Disadvantages of bigger diameter & lower profile (Heavier wheels)

[blade7]

A 968 Turbo RS had 18 inch speedline split rims, but the 968 Turbo RS was again designed for its hunting ground to be smooth and fast GP Circuits, not the B346 on a wet sunday morning.

I am all for modifying Porsche cars as many of you will know, but sometimes, Porsche did a pretty good job of specifying a car correctly for its intended market and environment which model by model, is pretty diverse. However, if you want to use the car in a different way to the masses at time of production and have a different set of expectations and requirements to those masses, then modification is the way to go.

Then there is the cool factor.. Cars do look cool with the right large diameter wheels, there is no getting away from that, but that is a completely different design criteria for a different audience, often at the expense of another dynamic.

Welcome back, I wondered if you'd retired to the caribbean []. I don't track my car nor do I kid my self it's dynamically comparable on 18's. I just like the way it looks and the steering isn't ruined like some claim. It's interesting you mention the 968 S, I don't know what brakes they came with, did they need 18's to clear them ? If not why fit 18's when not every track is glass smooth.

 

[924Srr27l]

BLADE7
It's interesting you mention the 968 S, I don't know what brakes they came with, did they need 18's to clear them ? If not why fit 18's when not every track is glass smooth.

It was a Rare one off special model which Porsche often go the whole hog and get really Racey with Looks and Components, so I suspect that's why they went bigger, they've done this for years on virtually all of their cars.

As Jon mentions it was not some mass produced road car that was to be sold in the thousands. I've no doubt this car's ride on public roads would be very harsh!

BLADE7
I don't track my car nor do I kid my self it's dynamically comparable on 18's. I just like the way it looks and the steering isn't ruined like some claim.

Pity that as 18's and tracky tyres would work well.

There's just no way whatsoever that 18" Wheels drive nice on any Classic Transaxle (24/44/68), I understand the "looky" thing is a very popular following and pastime for many (myself included but not the last 2 decades) but as Jon, myself and others will tell you, if you took them off and fitted a set 2" diameter smaller set you'd notice a staggering difference (acceleration, faster change of direction and more grip etc..)

I'd put it down to some people don't feel the same signals as others when driving cars and I'm also convinced the "Looks" are priority for many and the adversity's are ignored.

Jon's key words and the characteristics of these transaxles on 15 & 16's I couldn't describe better myself:
- Nimble, Stable, & at not the expense of other Dynamics with oversize wheels etc.. w

On Autobahns & European smooth roads yes I'm sure 17's are not as bad as what we have here in the UK. But I tried a set recently just in case there were some advantages, but there were none at all and nothing but 100% negative characteristics so off they came and sold on they were.

I also drove a 997 3.8 C4S with 19's last week and thought that wasn't that nice in a straight line at 85mph!

R

 

[blade7]

I have no interest in your opinions, and I'd prefer it if you didn't quote me in your posts.

 

[924Srr27l]

I have no interest in your opinions, and I'd prefer it if you didn't quote me in your posts.

Welcome back, I wondered if you'd retired to the caribbean []
It's a Public Forum where Porsche owners discuss their cars, and swap opinions on many related things.

And this thread is all about the Disadvantage's of fitting bigger diameter & lower Profile tyres, so you or anyone can be quoted
It's quite safe.

R

 

[edh]

..if you took them off and fitted a set 2" diameter smaller set you'd notice a staggering difference (acceleration, faster change of direction and more grip etc..)

Staggering difference in acceleration? I don't think so. More grip? You keep on with this, but it's not true is it? You said a bigger wheel / lower profile tyre will give more lateral grip.

That Chris Harris video has been kicking around for years - is he faster on spacesavers? Does he have "more grip"?

 

[924Srr27l]

..if you took them off and fitted a set 2" diameter smaller set you'd notice a staggering difference (acceleration, faster change of direction and more grip etc..)

Staggering difference in acceleration? I don't think so. More grip? You keep on with this, but it's not true is it? You said a bigger wheel / lower profile tyre will give more lateral grip.
That Chris Harris video has been kicking around for years - is he faster on spacesavers? Does he have "more grip"?

Yes, (Quicker Acceleration) have a look at the golf test (page 1) to give you an idea, the reality when you do a back to back really is very substantial.
Yes, more Grip in acceleration Bigger contact patch (Front to rear)
Yes, Wider has more hard cornering (Laterial grip side to side patch + better cooling) like a Track day and a Race car needs
Yes, CH has done affair amount of video's over the years, Not sure about any timing he didn't mention it ? but for
such a car to be wearing tyres 1/3 the width with how much RWD BHP? the car was fun, so controllable and the best handling he said!

R

 

[924Srr27l]

I know it's not an Easy Subject to understand, mostly due to what Racing Cars do and have, what Modern car's are fitted with, and what the Tuning industry
market and promote.

I think one major issue is some people (No fault of their own) do not realise you cannot drive any car on public roads no Where Near as HARD
as you can on a Racetrack, where even a decent standard Road car can be ruined within 10 laps driven by a Pro. (Brakes Tyres, Engine all Cooking)

Public roads are shockingly poor, they have many adverse conditions & surfaces, a Narrow road width (Not like a track) other vehicles coming towards you
on Single B roads and camera's and machines everywhere! also the sheer volume of traffic about these days is colossal!

Sometimes of the day, and some areas in the UK are better than others but generally I've never been able to drive a car so hard (G force) on public roads
as I have on Racetracks, and not just in fear of being Nicked ! (My lifetime total licence points tally is 19 so far!)

Tyres are the most important part of the suspension, either they are designed to fit it, or the suspension is designed around the tyre.

To think wider tyres make more grip because ''there is more rubber on the road' sounds logical but it's not exactly the case as it does not
automatically mean more grip or that there is actually more rubber on the road from a width and / or diameter change.

When you make a tyre wider, you alter the contact patch to be wider , but it reduces in length.
So depending on sidewall stiffness, a wider tyre can actually give less rubber on the road.

The main reason for tyres going wider is actually heat management. Wider, low sidewall tires will cool better than narrow tall tires.
If you can't get a tyre up to temperature it will give no grip, if you get it too high you will cook the rubber and ruin the set of tyres.

The actual contact patch that touches the road stays roughly the same area with wide or narrow tires (as long as the load & vehicle Weight stays the same).
The shape changes from Narrow will have longer contact patches (Front to rear) and wide tires will have shorter (Left to right).

The reason why F1 tyres are wide is primarily so that they dont cook (remember they arent just wide, they are fairly high sidewalled), but they do have wide
contact patches because this will give better Lateral grip going round corners as they are subjected to Huge amounts of Vertical load (Down force).

A wide tyre will generate more lateral force per slip angle making HARD cornering better on a Racetrack.
This will not be the same case on a public road because the G forces are not as High for continuous amounts of time.


F1 cars DO NOT have wide tyres for the best acceleration (forward motion) because but side grip (Lateral) is more important.
Conversely drag racers actually use the tyres not because of the width, but the tallness. As we know a wider contact patch gives better HARD cornering performance, a narrow but long contact patch is what you want for linear acceleration. So strangely, drag racers will actually be better with narrow tires. So why do some use wide tires? (remember the most important gip aspect of tyres) Temperature!

They want as longer contact patch as they can get, but need the width for cooling. (with 4000+ horsepower you do kind of build temp rather well) But if you look at the contact patch shape between say, an F1 car or drag racer. (both are considered to use wide tyres). The F1 patch will be wider and shorter for good cornering, the drag patch will be longer and narrower (relatively) for good linear acceleration.

It depends on the car, the situation, the conditions. Eg. Competition soft compound Rally cars use wider tires when on tarmac rallys, and use (surprisingly) very thin tires on ice rallys. F1 cars used to use narrow tires until aero began to be used in the 60's. Drag racers acutally want tall tires, width is there to stop the tire being destroyed.


Wide and Low profile tyres for the road run cooler than higher profile narrower tyres which warm up quicker and always have harder compounds to not overheat.

Then as the start of this thread "WEIGHT" is the Killer, when Larger (Heavier) wheels & Tyres are fitted it affects the car's ability to accelerate and hence makes it slower.
The extra lateral grip gained is not that much but if car A gets to 80 mph (1) second faster than car B because it's got heavy wheels fitted then I want car A for the road.

On the track car B 's slightly better cornering G is more advantageous lapping flat out on a 2 mile 15 corner racetrack.

Similar to the Chris Harris Video (which is very adverse - Space saver skinny wheels) I've also found narrower and lighter wheels react better and quicker, like Jon was
saying smaller diameter wheels are also more nimble and feel more Stable this is what I want for pushing as hard as possible on the road (Within the limited boundaries)

Stable is a much nicer, better handling Feeling approaching a Roundabout at 80mph, it means such a lighter (unsprung weight) car can dissipate the speed quicker and easier without putting the attitude of the car under stress due to (heavier) wide wheels taking long to slow and turn and pulling with cambers etc.. it means I can apply throttle earlier mid corner because narrower tyres give more feel than wide slabs and as I've proven with Road testing on many 924's 944's an a 968 .

Hence why I've fitted and are keeping 16" Wheels on my project 924 which has 200bhp per ton with a mechanical LSD and it has NO Wheelspin at all on 205/55/16 Tyres
(on all 4 corners). The Power to Weight is equivalent of a modified 944 Turbo with 300bhp which even on 255 rear tyres I've heard & Read of people saying it spins the rear wheels? (If this is FUN for some which I understand it may be) good for you, but I want no dramatics, wheelspin or a nervous chassis with adverse characteristics due to heavier wider wheels & Tyres because it's much more competent and enjoyable / faster on the road, this is above what it looks like I have no interest in how it Looks to me or anyone else.

R

 

[Eldavo]

Tl;Dr

 

[924Srr27l]

The way Modern Technology and car trends are going the Future looks like combining the two:
Big Diameter Wheels with narrow Rim widths & Tyres, because they are more efficient and have the same footprint as smaller wheels with wider tyres.

No Wonder Classic car values are rocketing, car's will never be, look or drive the same again!



According to tyre-maker Pirelli, instead of the squat, wide tyres we know today, the future is tall and skinny.
Because they face big challenges from tightening European laws covering the amount of noise and rolling resistance tyres generate.

The only way to work around the problem is to go tall and skinny.

The problem with tyres today is that they’re too wide, so they are terrible for noise and rolling resistance.

The solution long-term is for tyres to get taller and skinnier, which by making them taller you can make the footprint (the amount of area in contact with the road surface) narrow and long to keep it the same as the tyres today.

BMW is one of the first manufacturers to use the new breed of skinny tyres in a production car with its upcoming i3 electric car.

When driving an BMW i3, especially for the first time, there is much bewilderment as to how it drives and performs so well with what seems like a monumental list of things working against it. Firstly, there’s the ride quality. It’s fitted with these short little shocks, it’s stiffly sprung and it has eco tyres, yet the i3 remains surprisingly supple over rough pavement. It can also handle extremely well, too, despite being quite tall and narrow, which is the exact opposite of the recipe for grippy handling.???
How does it manage to handle and ride so well when, by the looks of it, it has the worst possible recipe to do so? Well the answer lies in two parts, but it’s the second part that’s particularly interesting.

Firstly, weight has much to do with it. It's extremely light weighing less than a Ford Focus. This allows the stiffly sprung suspension to handle any rough pavement with ease, as there is less mass moving about. This also helps in the handling department, as well. A lighter car is a more nimble car. It also helps that most of the weight is down low, in the floor of the car, where the batteries are.

At the front the BMW i3's wheels are 5" x 19" but the tyres are just 155mm wide, while the rears are 175mm wide.

The i3’s tire requirements were very specific. The tire needed to be narrow and have low rolling resistance , so as to be able to eek out as much range as possible. But narrow and eco-minded tires are terrible for both grip and comfort. So Bridgestone had to invent some new tricks to make these tires both eco-minded and grippy and comfortable. It was quite a task but that’s exactly what they did

The key lies in the size. The i3 is the only car on the market with this tire size of 155/70R19. The 155 is in reference to the width of the tire, which is 155 mm. The 70 is the aspect ratio of the tire which, long story short, means how large of a contact patch the tire has with the ground. And the 19 is the size of the wheel (an optional 20 inch is available), in inches, in which the tire fits to. At 155 mm wide, the i3’s tires are far narrower than anything else on the road. They’re motorcycle tire thin. But because they are so tall, sitting on 19s, and have a large contact patch length-wise rather than width-wise,

" It allows the i3 to grip the road with a similar grip as a much wider tire."

Another benefit of having such skinny, lightweight tires on skinny, lightweight rims, is the lack of unsprung rotating mass. Unsprung mass is any mass that isn’t carried by the chassis of the vehicle, but attached to the outside of it. So brakes, wheels and tires are unsprung rotating mass. So reducing this helps the car ride better, grip better and handle better.

0 - 60 mph 7.3 seconds !

So Chris Harris 's video idea using 4 Space savers on an AMG 400+bhp car with surprising results was not so silly after all. It's not easy to understand a wider tyre does not mean more contact or grip but it is all factual.

Isn't it bizarre that over 100 years ago this big wheeled, skinny tyre electric car was designed by a guy called Mr.Porsche ! and now this concept is making a BIG comeback !

R