On Wed, 1 Jul 1998, Lars G. Johnsen wrote:
> Bob,
Hi (again).
> Thanks a lot for a long and informed reply. Will check up the vendors, I
> have catalogues and adresses. In the meantime, some comments and some
> more questions :-)
Oh-oh.
> > And be careful that you don't make things so low that
> > your roll center is below the ground!
>
> What happens actually when the roll center is below the ground? However,
> it appears that the best position for the roll center is above the car,
> then the car would lean into the turn, or maybe I have got everything
> backwards...
You know, I don't have an answer for you. At least I don't have a
qualified answer... I don't have Mr. Puhn's treatise handy right now, but
I'll take a WAG and say that this will cause the car to handle poorly.
For instance on the front of the car, a roll center below ground level
will probably cause the car to push rather severely when it tries to turn.
Or it could cause a wheel to lift from the ground... I really don't know.
But that's a wild guess. I'm new at this stuff to, although I think I'm
starting to grasp some of the concepts. But every now and then, I try
some maneuver in my car and the car simply does not do what it should
do... then I start wondering if the driver is all there...
> > The easiest way to lower a car is to go with lower profile tires. With a TR6
> > and 225/45 tires, you lower the car by something like 1.5 inches. That's a
> > lot, believe me. You didn't say how you intend to use your car, but in the
> > 'States, using Hoosier DOT radials 225/45-15 will get you into the ballpark.
> > I happen to use 225/55-15's and they lower the car a lot too, though not as
> > much as the 45's.
>
> In Norway those tires would be illegal on my car. We are only allowed to
> put on tires that are within +/- 5% of the standard roll diameter, which
> in practice means that 205/65-15, or equivalent, is the smallest I can
> put on it. Strange, really, given that the PI-TR6 came with 165-15 and
> if my car where a PI then I could legally lower the car even further.
> 205/60-15 seem to be widespread on the English PIs (the only difference
> between the Carb and the PI seem to be the final ratio, the chassis and
> geometry should be the same). Right now I have a new set of Nokian NRT2
> T-marked 205/70-15 on it. Pretty tall, but still a little bit less than
> stock. The 225/55-15 are 7% smaller, well outside my (legal) options.
Bummer. Have you considered moving? I'll bet there's a lot of cars in the
US that would be illegal by those standards. I'll have to make it a
point not to bring my car to Norway for a tour sometime...
;-)
Does the MOT equivalent actually check that the tires are legal on every car?
> >.... For the
> > ultimate in adjustability, you can saw off the ends of the upper a-arms and
> > weld in some threaded inserts and use heims joints to give you even more
> > adjustment.
>
> Think I pass on that one! :-)
Yes, this would be for a competition only vehicle, as I imagine the heims
joints would not last very long in standard driving situations. And they
are pretty expensive.
> > Now the last word is: if you start playing with the suspension geometry, be
> > prepared to spend a lot of time sorting things out. Give the folks that
> > initially designed the things some credit - they most likely did a lot of
> > work to get what's there.
>
> Just wondered if my car is the way the engineers really designed it, or
> if it has been altered to meet bumper ride height and things like that.
> From my memory I am sure to have read somewhere (maybe it was
> Kimberley's TR6 book) that the TR6 at some point where raised in the
> front for that reason. If that is the case, there should be an optimal
> way of setting up the front suspension, and still keep the car the way
> it was originally intended (i.e. before government legislations changed
> the set up of the car).
Well, actually I wanted to comment on this. It turns out that the
vertical links on a lot of TRs seem to get bent such that you _can't_
dial in camber to go "negative". I'll bet that if this is the case for
your car, that you could put a big pipe on the spindle (with the wheel,
hub bearings and brakes all removed) and "reshape" the vertical link.
The tough part about bending the vertical link is that if you don't bend
it right, you could really introduce some wierdness into the front
suspension/steering. You see it should be possible to prescribe a
straight line from the upper ball joint through the lower trunnion and
that line _should_ continue down and bisect the tire contact patch. I've
seen cars that don't do this. I'll bet they are a "bitch" to drive.
I'm not sure how safe it would be to re-bend the link, but it turns out
that if you take a heavy shot on the tire/rim from the side, then that
link will bend. A friend "rolled" his TR6 one time and when we tried
putting it back together, the car had some serious positve camber. Turns
out that the vertical link was bent such that the car could only get
positive camber.
> Anyway, I got the car back from alignment shop, and it has now 0.30 deg
> positive camber (unladen) on both sides. Would have liked that number on
> the other side of zero though. But if I am to go through the front with
> new springs etc, I can wait to have it fixed.
Keep in mind that the distance between the lower a-arm and the upper
a-arm can vary with the compressed spring height. So, if you put a
shorter spring in there, you can get more negative camber - to a point. In
my car, simply leaving out the rubber spring pads got me from 0 degrees
of camber to around .75 degrees negative - I then installed one extra
shim on the lower a-arms and presto - negative 1 degree of camber. WHich
is "real good" with the radial tires that I use. I'd actually like
negative 1.5 camber if only just to try it. I'd have to put in more shims
to get that, and in my car, frankly I don't want to add more shims at the
bottom. So, on car #2, I'll put the threaded hiems joints (eventually) to
get some serious camber.
> Best, Lars
> 74 TR6 - soon to be taken on summer holiday trip around Norway.
Alright. Time to drive it!
I'm also soon to go on holiday with mine. looks like another 3500 mile -
or more - trip in the TR! Yeah!
rml
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