Board logo

String box wheel alignment questions
jps - 11/5/19 at 07:41 AM

I've read a bit on string boxes and have a couple of questions, hopefully those with experience on here can advise!
1- How do you set the centre line of the car? (I don't want to have my car crabbing down the road!)
2- How do you get the strings running the length of the car exactly parallel?
3- Is it useful to have a pair of identical bars (25mm box say) that go across the front and back of the car to fix the 'width' measurement?

martylemoo - 11/5/19 at 08:06 AM

Elise specific but everything still applies.Geo Setup

[Edited on 11/5/19 by martylemoo]

motorcycle_mayhem - 11/5/19 at 10:01 AM

Just take your time really, measure everything 3-5 times and take an average, assume nothing. The results are extremely accurate. Take your time, lots.

If you have access to the wishbone pivots (and make the assumption that they're an accurate starting point......) a string cross over them will define the centre line of the car.
Drill a small hole in a convenient place on a chassis member when you have decided it's an accurate CL, that way it's always visible and accessible when you measure it again.
Two large axle stands either side of the car with the string taught between them will present a line to work with, measure from your CL position.
Good quality spirit levels are cheap as a set, buy the set.
If you've got a level floor it makes it all so much easier. If it's a once-only thing then some floor levelling compound may be worth considering (or screed epoxy for a permanent-ish setting-up patch).

I've always added a couple of rivnuts (front and back) to screw the frame rails to for the two side strings, very easy on the race cars with bodywork relieved for that purpose, more problematical I guess if you're on the road with all the paraphernalia.
A mountable position allowed a very quick and simple realignment after pulling the car out of the tyre wall and replacing the suspension components in a rain soaked paddock.

Smoking Frog - 11/5/19 at 10:52 AM

I wrote a small app few years ago which may help to make sense of the measurements. Unfortunately I never learned how to code for android so this version is for the PC. Let me know if you find any errors.
Wheel Alignment app
Wheel Alignment app

I've finally got round (today) to uploading it somewhere. Here's the link https://ufile.io/6vqok4xu

jps - 11/5/19 at 02:46 PM

quote:
Originally posted by Smoking Frog
I wrote a small app few years ago which may help to make sense of the measurements. Unfortunately I never learned how to code for android so this version is for the PC. Let me know if you find any errors.

Thanks for sharing, unfortunately it would not run when I downloaded it - just came up with a popup box saying "Component 'COMDLG32.OCX' or one of its dependencies not correctly registered: a file is missing or invalid"... I'm running Windows 10...

Smoking Frog - 11/5/19 at 03:15 PM

Sorry it's not working for you. From what I've read it seems like windows 10 does not include COMDLG32.OCX I can get round this but it will take time. Meanwhile it is possible to download and register this component.

https://answers.microsoft.com/en-us/windows/forum/apps_windows_10-winapps/comctl32ocx-file-missinginvalid-for-windows-10/0d515059-0397-4b72-a39b-120a7 be96043

gremlin1234 - 11/5/19 at 04:13 PM

it works for me on win10 without downloading anything extra, but both chrome download and win defender didn't like it
did run virustotal on it before I ran it, and as say works for me

one note, can't set different size rimms front and back, but mine are all the same.
thanks smoking frog for sharing

edit get smoking frog's name right


[Edited on 11/5/19 by gremlin1234]

jps - 29/1/20 at 10:40 AM

quote:
Originally posted by motorcycle_mayhem
I've always added a couple of rivnuts (front and back) to screw the frame rails to for the two side strings


I'm thinking to try and do this - at least at the back. Is it literally a case of then setting the lines down each side against a T-square whilst moving the 'free' end to get them parallel to one another / at 90 degrees to the perpendicular line across the car?

40inches - 29/1/20 at 12:17 PM

A small tip: I used fluorescent fishing line, makes it very easy to see



Fred W B - 30/1/20 at 08:18 AM

quote:

I'm thinking to try and do this - at least at the back. Is it literally a case of then setting the lines down each side against a T-square whilst moving the 'free' end to get them parallel to one another / at 90 degrees to the perpendicular line across the car?



That will not be accurate enough. I put the string lines down each side of the car, and then measure that they are the same distance apart in front of and behind the car, and also the same distance on each side from the centre of the wheels to the lines. it takes some adjusting. The distance from the front wheel centre and the rear wheel centre to the line will most likely be different. Then you move to the wheel rim edges to check the toe on both front and back.

Regards


Fred W B

jps - 30/1/20 at 10:20 AM

quote:
Originally posted by Fred W B
quote:

I'm thinking to try and do this - at least at the back. Is it literally a case of then setting the lines down each side against a T-square whilst moving the 'free' end to get them parallel to one another / at 90 degrees to the perpendicular line across the car?



That will not be accurate enough. I put the string lines down each side of the car, and then measure that they are the same distance apart in front of and behind the car, and also the same distance on each side from the centre of the wheels to the lines. it takes some adjusting. The distance from the front wheel centre and the rear wheel centre to the line will most likely be different. Then you move to the wheel rim edges to check the toe on both front and back.

Regards


Fred W B


Thanks for this - it took me a while but makes sense to me. I'm not a natural geometrist, but does this negate the need to work from a centre line of the car? Given there are then 4 reference points along the length of the car, it seems to me that it would be impossible to have the box NOT aligned with the centreline of at least the centres of the 4 hubs?

Fred W B - 30/1/20 at 11:05 AM

Yes, it does presume that the wheels are symmetrical about the centre line, but if they are not exactly that the important thing is where the wheels are pointing in relation to each other.

Regards

Fred W B