String Alignment Wheelbase Track Dimensions Question.

[doug_porsche]

I am about to dive into the exciting world of String Alignment on my Gen II GTM.

Hoping to key a key piece of information.

If, for example, I am setting up the strings and trying to get them parallel.

The measurement from the string to the rear wheel center is 6 inches, what should the measurement, from the string to the front wheel center, be?

I am assuming that the front and rear track are not the same.

More research done.
as per two internet sources the front track is 60.5 inches and the rear track is 62.0 inches
If this is correct, answering my own question...

62-60.5 = 1.5 in diff
1.5 / 2 = .75 in diff

So if my string to rear wheel measurement is 6 in and the string to front wheel measurement is 6.75 in, I should be parallel

If this is incorrect, please let me know.

[Dave 53]

Sorry, I don't know the name, but I'm sure there are several that are easy to find. There are apps to calculate string alignments. Enter a few values and it spits out toe. Super easy to use. The strings don't need to be parallel.

[crash]

EDIT: Sorry I was at work on Friday and got called away before I could review and make edits. Please disregard what I said on Friday.

I have never done an alignment where the strings are not parallel.

The way I get the strings situated, and just so you know, this takes a lot of time, is to start by setting the strings the same distance on left and right from the center of the frame on both sides. I start in the rear because the rear suspension does not steer. Now get the strings close to parallel by eye. Then measure front and back between the strings until they are indeed parallel. Now measure the distance between the strings at the rear as close to a transverse frame rail as possible. Divide that distance by two and measure from one side's string to the frame rail and put a mark with a dry erase pen or pencil, or whatever. Now measure the width of the rail, divide it by two and make a mark on the frame rail. If the marks are not the same, adjust the strings side to side and measure again, making sure they are parallel again. Once this is done to where the strings are equal distance from the center of the frame rail and parallel, then you can start measuring and adjusting. To make things easier, make marks on the frame rails front and rear and then record measured distance from marks to strings. This allows you to reset the strings quickly after making adjustments to the suspension. With this method you will get the strings equal distance and parallel to the center of the frame, then you can use the shims or eccentric bolts and toe links to get everything where it needs to be. This is not an easy process and the first alignment of a vehicle using strings is tedious and time consuming. Sometimes it takes me days to get everything right. It is a lot of work and is best done with two people.

[VRaptor SpeedWorks, LLC]

Just my $.02. Save your energy and take it to an alignment shop. I'm a very particular person and I've tried doing a string alignment....and even had a very expensive camber gauge that clamped to the wheels. I spent hours and hours setting everything up and getting the strings, measurements and toe and camber settings right-on according to the strings and camber gauge. The car handled like crap. I took it to a shop with a computerized alignment rack and none of my settings were remotely close to what the strings and the camber gauge said they were. In fact, on all future cars, I just did the initial alignment completely by eyeballing it....no strings, no gauges, no measurements....just eyeballing the toe and camber.....and then take the cars in to the alignment rack. I had one car that I did this to that only needed a slight camber adjustment on the rear on one side and a slight toe adjustment in the front. Everything else was already in the green.

[crash]

Hey Shane. Just guessing here, but maybe you did not roll and bounce the car after every adjustment? You have to do this to settle the suspension. This is one of the reasons it takes me so long. Adjust, bounce and roll, reset strings, measure...rinse and repeat. BTW before I do alignment I do ride heights and corner weights. Then do the alignment. Then back on the scales to make sure weights and heights are still good. Then if anything is changed back to toe and camber. Yeah it is not the funnest part of a build.

I agree with you. Take it to an alignment shop if you can find one that will work on a GTM. The builder should still set ride heights(and weights if doing that) before they take it to the alignment shop.

[Shoeless]

I'd agree with the guys above on taking it to someone UNLESS you are like me and really want to learn it. I took an online class offered by the HPA Guys to get the real deal instruction on how to do this. Then I spent probably close to $2,000 in all the needed gauges, string alignment kit, turn plates, and corner scales. I even needed to get a $100 custom bung machined to sit on the inside of my rim to check caster.

Then I spent weeks learning how to mess up what I just changed in the last couple steps, but in the end, I figured it out and got mine DEAD SET to exactly what I wanted. Was able to test drive at 80 as a go cart and seemed pretty solid. I grabbed my laser level and popsicle sticks and gave Shane EXACT ride heigh measurements for his to set all 4 corners once the body is on and mine is essentially done.

I enjoyed the process and like many aspects of my GTM, I can say I 100% did it myself in my garage :cool:

[VRaptor SpeedWorks, LLC]

Hey Shane. Just guessing here, but maybe you did not roll and bounce the car after every adjustment? You have to do this to settle the suspension. This is one of the reasons it takes me so long. Adjust, bounce and roll, reset strings, measure...rinse and repeat. BTW before I do alignment I do ride heights and corner weights. Then do the alignment. Then back on the scales to make sure weights and heights are still good. Then if anything is changed back to toe and camber. Yeah it is not the funnest part of a build.

I agree with you. Take it to an alignment shop if you can find one that will work on a GTM. The builder should still set ride heights(and weights if doing that) before they take it to the alignment shop.

Yes, I did roll the car and bounce the suspension between adjustments and set the ride height before I started. I do not have scales to do corner weights, but I do make sure that the springs are adjusted the same side-to-side so that I'm not loading just 2 corners. I had a customer car come in once that the owner had already finished and was driving.....he just wanted some additional work done. I unloaded it from the trailer and on my way to the shop, I blipped the throttle a bit and got nothing but wheel spin. WTF? Yeah, it was a little cold outside, but this thing spun like it was on ice. Once I got it into the shop and up on stands, I looked at the coil-over settings and one rear coil over and the opposite front were cranked up about an inch farther than the opposite corners. The whole car was basically just balancing on those 2 corners and the other 2 corners were just hanging there like training wheels. I do wish I had the corner weights on that car!

[crash]

Yes, I did roll the car and bounce the suspension between adjustments and set the ride height before I started. I do not have scales to do corner weights, but I do make sure that the springs are adjusted the same side-to-side so that I'm not loading just 2 corners. I had a customer car come in once that the owner had already finished and was driving.....he just wanted some additional work done. I unloaded it from the trailer and on my way to the shop, I blipped the throttle a bit and got nothing but wheel spin. WTF? Yeah, it was a little cold outside, but this thing spun like it was on ice. Once I got it into the shop and up on stands, I looked at the coil-over settings and one rear coil over and the opposite front were cranked up about an inch farther than the opposite corners. The whole car was basically just balancing on those 2 corners and the other 2 corners were just hanging there like training wheels. I do wish I had the corner weights on that car!

Was the owner from North Carolina? It's called cross weight and I bet that car turned phenomenally in one direction. LOL!

Getting a car to turn properly on an oval track is largely about cross weight. Now getting the car to turn in, still put the power down mid corner, and then not push coming off the corner...that is the art of it.

[Shoeless]

Yes, I did roll the car and bounce the suspension between adjustments and set the ride height before I started. I do not have scales to do corner weights, but I do make sure that the springs are adjusted the same side-to-side so that I'm not loading just 2 corners. I had a customer car come in once that the owner had already finished and was driving.....he just wanted some additional work done. I unloaded it from the trailer and on my way to the shop, I blipped the throttle a bit and got nothing but wheel spin. WTF? Yeah, it was a little cold outside, but this thing spun like it was on ice. Once I got it into the shop and up on stands, I looked at the coil-over settings and one rear coil over and the opposite front were cranked up about an inch farther than the opposite corners. The whole car was basically just balancing on those 2 corners and the other 2 corners were just hanging there like training wheels. I do wish I had the corner weights on that car!

Sounds like my first attempt to see what a theoretical 50/50 weight distribution looked like on mine. It was more fun as I saw things getting way out of wack balance wise.

Again, I still loved learning this part of my build and doing it myself. My engineering brain just soaks this stuff up for a living :cool:

[doug_porsche]

EDIT: ...to start by setting the strings the same distance on left and right from the center of the frame on both sides. ...

I agree with everything you said.
I was terribly unclear on my setup.
I found a youtube site that give the recipe for a 3D printed string alignment (https://www.youtube.com/watch?v=Hk6HMuKnfRY no affiliation).

I setup my parallel string lines and note the measurements on the center rulers for both front and rear. For this example, let's say those measurements are 69mm front and 50mm rear.
So I now have a very repeatable wheel setup on the car.
And, I now have the correct measurements to set the strings to that would give me a string parallel to the center of the car.

So, at any time I should be able to:

Setup the wheel stuff in the picture.
Using string, or a laser overhead, to be at Xmm for the rear and Ymm for the front on the center rules.
This should be repeatable and once you have the string at X and Y you should be parallel to the car.
Then you can take your toe measurements using the front and rear scale/ruler.

My rulers are in mm (I am better at base 10 numbers than fractions).

So, being redundant and these numbers have not been finalized, if I find, after setting up the parallel lines, that the string is at 50mm rear and 69mm front on my center ruler then I have a base/repeatable measurements.
Now I have a repeatable fixture(s) and I have string measurements, for the center ruler(s), that set me up parallel to the car.

This should mean that ANYTIME I mount the fixtures and have the string/laser at the correct center ruler measurements (50mm and 69mm or any reading where the rear is 19mm less than the front), then I am parallel to the center of the car.
Then I can use the two outer measurements to play with toe.

I came up with 19mm because, and these numbers are untested, the internet is telling me that
Rear track is 62in, front is 60.5in for a difference of 1.5in (38.1mm) total for the car.
1.5in (38.1mm)/ 2 (two sides to a car) = .75in (19.05mm), or the front track is 19mm narrower than the rear on one side of the car.

Anybody see any issues with my thinking?


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[VRaptor SpeedWorks, LLC]

I guess the biggest issue I see with that setup is that 100% of your measurements are based off of the idea that there is a 19mm difference in the track. Changing the camber changes that measurement. When you adjust the cam bolts to give more negative camber, you're moving the lower ball joint outward......which moves the wheel hub outward...which is going to change that 19mm measurement. Changing the front camber from 0 to -1 degree is probably going to move the wheel hub outward by several mm. Same with the rear.

So, it's not necessarily a bad idea or bad set up.....you just need real numbers to work with as far as your actual front and rear track.....with the front and rear camber already adjusted to what you want them at...... before that method will work.....right? And if you're also setting caster, that is going to effect your camber adjustment, which is also going to effect the track measurement.....so before that setup can be used at all, first, you're going to have to have your camber and caster adjustments nailed down. And since adjusting your toe will also effect your camber adjustment....and adjusting that camber is going to change your caster setting...which is going to change toe setting...and your track measurememt........

This is the reason I suggest finding a local alignment shop that can work on cars like this. Knowing that the track is wider in the rear, you can eyeball your front and rear toe and camber settings by just sighting down the side of the car and get them close so that the alignment tech doesn't have to spend all day getting the sensors to read.

[doug_porsche]

Shane,

Good information.
This setup is focused on toe.

I admit I trust a trusted professional alignment setup over this.
Its just a rabbit hole I am going down because I hear frequently "string alignment"
Its, for me anyway, half: Can I come up with a reproducible process to measure.
Half: What can I learn
And one third (math is my passion): Had to replace a ball joint , or some such, and need to get the toe close enough to drive to the alignment shop.

[crash]

If all you are worried about is toe adjustments to get back to usable...buy some toe plates. This is what we use at the race track to get things back to usable when there is some sort of minor incident.

https://www.summitracing.com/parts/lng-79501

The way we use this is if we know that one side or corner has been hit by something and there is a potential for toe to be off then we use these plates and measure off the known good side and reset the other side. These adjust one side relative to the other and is good for resetting one side, but nothing beats setting to car centerline. The centerline reference makes sure the car tracks properly as far as going straight down the road, turning the same left to right, etc.

[Shoeless]

Shane,

Its, for me anyway, half: Can I come up with a reproducible process to measure.

Half: What can I learn

And one third (math is my passion): Had to replace a ball joint , or some such, and need to get the toe close enough to drive to the alignment shop.

Let's break your three halfs LOL down and answer each.

1. Absolutely you can 100% come up with a repeatable process in your garage, but you won't learn every intimate detail and the recommended exact steps to follow here on this form and in this format, well not yet at least. The first step hasn't even been mentioned I don't believe. You need to set up a flat patch in your garage or workspace so that all 4 corners of the car are essentially on the same plane. For that you need 1mm thick vinyl tiles as least the size of your tires, a couple sockets that are exactly the same height, a long piece of rigid aluminum that can reach all the corners from the others, and a digital camber gauge (ideally).

2. You can learn everything you need for about $90 out of your pocket and just your time invested. Just go over to HP Academy and buy the Motorsports Wheel Alignment Fundamentals class, linked below and have at it. I guarantee you, 100% you will be infinitely smarter on this topic at the end of those videos. They even offer a money back guarantee. Legit, if you thought it was a waste of your time or you didn't learn much, ask for a refund, and they will. I have their VIP access after buying and using so many of these guys classes, it just made sense. Plus, usually once a year they run a VIP Special, so I did it then).

https://www.hpacademy.com/courses/motorsport-wheel-alignment-fundamentals/

3. Follow crash's recommendation on obtaining a set up inexpensive toe plates and you will be fine. I literally eyeballed mine to get in the first go cart and I managed. Tow plates and a couple tape measures, and you are actually really solid.