Maximum useful rubber

[Jack Olsen]

A short time ago, I had to make the decision of whether to stick with my narrow-body SC/Carrera flares or go to the RSR/Turbo sizing. Since I don't have classification issues to contend with, I was able to go either way, and then have wheels made to match.

The trade-off with the RSR/Turbo configuration is less 'nimble' handling, and the additional drag from both poorer overall aerodynamics and the additional rubber on the pavement.

Of course, the up-side to the wider wheels is increased grip in the corners, which means better performance on about 80% of a normal track.

I'm not planning on changing my 8x10x17 configuration with 245/275 tires, but I'm still curious about the alternatives.

In the Open Track Challenge Tyson and I competed in, the fastest car of the entire event was a 914 with a 3.3-lter turbo-charged engine. It had 13-inch wheels in back, and 12-inchers in front, and it made short work of a number of race-prepped Vipers, Corvettes and a Ferrari F-40. They all had MUCH more power, but it was lighter (2200 pounds) and, of course, it was an early-seventies Porsche.

My question is this: at what point does a 2400-pound car hit meaningfully diminishing returns with additional tire width? I know this depends on the balance of low and high speed sections on a track, but where to modified 911's generally end up, in terms of tire width, when they're not specifically limited my classification rules?

[Bill Verburg]

Just MHO but I would want the tires to either just barely be hanging on or just barely breaking loose(leaning more to the latter) during the most aggressive moves, usually braking or cornering. This setup will vary a lot from day to day, corner to corner and track to track. That's probably why the pros have numerous choices for both wheels and tires. Mortals have to be content with 1 or 2 choices. I think your current setup is a great all round compromise regardless of fender flare used.

Don't forget that unless you go to an 18" rim wider also gets taller once the 275/40x17 barrier is reached. I hate taller tires, but must accept the tradeoffs to a point.

[Chuck Moreland]

There is such a thing as too much tire.

You've hit on a couple of negatives of larger tires; more rolling resistance and more frontal area. Bigger tires also add rotating weight and unsprung weight.

But can too big a tire actually reduce grip? Yup!

All things being equal a tire with a larger contact patch will grip better than one with a small contact patch. Due to reduced sidewall deformation and better heat dissipation, larger tires can be safely run at a lower pressure thus increasing contact patch and grip. The wider tires MUST be run at a lower pressure to achieve this benefit.

Tire grip is also a funtion of tire temperature. Grip increases with temperature up to some maximum, then it begins to decline with higher temps. Consult your manufacturer for optimal temperature but a good operating temperature for a typical DOTR tire would be in the 190-220 range.

So what is my point? Big tires run cooler due to less deformation and better heat dissipation. Too big a tire and you might not reach optimal operating temperature, thus losing grip.

You can lower pressure to increase temperature up to a point. But there are limits.

To answer your question use a tire pyrometer to take tread temps. If you have optimized air pressure (determined by taking three reading across tread surface) and your temps are at the high end of the operating spectrum, then you might benefit from wider rubber. If you are at the low end of the operating temperature range, then bigger tires will hurt and you might do better with smaller tires!

[JonT]

I've never raced but you can probably answer this yourself. Are you hanging in the corners with the faster cars or are you losing on the straights?

[katigbr]

Jack,
Thanks for the timely thread. I'm in the process of deciding between 275 or 315 width rubber for my ST conversion. The expected weight is going to be around 2200 pounds. I really don't know what advantages the 30mm of rubber per rear tire will make. Previous posters have indicated that you can have too much rubber. The downsides being increased weight, degraded aerodynamics and decreased steering quality.
I'm assuming that with a higher powered car like BB, the extra weight might not be a serious handicap. Obviously the benefits will be determined by the track config. More curves, more benefit, more straight, less benefit. In general, I get more kicks going faster in curves than going faster in a straight line. However being the lunatic that I am, I'm planning a 3.8 upgrade this winter to compensate for the "extra" weight.

I'm currently running 225/15's in the back and know that 275/17 would be a terrific improvement in grip. Will I notice the difference between 275 and 315? Are we talking a couple of seconds per lap? What were your observations during the open track challenge on your 275s with BB when compared to previous track sessions with the 16" tires? Recognizing that the aspect ratios are different and do affect handling, I'm curious as to the amount of improvement that tire sizing makes.

My assumption is that all else being equal, a car with wider tires will be able to carry more speed and brake in a shorter distance, keeping in mind the temp/grip issues stated by Chuck.

BTW, what were the offsets you finally ended up with on your Lindsey rims? Did you stick with 4.5" all around?

If you had it to do over again, would you go to the 315s?

Regards,
Rick

[Bill Verburg]

Rick, unless you use 930 flares this is the problem you will run into w/ 275/40


the situation is not much better at the fender lip

[Early_S_Man]

Good question, Jack, and worthy of an analysis by Dr. Feynmann on the 'n-body' problem in physics! Sometimes you just don't have enough data to complete the solution to the problem!

When dealing with cars in the 2200-2400 lb class at 210 hp vs 230 hp ... then the net drag increase, both aerodynamic and driveline ... can be measureable, and the '73 LWT RS 2.7 vs the '74RS 3.0 ... became a net loss. Top end as tested by Paul Frere at Weissach was 152 mph vs 149 mph ... with the same gearing!

Perhaps even the '73 RSR vs '74 RSR may have had similar slight loss too, but I've never seen comparison numbers! For '74 ... 330 hp and 14" rear wheels & 10.5" front wheels vs the '73's 308 hp and 9" & 11" wheels was a problem I will toss out for discussion ...

But, I favor the 'RSR' '73 look ... regardless of a slight loss, or not!

[Jack Olsen]

Yeah, Rick, if you're just flaring your fronts, ST style, then your choice is going to be limited to 275's. And even then, you're going to be rolling/grinding the lips, and machining those two bolts, and probably moving one of your oil lines.

As a data point, I've made two visits to Las Vegas Motor Speedway to run the ALMS course.

First visit: I had the 3.6 with a Carrera exhaust and Flowmaster, a stock 73 (7:31) transaxle, 7x8x16 Fuchs with 205/245 Toyo RA-1's. Camber settings were something like 0F, -1R. I was running a ducktail and an S front spoiler.

Best time: 1:45

Second visit: I had the 3.6, B&B headers, a close-ratio 8:31 transaxle, 8x10x17 2-piece Fuchs with 235/275 Yoko A032R's. Camber settings were -2F, -3R. I was running an IROC tail and an S front spoiler with a 1.5 inch splitter.

Best time: 1:36

The difference is 9 seconds. Now, some of that is attributable to more experience from a year of track events. But a lot of it is 30mm of extra rubber, camber, and downforce. I also had more aggressice brake pads on the second time out.

Do I wish I had gone to 315's? Well, since it would have meant more flares all around, the answer is no. I think what Warren is suggesting is true. Adding that much weight and drag is probably not worth the extra grip. I think I made the right decision in maximizing what's doable under RS/SC/Carrera-size flares.

What sizes do cup cars run?

[katigbr]

Bill,
Thanks for the reply. I had seen the posts on relocating oil lines and grinding down the bolts on the springplate bolts but thought that with ST flares I could squeeze in the 275/40 or even 315/35 rubber. Do the Turbo flares provide more clearance than the 9" ST flares? I do notice a flatter arch on the 930 flares when compared to the STs. My plan was to use the same offset that Jack/Randy Wells used for their early 911 config to get the wheels as inboard as possible and then use the ST flares to give me the clearance to install the fatter rubber.
My assumption was that if Jack/Randy can squeeze in 275/40 in the rear with Carrera RS flares then an ST-flared early 911 should be able to carry 315s on the rear.
I may run less offset on the fronts compared to Jack/Randy to fill out the ST flared front. Perhaps an inch or 1.5" less offset on the fronts.
Because I've got a 72, I'm concerned about the relocating the oil line bit but haven't looked at what needs to be moved yet.
Dave B. at TRE suggested getting the rims/tires before installing the flares to allow for some adjustments to the flare installation if needed.

Regards,
Rick

[katigbr]

Jack,
9 seconds!!! Thats like having a different car under you. What a ride that must have been.

I'm still not sure which way I'm going to go with the rear tire size. I'm adding the major aero drag with the ST flares, will I notice the extra weight of 315 vs 275? Maybe not.

I've got bigger problems since the gearing is radically affected going from the 15 to 17s. I'm adding and extra inch or so to my rolling diameter. I am NOT going to spend anymore money on new gearing so I'm going to have to live with the reduced acceleration with a higher top end. I'm hoping to be able to carry enough speed around the corners to offset the slower acceleration.

Rick

[Bill Verburg]

Rick, that is a pic of 275/40 on my Carrera which has 9" flares. That is the 275 barrier I wrote about previosly, the bolts need to be ground and the oil line relocated, it is very, very close on both side.

As I understand it the ST rear uses the same or at least close to the same flare in terms of tire room as the SC/Carrera, you will be limited to 275 width. I have searched high and low for a street tire 275/35 to get the height I want, no dice.

Another issue is that you want relatively equal tire heights f/r a bit of disparity in height is ok but I wouldn't want more than ~.5" unless there was a pressing need for it. This constraint also limits your choices, Ideally i would want a 235/40 to go w/ the 275/35. But have been forced to use a 235/45 to match the 275/40.

Weight is another issue already mentioned. I can save 10#/rear corner by going to a 265/40 tire.

If the normal venue for use was Daytona Talledaga or Le Mans I would be more concerned w/ aero issues, but for the types of tracks and of course street use aero is a secondary issue to ultimate grip. That is why the 235 or 245 in combination w/ 275 is arguably the better compromise
Pro
great grip(large friction circle)

Con
gearing, aero drag, frictional drag, weight

If you didn't have the torque of the 3.6, or the advantage of shortened gearing you might make a different decision.

The suggestion about getting the rims and tires first is a good one, Ruf managed to comfortably fit 10" rims and big tires by adding an inch to the standard SC/Carrera flare, its a fairly easy mod for a skilled metel man.

[CamB]

The ST has 9" front and rear flares, so you won't have any more room at the back than you already do.

It does seem that the STs which were raced had 10.5 inch minilite wheels. When I look at pictures, it looks like the rear flares were hammered out a bit more to make them fit.

I have no idea what size tyres they got on there.

[katigbr]

Bill,
Thanks that cleared up a lot of the questions that I had. I was under the mistaken impression that the 9" ST Flares were wider and give me more room. Based on your reply, I'll probably stick with the 275/40 and 235/45 to keep it all balanced.

Thanks,
Rick

[Jack Olsen]

Rick, your gearing might not change as much as you think. My car with 245/45x16's tops out in 5th at 135 mph. With 275/40x17's that figure changes to 141 mph. It's a difference, but one I'm happy with, since I hit redline in fifth on two of my local tracks with the smaller 245/45's. The 275/40's give me just a little more breathing room.

I just went out and snapped some measurements of my 8x10x17's. Since I'm not going to be able to eyeball my centerline, these measurements will give you the distance from the bolting point (the flat inside of the wheel) and the outside edge of the wheel itself.

Here's the difference in the outside measurements:



That's 2.81 inches on the 8x17 wheel. and 4.74 inches on the 10x17 wheel.

Here's the difference in the inside measurement:



That's 6.01 inches on the 8x17 wheel. and 6.07 inches on the 10x17 wheel.

I guess that adds up to 8.82 inches for the 8x17 and 10.81 on the 10x17, which is probably pretty close to the actual numbers, given my homespun measuring methods.

Since my piece of oak might not be 100% straight, I'd guess they went for 6-inch insets, with a variable outside measurement.

I'm sure this would allow someone with more than my rudimentary math skills to figure out what my offsets are.

Big note: My 930 brakes require that I run a 1/4 inch spacer in the front only to clear the front calipers. So if your car doesn't use any spacers for caliper clearance, and you're looking to get a set of these made, adjust the measurements accordingly for your wheels. It would be something like 5.75 on the inside, and 3.05 inches on the inside for the 8x17's. (Of course, keep in mind that no two 911's measure out exactly the same.)

[widebody911]

When I went from 16" 7's & 9's to 16/17 9's & 11's, my lap times *degraded*. This is horribly unscientific, of course. For one, going to a 17" wheel in the rear slowed down my accelleration, and I was much slower coming out of the corners than I used to be. I think more seat time could possibly get that back. But thru the long, off-camber sweepers, the stability and level of confidendence more than made up for it! I know I'm not at the limits of adhesion yet, and as I learn to drive, I'll find it.

No idea what the point of all that was, but I think wider tires are a good idea at the levels at which we play.

[Bill Verburg]

Jack, you are measuring backspace in the second set, on the 8" rim 6" of backspace is ~1.6" of + os or ~34.3mm of + os w/o the spacer. -.25" for the spacer(6.4mm) gives ~27.9mm mm of +os w/ the spacer. These #s are aproximate and only as good as the measurements given. The large native + os is the reason you need spacers to clear the calipers. I am running +25mm os on my front 8s with a ton of room for even the biggest Red. the trade off is that your rim/tire is set more inboard giving more lip clearance at the expense of inner wall clearance.

[Bill Verburg]

Thom, A perfect example of the tradeoffs involved. Does the increased size of the friction circle under braking and lateral transitions offset the decreased acceleration. At that one track on that day it sound like iot didn't, another track, another day the ans may be different. Sounds like you may want to get a copy of Jack's tranmission.

[Eric Coffey]

Jack-

I'm not sure about the earlier (964) versions, but the 993 cup cars ran 235's/245's in front and 285's in the rear. I think rim sizes were 8" (or 8.5") x 18" in front and 10"x 18" in back.

[Jack Olsen]

Thanks, Eric. It sounds like I'm right where I ought to be, then.

I can't wait to try some Hoosiers out on it.

[MMARSH]

Jack,

In your case I dont think most of the nine seconds you improved upon were from the tires. I would be willing to bet that if you ran your previous tire setup with your current chassis setup and your years worth of experience, you might only notice a couple of seconds if that. There are guys running 2.2 liter "S" motor early cars with narrow (16x7) wheels and fenders doing 133's at Willow springs. Those cars are weighing around 2250 lbs.

I dont think you realize it because your car has always been changing, but the biggest improvements you've made to your car getting around the track faster is your track time.