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Learning how to build Lotus Seven replicas...together!
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PostPosted: July 4, 2019, 12:29 pm 
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Joined: October 19, 2009, 9:36 pm
Posts: 2139
Location: meadview arizona
no i don't mean the wheels are not round,
what i mean is how square are the wheels to each other and as viewed from above aligned to the center line of the vehicle front to bach and side to side.
some time ago i had a slight mishap while tuning the PCM doing flat out full throttle runs and lost it at around 100 mph in the wet.
i know why was i doing it in the wet, well enough to say i won't be doing it again.
the end result was i ended in a ditch backwards.

so now i am checking the geometry to see if anything is out of line.
i am only concerned with all four wheels as nothing in the chassis appears to be bent.
i have set my front castor to 5 degrees and my camber to 1 degree with the steering wheel at center..
next i measured the total toe from one side to the other then devided it by 2 to give a center point for the front
i will now do the same for the rear this will give me a true center line for the car from which i can measure all the points of the contact patch center by projecting down from the center of each wheel.

then with a square i can mark these points on the center line, this will show if the wheels are off front to back as in stagger,

the front is the starting point, well you have to start somewhere, the rear is not so easy as there may be toe on one side and not the other after my little spin so i will set this toe to zero and find my center.

then as with the front i will make the same marks of the wheel centers to see if stagger is present.

all of my a arms are hiem jointed so by juggling these i can move the uprights fore and aft until the wheel points are "square" whilst keeping the track width equal about the center line.

the wheel base will not come into these calculations as i believe this only affects the ackermann which is a whole other story.

i have chosen this method over using the chassis as a basis as the chassis although it appears undamaged may not be completely true, most are not from the outset.

any thoughts that will help are appreciated.

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PostPosted: July 4, 2019, 12:40 pm 
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Joined: October 19, 2009, 9:36 pm
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Location: meadview arizona
the more i look at the car, the more i am thinking that i should draw my rear center from the diff center as the rear may be biased to one side or the other.

the diff is not quite as accessible but perhaps i should make the effort.

i do not want to induce torque steer into the rear.

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PostPosted: July 4, 2019, 12:51 pm 
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Joined: December 29, 2007, 10:41 pm
Posts: 895
Location: Vancouver, BC
John,
I tried to set up a string box a few weeks back and ran into the same issue, how to find the center of the rear axle. If I could nail the datum down, the rest of the alignment should be relatively easy. I need to check my car, as I'm certain the thrust angle is way off in the rear as I have toe in, but I'm certain it's biased to one side. I also have some nasty high speed instability, but that could be due to front end lift or play in the steering (I have a spare rack waiting for boots, then it'd getting installed).

Rod


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PostPosted: July 4, 2019, 3:56 pm 
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Joined: September 22, 2005, 8:12 am
Posts: 1620
Location: 4AGE in S.E. Michigan
A couple points. Pick up the center line on your frame rails. Measure the mark half way on both front and rear cross members. LIGTHLY scribe a reference line for future use on both tubes. Drop a plum bob down to fish line for your main center line. Then set your two out-board reference lines. But before you start you need to verify front axle set back. Both lower front B/J need to be the same distance from the frame zero line. You will probably have to shift one lower control F/A to get them to match. Then you can look at your track front to rear dim. Note that not all tire patterns are that accurate. Might want to check for run out before using the tire pattern as a reference point. Also make notes in your log book as to the adjustments made. The process will probably take 3 or 4 hours, but once you have your rough dims it is just a matter of counting turns [threads per inch] to dial the chassis in. Davew


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PostPosted: July 4, 2019, 4:44 pm 
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Joined: November 15, 2009, 9:58 pm
Posts: 394
Location: Port Angeles. Wa
The most important part of alignment for our cars, to me, is having the wheels pointing in the same direction with whatever toe in/out you want. I build a string box and reference it off of the widest axle with about 1/2" clearance to that set of tires. I make sure the strings are parallel (I use pieces of 1/2" EMT conduit front and rear at spindle/axle height) and drawn tight. The spacing to the tires of the other axle will be different but set equal side to side. I try to lock the steering wheel/column to keep it from moving when adjusting front toe. Measurements can then be made to the chassis to see if anything is offset.
A live axle will have to be moved forward or back on one side to align the rear wheels with the front wheels.
At least this works for me.

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PostPosted: July 4, 2019, 7:22 pm 
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Joined: October 19, 2009, 9:36 pm
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Location: meadview arizona
so i checked the alignment front to back on the front a arm ball joints, then drew a line between them, then i centered the rack and steering wheel.

on my diff there is a casting mark dead center, this gave me a front and back datum point, then set up a lazer to point to the front going through the mark on the diff this is at right angles to the line from front ball joint to ball joint, lucky me!

i fitted my alignment gauges to the rear wheels, it was off a long way front to back and side to side.

i measured the track width at the back and divided it by 2 then moved the short front to back side to equal the long side, this was at right angles to the front ball joint to ball joint line.

next i set the drivers side wheel to be parallel to the center line and half the width of the rear track so zero toe, then i set the passenger side wheel parallel to the center line and the same dimension and then double checked the overall track dimension and both were equal, well almost but within a 1/16 th of an inch on the overall compared to the sum of the two halves but still parallel.

now all i have to do is the rear camber but my digital indicator ran out of battery on the 4th of July so i will get to that tomorrow.

Dave,
i did not use the tire pattern as a guide on the measurements, i made some nifty alignment gauges that clamp to the rims and are indexed to be perfectly true and extend fore and aft to match the tire outside diameter and low enough to pass two tape measures under the car so i can read off front and rear of the wheels as i adjust and still know the mid point between the wheels on the center line is half the track.

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PostPosted: July 6, 2019, 3:23 pm 
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Joined: October 19, 2009, 9:36 pm
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Location: meadview arizona
well the car drives a lot better but there is play in the straight ahead position, when you turn the steering the play goes away even if you turn it a small amount so i think there is play in the rack or a universal joint.

as the rack is porsche 914 they are very cheap to repair as the play is usually in the lower ball bearing and the top needle roller bearing, about $15 for both so cheaper is better and i have a spare one to over hall and swapi did a search for servicing porsche racks and the symptoms described fit my car to a "T"

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PostPosted: July 18, 2019, 12:48 pm 
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Joined: October 19, 2009, 9:36 pm
Posts: 2139
Location: meadview arizona
well i got it aligned about a center line.

when i first built the car i used a rack for a dune buggy which wore out in about 6 months.

i thought the porsche rack was about the same length and hight but i was wrong, although caster, camber and toe were as with the dune buggy rack the bump steer was not, to reduce bump steer i had to move my track rods to a lower position, reduce the lengths of the track rods and increase the length of the rack, not hard to do but took a lot of time measuring.

now the car doesn't wander on uneven road surfaces all the way up to 90 mph and after a drive the tire temps are within one degree at around 116 degrees across the tire and corner to corner/side to side.

116 is good i think as the ambient temp here at the moment (9.40 am.) is at 100 degrees.

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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.


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