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Quote:
Originally Posted by Superman View Post
Many thanks to Dr. Verburg for his help in this thread and literally thousands of others over the years. Bill, I still don't fully understand the math and tradeoffs in wheel/tire decisions but I did pick up on the idea that a wider front tire would improve traction but at the expense of torque. And so....I think I am settling in toward a final decision and I just want to hear your thoughts, and those of my other Pelicanhead friends:

I am still thinking of the 17 x 7/9 Euromeisters with at least 215/45 and 245/40 tires. If I thought they would fit, I would be tempted to use 225 in the front. Is this a good combo? Is the 225 a bad idea? Will the rear tires have the proper overall diameter for my speedometer?
yes, that is the setup shown here and above


It's a good fit for an SC or Carrera
to read just look at the green sections particularly frt spc,

In front, for an SC the tire front space(how far the tire sticks out from the hub) max is ~3.5" your proposed 7ET23.3 w/ 215/40 x17 has a front space of ~3.3" well under the max so these can be used at most any height that you desire. The front tire height is ~24.7" so a fender height of ~25.5"(or maybe a little less) would look good. The front axle torque loss compared to 7x16 Fuchs is ~3.1 lb ft, this is from the weight increase. Compared to the 7x16 w/ 205/55 tires the 215 will be ~2.5mm closer to the lip

In back, for an Sc or Carrera the tire front space max is ~4.5", you proposed setup w/ 9ET15 & 245/40 x17 tires is ~4.23", well under the max so these will fir well at most any ride height. The tire height is ~24.7", so the rear fender height should be ~25"(or maybe a little less but set to ~.5" less than the front). The torque loss due to weight increase is ~.6 lb-ft but the gearing due ti a shorter tire more than makes up for that giving gain in torque available for acceleration of ~2.4 lb-ft. The net torque change for the summation of both axles is a loss of ~1.3 lb-ft, this will be unnoticeable. Compared to the 225/50 on 8x16 the 245 will be 1.9mm closer to the lip
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Old 05-13-2018, 11:52 AM
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Bill - would you mind doing a comparison going from 255/40-17 to 245/40-17 (both RE71)?
Old 05-13-2018, 12:45 PM
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Bill, I have been getting away with 23.75" (front) and 24.25" (rear) ride height. This, with very old tired shocks and stock 16" wheels. 22/29 T-bars. Along with the wheels and tires, this car will have custom-valved shocks. Fenders are not rolled.

Can I get away with that ride height using the wheels and tires we are discussing? Or should I raise it a half-inch?

You didn't discuss the 225 front tire idea. (unless I missed it by not yet reading Page 3 in this thread) I don't like understeer, but 225s might not fit, or might not be 'right' on a 7" wheel.
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Old 05-13-2018, 06:48 PM
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Quote:
Originally Posted by Superman View Post
Bill, I have been getting away with 23.75" (front) and 24.25" (rear) ride height. This, with very old tired shocks and stock 16" wheels. 22/29 T-bars. Along with the wheels and tires, this car will have custom-valved shocks. Fenders are not rolled.

Can I get away with that ride height using the wheels and tires we are discussing? Or should I raise it a half-inch?

You didn't discuss the 225 front tire idea. (unless I missed it by not yet reading Page 3 in this thread) I don't like understeer, but 225s might not fit, or might not be 'right' on a 7" wheel.
Should be fine at any ride height
Here's the comparison between stock 7 & 8 x16 Fuchs w/ 205/55 & 225/50 and Euromeister 7 & 9 x17 w/ 215/44 & 245/40
Look at the tire clearance sections

front: the 215/45 x17 is a more compact tire in all dimensions(except tread width). The 17 is shorter by 5.1mm and there is actually .3mm more clearance at the lip than the 205 when mounted on 7s. More room all around.
rear: the 245/40 is shorter but wider w/ 3.2mm less room at the lip, this shouldn't be a problem at any height

Here it is w/ 225/45 instead of 215/45 x17

w/ th e 225/45 the clearance at the front lip goes down by 6mm and the tire OD goes up by 2.5mm compared to the 205/55 x16,

you'll have to check your own car to see if this is ok.

Your ride heights are odd,
Quote:
23.75" (front) and 24.25" (rear) ride height
usually the front will want to be ~.25 to .5" higher than the back(w/ tires that have the same OD). This should give a slight nose down attitude.

of course the larger delta in installed tire OD the more that will change
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Old 05-14-2018, 03:51 AM
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Quote:
Originally Posted by kjchristopher View Post
Bill - would you mind doing a comparison going from 255/40-17 to 245/40-17 (both RE71)?
Here's the one I posted earlier w/ 255/40 x17 RE71
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Old 05-14-2018, 03:53 AM
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With the Euromeisters I'd recommend 225s/255s which will keep the proper front rear balance. 215 is too narrow with a 255, and 225 fits nicely on a 7" rim. You'd need to partially roll the edges of the front fenders between the 11 and 1:00 position to keep from catching the edge on a hard dip.
Old 05-14-2018, 11:01 AM
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Quote:
Originally Posted by Steve W View Post
With the Euromeisters I'd recommend 225s/255s which will keep the proper front rear balance. 215 is too narrow with a 255, and 225 fits nicely on a 7" rim. You'd need to partially roll the edges of the front fenders between the 11 and 1:00 position to keep from catching the edge on a hard dip.
It's more complicated than just comparing a couple of tire sizes
handling balance is affected by the grip ratio at each end of the car, more grip in front is less under/more oversteer, and vice versa, concentrating only on the wheels and tires relation to grip
using 205/55 x16 & 225/50 x16 on 7 & 8 as a base and assuming that the tire compounds and construction are identical or similar
215/45 x17 & 245/40 on 7 & 9 x17 will up the grip at both ends of the car but more at the rear. 225/45 & 255/40 will up that but the gains won't be as large because of the relation of the tire and wheel. Best grip is when the largest wheel spec'd for a tire is used. 205/55 x16 is spec'd for a 5.5 to 7.5, 7.0 is near the large end, a 215/45 is spec'd for 7 to 8 7 is at the small end so the s/w isn't stretched as much, 225/45 is spec'd for 7 to 8.5, 7 is at the small end but more so than the 215 on 7, so not as much grip gain as the step from 205 to 215. Similar thing happens in back.

the real killer though is the inertial cost and the gearing cost compared to the grip gains.

in front again compared to a base 205/55 x 16 on a 7 x16 Fuch the extra inertial cost of the 215/45 on the Euromeister 7 is ~+4.78 lb-ft(this is the loss available for acceleration), the 225/45 on the E/M 7 is ~+7.01lb-ft

in back compared to the base 225/50 x16 on 8" Fuch the 245/40 on 9x17 E/M has ~+6.3lb-ft loss the 255/40 ~+6.91lb-ft

also in back you have to consider the gearing coast due to a taller tire
again compared to a base 225/50 x16 the 245/45 is ~-2.4lb-ft(this is extra torque now available for acceleration) while the taller 255/40 has a ~+1.2lb-ft loss

net net the 215/45 + 245/40 has a +8.7lb-ft cost and the 225/45 + 255/40 has a ~+15.1lb-ft loss

You have to make a decision, is the small amount of extra grip worth the loss of torque available for acceleration, the more motor you have the more it makes sense. On my 2500# 3.6 I use 8(but very light wheels) 225/45 & 9.5(again a very light wheel) 255/40, it makes sense there, I used to use 235/40 & 275/40 on the same wheels that didn't make sense it was a very poor compromise as the extra grip didn't compensate adequately for the torque loss

Lastly there is the fitment issue
205/55 on a 7ET23.3 on the front can be a tight fit, 215/40 on 7ET23.3 is slightly narrower and shorter it eases the fitment, 225/45 on 7ET23.3 is a bit wider and taller it makes fitment a bit harder.

fitment in back is much less of an issue




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Old 05-14-2018, 01:43 PM
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Bill, thank you for the modest but important information my tiny brain has captured from your remarks and charts. For acceleration, at least to 100 mph, I'd go with 165/60s on 15" wheels. For autocross, I'd go with 225s on the front and perhaps 245s or even 235s rear on 7x9s. These cars push like crazy, stock. For aesthetics, perhaps 225s and 255s. Naturally, I have questions:

1) I am thinking of BFG G-Force-Comp 2 A/S tires. Does this make a difference?
2) Are these tired grippy and otherwise well-behaved?
3) In the chart above for 225s, the lip clearance appears to be -0.1". Might I need to roll the front fenders?
4) If so, 1/10 inch is pretty miniscule. At 11 and 1 o'clock. I think a belt sander would work for this. Am I crazy? Okay, Im crazy but what about the belt sander idea?
5) Again from the chart above, are you suggesting the Euromeister 7" wheels are actually 7.75"?
5) Stock front wheels for my car are 16x6. Euromeisters are one inch wider....or 1.75".
6) I agree with your assertion that tire treads should be narrower, not wider, than wheels. That's not a question, just my opinion. And yours.

You've given me plenty to think about. I'm thinking 215/245. Because of the clearance thing, the torque loss thing and #6 above. Sure would love to address this understeer thing, but you believe the 225 solution, at least on 7" (nominal) wheels, offers minimal grip and overall performance increase because of the narrow wheel. Now...if there were an 8" wheel (and a belt sander), then 225 might make more sense.

7) Have I got this right, now?
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Old 05-14-2018, 04:16 PM
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Quote:
Originally Posted by Superman View Post
Bill, thank you for the modest but important information my tiny brain has captured from your remarks and charts. For acceleration, at least to 100 mph, I'd go with 165/60s on 15" wheels. For autocross, I'd go with 225s on the front and perhaps 245s or even 235s rear on 7x9s. These cars push like crazy, stock. For aesthetics, perhaps 225s and 255s. Naturally, I have questions:

1) I am thinking of BFG G-Force-Comp 2 A/S tires. Does this make a difference?
2) Are these tired grippy and otherwise well-behaved?
3) In the chart above for 225s, the lip clearance appears to be -0.1". Might I need to roll the front fenders?
4) If so, 1/10 inch is pretty miniscule. At 11 and 1 o'clock. I think a belt sander would work for this. Am I crazy? Okay, Im crazy but what about the belt sander idea?
5) Again from the chart above, are you suggesting the Euromeister 7" wheels are actually 7.75"?
5) Stock front wheels for my car are 16x6. Euromeisters are one inch wider....or 1.75".
6) I agree with your assertion that tire treads should be narrower, not wider, than wheels. That's not a question, just my opinion. And yours.

You've given me plenty to think about. I'm thinking 215/245. Because of the clearance thing, the torque loss thing and #6 above. Sure would love to address this understeer thing, but you believe the 225 solution, at least on 7" (nominal) wheels, offers minimal grip and overall performance increase because of the narrow wheel. Now...if there were an 8" wheel (and a belt sander), then 225 might make more sense.

7) Have I got this right, now?
You have done the right thing w/ t-bars to reduce understeer
to continue the process w/ wheels and tires you'd want wider front

The BFG G-Force-Comp 2 A/S tires aren't particularly grippy, the RE71R that I used above are near the state of the art for grip(track or A/X or street) and yes tires make a difference, even small tire and wheel differences can have a relatively large net effect

Here's the update, 225/45 front to reduce under steer, BFG tires, compared to 6 & 8 Fuchs


notes
see the front end front space, it's now red yes, you'll need to address the lip clearance, and probably restrict ride height. I don't like that tire on that wheel

also note the rather large effect on torque available for acceleration, you're losing almost 18lb-ft just to rotate the wheel & tires
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Old 05-15-2018, 04:29 AM
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Just for general comparison here's mine
wheels 8ET25 & 9.5ET19.05
tires Dunlop * Spec 225/45 & 255/40
weight 2500#
torque 238lb-ft @6kmax
21/26 t-bars, stock sways


Notes
car is set up for street and occasional track, ie low speed understeer, transitioning to neutral, aero helps
the front 225 on 8 at nominal s/w could fit better but when mounted on the 8 the s/w is stretched to 231mm, front space is 3.57". A tad above SC/Carrera max. To make them fit the lips are shaved and ride height is a tad higher than I'd like. Rears are a perfect fit. 8s w/ ET27 to 30 would also help as that moves the whole assembly in another 2 to 5mm

torque loss was acceptable to me as the motor and weight make for a very lively package, a strong 3.0 would have ~175lb-ft @4500-5000 and ~2900#, the torque loss would be noticeable


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Old 05-15-2018, 05:31 AM
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Thanks, Bill. Perhaps I will look at a different tire. I like grip. Also, perhaps the 215 is the best front tire choice, given the geometry of these wheels. I can make the rear sway bar a bit stiffer. There are two mounting holes at the drop link end. Also, BTW, my car is 2500 lbs or a little less, depending on fuel.
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Old 05-15-2018, 06:12 AM
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That's more than 100lb lighter than my car at it's absolute lightest. Neat!

My $.02: If time on the clock is important, always choose compound over size. A slightly undersized good tire is going to blow out of the water a perfectly sized crappy tire every time.

Looks like you're doing some AX. Are you trying to drive in any particular class and are bound by rules?

Zero rules - try Avon crossply or radial slicks
DOT R - Hoosier A7 is king
200TW - Bridgestone RE-71R or BFG Rival S 1.5
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Old 05-15-2018, 06:34 AM
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Quote:
Originally Posted by Superman View Post
Thanks, Bill. Perhaps I will look at a different tire. I like grip. Also, perhaps the 215 is the best front tire choice, given the geometry of these wheels. I can make the rear sway bar a bit stiffer. There are two mounting holes at the drop link end. Also, BTW, my car is 2500 lbs or a little less, depending on fuel.
I agree, because of the ET of those wheels 215/40 is the best fit for a 17 in front & the best match for the 215/40 in front is the 245/45 in back.

This setup will have the best fit, least inertial and gearing cost

Depending on tires you will see an increase in grip at both ends w/ a bit more of an increase in back.

If you really want all seasons, the Michelin Pilot Sport A/S 3+ seem to get great reviews, they are a few #s lighter in 215/40 & 245/40 x17(as will your wallet) than the BFG A/S. Tread wear is 500 compared to 400 for BFG but that is a useless datum across different manufacturers lines
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Old 05-15-2018, 06:48 AM
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Quote:
Originally Posted by Driven97 View Post
That's more than 100lb lighter than my car at it's absolute lightest. Neat!

My $.02: If time on the clock is important, always choose compound over size. A slightly undersized good tire is going to blow out of the water a perfectly sized crappy tire every time.

Looks like you're doing some AX. Are you trying to drive in any particular class and are bound by rules?

Zero rules - try Avon crossply or radial slicks
DOT R - Hoosier A7 is king
200TW - Bridgestone RE-71R or BFG Rival S 1.5
Good input!

I'll just re-add that 'If time on the clock is important' ie max performance, after compound, the relationship between the tire and wheel is most important, for max performance from a 215/40 you want an 8( for the really serious stretch that tire onto an 8.5 but that complicates fit), to fit that on a 911 SC/Carrera the wheel needs ET25-30 , for max performance from 245/40 use a 9.5(again that can be stretched to a 10 but then again fit is more complicated)

Of course in the real world cost and availability are as important as anything else and for the price the Euromeister & & 9 x17 though compromised is not that bad a choice

The tire's Driven mentioned above are the best for grip, the penalty is life and wet performance. I have Dunlop * Specs which at the time of purchase were grippiest but the RE71R on my 993 are worlds grippier, right up there w/ the DoT-Rs that I usually use, But they are also nearing the end of their useful street life after only a few thousand miles
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Old 05-15-2018, 07:00 AM
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Here's what a maxed out setup for 215/4 & 245/40 tires might look like using Braid wheels. Because of the custom ET's available you could also do 225/45 & 255/40

here's the 215/4 & 245/40 on 8 & 9.5 x17


here's the 225/45 & 255/40, more grip but w/ a net loss of torque



Main downside here is cost
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Old 05-15-2018, 01:04 PM
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I live in the pacific northwest. It rains here more days than not. I need a tire that can pump water away.

Matt, we don't need no stinkin' A/C here. That's 50-100 lbs
Sound pads on both sides of the firewall....gone
spare tire and jack....gone (towing insurance is available)
Radio and speakers....gone
Backdated heat
Rear deck lid is fiberglass with a wing

These cars are highly weight-sensitive. 356 guys think SCs are heavy, sluggish-feeling.

Naw. I might still visit an AX event occasionally. BTDT. All the rules for my car are set by.....me.

Thank you very much for the tire suggestions. Much appreciated.

Yes, cost is important. I could afford $6K in tires and wheels, but I'd rather pay $1.5K. I like the look of the Euromeister wheels. I think I'll get them, and find the right tires. Looking forward to the improvements, aesthetic and performance.
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Old 05-15-2018, 04:27 PM
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Consider using 7" wheels in front, 8" in rear - an available fitment in euros, and a better fit for most tires.

IMO a 225 is a bit wide for a 7" wheel and looks clunky on a 911. 245 is too narrrow for a 9" wheel.

On one of my cars I went with 205 front 255 rear on 7 + 9 euros, but it's a full weight G50 coupe and more of a cruiser than anything so I wasnt as concerned about weight.

I think it looks good.



My other car is a bit racier, lighter and I am trying to keep it as light as possible. I went with a 215 in front on the 7" wheel and a 245 on back on an 8" wheel. Tires fit the wheels, and the car, well. It looks a bit goofy in this pic because I lightened the car a bunch and didn't get around to lowering the rear and corner weighting it yet.

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Old 05-15-2018, 04:56 PM
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This is a Red Line archive from Pete Zimmermann's "Used 911 Story". We've seen it all.
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Old 05-15-2018, 05:40 PM
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This is a Red Line archive from Pete Zimmermann's "Used 911 Story". We've seen it all.
Yeah, but don't you know the Chinese have solved that cracking issue? Just like the issue that had with having way too much industrial lead-slag waste in their country? Their solution: putting a little bit of it in every product they sell in order to get rid of it all.
Old 05-16-2018, 03:40 AM
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Quote:
Originally Posted by Jonny042 View Post
Consider using 7" wheels in front, 8" in rear - an available fitment in euros, and a better fit for most tires.

IMO a 225 is a bit wide for a 7" wheel and looks clunky on a 911. 245 is too narrrow for a 9" wheel.
It all depends on what you want from a tire, it appears that you evaluate by looks and comfort.

Each tire has a design spec, Racing engineers will often stretch the design spec and use .5" wider than spec wheel.
225/50 x16 design spec is 6" to 8" wheel, you get the best comfort/least grip from a 6 and the best performance from an 8.
225/45 x17 design spec is 7" to 8.5" wheel, cushiest ride/least grip is w/ a 7 and best performance is w/ an 8.5.
245/45 x16 design spec is 7.5" to 9". softest ride/least grip is w/ a 7.5" and best performance is w/ a 9.
245/40 x17 design spec is 8" to 9.5", best ride/least grip is w/ an8 and best grip is w/ a 9.5

The reason wider/lower profile is better for grip is that the wider wheel stretches and stiffens the sidewall, this lowers the slip angle that the tire runs at.

here are the forces acting on a wheel/tire assembly

the main ones to be concerned w/ here are the slip angle and rolling resistance moment aka rotational inertia cost on available drive torque. For the drive tires the gearing cost on available drive torque is an additional concern. In general to minimize these parasitic losses keep weight and OD down.

here's a simplified slip angle diagram, the foundational elements, which are the result of the construction and interaction of the tire w/ the wheel, determine slip angle, the stiffer the foundation, the lower the slip angle and the more grip will be available. Of course other factors such as temperatures(tire carcass and road) and tire pressures also affect this.


The grip provided by a tire can best be seen w/ a friction circle diagram. These aren't really a circle but you get the idea. Supe posted that he has experience at lower speed but not higher speeds. The circle changes a lot as speeds change, particularly in the longitudinal direction(braking/acceleration)


Here's a graph showing relative grip levels from various tire specs @ varying normal force levels(normal force is a combination of the weight of the car +/-any dynamic loads imposed by what the car is doing ie turning, uphill, downhill braking, accelerating. These all add or subtract normal forces depending an the situation), all using the same compound and middle of the spec range for that tire wheel. For instance when braking the normal force on the front wheels goes up & the normal force on the rear wheels goes down, in a left turn the normal force on the right side wheels goes up & the normal force on the left side wheels goes down.


Quote:
Originally Posted by Jonny042 View Post
On one of my cars I went with 205 front 255 rear on 7 + 9 euros, but it's a full weight G50 coupe and more of a cruiser than anything so I wasnt as concerned about weight.
That's an oe 964 fitment(obviously w/ different ETs), designed for safety(understeer), cost and comfort


Quote:
Originally Posted by Jonny042 View Post
My other car is a bit racier, lighter and I am trying to keep it as light as possible. I went with a 215 in front on the 7" wheel and a 245 on back on an 8" wheel. Tires fit the wheels, and the car, well. It looks a bit goofy in this pic because I lightened the car a bunch and didn't get around to lowering the rear and corner weighting it yet.
yes a little racier w/ a little less understeer, wider wheels would make it racier yet.
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'76 Carrera 3.6RS(nee C3/hotrod), '95 993RS/CS(clone)
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Old 05-16-2018, 04:00 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #80 (permalink)
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