GTD suspension setup wheels

[Andy]

Hi Guys,
Looking for some advice which procedure to use to setup initial the suspension. I am not talking about caster, camber or toe yet but the position of each wheel in order to get a symmetrical thrust line (x,y position).
Lets says x-direction is the driving direction.
And y-direction is 90 degrees in the horizontal plane.
All wheels positioned in a perfect square...how to achieve this ?

I measured some points on the GT40 chassis, the rear end beams are not all symmetrical. I noticed this when putting the dimensions in 3D software and copy the right to left side.
Depending on which x-direction chassis rails you take, a new centerline is obtained. There are offset of easily 5 mm so which one to choose ?

What I did so far.
First I used some wood plates and shimmed the garage floor below the wheels to create a level surface for the car.
Secondly I set the ride height roughly 130 mm on all corners.
Third, I used the small long beam above the fuel tank below the door as a reference to start positioning the rear wheel using a small laser.
Noticed some small waveniss along the beam at welds which is normal so I used a straight ruler against this rail to have a straight line.
Set the rear wheels with roughly zero camber, zero toe in and rear uprights 90 degrees (rear lower arm horizontal in x-direction).

The left wheel needed to move 10 mm outwards (y-direction) to be symmetrical to this reference and looking to the body this made sense.
Made the change by turning out the rodends 10 mm both top and lower arm at the left side.
The result is that the long left lower suspension beam (beam connected to the bulkhead of the car pointing in X-direction) is hitting the chassisrail (on the left outer side). The driveshaft length is now 10 mm longer on left side (445 mm left, 435 mm right). Measured driveshaft and is about 440 mm ?
How much play is here maximium allowed ?
Checked the gearbox position towards the lower rear chassis rails and it is perfect centered referenced to that part of the rear chassis.
But it might be off the center by 10 mm choosing another rail ?
Assuming the gearbox is centered.
Which means I have to put the left wheel back 10 mm inwards so somewhere I will keep a misaligned between left and rear wheels towards the current reference.
At the right rear wheel, I am not able to move any further inward (rodends to their minimum) and the low long suspension beam is clearing the chassis rail.

Another boundary is the position of the damper/spring, they can have almost no inclination (Nylon bush type), there is only 5 mm forward position of the wheel upright possible. Need to take this in consideration.

I gonna check if the offset of the rear wheels are identical since I noticed the right wheel needs a 2 mm offset plate to clear the upright while the left doesn't need this offset plate at all. The car came with the offset plate.

Measuring the distance between the front and rear wheels, the right rear wheel needs to move 16 mm forward. But are the front wheels at the same X direction ?

Spoke with my local wheel alignment shop, they said they couldn't check the wheel position but I noticed on their screen there is a module for wheel positioning checking. They never used it before...

What is the easiest way to the measure and setup the positioning of the wheels (x,y position) with DIY tools.

Regards,
Andy

 

[Mike Pass]

One of the strange things about GTD chassis is that one of the front wheels is about 1" forward of the other! Somebody couldn't measure. So any measurements taken from the front axle will fairly useless. It seems to make no difference to the car's behaviour.
The correct way to do it is to centre line the car. Find the centre points of the chassis front and rear and then mark a line joining them. This way you know where the centre line of the chassis actually is and the thrust line should be along this line. Using a large set square such as a builders or roofers square you can set the fore/aft position of the rear wheels by marking points at 90 degrees to the centre line. The rear wheels will now be at the same forward/aft position. Set the rear camber to 1/2 degree negative.
They then need to be set with the same amount of toe in. The toe in is set on a GTD by moving the inner end of the lower reversed wishbone forwards and backwards. This is done by moving the washers from the front of the inner end of the lower wishbone to the rear and vice versa. If I remember correctly moving one washer changes the toe in by about 1/4 of a degree. The toe in can be checked by clamping a bar across the car at exactly 90 degrees to the marked chassis centre line. Use the large square set against this 90 degree bar to check the toe in. The toe in setting suggested by GTD is 1.6mm and should be equal both sides. I set mine to about 1/4 degree toe in. Make sure you have toe in and not toe out!
This method is not totally accurate and I get my car onto a good professional four wheel alignment rig. When checked this way my rough setup was pretty close.

As an aside before I bought the car my friend brought the car round for a check and I found that the front toe in was 15mm on one side and 22mm on the other. He was complaining about weird handling at 120mph!

Cheers
Mike

Education is what remains after one has forgotten what one has learned in school. Albert Einstein

 

[Malcolm]

1 inch in front of the other? New one on me so would beg to differ on that statement!

 

[Andy]

The previous builder of my car drilled new position holes for the left front wheel 2 top brackets (sleeved brackets) 1" further backwards. This indicates what Mike says.
As a starting point, I used the GTD hole position for the upper brackets.
But still need to measure everything up.
First gonna try using the bottom plate as my reference plane and since the rivets are exactly positioned in the middle of the plate along the x-direction.this will be the reference line. Need some long straight to setup in 90 degree to this line so I can check the position of the wheel (x-y direction).

 

[Terry Oxandale]

I don't know if this is something that can be done easily, but I would cross-measure the corners (FL to RR) and FR to LR), then combine that information with some fore/aft, and lateral measurements to get an idea of where this chassis should be adjusted.

 

[Andy]

Measured the location of the frontwheels with a long ruler aligned with a front rail of the chassis. It looks like it is within millimeter correct using the original location and the brackets centered. That is a good sign.
My 265/50 rear tyres look awfull and there is too much space left in the wheelarch. 275/55 would fill the gap. Bit worried with the ride height, rear springs are barely compressed and still too heigh ride height. Think I will need longer springs in the rear especially with 275/55 rising the car even more with an inch.

 

[Jim Cowden]

Andy using a chassis rail to measure off is not the best way,it means nothing.
Tube work can be out and adjustments are made up in bracket lengths.

Mikes method is better,drop off 4 inner suspension points in front and 4 in the rear then you can diagonal measure as well like terry mentioned.

It can all be done with a plumb bob, masking tape and a string line.

Often stagger is adjusted to suit wheel arch and body work on hand built cars as an easy solution.

Jim