> My deductions,
> assumptions and half-remembered factoids are detailed below - please
> feel free to tear them to shreds and replace them with valid
> information!
All I've got are more half-remembered factoids ... <g>
> OK: first principles. Each wheel can either be (a) leaning in/out at
> the top; (b) pointing in/out at the front; (c) too far forward/backward;
> (d) too high/low; (e) too far out from/too far in towards the central
> axis of the car. Is this right? What, if anything, have I missed?
You left out the relationship of the steering axis (the imaginary line that
the hub rotates around as you turn the steering wheel) to vertical. There
are two components to this, the fore/aft component is called caster, the
lateral component is called steering axis inclination (aka king pin
inclination even though our cars don't have king pins). These angles are
non-zero to make the steering self-centering, and to improve stability at
speed.
There's also the Ackerman (sp?) angle, which is a measure of how much the
inner wheel turns (around the steering axis) compared to the outer wheel.
To avoid scrubbing the tires in turns, the inner wheel has to turn in
further than the outer wheel. This is designed in, it's rarely even
measured afterwards (except of course by those who are redesigning their
steering).
> (a) is called camber. It is a good thing for front wheels to lean out
> at the top, and for back wheels to lean in at the top.
Not necessarily. IMO it's best to start with the factory specifications
(which for the Spit is positive in the front and negative in the rear); but
many racers modify these angles for better
handling. As a gross simplification, ultimate grip is usually obtained when
the tire/wheel is perpendicular to the road. Usually (not always), running
negative camber in front allows the outside wheel to be more nearly
perpendicular in a hard turn, because the inside suspension is extended
while the outside suspension is compressed, thus putting the frame at an
angle to the road. Thus, when one first starts modifying the front
suspension geometry for better road handling, going to negative camber is a
common first step. However, as the tire contact patch moves away from the
steering axis, steering effort goes up, as does the force on the wheel
bearings.
> The amount of
> camber on a left wheel should be exactly equal to the amount of camber
> on the matching right wheel.
It's not critical that they be the same, and I believe sometimes they are
set unequal on purpose, for example to track straight on a crowned road or
for use on a circle track where all turns are in the same direction.
> Leaning in is called negative camber;
> leaning out is called positive camber.
Yes.
> (b) is called toe. It is a good thing for front wheels to point out at
> the front, and for rear wheels to point straight ahead.
Again, best to start with factory specs. I don't find a spec for the rear
wheels on a Spit offhand, but the front wheels should have 0 to 1/16" toe-in
according
to my factory manual. Some small toe-in is usually specified for the front
of most rear-wheel drive cars; I believe on the theory that the suspension
will deflect slightly at speed and that for maximum tire life and fuel
mileage, the wheels should be exactly parallel under operating conditions.
The rear wheels of IRS RWD cars may have a toe-out spec, for the same
reason; and FWD cars may be reversed.
A notable exception was the first generation Toyota MR2 Spyder, which
deliberately had the wheels not parallel, to give more rapid steering
response. One of the side effects was that the tires would only last
roughly 15,000 miles. With original tires at something like $150 each (and
unique to each corner), there were a lot of unhappy MR2 owners.
> The amount of
> toe on a left wheel should be exactly equal to the amount of toe on the
> matching right wheel.
Front toe is not measured separately, only the total amount with the
steering
centered. A differential change in toe is considered a steering centering
adjustment.
> Pointing out is called positive toe (or
> toe-out); pointing in is called negative toe (or toe-in).
That is exactly backwards to my understanding. Every manual I've seen
(including the Spit factory workshop manual) give toe-in as being positive
(and usually specify that it's toe-in).
> (c) is called ??? It is a good thing for each pair of wheels to be
> exactly the same.
I believe this is called "set back". It's only critical from the aspect
that any significant difference is likely to indicate damage at one or more
points (eg bent frame, cracked suspension link, etc.)
> (d) is called ??? It is a good thing for each pair of wheels to be
> exactly the same.
Ride height. Again not critical, except to the extent that it affects the
other aspects of suspension and steering geometry.
> (e) is called ??? It is a good thing for each pair of wheels to be
> exactly the same.
No special name that I know of, not a critical item.
> What are: (a) Caster, (b) SAI,
See above. SAI is an abbreviation for steering axis inclination.
> (c) Included Angle (appears to be the sum
> of Camber and SAI)
Yes. Significant mostly in that it's difficult to measure SAI directly,
it's easier to measure the camber and included angle, then difference them
to get SAI.
> and (d) Thrust Angle,
Thrust angle is the angle between the rear wheels and the axis of the car.
> and what do they tell us?
For most cars, most of these angles are fixed by the factory design and are
not normally changed. Toe-in is normally adjustable, camber and thrust
angle adjustments are fairly common, the others are less common. For the
fixed angles, measuring them gives an indication of other problems : bent
frame/body, sagging bushings, etc.
> Any information on this most confusing subject would be greatly
> appreciated!
Here's a web site I stumbled across :
http://gworobec.users.50megs.com/1110algn.html
There are several good books on the subject, one that was just recommended
to me by Michael Porter (Hi Mike : Still working on a reply to you) :
>> I greatly recommend _Sports Car
>> Chassis Design_ (as I recall the title) by Costin and Phipps. Costin is
>> the Cos in Cosworth, and Phipps was a Lotus chassis designer, ISTR.
> >1. Front Camber
> >The readings fall within the "specified range" according to their
> >computers, but the left wheel is at 2 deg 21 min, and the right wheel is
> >at 1 deg 58 min. Is this an acceptable variance?
I believe so, yes. I don't have the book to hand at the moment, but I
believe it allows something like 1/2 degree (30 min). Since you drive on
the left (?), this may even be a desirable variance, since most roads are
crowned (to help rain run off).
> >2. Caster
> >The readings are well below the "specified range" of 4 to 5 deg at 2
> >deg 36 min (Left) and 2 deg 53 min (Right). Why might this not have
> >been corrected, and what is the effect?
I don't believe the Spit has a caster adjustment, except perhaps to replace
all the suspension bushes. You might find that you have less centering
force, but with roughly 2.75 degrees I don't think it will be a problem.
The TR2/3 have 0 caster as I recall, TR4-6 around 3 degrees. My TR3 does
have a bit of a tendency to follow the road surface rather than a straight
line, but it's not a big problem.
> >4. SAI
> >I don't even know what this stands for, much less its significance.
> >Anyone? Again, my corrected readings fall within the specified range,
> >but the Left wheel has half a degree more SAI than the right. Is this
> >a problem?
Most likely not.
> >9. Rear Camber
> >I'm a bit concerned about this one. The specified range is -4 deg 45
> >mins to -2 deg 45 min. I've got +53 mins on the Left and +5 mins on
> >the right. Problem?
Not a serious one, IMO, but perhaps worth looking into. First step should
be independent verification, as I've seen a lot of shops that are sloppy
about measuring such things. Oddly enough, this seems to be especially true
with "computerized" equipment, the attitude seems to be that the computer is
so smart it doesn't matter if it's sensors are improperly attached, or the
other steps (proper loading in the car, suspension settled, etc.) are not
followed. Next step would be to look into ride height, as both front and
rear camber change as the suspension moves through it's travel.
> >10. Rear Toe
> >This too. The specified range is zero to 8 mins. I have -12 mins on
> >the left and +1 min on the right. Surely this means that my rear
> >wheels are driving the car crab-wise?
Yes, but 6 minutes (0.1 degrees) is a small angle, probably not enough to
worry about. I'm surprised the factory spec is that tight; makes me wonder
if perhaps someone (Mr. Haynes perhaps) read something wrong. I don't know
the wheelbase of a Spit offhand, but 0.1 degrees is only 0.2 inches at 10
feet.
> >
> >11. Total Rear Toe
> >This should apparently be somewhere between zero and +16 mins. I've
> >obviously got -11mins overall.
> >
> >12. Rear Thrust Angle
> >No specified range for this, but they have altered it from -6 mins to -7
> >mins. Is this good? Bad? Meaningless?
I believe both of these angles are too small to worry about. Even measuring
to this precision can be problematic, 6 minutes is less than .025" across a
13" wheel.
> >Having paid GBP40 plus VAT for the service, I'd like to understand what
> >I've got, and whether they should have done things differently.
A "wheel alignment" is normally limited to adjusting the adjustments
provided ... while I don't know what adjustments are provided on a 1500
Spit, your description sounds reasonable to me.
Do you really pay VAT on labor ? I didn't know that ... thought it was only
collected on goods. GBP40 is about USD64, that's a fairly typical price
over here but most states only collect tax on goods, not labor.
Randall
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