Pirelli 18 inch tyres - visualization

[SectorOne]

why 18inch tyre slower than 13inch ?

I'll just address the acceleration part of 'slower.'

As previously posted, the rotational inertia of the wheel and tire varies as the square of the distance of the mass from the axis of rotation. The 18" wheel will have its rim ~1.38 times as far away from the axis or ~1.9 times as much inertia if the mass was the same. But the rim also has more mass because of the larger diameter (by 18/13), so the total increase in inertia is ~2.6 times as much. This increased inertia of the wheels and tires means that more energy must be expended to accelerate them to 60 mph, thus leaving less of the maximum available to accelerate the rest of the car.

Years ago I did a calculation guesstimate of the reduction in 0-60 time for a Mini Cooper S in going from ~ 50lb. stock wheels and tires (!!!) to ~39lb. wheels and tires. With the wheels and tires the same size, the inertia ratio was ~ just linear at 50/39 ~1.28. The guesstimate was an ~.2sec reduction, which is what was achieved on track.

So, with the large number of accelerations in an F1 lap, I'd expect a massive increase in the acceleration time over a lap. (Yes, there will be an offsetting reduction, perhaps, from better cornering.)

Could there be any chance of the rim actually weighing less then the rubber it replaces? (just the part getting replaced)
With so big sidewalls i would assume there´s some pretty hefty strengthening going on and F1 wheels i believe are magnesium today.
Could it be that the more rim you have, the lower weight you get in total per wheel-set?

Also regarding stiffness when on the power, i would assume that the tire will absorb some of the energy from the drivetrain with such big sidewalls?

[bhall II]

Could there be any chance of the rim actually weighing less then the rubber it replaces? (just the part getting replaced)

[...]

Doubt it. F1 tires are made from seriously thin rubber.


Shocked me, too.

[flyboy2160]

...."Could there be any chance of the rim actually weighing less then the rubber it replaces? (just the part getting replaced)
With so big sidewalls i would assume there´s some pretty hefty strengthening going on and F1 wheels i believe are magnesium today.
Could it be that the more rim you have, the lower weight you get in total per wheel-set? ....

SO,

I doubt it. Remember, the portion of the tire that's getting 'replaced' is in the height of the sidewalls, not the tire tread. I seriously doubt that the F1 tires have hefty sidewalls like runflat tires. If I recall correctly, Pirelli said the new tires weight about the same as the old ones.

The key is that the rim on the 18" wheel both weighs more and is farther from the axis of rotation. The rotational inertia has to be going up by some whole number multiple.

I predict that the Teams will bitch mightily about changing, especially after they run the new wheels and tires through their simulators. I'll guesstimate that the lap time deltas will be measured not in tenths, but in seconds.

edit: Ha! The Deuce was posting while I was s...l...o....w...l...y typing!

[bhall II]

Ya gotta be handy with tha steel, if ya know what I mean.

[SectorOne]

I doubt it. Remember, the portion of the tire that's getting 'replaced' is in the height of the sidewalls, not the tire tread. I seriously doubt that the F1 tires have hefty sidewalls like runflat tires. If I recall correctly, Pirelli said the new tires weight about the same as the old ones.

Oh thanks, yea obviously i was talking about the sidewalls. I figured there was some pretty solid weight in that sidewall to keep it somewhat pinned down when pulling multiple G´s. Now i know

[Jersey Tom]

And you´re saying all flagships from any supercar manufacturer could perform better with big profile tyres, but they use low profile tyres only because of marketing....

Are you serious?

From first hand experience, yes. Cars in the UHP category with the big price tags are heavily driven by marketing. Don't forget that the vast majority of those cars don't see track time - they're on display. The tire companies involved usually don't even have any say in it. They're told, "Okay here's our prototype car.. we've already picked the springs and bars, kinematics, etc.. already have picked the rim and tire size.. figure out a tire design in that box which will give us the handling we want."

[Cam]

Fashion. If you had a choice between comfy runners or skinny high heels, your choice depends on what you want others to think, not performance. James May:

Anyway - tyres. I'm not suggesting this is a directly related topic and yet, in a way, it is. A few days ago, I was looking at a slightly tricked-up Nissan sitting on tyres that had about as much give in them as a supermodel's elbow, and I just didn't fancy it at all. Didn't look like it would keep you warm at night.

Then, in America a few weeks ago with Jeremy, we saw a Dodge Charger (the new one) on tyres that had obviously been up vomiting all night, and that looked really uninviting. I mean, the wheels looked positively unwell. Terrible, it was.

It's been going on for years, this sort of thing, and I think it's time we cried ‘enough'. Some tyres are now so thin it's difficult to tell what they actually are. It's fashion, obviously, because really low-profile tyres are of no benefit outside of the racetrack and only make the car steer and ride badly, so it will probably pass. I think this may be about to happen, and I do hope so.

May is right and wrong. F1 has proven for years that 'really low-profile tyres' are not required at the top level of Motorsport. The cars zip around quite fine. So why change? Fashion. Gucci needs to sell more hand bags.

Hockenheim (and others) has become a runway, to sell next seasons shoes. And they wonder why no one turns up to watch.

[mrluke]

F1 has proven for years that you can use bigger tyres if they are strictly mandated by the regulations, it has not proven that deep sidewalls are better than thin ones.

Better yet, remove regulation on wheel size but specify the required rolling radius, let the teams decide what is best.

Why is the rim size regulated anyway?

[Cam]

Agreed. The rules dictated the tyre, but based on what? Did they pick a rim and wall size from thin air? Maybe we need to go back and look at the reasons they thought low-profile tyres, we're not acceptable.

[SectorOne]

NASCAR has some serious loads through the tires as well and even they don´t run tiny sidewalls.

[henry]

I predict that the Teams will bitch mightily about changing, especially after they run the new wheels and tires through their simulators. I'll guesstimate that the lap time deltas will be measured not in tenths, but in seconds.

I think you are somewhat overestimating the impact of the increased wheel mass and inertia.

A rule of thumb I remember is that 10kg of vehicle mass equates to 0.4seconds of lap time.

The lowest number of seconds is 2. Which would mean a 50kg increase in effective mass due to the larger rim size, 12.5 kg per wheel. I would be surprised if the change were more than 2.5 kg per wheel. (1 from spoke and rim mass and 1.5 from the mass effect of the inertia change)

Even so I think the teams would still bitch. Businesses hate change which equates to risk. The increased rim diameter would present opportunities to change suspension geometry with different outboard pick up points. Higher wheel mass, but stiffer sidewalls, would need changes to springs, dampers and inerters. The large open area in the wheels would pose a new set of aerodynamic challenges, sealing the diffuser, front tyre wake control, brake to tyre heat management, etc.

The engineers would love the challenge. The managers would think "what if someone makes some better choices than us?". They've seen the consequences of that scenario this year.

I would like to see them try. Would make a change from endless speculation on front wing endplate changes. This year's power unit change has led to some really excellent topics on this forum. I would hope a radical tyre change might spawn more of the same.

[flyboy2160]

I predict that the Teams will bitch mightily about changing, especially after they run the new wheels and tires through their simulators. I'll guesstimate that the lap time deltas will be measured not in tenths, but in seconds.

I think you are somewhat overestimating the impact of the increased wheel mass and inertia.

A rule of thumb I remember is that 10kg of vehicle mass equates to 0.4seconds of lap time.

...The lowest number of seconds is 2. Which would mean a 50kg increase in effective mass due to the larger rim size, 12.5 kg per wheel. I would be surprised if the change were more than 2.5 kg per wheel. (1 from spoke and rim mass and 1.5 from the mass effect of the inertia change)
...

Did you actually do any calculations or are you just quoting something you read? I actually did the calculations for real wheels and real tires on a real car and came up with a number which matched real track times. Based on that, I guesstimate-extrapolated to F1.

What you said is a non sequitur: you can't equate the effect of purely translational mass on lap time with the effect of rotational inertia on lap time. You need to do the calculations, not just incorrectly reuse some "rule of thumb."

When I said "seconds," it was meant in the engineering order of magnitude sense: tenths would be .1 to under 1 second, seconds would be 1 to under 10, tens of seconds is ten to under 100, etc.

[xpensive]

An 18" rim being 12.5 kg heavier that a 13", somehow I doubt that.

[basti313]

I predict that the Teams will bitch mightily about changing, especially after they run the new wheels and tires through their simulators. I'll guesstimate that the lap time deltas will be measured not in tenths, but in seconds.

I think you are somewhat overestimating the impact of the increased wheel mass and inertia.

A rule of thumb I remember is that 10kg of vehicle mass equates to 0.4seconds of lap time.

...The lowest number of seconds is 2. Which would mean a 50kg increase in effective mass due to the larger rim size, 12.5 kg per wheel. I would be surprised if the change were more than 2.5 kg per wheel. (1 from spoke and rim mass and 1.5 from the mass effect of the inertia change)
...

Did you actually do any calculations or are you just quoting something you read? I actually did the calculations for real wheels and real tires on a real car and came up with a number which matched real track times. Based on that, I guesstimate-extrapolated to F1.

What you said is a non sequitur: you can't equate the effect of purely translational mass on lap time with the effect of rotational inertia on lap time. You need to do the calculations, not just incorrectly reuse some "rule of thumb."

When I said "seconds," it was meant in the engineering order of magnitude sense: tenths would be .1 to under 1 second, seconds would be 1 to under 10, tens of seconds is ten to under 100, etc.

Well, I did these calculations for bikes and "rotational inertia" did not play a roll there:
If you do the calculation for rolling cylinder translatory energy and rotational energy are the same. (1/2mv^2) So you always end up with the ratio from the change of the tire mass to the full car mass...this is already nothing. If we add drag to this, the change in the wheel mass does not play any roll regarding the acceleration.

Laptime will only change due to the problems with the suspension due to extra mass and reduced damping by the tire.

[henry]

I predict that the Teams will bitch mightily about changing, especially after they run the new wheels and tires through their simulators. I'll guesstimate that the lap time deltas will be measured not in tenths, but in seconds.

I think you are somewhat overestimating the impact of the increased wheel mass and inertia.

A rule of thumb I remember is that 10kg of vehicle mass equates to 0.4seconds of lap time.

...The lowest number of seconds is 2. Which would mean a 50kg increase in effective mass due to the larger rim size, 12.5 kg per wheel. I would be surprised if the change were more than 2.5 kg per wheel. (1 from spoke and rim mass and 1.5 from the mass effect of the inertia change)
...

Did you actually do any calculations or are you just quoting something you read? I actually did the calculations for real wheels and real tires on a real car and came up with a number which matched real track times. Based on that, I guesstimate-extrapolated to F1.

What you said is a non sequitur: you can't equate the effect of purely translational mass on lap time with the effect of rotational inertia on lap time. You need to do the calculations, not just incorrectly reuse some "rule of thumb."

When I said "seconds," it was meant in the engineering order of magnitude sense: tenths would be .1 to under 1 second, seconds would be 1 to under 10, tens of seconds is ten to under 100, etc.

I did some very rough calculations. I will do something a bit better.

I think the rule of thumb can be applied. It would, I suggest, overestimate the effect of rotational inertia. The wheel mass would typically have a similar effect to,say, fuel mass. It would have an effect both in lateral accelerations during cornering, and longitudinal accelerations all the time. The wheel inertia only affects longitudinal accelerations.

As a quick check. Today at Hockenheim Rosberg's pole was 1:16.5 and his race simulation runs around 1:22.5. This would give 0.6 seconds per 10 kilos as a coarse approximation. So perhaps I'll use that.