DE Performance: 15x7/8 or 16x6/7 Pros/Cons?

[euro911sc]

This discussion was started here:

Current best tire/size for 15x7x8 DE/Street SC?

But I have moved it to it's own thread.

What are the pros and cons between these two common rim sets when running DE (as opposed to AX)?

Please note the body style (narrow, SC/Carrera/RS), tire make and sizes, and susp. updates if speaking from experience so we can all compare with knowledge

I currently have a 26 year old SC stock susp so I'm interested in examples of that kind, but I'll take everything I can get

Thanks guys!

-Michael

[Bill Verburg]

on my '73 RS clone I used 7 & 8 x15 w/ 215/60 & 235/55 tires but that was more of a street setup, except it had fully adjustable sways

on my '76 C3, stock suspension w/ 7 & 8 x15 I used 205/50 & 225/50 til the early 80s when you could start to see the writing on the wall wrt the coming unavailability of 15" tires for performance apps. I switched to 7 & 8 x16 205/55 & 225/50 to get the tires I wanted to use. Later 225/50 front and 245/45 rear(on 9s) were also used.

It all comes down to the tires you want, if they are not available in 15 you have to go to 16

I would have been perfectly happy w/ R rated 205/50 & 225/50(on 7 & 8) x15 or extreme performance street 205/55 & 245/45(on 7 & 9) x16. The difference in gearing is not so important for most street use or at most tracks. My C3 w/o lsd had a lot of throttle oversteer, either an effective lsd or 245/45 tires would have cured it, it was just a lot easier to do the tires.

If you A/X or like the stop light drags the 15s will give you an edge.

[KTL]

One thing you also need to consider is appearance. While secondary to performance, you'll find that your 15 in. setup will be noticeably shorter than the 16 inchers. The 15s can create quite a gap between the top of the tire and the fender lines, and also ground clearance if your car is really lowered.

On an SC which is relatively light compared to the later Carreras, i'd be inclined to go with the width of the 15x7,8 setup. Tire selection is still very good for these wheels in 205 and 225 section widths

Nitto NT-01
Toyo RA1 and R888
BFG GForce R1
Kumho Victoracer and Ecsta V710
Hoosier A6 and R6
Michelin Pilot Sport Cup
Hankook Ventus Z214

Lots of choices to pick from! You'll also benefit from less unsprung weight in both the wheel and the tire.

If your going to consider the 16s, you should focus on 16x7,8 since I feel the 16x6 up front is on the low end of width for use with a 205 series tire. A 205 is much better suited on a 7 in. wide wheel and a 225 is better suited on an 8 in. wide wheel. 16x6,7 will perform well, as i've seen friends navigate the track very well with them. But the 16x7,8 will perform better.

[ninesixfour]

Originally Posted by KTL

One thing you also need to consider is appearance. While secondary to performance, you'll find that your 15 in. setup will be noticeably shorter than the 16 inchers. The 15s can create quite a gap between the top of the tire and the fender lines, and also ground clearance if your car is really lowered.

On an SC which is relatively light compared to the later Carreras, i'd be inclined to go with the width of the 15x7,8 setup. Tire selection is still very good for these wheels in 205 and 225 section widths

Nitto NT-01
Toyo RA1 and R888
BFG GForce R1
Kumho Victoracer and Ecsta V710
Hoosier A6 and R6
Michelin Pilot Sport Cup
Hankook Ventus Z214

Lots of choices to pick from! You'll also benefit from less unsprung weight in both the wheel and the tire.

If your going to consider the 16s, you should focus on 16x7,8 since I feel the 16x6 up front is on the low end of width for use with a 205 series tire. A 205 is much better suited on a 7 in. wide wheel and a 225 is better suited on an 8 in. wide wheel. 16x6,7 will perform well, as i've seen friends navigate the track very well with them. But the 16x7,8 will perform better.

Just to add to KTL's info, there are plenty of choices in 205/225 but if you want to go 225/245 your ONLY choice is the Hoosier. I'm in this dilemma now and am considering 225 RA-1s all around before making the jump to Hoosiers.

[RSstop]

Michael,
From experience, when I had my SC I ran 16 & 15's both using 7&8 widths and Toyo RA1's. The 15's were much quicker on tracks like Moroso, Sebring, and Homestead because of gearing and a little because they are lighter weight.

You will need to lower it to take advantage of the 15's.
The new Toyo R888 is available in a 235 for the rear.
I lowered, and aligned an SC recently on 15's that you may have seen around. It is amazing how much lower it is on 15's.
Lyn

[mede8er]

OK...what Lyn said.... : )

[GrantG]

Toyo RA-1's work great on 15 x 7 & 8's (205/50 and 225/50 or some put 225's all around). My favorite tire for road & track. The small size gives a little extra zip with the light weight and smaller diameter (gearing advantage).

[euro911sc]

I'll need to lower my car for sure. Right now I'm at the Euro height, but my street car is lower so I kinda feel funny... it should be the opposite

Yes, Lyn.... *drool* I have seen that car around town Very nice machine.

My only concern about running the same size on all 4s is it will set the car up for oversteer after it already tends there on balance alone. I do not have adjustable sways so I cannot tune it out either.

I would love a 225/235 setup, but alas, the Hoosiers would laugh at me all the way around the track as I do not have the skills to put them to work so it would be a waste

If I could afford a set of 16x8s I would go that way as the tire selection is better for street and track and I already have 7's that would fit the front... but I was able to pick up a 15x7/8 set for a song so now my choice is refinish & sell that set and stay w/ 16x6/7 or sell the 16x6/7 and run the 15x7/8s on the street and track. Perhaps if I refinish the 15's I can sell them for enough to get a set of 16x8s, but that would be at a much later time. We'll see... but for now I'm leaning toward the 15s.

I guess one other concern would be running Daytona on the 15s... Lyn? Brodie?

-Michael

[KTL]

I didn't mention the 225 all around because that seems to be a setup that most give a thumbs-down to. Glad to see some guys here have done the 225 all around and endorse it. I myself have run it with 16x7 wheels and its very capable. But I do think rear sway adjustment is key to help dial the attitude of the car.

Remember that Fuchs aren't your only choice for 16x8. BBS made thousands upon thousands of these wheels. I scored two 16x8 off ebay a few years back for $200 total. They're very good, strong wheels with the perfect offset- same as the 911/930 16x8 which is ~ET +11mm

[JB36]

I'm in the same boat. I've been thinking of buying a set of 15x7/8s because I already have a set of 16s with 8s. These have Bridgestone RE01s that are grippy, but not grippy enough for autox. At my last event I was struggling for grip in a lot of places; especially in the rear end (on and off throttle). Like mentioned, I figure the lighter wheel and gearing would be a plus and cheaper. I may consider a used BBS wheel. Would an 225 R compound tire help on 15x8? I like a 245, but there's not much available. Would sway bar upgrade of some type help? My current setup is Bilstiens (hd/sport combo), 22/28 sanders T-bars w/ elephant racing poly bronze all the way around.

[911st]

I will try at the 225's on all wheels again at the risk of getting beat.

I know of a race instructor that ran all 225's on 7's on his 2300+ lb 73S with 22-28's and 22/22 adjustable sways. On the track he fully disconnected the rear sway bar. For auto cross he would hook it back up.

He was scary fast.

It seems equal tires around lends lends it self to a running stiff front setting on the track. This can give one more stick in the back than a more ballanced set up running smaller tires in the front.

Yes this was a narrow body car but he could have run 205/225's as he was running all 225's. Also, having the extra wheelbase width in the back is a good thing with SC/Carrera flairs.

I know running 225's front and back is not the proper balanced set up but if approached right it might just be faster than 205/225's.

One may also pick up some braking potential with the larger front tires.

I have given thought to going to 7&8 15's for the gearing advantage noted above.

Unless one goes to the 9's in the back sounds like 225's are the best rear tire choice for an 8" rim if a 15 or 16 anyway.

I still like the R888 225/235 option. They are suppose to ware well and require less neg camber than the R1's did. Good for a more stock car.

Just one more crazy idea. Concider selling the 15's. Put the Fuchs 7/16 in the front w 205 or 225's.

Take the money from the 15's and set the 6" Fuchs and sed to Rebbel Racing and have them built your a 17x9.5 and run 255's on the back.

225/255's should be a great combo.

[ninesixfour]

Originally Posted by 911st

I still like the R888 225/235 option. They are suppose to ware well and require less neg camber than the R1's did. Good for a more stock car.
225/255's should be a great combo.

I was excited about the 225/235 R888 combo but I've only heard bad things about those tires. Shorter life, less grip, slower times, more expensive....not very promising.

[911st]

I am not an expert but they are different tires and seem to require different set ups.

There are advantages to the R888's. Just putting the R888's on a car set up for R1's is not going to perform as it was designed.

The R888's are designed to require less neg camber, require a lower slip angle, and have stiffer side walls.

My thought is they may work better with stock suspensions that have less adjustment.

With less neg camber they might be a bit better for a street / track tire to.

[Bill Verburg]

Originally Posted by KTL

I didn't mention the 225 all around because that seems to be a setup that most give a thumbs-down to. Glad to see some guys here have done the 225 all around and endorse it. I myself have run it with 16x7 wheels and its very capable. But I do think rear sway adjustment is key to help dial the attitude of the car....

Your need to keep in mind the usage of the car. An A/X will not have the same needs as a DE, street will be somewhere in between.

handling is fluid and changes w/ speed. A car set up for DE will exhibit too much push for A/X because of the speed and tightness of the course.

tools for changing handling characteristics are
alignment/ride height/corner balance
tire pressures
tire sizes
tire construction
wheel sizes
wheel o/s
sway bars
main springs
differential design
weight distribution
suspension geometry

a DE car needs to be set up accepting low speed understeer because as speed builds so does over-steer. A street car, especially as delivered by an oem, needs to have understeer all the time for liability reasons(it's considered by the legal system to be safer).

What works for one person may not work for another in a different milieu, some things that can be done on one car cannot be done on another because of body work or other design factors, the further you go from a conventional setup for your car and usage the more out on a limb you will find yourself(toward over or under steer). Walk the pits where you participate most w/ an open eye(and camera because you will miss a lot) and you can learn a lot about what works, be wary of the outliers but do look carefully w/ an inquiring mind as to why they are doing something.

Again, yes, there are a fair amount of good options for 205/225 x15 R rubber but that is not the case w/ street rubber.

If the tire you want is available in 205/225 in both 15 & 16 in both cases for use on 7 & 8. The 15 will outperform it's taller & heavier twin.

[euro911sc]

Should have know it was too good to be true... guy had 16x6/7s... not 15x7/8s!

Oh well. good info here still

-Michael

[CRNT918]

I agree with KTL, I've run 225's all around on 7's for years. I use spacers/adapters on the rear to fill out the wheelwells. Another advantage is being able to rotate tires front to rear.

[KTL]

Originally Posted by Bill Verburg

Your need to keep in mind the usage of the car. An A/X will not have the same needs as a DE, street will be somewhere in between.

handling is fluid and changes w/ speed. A car set up for DE will exhibit too much push for A/X because of the speed and tightness of the course.

tools for changing handling characteristics are
alignment/ride height/corner balance
tire pressures
tire sizes
tire construction
wheel sizes
wheel o/s
sway bars
main springs
differential design
weight distribution
suspension geometry

a DE car needs to be set up accepting low speed understeer because as speed builds so does over-steer. A street car, especially as delivered by an oem, needs to have understeer all the time for liability reasons(it's considered by the legal system to be safer).

What works for one person may not work for another in a different milieu, some things that can be done on one car cannot be done on another because of body work or other design factors, the further you go from a conventional setup for your car and usage the more out on a limb you will find yourself(toward over or under steer). Walk the pits where you participate most w/ an open eye(and camera because you will miss a lot) and you can learn a lot about what works, be wary of the outliers but do look carefully w/ an inquiring mind as to why they are doing something.

Again, yes, there are a fair amount of good options for 205/225 x15 R rubber but that is not the case w/ street rubber.

If the tire you want is available in 205/225 in both 15 & 16 in both cases for use on 7 & 8. The 15 will outperform it's taller & heavier twin.

Bill,

I don't disagree with ANYTHING you've said above. Only reason I mentioned some people (not calling you out indirectly) tend to poopoo the 225-all-around setup is because the general consensus/feeling I get is that there's fear the setup is too oversteerish for DE use. My experience has not been so with myself and others local to me that have run this setup.

With max camber the stock underpinnings will allow, stock or uprated springs, R compound tires, rear 21-28mm spacers, and an adjustable rear sway bar, the 225-all-around setup has proven to be quite agreeable and fast at the tracks we frequent.

As you said, one needs to understand what more needs to be done to make this setup work fairly well. Max camber, good alignment, rear spacers, adjustable rear sway bar, rolled front fender lips and a liking for a bit of high speed oversteer are all required to enjoy the benefits of 225-all-around. Agreed that what appeals or works for one guy may not be to the liking of another. I'll be honest that the original incentive for 225-all-around was because of what Pete said- same wheel and tire on all four corners allows one to balance tire wear more easily. The fact that the car handles quite nicely and is quite fast to boot was a somewhat unexpected side benefit!

At our last event a few weeks ago, I was absolutely shocked that a friend who runs this setup with Hankook Z211 Rcomps was able to again shave some time and keep pace with me on my new 16x8/9 225/245 Victoracer setup! His car is also on stock t-bars while I run 23/31 and Konis single adj. that I can somewhat tune. I know I had more time to shave since I was learning the new car setup at the track and was not finding the limit in a number of corners. But I still bested my best times at this track by almost 2 sec., with fairly respectable times Point is, i'm shocked that this guy keeps shaving time on his 225's and stock springs!

[911st]

Thinking out loud.

Running all the same size tires on a 911 is one extreme. To make this work at its best I suspect we have to in effect turn our 911's it into a three wheel car so as much as possible of each of the rear tires can be on the ground.

With the larger front tires there is plenty of tire patch with one front wheel to hold a balanced line in a corner. Coming off the track with this set up one should find hot rear tires and likely front tires that are running much cooler as they are only working half as much.

Normally equal ft & rr tires would be best on a 50/50% balanced car.

However, a stock Carrera is 40/60% or like a 2240 car riding on the front wheels and a 3360 lb car riding on the rear. (Assuming a 2800 lb car.)

If one was trying to match tire patch area to weight a more balanced tire combo would be something like a 205 / 255 mix.

This would require a full rethink of the suspension and a car set up for the tire combo above would likely need more than adjustment of the sway bars to make it reach its potential.

If achieved, tire temps should get close to equal I would think.

With 205 and 225's I think I get about a 30+ deg difference in front to rear tire temps.

Of cource if you have a monster motor, more rear tire would be a plus as the rear tires now have to do even more work.

Just some thoughts for what it is worth.

[KTL]

Keith,

Your assessment of the attitude of the car with same size tires all around is understandable, but based on my experiences it is not as extreme as you calculate.

The car is very predictable on this setup. With my Koni valving at near full firm (rears full firm, fronts 75%) and 23/31 springs, plus the Tarret/TRG rear swaybar at almost full stiff (which induces oversteer) the car handles very well and is completely catchable at the limit. Note that I don't have a limited slip diff, so my car would not be the most stable under braking, nor on turn-in.

I'm certainly not saying this setup is optimum. But it has merit because i've turned what are considered "fast" times at the tracks I frequent. And keep in mind my suspension tuning is what is typically desired for a "staggered" wheel/tire setup. Yet the car still performs quite well. Again, i'm not saying it's the fastest.

My latest track event showed I lost almost 2 sec. per lap vs. last year. This year I have lost a boatload of weight with fiberglass parts and other diet means, plus installed the 16x8/9 225/245 combo. Last year I had the stock bodywork (with some items removed like fogs and bumper pads) and ran the 225 all around. Were my lap time improvements due to the wheel/tire setup or the weight loss? I'd tend to lean more toward the diet. Whatever the case, the car feels sure-footed with the bigger wheels/tires and the diet. Some places I used to struggle for grip (long high speed sweepers) are no longer and that means even more time to shave! My situation is a VERY bad test case because I ran my car for a long time with a mediocre alignment and poor corner weighting. Now with stiffer springs, big weight loss, staggered wheel/tire, aggressive alignment, good cornerweighting, the car is a completely different car- as should be expected otherwise i'm just throwing $ away!!!

Cheers,
Kevin

[911st]

Keven,

Good real world info. What size ft and rear sways?

I can very much be way off on this stuff.

If my info is right, a 23/31 like a 43% ft 57% rear spring tension distribution. Compared to a 40/60 weight dist this is a front stif set up before the sways are added.

245's are only about 10% bigger in contact patch I suspect. I am thinking that is not much of a stager considering the rear has 50% more weight on it than the front. Should still favor a stiff front set up.

For contrast, a 993 C2S runs 225/285 and an 04 Gt3 is 235/295. That is a stagger. Thus, 225/245 is not really much of a stager considering our weight distribution.

I bet your rear tire temps are running 200 deg and the front is 20-30 less. A balanced tire stager in theory sould run same front and rear temps.

Another thought, are you getting the same neg camber in the front in a turn as the back? Unless the front spindle is raised there is not much camber gain up front with compression compared to the back. If the front tires are not holding there traction as well as they should, the rear would have to be stiffened to compensate I would guess.

Sorry for my free association but I find it very interesting trying to figure out what make these cars work.