~ > Scott Fisher writes:
~ >The GT's springs are about 23%-25% stiffer than a roadster's springs,
~ >front and rear. ......
~ Since the GT's springs are stiffer than roadsters you will get a raised rear,
~ vrs the normal springs (unless you get the springs re-arched at a local spring
~ shop) due to the reduced weight of the roadster.
Oh, of course. On the other hand, if they came off a 20-year-old GT,
they've probably sagged an inch or two themselves. :-)
~ IMHO I don't believe this
~ is good for handling, (others may disagree).
It's probably not good for handling, but not necessarily for the reasons
you suggest:
~ It will roll more in the corners not less, do to the higher rear end.
If the springs are stiffer, it will ROLL less no matter how high you jack
the rear end. The spring rate determines the roll rate; if it takes 125
pounds to deflect the spring 1 inch, it doesn't matter whether that inch
is at stock ride height or four feet off the ground (and yes, I *have*
seen a mini-Bigfoot style 4x4 Midget). And if you put in springs that
take 125 pounds to deflect one inch, replacing springs that took 100 pounds
to deflect one inch, it'll still be 25% stiffer, no matter what the ride
height is.
Also, it's appallingly easy to lower a car on leaf springs; you put
in a spacer between the axle pad and the top of the leaves, and there
is a 1:1 relationship between spacer height and ride reduction. The
hardest part will probably be locating longer U-bolts of the right
dimensions to hold the axle to the locating pad; the rest can be done
with a bandsaw (or a hacksaw and lots of elbow grease) and some metal
stock of the appropriate thickness.
However:
Rear center of gravity and more important, rear suspension geometry will
be different from the original car. The B's rear suspension was designed
to provide progressive but mild roll understeer, by manipulating the
lengths of the spring sections between the forward spring eye and the
axle. On stock MGBs, the spring *compresses* slightly under roll --
that is, the outside wheel steers toward the inside of the turn, which
aids stability in high-speed corners (particularly in corners that
are being taken at a speed higher than the driver probably meant, which
is part of the "Safety Fast" philosophy :-).
If you re-arch the spring significantly, you'll very likely change this
to roll *oversteer* -- which is part of the problem with rubber-bumper
Bs -- meaning that as the car rolls, the back end is steered toward the
outside of the turn. This is an inherently unstable condition, and most
the instinctive reaction to it (brake and turn the wheel farther) is
exactly wrong for this condition.
Actually, for autocrossing use, this might be beneficial if it's done
with great restraint; most autocross cars are modified in the direction
of more oversteer, to make them more "pointable" in short-duration,
low-speed maneuvers.
But we're still left with rear CG being higher than stock (discounting
the issue of sagged springs). What this does is lengthen the virtual lever
between the center of the axle and the center of gravity over that axle.
This means that for similar deflection -- that is, if the car rolls at the
same angle -- there is greater leverage being applied to the roll couple,
meaning more weight transfer to the outside. Since Smith tells us that
weight transfer reduces overall grip on the affected pair of wheels, and
since this weight transfer in particular moves *off* the inner wheel and
overloads the outer tire (effectively doing the opposite of a sway bar),
it sounds particularly bad. (Also, the longer lever arm will make the
car *feel* more tippy, whether or not this affects the actual adhesion.)
In short: If you put GT springs in the rear of a roadster, be prepared
to lower the car to get it back down to stock ride height for a roadster,
or lower if possible.
~ For better handling you want the car lower to the ground not higher
Well, generally but not exclusively true. The main advantage is that
this shortens the virtual lever described above, as well as lowering
the CG everywhere. However, don't overlook geometry. On the B, for
example, the front suspension is designed to add roll understeer at
the front by adding positive camber to the outside wheel (that is,
tilting the top of the wheel OUT away from the vehicle centerline).
In the old days with skinny bias-ply tires, this meant that as the
car went into a turn too fast, chances were good that the natural
response of an inexperienced driver -- brake and turn the wheel more --
would not get him or her into trouble. The car would eventually slow
as the front end scrubbed off speed, and the driver would think he
was Stirling Moss (or she was Pat Moss, depending on which way the
driver's suit zipped up :-).
What this means for racers of MGBs, however, is that if all you do is
cut a couple of coils off the front, you end up with a car that is
already into the fat part of the camber curve. If you do this,
eventually you will lend the car to a codriver who is used to a different
suspension geometry, and this codriver will plow the car into the hillside
at the exit of Turn 7 at Sears Point (to use a purely hypothetical example,
certainly not anything that would have involved a couple of members of
this list who shall remain nameless... :-).
So while lowering can be a good thing, be sure you understand everything
you're doing before you start cutting coils or swapping springs. I've been
cautious and/or lucky, in that I got to learn about this from someone who
had made the changes after reading up on the subject; my old EP race car (is
it on the frame jig yet, John?) had already had the suspension modifications
done to it by a knowledgeable designer when I bought it, and he simply
explained it all to me.
~ You can get from most suppliers lowered springs, which lower the ride height,
~ or you can get the rear springs rearched (stiffer or normal your choice). Note
~ that if you do lower the ride height the front MUST BE DONE FIRST otherwise
~ the handling of the car will be upset.
The handling of the car will be "upset" if you do only the front, too.
The trick is to "upset" it in the direction you want. In most cases,
modifying the front suspension makes the car more stable to drive, while
modifying the rear can make it more responsive, which is inherently less
stable. Depending on the car in question, this may be the right choice.
More specifically, lowering only one end screws with the roll axis, which
is the imaginary line between the front and rear roll centers (geometric
ways of modeling how a car behaves). Roll axis is always something I have
to look up, and I'm at work and the books are at home, so I'll beg off on
this issue till I get a chance to re-read Carroll Smith.
~ Also note that lowered springs and
~ weak springs that have lowered themselves have vastly different effects.
Exactly so; see the discussion of CG versus geometry, above, for why.
In addition, sagged springs bring you closer to the bump stops without
raising the spring rate required to reach them.
~ As will all things there are limits, as lowering you car will mean that it is
~ closer to the pavemnet for things like speed bumps etc. Also care must be
~ taken not to foul the tires.
This is also a specific problem with MGBs, particularly at the rear,
where the wheel arch drops down over the top of the tire. Oddly, it
seems that older (1971 and earlier) MGBs can use wider tires at the
rear than newer (1972 and later) ones can. At one time, when I was
building my race car and was in contact with three other MGB racers
with about six cars between us, we noticed that early Bs can run 185-60
tires with no problem, and 195 to 205 series with some modifications
to the fender lip. Later Bs, however, rub with 185-series tires, and
need serious cutting or banging to use anything wider.
~ I've heard you can lower rubber bumper models 2-3" but as
~ I don't have one I don't know for sure. (I think I heard that there is a
~ couple of other things you have to do first)
The biggest problem with RBBs is that the body is raised up higher on
the front suspension crossmember; I can't remember now whether that's
simply by putting pads between the crossmember and the chassis legs
or whether it's a revised crossmember design (I've intentionally avoided
learning about post-75 MGBs to a large extent). So even if you put
in lower springs, you'll run into the positive camber problem long before
you get the ride height down to a reasonable level.
~ Also I have heard that nylatron (SP) or urethane (are these the same) shackle
~ bushes, hold the rear springs from side ways movement better than rubber, and
~ give better handling. Any comments, or users ?
I like the nylatron bushings I've got at the rear of my MGB. They do
restrain the Abingdon Side-Step somewhat -- you know, that tendency for
the rear axle to take a short jog to one side or the other if you clip
a bump as the car is heeled over in a corner. I've got them at the
rear shackles as well as in the spring pads, and they don't make the
car that much harsher than it already is. I will probably put them in
the front when I rebush the inner A-arm mountings, which I need to do
on at least one side. Note that apparently the MGB's rear leaf springs,
once properly located at any rate, are stiff enough laterally that they
do not work well with a Panhard rod; Phil Ethier reports an MGB autocrosser
who has had Panhard rod mounting points rip out of the chassis because
the lateral motion they require (visualize the geometry) is greater than
that permitted by the B's rear springs, and something has to give.
>From what I read when I purchased mine, nylatron is stiffer than urethane
but doesn't squeak or bind as much due to its construction. Nylatron is
a nylon bushing material impregnated with molybdenum disulfide, a dry
lubricant commonly added to gearboxes (it's the moly in Moly-Kote, for
example). I also have urethane bushings in my uprated front anti-roll
bar (what is it again, 3/4" I think, up from 5/8") and they did squeak
till we added some grease to them.
--Scott "Time to fix this car and DRIVE it instead of talking about it" Fisher
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