Drove the car properly for the firs time today and noticed that it felt very light around centre, like it had no self-centering at all and almost wanted to go either left or right.

When in a turn, the steering weighs up nicely, but doesn't give a stable feel around center, this is pronounced under breaking.

I've just measured the caster of both front wheels using the camber measurement at 20 degrees left, 20 degrees right, then measure the absolute difference between the two, readings as follows:

Left = 2.1 degrees difference in camber, right wheel =. 1.8 degree difference in camber.

If my research is correct, I multiply these numbers by 1.5 (at 20 degrees, 40 deg total) which gives me...

3.15 degree positive castor on the left and 2.7 degrees positive caster on the right.

This appears to be in the ball park for the various numbers out there on the net.

I've just re-checked my toe which is the same as before at a tiny bit under 1/4" total toe-in (laser tool measures 15 minutes toe-in consistently) , again, in the ball park - camber less than 1 degree on both sides.

So, based on the above appearing to be about right, why might the steering feel so light feeling like it has no self-centering?

Tires all new, pressures all correct and feels the same as it did on the old tires when I drove it very briefly.


EDIT: Just thinking out loud.... would 40 year old, perished original A arm bushes cause this - i.e good static numbers but dynamically challenged under load?

 

You must get the thrust angles correct. Once this is correct adjust for total toe. Don't try to set toe for each wheel. Without a alignment machine this is very difficult to read correctly.

My full procedure is here.
My DIY Alignment method - CorvetteForum - Chevrolet Corvette Forum Discussion

 

I'm not 100% confident I understand the thrust adjustment and what this represents - are you referring to rear toe values, ie making sure both rear wheels have the equal measurements of toe from side to side, irrespective of that measurement?
i.e it could be toe in or out at the rear, but as long as it's the same side to side, the thrust is correct?

Forgive me for not quite grasping thrust and what to do about it.

Will this require adjustment of rear shims to correct? I could just do with a more basic Jack and Jill version of what I'm trying to achieve, as my gut instinct is telling me this doesn't explain the behaviour I describe in my last post, although I fully accept this might be due to my own lack of understanding.

Thanks in advance.

 

Here is the rear thrust angle setup. I place a laser on the rear wheels and measure the distance from the laser to the middle of the wheel at the front axle. The left rear wheel should measure the same distance (@ the front axle) as the right rear wheel. In the example picture it measured 9mm.

Once you have this equal on both sides, then measure total toe. Then to adjust the rear total toe, move shims equal amounts both sides to get the total toe correct. To varify thrust again measure the thrust angles, let say the laser line now reads 12mm each side.



So the thrust angle of the rear can steer the car which requires you to adjust the front steering to compensate. So if the rear thrust pushes the rear axle to the left you will need to steer the front wheels to the left to get the car straight again but the car will drift to the left so you steer
the car to the right but the rear will try to steer to the left. You will repeat this process many times. And with power steering this can feel like very light straight steering adjustments.

To have a tight front tracking alignment you need 6 degree's plus caster ( not possible with C3 factory suspension arms ) need offset arms , or SPC tubular adjustable arms. Also the power assist pump needs reduced flow to make the PS heavier.

What steering setup is on your car ? Factory , Rack & Pin , etc ?

 

Thanks you.

Just to cement my understanding, thrust is whether the rear wheels are equally behind the front wheels, i.e the contact patches of all four tires are not in a parallelogram, but in a slight and equal angled trapezoid?

If my understanding is true, and if the thrust turns out to be off, how do I adjust it?

I think I understand the steering to compensate theory you describe above, however I can't rationalise this in my head with the way my steering feels. Imagine you are driving along the top of a tube, down it's length and you're trying to keep the car from falling off to the left or the right, If you let go of the wheel, it will very quickly decide that it wants to fall one way or the other and will be very nervous and massively, excessively light at centre, almost like trying to balance two opposing magnets - once it goes in one direction, everything then feels normal and correctly weighted, with the correct amount of self centering, Now imagine that as you brake and get below about 15 mph, the steering really wants to pull either left or right, exaggerating the feel of the conditions above, either left of right, requiring you to hold the steering straight, which as soon as the car comes to a halt, the resistance effort in the steering relaxes. It will do this either left or right, depending on whether you are are in a slight left or right position when you brake. It feels life as the weight transfer compresses the front suspension, or further deflects the bushings, the geometric issue is exagerated.

By comparison, when I had my '80, it felt daed straight and true and the steering was correctly weighted and had good natural feel under all conditions - the difference felt like rather than driving on top of a tube, I was driving inside the bottom of it.

I hope that makes sense and explains why I'm struggling to associate with thrust.

The car is a bone stock '81, 42k original miles and unmodified, as it left the factory - everything looks virginal and original - including all the 41 year old bushings.

 

Rear Setup.
How are you measuring Toe ?

So I set thurst buy setting the toe angle equal on both side based on the front axle position as a reference. If you are not using the front axle as a reference then its very difficult to take measurements.

Here is the measurement with the laser attached to the rear right wheel. I measure the distance from the right front axle reference point. Here it shows ~ 100mm. I then adjust the left rear toe shims to read within 1-2 mm on the other side. (impossible to make it perfect with only 1/32" shims. Once you are happy they are equal both sides (Thrust is ~ 0.001 degrees 1-2mm difference) then you adjust again BOTH sides to get the desired TOTAL TOE. example total toe = 1/32nd" then toe in both wheels ~ 3/64nd " this would give you total toe 8/64nd" or 1/4" total toe.

 

Ah, I get you, so my understanding is correct - you need to ensure the rear toe is equal both sides - got it.

I am measuring my rear toe using the string method to ensure a perfect square round the car and equal distances to the hubs across both axles, then measuring the distance from the rim fore and aft edges to the string - like you describe, this is a painstaking process to set up, but as ane ex-engineer, I cam confident my digital verneer readings and meticulous care and attention and quadruple checking is giving me some reasonably accurate results.

If i recall correctly, one of my rear wheels is toe out, the other about neutral, which I believe would give a left hand steer tendency? Mine, if anything wants to steer right a little - weird....

However..... those measurements were taken after lowing the car from jack stands and not after driving it, so I'll re-check the measurements tonight and report back.

My confidence in my string measurements was based on my laser toe gauge used on the front axle - my string calcs corresponded almost exactly with the laser gauge, within a tiny fraction.

I'll use the same laser gauge on the rear axle once I've done the string to at least confirm total toe along with thrust.

Still not convinced this explains my "either side" driving on top of a tube symptoms though....?

 

The problem with using the string method is margin of error.

Using a 15" Rim (front rim vs rear of rim) from the string.
Lets say the best you can repeat is with-in 1/64th reading = 0.015625 inches over 15" = 0.059 degrees. best reading you can get 1 side so total toe = 0.119 degress which is ~ 1.779 32nds. or 1/16 total toe. This is assuming you can be accurate to 1/64th.

So lets increase the odds in our mistakes by increasing the distance where we take our measurments.

In my DIY method
My DIY Alignment method - CorvetteForum - Chevrolet Corvette Forum Discussion


Using a laser bar attached to the wheel and spacing the measuring bar using 4ft spacers front & back to the tire. Spaces ~ 2770 total mm.
So in the above example readings difference in toe = 120-129mm = 9 mm over 2770mm to calculate total toe angle = .18615 degrees which is ~ 2.72/32nd total toe.

but note each 1mm reading which is a lot equals 0.0206 total toe degress ~ .31 / 32nd so very easy to measure 1/32nd total toe. As the string method impossible to repeat. But worse takes way to long to setup.

I can read thurst / caster / camber / toe for all 4 wheels within 5-7 minutes and still make reading errors without affecting accuracy.

in the above example, lets say inside of reading 180mm on the right side it was 182mm (2mm error is easily noticed)

total toe is 11mm over 2770mm = .2275 total degrees 3.43 / 32nd's vs orginal measurement 2.72 / 32nd's. still averages to human readable as 3/32nd.

But here is another thing to check based on suspension age. Take 2 readings.

step 1 drive the car around the block, take your toe measurements.
step 2 drive the car around the block but drive onto some skid pads and take the toe measurements again.

Picture shows skid plates. (plastic cutting sheets with greese in between) this forces the suspension to move if worn out bushings / joints etc.

If you get a large variation in toe out / thrust readings this shows you need to fix / replace parts. In a tight suspension you will still get a change in readings, but as shown even a 2mm reading change wont effect the accuracy much.



Well worth the time to stop using string alignment methods to laser pointers. Use math to your advantage to improve your odds.

I check my alignment after every track event ( twice each month ) on 2 cars. 1978 corvette and 2010 C6 grandsport. With-in 5-10 minutes I have checked the alignment accurately.

I when I started doing this I would also take my car to a chev dealership and have them take measurements. This is when I determine I can DIY this as well as an alignment machine.

As for making the adjustments on the car, that is another story. Rear Thurst / Toe is the most time comsuming procedure.

 

Thanks.

With the equipment I have available to me here and now tonight, I've just done the following, below..... perhaps you can help me interpret the results?

Firstly, I measured total rear toe relative between the rear wheels using my laser toe measuring tool, This gives absolutely repeatable results time and time again, so I trust the number as below....

https://i.imgur.com/fluOYYt.jpg/img]

This shows just 5 minutes of toe-in across the rear wheels, well within spec, yes?

I then replicated your thrust measurements (or at least part of them) using a laser line and measurement to the front wheels - same spot on both wheels each side, making sure the bar was square to the wheels in all planes....



The results are interesting and likely point to something.....





As we can see, the left hand rear wheel is pointing further away from the front wheel than the passenger side, buy (229.5mm, minus 201.5mm) 28mm, which over the wheel base of the car 2,489mm gives (assuming the axles are equally spaced side to side) a toe-in angle of 0.3223 degrees, based on trigonometry of A= 2489, B = 14mm (half of the 28mm differential), or 19 minutes toe-in, which isn't a mile away from my laser toe gauge which shoes 5 minutes total toe - a difference between methods well within the accuracy of execution / environment.... again, assuming the wheel rims are identical and the a-arm shims are the same side to side etc, etc...

Therefore at this stage, is it fair to say that my rear wheels are toe-in ever so slightly, albeit perhaps not equally within the centre line of the car?

Now this is where I had to fall back on my string methods, which to your point of accuracy, I can repeat my measurements to within 1/64th like a forensic scientist on my hands and knees, up close and personal to the string looking to detect the slightest hint of deflection as the edge of my digital vernier just brushes the string!

The results show that the left rear wheel is 1.23mm toe-in across the 410mm rim edges - which based on trig - a = 1.23mm, b = 420mm angle = 0.17 degrees or (10 minutes) toe-in referenced to the vehicle centre line (assuming the axles are equally spaced side to side).

However, the right rear wheel shows a 2.06mm toe-out across the 410mm rim edges - which based on the same trig above is 0.288 degrees (17 minutes) toe-out referenced to the vehicle centre line (assuming the axles are equally spaced side to side.

If we subtract the 10 minutes toe-in from the left from the 17 minutes toe-out on the right, this leaves us with 10 minutes toe-out total, based on a tone of assumptions and the inaccuracies of the string method - but either way, only 20 minutes (0.33 degrees away from my laser toe gauge, which is likely to be more accurate.

The rear toe measurements are pretty much the same as the last time I did the string method, for the record.

So, what can you help me deduce from the above and what next?

(Really appreciate your help).

 

Double post - can't figure out how to change the font color in the previous post....

 

OK step 1 to get the thrust angle correct both sides should read the same values, either 230 or 202 , or any number to start. Your rear thrust angle is way off. I set mine to be 1-2mm off at most. So 28 - 29 mm off is miles off.
1 mm off is a thrust angle of 0.02 degrees which is very difficult to get with 1/32 shims. but try to get them with-in 1-2mm side to side.

But you have everything you need to measure toe the same way I do. Place 2 tape measures the same distance from the tires front and back.

I am using 4ft plastic rulers as spacers. But anything the same length will work.
I have a bar with measuring tape stuck to them, but just using a measuring tape layout will also work. Measure the distance the 2 tape measures are from each other. Left vs right side should be equal to be square. Lets call this distance 2770mm

Then on 1 side point the laser to the front tape measure and pick a number as a starting point. lets say the 100mm on the measure. Then reverse the laser to point to the back tape measure. Move the tape to the same 100mm point. This sets the reference point the same on 1 side.
Then move the laser to the other side and take the reading of the front tape measure spot vs the rear tape measure spot. Lets say the front reads 3400mm and the rear spot reads 3410.
Since the front distance from the reference side is less than the rear distance ( 3400 vs 3410 ) wheels are toed in. Total toe = inverse tan of 10mm / (2770mm) this equal total toe = .2068 degrees.

Using this online calculator
Convert Toe Degrees to Inches (robrobinette.com)
Assuming 27" wheel dia (factory C3 Tire)
this comes out to 3.11 / 32nd's total toe.

But here is the nice things about doing it this way. Once you know the static distance between the tape measure (based on the spacers you use ) you can make a simple chart for reference.
1mm = xxx
2mm = yyy
3mm = zzz

my target is anything in the 1-5mm reference for the front and 10-15mm for the rear. No more calculations required once you know the range you want. This takes only 2-3 minutes to measure / setup. More time is spent attaching the laser to the wheel then anything else.

and the longer your spacers are the more accurate you can be based on math.

So in the example below I set the tape measure on the right side reference to 180mm. And a reading of 60mm top and 51mm lower, a difference of 9mm toe-in. read your chart and you are done. based on the assume measurements = (2.8) 3/32nd toe in.

 

Thanks again, and please forgive me for double checking my understanding....

We are saying that whilst the toe-in between each wheel seems to be okay using my laser tracking tool known to be pretty darn accurate, that the actual issue is the relative difference between the front and rear axles in that in unison, they are steering the car a little like an articulated vehicle?

And if so, based on my 28mm difference, this differential is in the rear wheels, i.e to adjust the thrust, my focus needs to be on the rear shims?

And if so, is (assuming my slight toe-in measurement is correct and I want to maintain this?) that whatever I do to one side, I replicate to the other side, effectively keeping the toe the same, but steering the axle so it is in line with the front?

 

Yes Adjust each side similar amounts. If you toe-in one wheel toe out the other wheel the same shim thinkness until you get the rear thrust measurement with-in 1-2 mm difference.

You can then do the same with the front wheels. Center the steering wheel to the center position. Take the same measurements for thrust but measure to the rear wheel axle reference. This is much easier to do because you need to only turn the tie rod sleeves.

I used a tape maker on the steering wheel and column to make sure I set the steering wheel correct (same) each time.

 

Thank you, you, this is hugely helpful and I'm really grateful.

Whilst I thoroughly trust my laser toe gauge, I'll do a manual check using your spacer/tape method and compare the results. If they are the same, I'll then pluck up the courage to look at the shims.

Assuming my toe is correct, which based on the image below it is, am I effectively looking to toe in the left rear wheel 14mm worth of thrust and toe out the right wheel 14mm of toe to equalise the measurements?



The only thing puzzling me is that my string method (whilst not perfect, granted) is showing repeatedly that the right rear is already in a toe out condition and this will make it worse...

 

correct move the 230mm mark to 216mm and the 202mm mark to 216mm.

Also measure you wheel base to see if they are the same side to side. if not I would expect a bent frame / suspension parts.

Like this



The only way the right would be more toe-out is the front axle is shifted to the right in relation to the rear axle (axle center lines)

Based on your measurements the 230mm mark show this tire is much more toed out vs the wheel that measure 202mm. (assuming the axle center lines are the same)

As a quick check place 2 tape measures front and backup of wheels similar distance and using the laser and see if the tape measure system agrees with toe out vs toe in.

I read up on the trackace alignment tool to see how it works. I would check the calibration / setup if they both dont agree toe-in vs toe-out.

 

Your 'miles out' comment had me second guessing myself regarding the side to side measurement and I suspect I didn;t use the same reference point each side, although I thought I had.

I've just been out to double check (twice) and whilst, still plenty out, is not as bad I first thought, measured from the centre rim flange...
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14.5mm is the actual difference.

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And a moody shot for the hell of it...​​​​​​

 

I set up the measurements the same way you have advised, and zeroed the tapes (at the 100mm mark) at one side in order to measure the toe settings at the other.
As can be seen, there is a 7.5mm difference over the total difference between the tape of 2,667mm, which as per below gives me 0.161 degrees toe-in, which on a 27" wheel equates to 0.151" total toe-in, or as a fractional measurement, just over 1/8 - within spec. What is encouraging is that 0.161 degrees is 9.6minutes, very close to my laser gauge which suggested between 5-6minutes.

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​​​​​​Therefore, can I deduce that the next step is to adjust the trust by shifting rear training arm shims equally on both sides to bring the thrust equal?

 

The total toe of 0.16112 is actually 2.4296 / 32nd or just over 1/16" total toe. The online calculator needs degrees of 1 wheel not total toe as input then displays toe inches for 1 and total.

So I entered .08056 as the degrees and 27" wheel which calculated total toe as 0.0759" and 2.4296 / 32nd.

So based on your measure tape distance a simple chart. Once you have a chart no more calculations just simply add your spacers , add the tape measures, mount your laser and read the values, quick simple and very accurate.
So now you can measure to .32 / 32nds and make a error between 1 to 4 mm and be out by 1/32" total toe. Easy to repeat and setup. (unless you have worn suspension and it constantly changes)

 

Also try your laser mounting bar, place it vertical on the wheel and use your phone to measure the camber.

When you adjust camber on a wheel it will change the toe as well. So for the rear, adjust camber first and adjust toe.

 

Thanks - is that a table based on yours or my data and is that for the difference from rear to front tape?

And am I correct in assuming next steps are effectively move the pivot points of both training arms to the right?

Based on my now more accurate measurement of 14.5mm total thrust difference, as I'll need to move each wheel by the same amount, in your experience, what in terms of shims would equate to a thrust change of just over 7mm at the front axle? Given the relatively short training arm length, a 1/32 shift (0.8mm) would surely do more than 7mm at the front axle?