Bump Steer - Adamson Method

[Tmac897]

I have read through the threads on Bump Steer, and although I had a couple of theories of my own, I decided to go with something that’s proven. So I decided to go with the Adamson method, since in theory it’s simple enough to do.

First, there were no pictures in the archives so I had to go on what was written. I’ve included a picture of my setup, just in case I have a setup issue.

Did the “No Shims” test and the “Bunch of Shims” test side by side on the erasable board. Picture below (or above, on the whim of the software). They look exactly the same. I would have thought there would be a more pronounced “bump” in at least one of the cases. I did not expect to get two smooth curves.

What am missing?

[Billmack]

I dont see that method recording any kind of toe change. I recommend the board with a hinge on the ground and two pointers on the rotor method. Very very simple

[661]

That is interesting!
How many inches did yo go into bump and droop from the horizontal wishbones position?
It is possible , I suppose, that you've not got the sweet spot and the right amount of shims is somewhere else.
I was surprised how many shims it took with mine, from memory about 10mm, but the 26R set up is different because of the 'bent' steering arms.

[Graham B]

Hi Tony,
If I am correct, your photo is looking vertically down. I believe your setup is incorrect and will not measure bump steer.
The pen should be 90 deg clockwise from where you have it (ie. inline with the steel rule and parallel to the floor) and the board should moved 90 deg cw and touching the end of the pen. The hinged board referred to in other descriptions is to allow the board to stay in contact with the pen as it moves horizontally, vertically as well as in and out as you move the suspension up and down. You will need to change how the board is mounted so it can lean against the pen.
The pen's movement horizontally is the indication of bump steer, movement vertically is from the suspension up and down movement and movement in and out is a combination of bump steer and the arc of the wishbone movement as it moves up and down.

To test the layout, just move the steering a little and and watch the pen to make sure the movement is followed by the pen.
You will need a vertical line on the board to measure the horizontal distance of the pen trace to the vertical line. The change in this distance is the indication of bump steer. Note that the car should also be level as per the ride height dimensions.
Do update us with your progress and a picture of your setup.
Regards
Graham

[Tmac897]

That is interesting!
How many inches did yo go into bump and droop from the horizontal wishbones position?
It is possible , I suppose, that you've not got the sweet spot and the right amount of shins is somewhere else.

That was full droop to full compression. But as I mentioned above, I think my setup is wrong, or at least suboptimal. In one of his posts, Paul Adamson says the ruler should be “perpendicular to the car.” But while that might register some change, it’s not going to amplify the change. I’ve now seen some other posts, and even some other posts from Paul that indicate the ruler should be attached to the disc. That would make more sense since any “sudden” toe-in or toe-out movements would be magnified - similar to a pantograph. I’ll try that and post again.

That having been said, my lower arm and tie rod end are only a few degrees out of parallel without any shimming. So this might turn out to be just for peace of mind. I’ll post some pics of the new traces when I get the time to try this again.

[Tmac897]

Hi Tony,
If I am correct, your photo is looking vertically down. I believe your setup is incorrect and will not measure bump steer.
The pen should be 90 deg clockwise from where you have it (ie. inline with the steel rule and parallel to the floor) and the board should moved 90 deg cw and touching the end of the pen. The hinged board referred to in other descriptions is to allow the board to stay in contact with the pen as it moves horizontally, vertically as well as in and out as you move the suspension up and down. You will need to change how the board is mounted so it can lean against the pen.
The pen's movement horizontally is the indication of bump steer, movement vertically is from the suspension up and down movement and movement in and out is a combination of bump steer and the arc of the wishbone movement as it moves up and down.

Hi Graham, thanks for the clarifications, and I will give it another try. The board isn’t hinged but the setup allows it to lean against the tip of the pen as it travels, so while it may not be elegant, at least that part works.

Back to the garage!!

[baileyman]

I set mine so that it toes in immediately on droop, then I get a bit of near-neutral behavior in bump, which is where it seems to matter. John

[Andy8421]

But as I mentioned above, I think my setup is wrong, or at least suboptimal.

Tony, I am afraid your setup isn't suboptimal, its definitely wrong. As pointed out above, bumpsteer is the change in toe in / toe out as the suspension goes up and down, you are measuring the arc of the wishbones.

Think of bumpsteer as measuring exactly the same motion of the hub as turning the steering wheel, but in this case holding the steering wheel steady and making the hub 'steer' by raising and lowering the suspension.

This is why bump steer matters, its as if you turn the steering wheel a bit every time you hit a bump.

There are a number of youtube videos that explain this and demonstrate ways of measuring it.

Good luck.

Edit: I spent quite a bit of time looking at this a few years back. It is important that when you are measuring bumpsteer that the steering is straight ahead, and that the rack is centralised - the effective track rod length (from balljoint on the hub to balljoint on the rack) needs to be the same both sides. It is easy to get these different lengths when fitting the balljoints.

[snowyelan]

Hi Tony,
If I am correct, your photo is looking vertically down. I believe your setup is incorrect and will not measure bump steer.
The pen should be 90 deg clockwise from where you have it (ie. inline with the steel rule and parallel to the floor) and the board should moved 90 deg cw and touching the end of the pen. The hinged board referred to in other descriptions is to allow the board to stay in contact with the pen as it moves horizontally, vertically as well as in and out as you move the suspension up and down. You will need to change how the board is mounted so it can lean against the pen.
The pen's movement horizontally is the indication of bump steer, movement vertically is from the suspension up and down movement and movement in and out is a combination of bump steer and the arc of the wishbone movement as it moves up and down.

To test the layout, just move the steering a little and and watch the pen to make sure the movement is followed by the pen.
You will need a vertical line on the board to measure the horizontal distance of the pen trace to the vertical line. The change in this distance is the indication of bump steer. Note that the car should also be level as per the ride height dimensions.
Do update us with your progress and a picture of your setup.
Regards
Graham

If a vertical line is used as a reference ro setting toe to parallel/neutral it will give bump toe in. The reference line should be parallel to the turret/suspension upright inclination.

[Craven]

You are after a vertical plot of horizontal movement. X. vertical Y

[Foxie]

I agree with Graham (above ) the pen should be inline with the axle.

I adjusted my bump steer many years ago, using a laser pen. The pen was taped to a small stool facing the brake disc. A mirror was taped to the brake disc. A sheet of squared paper was mounted vertically beside the laser pen, on which the reflection of the beam was plotted, while the hub was jacked through its vertical travel, say every 10 cms. It is not necessary to calculate deflection angles, just change the shims to minimize the deflection.

[snowyelan]

I edited my above response to clarify it a bit. I agree with Graham with the exception of the reference line being vertical. The reason is the suspension travel is inclined from vertical. It's simple to plot by disconnecting the steering tie rod and plotting the line using the same method as Graham. Reconnect the tie rod and follow the same method. The difference between the lines is the toe change. Some basic trig and you can calculate the angle.

[Graham B]

Hi Scott,
I understand what you mean and had not considered inclination of the suspension travel.
Yes, the centre of the wheel will move for and aft as the car moves up and down on the suspension. So the reference line should be parallel to the vertical centre line of the suspension. Isn't this the Castor Angle which I think is 3 degrees on later Elans.
As you suggest, its just as easy to clamp the suspension in the straight ahead position and disconnect the steering arm ball joint to plot the reference line.
Regards
Graham

[ericbushby]

Hi All.
On that basis it would suggest that mine is wrong then. First it was jacked up with the chassis level.
I took the spring out and with the wheel off I clamped a piece of wood to the bottom wishbone to use as a handle.
A Laser pen was jammed into the hub centre with a kitchen sponge and the button held on with a clothes peg.
A piece of cardboard with a vertical line on it was propped up on the garage wall. The car came with no shims fitted.
As I raised and lowered the suspension I could actually see the hub and brake disc turning and the Laser traced a sloping line on the card. I fitted shims at alternate sides until the traces stayed on the vertical marks on the cards. It appeared to me that only when it was vertical were the wheels not turning slightly left or right when the suspension was compressed or extended.
I now wonder if that is not the correct state after all. Maybe there should be a little bit of bump steer built in.
It seems to run alright but I have nothing to compare it with.
Eric in Burnley
1967 S3SE DHC

[snowyelan]

I haven't found a solid answer from a traceable source on how much, if any, bump toe Lotus designed in. I CAD modelled up the front suspension a while back using the information I could find. The result was some toe, but a couple dimensions used were measured from my Elan and not from period component drawings. IIRC the ball joint height in the steering link and the trunnion to axle height were the 'as measured' dimensions.