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String Line Setup

Motorsport Wheel Alignment Fundamentals

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Discussion and questions related to the course Motorsport Wheel Alignment Fundamentals

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In step 2, setting up the equipment, the method of finding the vehicle centre line is to measure off the hub outside surface. If the centre line is not set accurately the resultant alignment will not be accurate. I don’t understand how this method can be accurate as before the alignment has been completed there is no guarantee that the lower arms on both sides are adjusted to be sticking out from their inner mount on the chassis by the same amount. I think this needs to be measured accurately from the centre of the chassis. Is there something I have missed or misunderstood?

Hi Neil, your concern is a valid one and that's why we start the process of fitting components like suspension arms by adjusting them so they're an equal length. While you'll often find that purpose built race cars will have a datum that you can use to measure from in order to set up the strings initially, this is seldom the case on a production car which is much more tricky to reliably measure from.

I thought that would be the case. I am working on a old Datto 1200 where the lower arms have already been installed by a previous owner many years ago so I’ll have to take them off first and adjust them equal.

Do you think a better procedure for a older car with a unknown past will be to find the true centre of the car so you can also check the inner mount position accuracy as the chassis rails are not necessarily as straight as when new?

p.s great course, I’ll never pay someone else to do a alignment on a race car again!

I do have one concern on this topic as well. What if say the front wheel base is narrower than the rear wheel base? Then it sounds like all wheels will have “toe in” and then your alignment is not proper due to different wheel base measurements. I understand why you measure the control arms to make sure they are equal prior to install but like i said, what if the the front and rear wheel bases are not the same even with measuring control arm adjustments and comparing it to a factory control arm?

Thank you!

Hi Eddy, I don’t think it matters if your rear wheels are wider apart than your front as the string lines are just there to provide a reference point that is parallel to the cars centre line and equal distance out each side to take your measurements from. Your rear settings are not effected by your front settings and vice versa.

Cheers Neil

First let me answer Eddy's question. The wheel track is not going to affect our string line setup since all we care about is making sure the strings are equal distance from the datum point on the left side and right side at the front, and then the same at the back. We don't expect or need the measurements to be the same at the front AND the rear, they just need to be the same on the left and right for each axle. This means track width won't matter.

@Neil, while finding a true centre line makes sense, this can be easier said than done with sufficient accuracy to be useful. We are of course assuming that you're dealing with a car with a straight chassis though to begin with.

But i think it would matter because then the strings wont be parallel, therefore you’re “Toe” adjustments will be off. I understand that the distance from the front and rear axles may be different but how will you know how much the difference in distance the axles will be?

I guess what im asking is, how will you know if your strings are set up square to your car if you dont know how much different the Front (hub to hub) distance is compared to the rear (hub to hub) distance is to make the string square?

In my situation where i built a string alignment setup i don’t have the machined groves to set my string on a grove and move the bars to the properly center them and start my toe alignment. The video says to use the hubs to square the strings to the car. If the axles aren't parallel then the strings wont be parallel.

Maybe im just over thinking it or missing something but ive gone over the video a few times and it doesn’t seem to make sense to me if, like i said by any chance, the axle distances are different how will you square the strings?

Thank you

I can understand that it won’t be easy finding the true centre line of a 45year old car (or a newer car really) that may have had many accidents but at least you only have to do it once then you know your wheel alignments from then on will be accurate.

Checking the cars chassis straightness wouldn’t be a bad idea if you don’t know it’s history, at least then you know what you are working with.

Eddy, the critical aspect you're missing is having those matching grooves on the bars your string attaches to. Having that gives you a parallelogram by setting equal distance between the strings assuming your strings are of equal length as well. The process of ensuring the distance is equal side to side from the hub to the string for each axle will square up your parallelogram giving you a nice rectangle to take measurements from. Hope that helps.

As Sam says, you need the grooves in the bars to begin with for this to work.

Otherwise it will be a guessing game.

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