Alignment Caster problem - getting -ve caster
10-04-2010, 04:04 AM
#16
Three Wheelin'
Cheers,
Mike
10-04-2010, 08:09 AM
#17
Three Wheelin'
Thread Starter
The camber plates allow the centre of the strut to moved rearwards by approximately 5mm from the normal position. I calculate this to be about 1 degree additional caster based on change of strut inclination - but could be wrong here. I did do some measurements with the camber plate in neutral vs full caster. I think the difference at around 20 degrees steering angle was about 0.3 reduction in camber "loss".
10-05-2010, 10:42 PM
#18
Three Wheelin'
I think Mikey is right on here, let me add a few thoughts:
I recently measured caster on my '86 851 (early offset) with a Smart Camber gauge, and I did get a bit of negative camber gain on the outside wheel when turned in: I have -2.3 static camber, and they were -2.5 and -2.7 when turned 20 degrees. But, I have an early offset car, and the KPI (more commonly referred to these days as steering axis inclination, or SAI) is less than on a late offset car, which yours must be if the "S2" in your screen name has any meaning.
In other words, when viewed from the front on a late offset car, the angle of the steering axis, or the imaginary line drawn from where the top of the strut pivots through the lower ball joint, is more extreme than on my early offset car. This causes the change in camber while the wheel is turned to be even greater (the spindle actually moves in an arc when viewed from the side; this is why if you turn the steering wheel from lock to lock when sitting still one side of the car lifts). Plus, you have -3 degrees of static camber set from your camber plates (my camber is set w/ the stock eccentrics), so the KPI/SAI is even greater still.
This also illustrates why McPherson strut cars used on track/auto-X need to run so much static negative camber when compared to double wishbone cars (double wishbone suspension gains camber when compressed, and yet camber changes a lot less when the wheel is turned due to the SAI being much less, making things more predictable). The RevoKnuckle system used on the Ford Focus RS is simply a halfway-house between a traditional strut front suspension and a full-on double wishbone arrangement: the upper pivot for the steering axis is on the bearing at the top of the knuckle instead of at the top of the strut.
As an aside, the main reason so many strut equipped FWD cars run so much KPI/SAI (resulting in negative scrub radius, which means that the imaginary KPI/SAI line intersects with the ground outside of the centerline of the tire contact patch, this is illustrated in the link Mikey provided) is for safety: if the left front tire were to blow out on a FWD car with POSITIVE scrub radius, the car would pull violently to the left. With negative scrub radius, the blown tire would tend to pull inward, the the car would continue to go relatively straight. Since the front end geometry on our cars is based on that of a FWD VW Golf, I would guess it is a legacy of that.
Eric, if I were you I'd max out the caster adjustment on the camber plates (I desperately want some, but my current class rules don't allow!) and on the factory adjustment, but make sure they are even side to side.
I recently measured caster on my '86 851 (early offset) with a Smart Camber gauge, and I did get a bit of negative camber gain on the outside wheel when turned in: I have -2.3 static camber, and they were -2.5 and -2.7 when turned 20 degrees. But, I have an early offset car, and the KPI (more commonly referred to these days as steering axis inclination, or SAI) is less than on a late offset car, which yours must be if the "S2" in your screen name has any meaning.
In other words, when viewed from the front on a late offset car, the angle of the steering axis, or the imaginary line drawn from where the top of the strut pivots through the lower ball joint, is more extreme than on my early offset car. This causes the change in camber while the wheel is turned to be even greater (the spindle actually moves in an arc when viewed from the side; this is why if you turn the steering wheel from lock to lock when sitting still one side of the car lifts). Plus, you have -3 degrees of static camber set from your camber plates (my camber is set w/ the stock eccentrics), so the KPI/SAI is even greater still.
This also illustrates why McPherson strut cars used on track/auto-X need to run so much static negative camber when compared to double wishbone cars (double wishbone suspension gains camber when compressed, and yet camber changes a lot less when the wheel is turned due to the SAI being much less, making things more predictable). The RevoKnuckle system used on the Ford Focus RS is simply a halfway-house between a traditional strut front suspension and a full-on double wishbone arrangement: the upper pivot for the steering axis is on the bearing at the top of the knuckle instead of at the top of the strut.
As an aside, the main reason so many strut equipped FWD cars run so much KPI/SAI (resulting in negative scrub radius, which means that the imaginary KPI/SAI line intersects with the ground outside of the centerline of the tire contact patch, this is illustrated in the link Mikey provided) is for safety: if the left front tire were to blow out on a FWD car with POSITIVE scrub radius, the car would pull violently to the left. With negative scrub radius, the blown tire would tend to pull inward, the the car would continue to go relatively straight. Since the front end geometry on our cars is based on that of a FWD VW Golf, I would guess it is a legacy of that.
Eric, if I were you I'd max out the caster adjustment on the camber plates (I desperately want some, but my current class rules don't allow!) and on the factory adjustment, but make sure they are even side to side.
10-06-2010, 08:46 AM
#22
Three Wheelin'
Thread Starter
Already have caster set to max. The GC plates allow about 5mm setback from centre for caster.
Another question - I have the camber eccentric bolts set to max camber, with the camber plates used to add another degree or so. If I were to dial back the camber eccentrics (say to zero) and compensate using the camber plates (to get -3 camber), would that make the SAI larger and hence further increase the camber loss?
10-06-2010, 05:08 PM
#23
Three Wheelin'
Thanks for the further explanation.
Already have caster set to max. The GC plates allow about 5mm setback from centre for caster.
Another question - I have the camber eccentric bolts set to max camber, with the camber plates used to add another degree or so. If I were to dial back the camber eccentrics (say to zero) and compensate using the camber plates (to get -3 camber), would that make the SAI larger and hence further increase the camber loss?
Already have caster set to max. The GC plates allow about 5mm setback from centre for caster.
Another question - I have the camber eccentric bolts set to max camber, with the camber plates used to add another degree or so. If I were to dial back the camber eccentrics (say to zero) and compensate using the camber plates (to get -3 camber), would that make the SAI larger and hence further increase the camber loss?
Absolutely correct. This is the main reason I went to camber plates - to give me the flexibility to adjust SAI independently of camber. At the moment I run minimum SAI. It's even possible to run the camber plates "reversed" to reduce SAI even further, should you have enough tyre clearance.
The effect on scrub radius (and hence steering feel) might be the defining factor, however.
10-07-2010, 06:25 AM
#24
Three Wheelin'
Thread Starter
Absolutely correct. This is the main reason I went to camber plates - to give me the flexibility to adjust SAI independently of camber. At the moment I run minimum SAI. It's even possible to run the camber plates "reversed" to reduce SAI even further, should you have enough tyre clearance.
The effect on scrub radius (and hence steering feel) might be the defining factor, however.
The effect on scrub radius (and hence steering feel) might be the defining factor, however.
10-07-2010, 06:40 PM
#26
Three Wheelin'
Spin the camber plates around so they give positive camber adjustment and correct at the strut to spindle eccentrics.
reduces SAI = less camber loss on lock.
Don't think I'll need to go there though, recent mods have reduced my dependence on turning the steering wheel - car is really well balanced.
reduces SAI = less camber loss on lock.
Don't think I'll need to go there though, recent mods have reduced my dependence on turning the steering wheel - car is really well balanced.
