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Slicks tires 17

Gents.

Happy new year.

Will be running this Porsche 911 SC 3.0 1981 all Racecar in 2018.



The car is running 8x17 fronts and 9.5x17 rears.

Old tires:





As class rules we can run Slicks.

Please let me know your choice of slick tires.?

Thanks - Cheers - Olsen911 - Tommy Olsen
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Old 01-01-2018, 03:57 AM
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I guess you will want to run radial slicks to be more competitive and for better lap times.
I've had taller tires on the 911 but prefer shorter slicks, 25 rear, 23.5 front Avons.

Although I prefer bias ply for greater slip angles and lurid slides and saves.
I have 15 inch diameter bias ply Avons on my 911.
16 inch Goodyear bias ply on one non Porsche race car,
and 16 diameter Goodyear radial slicks on the other non porsche.

Don't bother with these cheap American racer slicks, they start off crappy and then get worse.
Old 01-01-2018, 07:45 AM
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The "used slick" market consists mostly of the 18" size. I use mostly Pirreli's due to the choice of sidewall heights. These slicks come from Cups, GT4's and Caymans.
Meet the race shop folks at the track and pick up the next events tires.

PS, Formula Vee is your friend if application directions are exactly followed

E-Ticket fun in the summer, but winter use is an adventure
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Old 01-02-2018, 12:18 PM
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Not as much choice in true slicks at 17", tons of 18's though. Call some of the race tire shops and see what they offer (Frisby, AIM, Roger Krause, Sasco, etc). I know Yokohama makes some 17's. Dunlop used to be available in that size but that was a long time ago. Probably be a special order thing. If you can't find slicks you can run Hoosier R7. You can also modify those rear wheels to be wider and open up your options, since they are only 9" now. Nice car. I remember seeing it at POC events.
Old 01-08-2018, 03:13 PM
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Quote:
Originally Posted by Olsen911 View Post
Gents.

Happy new year.

Will be running this Porsche 911 SC 3.0 1981 all Racecar in 2018.

The car is running 8x17 fronts and 9.5x17 rears.

As class rules we can run Slicks.

Please let me know your choice of slick tires.?

Thanks - Cheers - Olsen911 - Tommy Olsen

Hi Tommy,
Out of interest what did you find?

(Or will you look at R7s or a different wheel size...)
Old 02-10-2018, 02:26 PM
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Quote:
Originally Posted by Olsen911 View Post
Gents.

Happy new year.

Will be running this Porsche 911 SC 3.0 1981 all Racecar in 2018.



The car is running 8x17 fronts and 9.5x17 rears.

Old tires:





As class rules we can run Slicks.

Please let me know your choice of slick tires.?

Thanks - Cheers - Olsen911 - Tommy Olsen
R7 245/40 & 275/35 x17 would be a tasty choice for that car
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Old 02-11-2018, 10:05 AM
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Quote:
Originally Posted by DHE11 View Post
Hi Tommy,
Out of interest what did you find?

(Or will you look at R7’s or a different wheel size...)
Still looking, having a hard time finding some 17" Slick.

Bill:

"R7 245/40 & 275/35 x17 would be a tasty choice for that car"

Please do tell me more.?

Thanks - Cheers - Olsen911 - Tommy Olsen
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Last edited by Olsen911; 02-11-2018 at 08:47 PM..
Old 02-11-2018, 08:39 PM
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I was running the 245/275 R7’s on my previous car.
Still have those rims and tyres.
Agree with bill, a good set up.

I’m looking at moving up to 255 and 295 on my new car which will be dedicated track car, with wider front track.

I understand some people are running 275/315 set ups on turbo bodied 911’s....
Old 02-12-2018, 01:48 AM
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We're using Hoosier R7 and A7 245/40/17 F 315/35/17 R = Class track record

But, the 245 R/A7s are almost as wide as the 275 tires . . .

Wheels = F 9x17 R 10.5x17
Turbo wide body mod on '78SC

OP - for your wheel size you might consider 235 F and 275 R to get 40 F/R bias

Also, tire rubbing becomes an issue when you go really big on the tires - we have some
but use wheel spacers to get the rub out . . . for our car info see my Garage

Regards,
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Old 02-19-2018, 06:22 PM
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Quote:
Originally Posted by Olsen911 View Post
Still looking, having a hard time finding some 17" Slick.

Bill:

"R7 245/40 & 275/35 x17 would be a tasty choice for that car"

Please do tell me more.?

Thanks - Cheers - Olsen911 - Tommy Olsen
That's a sweet spot for grip vs inertial and gearing costs, bigger may give incrementally better grip but will certainly incur both inertial and gearing costs that come right off the engines available torque.
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Old 02-20-2018, 06:36 AM
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Quote:
Originally Posted by Bill Verburg View Post
That's a sweet spot for grip vs inertial and gearing costs, bigger may give incrementally better grip but will certainly incur both inertial and gearing costs that come right off the engines available torque.
Is 915 Short gears sets with 680 or 705 height profile is another way to kinda do the same thing, good accell corner to corner with the taller tire slip??
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Old 02-20-2018, 09:26 AM
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And don't forget the suspension optimization that needs to happen on an older 911 to have it work well with taller tires on a lowered car for track use.
Old 02-20-2018, 11:36 AM
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Quote:
Originally Posted by 3rd_gear_Ted View Post
Is 915 Short gears sets with 680 or 705 height profile is another way to kinda do the same thing, good accell corner to corner with the taller tire slip??
You can negate the gearing penalty from tall tires by regearing the transmission, but you can't do anything about the inertial penalty which is a function of mass and distance of the mass from the axis of rotation and radius*radius, The result is that fairly small increases in tire diameter will result in a noticeable reduction in net acceleration.

The gearing penalty is easy to put a number on it's ~ -12lb-ft per extra inch of tire diameter, the inertial penalty is more difficult to quantify in general, you actually have to ether dyno or run the specific #s to see the effect.
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Old 02-20-2018, 02:28 PM
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Thanks guys.

Is the choice within slicks much bigger to 16 ".?

Thanks - Cheers - Olsen911 - Tommy Olsen.
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Old 02-20-2018, 11:21 PM
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Hoosier make both radial and bias ply slicks in 16" sizes. I am referring to full racing slicks, not the DOT slicks people use in "stock" classes. Decent selection in the 10-14" width sizes. Goodyear used to make 16", but pretty much stopped a few years ago. I understand Avon also makes 16" rubber, but I don't have experience with them.
Old 02-21-2018, 04:51 AM
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Quote:
Originally Posted by Bill Verburg View Post
You can negate the gearing penalty from tall tires by regearing the transmission, but you can't do anything about the inertial penalty which is a function of mass and distance of the mass from the axis of rotation and radius*radius, The result is that fairly small increases in tire diameter will result in a noticeable reduction in net acceleration.

The gearing penalty is easy to put a number on it's ~ -12lb-ft per extra inch of tire diameter, the inertial penalty is more difficult to quantify in general, you actually have to ether dyno or run the specific #s to see the effect.
First, I appreciate this discussion, thanks to all
.
So, the math & physics make sense , so then why do the Cup cars, GT3 & GT4 of today use 18" tires & wheels? I realize they have modern suspension as a big factor, if so how are the inertial physics offset, just more Torque/HP?
These are the guys I get my "gently" used slicks from and the basis of going with 18" wheels.
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Old 02-21-2018, 08:50 AM
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Modern race tires keep getting taller and taller with larger sidewalls because they are all chasing maximum grip. As the diameter increases, the length of the contact patch increases as the tire is "flatter" where it meets the road. Larger contact patch reduces the pressure each part of the contact patch sees distributing the heat load over a larger area and allowing lower air pressures to be run thus increasing the tire patch further and increasing the tire's ability to conform to small road imperfections. With a bigger tire you can use a softer compound because the tire will not overheat as easily since there is more tire to dissipate the same energy into. Because of the larger air volume in the tires and greater heat dissipation the pressure change hot to cold is reduced. The larger side wall increases the load holding capacity of the tire which is a big thing on high downforce cars and/or banked tracks. The larger sidewall also acts a bit as a torque damper allowing power or brakes to be snapped on and off more quickly without shock loading the contact patch. All of these reason contribute to why modern race cars have taken the weight and inertia hits and gone to 680 or 710 height 18" slicks but it is all a trade off. You have to completely redesign the suspension to be able to achieve the same ride hight and geometry increasing the tire diameter by inches. This will require lots of fender modification to achieve also. The leverage the tire contact patch has on the suspension in now a lot higher as the contact patch is farther away from the axle centerline so the strength and stiffness of the suspension components and wheel bearings has to be higher to maintain alignment under load. You start running in to axle angle problems because the axle is now much higher in the car for the same ride height so upside down gearboxes (a la 935) or specialty racing gearboxes with raised diff locations like Porsche finally started using in 997 RSRs have to be used, and above all, you have to have the power to be able to offset the added weight and inertia and need the higher heat capacity be able to use the added grip available to achieve lower lap times.
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Last edited by Evan Fullerton; 02-21-2018 at 09:50 AM..
Old 02-21-2018, 09:46 AM
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Tires have gotten taller mostly because the greater circumference allows of better cooling and gearbox ratios have been adjusted accordingly to compensate.

but the growth isn't that drastic
GT3 Cup uses
27/65-18 which in street lingo is a 298/38 x18 & 31/71-18 which is a 347/42 x18 @27.8" od

RSR uses
the same rears but a 30/68 -18 which is a 333/30 x18

They they contact patch shape does change w/ these bigger tires but again not all that much

these cars have 500+ hp and can afford to give away a lot more than our more run of the mill cars( mine anyway)

modern street tires have gotten really tall too for example 335/30 x20 is 28"od, doesn't make it a good choice for older cars
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Old 02-21-2018, 12:38 PM
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