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String box wheel alignment
alistairolsen - 22/3/11 at 01:33 PM

Having read various guides..... The aim is a rectangle aligned to the rear wheels.

Question is, how do you achieve this without measuring the diagonals? You can ensure the sides are parallel but if your front and rear axle were slightly offset how would you pick it up?

Cheers

MakeEverything - 22/3/11 at 01:46 PM

You need a datum point, normally, the centre of the car - measured from the centre of the rear axle (if symmetrical) or a known centre. From this you should be able to measure the wheel track and make markings until its square and symmetrical.

After youve done this, it gives you more datum points to measure the fronts.

Doctor Derek Doctors - 22/3/11 at 01:53 PM

quote:
Originally posted by alistairolsen
Having read various guides..... The aim is a rectangle aligned to the rear wheels.

Question is, how do you achieve this without measuring the diagonals? You can ensure the sides are parallel but if your front and rear axle were slightly offset how would you pick it up?

Cheers


I've been 'stringing' race and track cars for years, its a good simple method. It probably won't pick-up offset axles though..... are you really expecting this to happen?

I have seen Porsche British GT cars crabbing down the straights at about a 3 degree angle though as the set-up was done on the plss.

What car are you doing it on? If its a Seven and you are worried about axle offset, take the bodywork off, work out and mark the centreline down the middle of the chassis and then measure form that to make sure that each wheel is an equal distance from the centreline. If this is the 1st set-up of a brand new build I'd recomend doing it thouroughly with body work off anyway. Bodywork lines can be very decieving.

Also if you have the bodywork off you can mount the string bars directly to the chassis which is a much better method, most race cars have two small mounts front and rear for a stringing bar to be cable tied/bolted onto and then a centre mark on the chassis and bar to line the two up.

alistairolsen - 22/3/11 at 02:06 PM

I'm considering doing it on my daily cos it will need done about 3 times in the next few weeks and while I cant warrant the tyre wear, I don't mind if the wheels in the piss. I was just curious about the thrust angle thing, I can see how its done on a spaceframe fairly easily, just not on a curvy monocoque!

If there was something simple I was missing it would have satisfied my curiosity as to just how bad the rear axle alignment tolerance is from the factory!

Cheers

britishtrident - 22/3/11 at 02:18 PM

It is quite handy to provide front and centre line datums on the chassis at the building stage,
It really is quite tricky to get everything pointed on the right direction but thankfully very smal errors are critical. My personal (lazy) method was to get one rear wheel pointing as close as possible to the correct direction then set the the toe-in accross the axle. A rear wheel should never toe-out, always err on the side of toe-in.

On the front just equalise the track rod length and adjust the toe-in accross the axle equally on both tracknrods, if the steering wheel is off centre you can turn it on its splines. Again it pays to err on the side of toe-in but nothing like as critical as it is on the rear.

v8kid - 22/3/11 at 02:21 PM

Measuring the diagonals between tyre contact patches will tell you if it is square or not. To get a consistent point to measure from use two try squares cut out of mdf with a small screw in the corner. A bit like a solid capital L shape with a screw at the elbow.

Fit the squares flat on the ground snug to the inside of the wheel between a diagonal pair and measure length. Do it a few times until measurements become consistent. Compare diagonals.

If the rear axle is 1 deg off square the diagonals will vary by 22mm so you can see this is surprisingly accurate. Usual caveats of blowing up tyres adjusting other stuff bla bla bla.

Cheers!