Disadvantages of bigger diameter & lower profile (Heavier wheels)

[924Srr27l]

(See attachment)

An interesting test on a Golf when fitted with several different
Wheels & Tyre widths and sidewall profiles from the standard 15" (65) to 19" (35)

It's the extra weight which Kills the performance & speed.
Decreasing acceleration 0-60 & 100mph and 1/4 mile times etc..

The extra stability and different Contact Patch shape on the bigger Diameter rims
does provide slightly more cornering but only 0.06G better which can't be felt from the Hot seat!

MPG is worst of course, but braking slightly better retardation, and finally the increased Unsprung weight
will affect the entire suspension system and produce adverse effects on the road.


R

 

[blade7]

What were Porsche thinking when they fitted 17" wheels to the 968, and then 18" wheels to the 968 turbo...

 

[oliver]

Unsprung weight savings also contribute to better ride and handling.

"Bumps and surface imperfections in the road cause tire compression—which induces a force on the unsprung weight. The unsprung weight then responds to this force with movement of its own. The amount of movement, for short bumps, is inversely proportional to the weight - a lighter wheel which readily moves in response to road bumps will have more grip and more constant grip when tracking over an imperfect road. For this reason, lighter wheels are sought especially for high-performance applications. In contrast, a heavier wheel which moves less will not absorb as much vibration; the irregularities of the road surface will transfer to the cabin through the geometry of the suspension and hence ride quality and road noise are deteriorated. For longer bumps that the wheels follow, greater unsprung mass causes more energy to be absorbed by the wheels and makes the ride worse”.Wikipedia.org/wiki/Unsprung_mass

 

[924Srr27l]

What were Porsche thinking when they fitted 17" wheels to the 968, and then 18" wheels to the 968 turbo...

They were thinking of the future ! and the fashions and trends required to sell cars!

Just like today 991's have 20" Wheels..

A lot of Porsche wheels compared to other manufacturers are light, but especially in the (70's 80's 90's)
where performance was more important over fashion.

The Forged Fuchs Windmill's , Cookie Cutters and 928 / 944 forged 16's are all nice and light.

The 968 also had some nice light 16" Cups, the 17"Cups available as an option and the cheaper CS were very heavy on the 968!
I took mine off, stayed with 17" but fitted Boxster BBS one piece Rims 22kg less per set! which with narrower tyres and rim widths
made the car so much faster...

Without all the electronic aids on car's today, Large wheels would be such a death-trap.
Can you imagine fitting 20's on a 930 ! All show & No go & Scary!

R

 

[924Srr27l]

Unsprung weight savings also contribute to better ride and handling.

"Bumps and surface imperfections in the road cause tire compression—which induces a force on the unsprung weight. The unsprung weight then responds to this force with movement of its own. The amount of movement, for short bumps, is inversely proportional to the weight - a lighter wheel which readily moves in response to road bumps will have more grip and more constant grip when tracking over an imperfect road. For this reason, lighter wheels are sought especially for high-performance applications. In contrast, a heavier wheel which moves less will not absorb as much vibration; the irregularities of the road surface will transfer to the cabin through the geometry of the suspension and hence ride quality and road noise are deteriorated. For longer bumps that the wheels follow, greater unsprung mass causes more energy to be absorbed by the wheels and makes the ride worse”.Wikipedia.org/wiki/Unsprung_mass

Yes good old Wiki, is spot on.

Other advantages and features for Lightweight and downsides for heavy wheels & Tyres are:

- They can be stopped faster,
- They allow more torque to be available to push the car down the road
- Turning a wheel requires less TORQUE,
- A decrease in the amount of HEAT in the brake rotors due to less energy being absorbed
- More efficient (better MPG)
- Increasing tire size usually also hurts top end acceleration and top speed as it decreases the engine's ability to overcome the air drag holding the car back
- weight distribution a bigger diameter wheel+tyre combo is worse - the heaviest part (the tyre) is now further away from the center of the wheel, In short, it robs precious power from getting to the road
- To think that a wider tyres give you more grip as it puts more rubber on the ground is not entirely the case, as the contact patch
shape that touches the road is different compared to a narrower tyre and directly related to tire pressure and the weight on the wheel.
The primary limiting factor for grip is, the tyre's Coeficient of Friction compound.

- lighter cars require less grip because they have less mass, and hence can use smaller width wheels with further weight and performance gains.



R

 

[blade7]

What were Porsche thinking when they fitted 17" wheels to the 968, and then 18" wheels to the 968 turbo...

They were thinking of the future ! and the fashions and trends required to sell cars!

A lot of Porsche wheels compared to other manufacturers are light, but especially in the (70's 80's 90's)
where performance was more important over fashion.


The 968 also had some nice light 16" Cups, the 17"Cups available as an option and the cheaper CS were very heavy on the 968!


Without all the electronic aids on car's today, Large wheels would be such a death-trap.

[8|]

 

[924Srr27l]

What were Porsche thinking when they fitted 17" wheels to the 968, and then 18" wheels to the 968 turbo...

They were thinking of the future ! and the fashions and trends required to sell cars!
A lot of Porsche wheels compared to other manufacturers are light, but especially in the (70's 80's 90's)
where performance was more important over fashion.
The 968 also had some nice light 16" Cups, the 17"Cups available as an option and the cheaper CS were very heavy on the 968!
Without all the electronic aids on car's today, Large wheels would be such a death-trap.

[8|]

I think there must be a error here? there's no text showing


R

 

[terry993]

All very interesting this. One question I have if this is all correct why do Porsche fit 21" wheels to the GT3RS?

terry

 

[924Srr27l]

All very interesting this. One question I have if this is all correct why do Porsche fit 21" wheels to the GT3RS?
terry

Yes very good question Terry,

Wheels have got bigger on Road cars for numerous reasons:

To accommodate bigger brakes, but also to deal with Increased heat.
Narrower higher profile Tyres have a longer (Front to rear) contact patch and this does not allow the tyre to cool much
between each revolution, Lower profile Tyres have a narrower contact patch front to back, but a wider contact patch from
left to right which cools a lot better.

The heavier and more powerful Road car's evolved the higher profile tyres (70 then 60 series) had to be made with a hard compound
(Treadwear rating) to avoid them overheating, wearing out very quickly and potentially having a blow out.

Widening the tread and making the sidewall smaller decreased the amount of tyre on the road front to rear and allowed softer compounds
to be used, as mentioned previously car's got heavier and more powerful so a bigger diameter Wheel was also beneficial to accomadate
bigger brake discs.

They have also been used as a fashion feature, especially when touring cars had to go large diameter (due to ride height regs)
they started putting them on Hot hatches, and from then on Big wheels were marketed as being cool like Touring cars (but not F1)

In time with more understanding and evolvement in the case of Porsche they have engineered the car's suspension to suit such large wheels
and of course all the electronic aids now work overtime.

If it was possible to fit 21" wheels in your 993, you would then experience an awful dangerous and very uncomfortable driving experience
where the steering wheel be constantly trying to snatch itself out your hands, the car's handling would be terrible as the scrub radius and other
geometry would all be wrong, out of whack and generally it would not be possible to drive the car safely.

Formula one attempting to consider using 18" Wheels (To improve he Looks for younger generations!) and they did try a development set,
but the car not only lapped 12 seconds a lap slower but the driver said the car was far too nervous, skitty and dangerous!
So they've stuck to 13", although they are going wider for next year's car's again to improve the Look, and make better use of more downforce.

The downside of a low profile tyre and large wheel is acceleration (Traction) and braking is not as good as the contact patch of
a larger profile tyre, but today's modern cars are too heavy and powerful to handle these large profiles without causing heat and wear issues, so
they all now engineer many electronic traction, skid, stability, Yaw controls etc....to make them safe for the road.

R

 

[blade7]

All very interesting this. One question I have if this is all correct why do Porsche fit 21" wheels to the GT3RS?

terry

Don't encourage him FFS.

 

[924Srr27l]

All very interesting this. One question I have if this is all correct why do Porsche fit 21" wheels to the GT3RS?
Terry

Don't encourage him FFS.

I don't think your comments are very helpful,

Any online community often encounters a fair number of negative people. Negative people often inundate members with negative posts or bad comments.

Criticizing and destroying someone else’s post, thoughts & ideas take very little thought, whereas building upon an idea and making it better takes creativity and a fair chunk of time.

I would ask that you keep your comments to yourself unless you have something better to add to an adult conversation.

R

 

[terry993]

Hi I find all the information and reasons very interesting to read.

However and just for the record my 993 turbo and 991 GT3RS are both standard specification and unlikely to be changed.

Terry

 

[oliver]

I think it was Gordon Murray who once described unsprung weight on sports cars as being like going for a run in a pair of trainers or a pair of wellingtons. It's one of the principal reasons for speccing ceramic brakes, which can save 4-5 kilos per corner.

 

[924Srr27l]

Hi I find all the information and reasons very interesting to read.
However and just for the record my 993 turbo and 991 GT3RS are both standard specification and unlikely to be changed.
Terry

Yes me too, (I'm Very interested in the true chassis & Handling characteristics and not the Hype or marketing)
especially when it's kept quite secret and for good reason!

The motorsport and Road tuning Industry is a HUGE multi Million pound empire that sells lots of tuning
parts and big wheels & tyres, any shop & Retailer in the country Won't be telling anyone their products
will make your car Slower!

Keeping your fleet stock is a very good train of thought Terry.

R

 

[edh]

The Golf test is interesting - narrower tyres (on smaller wheels) give worse braking & "roadholding" numbers in this test but better acceleration. So which is "slower"? Worth looking at what the racers use (within their regs of course..)

I have no doubt that lighter is better, but it's always a trade off to provide the optimum set of characteristics for your needs. I agree the modern trend for huge wheels is, in general, a styling led thing, coupled with the need to accommodate massive brakes (GT3RS et al). Clever suspension design has made them work though.

The "feel" is more a subjective thing - I don't like the way a 944 (or 924) steers with heavy front wheels, and I've tried a fair few different sets of wheels, from memory;15" teles (6 & 7J), 16" winter cups, clubsports, D90's, Cups, 17" Boxster II & Cayman, Speedline & Cup2 style split rims (both really heavy)

FWIW, my measurements, when choosing wheels for my track car - Cups are thought to be the lightest 16" late offset wheel I think.
16" Cup front (7J?) & contisport contact - 16kg
16" Cup rear (8J) & contisport contact - 18kg
17" Boxster 2 (8J) & AD08 (225/45/17) - 17kg

 

[PSH]

i agree with Terry..all very interesting, I would bet that most car enthusiasts already understand that bigger wheels means a drop in performance, they mostly buy such things for looks. I prefer to use the wheels that the car was designed around, not just their size, though, for me the car needs to maintain the external appearance as it left the showroom, with the one allowance of dropping the height, this does improve a cars performance. Mods in general I'm all for if done properly, sticking a dustbin on the tailpipe doesn't pass muster.

Pete

 

[Lancerlot]

Mods in general I'm all for if done properly, sticking a dustbin on the tailpipe doesn't pass muster.
Pete

Aw! I was a big fan of the Peco Exhaust Booster on my 105E Ford Anglia in bygone days. []
Regards,

Clive

 

[PSH]

Mods in general I'm all for if done properly, sticking a dustbin on the tailpipe doesn't pass muster.
Pete

Aw! I was a big fan of the Peco Exhaust Booster on my 105E Ford Anglia in bygone days. []
Regards,

Clive

haha... I think we've all been there Clive, I used to have a straight through side pipe on my Mk2 cortina, jacked up rear, large bonnet scoop, aka TransAm style.....you could hear it coming from miles away....fun days, I've grown up a little since then, or is it that I've just got a little older? Wife says that I drive like an old man these days,,,,long gone are the days of 170+ on the continent, I have driven many high-performance cars, only when I was introduced to the 951 did I find a car that was, well was right, straight out of the block, a car that feels just as secure on the road at it's highest possible speed no matter what the road throws at it and yet will happily sit in traffic without dropping a rev....heaven....

Pete...very much looking forward to getting my car back from the Bodyshop soon...

 

[924Srr27l]

I would bet that most car enthusiasts already understand that bigger wheels means a drop in performance,
Pete

I don't think this is the case, quite the reverse, hence 2 comments from members on this thread alone why would Porsche put
Fit bigger wheels to their Production cars? (991 gt3RS + 968 Turbo).

It's a mentality that's been made and marketed to sell products, sure a big percentage of wheels are bought and fitted
to achieve a cosmetic look, but I'd say a very high percentage also think they are going to go faster and this is not always a given.

I am also talking from experience (Working in the Road and Race tuning Industry) having absorbed all this information from umpteen product supplier training sessions and reading many magazines that illustrate big wheels and fat tyres are the way forward, it's very hard not to get sucked in! however with age and maturity (I'm sure I had some polished Chrome Exhaust tip on my Capri 21 years ago!) I've since learnt a lot of things are not all the Hype is cracked up to be.

If i'd of not done a 3.5 year Lightweight 924 project, worked with Wheels & tyres for the last 9 years, and also owned 9 Transaxles where I tried a lot of different wheel and Tyre Combo's from 15" to 18" I'd not know what I do now! (That old adage!)


R

 

[TTM]

Been running 17" Cup 2s on the turbo for 2 years, and I have reached the point where the extra stability at Autobahn speeds is largely outweighed by the harshness of the ride, the extra weight in the steering feel and the overall loss of agility and swiftness. Will be moving back to the standard 16" wheels very soon.