Silly question, but I always thought the primary reason for castor was to
make the wheels pull back straight when moving and not just running all over
the road. Like a bicycle or motorcycle with the inclined forks causing the
front wheel to point straight. Any angling of the handlebars ends up raising
the bike, thus gravity is the force keeping the wheels aligned in the
direction of motion. Without that positive feedback steering is pretty
strange. For a while I had a summer job driving a small factory transport
with no incline on the steering. It was designed to go forward or backward
with equal facility. You really had to watch that thing, it would run off
course in a millisecond. Then overcorrecting on recovery was really
difficult to manage.
Mark Hooper
72 TR6
-----Original Message-----
From: Jim Muller [mailto:jimmuller@pop.mail.rcn.net]
Sent: Monday, August 27, 2001 4:50 PM
To: triumphs@autox.team.net
Subject: Re: Suspension Questions Again (longish reply)
snip...
Okay, so what does castor do and why might you change it? Primarily,
positive castor causes camber to change as you turn the steering wheel. The
outside wheel which takes most of the cornering load goes more negative, and
the inside wheel goes more positive, causing both to lean into the turn.
This helps keep the tread on the road, compensating for both tread tuck-
under and body roll. (This isn't the only camber effect, of course.)
Another effect of castor depends on your steering radius. Imagine the
steering axis projected downward toward the road surface, and then imagine a
line perpendicular from this axis to the contact patch center. This is the
steering radius and is the moment arm by which the steering manipulates the
tire/road contact. With positive castor, steering makes the ouside wheel
rise w.r.t. the car, and the inside wheel fall. This exacerbates body roll
slightly. It also transfers weight from the outer-front and inner-rear
tires to the opposite corners, which may or may not be good depending on
other factors. The larger the steering radius, the more pronounced this
effect is (and also the more steering kickback you get), so on most cars the
steering axis is tilted inward at the top to short the radius.
[Note, an interesting benefit of trunnions over lower ball joints is that by
threading them to the hub you can cause vertical movement to happen in the
"right" direction when the car is steered. Also note that steering radius
is affected by tire height and offset. More offset or lower tire height
increases the radius, thereby increasing steering effort and also the side-
pull you experience when you lose traction with one wheel while braking.]
So is slight negative castor bad? Not really. It simply means that you may
not be getting the best possible camber behavior while cornering. If the
body didn't roll and the tires didn't tuck under, then camber changes
wouldn't be necessary anyway. Low values have very little effect, and "big"
castor values would be upwards of 5 to 10 degrees. In any case, it won't
affect your braking and won't induce pulling. Its only impact on your tires
may be a slight increase in wear on the outside of your tread while
cornering hatd. If you dial in a bit more negative camber you can trade
inside for outside. So unless you are seriously into competition or you
have a seriously worn suspension mount, you needn't worry.
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