I'll be using SlapSmak's toe measuring setup, due to it using far less storage space, and string positioning is always consistent with the car's position. My car has come across many pot holes and curbs, and also lowering a car may affect track widths; mines not lowered though. Because of these factors, I wanted to utilize the car's center line rather than factory track widths, but had a couple concerns.

At the front, I was thinking of attaching a plumb bob to a string, route it onto the inner most LCA bolts, on the passenger side (part of thread facing catalytic converter) and drivers side (again, the part of the thread facing catalytic converter), until it comes in contact with the ground (asphalt). Place tape on asphalt and use white paint marker, to mark where the plumb bobs touch the ground. Then measure the distance, between the two points, and mark the half way point on the ground. Repeat on the rear section of the car.

Go back to front of vehicle, place 2x4 and tool box (or anything heavy) on top of it, near the 'midpoint marking', so tape measure can push up against it (i'll be working by myself); and measure 3 feet out towards drive side wheel, go through and under wheel rim. I'll use the tape measure in conjunction with a 'chalk line' tool for greater accuracy. Repeat for passenger side wheel as well. I chose 3 feet because this is longer than the 1475mm track width divided by 2 (i.e. greater than 2 feet 5 inches), and it's not too far from the wheel rim.

For the rear of vehicle I'll repeat what I did in the last sentence.

Then from there I'll get the difference from that 3 feet marking to the respective wheel hub, so I don't have to do this again unless I get new wheels and such.

The surface won't be leveled, as the driver side, is around 5/8" (16mm) of lower elevation compared to passenger side, but I'll make sure the jack stand ratchet heights are equal on both sides. When doing the alignment itself, I'll be sure to use floor tiles and such to make tire contact leveled.

Just wondering if this seemed reliable enough for obtaining center line measurements. This guide (link), had the person measure directly on the cross-member, and route a string front to back under the vehicle.

Edit: since ground is not leveled, plumb bob's positioning would be 'too far' to the right; but this could be overcome with sine/cosine/tangent.

 

why do you care about the centerline of the car?

string alignments are still a relative measurement to get the relative toe of the front or rear wheels, and between the left or right wheels. you dont need a centerline to measure that.

youre over thinking it.

just make sure that string is parallel to the side rocker along the car's body.

youre over thinking it.

and personally, i dont mind measuring the track of the rear to front alignment by the actual track widths. works for me with a laser and some toe plates, no strings. the only benefit i see of that slapstick method is you can roll the car front and back and not have to redo the strings, which is nice. but not going to change how i do alignments.

 

I want the toe measurements to be as accurate and consistent as possible, and I'm willing to spend an extra 8 to 12 hours of my time to obtain the precise track widths. I know strings won't be as good as a laser, but if it's good enough for racing teams, its good enough for me.

 

i dunno why you think thats anything more precise. go ahead and do it your way, no better than other ways.

 

I may have come up with a more consistent solution to getting a string lined up with the centerline / center line. I've included a diagram that I created in 'draw.io'

https://imgur.com/FnDH0G6


Step 1:
Attach plumb bob to string and route over LCA bolts, use one hand to guide it down, then use other hand, to mark it on the ground. Be sure you're consistent on how you place the bob on the bolt's thread. E.g. if on the passenger side, part of string going downward, will be on part of bolt's thread facing driver side; on driver side, its the part of bolt thread facing passenger side.

Plumb bob contacts are represented by the green dots in the image.

Step 2:
Get mid way point between plumb bob markings, hence blue circle in image.

Step 3:
Use chalk line, to mark a line towards exterior of vehicle. The more dots/marks the chalk-line's thread instersects, the more 'straight' the chalk line will likely be.

Step 4: repeat on passenger side.

Step 5:
From mid-point marking, measure out towards exterior of vehicle. I want a measurement that's not too far from the wheel hub, but at least 40mm away from hub cap of front wheels. I'll just go with 3 ft.
If you're on a surface that's not perfectly flat, you'll really need to be consistent on how you use your measuring tool.

Step 6: repeat on passenger side.

Step 7: With where you marked the 3 foot spot between the respective wheel base of the vehicle (front to back), make a new chalk line that will reach front and rear bumper of car.

Step 8: see step 6

Step 9: You should now have 2 parallel lines to the car's center line.

Step 10: If you're in my situation, i.e. un-leveled ground, you'll likely have a result closer to diagram #10. Disregard this for now.

Step 11:
Setup alignment strings on both sides of the car; they should match up with the height of the hub cap. Have a plumb bob suspended from a platform, similar to that of a desk lamp. Have plumb bob come in contact with the chalk-line drawn in step 7 and 8. Try and get the alignment string to touch the plumb bob's string.

Rear wheels hub caps, should clearly be further away from the front wheel's hub cap, on both passenger and driver side.

Step 12:
Consider having the car's suspension settle for this next step, by either rolling the car or putting 2 pieces of plates under the car's tire with grease in-between.

In regards to the front hub cap, the distance from the hub cap to the alignment string will be recorded for both driver & passenger side, then they will be subtracted from each other. Divide result by 2, to get x. Then the side further away, string will be moved x amount closer to hub cap, and other side will be moved x amount further away. Repeat for the rear wheel string.

Edit: When performing the first 2 steps, all four tires must be suspended from the ground equally, ideally under load, even if front and rear axles are not on leveled ground. E.g. they must all be 6" off the ground. Feel free to look up '2x4' cribbing.

 

I spent the better part of 10 hours obtaining the centerline info, and unfortunately it's not as consistent as I was hoping for.

Front inner LCA bolts were 64.2cm apart, and rear were 65.5.
If using the SlapSmak's appartus, consider only measuring out to only 3 feet, instead of 3' 1".
Passenger side of car had lower elevation, front and rear were more equal; car didn't move when placed in neutral.
Front Acura emblem was around around 2" off from the center line, whereas rear was around 1".
The plumb bob tool described in step 11, consider having 2 instead of them just 1, it will save 1 to 2 hours of your time.


When measuring distance from hub cap to string, consider placing a bubble/spirit level on the rigid ruler.

Right (passenger side) trailing arm was swapped to that of a 1998 Integra at a junk yard. I have difficulty installing new aftermarket wheel hub on it, i.e. have to use a rubber mallet, since it's rod diameter was a millimeter or so too thick. Have to use gear puller to remove new wheel hub; inner race of new wheel bearing hub assembly gets stuck to the trailing arm's rod, when removing it to install splash guard.

Note: step 7 and 8 are optional.
Pro-tip: It was really nice to see the car's tire/suspension settle, when 2 pieces of plexiglass coated with lithium grease in-between, and placed under the tires.

I'll stick to using the factory's front and track width when doing the alignment, if the car feels undrivable/unstable, I'll consider using the 'conduit' method, where the car is encircled by rigid rods, and string is tied to them..

Only thing 'to scale' in the image, is the distance of the green dots from their respective tire.