| Re: [GMCnet] Wheel alignment specs [message #106697] |
Thu, 25 November 2010 06:17  |
Gary Casey
Messages: 448
Registered: September 2009
Karma:
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Senior Member |
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I've never heard of the desired castor angle being different for a front wheel
drive vehicle. The castor angle is defined as the angle from vertical of the
steering pivot. In our case it is, in the side view, the angle of a line
between the upper and lower ball joints, and when the line is tilt rearward at
the top it is considered positive. The purpose of a positive castor angle is to
increase the centering effect with side loading, like in a turn. After all,
even in a front wheel drive vehicle the wheels are only "pushing" forward part
of the time - during braking they are "pulling" back. I don't think that has
any real effect.
And there is another dimension we don't talk about, but two-wheeler designers
do, and that is "trail". This is the distance in a side view between the pivot
line extended to the ground and a vertical line extended down from the axle
centerline. This also influences the centering behavior, but in a slightly
different way. We don't talk about it because it is built into the geometry of
the knuckle and it can't be changed. A shopping cart front wheel has zero
castor angle(the pivot is vertical), but a lot of trail. A bicycle has a lot of
castor angle and also a lot of trail. I'm guessing that our knuckles have the
ball joint centers lined up with the axle, but I don't know that for a fact. If
that is the case, if the castor was 0 the trail would also be 0. When the
castor angle is increased the trail also increases.
And then there is the effective trail caused by the tire. When loaded sideways,
the effective resistance to that load isn't at the center of the tire patch
(directly below the axle) like one might think, but is in the back of the
contact patch, something like 2/3 to 3/4 of the way back. So even with 0 castor
and 0 trail a tire will tend to track. A higher tire pressure shortens the
contact patch, reducing this effective trail, reducing understeer and making the
vehicle less stable.
All this is a discussion that could go on forever - and some make a career out
of the study of suspension geometry. The above statements are facts as I know
them - but my facts aren't always true :-)
Have a great turkey day!
Gary Casey
Robert Mueller wrote on Wed, 24 November 2010 18:13
> If you are running radial tires Dave Lenzi (alignment guru) recommends:
>
> Caster: as much as you can get up to 5 degrees
> Camber: 0 degrees
> Toe In/Out: 0
From George:
In the front, I go with Dave Lenzi, too, but could not get that much Caster. (
I don't have his re engineered A-arms.) Just to be clear, that is Positive
(like a motor cycle) Caster. Which makes sense, because the wheels are pulling
the coach.
A shopping cart has negative caster when you push it, because the front wheels
are trailing. They flip around naturally when they trail. Our front wheels
don't trail, they pull. We don't want our wheels "trying" to flip around when
they pull.
I have a new bicycle with almost no positive caster. Pain in the neck... I
cannot ride with no hands. Can't push it by the seat. Squirrelly as all get
out.
--
'74 Eleganza, SE, Howell + EBL
Best Wishes,
George
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