932 series suspension tuning

[Colin Edwards]

Hi Johnl,
Although different in design to the 147, installation of eccentric poly bushes to my 159 upper control arms has been very successful.  Their is enough compliance in the polyurethane to cope with the off axis alignment when setting the respective eccentrics.  This plus the grease supplied wth the bush kit eliminates any stiction.  Very simple to dial in camber and caster adjustments.  A very cost effective mod that improves the cars' ability to use the tyres more efficiently.  Had them in the car for around 3 years without any dramas whatsoever.

[johnl]

Hi Colin,
I'm pleased this is working for you.

You say; "Their is enough compliance in the polyurethane to cope with the off axis alignment when setting the respective eccentrics." Which suggests these bushes are not the harder 'racing' grade?

I suspect that softer grades of poly may have enough give that some misalignment isn't a significant problem. My experience with poly bushes has been with harder 'racing' grade plastic. I suspect my misgivings would likely be valid for such harder polyurethane, but I could possibly be wrong, and it may also be very case specific.

What I have experienced with hard poly 'D' bushes (for ARB mounting), is that only a very small amount of 'crush' between the bush and the ARB causes a great deal of friction, and in a fairly short amount of time the grease squeezes out and causes the bush to become noisy ('creaky' / 'groany').

Regards,
John.

[Divano Veloce]

I had some time last night to investigate. It looks like the front sub-frame is a replacement spare part item, along with the lower engine mount rubber as they have spare parts type labels on them. I removed some of the sub-frame bolts and it looks like there's some scope for moving the sub-frame forward and to the drivers side but to get the caster back in spec it suspect it would need some slotting. I would like to get it back in spec and maybe a bit more, see if this improves the super light and un-communicative steering that is now the main cause of my discontent with the car. As the steering rack moves with the sub-frame, and that the steering arms are fairly low down on the spindles, what sort of bump steer impact would moving the sub-frame have compared to moving the strut tops?

[Citroënbender]

Some stuff seems to be ex-inventory on all 147s, look at the label date if it can be read.

[Divano Veloce]

CB, I cant find a date but there is "Aust" on the label which might be a reference to Australia. The label is pretty deteriorated from exposure.... I couldnt see any evidence of repair or damage underneath the car.

[johnl]

I removed some of the sub-frame bolts and it looks like there's some scope for moving the sub-frame forward and to the drivers side but to get the caster back in spec it suspect it would need some slotting. I would like to get it back in spec and maybe a bit more, see if this improves the super light and un-communicative steering that is now the main cause of my discontent with the car.

DV,
the lower sub-frame bolts insert into significantly long tubes (in the frame) that do not lend themselves to being 'slotted'. I don't think you'll achieve any significant caster adjustment increase by such a means, let alone any caster increase worth having....

If you were extremely enthusisatic it might be possible to move all of the tubes in the sub-frame, i.e. cut the tubes out, elongate the resulting holes in the sheet metal, weld the tubes back in (with gussets to cover where the holes had been slotted). Of course this would be a big project, requiring the sub-frame to be removed, possibly requiring the rack to be reloacted on the sub-frame, plus other unknown etceteras....

Of course this is assuming it is even possible to move the subframe forward by moving the mounting tubes in the frame, it may not be practically possible at all...

As the steering rack moves with the sub-frame, and that the steering arms are fairly low down on the spindles, what sort of bump steer impact would moving the sub-frame have compared to moving the strut tops?

I don't think that moving the subframe forward would directly affect bump steer (?). There may well be some indirect affect caused by any consequent change in caster angle.

An increase change in bump steer will occur with any change to the caster angle. This is so whether the change in caster has been created by moving the upper ball joint longitudinally, or the lower ball joint. For example, if caster were to be increased (by whatever means), then the end of the steering arm (in side view) will become lower to the ground. This changes the horizontal angle of the steering tie-rods (in front view), which become angled more downward from the rack end to the steering arm.

As a result of this change to the static tie-rod angle, the outer end of the tie-rod will now move (in bump or rebound) through a different arc relative to the lower and upper ball joints, than it previously did. In this example of increased caster angle, this change in relative arcs as seen in front view will cause the front wheels to gain more toe-in with 'bump' motion and more toe-out with 'droop' motion. As the chassis leans with body roll the outside wheel goes into bump and gains toe-in, the inside wheel goes into 'droop' and gains toe-out. This means that with body roll both front wheels dynamically 'toe' toward the cornering direction, so there is a 'roll oversteer' moment introduced, i.e. the front wheels 'auto-steer' in the direction of the corner, more than can be accounted for by the angle of the steering wheel, and it is all rather 'non-linear'...

This may be very minor, or it may be problematic. When I increased the caster angle on my CB7 Accord (more than doubled it, by moving the lower ball joint about 3cm forward), there was a significant increase in roll oversteer. This manifested as the chassis turning into the corner very well, then as roll motion occurred (even though roll motion was small due to large ARBs and stiffer springs / dampers) it would turn in more than the driver had intended, so the driver had to back off the steering as the car approached mid corner. At first this characteristic was entertaining, but the novelty soon wore thin.

I fixed this problem by lowering the Accords' steering rack, but I can't see this being feasible with the 147...

Regards,
John

[Divano Veloce]

Thanks John for another detailed response to my predicament..!
Another option to correct caster may be to move the lower control arm mounts about on the subframe.

I notice on the UK forum that the "sports kit" suspension alignment settings are more like -1.6 deg camber and 4.3 deg caster. Is this the result of lowering springs or does the sports suspension have different components to achieve these alignments?

First thing though is to verify the caster. I don't trust the results from the alignment shop as the camber alignment is way out side to side. John, can you tell me more about your caster measuring system?

[warsch]

I believe that's because when you lower 156 that results in more negative camber (and may be caster too). I put -40mm suspension on my car and camber is visibly different without any adjustment. Not that you can adjust it anyway. Sportpack suspension is lowered too so I assume that's why factory settings differ just to reflect that.

[johnl]

Thanks John for another detailed response to my predicament..!
Another option to correct caster may be to move the lower control arm mounts about on the subframe.

DV,
Having given this some thought myself, I doubt that it's practical to try and achieve significant caster change at the lower subframe. Yes, it is possible to make a small change by moving one side of the subframe forward and the other backward.

You might also get a mm or two  by loosening the bolts holding the lower wisbones and then levering the wishbones backward or forward while re-tightening the bolts.

All up, moving the subframe and the wishbones on the subframe, I would guess you might move the lower ball joints longitudinally by maybe 3 or 4mm, certainly not a lot.

I think that slotting the upper holes in the 'strut' tower offers more scope for changing caster, and will achieve X caster change with less collateral affect on wheelbase length. This is because the upper ball joint is significantly farther from the hub centre than is the lower ball joint, so moving the upper ball joint longitudinally by X has less affect on wheelbase than moving the lower ball joint by X. This is what I did, i.e. slotted the four holes in one of the towers with a rats tail file, elongating them by 5mm to increase caster by 0.5° to equalise caster side to side.

At the other tower I slotted the holes laterally (10mm), to change camber by 1°, in order to equalise the front camber. I would avoid slotting any individual tower hole both longitudinally and laterally, which I expect might weaken the tower top excessively. This is why I have adjusted only caster at one tower and only camber at the other tower, in both cases just equalise.

I notice on the UK forum that the "sports kit" suspension alignment settings are more like -1.6 deg camber and 4.3 deg caster. Is this the result of lowering springs or does the sports suspension have different components to achieve these alignments?

My understanding is that the nominal caster angle for the 'normal' 147 is 3° (plus / minus 30'). I don't know for sure where the "sport kit" (whatever that is) finds another 1.3°. It might have some special parts, but I tend to doubt it.

I do know that this suspension design must gain additional caster with bump motion. This is because the upper wishbone pivot axis is not parallel with the chassis centreline (actually, not parallel to the lower wishbone pivot axis, which happens to be parallel to the chassis centreline). Because the upper wishbone pivot axis is at a substantial angle to the centerline, the upper ball joint moves in an arc that causes it to move not only laterally (affecting camber) but also longitudinally (affecting caster). The arc of motion of the lower ball joint differs because the lower wishbone pivot axis is parallel to the chassis centreline, the lower ball joint moves in an arc that only causes the ball joint to move laterally (and not longitudinally).

This (arc of the upper ball joint having some longitudinal moevement, and the lower ball joint not having any longitudinal movement) means that lowering the ride height will cause an increase in caster as well as camber, but I don't know by how much per unit of lowering (though I could figure it out if my life depended on it...).

First thing though is to verify the caster. I don't trust the results from the alignment shop as the camber alignment is way out side to side. John, can you tell me more about your caster measuring system?

The thing with commercial alignmnent rigs, in my understanding, FWIW, is that they don't measure caster directly, rather they infer it from measuring the camber changes that occur when the steering is turned through a specific range of wheel angle (road wheels turned X° to the left and to the right). If the operator is very careful and knows what he / she is doing, and the rig is in good condition and properly calibrated then all should be reasonably accurate. If one of these things is not happening, then there is plenty of scope for the caster 'measurement' to be less than accurate...

My 'caster gauge' is very simple, but a bit hard to describe. Look at the wheel in side view. You'll see the upper ball joint above the height of the top of the tyre, and the lower ball joint through the spokes in the wheel. Now at each ball joint, can you see the ball joint post thread protruding through the nut? Keep these threads in mind when reading the following.

My 'gauge' consists of a square section tube that is at least as long as the distance between the threads in the upper and lower ball joints (in this case my tube is 520mm). Welded to each end of the square tube is an arm made from a flat piece of steel (25 X 2.5mm), one arm is 215mm long and the other is 150mm long. Each of these arms is at 90° to the tube axis. At the end of each arm is a semi circular notch (made with a rat tail file). To muse the jig, these two notches are each pushed up against the threads in the ends of the ball joint posts, so that the square tube is to the outside of the wheels, with one arm protruding through the wheel spokes and the other arm passing above the top of the tyre.

The notches at the end of each arm fit closely around the threads at each ball joint post, and should be a good match for the OD of each thread, with the lower ball joint thread being of larger diameter. The jig is held and pushed firmly against the ball joint post threads, and a digital inclimometer is held against a forward or rearward face of the square tube. An angle can then be read from this. Practice required to get repeatable measurements.

The lower arm is shorter and the upper arm is longer because the lower ball joint is farther outboard than the upper ball joint, so with the lower arm being shorter the square tube will be near vertical outside the car. Care must be taken to ensure that the axis of the two notches matches quite closely to the axis of the tube, otherwise the tool won't measure quite the same when measuring left side vs right side caster.

Regards,
John.