Hullo. Steve writes:
>If Jim Muller's out there, I expect to see another of your excellent
>technical descriptions of how I screwed up some alignment principle.
I'm disinclined to back down from a challenge like that! :-)
It's nice to feel needed again so soon. Also several more :-)'s.
>A few weeks ago, Phil Ethier suggested putting 165 tires on a midget...
>[instead of the stock 145, but] I decided to put 155's on instead.
We're talking 80-series (what used to be called 78-series), no? Perhaps
a better choice would have been to go to 165 but in a 70-series, which
would have given you approximately the same diameter with a wider tread.
The trouble with just going up to 155 with the same aspect ratio is that
it gives you wider tread and larger diameter too. But that would have
broken your bank account even worse, and so probably wasn't an option.
Still, the intersting problem is what follows:
>I put the 155's on the rear and then drove a few miles (20-40) with
>155's on the back and 145's on the front. During that time, I could
>hardly keep the car on the road at speeds higher than 35-40mph. The
>slightest change in direction made me feel as if the rear end was
>coming out around me. The next day, I was able to put the new 155's
>on the front and all seemed well again.
>So the puzzle - what do you figure was going on? Were alignment and
>center of gravity really affected enough by such a small change as that?
This was a stock mid-to late model Midget, not the Bugeye with the
quarter-elliptic springs, right? So we can rule out some bizaare rear-
axle behavior, maybe, except that this *is* a Midget we are talking about!
:-) (Yeah, I know, I have no room to talk about rear-end behavior if I
drive a Spitfire...)
I don't have any tire tables in front of me just now, but I figure that
change may have pitched the car forward by perhaps 1/4 degree max. A
forward pitch increase should have made the rear axle more stable. Any
body roll to the outside of a turn would now cause the outside of the
rear axle to move forward and the inside to move backward, giving some
rear-axle steer toward the inside. I wouldn't have guessed 1/4 deg. to
be enough to matter anyway, but since this is opposite to what happened,
we can rule it out.
What about the front? With the same front tires, there should have been
no first-order change to the steering radius or any other contact-patch to
steering-axis relationship. The effective caster would have decreased,
but I'd guess it would have been a small fraction; what is the Midget's
caster, maybe 5-10 degrees? In any case, caster has the effect of making
the tires lean into a turn (negative on the outside, positive on the
inside), so pitching the car forward should have reduced, not increased
the front tires' grip, thus encouraging understeer rather than the
oversteer you described. (Unless, of course, you are already running
so much negative camber that you are losing tread-road contact, so that
pitching the car forward helped!) And anyway, you might expect to see
that much change in the pitch just by loading or unloading the trunk
(though admittedly that would make the rear go down, not up) and it never
upsets handling that much to load the rear. Have you ever taken the
spare out of the trunk, for, say, an autocross? That should both shift
the weight bias forward and raise the rear, but I've never seen any
admonishment against driving without the spare in the trunk! The roll-
stiffness and roll-center height at each end would have been largely
unaffected too, so front/rear roll balance should not change.
That leaves us with some tire factor. Normally I'd guess you did the
right thing to put the bigger tires on the rear, but there are several
other possibilities. We don't know what kind of tires these were, so
it's possible that the new rears just don't grip as well (run with
higher slip angles), maybe because of a harder tread material or more-
flexible sidewalls, or perhaps they just had lower inflation pressures.
More likely though, you may have been experiencing some transient that
resulted from the fact that the 155's are taller, i.e. have more sidewall
flex even if they are nominally the same tire. The net affect is that
the rear has more lateral play. In a turn, you'll feel the rear seeming
to be looser, even if the final maximum traction condition has more grip.
(The rear, especially the outside, should certainly be loaded less, and
understeer would have been the design target anyway.)
So I'd have to vote for some tire factor, probably the transient behavior
of the taller tire giving you a seemingly looser rear end. Of course, if
you *did* change the aspect ratio too, i.e. went to 70-series 155's, all
this would be different. The car would have pitched backward and many of
the geometry factors would have been different.
Enough BS'ing.
Go out and drive or get some work done!
Jim Muller
(It's nice to be back.)
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