My take on the "old geezer's method" for steering alignment

[john.p.clegg]

Forget parallel lines , think Trapezoid , If the wheelbase is 96" and the track difference is 1" then the angle of toe-in is Tan 1/192 which is tan 0.0052083333 which works out as 0.29841282 degrees.....

John

[JonB]

John, I added rear toe in measurement to my previous post as you were posting. Could you read it and comment, please?

[MarkDa]

Jon
That was quick!
Yes the tyre seat is 13" so by the time you get out to the rim it goes up to 14".
Personally I'd be aiming at the mid point of c3mm on rims so 5mm or so at tyres.

4.8 mm is near enough 3/16" so your rears are bang on.

Mark

[MarkDa]

I've been thinking about these angles.
Mid point toe in of 3.2mm total generates an angle of 1.6/355 radians per wheel that's 0.26 degrees.
Which is what Hunter tracking machines specify.
Not sure how that relates to John's trapezoid calculation though

[john.p.clegg]

Jon
I agree,front and rear toe-in should be the same.

John

[JonB]

Would that give a true parallelogram for the tracking?

[john.p.clegg]

Yes.

tracking.jpg (45.11 KiB) Viewed 992 times

John

[MarkDa]

After Hunter alignment with 0.25 degrees each at rear my car was still very nervous, i just was not comfortable even on going straight at 60 mph on an ordinary road.
I cranked the toe-in up to 3/16" and much more stable under all conditions.

[john.p.clegg]

Mark
So what degrees did you end up with ?

It's all very confusing...inches/mm's/degrees/radians/per side/overall....

John

[MarkDa]

John
The tyre shop set at 0.25? = 1/16 toe-in.
Using the thread pitch and 2:1 ratio of wishbone width and wheel diameter i cranked it out to 3/16" or 0.75?
I have an old Dunlop wheel alignment gauge which is ok for setting the front as they are interlinked but the rear is set against the chassis so either needs a 'frame' set up or a computer.

Sorry about the mixture of units.
Obviously the workshop manual used inches of toe in.
I then took Jon's metric measurements to help us get directly to an angle via radians (or tan in your calcs) where the toe-in is the circumferential length.

I had hoped I was clarifying things but I suspect I've achieved the opposite!
Mark

[69S4]

Picking up on JonB's steering alignment discussion from a month or two back, I've been trying to string my way into finding out where the front end of my S4 is after a winter strip down / rebuild. It'll probably end up on the local tyre depot's Hunter machine in due course but to begin with I just need a first go at getting it roughly correct.

This is what I came up with:

Firstly - two lengths of wood (borrowed from our loft project!) with exactly matched saw cuts along the top for the string to sit in - so the string would be exactly the same distance apart front and rear. They were placed at spinner height and the same distance in front / behind each wheel so approximately square to the track.

I then ran string along the left hand side of the car between the planks and adjusted the rear plank left and right until the string to spinner distance on both wheels was exactly the same . The front and rear track on the S4 is only 1mm different so for a first go I ignored it. So far so good (in my tracking fantasy anyway!).

Next (if you're still with me ) I ran a line between the cuts on the drivers side. I expected the string to spinner distance would be pretty much the same for the front and rear on this side - after all the string fixing points on the planks were exactly the same width and the string was parallel with the spinners (and therefore the car) on the left side. That's three sides of a square.

On the right side though there was a 20mm difference between the string to spinner distance at the front compared to the rear - front greater than the rear. I expected I might see the 1 or 2 mm track difference showing up here but not 20mm.

So, at last, my query. Can anyone see where I've gone wrong with this? What error has caused the non parallel second string?

[nmauduit]

the setup seems fine to get started, but the reference should be the center line of the car, not one side : you need to equalize your gaps at the rear, then at the front: you can then measure the angle of each wheel with respect to the reference, which will be the centerline of the car (plus your assumption of the car being approximately equally wide front and rear is too approximate, see the schematic of John Clegg above).

or if it's just to drive to the laser shop, eyeballing it roughly parallel and driving slow should be ok...

[69S4]

It was John's diagram that inspired me but it's for a +2 rather than my 2 seater. The smaller Elan's track is almost exactly the same front and rear - 1mm wider at the rear according to my workshop manual, so his trapezoid should become my near as damn it rectangle.

If I move the string to equalise the gaps I'll have a rectangular car set within a trapezoid string frame which surely can't be right. It would probably do to get the toe within what is a fairly wide spec but part of it now is trying to work out how I've cocked it up.

I could take it down to the local shop but like many others have commented I'm more impressed with their equipment than by the people who use it. I'd like to have it somewhere near correct before I let them loose so I've some idea whether I'm being strung a line (so to speak!). I suppose what also niggles is that this should be a fairly straightforward thing to do given it's just playing with string and rulers.

[nmauduit]

If I move the string to equalise the gaps I'll have a rectangular car set within a trapezoid string frame which surely can't be right. It would probably do to get the toe within what is a fairly wide spec but part of it now is trying to work out how I've cocked it up.

no, you use your setup making sure that the strings are parallel, and center the car within so that they both share an identical "symetry" axis by equalizing independantly
1) the rear wheel gaps (same distance A between left rear nut and string, as the right rear nut and other side of the string rectangle)
2) the front wheel gaps (same distance B between left frontnut and string, as the right front nut and other side of the string rectangle)

Make sure the steering is centered, and do not touch it while doing adjustments

Then say you check toe in for front left wheel by measuring A-a between the string and the rim at front near the bumper at nut height, and A+a between string and rim at the rear near the door - same nut height, then work out the math

Same for the other side. Do it several times, average out and get a feel for where the imprecision comes, do some adjustments ....

[Bigbaldybloke]

Ok, I?m looking at this based on reading a very old book on the design of car suspension aimed at people building their own kit type cars in the 1950?s plus my knowledge gained in racing modern radio control cars that have fully adjustable suspension right down to anti squat, anti dive, roll centre position and the more often talked about camber angles and toe in or toe out.

If your rear wheels are running negative camber, which at normal ride height they should be, think of the wheel as the bottom disc of a cone, hence the point of the cone for each wheel will be somewhere on the other side of the car. Now imaginge letting this cone role forward, if you are imagining the right hand rear wheel, this cone will tend to move in a circle with the bottom ie the wheel turning to the left. So, for the wheel to follow this line it needs a small amount of toe in and the more the camber the more the toe in is required.
Incidental, in this book the toe in figures are defined as an amountat the wheel rim diameter, hence 1/4? toe in on front wheels is a difference between the front of the rims and the rear of the rims at axle height with the car loaded with a specified amount of fuel and passenger weight.
Lots of other nuggets of information on suspension in the book, things like how king pin inclination should be designed - a line through the kingpin or top and bottom pivot points should intersect the ground at the centre line of the tyre, so having wheels with more offset causes more kick back through the steering and makes the steering harder to turn. Drawing a line from the kingpin through the centre of the ball joint on the steering arm it should intersect the centre of the back axle to give the ideal Ackerman angles ( also works if they intersect at a smimilar distance in front of the car as on an Elan)

Moving to radio control cars which nowadays are fully carbon fibre and titanium with every suspension setting adjustable, more rear toe in makes the car more stable and gives rear grip especially when accelerating out of corners, but too much then causes excessive scrub and limits top speed. On the front the toe in is typically small or none, but for better turn-in on corners, slight toe out can be advantageous, but is not so stable at high speed in a straight line, but bear in mind these are 4wd cars with limited slip diffs front and back. Not fully transferable to an Elan I know but an excellent way to find out what the different settings do to the handling. For info my latest car has rotary dampers and quarter elliptical springs for each wheel to keep the weight as low as possible so does not need the high support typically needed for telescopic dampers and springs, I run about 1.5 degrees negative camber front and rear with 3 degrees rear toe in and no front toe in, the front diff is much tighter than the rear and front caster is currently around 4 degrees, but I?m experimenting with that.