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Jamesx19
Spa-Francorchamps


Joined: 10 Jul 2015
Posts: 374
Location: Brighton


PostPosted: Sun Dec 02, 2018 6:29 pm    Post subject: 996 Front wheel and tyre size question Reply with quote

Evening

Opinions required of front wheel and tyre sizes on the 996.1 if your able to assist? I assume some of you guys may be able to offer some thoughts and comparisons.....

At the moment the car (a fairly boggo 996.1 C2) has 8" rims on standard ET50 offset and 225/40 18 tyres together with 9mm spacers at the front and 11" rims ET63 at the rear.
My current geo is 1'20" neg front camber with 1'30" rear.

Car handles well, with lots of front end response and little understeer at "normal" road speeds, though will push on if really pressing on into a corner.

I am looking at buying a set of lightweight rims secondhand, but the fronts are in 9" width ET46 which will need 235/40 tyres I'd have thought?

Opinions and knowledge sought:

What differences in front end bite / traction and handling with the wider rims and tyres?

Will there be less feel through the steering?

Will the car be a bit "traction heavy" and less mobile? One of the things I enjoy is the feedback and feeling of the cars grip ebbing and flowing at sane road speeds...

Car is only road use. Many thanks
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asterix_the_gaul
Suzuka


Joined: 20 Dec 2008
Posts: 1136
Location: Cheshire

1999 Porsche 996 Carrera 2

PostPosted: Wed Dec 05, 2018 4:02 am    Post subject: Reply with quote

235 profile is the minimum fit to a 9" rim, so the tyre will look stretched a bit, 245 or 255 is the ideal size for 9"

Are these still an 18 inch rim or are you going to 19? If going up to 19 you need 235/35

9" rim 46 offset will come out a bit more than your previous with the spacer, like 8mm or so : turn full lock and see how much clearance you have to the wheel arch plastics on current rims and imagine another centimeter..
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1995 993 C2 Cabriolet, Midnight Blue/Marble grey -gone!
1999 996 C2 Coupe, Ocean Blue/Graphite grey
2002 E46 M3 Cabriolet, Laguna seca blue/LSB
 
  
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eabeukes
Silverstone


Joined: 30 Jan 2018
Posts: 128
Location: Aylesbury

1999 Porsche 996 Carrera 2

PostPosted: Wed Dec 05, 2018 12:16 pm    Post subject: Reply with quote

May I ask how on earth do you know these numbers? Is there a magic chart somewhere that shows the matrix?! eh!
 
  
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crash7
Montreal


Joined: 28 May 2011
Posts: 523



PostPosted: Wed Dec 05, 2018 2:23 pm    Post subject: Reply with quote

Assuming your running 285 at the rear?

With 225 x 285 the car will understeer, period.

More tyre at the front helps reduce this, and brings the car a little more towards neutral, albeit a very little bit! - you can fit 235 on an 8" rim.

9" is doable on the front, but you will get rubbing on the arch liners, unless your tracking the car and planning on 245's your running extra wheel width and weight for no benefit.
 
  
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Martin996RSR
Spa-Francorchamps


Joined: 08 Dec 2016
Posts: 288



PostPosted: Wed Dec 05, 2018 3:00 pm    Post subject: Reply with quote

Your proposed 9" wide front wheels may be light weight, but they're still two inches too wide. I've run a 235 tyre on the front of my 996.1 and it (just) rubbed on the wheel arch liner. 245 will definitely rub. My reason for going to 235 was because my car understeered badly. It turned out that the cause was too soft rear springs and dampers, and poor geometry settings. The tyres had a much smaller influence on the handling balance than you would expect.

If you want a nice driving 996 for the road then you should be looking at less wheel rather than more. After a lot of experimentation, I run 255/17 on the rear and 205/17 on the front. I'll be going up 20 215 on the front as and when I can be 4rsed. You have a colossal amount of camber and tyre on your rear and if you really appreciate a car that talks to you about grip limits then you need to reduce your tyre width and reduce your rear camber. With more upright rear tyres you'll find you have more predicatable and better access to oversteer, which is where the fun is.

As my car continues to get lighter, I may well go to 245 rears. The obsession with stupidly wide rear tyres on Porsches needs to stop.
 
  
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asterix_the_gaul
Suzuka


Joined: 20 Dec 2008
Posts: 1136
Location: Cheshire

1999 Porsche 996 Carrera 2

PostPosted: Wed Dec 05, 2018 6:49 pm    Post subject: Reply with quote

eabeukes wrote:
May I ask how on earth do you know these numbers? Is there a magic chart somewhere that shows the matrix?! eh!


Yes, there is... See below.
For rolling radius calculator and poke see will they fit. Com
But essentially the calculation for poke you can do in your head, the lower the offset et number, the further out the wheel pokes out of the arch. A spacer reduces offset and makes wheel poke more.

Current offset 50, minus 9 for your spacer makes 41.
New wheel is 9in vs 8in so 26mm wider, but that's halved as its equal either side of the hub.
New offset is 46 (without the spacer), but 9in wheel so sticks out 13mm more for the wider wheel, but the previous effective offset was 41 so stuck out 5mm more, 13 minus 5 makes 8mm more poke with the wider wheel when dropping the spacer and putting on the 9in wheel.
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1995 993 C2 Cabriolet, Midnight Blue/Marble grey -gone!
1999 996 C2 Coupe, Ocean Blue/Graphite grey
2002 E46 M3 Cabriolet, Laguna seca blue/LSB
 



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Jamesx19
Spa-Francorchamps


Joined: 10 Jul 2015
Posts: 374
Location: Brighton


PostPosted: Wed Dec 05, 2018 8:18 pm    Post subject: Reply with quote

Many thanks for all the responses.

Together you have coalesced some of my suspicions and concerns.

I have I confess, been trying to make a case for them on the basis that they are financially viable rather than dynamically the best option.

Asterix-the-gaul is bang on with the maths and Crash7 / Martin996rsr many thanks for your experiences. That was the sort on information I was after.

The other point that Im not totally convinced by is how much difference a lighter wheel will make in the grand scheme of things. I'll go back to thinking about Fuchs again, but ultimately I'm looking at 3.2kgs off each of the fronts and 4.8 kg off each of the rears.

Albeit the Fuchs allow me to fit smaller rear tyres which I feel he right to say may provide a more involving road driving experience.

Im currently 225/40 fronts and 295/30 rear which as Crash7 points out will always result in understeer, and I suspect doesn't allow me to bring the rear into play with a lift of the throttle as steering lock is applied. Well not at sane road speeds anyway!

Thanks chaps.
For interest the wheels Im looking at came off a 993 turbo and are 9x18 et46 and 11 x 18 et59. I think the rears may stick out a bit too much for my narrow body car and would need 15mm spacers for a turbo body.
Might be good track wheels though as reputed to be 8kgs front and under 9 kg rear.
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asterix_the_gaul
Suzuka


Joined: 20 Dec 2008
Posts: 1136
Location: Cheshire

1999 Porsche 996 Carrera 2

PostPosted: Wed Dec 05, 2018 9:16 pm    Post subject: Reply with quote

had a similar weight reduction going from solid spoke turbo twists to the hallowed hollowspoke twists on my 993. Made naff all difference. Dropping to a 265 rear was noticeable as the rear could then slide a bit easier.remember the tyres weigh nearly as much as the wheels so bigger tyres add weight

The quoted 8 and 9 kg are really light for that size wheels most are at least 1-2 kg heavier. Not that you would be able to tell without scales
.http://www.944racing.de/wheelweights.php[/i]
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1995 993 C2 Cabriolet, Midnight Blue/Marble grey -gone!
1999 996 C2 Coupe, Ocean Blue/Graphite grey
2002 E46 M3 Cabriolet, Laguna seca blue/LSB
 
  
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Martin996RSR
Spa-Francorchamps


Joined: 08 Dec 2016
Posts: 288



PostPosted: Thu Dec 06, 2018 9:13 am    Post subject: Reply with quote

As has been said in another thread on here: 'It's the sum of incremental changes, innit..' i.e. Lighter wheels will have a small but positive effect, but when combined with many other small changes the car can become dramatically better.

I reckon the best choice if you're just going to limit your development of your car to a wheel and tyre choice, then choose a wheel you like the look of that allows you to fit a tyre size that will give the driving experience you seek. An 11" wide rear rim will limit you to a minimum of a 285 section tyre, something which we know will still give you understeer. If you can find a set of Boxster Sport Classics then that would be a great choice as they look great, are 18", so will look nice in the arch, and they are the right width to be able to put sensible tyres on that will allow you to adjust in some corners with the throttle if you want (given appropriate suspension geometry settings).
 
  
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Jamesx19
Spa-Francorchamps


Joined: 10 Jul 2015
Posts: 374
Location: Brighton


PostPosted: Thu Dec 06, 2018 4:45 pm    Post subject: Reply with quote

Some good points both.

I'm chipping away at the weight. I know it's not to all tastes, but I'm happy with the compromises so far;

Rear wiper delete
Air con compressor delete
Rear seats and belts gone.

Maybe 20 -23kgs overall. Minus 16kgs wheels, 20 kgs lighter seats minus 15 kgs battery etc. Might get it down to 1250kgs on narrower tyre.

There are other areas I'm not so keen on changing I.e.
Sound deadening or removing the spare wheel etc.

May leave big ticket items like wheels a bit longer based on how much effect each mod has on the car....
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crash7
Montreal


Joined: 28 May 2011
Posts: 523



PostPosted: Thu Dec 06, 2018 5:40 pm    Post subject: Reply with quote

All 996's will inherently understeer, its a quirk, you can eliminate a lot through setup etc, but in essence you need to adapt your driving style to live with it as oppose to trying to engineer it all out.

I run with 235 / 285 on 8" & 10" which works for me, I may eventually try 245's on the front but I will need adjustable thrust arms prior to allow for correct caster adjustment and to stop the tyres rubbing the arch liners.

996 Cup Cars run 9 & 11" most PCGB guys run 8 & 10, with with appropriate width slicks.

The front tyres also take care of a lot of the braking if your running a decent about of camber when the car is in a straight line you have a reduced contact patch a wider profile will give you a little back.

In regards to weight, the car will always weight more than you think - If you get your car to 1300Kg excluding fuel you will have done well.

I was expecting mine to be circa 1275, C2, no-sunroof, manual, as I have gone after pretty much everything easily achievable, partial stripped interior, buckets, light weight battery, giro discs, coilovers, LWFW, GRP bonnet etc etc with the exception of replacing panels for carbon and wheels. - I have a set of OZ that will drop me closer to 1275 but I like the standard wheels. At present mine weights 1297kg, no fuel.

Chris at CoG mentioned that everyone thinks their car will be lighter than it is.

In my experience you won't get near 1250 unless you invest significantly in replacing parts, carbon wings, plastic windows etc etc

Its also easier to strip weight from the front of the car, spare, tool kit, CD Player, light weight battery etc, but this only serves to compound the understeer issue.

I find the car is faster, with half a tank of fuel than with 1/8, so although fuels adds roughly 1Kg per Litre, the extra weight over the front wheels helps with traction.

I wouldn't get hug up on weight or tyre width, do what you like and works for you.

Last edited by crash7 on Fri Dec 07, 2018 6:56 pm; edited 1 time in total
 
  
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Dammit
Kyalami


Joined: 23 Sep 2016
Posts: 1938



PostPosted: Thu Dec 06, 2018 6:58 pm    Post subject: Reply with quote

I'd really like to try my 17's back to back with my 18's over the same road/track, given the nature of the conversation in this thread (and many others) about the car being more engaging on 17's.

I'm sure it's true, but I'm unsure that I can actually detect it!
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Jamesx19
Spa-Francorchamps


Joined: 10 Jul 2015
Posts: 374
Location: Brighton


PostPosted: Thu Dec 06, 2018 11:02 pm    Post subject: Reply with quote

That's really good interesting info Crash7. And valuable in the sense that is your experience rather than something someone has heard from a third party and regurgitated.

I do wonder at how much this stuff effects the overall experience of the original car. It's still a 911 996 with the same weight distribution and ballpark power/weight, etc.

When I carry a 85kg passenger, I can detect the car is slightly slower and less responsive but not massively different overall.

How often do I use full throttle? Twice in the last month of driving, for about 4 seconds total before I was doing some crazy speed and prudence dictated a coast down to the next corner.

Modded C2's have been compared in magazine articles to a GT3 of the same vintage and whatever the C2 has had done or spent on it, the article always concludes it's nice, but not as good/fun/focused as a GT3.

Though as Martin996rsr says, a host of small changes can add up to a different compromise and overall experience. Biggest difference so far has been the current geo settings.

Food for thought I feel.

Biggest issue with 17" rims? I don't like the looks. How shallow? Yes I know sorry!. Dynamics and driving feel are all well and good, but it needs to look the part right?

Some sound opinions thanks
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coullstar
Barcelona


Joined: 15 Sep 2015
Posts: 1321
Location: Aberdeen/Torphins


PostPosted: Thu Dec 06, 2018 11:14 pm    Post subject: Reply with quote

Reducing weight is always going to be good but as said depends on where from.

I suppose the thing here is the feel through the wheel and how the cars is over normal UK roads i.e. Unsprung mass must have a difference on how the car feels over bumpy roads. Is that not one of the big things to us when driving these cars?

I've had lightweight OZ, STD wheel and loads of aftermarket and I have noticed a difference albeit not massive.

Currently have fairly light wheels with reduced width rear but running 225 and 265 tyres and it feels like a good balance. Is increasing front track not another way to help regards the understeer?
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Jamesx19
Spa-Francorchamps


Joined: 10 Jul 2015
Posts: 374
Location: Brighton


PostPosted: Sun Dec 09, 2018 5:12 pm    Post subject: Reply with quote

Hi Coullstar,

Sorry only just seen your post.

Totally agree. Most fun roads round where I live are B roads, but a lot of C and Unclassified too. As you can imagine the road surfaces are the same as everywhere else in post recession UK, bumpy potholed and a multitude of surfaces.

I'm very chuffed indeed with the Bilstein B8 dampers and H&R springs. They have gotten better with a few thousand miles on them now. The car never "bottoms out" or scrapes he road, and rarely gets to smack the bumpstops with any great force. In addition the ride, whilst firm, rounds off the bumps without feeling crashy.

It results in fast comfortable progress over these type of roads, without having to back off and make concessions for less than favorable road conditions. If lighter wheels made this better, which they should (?) then Id be happy. As long as I like the looks though!

265/35 rears seem to be favoured by those that have responded to this thread. Not much difference in weight between those and 285/30's.

Cast 17" wheels seem very much the same weights as forged 18" ones, and 17" tyres weigh slightly less again. Just can't help being a bit superficial though!
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Dammit
Kyalami


Joined: 23 Sep 2016
Posts: 1938



PostPosted: Sun Dec 09, 2018 5:22 pm    Post subject: Reply with quote

I originally asked for 265's, Chris from Exel convinced me that the 285 fit the wheel significantly better - see the invoice:



I'm now going to add power to counteract my tyre.
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coullstar
Barcelona


Joined: 15 Sep 2015
Posts: 1321
Location: Aberdeen/Torphins


PostPosted: Sun Dec 09, 2018 8:02 pm    Post subject: Reply with quote

265 on a 10in is very slightly stretched so I can see the 285/30 reasoning.
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DynoMike
Barcelona


Joined: 25 May 2012
Posts: 1488
Location: The Cotswolds

2003 Porsche 996 Turbo

PostPosted: Mon Dec 10, 2018 7:19 pm    Post subject: Reply with quote

Fascinating thread, some good info from Crash and Martin RSR.

One other variable on the front end is caster, adding more increases the steering angle induced camber gain, meaning less static camber can be applied in the first instance, giving more braking grip in a straight line. I know mine is a Turbo but running 8' 15" with skinny camber of 55", the car simply doesn't suffer from understeer.

The guys are spot on with driving style adjustments - load the front good and proper on turn in, then use throttle to steer the car. Rear toe has an amazing affect on the front end characteristics, less toe back there tends to make the car rotate faster and grip more keenly on the front.

Will be following this thread with interest Thumb
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Jamesx19
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PostPosted: Mon Dec 10, 2018 8:08 pm    Post subject: Reply with quote

Hi Mike,

Some good info on Castor angle affecting Camber. I hadn't considered that at all.

I know my C2 castor angle is not adjustable but when it was set up at 9e it was circa 8 deg 15'.

I wanted as much front neg camber as I could, and 1deg 20' was achieved. The other value that I wanted adjusting back from the X74 settings was rear camber. It was set at around 1 deg 30'. I asked for it to be wound back to the standard 1 deg 10' as the rear tyres were wearing the inner shoulders fast, but that refused on grounds of safety! In the end we went with circa 1 deg 20' at the rear as well.

This set up feels very nice for road use. Good turn in and really only starts to push in tighter corners where perhaps I am not making best use of proper 911 driving technique.

I have been trained in mostly front wheel drive / front engined cars where the weight is at the front of the car. The safe method of road driving here being to brake in a straight line, off the brakes, then apply a small amount of throttle to balance the car and move weight to the rear and turn in. More throttle applied once through the apex.

Explained to me as "Might not the fastest method, but the fastest method to use and not crash."

Conditioning myself to either brake into the start of a bend, or lift on entering is a challenge. If the corner is sharp with a well sighted exit, its a lot of fun and I can feel the move to a rear balance. But, not as much as I'd like at the moment. The current 295/30's are perhaps preventing that? (Based on Martin996RSR and Crash7's comments)

Mike, maybe less toe might be something I try before swapping rims and tyres. Do you know what yours is for reference?

Cheers James
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Martin996RSR
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PostPosted: Mon Dec 10, 2018 9:20 pm    Post subject: Reply with quote

Before I did a lot of development of my car I took it on track and found it was very keen to swap ends on corner entry, leading to some pretty lurid drifts, some at pretty high speeds too. This was with 285 section rear tyres and 235 fronts. I later found the problem to be duff dampers, but I would still not recommend using a lift to get the car to rotate when you're really on it (lets not get into a debate about driving fast on the road, that's for another thread).

To be fast and safe on the road you need the car set up to behave predictably, and you need to drive smoothly, whether you prefer a little oversteer or a smidge of understeer, if you're spending too much of your time thinking about how to control the car then you'll be spending too little on what's going on further up the road.

I currently have 255 rears, which I think is ideal, and about -1.5 degrees camber and zero toe on the rear. I'll soon be fitting adjustable rear toe arms to allow me to go back to the -0.5 degrees rear camber that I used to have before I went to coilovers. My car is now faster point to point than it's ever been, but the ultra-stable rear end is just a little dull for me and I want to go back to easier-access oversteer.

BTW, you can adjust castor by making use of non-standard coffin arms with pucks in the centre that allow the dog bones bolts to be moved, which moves the outboard end of the coffin arm forward or back as desired.
 
  
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