Bump Steer - Adamson Method

[Craven]

Camber angle is a fixed constant the changes seen are from the inability of the double wishbone to track and keep a constant angle throughout its travel, when the target is parallel to the hub/disc the change in camber angle won’t be seen, OP first plot probably shows the change. Bump contribution will cause a change in the plotted X axis against a vertical straight line Y.
Personally I think the setup was with a small degree of Roll understeer much more controllable than Roll oversteer.

[snowyelan]

Hi Andy,

In general, front vs rear rack position doesn't dictate bump steer.

Front racks typically create a problem with steering ackerman in that they cause increasing toe in conditions when steering angles are introduced. I'm wondering if your book suggests bump toe out as a way to compensate for this effect?

Scott,

The effect was due to the compliance in the mounting of the wishbones. Under heavy braking, the hubs are 'pushed' toward the back of the car, while the rack remains in place. This has the effect of causing the wheels to toe in.

Andy.

Hi Andy,

Interesting idea. I suppose it comes down to the individual suspension design geometry, bushing compliance, etc. Which book was this from?

[snowyelan]

Hi Tony,

Its not specifically the castor angle, but the inclination of the suspension. On elans these angles are the same. If you look at my first post on page three in the link below the front chassis diagram shows the inclination.

viewtopic.php?t=45889&f=42&start=30#p346496

[Tmac897]

Hi Tony,

Its not specifically the castor angle, but the inclination of the suspension. On elans these angles are the same. If you look at my first post on page three in the link below the front chassis diagram shows the inclination.

viewtopic.php?t=45889&f=42&start=30#p346496

Scott,

Just for the record, my post references camber, not castor. I’m still not following your explanation, though. I’ll have to find your earlier post with the diagram(s) and see if the light bulb comes on. Thanks for your help.

[snowyelan]

Hi Tony,

My mistake on the castor vs camber.

I've been trying to come up with a better description but Andy explained it the best.

"because of the geometry of the wishbones and their attachment points to the chassis, the hub doesn't go up and down vertically, but at an angle to the vertical. If you adjust your bump to a vertical line, then because the hub itself doesn't track up and down vertically, you will be introducing bump steer."

The only thing that I can think of to add to this is the angled hub movement we are concerned with is as viewed from the side.

[Tmac897]

Scott,

I had some time this afternoon and I attached a pen to the upper steering arm. I see what you’re saying - the hub does track up and to the rear.

I’ll do some more testing to see if I can get closer to that line with the steering rack height.

Thanks for hanging in there.

[Tmac897]

I did a bit more testing today, using a method independent of the methods that we’ve been discussing. I set up a “plate” parallel to the hub, the measured distance on the forward and rear edges of the disc measured along a pair of attached framing squares.

I took measurements at stops along the extension of a jack stand. I’m currently out of jacks, as I have multiples holding up the frame at level so improvisation was required.

The theory here is if the distances between the hub and plate remained equal on a measurement by measurement basis, then my suspension members are not introducing bump steer. If you see a flaw in my logic, please don't hesitate to show me the error of my ways. (I went to Catholic School through 12th grade, so I’m certainly used to that!)

I took 7 measurements, in inches

Position Front Rear
Droop 13 13
1 12 5/8 12 5/8
2. 12 5/8 12 9/16
3. 12 5/8 12 5/8
4 12 9/16 12 9/16
5 13 13
Full Comp 13 9/32 13 9/32

The up and rear motion of the hub does not appear to alter my original results.

[Tmac897]

Here’s a picture of my setup. Looks a little medieval, but seemed to work.

[snowyelan]

Hi Tony,

I believe your method will work, but will be hard to measure accurately. You're working with a distance of 9-10" between the points of the squares.

[Tmac897]

Hi Tony,

I believe your method will work, but will be hard to measure accurately. You're working with a distance of 9-10" between the points of the squares.

I agree. Certainly it would benefit from having the reference plane further away and by using one of those laser measuring devices, if not cost prohibitive and the accuracy is adequate. And it is a bit more tedious than tracing a line with a single movement of the suspension - I wouldn’t want to do it 3 or 4 times each side.

All I wanted to do was verify that my shim settings were correct, so I only had to do it once each side. I had some free time and wanted to try out something. I was also thinking that since this method produced actual measurements, it could actually be used to calculate the required shim size, eliminating the trial and error process. I may try to do that at some point, as winter sets in here.

It’s been interesting and I’ve learned a lot about suspension geometry and adjustment.

Thanks for all your help along the way.

Best,
Tony