me more to ponder...

One reason I chose to mount the strings from the car was to ensure they’ll stay parallel, as long as they don’t receive any major bumps(they’re attached to the tow hooks front & rear fwiw). I used the dollies so I wouldn’t have to roll the car around between adjustments & added the wood so I’d have room to reach the eccentrics(hopefully).

I definitely like to check out your spread sheet, though it sounds like it may be over my head. 

Dirtydog
Dirtydog HalfDork
6/17/18 6:32 p.m.

Pete, here's one to ponder.

https://www.miata.net/garage/alignment/

freetors
freetors Reader
6/17/18 7:17 p.m.
Pete Gossett said:

In reply to freetors :

Thanks, that definitely helps! And gives me more to ponder...

One reason I chose to mount the strings from the car was to ensure they’ll stay parallel, as long as they don’t receive any major bumps(they’re attached to the tow hooks front & rear fwiw). I used the dollies so I wouldn’t have to roll the car around between adjustments & added the wood so I’d have room to reach the eccentrics(hopefully).

I definitely like to check out your spread sheet, though it sounds like it may be over my head. 

I thought about it and you should be good to go with mounting your strings on the car as long as you are diligent to keep them parallel. I will post up that spreadsheet soon. It's really not that complicated but it is packed with features.

freetors
freetors Reader
6/17/18 9:20 p.m.

Here is the spreadsheet that I use. Some of it based on Rob Robinette's web based alignment calculator and corner weight calculator, but I've added a bunch of stuff to really expand its versatility. I recommend downloading it and running it in Excel, not in google sheets as some of the formulas may not work in google. One thing that's really important to understand is on the "Toe" sheet where you input wheel size, this value is NOT your nominal wheel diameter, it's the actual outer diameter of the rim where you will be measuring from. For most wheels this will be 1-1.5" larger than the nominal wheel diameter. There are a lot of calculations that are based on this diameter so if you get it wrong just about everything else will be too.

Link to spreadsheet here

Pete Gossett
Pete Gossett MegaDork
6/18/18 5:13 a.m.

In reply to freetors :

Thanks!

Pete Gossett
Pete Gossett MegaDork
6/18/18 7:00 a.m.

After some measuring & pondering this morning, I decided the pinch welds would likely be my best reference point, as they appear to be mostly parallel. It’s difficult to get an accurate measurement as there are other parts of the chassis lower than that, but they’re about 53” apart. 

So I cobbled together this tri-level to allow me to align the string from either end. 

 

After I got them squared up I had a couple minutes left, so I checked both rear wheels. I was surprised to find the toe was zero on both sides, but the actual distances were different - 49mm on the passenger side vs. 45mm on the driver’s side - so I need to double-check my string height and see if they’re different side-to-side. 

APEowner
APEowner HalfDork
6/18/18 2:46 p.m.

I center a laser level (the kind that projects a vertical line) between the lower control arm mounts front and rear and then measure out to locate the strings.

Pete Gossett
Pete Gossett MegaDork
6/18/18 3:57 p.m.

I went out at lunch & measured the front wheels. I’d expected to find the RR wheel toed in, and thought I’d set the front for toe out. Measuring proved otherwise - the rears are nearly perfectly straight, while both fronts have significant toe in, the driver’s side being worse than the passenger’s. 

The rear camber is pretty even, from as accurately as I can measure it they’re both between -3.0* to -3.5*. The LF reads about  -4.2* while the RF is -5.x*, however I’ve not attempted to truly level the car, so I don’t believe camber to be 100%. Regardless, I’ll leave camber alone as it’s within spec of other STS cars, dial in a bit of front toe out, and leave the rest alone for now. 

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