Low Profile Tyres. Why?

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IcedKiwi
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Re: Low Profile Tyres. Why?

Postby IcedKiwi » Mon Apr 11, 2016 9:37 am

StressedDave wrote:Tyre widths have significantly increased over the last 15 years to give a bigger contact patch and more safety for the average driver who just turns the wheel and wants the car to go around corners with a minimum of thought. If you have a wider contact patch, you can't support it easily with a higher sidewall and the 'idiocy factor' would play a significant part.


Contact patch stays the same size though, it's only the shape of it that changes. For a given vehicle mass and given tyre pressure, you will get the same area on the ground to balance the N/mm² equation. Is it the shape of the contact patch that gives the safety (i.e a wide short one on a low profile tyre vs a long narrow one of higher profile tyres)? Is it stability of the contact patch as you apply loads to it? Or is it the rate at which the contact patch is refreshed, i.e for long thin contact patches, any given bit of rubber will stay on the ground for longer compared to low profiles tyres and hence asking each bit of rubber to do more work, heats up faster etc?

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StressedDave
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Re: Low Profile Tyres. Why?

Postby StressedDave » Mon Apr 11, 2016 9:50 am

If only it were that simple... because you're not just looking at a balloon, but a balloon fixed to a stiff belt of metal & rubber, the pressure effects are a bit interesting - too much and the edges of the tread lift clear, too little and the centre lifts clear. While force over area has to balance, you aren't getting linear force as a result so you don't get a linear area. It's better not to try and visualise - just run with tyre data.

I've never considered the effect of a longer, thinner contact patch. In my experience, they're pretty close to circular.
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Gareth
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Re: Low Profile Tyres. Why?

Postby Gareth » Mon Apr 11, 2016 9:53 am

IcedKiwi wrote:Is it the shape of the contact patch that gives the safety (i.e a wide short one on a low profile tyre vs a long narrow one of higher profile tyres)? Is it stability of the contact patch as you apply loads to it? Or is it the rate at which the contact patch is refreshed, i.e for long thin contact patches, any given bit of rubber will stay on the ground for longer compared to low profiles tyres and hence asking each bit of rubber to do more work, heats up faster etc?

Does the difference in sprung/unsprung weight make any significant difference? Lower profile tyres are normally wider, fitted to larger, wider wheels, typically resulting in a heavier rotating mass, maybe not so good at keeping contact with the ground?
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Astraist
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Re: Low Profile Tyres. Why?

Postby Astraist » Mon Apr 11, 2016 10:23 am

It sure does if you ask race driver, driving coach and automotive Engineer Ed Dellis:

20+” rims fill the rotating mass with more metal and tosses a thin rubber band of a tire on top.

Compliance goes away, and unsprung mass goes through the roof.

Sure, cornering stiffness skyrockets and steering response gets noticeably snappy since the relaxation period — time it takes for forces to take a “set” — goes away.

Somewhere in between is where you need to be if you drive on the street.


As for air pressure, I find that because of the steel casing in radial tyres nowadays, it takes a highly exaggerated pressure to actually lift the edges of the tread, so small modulations to air pressure have more to do with how stiff the tyre is rather than how big the contact patch is when static.

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StressedDave
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Re: Low Profile Tyres. Why?

Postby StressedDave » Mon Apr 11, 2016 12:39 pm

Astraist wrote:As for air pressure, I find that because of the steel casing in radial tyres nowadays, it takes a highly exaggerated pressure to actually lift the edges of the tread, so small modulations to air pressure have more to do with how stiff the tyre is rather than how big the contact patch is when static.

Your mileage may vary... I saw it on an everyday basis for 10 years.
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devonutopia
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Re: Low Profile Tyres. Why?

Postby devonutopia » Mon Apr 11, 2016 12:43 pm

I have them mainly for a bigger floor patch / contact area. The stiffer sidewall helps with general handling too. For info, I run 215/40 R17 on both road and track.

To counter this, I dislike the lack of comfort, especially on rougher roads.

I also need 17" wheels to clear my front brakes, but not everyone fits 330mm 6 pot stoppers to their Skoda Fabia.

On track, lower profiles definitely help with feedback, grip, etc.

For comfort I drive the van :D 195/60 R15 are wonderful haha!

michael769
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Re: Low Profile Tyres. Why?

Postby michael769 » Wed Apr 13, 2016 8:24 am

martine wrote:I suppose the question is why do racing cars have low profile tyres? (F1 doesn't but it's specifically in the regs).

Is it better handling and more feedback?


It is indeed! It's all about minimizing unsprung weight.

Tyre rubber is quite heavy so minimizing it is beneficial to handling on the track, but if you reduce the width you reduce grip, so reducing the profile is the other option. Modern alloys mean that the rims can be much lighter than the tyres - hence you want bigger rims and smaller tyres.

On the track comfort,noise and poor performance on snow/mud (the main downsides) are not concerns - though there are other trade offs.

Many drivers like the "sport" look on their car so many things that make sense on the race track end up on high performance road going cars and then filter down the spec range. Some drivers are willing to make the same compromises in terms of comfort and noise to get such a look. On road going vehicles some of those compromises can be mitigated by suspension tuning and different rubber compositions - which helps to make such features more palatable to the more average driver.

My fairly modest (205/55 R17) wheels would, I suspect, feel like driving with solid cart wheels were I to have 1960s rubber and suspension design.

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Re: Low Profile Tyres. Why?

Postby Gareth » Wed Apr 13, 2016 9:27 am

michael769 wrote:Modern alloys mean that the rims can be much lighter than the tyres - hence you want bigger rims and smaller tyres.

I'm wondering why you think this - have you weighed any wheels and tyres to check?

Our Octavia came with oem 6.5Jx16 alloys, and when fitted with Goodyear Optigrip 205/55R16 tyres weigh 42.8 lbs. Our oem 6Jx15 steel wheels with Vredestein Snowtrac 3 195/65R15 winter tyres weigh 37.4 lbs. The steel wheels on their own weigh 19.8 lbs.

Against the trend, we use the winter tyre sizing with our Fabia all year around. 6Jx15 alloys fitted with Hankook Optimo 4S 185/55R15 tyres weigh 34.0 lbs. The alloy wheels on their own weigh 17.8 lbs, a bit less than the steel wheels for our Octavia, and these were the lightest I could find in the Skoda oem catalogue. According to a contributor on another forum, the default alloy wheels, 6.5Jx16 with Toyo T1-R 205/45 tyres @ 6 mm weigh more than 37 lbs.

My 80s Alfa has oem 6Jx15 alloys, each weighing 13.3 lbs - interesting how the modern oem wheels weigh more in the same notional size!

The trouble with all these measured weights is that they don't compare the same style wheels, nor the same model tyres, in different sizes. I haven't been able to find tyre weights online - I've had to weigh them myself. I've been able to find a few wheel weights online, but they're often not easily found or recorded for different wheel sizes in a range. This works against being able to directly compare, say, a 195/60 specific tyre on a specific 6Jx15 wheel with the same model 195/55 tyre on the different sized but same model 6Jx16 wheels.
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Silk
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Re: Low Profile Tyres. Why?

Postby Silk » Wed Apr 13, 2016 11:55 am

martine wrote:I've got 18" moderately low-profile tyres and I wouldn't describe the ride as bone-shaking...would you?


I don't remember your car being especially harsh. Perhaps you're just better at avoiding the bumps than I. ;)

Silk
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Re: Low Profile Tyres. Why?

Postby Silk » Wed Apr 13, 2016 12:00 pm

StressedDave wrote:Ignoring the marketing reasons (which probably add to the 'how many inches' have I got argument, the main reason for Low profile tyres is that higher profile tyres won't work within the necessary parameters today.


I'm trying to follow the science as best I can, but what I'd really like to know is, are there any real advantages in having lower profile tyres on a road car and is it inherently unsafe to have, for example, a 200BHP hot hatch on 16" wheels?


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