Bump Steer

[tibimakai]

I was measuring on the disc itself.
My car does not have a suspension yet, so it is sitting at ride height on blocks. 1" higher at the front, since the garage floor is not leveled.
I will resume tomorrow the adjustments.
I made an aluminum plate that is longer, than the disc surface, though I can only measure 1.5" bump with it.
Made some proper aluminum spacers.
That picture shows the vsusp height, but what I'm noticing is that height is not the right height. I had to raise it over an 1" from that picture.

Miatav8, you are saying that the laser should point toward the front of the car? To me, that it would make more sense, but then the camber is added together with the toe change, no?

[Miatav8,MstrASE,A&P,F]

See kartracer47's link to a product that is the same basic setup as what several members have endorsed:
https://advancedracing.com/product/lase ... eer-gauge/

The view is from the front. The laser is on the floor and points toward the mirror on the hub. The target is on the wall or a stand on the floor.

[Sean in CT]

FYI. Make sure you have your caster set where you want It, as this affects bump steer too

Anti dive geometry makes the bump steer curve "S" shaped, which is impossible to get perfect.

I use the single dial indicator method. Very easy and repeatable.

I made mine out of scrap. It uses a $10 harbor freight dial indicator.
I can take a pic if anyone is interested

[tibimakai]

There is a way to measure caster?
I screwed up the UCA with to much welding. The arms closed up like 1/4", so I had to install it a 1/4" more rearward to fit in the U brackets. This increased the caster angle.
Pictures are always good. I understand everything better with pictures.
I'm trying to avoid making this gage, but I may end up still making it. MDF boards are flat enough to be useable? It is hard to believe to a machinist.

[Sean in CT]

There are at least 2 ways to measure caster. It doesn't matter what the measurement is - this issue is that changing caster requires re-setting bump steer

The gauge was easy to make - ill get a pic tomorrow

I never checked the flatness of MDF - I have a piece of tooling plate (1/2" cast aluminum, ground flat) that I use. Picked it up at a scrapyard for $5 many years ago.

[tibimakai]

Question, how trustworthy is the vsusp steering rack length is?
I made it to that length, but visually it looks shorter, though the distance between the two swivel points of the LCA is 19.8", and the rack is 20.472".
Here are some pictures and video link to the most recent setup:
https://1drv.ms/v/s!Ap5Pmtb1HL5wrQd_r1a ... 8?e=gbNJcI
Duratec- 7 Build version-w/15mm wheel spacer

[Kartracer47]

Sorry - I give up

[tibimakai]

I don't have a mirror and a proper laser either.

[tibimakai]

I will go out tomorrow, and I will buy a laser and a mirror.

[anduril3019]

Back to measuring bump steer via the redneck system, if the pictures below are showing your setup, using a single, rigidly mounted dial indicator you're going to be measuring toe changes, plus track changes, plus camber changes as the control arms and tie rod swing through their various arcs. That's the beauty and simplicity of the hinged board or plate. By using the indicator on one side of a hub mounted plate, and a rigid depth stop on the other, you end up only measuring the toe change, with the camber and track changes being zeroed out as the indicator and stop pivot on the hinge.

Re. Vsusp rack height, my final location was about an inch off from the Vsusp number given, I'm attributing it to tolerance stack on my end in the build and in measuring, either way, I took the vsusp numbers as a starting point, that's why bump steer stil needs to be dialed in when you build, even if yo have good numbers. I started with large changes, +/- 1" of rack height, and reduced it until I got to where I was making changes of .050" with shims. It took several days to get to where I decided it was as good as it was going to get.

As MiataV8 mentioned, you may be running out of room to lower your rack (if that's the direction it needs to go) and will need to move the outer tie rod ends on top of the steering arms. Also as mentioned before, you can experiment with rack width by offsetting the rack to one side and extending or decreasing your tie rod by a corresponding amount, Extending is easy with a spacer, shortening is harder, I had a spare that had enough threads on it that allowed me to cut it shorter. Once you get the offset working, measure from teh center of the car to the inner tie rod pivot, double it, and that will be your rack length, pivot to pivot. Before you play with rack length, get your rack set to minimize bump steer, then start playing with width, one thing at a time. Move it in a little, how does bump steer change, move it out a little, how does bump steer change?

Another good piece of advice I got was to move on at a certain point if your numbers aren't too crazy.

[anduril3019]

Double post, sorry!

[tibimakai]

Mine looks about the same, I have it around an 1" higher.
Thanks for now.

I'm willing to give a shot to the laser method.
I have a distance laser, that shots off every 20 seconds, not good. I will pick one up, that is a proper laser pointer.
Will have to find a mirror, that is not to small and not to big. Right now I have one that is 5"x6", not sure though if it is glass or not.
Where should I place the laser, I'm keep trying different spots and I can't seem to pint it on the paper.
On the ART site shows at the floor, but that will point the dot almost at the ceiling, or on the wall, pretty high though.
Found this video, that shows that it should be placed on the side:
https://www.youtube.com/watch?v=IkFVyxC0tlw
What should be the height of the laser, in relation to the paper?

[tibimakai]

Found this Excel sheet calculator for bump steer, but I don't remember where, it could be at one at an aussie site.
It is named Bumpst~2.
I can't add it here, but I'm including a picture of it. Maybe somebody is familiar with it, and can tell me if it is any good.

[tibimakai]

OK, I did it with the laser and mirror.
To me it looks good, but what I know?
It was done over 5' away.
I don't know why the laser is so dotted.
Rack is at vsusp spec height.

[RTz]

I don't know why the laser is so dotted.

Those are Mach Diamonds