Hi Guys,
That could start a long topic of discussion in the tire and wheel section, or on this thread. Maybe someone like Ron Sutton will chime in with his thoughts.
Rob you’re right on this being a involved topic of discussion … but hey … what are we on here for besides seeing cool projects progress. I’ll chime in & add info to help in your decisions, but it’s your car & your decisions.
First I have limitations to tire size, self imposed limitations because I don't want to do any noticeable modifications to the body of my car.
This personal preference is what makes our cars different. Otherwise, if optimum performance was our only priority, we’ll all be building F1 cars.
Starting with that in mind and the black art of tires in general I evaluated the need for steam roller tires in the front. I am by no means an engineer or an expert on this. I have been playing with things that go fast around corners be it Cars, carts or motorcycles for 30+ years so I have some first hand experience and I've done a lot of research on this crap over the years.
First I believe JMO that wider is not the be all end all JMO. I have read and agree that there are benefits to width that go hand in hand with tire height and rim width that dictate the optimum contact patch for a combo. Weight on the tire,more importantly load which is affected by suspension design the cars weight and moment arm all play into it. then you get down to important factors like air pressure adjustment and heat buildup. Just slamming the largest tire you can stuff doesn't guarantee success.
Agreed. More to it than that.
But the amount of tire contact patch & the compound of that tire … is the #1 factor to grip … and therefore to speed. Everything we do in suspension tuning is to optimize the tires contact patch & load.
Other than singular purpose built Formula, Indy, GTP, Midgets & Sprint Cars … production type car’s typical limiting factor … is front end grip. In all my Stock Car & GT & Sedan Road Racing ventures, we know we can’t go any faster through the corners than the front end has grip. If we haven’t gone off the range with TW split or tire size split … it is relatively easy to balance the rear grip to the front. The front grip is top priority & typically where I have an edge over my competitors because I understand it well.
Think of it this way. The contact patch to contact patch is the true track width.
Dynamically, this is very true. Hopefully, you’re using the full tread width for contact patch with optimum front end geometry. I see a lot of cars with 8”,10”, even 12” of front tire tread … using only 40-60% of it in the corners because their geometry is out to lunch.
If I widen the tires by using more offset I narrow the track width. I also start messing with scrub radius as that patch moves to the inside of the line from the top balljoint through the bottom to the eventual point on the ground.
You are correct in what moving things affect, but “typically” those changes are positive changes. (Always exceptions). A typical scrub radius on PT cars with 275/285 tires falls in the 1.5-2” window. With zero being optimum, most cars can move the tire “in” with deeper offset wheels & improve the scrub radius. Your target doesn’t “have to be” zero … just know smaller scrub radius help the car to “cut” in the middle of corners where you’re running a late apex & makes a huge improvement in tight corners on road courses, back roads & AutoX.
On track width (center of tread to center of tread for any readers not familiar) … the optimum F/R TW split falls somewhere in the window of -1” to +1” depending on the cars weight distribution & suspension travel strategy. When a car has significantly more front weight distribution … say 53-56% … the front naturally wants to roll more than the rear. Having the front track width a little wider helps balance the car’s roll angle. I don’t like to have the front TW more than 1” wider than the rear, or the roll angle balance goes the other direction & the car wants to roll over on the outside rear tire too much.
With the much wider rear tires moving the rear TW "in" ... I suspect your rear track width is substantially narrower than your front & will contribute to things that cause a push condition. You should measure both front & rear track widths & post the info. If the rear is more than 1” narrower than the front … and if you care about this … then you would want to narrow the FTW, or widen the RTW or a balance of both. I later learned Rob's Torino body is wider in the rear, so he has a 2" wider TW in the rear than the front. 58" rear & 56" front, which we discuss how that will free the car up later in the thread.
In case this is new to anyone, Wider front TW than rear = tightens the car … too much makes it push. Wider rear TW than front = frees the car … too much makes it loose. If the car has more rear weight, I may, or may not, run the rear wider than the front. It does depend on if the suspension travel strategy is “high roll/low pitch” or “low roll/high pitch”.
If you’re not clear on this … a handling car HAS TO travel the suspension to work. You can not run it flat “low roll & low pitch” as it will push … and you can not run soft everything because “high roll & high pitch” will be loose. The old school strategy with stiff front springs & small-med sway bars is “high roll/low pitch.” The new-ish strategy (last 20 years) is soft front springs & large to OMG sway bars is “low roll/high pitch”.
I use TW split as a design tool to help balance un-balanced cars … and … as a track day tuning tool to balance the car’s roll angle. Ultimately, you need to have optimum grip on all tires and disengage the inside rear tire (to a degree) to turn well … then re-engage the inside rear tire (to a higher degree) for maximum forward bite on exit. So, on entry & mid-corner, the car needs to roll less in the front to keep both front tires engaged for optimum front end grip, while allowing the car to roll slightly more in the rear to disengage the inside rear tire, to a small degree, to turn better. For optimal exit, the car will have more roll in the front & less in the rear to re-engage the inside rear tire to a higher degree than it was on entry & exit, for maximum forward bite (traction) on exit.
I'm not racing in the Rolex series so no use over-thinking it but I didn't want to go far out on a limb in any one direction.
With the big difference in tire size, you are going out on a limb in one direction. But it’s your car, and I’m a fan of it making you happy. I know I’m building my car for me & my priorities.
So I took in some ways a conservative approach and right or wrong a big part started with aesthetics.
It’s your baby. 
I didn't want a 1" or 1.5" outer lip on my front wheel. To me that give a Jiffy POP look of the center popping out of the hoop. JMO I don't like it.
Again, it’s your baby.
I also didn't want to be stretching or flaring the front fenders.
I believe this comes down to each person’s priorities. Another guy I’m helping is putting on fenders allowing him to add 1.25” of tread on the outside, to go along with the added 1.25” of tread on the inside.
My post was so long ... SURPRISE ... It had to be split into two posts. Part 2 is in the next post.