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as the title says, the car has been twitchy in a straight line ever since i spun it at 100 mph and ended in a ditch.
i have done a front end alignment which has improved things but i feel that the back is out of alignment.
the rear is indipendant based on the Roughty design.
it is fully hiem jointed so can be adjusted but where do i start, perhaps establishing the thrust line?

the wheels are the same distance from the back to the front on both sides ( wheelbase ).

i cannot guarantee that the center line is still in the center and as a result, i can't say that the thrust line is down the center of the car.

when i originally aligned the car, i had a lift so was able to establish diagonals, this indicated that the chassis was in fact not straight but i adjusted the relevant hiems to get the thrust at right angles to an imaginary line from one rear upright to the other when the wheelbase was equal on both sides and pointed to a spot in the center of the front axle line to complicate matters, i am now older and not so nimble and my available space to work on the car inside is limited.

ideas please

_________________
this story shall the good man teach his son,
and chrispin chrispian shall ne'er go by,
from this day to the end of the world.
but we in it shall be remembered.

What's rear toe set at? If it's zero, give it a 1/16 toe-in. If it's already there, try another 1/16. What's the rear bump steer? Did the rear toe get measured with the driver weight in the seat?
Could instability be due to front end aero lift at high speed?

thanks for posting.

the problem is that of parallelograms if the rear axle is off to one side relative to the center of the front axle, they can be pointing straight and be parallel and the wheelbase can be the same on both sides.
to complicate matters, it should be measured on the ground at ride height with the wheels on and i can't get under the car when it's on the ground.

if i measure from the forward inboard LCA chassis which i hope is straight to the forward pivot on the upright on both sides i could be in luck if they are both the same, this would give me a starting point "if" the chassis is square.

then i will adjust the rear "chassis to upright dimension" on each side to give me the same dimension as described above, thus making the upright parallel to the chassis because until the wheels are parallel to the chassis and equally spaced from it whilst maintaining the same wheelbase, the axle may not be centered in the chassis.

when i get to that point i will have no toe and can add to each side

remember, the left wheel could be towing out a 1/16" and the right wheel could be towing in 1/8" which would read as 1/16" toe in

_________________
this story shall the good man teach his son,
and chrispin chrispian shall ne'er go by,
from this day to the end of the world.
but we in it shall be remembered.

A quick n dirty parallel string lines on each side of the car will preclude the last example from going unnoticed.

well i have to get the car square with the parallel string lines, the center line of the car, which is the the line of thrust.

i have measured from the chassis to the LCA rear most and front most ball joints on both sides and they are now all equal, thus the axle is equally spaced to the chassis left and right so the origin of the line of thrust should be at the center of the chassis and the rear should have no toe in or out

next i will fit the wheels and lower it to the ground to check that it is zero toe.

lets hope that the chassis is straight and the line of thrust is also in the center of the front axle and at right angles to the rear axle center line, however i do not have a reliable method to check the wheel position fore and aft

_________________
this story shall the good man teach his son,
and chrispin chrispian shall ne'er go by,
from this day to the end of the world.
but we in it shall be remembered.

i have checked the distance from the rearmost chassis member at the back of the floor to the forward edge of the LCA and it is equal.
this is the first tube welded in position when building the car.

so everything is equal and square in the rear from that cross member back.

the next question is does the chassis have a bend in it, that would result in the thrust line being off at the front, if it does run off center maybe i can move the front over a little.

_________________
this story shall the good man teach his son,
and chrispin chrispian shall ne'er go by,
from this day to the end of the world.
but we in it shall be remembered.

For a quick and dirty check for parallelism (aka crabbing or dogging), I would drive the car straight ahead on a flat surface for a few tire rotations and stop. Do not move the steering wheel. Then I would use a piece of aluminum roofing drip edge, place it against one tire, mid height, across both sidewalls of that tire and pointing to or extending to the other tire. If you do this on all 4 tires you will see how much different the spacing is on the left and rt side of the car.

_________________
Chuck.

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To recap, it wasn’t PERFECTLY square when you built it, just like any mass produced car or anything I have ever made. It may be more or less square now after the excitment.

As you know, the frame can’t be made square by adjusting the heims.

You are asking for ideas on how to align and stop the twitch without putting it on a lift. I’m going to repeat the others with a little more detail.

1) Center the steering.
2) Tie the end of the twine to a jack stand about six inches off the ground and put it behind a rear wheel.
3) Pull the string around so it wrap the rear of the rear tire bulge and barely contacts the front of the rear tire bulge. Run the string out to past the front tire and tie another jack stand at six inches off the ground. Ensure the string still barely touches the front bulge of the rear tire.
4) Make the same rig for the opposite side of the car.
5) Adjust the heims as needed then readjust the strings to get the distance equal on each side to the front wheel centers. Yes, that means the UCAs or LCAs will no longer be symmetrical from side to side, just like any alignment.
6) Put the string rigs aside.
7) Place a piece of 2x2x24 angle iron or other straight surface on the ground outboard of a rear tire, touching both front and rear tire bulges.
8 ) Do the same for the other side.
9) Measure between them at the front and rear.
10) Adjust the heims at the front or rear leg of the LCA on each side the same number of turns to make the measurements equal. Use the same leg for each side of the car.
11) Shorten the front heim or loosen the rear heim one turn each side for some toe in.
12) Recheck your strings.

If you can't reach the LCA heims under the car, get a step ladder.

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This is probably not relevant, but when I sold my car the buyer had it thoroughly checked out, at which time a hairline crack was found in the suspension. Looking back, I'm fairly sure that it occurred when I spun off the track at Laguna Seca. It really gave me pause that I'd sold a potentially unsafe vehicle, but then all sevens are pretty unsafe. Point being, look for the hidden stuff.

today i managed to get a pair of reliable diagonals from the rear LCA"s front ball joint to a place on the front LCA's,

it would appear that there is a discrepancy of 1/2" towards the left side, it is longer so if the wheelbase is equal on both sides this would mean the front has shifted to the left 1/4" approx, the front LCA"s are both the same length.

or maybe the rear has shifted to the right, this is i think is what happened as i had to extend the left LCA 1/4" to make both LCA"s the same length or maybe it was always off center and i screwed up when i built it

any how, if the rear did move, as this is the datum for the thrust line, then the thrust line may no longer be in the center of the car and may be just off center at the front by 1/2"

if i extend the right front LCA 1/4" and reduce the left side by the same we could be square but to do this will require the riveted body side panels to be removed.

alternatively, if i reduce the right rear LCA by 1/4" and extend the left one 1/4" would this be o.k.

so the question is would this offset be a reasonable tolerance ?

consider, if you removed the right front wheel and replaced it with a narrower wheel that put the center of the contact patch 1/4" inboard and then replaced the left front wheel with a wheel with the contact patch 1/4" outboard would the car handle poorly ?

_________________
this story shall the good man teach his son,
and chrispin chrispian shall ne'er go by,
from this day to the end of the world.
but we in it shall be remembered.

I would argue that it is very relevant. I am skeptical that all this focus on the thrust line causing a bit of crabbing would result in significant instability. A failed, failing, or compromised component more directly related to stability under load on the other hand seems highly plausible after a high-speed off.

_________________
-Justin

"Orville Wright did not have a pilots license." - Gordon MacKenzie

that's exactly what i thought, to the point of purchasing 14 new hiem joints at a cost of $181.00

but none of these were in the least bit worn , they were replaced however.

this is what sent me on the journey of alignment, having found no problem with the front,

camber = 1.25 deg. both sides

caster = 5.5 deg. on the right front and 5.25 deg on the left front ( discrepancy to compensate for road camber )

tow = 1/8" over all

i thought it wise to start at the thrust line which is where all vehicles with independent rear suspension should be started and down the rabbit hole i went.

_________________
this story shall the good man teach his son,
and chrispin chrispian shall ne'er go by,
from this day to the end of the world.
but we in it shall be remembered.

well folks, after a lot of house remodeling, now i have a ceiling in the whole house.
alas i have been stalled on the alignment problems, but i have a garage now, 24' x 36' and all my stuff is in one place.
i spent some time going over all the dimentions of the car and am satisfied that it is square enough.
the car does run straighter than before without sudden violent direction changes.
however there is a tendency to change direction, very slight but it is there and correction attempts seem to just make it go the other way, it's like it has toe out on the front but it doesn't, i have added another 1/8 toe in without any change in characteristics so i think that is not the problem.
having driven the car i will check all the alignment settings for the rear but it is difficult to measure bump steer.
as this presents it's self in both directions, i cannot say it is something on one side of the car so must be something happening on both sides of the car such as rear toe adjustment or bump steer, if indeed it is the rear of the car.
i have reduced the camber to 1* on the drivers side and 1.5* on the passenger side, and the caster is now 6.5* on the drivers side and 7* on the passenger side, toe is 1/4", when doing these changes i have become suspicious that the rack is too long and the attached arms are too short by about 1/4"
so will check the front for bump steer as well when i find a way to accurately measure it without expensive equipment investment.
i am looking at using a laser and a mirror.

_________________
this story shall the good man teach his son,
and chrispin chrispian shall ne'er go by,
from this day to the end of the world.
but we in it shall be remembered.

It's good that you're checking static dimensions, but I don't think that's it - something is changing dynamically.

If the rear axle isn't square, you might have to hold the steering wheel 1/2" off, but it'll stay there. What you're describing - "twitchy", means something is actively changing while you're driving.

First thing I'd do is put the car on stands, grab each wheel in turn, pulling it fore-aft, checking for play. It's pretty amazing what your fingers can sense. I one time had strange handling and it was traced to the rear lug nuts bottoming out, such that the wheels were slightly loose. That took a long time to track down...

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i still haven't got to checking bump steer yet.
yes it does feel like something is flopping about from side to side, even on a road with a severe camber.
i have gone through the rear end again and it has 1/8 to 3/16 toe in which seems to be equally spaced about the chassis, not all on one side, it has 1/2 to 3/4 of a degree negative camber, the front has a lot of caster like 6* to 7* so should track straight
the faster i go the more it seems to wander, you may be on to something , i have checked all the hiems and replaced any that were worn, they are all stiff not sloppy.
i will check for wheel bearing play as you suggest, also for play in the steering rack.

_________________
this story shall the good man teach his son,
and chrispin chrispian shall ne'er go by,
from this day to the end of the world.
but we in it shall be remembered.