Increasing steering axis inclination on '88 front suspension

[Series8217]

I'd like to increase the self-aligning torque on the front of my '88 with power steering. I'd like it to recenter a bit better, and require a little more force to move off center. I've already maxed out the caster that's available with the factory adjustment. Should I modify the control arms to keep adding more caster? What's "too much caster"?

I thought about increasing the steering axis inclination on the front knuckles by cutting back the hub mounting face at an angle and then changing the control arm length. This would also decrease the scrub radius when using wheels with too low of an offset.

However, there are two other important features that are not so easy to change the angle of it: the bolt holes, and the hub bore. If I recall correctly, the hub bore is a slip fit for the bearing. If it is, I assume it is important for carrying loads. If not, I could just clearance the bore where necessary. For the bolt holes, a self-aligning washer is a possible solution.

[The Dark Side of Will]

Build the race control arms we've mentioned elsewhere...

I've been thinking about building a custom knuckle.

Basically, it's doable with a 1/2" steel plate laser or water jet cut. Bolt-on blocks could mount the ball joints and be shimmable for infinitely adjustable kingpin angle. Bolt-on steering arms would allow adjustment of bump-steer, ackerman and arm length in one setup.

I think I've seen ready-made blocks somewhere, but they shouldn't be hard to make once one acquires a tapered reamer:

http://www.travers.com/m/sku-list.asp?r ... 3+id+30204

[fieroguru]

You can extend the slots in the upper control arm mount at the crossmember, but you also have to clearance the dogbone nut retainer as well, but this only gets you about 3/16".

You could also replace the stock cross shaft with a rectangular aluminum block with two bolts for the bushings at the end. The rectangular block would allow you to shift the mounting bolts for the cross shaft rear ward to be centered in the current slots and allow you to adjust in more caster.

I do like the idea of machining the bearing mount surface at a slight angle as well.

[The Dark Side of Will]

I do like the idea of machining the bearing mount surface at a slight angle as well.

I don't think that's a good course of action.

A) Making the bolt holes and hub bore not perpendicular to the mounting face is just bad form

B) If you have the machine tooling to do that, you have the machine tooling to make the modified inner pivot block.

[fieroguru]

I do like the idea of machining the bearing mount surface at a slight angle as well.

I don't think that's a good course of action.

A) Making the bolt holes and hub bore not perpendicular to the mounting face is just bad form

B) If you have the machine tooling to do that, you have the machine tooling to make the modified inner pivot block.

I thought the concept was interesting and something I hadn't heard of before. You could use your misalignment washers on the bearing, and boring the bearing hole after facing off the upright wouldn't take much to do either. The rectangular cross shaft would be much easier and is probably the direction I will take on my car.

[The Dark Side of Will]

I haven't looked at the fit of the hub journal in the knuckle bore, but obviously if you bore it, that fit goes away.

[Series8217]

According to Bloozberry's data on the other forum, the '88 front suspension has a kingpin angle of 5.45 deg.

I need to re-measure my caster but I think it was less than 6. I need to double-check.

For reference, a 987 Boxster has front caster fixed at a little under 8 degrees. My Bentley manual for the E46 lists ~5.5 for standard cars and 7.5 for the M3. I can't find a spec for the kingpin angle for any of these vehicles, but being strut cars they are all probably at least 12 degrees.

The Honda S2000 runs an SAI of ~7 deg, and caster around 6.

More caster is needed with high SAI to negate the positive camber gain from that geometry. Strut cars seem to be screwed.

Hopefully enough caster for a good steering feel is also enough to let me run 1 deg or less static camber up front; I could use a larger contact patch for braking.. I've only got 215s up front, and stock brake bias.

[fieroguru]

I haven't looked at the fit of the hub journal in the knuckle bore, but obviously if you bore it, that fit goes away.

You can sleeve it to close the gap.

[Series8217]

I haven't looked at the fit of the hub journal in the knuckle bore, but obviously if you bore it, that fit goes away.

You can sleeve it to close the gap.

Good idea.

After cleaning up the bore, a sleeve could be pressed in with some retaining compound before any further machining. Then, install the knuckle on an angled fixture in a mill, cut the hub mounting surface angle, and machine out the ID of the sleeve at the same angle, then hone it to final dimensions. Should be trivial on a CNC mill, besides maybe the fixture.

I'll check my spare knuckle and bearing tonight to see if the bearing cartridge fits tightly in the knuckle or not, to see if it would even be necessary.

That just leaves the bolt holes..

[fieroguru]

The bearing to upright is fit it pretty close, but isn't a press fit. I think they leave some room for the bearing assy to expand slightly as it warms up during use.

A single mounting fixture to locate the upright at the desired angle, then face mill the surface. Using the same fixture and setup, bore the bearing hole over size. Now the sleeve... I would suggest finding some tubing material with close the right ID as the OD of the bearing and a wall thickness in the 1/8" to 3/16" range. Once you cut the ring for the sleeve, then cut it along its circumference. This slit will allow the ring's ID to be adjusted down to the needed size for the bearing. Knowing the wall thickness and bearing OD, you can make sure to bore the upright to the proper dimension. Once the hole is bored, slide the sleeve in place. It will be rather snug due to making its ID smaller, but if you wanted you could put a couple of tack welds on the back to hold it to the upright (upright is ductile iron and can be welded under the proper conditions).

For the bolts, the misalignment washers, or while the upright is in the fixture, install the bearing with 2 of the 3 bolts and spot face the missing bolt. Swap the bolts and do it again for the other 3.

Single setup with the right fixture and a basic vertical mill...

[The Dark Side of Will]

Wouldn't just building the fully adjustable control arm be less fussy and yield a better (fully adjustable) result?

//

At zero slip angle, the center of traction of a tire is at the geometric center of the contact patch. That is, it's 0.5 of the length of the contact patch from the front edge.

As the slip angle increases to the point at which the rear edge of the contact patch is as deformed as it can be without starting to slide (essentially maximum traction), the center of traction is 0.707 of the length from the front edge.

As the slip angle progresses until the tire is in a complete slide, the center of traction returns approximately to the geometric center.

The center of traction of the tire is the point at which the lateral loads on the tire act against the steering. Knowing this about tire behavior and enough about the suspension geometry, you can figure out what the lever arm acting on the steering axis is.

The caster TRAIL (different than caster angle) can be used to adjust that lever arm such that the change in effort going beyond maximum traction is very apparent to the driver.
Making this adjustment such that you can still feel this change in effort may be more difficult with the power steering that you've installed...

[fieroguru]

Wouldn't just building the fully adjustable control arm be less fussy and yield a better (fully adjustable) result?

It would be, but then it stands out as you go through tech.

I think machining the upright has merit for keeping the car looking bone stock and avoiding a class upgrade in some racing series. Plus it could be done for under $20 in parts if you have the needed machining equipment (which I do). The rectangular block cross shaft is about the same cost and less work, but would be an obvious component change.

[Series8217]

Wouldn't just building the fully adjustable control arm be less fussy and yield a better (fully adjustable) result?

A fully adjustable control arm can adjust camber and caster, but not kingpin angle / steering axis inclination & scrub radius. So comparing it to the knuckle modification is comparing apples to oranges... unless adding a few degrees of caster is enough.

[The Dark Side of Will]

True... hadn't thought about it that way, but yes, it can't adjust kingpin or SAI without simultaneously adjusting camber and caster.

Just make some assembled knuckles from scratch along with the fully adjustable upper control arm.

[Series8217]

Another thing I just realized for the knuckle mod is that it would require machining the caliper mounting ears at an angle as well, and dealing with those bolt holes.

[fieroguru]

The caliper mounting will be tricky. With stock sized rotors you will probably need to weld the current bolt holes shut and drill/tap them at the proper angle (and surface the mounting boss for the sliders at the proper angle as well. Or if you are running a big brake kit you can use the caliper brackets to make the angle change.

[Series8217]

I put my '88 Fiero up on the laser alignment rack at Lang Racing Development in Irvine over the weekend.

Front caster is 9 deg per side. I didn't know the stock suspension would go that high.

The laser system also measured the SAI, but I can't remember the specs off the top of my head. I will check it again tonight. I think it was 6 degrees though.

[The Dark Side of Will]

sweeet. Actual caster induced camber gain.

[Series8217]

From my other thread, these are the most aggressive front alignment settings possible on stock '88 Fiero front suspension with a 14-inch ride height (fender arch to hub center):

Code: Select all

Front           Left       Right
-----
Camber          -0.9       -0.5
Caster           8.9        9.2
SAI              6.6        6.0
Toe              0.02       0.01

These were measured using a Hunter 4-wheel laser alignment system at Lang Racing Development in Irvine, CA.

[The Dark Side of Will]

I have a design in my head for a fully adjustable knuckle with shim adjustments for steering offset, stering axis inclination and Ackerman geometry...

Because of my desire to completely rework the 84-87 front geometry in The Mule, it will be a while before I design anything for that car.

However, I could design a set to replicate the stock '88 geometry easily. Would you like the prototypes?