When you're pushing your XJ6/12 around the bends, and the
tires start to let go, does the front end or the back end
slide out first? I understand that heavy throttle can make
the back end come out around a bend, so I guess we'll assume
that you're at no or low throttle.

On my XJ6, the front end seems to slide out first, so the
car understeers. (I think that's what it's called!)
However, my tires are badly and unevenly worn, and my
alignment is out, so it's probably not a good benchmark.

The reason I am asking is because I plan on buying new
wheels and tires soon. Without any serious modifications, I
can get 1" wider wheel and tire on the back of the car.
However, if the car already understeers, that might not be a
good idea.

So far I have an upgraded front sway bar w/poly mounts and
end links, all new ball joints and tie rod ends, poly upper and
lower control arm bushings, poly steering rack bushings, and
new KYB shocks. The IRS is totally rebuilt. The car has stiffer King lowering springs. It also has an 5.7L LS1 and 4L80E transmission.

The wheels I am looking at are OE Jaguar patterns in 16x7
for the front and 16x8 for the rear. The tires are 225/55
and 245/50 respectively. I understand this will limit me in
rotating tires.

So, what does your car do? What wheels, tire, suspension,
etc do you have? Any thoughts?

 

Yes, under steer or push as the racers term it. It is gonna be hard to get to neutral
in your Jaguar or most any similar car. The manufacturers in their ultimate wisdom think under steer is safer for most drivers. Just back off a tad on the throttle and the car will correct. It is in the front weight bias and possibly in the geometry.

My near new 8 Tbird pushed noticeably. it's original wheels and tire swere a tad on the skinny side. when reshod with contemporary wheels from a Mustan and larger tires it got quite good at sharp corners.

Negative front camber reduces push. Some early dirt racers used exagerated camber on the right front. On left turn only. On right turns, hold o!!!

Over steer or loose as the racers term it is somewhat conducive to fast cornering, if in the hands of an expert. A tad more difficult to control.

Corvair and Beatles were somewhat notorious for over steer. hence different front to rear tire pressures recommended to ameliorate.

Carl

 

Yes, you're getting understeer and, yes, Carl is correct....it's intentionally designed into the car.

Adding a rear anti-roll bar is a good way to reduce understeer.


Cheers
DD

 

Ok, there is quite a bit going on here. First, get your alignment and tire situation figured out. Before doing any more mods you need a known zero. Next, even with modern cars, tire pressures are definitely different for front and back. When I autocross, it depends on the weather, the surface, and other variables as to what pressures I run front and back, be it in my MX-5, Triumphs, etc.

Now, how to control under and oversteer:
On a front drive car, if understeer begins to happen, you lift off and the weight transfer gets the front to bite. On a rear drive car, even a big one, you give it more throttle to get the back to rotate and tuck the front in. Too much and the back will break loose and begin to drift.

When I race I go into a turn and by the apex I'm rotating the car, and come out with a little opposite lock (where you steer into the drift) and adjust the attitude of the car with the throttle.

Honestly, to drive an XJ6 on twisty roads hard enough to need to correct for oversteer or induce massive understeer sounds a bit scary, and I've been racing all kinds of cars for almost 20 years.

 

I can tell you from experience bigger tires alone will make the problem worse. When you add larger tires the basic adhesion to the ground is no longer the primary limiting factor. Tire technology has come along way in 30 years. With larger tires and more traction the car will roll violently, then the rear trailing arm on the inside shortens and gives you rear steering that can be very unsettling and abrupt. You have to put on a rear anti sway bar to control the rear pitching and limit the amount of rear steering created by the trailing arms. Add a small stock sized rear sway bar to the rear and add 1 size up sway bar to the front and then put on some sticky tires and you will have one a great handling car that still rides like a jag.

 

OMG, I'd never think of pushing 'Old Betsy' to that limit. To impossible to replace, to brittle in so many areas. I drive her a bit hard but never, ever, anywhere near the limit.