quote Originally posted by theogre: Polyurethane, aside from its well documented squeaking issues actually wears considerably. Polyurethane operates as a bearing in the control arms. Dry bearings wear. Fast. If you are squeaking you are wearing but it isn't always that obvious as dry ploy doesn't always squalk, especially when it is already worn. Polyurethane is a bloody poor bearing material to start with. Adding graphite and other compounds to the material don't really change that much.

any dry on dry sliding surface will wear quickly compared to a flexing joint or a lubricated one

this is why I put grease fittings on my poly bushings.. yes I could go harder with delrin but don't feel the need to -

as for what the rubber bushings obsorb.. it touches the tires first.. the tires obsorb almost everything that is small - but as you go to high pressures and lower profile tires they obsorb less..

quote Originally posted by Pyrthian: how about a ball joint that just plain old doesnt rotate, except thru adjustment?

Such a ball joint exists..??..??

quote Originally posted by Yellow-88: Such a ball joint exists..??..??

lol - nope. but, you see where we need to go with this. we need a joint, that will rotate ONLY in the up/down plane, hold the knuckle securely, and that joint need to be adjustable in the left/right plane,maybe by a 2nd joint, while keeping the knuckle square under the struts/coils.

quote Originally posted by Kohburn: .. it would need to be a dual axis like a universal joint that isn't allowed to turn..

This is a fascinating concept, but is really just a bi-axial joint, with one axis in the plane we are trying to eliminate. It also doesn’t address the kingpin axis.

thanks howard...

quick .mov about bumpsteer

damn I suck at the internet

[This message has been edited by Jncomutt (edited 03-14-2005).]

quote Originally posted by Pyrthian: we need a joint, that will rotate ONLY in the up/down plane, hold the knuckle securely, and that joint need to be adjustable in the left/right plane,maybe by a 2nd joint, while keeping the knuckle square under the struts/coils.

We have addressed this. Will’s "H-arm" does that. Mine is basically that with one of the lateral legs adjustable. But essential to any design, is the need for the inboard end to be parallel to the chassis centerline.

quote Originally posted by Yellow-88: This is a fascinating concept, but is really just a bi-axial joint, with one axis in the plane we are trying to eliminate. It also doesn’t address the kingpin axis.

yes, but that doesnt exist in the rear, does it? the front & rear bumpsteer, tho caused by the same thing, are slightly different. the front bumpsteer is damn near impossible to fully eliminate, due to that fact we need steering. but in the rear, we dont need steering, just some adjustable range. there is no upper balljoint for the kingpin axis. there is the top of the struts, but being so far away, it doesnt play any real role.

hmmm . . . so many good sounding ideas, and not a single picture!

when you say kingpin angle, is that mostly the same concern as scrub radius?

im wondering, because i noticed with my 88 (understand that we are talking about the early though) i assumed that the force of acceleration was going to cause toe-in. and toe out with braking. i believe i can prove i was wrong. i developed a severely worn joint in my front toe link. under moderate accell, i could feel the wheel "flapping" in and out, instead of being "pinned" in as i expected. i believe this was because the scrub was near perfect on the rear wheel. to me this meant that the front bar (?) was taking all of the forward motion of accel. this would also mean that the to links and inboard pivots take little force except when cornering. if this is a proper conclusion, the kingpin/scrub in the rear is very important, and "torque steer" could be mostly eliminated without very hard bushings. the 88 design fixed bumpsteer too did it not?

quote Originally posted by Pyrthian: yes, but that doesnt exist in the rear, does it? the front & rear bumpsteer, tho caused by the same thing, are slightly different. the front bumpsteer is damn near impossible to fully eliminate, due to that fact we need steering. but in the rear, we dont need steering, just some adjustable range. there is no upper balljoint for the kingpin axis. there is the top of the struts, but being so far away, it doesnt play any real role.

I opened this thread to address FRONT bump steer correction. That can be corrected by relocating the steering rack. The rear of the pre-88 is where we have evolved to.

Kingpin inclination axis is a term that does not necessarily refer to real king pins or real steering. It is the line drawn between the lower “ball” joint and the upper “ball” joint. That line exists in any suspension system, and must be addressed. In the case of the Chapman strut, the top of the strut is the upper ball joint.

quote Originally posted by Yellow-88: I opened this thread to address FRONT bump steer correction. That can be corrected by relocating the steering rack. The rear of the pre-88 is where we have evolved to.

sorry about continuing the hijack, just got "into it" your rack relocation has much interest. if you snail mail me some reglar old photos, i will post em for you! it would be the least i could do.

quote Originally posted by ricreatr: sorry about continuing the hijack, just got "into it" your rack relocation has much interest. if you snail mail me some reglar old photos, i will post em for you! it would be the least i could do.

Thank you very much for the offer. It has never occurred to me until now to document this kind of thing, but it is now clear that I may have been withholding valuable info for too long now. I do hope to show some of this at the NEFA, Kick Hill Farm swap meet this year, so in preparing for that I will make an effort to do a photo series.

In the mean time….look at how your rack is mounted and imagine that instead of the fixed weld-nuts allowing only one location, the bolting system allowed the rack to be positioned slightly above or below is present position. Moving it as little as Ľ” will have a dramatic effect on bump steer.

With regard to the early rear suspension...

The angle that interacts poorly with the ball joints arcs is caster not kingpin angle. The strut must be perpendicular in side view to the control arm inner pivot axis, and the inner pivot axis must be parallel in top view to the centerline of the car in order for an H-arm to work. I need to take some careful measurements, but this may not be easy to do alongside getting rid fo the pro-squat geometry.

My design will also include a toe adjustment in the H-arm.

quote Originally posted by Will: With regard to the early rear suspension... The angle that interacts poorly with the ball joints arcs is caster not kingpin angle. The strut must be perpendicular in side view to the control arm inner pivot axis, and the inner pivot axis must be parallel in top view to the centerline of the car in order for an H-arm to work. I need to take some careful measurements, but this may not be easy to do alongside getting rid fo the pro-squat geometry. My design will also include a toe adjustment in the H-arm.

Yes. Caster is the side view of that line. Kingpin inclination is the front view.

Accurate measurements really are very difficult. One of the reasons I’m adding an upper control arm is to be sure about how to handle the necessary side view perpendicularity that you so rightly mention. A coil-over will be compliant mounted above the upper arm. I can’t see how pro-squat can be done with the Chapman strut, because some change in caster is needed do that.

Your H-arm sounds a lot like mine. I’m using rod ends outboard just to accommodate the toe adjustment. I suppose poly would handle it though. How are you addressing the inboard parallel issue..??

Initially I plan on build a bolt- or weld-in bracket that will relocate the rear control arm pivot. After that... we'll see.

The pro-squat is an artifact of the fact that the rear suspension was originally designed as a front suspension. Anti-dive in the front end application turns into pro-squat in the rear application. My pivot relocation bracket will space the rear pivot outward as well as lowering it to eliminate pro-squat. Obviously this has to work in concert with the caster angle of the strut so that the strut stays perpendicular to the pivot in side view, but I have some ideas on that as well. The '88 hub carrier has the strut boss offset from the axle centerline. I think they could be swapped left for right and work reasonably well... but I'll have to wait until I get my engine back together and my car back on the road to take the measurements I need.

quote Originally posted by Will: Initially I plan on build a bolt- or weld-in bracket that will relocate the rear control arm pivot. After that... we'll see. The pro-squat is an artifact of the fact that the rear suspension was originally designed as a front suspension. Anti-dive in the front end application turns into pro-squat in the rear application. My pivot relocation bracket will space the rear pivot outward as well as lowering it to eliminate pro-squat. Obviously this has to work in concert with the caster angle of the strut so that the strut stays perpendicular to the pivot in side view, but I have some ideas on that as well. The '88 hub carrier has the strut boss offset from the axle centerline. I think they could be swapped left for right and work reasonably well... but I'll have to wait until I get my engine back together and my car back on the road to take the measurements I need.

Look at the side view instant center of the suspension. If that's behind the axle CL or forward of it but below ground level, then you have pro-squat. If the caster of the suspension was retained when it was moved from front to rear, then the SVIC will be behind the axle CL. I need to take measurements to see exactly what the situation is.

To graphically locate side view instant center, extend the control arm inner pivot axis. Where it intersects--in side view--a perpendicular drawn from the strut upper pivot point is the side view instant center. For SLA, extend the inner pivot axes of both control arms. The side view instant center is where they intersect (in side view, of course).

quote Originally posted by Will: Look at the side view instant center of the suspension. If that's behind the axle CL or forward of it but below ground level, then you have pro-squat. If the caster of the suspension was retained when it was moved from front to rear, then the SVIC will be behind the axle CL. I need to take measurements to see exactly what the situation is. To graphically locate side view instant center, extend the control arm inner pivot axis. Where it intersects--in side view--a perpendicular drawn from the strut upper pivot point is the side view instant center. For SLA, extend the inner pivot axes of both control arms. The side view instant center is where they intersect (in side view, of course).

So the term “pro squat” means…anti squat geometry is present..??
Mesuring a live car acurately is very difficult. The pre-88 control arm inner pivot axis appears to be parallel to the ground, or at least to the cradle bottom. There appears to be a bit of positive caster, so that places the SVIC behind the axle CL. Correct me if I’m wrong, but as I recall, the instant center of a strut system is always above ground.

The strut is limited for a couple of reasons, but if you can keep it, you will save a lot of work in fabrication.
Just losing the ball joint, and getting a rear end that always points straight ahead, will make a much better Fiero. I do wonder about your H-arm length. I was able to get 12.5” by using a pre-88 cradle, moving the front pivot inboard, and moving the rear pivot outboard. I’d like to see 14” for the lower arm, but the engine mount gets in the way.

looking from the side.. if the top pivot of the strut is behind the centerline of the lower ablljoint travel then the force of braking will make it try to extend the strut.. if its infront of that line then braking will try to compress the strut..

now isn't squat the rear end wanting to dip while braking.. like pro dive in the rear and anti-dive upfront to kep the car level while braking which in theory is great but adds more complicated forces when trying to setup a track car

quote Originally posted by Yellow-88: So the term “pro squat” means…anti squat geometry is present..?? Mesuring a live car acurately is very difficult. The pre-88 control arm inner pivot axis appears to be parallel to the ground, or at least to the cradle bottom. There appears to be a bit of positive caster, so that places the SVIC behind the axle CL. Correct me if I’m wrong, but as I recall, the instant center of a strut system is always above ground. I do wonder about your H-arm length.

pro-squat is exactly the opposite of anti-squat... acceleration loads on the rear suspension tend to compress it.
Instant center of a strut system can be pretty much anywhere if you screw it up bad enough...
My H-arm will be almost as long as the stock control arm.
Shorter arms improve camber performance, which the early chassis could use as long as the stock upper strut pivot is retained.
I will also make the static roll center height adjustable via multiple locations for inner and outer pivots...

quote Originally posted by Kohburn: looking from the side.. if the top pivot of the strut is behind the centerline of the lower ablljoint travel then the force of braking will make it try to extend the strut.. if its infront of that line then braking will try to compress the strut.. now isn't squat the rear end wanting to dip while braking.. like pro dive in the rear and anti-dive upfront to kep the car level while braking which in theory is great but adds more complicated forces when trying to setup a track car

Your logic wrt the strut is backwards.
Squat is the car pitching up under acceleration. Anti-squat reduces the change in pitch attitude, pro-squat exacerbates it.
Anti-dive at either end reduces the tendency of the car to pitch down under braking. Same with anti-squat. Fiero can not have any front anti-squat because it does not transmit drive torque to the front tires.

quote Originally posted by Will: Your logic wrt the strut is backwards. Squat is the car pitching up under acceleration. Anti-squat reduces the change in pitch attitude, pro-squat exacerbates it. Anti-dive at either end reduces the tendency of the car to pitch down under braking. Same with anti-squat. Fiero can not have any front anti-squat because it does not transmit drive torque to the front tires.

AH - sqaut as in under acceleration -- I was thinking under braking.. gotcha
the squat then would be controlled by the angle of the inboard swing arm pivots being angled from parrallel to the ground correct?

pro-squat would be preffered for a FWD setup to keep the weight from transferring off the drive wheels - and would be potentially usefull for a drag specific RWD car - (or would it cause excess wheel hop on a FWD car?)

[This message has been edited by Kohburn (edited 03-16-2005).]

Pro-squat UNLOADS the tires because it tries to make the suspension pull them off the ground.
Can't change weight transfer with suspension settings... that's a function of acceleration, wheel base and CG height. Anti-squat and anti-dive just manage chassis motion resulting from weight transfer.

Instant Center, and roll center.
I can’t come up with any scenario that puts an instant center below ground. Angling the 88 struts more inboard at the top did’nt move it relative to the ground, but did raise the rear roll center. On the pre-88 I figured RC roughly 3.75” above the ground, and the 88 is roughly 4.5” above, both with 25.5” tires. I don’t have good numbers for the pre-88 front, but the 88 front RC is about 5” above ground with 25” tires. I see why they changed the rear strut angle.

Camber Change.
I haven’t plotted the 87 rear with shorter control arms, but the stock numbers are: At 5 deg roll, camber change is .4 deg negative. The stock 88 does .75 deg negative at 5 deg roll.

I’ll bow out of attempting to figure out anti-squat, pro-squat with a Chapman strut. I’ve abandoned the strut in favor of double A-arm, or H-arm A-arm system. That way I can do basic IRS design with out having to deal with the strut.

could someone tell us how much and what direction the strut mounts were moved for 88?
thanks

Generaly, because we've evolved to this point...check this out.
http://www.exoticreplicacars.com/html/chassis/upgrades.shtml

inboard about 0.75" per side and forward about 1/4"

You can compare chassis straightening measurements from '87 and '88 shop manuals to figure this out...

will comes through again.

top of struts are closer together by 74mm (37 per side) hair under 1.5"

forward move is not so easy to figure. they show a diagonal measurement. in fact there are two diagnal measurements, one indicates the 88 slightly (three mm?) forward and the other slightly back. must be a missprint. if the drawings are to scale, they move forward slightly. to boot, they only give two sides of the triangle. even if i was a math wizz, i dont think it is possible to figure it.

allowing my ignorance to shine . . . i am surprised that even the big measurement would matter.

quote Originally posted by ricreatr: will comes through again. top of struts are closer together by 74mm (37 per side) hair under 1.5" forward move is not so easy to figure. they show a diagonal measurement. in fact there are two diagnal measurements, one indicates the 88 slightly (three mm?) forward and the other slightly back. must be a missprint. if the drawings are to scale, they move forward slightly. to boot, they only give two sides of the triangle. even if i was a math wizz, i dont think it is possible to figure it. allowing my ignorance to shine . . . i am surprised that even the big measurement would matter.

you can figure the 3rd side of the triangle if you know one of the angles..

'Yellow-88'
I have mounted a pwr rack with adjustable mounting slots (vertical slots). My car is no longer much of a fiero, (2" front width added, coilovers w/ tubular A arms, 4"/side rears). I have completed this setup (pwr steering) but have not driven it yet (car is waiting on 'warmer'weather). My question is: Is there a 'sweet spot' to setup/lineup my rack (to get decent results for 'bumpsteer') before going to the alignment shop? Or do I just have to try different settings?" I guess I should add that the previous (Fiero)rack was extended same length, but was using existing Fiero mounts on said suspension. In other words, should my rack be placed just above the running gear (when on the ground), to 'toe out' when hitting a bump? I have read that 'toe-out is better than toe-in'?
suggestions welcome,
Sandy

quote Originally posted by cowans: 'Yellow-88' I have mounted a pwr rack with adjustable mounting slots (vertical slots). My car is no longer much of a fiero, (2" front width added, coilovers w/ tubular A arms, 4"/side rears). I have completed this setup (pwr steering) but have not driven it yet (car is waiting on 'warmer'weather). My question is: Is there a 'sweet spot' to setup/lineup my rack (to get decent results for 'bumpsteer') before going to the alignment shop? Or do I just have to try different settings?" I guess I should add that the previous (Fiero)rack was extended same length, but was using existing Fiero mounts on said suspension. In other words, should my rack be placed just above the running gear (when on the ground), to 'toe out' when hitting a bump? I have read that 'toe-out is better than toe-in'? suggestions welcome, Sandy

There is only one ideal position for the rack that produces a zero bump steer condition, and that condition will only happen over part of the control arms travel. That “sweet spot” should be positioned at normal ride height. With a change as big as the one your doing, you need to do pretty much everything yourself, because caster and camber will affect bump steer. You have in effect, started from scratch, and have nothing to go on. This is a reprint of how I measure bump steer.

To measure bump steer, you will need to remove the springs, and reassemble the suspension with out the tires. The car should be on jack stands in a way that allows you to move the control arms through their full range. You will need a reference line on the floor, parallel to the chassis centerline, at the tires contact point. You will also need a piece of square steel tubing clamped to the brake rotor, parallel to the floor. Bump steer will be visible by sighting down, so that relative parallelism between the steel tube, and the reference line on the floor is clear. By moving the control arms up and down, any bump steer will be seen. The extreme limits are not too important, but the point near normal ride is critical.

This another of my threads that might help you.
https://www.fiero.nl/forum/Forum2/HTML/060836.html

quote Originally posted by ricreatr: will comes through again. top of struts are closer together by 74mm (37 per side) hair under 1.5" forward move is not so easy to figure. they show a diagonal measurement. in fact there are two diagnal measurements, one indicates the 88 slightly (three mm?) forward and the other slightly back. must be a missprint. if the drawings are to scale, they move forward slightly. to boot, they only give two sides of the triangle. even if i was a math wizz, i dont think it is possible to figure it. allowing my ignorance to shine . . . i am surprised that even the big measurement would matter.

I agree for the most part:
'87 manual: Forwardmost of three strut mounting holes is 354mm from reference point N and 1,104mm from the corresponding hole on the other side.

'88 manual: Forwardmost of three strut mounting holes is 359mm from reference point N and 1,030mm from the corresponding hole on the other side.

Since the 354 and 359 dimensions are not parallel to the car's long axis, the fore/aft difference is less than 5mm... so the '88 strut tops were moved back 4 to 5mm relative to the early cars'.
1,104 - 1,030 = 74mm

The two long diagonal measurements don't conflict enough to worry about.
1298 - 1259 = 39mm difference, of which 37 come from the top of that strut moving closer to the reference... in reality the car's at least 5mm off in any given dimension so don't worry about it...

quote Originally posted by Yellow-88: There is only one ideal position for the rack that produces a zero bump steer condition, and that condition will only happen over part of the control arms travel. That “sweet spot” should be positioned at normal ride height. With a change as big as the one your doing, you need to do pretty much everything yourself, because caster and camber will affect bump steer. You have in effect, started from scratch, and have nothing to go on. This is a reprint of how I measure bump steer. To measure bump steer, you will need to remove the springs, and reassemble the suspension with out the tires. The car should be on jack stands in a way that allows you to move the control arms through their full range. You will need a reference line on the floor, parallel to the chassis centerline, at the tires contact point. You will also need a piece of square steel tubing clamped to the brake rotor, parallel to the floor. Bump steer will be visible by sighting down, so that relative parallelism between the steel tube, and the reference line on the floor is clear. By moving the control arms up and down, any bump steer will be seen. The extreme limits are not too important, but the point near normal ride is critical. This another of my threads that might help you. https://www.fiero.nl/forum/Forum2/HTML/060836.html

Thank you, I will set that up, measure out, and adjust.... + for you!
Sandy

I have an 86 that is in the process of being lowered (by balljoints and springs). Does anyone know what dimensions that shims would need to be for lifting the rack up to try and make up for the lowering?

I can machine different thickness shims at work, but I haven't looked at the rack to get sizes.

Anyone know what shape/size to make them?

Thanks in advance

I finished looking at my 86GT (lowered with balljoints and one coil off in the front).

As I shimmed, the bumpsteer got worse, so I think that maybe the lowering on this will be fine. I wish I had tested it stock to conpare though. I c-clamped a laser pen to my rotor, and measured the distance it changed with various shimmed placed under the rack. I don't know how the 88 is set up, but the 86 is at a 45 degree angle. I "could" cut the rack brackets off and weld up a new, adjustable bracket that could be dialed in.

hmm....

I will have to see how it handles. I might just do that.

Yellow88, are you still going to bring any of this to the Kick Hill Farms swap meet? It is going on this Saturday, August 20, 2005. I would be very interested in seeing what you have done so far.

[This message has been edited by avengador1 (edited 08-16-2005).]