DeTomaso Mailing List: April 2001, Message #41

There are several factors that affect maximum backspacing on our Pantera's.
If you plan to run less than 11" wheels, focus on your frontspacing and
ensure the tire sits in the wheelwell in the manner most aesthetically
pleasing to you while ensuring it won't hit the fender when the suspension
compresses. You are probably fine on the backside.  Also recognize that
changes in wheel offset impact your suspension tuning as your are
effectively shifting the fulcrum on the lever (tire)

These factors impact your optimal backspacing on your rear wheels.  There
are probably other issues - this is what I have learned.

1)  The brake system being used.  For example, Wilwood hats push the wheel
further outward than stock.  Allows more backspace.
2)  The diameter of the wheel being used.  For example, 18" wheels clear the
lower A-arm to Hub carrier nuts and can thus accommodate more backspace.
3)  The location of the muffler strap mount on the inner frame.  ( This is
different from car to car - At least on Fred Hall's and mine) Fred's
accommodates more backspace than mine.  My muffler mount lines up with the
inner frame rail.
4)  The width of the tire relative to rim size.  Less backspace available
for a 335 vs. 315 on an 11" rim.
5)  Brand of tire being utilized.  Dunlop's 335 section width is less than
competitors' 335s.  Fred Hall's 335's protrude 3/4" on each side of an 11"
rim; while, Mike D's Dunlop's protrude less than .5 inch.
6)  Whether or not you are running stock or aftermarket extended or
adjustable upper A-arms.  Aftermarket allows more backspace.
7)  The degree of negative camber that you are running.  More negative
camber equates to less available backspace.
8)  Tire diameter - Larger tires move their maximum section width further
out from the center of the wheel increasing the risk of hitting the
Pantera's inner frame rail or muffler bracket.  Unfortunately, taller tires
need to have less frontspace to ensure they do not contact the fender when
the suspension compresses.
8)  On larger diameter tires, the location of your mufflers.  Longer exhaust
pipes move the mufflers further behind the car and permit more tire and
backspace.
9)  Stiffness of tire sidewall and style of driving.  Probably, a function
of aspect ratio.  The top of the tire will flex in a turn with the tread on
the surface.  We need to provide ample space between the tire and inner
Pantera upper frame rail.  Today, I spoke to a technician at Goodyear about
this and he was not helpful.  My aim was to attempt to learn how much the
top of a tire flexes proportionately to the tread at the contact patch.
With this data, we could better approximate available maximum backspace or
distance necessary from the Pantera's inner frame rail.  He offered no
insight inferring that such data, if available, was proprietary.  Perhaps,
others on the list have access to better resources.  Even generalizations
would be good based on aspect ratio of a tire.
 I know of two Panteras running 335s that have polished their inner frame
rail with the inside of the tire when driving aggressively at track events.
However, their backspacing is probably fine when driving around town.
10) Ride height and suspension stiffness.  As the suspension compresses the
tire moves away from the frame rail until the lower suspension arm is
parallel to the ground.  After parallel, the suspension begins to pull the
tire back in toward the frame rail.  The shift is significant and I have
measured movement exceeding .5".  If you cars suspension is low and your
lower control arms are parallel to the ground, compression will pull your
tire inward sooner.  Alternatively, if your lower control arms have a
downward angle toward the hubcarrier, initial compression will move the tire
further away.  Thus, lower cars can use less backspacing.  A case could be
made that stiffer shocks accommodate more backspacing as well.

These are my observations.  Recognize that our cars are very different and
you need to examine your own car first, rather than follow what has worked
for others.  At best their experience should be a guide.

J, not needing new boat anchors,T




----- Original Message -----
From: "Antenucci, Dennis" <antenuccid@samtrans.com>
To: "'John Taphorn'" <jtaphorn@kingwoodcable.com>
Sent: Monday, April 02, 2001 10:22 AM
Subject: RE: Wheel Optimization


> I think one thing to mention/REMIND folks of John is that your backspace
> numbers are for an 11 inch wheel...otherwise it looks good
>
> > -----Original Message-----
> > From: John Taphorn [SMTP:jtaphorn@kingwoodcable.com]
> > Sent: Friday, March 30, 2001 9:03 PM
> > To: Multiple recipients of list
> > Subject: Wheel Optimization
> >
> >
> > Mad Dawg asks
> >
> > > WHAT tire/wheel combo is it that you are putting
> > > on your car?
> > > Would that be the Goodyear F1? The 315/40/18 tire is also a lot of
> > rubber.
> >
> > At present, I am about to pull the trigger on a set of Fikse FM/5
wheels.
> > May be viewed at http://www.fikse.com/photos.html .   Dimensions are as
> > follows:
> >
> > Front: 17" X 9" with 4.25" Front Spacing (FS) & 5.75 Back Spacing (BS)
as
> > measured from outside edge of rim.
> >
> > Tire will be a Michelin Pilot 245/35/17
> >
> > Rear: 18" X 11" with 5" FS and 7" BS
> >
> > Tire will be Goodyear F1 SuperCar 315/40/18
> >
> > As an FYI, rim width is typically measured between the tire bead and my
> > offsets are measured from the outside lip which is a half an inch thick
> > adding an inch to the rims total width.
> >
> > While my rear wheel measurements suggested 5.25" FS and 6.75" BS as
ideal.
> > Fikse, in their 18" wheel diameters, create offset only on the half inch
> > off
> > full inch measurements (i.e.. 6.0" ,6.5" , 7.0" etc.). Thus, I had to go
> > 7"
> > BS or risk the taller tire hitting the outside fender when the
suspension
> > compressed with a 6.5" BS.  I modified my current rear tire to emulate
the
> > larger tire with proposed offset to confirm clearance.  Additionally, I
> > moved the suspension through a range of motion to confirm clearance.  It
> > is
> > tight.  However, spacing appears adequate.  I believe this much
backspace
> > is
> > only possible with an 18" wheel as interference with the nuts that
connect
> > the hubcarrier to the lower A-arm would be the first interference point.
> > An
> > 18" wheel's inside diameter should be adequate to go around the two
nuts.
> >
> > I also had David Bell and Fred Hall on a recent visit inspect and
confirm
> > my
> > fender/frame/muffler clearance issues.  They encouraged me.
> >
> > On the tire decision, I am not thrilled at running two different brands.
> > However, the 28" diameter rear tire not only does a better job of
filling
> > the rear wheel well but, it may give me the enhanced gearing I am
looking
> > for without spending the extra cash on the new ring and pinion.  How's
> > that
> > for a rationalization.  See, I am saving money.  Unfortunately, Goodyear
> > doesn't make a suitable 17" front tire in the size I prefer.  I also
> > surmise
> > the Pilots are stickier and will reduce understeer.
> >
> > To play it safer, Fikse is going to produce and send me one front and
rear
> > wheel with tire mounted to confirm fitment and limit financial damage
> > should
> > I have messed up.
> >
> > If you think you know something that I may have missed, please let me
know
> > before I dive in.
> >
> > Thanks
> > JT
> >
> >