Amici - With the recent string discussing weights and currently dealing with
a TR3 which understeers in hair pins and decreasing radius corners (right
turns being the worst) but is neutral and very controllable in high speed
corners, left or right, I took the opportunity at a recent event to explore
the weight distribution on the TR3 and using the data try to determine
whether I had a tire issue or there was something inherent with the weight
distribution that was contributing to the understeering problem.
First a bit of background. The car is a 1959 TR3A that I use as my daily
driver in the summer and it takes me a day to convert it to racing
configuration. The weights below are for the car in its racing configuration
but includes the battery in its stock position, heater, windshield wipers,
headlights and front and rear bumpers. The car has a complete TR6 front
suspension (2 deg. neg. camber, 3 deg. pos. castor, 1/32 toe-in), TR6 comp.
front coils and comp. rear leaf springs and a 7/8 anti-roll bar.
Static weight, ie no driver, but with 5 gals. of fuel is 2009 lbs.
LF - 564 RF - 518
LR - 462 LR - 465
Front wt. - 1082 53.9%
Rear wt. - 927 46.1%
Left wt. - 1026 51.1%
Right wt. - 983 48.9%
Right Cross wt. - 980 48.8%
I include these weights more for information than anything else since the
more relevant weights are with the driver and all his equipment in the car.
With the driver and equipment the total weight was 2221 lbs.
LF - 638 RF - 515
LR - 547 RR - 521
Front - 1153 51.9%
Rear - 1068 48.1%
Left - 1185 53.4%
Right -1036 46.7%
Left Cross wt. - 1159 52.2%
Right Cross wt. - 1062 47.8%
So what does this mean and how do I interpret it especially considering the
understeering problem that I'm dealing with. Well that's not easy. I'll
include a couple of quotes to see if that clarifies the issue - or maybe
not......
>From Per Schroedar, Grassroots Motorsport - " ....Left cross weight >50% the
car will likely understeer in a right hand turn because right rear tire
carries more load so the car drives off the right hand turn better. But in a
left turn the opposite occurs and the handling is worse. In almost all cases
the loss of corning performance in one direction is greater than the gain in
the other direction." Not sure that I completely understand this.....
>From Dennis Grant, Autocross to Win, Farnorthracing.com -
"Adjusting corner weights is one of those things that is part of your
baseline setup, but finding any sort of agreement on just how to set corner
weights is next to impossible. It seems everybody has their own favourite
formula and magic sequence.
I'm no different.
My method takes as a given that your car is, being production based, almost
certainly left side and front end heavy, thanks to the packaging of the
driver (offset left) and the engine/transmission (offset front). There are,
of course, exceptions, but for the majority of us, the physical distribution
of weight inside the vehicle is asymmetrical - and so then will be the
corner weights.
No amount of twiddling spring perches or cranking down on load bolts can
compensate for 200+ lbs of driver offset 10 inches left of the vehicle
centreline. Unless there is a corresponding mass in very nearly the same
location (in plan view) the corner weights will be offset left.
So then, the question becomes how to best minimize the effects of the
asymmetric weight distribution. If we have to live with it, at least we can
reduce the effects and keep things reasonably balanced.
Unless you only have left turns to deal with, you don't want to talk about
"cross weight" or "wedge". Those can be helpful in setting up a car that has
purposeful lateral static weight distribution bias because all turns are in
the same direction, but we don't play that game. It doesn't help at all that
most electronic racing scales sold in North America are intended for the
roundy-round guys and so will happily show you cross weight like it was the
magic ticket - it isn't.
Instead, I like to set up a car so that it has equal left weight front and
rear; that is, the portion of the left side weight on the front wheels is
the same as the portion on the rear wheels, by percentage. If the car has
52% front left weight, then it should have 52% rear left weight. Why? So
there's no diagonal bias and no static twisting moment. It seems to keep the
car more predictable."
OK, using Grant's thought process and his Corner Weight Calculator here's
what the TR3's corner weights would look like:
Take 23 lbs out of the left front and right rear and add 23 lbs to the right
front and left rear which would give you the following distribution -
Left Front - 615.2 Right Front - 537.8
Left Rear - 569.8 Right Rear - 498.2
Front Left - 53.34%
Rear Left - 53.37%
Left - 1185 53.35%
Right - 1036 46.65%
Left Cross wt. - 113.0 50.11%
Right Cross wt. - 1108 49.89%
Hmmmmm need to think about this.........
Barry.
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