On 24 May 2004 at 21:54, Randall Young wrote:
> It's called Ackerman steering, and the TR3 has it as well, so it does
> make a difference.
As usual, Randall is on the mark. Rudolph Ackerman developed this
scheme in 1810 (well before Daimler or Ford or anyone else
automotive!) to improve the steering of carriages. In its
theoretically complete form, a line through the axle of each wheel
passes through the center of the circle transcribed by the car as it
turns. Since the rear wheels don't steer (and assuming zero toe-in
at the rear), this means the car pivots around a point inline with
the rear axle. In practice this isn't ideal because it assumes no
slip angle for the tires. But if you assume slip angles, the
rotation point is moved forward, sometimes referred to as "modified
Ackerman steering". As Randall points out, in practice the slip
angles are somewhat ill-constrained because so many things can make
them bigger or smaller. Thus one wouldn't necessarily design for
theoretically correct geometry. In any case, the designers would be
expected to have chosen a geometry for slip angles expected from the
contemporary tire technology. Presumably they would also have taken
into account the weight transfer and camber changes expected at
maximum lateral G's. Then again, maybe not! They certainly wouldn't
have been as good at it as modern computer modelling allows.
Even so, any asymmetry in the system would mean asymmetric behavior.
And if one side gets better when it doesn't meet the original specs,
the other side should get worse. All this would become worse as you
cranked in more lock, but fortunately we tend to do our high G's at
high speed with a large turning radius rather than at full lock at
lower speeds. The only reason I mentioned it as an issue for the
Spitfire/GT6 is because it allows such severe lock, but then, one
doesn't usually try to pull high G's around a 24ft skidpad. :-)
Unless perhaps you own a tire factory.
> As I recall, Triumph Herald & derivatives implement
> "anti-Ackerman" steering ... don't know about the Spit & GT6.
Yow, I don't recall ever hearing the term anti-Ackerman. Would it
mean that the outer wheel turns in more, that toe-in increases as
your turn the steering wheel??? I've read the Spitfire/GT6 described
as modified Ackerman. At full lock it might be anti-Ackerman. Now
I'm curious. I'll have to go out to the garage and start measuring
things. (Thanks, Randall. I really needed one more thing to do! :-)
Don't anybody hold ye'r breath for me to post results though. Joe
C., help me out here! I'm sure you've done it already!)
--
Jim Muller
jimmuller@pop.rcn.com
'80 Spitfire, '70 GT6+
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