1969 Torino

[Ron Sutton]

Originally Posted by Track Junky

Thank you sir. Much obliged. You know gentlemen like me take a liking to that sort of thing.

EDIT: Disclaimer: The message above was written after numerous cocktails last night and should be a testament that drinking and typing is not a safe practice.

That's funny right there.

[Ron Sutton]

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.

[Ron Sutton]

Part 2 of previous Post / Related, but new topic:

(This is still Rob in SoCal talking)
This also go me thinking about rim width. Why does one guy run a 275 on a 9" wheel and the Nissan guys run one on an 11"?

From all my reading, to a point,not always, wider is better for rim width if it is within the tire manufacturers recommendations. Look at Porsche they regularly spec tires on Wider than recommended wheel widths from the factory. My goal was the flattest tread profile for whatever tire size I choose. In reading every .5" of added rim with will add .2" of measured tread width on the same size tire. I looked to match the wheel width and tread with dimension for my chosen tire heights.

Some tire & wheel basics: Whatever tire width you put on whatever rim width … there is a “happy window” of tire pressure to achieve a full & even tire contact patch on the pavement. Every tire sidewall acts like a spring and the spring rate is affected by tire pressure. More pressure = higher spring rate. All tires do weird things when you get them out of their optimum spring rate. Until just recently I had & utilized an Intercomp tire sidewall spring rater to test sidewall spring rates of different tire & rim combinations to find their “happy window” & to know at what point they got weird.

When you have the tread width wider than the rim width, you have to run lower tire pressure to achieve a full & even tire contact patch on the pavement ... otherwise you’re “crowning” the tire & using less than the full tread width. This lower tire pressure makes the tire sidewall spring rate “softer.” The softer sidewall COMBINED with the fat tire sidewall bulge, leads to the tire “moving around” on the rim substantially. This is ugly when the car is pushed hard, like in track days or AutoX, but provides a softer ride.

When you have the tread width narrower than the rim width, you have to run higher tire pressure to achieve a full & even tire contact patch on the pavement ... otherwise the tire tread goes “concave” … not utilizing the center tread … ending up with less dyanamic tread width. This additional tire pressure makes the tire sidewall spring rate “stiffer.” The stiffer sidewall COMBINED with little to no tire sidewall bulge, makes the tire substantially “more stable” on the rim. This combination performs best … BY FAR … when the car is pushed hard, like in track days or AutoX, but provides a harsh ride.

I wanted a 26" tall front and 27" tall rear tire to fill my wheel wells and maximize the length of the contact patch and the tires ability to radiate heat. Out back that was a no brainer. 345 30 19 on a 13" wide wheel. This is the Dodge Viper spec. Michelin built that tire originally for the Dodge Viper. Am I smarter than those companies engineers? No.
Not to be a smart ass … ok … being a little bit of a smart ass … is your car a Dodge Viper … with the same suspension set-up & geometry? No.
In my opinion, I always suggest when you’re copying something, be clear on why.

So what to match it with up front. Well a front engine rear wheel drive 3400lbs Viper uses a 275/345 combo with a 50/50 weight distribution. I may not get to 50/50 but I'm trying to get as close as possible.
That will be awesome. The closer you get the weight balanced front to rear … and side to side … WITHOUT placing weigh out past either axle to get it … will be beneficial. Get as close as you can.

I'm also hoping to come in around that weight. Before anyone else says it I know there are more factors than that. To start with CG to Roll center creates a moment arm which actually influences load on the tire as much as total weight. Like I said before I'm not building a Rolex competitive racer so I'll keep it simple.
Sounds smart. Working with track width, tire & rim width are simple, predicatable tools.

I borrowed a 275 35 18 mounted on a wheel from Ron, thanks again Ron and a 295 35 18 tire from Brett, thanks bud. the 275 was an easy fit and the 295 ,using my wheel fit tool, was a little big. A 285 should be just right I thought.

Here is where Travis really came through. I had communicated with him a few months ago about a quote for my wheels. I really like the fact his wheels were clean timeless designs and engineered for and used in racing. From my wheelfit measurements I was pretty certain I had a BS measurement and wanted a 10.5" wide wheel wheel.

The 10.5" wheel did two things. The measured tread width of a 285 Mich on a 10" wheel was 10.2" So adding .2" for a .5" increase in wheel width this gave me 10.4" tread width on a 10.5" wheel width. A good match in my mind. It also gave me a 2" outside lip with my backspace measurement to avoid Jiffypop.
I really need to expand my terminology. I’m behind. Does “Jiffypop” mean the wheel center is crowned out … or flush with the outer edge of the wheel … or both?

In speaking with Travis he said he could build a test wheel to the same specs I thought I needed actually try it on the car. Hell yea. A week alter I had a borrowed wheel made from both new , blem and used parts. Travis sealed it so I could mount and air up my tire.

This was a huge help. I was able to confirm my thinking on backspace, the profile of the 285 on the 10.5 matching that of the rear tire, double checking caliper clearance and getting a real world visual can't be topped.

Tires are disposable items. My car won't be painted before I actually start tuning the set up. I plan on running it in shake down form for a while before I really finish it pretty. If I decide to go wider and do more tweaking on the body it won't be big deal. The 10.5" lets me use a 295 or 305 and still get a good footprint if in the end I feel the 285 is a limiting factor.

My suggestion would be … if anything I wrote influences you to go to wider front tires … do it now. Do it once & do it right, by getting wheels that are a little wider than the tread width. But if you’re happy with the 285’s … party on Garth.

I still think the 285 on the 10.5" wheel will give me all the front bite I need if my shocks, springs and bars are tuned correctly.
This will be your limiting factor in performance driving. If you really dial in the front suspension … use enough caster to end up with a KPI/Caster split favoring the caster by 1.0+ degrees, moderate camber gain, -1.0 camber, correct toe-out/Ackerman balance, zero bump steer & the optimum roll center … plus the optimum sway bar, spring & shock combo … the car can go no faster in the corner than the 285’s can grip. So you will be reducing the grip of the rear end to match the 285’s. As far as the performance is concerned, you might as well run 285’s on the rear, because all the tuning you do, will be to reduce the grip of the tires to the level of 285’s.

Now I’m going to take a different position.
This isn’t a race car. It’s your car. It’s your car to drive & have fun in however you want it to be. That may include track days, competitions, spirited driving, etc. I think you should build it the way you want it, which is what I love about Hot Rodding in general & Pro Touring in particular. I get the look you’re going after … because I like it too.

For looks: I also prefer a small 1.5-2” lip on the front wheels & a deeper dish (lip) on the rear wheels.

For handling: I prefer to get the scrub radius down to zero or close to it, which requires the face of the wheel to be moved out close to flush, kind of like the SCCA GT1/Pro Trans Am cars are. For me, handling always ends up winning on the priority list.

I attached a photo of my "Street tires & wheels" from American Racing. I think they look ok. But they "Jiffypop" so I could achieve a zero scrub radius front suspension.

In the photo:
On the Left, are my flush face front wheels: 18x10" AR "Burnout" VN472 Wheel with 8" backspacing. Tires are Nitto NT555 285/35ZR18
On the Right are my deep dish rear wheels: 20x12 AR "Burnout" VN472 Wheel with 4" backspacing. Tires are M/T S/R 29X15.00R20 = 365/30/20

* For Autocross & Track Driving these tires & wheels come off & "front & rear matching size" tires & wheels go on.
BFG Rivals 335/18/30 on 18x13” CCW lightweight LM5 rims.

...

[Ron Sutton]

Originally Posted by Vegas69

I agree with the notion that slapping a tire on a wheel that is to small or on the smaller side of the range may be counter productive. I do however think the biggest tire on the front(Right width wheel) with a reasonable stagger is the best way to go. You notice the Vette/Viper have decreased their stagger and increased front width lately. These old muscle cars are nose heavy and they want to push. You'll find yourself continually looking for an increase in front grip while you own the car. You'll naturally have more lateral rear grip at your disposal which means you will be decreasing rear grip to get chassis balance. I'd say a smaller rear tire would net better side and forward bite due to a slightly more desirable spring rate/shock setting.

Don't get me wrong, I think you can get the car to handle nicely.

Couldn't agree more.

[Ron Sutton]

Originally Posted by Track Junky

Now the dilemma......I have chosen a big ass 345 to handle the rear load but what size up front. IMO smart money goes with the widest I can get that will compliment the rears and I dont believe 285's are going to do it. Reason being I used to run 275's all around and had my car neutral. When I mini-tubbed my car and installed 315's out back just for the cool factor the car began to push. I'm still working on getting my car neutral again.

This is what I see all the time. If you think about your previous car in contact patch comparisons with 315's with approximately 11.8" of tread on the rear ... and 275's with approximately 10.2" of tread on the front ... your rear tires had 15.6% more grip.

You had a balanced car ... and added 15.6% more grip to the rear. They only way to fix the handling balance ... without swapping tires ... is to adjust the suspension to reduce the rear grip by 15.6%.

That's a lot. I know firsthand, because I've done it ... and it takes BIG spring, sway bar & roll center changes to achieve.

[Vegas69]

Great stuff Ron. Thanks for the education.

[Ron Sutton]

Originally Posted by Vegas69

Great stuff Ron. Thanks for the education.

No worries. I enjoy sharing when the topic is in my wheelhouse. I also enjoy learning about stuff that is not in my background.

Do you AutoX your car on a regular basis in Vegas? If so ... where?

[Ron Sutton]

Hey Guys,

I'm on the Forum today for a few more hours ... then I'll be gone for a week.


I'm meeting up with my friend Neil Porter at Sears Point where he's running 2 formula cars in SCCA racing. Then I'm camping with my girls the rest of the week.


I'll be back online late Thursday (7/11) or Friday morning (7/18).

[Matt@BOS]

Hey Ron, another question for you...

You mentioned that in Gaetano's case he might as well have just thrown 275 tires on the back of the car instead of the 315s because he would need to remove the 15.x percent more grip that the 315s added in order to get the car balanced again.

My question is, if you're working with more tire in the back versus the front, can you tune the rear to have more grip in the straights and less under steady state cornering?

When you discussed the pros and cons of high roll/low pitch versus high roll/low pitch, I was curious if you could run one setup on the front and the other on the rear. I feel like there is a reason you haven't mentioned doing this, but if you have the time, it would be cool if you could explain what would happen from running low roll/high pitch in the front and high roll/low pitch in the back.

Thanks for all the time you've spent sharing knowledge with us amateur hour club guys

Matt

[FETorino]

Originally Posted by Vegas69

Great stuff Ron. Thanks for the education.

Yes I gotta agree with that.

Thanks Ron. This is a lot of good information that should lead to some further discussion.