Physics of Car Tires...

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Syntax
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Physics of Car Tires...

Postby Syntax » Sun Dec 23, 2007 12:05 pm UTC

Question: When a race car "spins out" at the beginning of a race, is that just for show? One would think that it could get a bit more velocity by utilizing static friction by keeping its tires from sliding.


If I'm right, this fact should imply that a car can stop more quickly on a paved road with ABS than it could without. But, from what I gather, this isn't true. What have I overlooked?

I moved the tire discussion from Friction to here, to avoid things being said that had already been discussed pretty well.

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Re: Physics of Car Tires...

Postby Kabann » Sun Dec 23, 2007 12:27 pm UTC

I'm no expert, but what I have learned is that race tires have a much harder rubber compound than normal street tires... the burnout at the beginning of a race (or in drag racing, just before the start of the race) is to heat the tires up, so the extra-hard rubber softens and gives them more grip.

As far as an ABS system, its primary job is to overcome human error and prevent brake lockup in hard-stopping or poor traction situations. Good control plus good traction on a non-ABS car will potentially allow one to stop in a shorter distance.

Anyone with more extensive knowledge is always welcome to slap me around if I've missed or misinterpreted anything, but I'm pretty confident about the basics.
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Re: Physics of Car Tires...

Postby Ar-Pharazon » Sun Dec 23, 2007 4:02 pm UTC

Friction is weaker when you're sliding than when you're static relative to the surface. ABS keeps the wheels just at the edge of slipping, to get more deceleration than what you could get by braking manually and slipping, since rolling wheels' contact surface is not moving relative to the ground.

I don't know why they spin the tires before before starting though. It seems like a stupid thing to do actually, you'd want the nicer static friction so that you can accelerate faster. Aside from the -likelier- reason above, maybe it has something to do with rolling friction? Not that I have any idea what rolling friction is beyond "some crazy friction force that happens to wheels".

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Re: Physics of Car Tires...

Postby The Ethos » Sun Dec 23, 2007 4:02 pm UTC

While we're on the subject of tires, having had a blowout at 85 mph on the god damn Mass Pike the other day....

Could anyone explain why it is that you are only supposed to go 50 mph for 50 miles on the donut? It's not like you throw it out afterwards, you just put it in the trunk again.....Is it about steering? Does the tire heat funny?

I'm thinking it's a conspiracy put on by the South American Vulcanization plants.

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Re: Physics of Car Tires...

Postby Dropkneedave » Sun Dec 23, 2007 4:43 pm UTC

Kabann: That's more or less it, the tires are designed to run hot so then only function properly at certain temperatures. Same goes for the gear box oil.

There's also another reason. When tire spins it leaves rubber on the tarmac. The friction between rubber and rubber is greater than the friction between rubber and tarmac.

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Re: Physics of Car Tires...

Postby MotorToad » Sun Dec 23, 2007 4:53 pm UTC

Rubber has some funny properties, one of which is that its peak grip is when it's *slightly* sliding. Not spinning like a burnout, but when the tire's making that squealing noise that says it's happy.

Race tires are very dependent on temperature to operate properly. If they're not at the right temperature they not only offer no grip, but they can "cold tear" and screw the grip surface up completely and the tire will never be worth a damn. They're not at all hard, but they would seem so if you used them on the street because they can't come up to temperature.

Passenger car tires are designed to operate across wide temperature ranges. Most summer tires work between around 60ºF and 110ºF (more or less) ambient temperature, probably 40-50º higher than that actual tire temperature. At freezing temperatures, they pretty much don't work at all. Conversely, a snow tire works great at freezing temperatures, but if you run them in the summer they'll wear out in a few hundred miles, and if you run them hard they'll literally come apart in chunks.

Race tires, however, are designed to operate at somewhere like 180-220ºF, and a tire that works at 180º might not work at 220º because the envelopes they're designed for can be so narrow. In Formula One they preheat the tires to 180º and they're considered "cold" when they go on the car, requiring most of a lap to come up to temperature. That's why you'll see them spinning them up to warm them, and making zigzagging motions when they're following a pace car.
The Ethos wrote:Could anyone explain why it is that you are only supposed to go 50 mph for 50 miles on the donut? It's not like you throw it out afterwards, you just put it in the trunk again.....Is it about steering? Does the tire heat funny?
It's about heat. The tire is tiny and can't sustain the kind of heat that high speeds put on it. A space-saver won't blow up as soon as you hit 90 mph, but it will blow out at sustained high speeds. As the tire flexes the little rubber molecules rub against each other and heat up. If the pressure is low they move a lot more and the tire can really heat up and blow out.

Space savers are very soft compounds and actually provide pretty close to the amount of grip as the car's other tires, but they won't do it for long.
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Re: Physics of Car Tires...

Postby Ieatsoap6 » Mon Dec 24, 2007 2:30 am UTC

In addition to the Formula One part of the previous post, for drag racing, they burnout is to put down a fresh layer of rubber at the proper temperature to get better grip.

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Re: Physics of Car Tires...

Postby hipp5 » Mon Dec 24, 2007 11:24 pm UTC

Ar-Pharazon wrote: ABS keeps the wheels just at the edge of slipping, to get more deceleration than what you could get by braking manually and slipping, since rolling wheels' contact surface is not moving relative to the ground.


The more important aspect of ABS is not that it allows you to decelerate faster but that it allows you to steer. You can't steer out of a situation if your wheels are locked up so ABS prevents them from locking up.

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Re: Physics of Car Tires...

Postby TemperedMartensite » Tue Dec 25, 2007 12:36 am UTC

hipp5 wrote:
Ar-Pharazon wrote: ABS keeps the wheels just at the edge of slipping, to get more deceleration than what you could get by braking manually and slipping, since rolling wheels' contact surface is not moving relative to the ground.


The more important aspect of ABS is not that it allows you to decelerate faster but that it allows you to steer. You can't steer out of a situation if your wheels are locked up so ABS prevents them from locking up.


Although ABS is terrible if you need to stop fast on gravel. You can easily stop faster on gravel by locking your brakes and "plowing" than having ABS keep you rolling. Your average driver simply does not have the skill to out-brake ABS, you need to be an incredible driver to stop quicker than ABS without locking the tires.

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Re: Physics of Car Tires...

Postby null » Wed Dec 26, 2007 3:50 pm UTC

With regards to ABS, the way it works is by applying brake pressure, then releasing brake pressure, then applying it again in very quick sucession.

If you look at the mu(friction)-slip curve below:

Image

As was said before, tyres have the most grip when they are slipping slightly (but not too much). As can be seen on the graph, when you are slipping at around 10% of the wheel speed, mu (friction) is a maximum. ABS causes the wheel to slip when it applies brake pressure, and it will quickly pass the peak mu point (at around 10% slip). When the ABS releases brake pressure, the wheel will stop slipping, and you will return to the left hand side of the graph. This process continues creating a 'loop' at the top of the mu-slip curve.

The reason ABS works this way is because its bloody difficult to predict where the top of the mu-slip curve is, or will be, because road conditions vary so quickly. Its much easier to apply and release your brakes very quickly, as you will maintain a slip value that is near the peak of the mu-slip graph.

The mu slip curve varies depending on the surface you are driving on. For wet or loose surfaces it will be much flatter, and the peak mu will be at a slip value closer to 5%. In dry conditions, your peak mu could be at around 15% slip. In dry conditions, you may be able to apply full brake pressure and never get past 15% slip, in which case you would not skid at all. In dry conditions using ABS would increase your braking distances.

If you are very talented, you can apply just enough brake force to give a good coefficient of friction, to slow your car, without locking up your wheels. In this case, you will stop faster than if you used ABS.

Hope that made sense!

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Re: Physics of Car Tires...

Postby Seraph » Wed Dec 26, 2007 7:23 pm UTC

The Ethos wrote:While we're on the subject of tires, having had a blowout at 85 mph on the god damn Mass Pike the other day....

Could anyone explain why it is that you are only supposed to go 50 mph for 50 miles on the donut? It's not like you throw it out afterwards, you just put it in the trunk again.....Is it about steering? Does the tire heat funny?


There are a number of reasons:
1) Your wheels will be spinning at different speeds, making the differential (or equivelant) work a lot harder, and possibly throwing off your ABS.
2) Your suspension/brakes/stearing isn't really designed for tires of two different sizes, so your car can handle funny.
3) If you have the spare on the car, then you're out of luck if you have a flat.
4) The spares don't last as long as regular tires.
5) Spares arn't guarenteed to withstand the speeds that normal tires can.

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Re: Physics of Car Tires...

Postby MotorToad » Wed Dec 26, 2007 8:40 pm UTC

null wrote:With regards to ABS, the way it works is by applying brake pressure, then releasing brake pressure, then applying it again in very quick sucession.
This excellent post reminded me of one other thing about ABS that hadn't been mentioned with regards to Syntax's question.
Syntax wrote:If I'm right, this fact should imply that a car can stop more quickly on a paved road with ABS than it could without. But, from what I gather, this isn't true. What have I overlooked?
Yes, a car without ABS can stop in a shorter distance than one with ABS, on a flat road with a damned good driver in it. This didn't necessarily need a pro driver when ABS first came out to beat it, though, and I'm sure there are more than a few crappy old systems still out there. The first systems commonly available to the public weren't very good at all and the "loop" null mentioned was quite a fat one. These systems would help a mindless driver steer while standing as hard as he can on the brake pedal, but they didn't help the car slow very well. ABS will always win if grip differs for the left and right wheels, though.

The reason I bring this up is that I recommend you go find an empty parking lot and learn about your brakes. Most people have no idea how to use maximum braking in a car, and I've even heard of people that let off the brakes because they were startled by the ABS pump when it came on. If you find yourself in a situation where the ABS comes on, keep in the brake pedal as trying to modulate the brakes as the ABS is modulating the brakes can cause stopping distances to be quite a bit longer.

They made ABS systems in Formula One that were far better than any driver could hope to be, but that sort of thing is way beyond what you'll find at a dealer's lot. :) They had differential braking and even wheel-specific braking to aid cornering. It's thankfully all been banned now, though.
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Re: Physics of Car Tires...

Postby RockHawk » Thu Dec 27, 2007 3:19 pm UTC

This video has some good slow motion shots of how tyres on dragsters deform and slip as they come off the line.

If you watch how much they twist it's amazing they don't rip!

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Re: Physics of Car Tires...

Postby wst » Fri Dec 28, 2007 3:48 pm UTC

Just a thought- would it be more effective to stop by cadence braking (which can be better than ABS), or to flick the car around 180 degrees and dump the clutch, or using engine braking and normal braking to stop the wheels skidding on the road. This is on an infinite piece of perfect tarmac, with perfectly identical tyres for the sake of this, and the car would be, say, heading up shit creek rapidly if you didn't stop.
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Re: Physics of Car Tires...

Postby e946 » Sat Dec 29, 2007 6:34 pm UTC

MotorToad wrote:Race tires, however, are designed to operate at somewhere like 180-220ºF, and a tire that works at 180º might not work at 220º because the envelopes they're designed for can be so narrow. In Formula One they preheat the tires to 180º and they're considered "cold" when they go on the car, requiring most of a lap to come up to temperature. That's why you'll see them spinning them up to warm them, and making zigzagging motions when they're following a pace car.


In nascar, the zigzagging is more to clean off the tires. When they race, little chunks of rubber (called "marbles") come off the tires (Think of the stuff that comes off when you use an eraser) and collects on the unused areas of the track. So if they have to drive through some of it to avoid a crash, or drive through some when coming out of the pits, they have to clean it off.

edit: the marbles do appear in f1 too, but i don't know about the tire cleaning or whether it's purely to heat the tires.
Last edited by e946 on Mon Dec 31, 2007 6:25 am UTC, edited 1 time in total.

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Re: Physics of Car Tires...

Postby null » Sun Dec 30, 2007 2:19 am UTC

*little* chunks?!?! I went to the british grand prix (ok its F1 not nascar) a few years back, and we walked along the track after the race. These 'little' chunks are about the size of a pencil eraser...it really made me appreciate anyone deviating from the racing line must struggle to find any grip at all...

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Re: Physics of Car Tires...

Postby AbNo » Mon Dec 31, 2007 1:17 am UTC

Seraph wrote:There are a number of reasons:....


6) Layman aren't expected to be able to properly put a tire on a wheel, or to properly tighten the lugnuts.

As insulting as it sounds, it's the truth. I've had to help more than a handful of people on the side of the road change a flat, because they DID NOT KNOW WHAT TO DO.

I don't just mean they didn't know how put the jack under the car, or loosen the lug nuts before raising the car, I mean they didn't even know if they HAD jacks.

I get the feeling they would still be sitting on the side of the road if I hadn't come along. :roll:
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Re: Physics of Car Tires...

Postby swifty360 » Mon Dec 31, 2007 1:28 am UTC

Simply amazing watching those videos. Makes me want to get one of those super slo mo cams but I don't have 25k to drop for one.

RockHawk wrote:This video has some good slow motion shots of how tyres on dragsters deform and slip as they come off the line.

If you watch how much they twist it's amazing they don't rip!
My two cents.

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Re: Physics of Car Tires...

Postby null » Sun Jan 13, 2008 11:05 pm UTC

Would i be correct in thinking that the tyre deformation is beneficial for traction? I had always heard that adhesion friction was independant of tyre footprint, so tyre deformation and the associated increase in footprint would not affect adhesion friction from hot, sticky slick tyres...Only interference friction would be affected by footprint...I always thought dragsters mainly relied upon adhesion friction.

From what ive heard in the past, low profile tyres are 'better' - but it now appears that this is only the case for braking and cornering. Are high profile tyres better for drag racing? As stopping and cornering are non-issues, and traction is all that matters.

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Re: Physics of Car Tires...

Postby cypherspace » Mon Jan 14, 2008 1:22 am UTC

RockHawk wrote:This video has some good slow motion shots of how tyres on dragsters deform and slip as they come off the line.

If you watch how much they twist it's amazing they don't rip!


That's a fantastic video. Isn't rubber amazing?
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Re: Physics of Car Tires...

Postby wst » Mon Jan 14, 2008 6:05 pm UTC

null wrote:t, low profile tyres are 'better' - but it now appears that this is only the case for braking and cornering. Are high profile tyres better for drag racing? As stopping and cornering are non-issues, and traction is all that matters.


Well, the flexing of the tyre takes away a bit of power at the launch, and stops the wheels going everywhere, because it's doing work to the tyre, not the track.

It soon balloons out when it gets moving, giving it a bit more speed as well.
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Re: Physics of Car Tires...

Postby MotorToad » Mon Jan 14, 2008 6:27 pm UTC

null wrote:Would i be correct in thinking that the tyre deformation is beneficial for traction? I had always heard that adhesion friction was independant of tyre footprint, so tyre deformation and the associated increase in footprint would not affect adhesion friction from hot, sticky slick tyres...Only interference friction would be affected by footprint...I always thought dragsters mainly relied upon adhesion friction.
The first part is correct, to a point. Deformation allows much more footprint, and footprint is where adhesion comes from. Pretty much nothing about a tire is independent of the footprint. On the extreme, though, the footprint goes down with deformation. If there isn't enough pressure in the tire it buckles the contact patch so that the tire rides more on the sidewall, especially in cornering.

Basically, acceleration and braking grip improve with less pressure, and cornering grip requires more pressure to keep the sidewall off the ground. Getting the proper cornering pressure can be tricky.

null wrote:From what ive heard in the past, low profile tyres are 'better' - but it now appears that this is only the case for braking and cornering. Are high profile tyres better for drag racing? As stopping and cornering are non-issues, and traction is all that matters.
Low-profile tires are better for almost nothing. I know it's going against everything you're seeing, but most of what you're seeing is marketing to upsell people on 19" rims for their Porskis Porsches and Ferraris. The one and only real reason to use, say, a 19" wheel over a 17" wheel with the same tire diameters is to put bigger brakes in it. If you see a 19" wheel with less than a 16" rotor behind it, it's rice (or "potato" for American, "pasta" for Italian, and "kraut" for German versions of rice).

What people will tell you is that your tire will have more grip and be more "responsive." What you'll get is much more noise, the same or maybe even less grip, and tons more rotational inertia. And a slower car, guaranteed. People like the feel of "responsive," but that's unrelated to grip and in many cases counterproductive to grip.

Formula One, the very pinnacle of technology (certainly if measured by budget and espionage), uses 15" wheels. Admittedly, these are required by the rules, but they are the quickest cars ever produced by man, no other series produces lap times that are even close to the F1 cars.


MCSL wrote:2007 Lap Times

750hp F1 McLaren-Mercedes _ 1:21.356

600hp GP2 Dallara-Renault _ 1:30.546

700hp LMS P1 Peugeot 908 _ 1:34.503

425hp WSR Dallara-Nissan _ 1:35.984

500hp LMS P2 Lola-AER _ 1:39.271

600hp LMS GT1 Saleen S7-R _ 1:45.443

480hp LMS GT2 Porsche 997 GT3 RSR _ 1:50.381
These are lap times at Monza in Italy collected by MCSL on a BMW forum. The neat thing here is that Monza is a very low-downforce set-up so F1's cars have the least advantage of the incredible downforce they're capable of... i.e. the other cars'll be closer than they would at, say, Imola or Jerez. GP2 is the new F1 step-up series, the rest are the premier sportscar series in the world. So every four laps a 600 bhp Saleen (that's the mid-engined supercar, not some fluffed-up Mustang) would go a lap down to an F1 car!

Anyway, my point is you don't want bigger wheels. You may want wider wheels and you definitely want better tires, but rim diameter has nothing to do with it.
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Re: Physics of Car Tires...

Postby xkcd_n00bz » Mon Jan 14, 2008 7:12 pm UTC

A big part of the burnout is show. Everybody looks when a thousand horsepower spins drag slicks into smoke. That makes sponsors happy.

It's also for non-tire safety; a little pre-test of the vehicle. It's better to blow apart a diff, tranny, or engine in the pit then when you are down track at 200 mph.

As said, heat is one of the reasons on circle and road courses. But in the quarter-mile, that is not true. You can go look at the track behind the lights; it's got a shallow pit of soapy water to burn out in. The reason for the soapy water is that it reduces traction. Spin the tires to clean the rubber. If you have ever touched drag slicks, they are soft. Like a pencil eraser. They pick up all manner of gravel and dirt and dust being pushed or towed around to and from the trailer. Better to burn that stuff off early.

Also, because those tires are so soft, they get "hard" in a hurry. Think of really old rubber; it's hard and flakey. Burn off the outside 1/16" so they have a fresh surface to grip.
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Re: Physics of Car Tires...

Postby arkady » Mon Jan 14, 2008 7:46 pm UTC

In F1 there's hardly any wheel-spin. You're correct: the getaway is faster if you don't do it.

Apart from where it's useful to clean the tyres or get some last minute heat into them, you're faster if you don't do it, and the main reason it's done is because the driver has gone too hard on the throttle.
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Re: Physics of Car Tires...

Postby wst » Mon Jan 14, 2008 8:29 pm UTC

xkcd_n00bz wrote: If you have ever touched drag slicks, they are soft. Like a pencil eraser. They pick up all manner of gravel and dirt and dust being pushed or towed around to and from the trailer.


Sticky tyres nearly got me whiplash once. I was looking at the gokart tyres I was using and a bit of wood stuck to one of them, and the texture and stuff, and totally missed the lights.

They're amazing- next time when I get to go karting I'll skid a smidge before stopping properly and take a macro shot of the tyre mayhaps. It's like putty more than a rubber, even. Almost able to make fingerprints in it, I think.
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Re: Physics of Car Tires...

Postby null » Sat Jan 19, 2008 1:18 pm UTC

Id just like to say WOW thankyou for some great responses. Ive learned a lot, its not often one can find a proper intellectual discussion about cars, because most modders just seem to want a noisey exhaust and BOVs.

Deformation allows much more footprint, and footprint is where adhesion comes from.


I thought adhesion friction was related to the pressure the car exerts on its tyres? Ie. More deformation=more footprint area, which would be a lower pressure for the same weight of car.

tons more rotational inertia (with big alloys and low profile tyres)


Because alloys are heavier than rubber? Id never really thought about that...

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Re: Physics of Car Tires...

Postby wst » Sat Jan 19, 2008 1:37 pm UTC

Of course, the wheels are solid, so there's more mass to them than hollow tyres.

That's curious, never thought about wheels being bad for more than air resistance/grip. But acceleration too.
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Re: Physics of Car Tires...

Postby kamocat » Sun Jan 20, 2008 1:14 am UTC

Seraph wrote:
The Ethos wrote:While we're on the subject of tires, having had a blowout at 85 mph on the god damn Mass Pike the other day....

Could anyone explain why it is that you are only supposed to go 50 mph for 50 miles on the donut? It's not like you throw it out afterwards, you just put it in the trunk again.....Is it about steering? Does the tire heat funny?


There are a number of reasons:
1) Your wheels will be spinning at different speeds, making the differential (or equivelant) work a lot harder, and possibly throwing off your ABS.
2) Your suspension/brakes/stearing isn't really designed for tires of two different sizes, so your car can handle funny.
3) If you have the spare on the car, then you're out of luck if you have a flat.
4) The spares don't last as long as regular tires.
5) Spares arn't guarenteed to withstand the speeds that normal tires can.

Why would spares be manufactured any differently than other tires? Okay, so it may be less expensive, perhaps, but it's also less standard. Is it not possible to go into a tire store and buy just a single tire?

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Re: Physics of Car Tires...

Postby bananarchy » Sun Jan 20, 2008 3:08 am UTC

kamocat wrote:
Seraph wrote:
The Ethos wrote:While we're on the subject of tires, having had a blowout at 85 mph on the god damn Mass Pike the other day....

Could anyone explain why it is that you are only supposed to go 50 mph for 50 miles on the donut? It's not like you throw it out afterwards, you just put it in the trunk again.....Is it about steering? Does the tire heat funny?


There are a number of reasons:
1) Your wheels will be spinning at different speeds, making the differential (or equivelant) work a lot harder, and possibly throwing off your ABS.
2) Your suspension/brakes/stearing isn't really designed for tires of two different sizes, so your car can handle funny.
3) If you have the spare on the car, then you're out of luck if you have a flat.
4) The spares don't last as long as regular tires.
5) Spares arn't guarenteed to withstand the speeds that normal tires can.

Why would spares be manufactured any differently than other tires? Okay, so it may be less expensive, perhaps, but it's also less standard. Is it not possible to go into a tire store and buy just a single tire?



Yes, you can get single tires, and some cars have full size spares, which are identical to the regular tires and have no mileage/mph restrictions. Most cars, though, have spare tires designed to be cheap but mainly designed to be small to take up as little space as possible, so they're narrower and smaller in diameter.

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Re: Physics of Car Tires...

Postby null » Sun Jan 20, 2008 7:34 pm UTC

Ive been wondering about this...I think im being stupid somewhere, please correct me!

If you fit wider tyres to your car, it would appear the tyre footprint would get bigger. But the tyre would deform less, because the weight of the car is now spread over a larger area, so the pressure is lower. If the tyre deforms less, the footprint would go down again...maybe back to where it started (roughly)?

(Assuming tyre inflation pressure is the same for stock and wider tyres.)

So basically fitting wider tyres would give you a shorter (front-back) but wider tyre footprint than stock?

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Re: Physics of Car Tires...

Postby MotorToad » Sun Jan 20, 2008 8:10 pm UTC

You're probably not going to get much more than 10% wider for tires and have them fit under the car, so it's not at all a big deal so long as the tires are designed to fit the wheels. a 3 psi change is pressure will have as much difference. If the wheels aren't wide enough for the tire then Genuinely Bad Things happen.

I'd be more interested to see the difference in the length of the footprint between something like a Civic that someone changed 14" for 17" wheels. If the footprint wound up being smaller, I wouldn't be shocked.

As for space-savers, the reason they're the way they are is the result of a string of compromises. You really do need a spare tire, but you don't really need a full-size unless you're off-roading. While a full-size spare would relieve you of having to drive slower in that four hours or so between getting a flat and getting a new tire, you'd have to carry that spare for the other 99.9999999999999999% of the time you drive the car. It's heavy, takes up loads of space, and affects fuel economy. But in order to function close to a real tire and be small and light, it has to be made of a very soft compound and operate at a very high pressure (like 85 psi or so).

Tires are the most important item on your car, by a huge margin. They affect every aspect of performance, usually in more than one way. Never ignore your tires and never think you're going to get by on the cheap when dealing with tires, they really matter that much. *Nothing* else on the car matters if the tires are bad. Well, I guess you can sit on the side of the road with the AC on... until you run out of fuel. :)
wst wrote:Of course, the wheels are solid, so there's more mass to them than hollow tyres.

That's curious, never thought about wheels being bad for more than air resistance/grip. But acceleration too.

More than that, the mass is concentrated in a ring at the perimeter, so mass basically goes up at pi * the radius, and IIRC inertia goes up with the square of pi.
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Re: Physics of Car Tires...

Postby asad137 » Wed Jan 23, 2008 5:21 pm UTC

null wrote:So basically fitting wider tyres would give you a shorter (front-back) but wider tyre footprint than stock?


Yes, and it changes the shape of the so-called "friction circle".

MotorToad wrote:You're probably not going to get much more than 10% wider for tires and have them fit under the car


That certainly depends on the car. My car had 195mm section width tires stock and currently has 245mm in the rear, an increase of over 25%. My negative camber is still within the stock spec, though my wheels have a different offset, bringing the centerline of the tire inboard toward the middle of the car.

MotorToad wrote:I'd be more interested to see the difference in the length of the footprint between something like a Civic that someone changed 14" for 17" wheels. If the footprint wound up being smaller, I wouldn't be shocked.


If the tire width is the same, the footprint will be the same length for a given inflation pressure. If the tire is wider, the footprint will be shorter (front-to-back), but the total area is the same (again, given the same inflation pressure).

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Re: Physics of Car Tires...

Postby Seraph » Wed Jan 23, 2008 7:14 pm UTC

kamocat wrote:
Seraph wrote:
The Ethos wrote:While we're on the subject of tires, having had a blowout at 85 mph on the god damn Mass Pike the other day....

Could anyone explain why it is that you are only supposed to go 50 mph for 50 miles on the donut? It's not like you throw it out afterwards, you just put it in the trunk again.....Is it about steering? Does the tire heat funny?


There are a number of reasons:
1) Your wheels will be spinning at different speeds, making the differential (or equivelant) work a lot harder, and possibly throwing off your ABS.
2) Your suspension/brakes/stearing isn't really designed for tires of two different sizes, so your car can handle funny.
3) If you have the spare on the car, then you're out of luck if you have a flat.
4) The spares don't last as long as regular tires.
5) Spares arn't guarenteed to withstand the speeds that normal tires can.

Why would spares be manufactured any differently than other tires? Okay, so it may be less expensive, perhaps, but it's also less standard. Is it not possible to go into a tire store and buy just a single tire?

It isn't a matter of "would", it's a matter of fact. A lot of the spares out there are not regular tires.
Spares come in two flavors, "full-sized" and "compact" (the later are also known as "donuts").
The advantage of the compact spares is that they take up a lot less space in a car trunk, but have all the disadvantages I listed.
The full-sized spares take up more space but are tougher they are often used on things like SUVs and Trucks where space is a little less tight and the tire has more demand put on it, Often they can be carried on the exterior so the space thing isn't an issue.

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Re: Physics of Car Tires...

Postby MotorToad » Wed Jan 23, 2008 7:33 pm UTC

asad137 wrote:If the tire width is the same, the footprint will be the same length for a given inflation pressure. If the tire is wider, the footprint will be shorter (front-to-back), but the total area is the same (again, given the same inflation pressure).

Asad
Uhm... No. Tire construction between a 14" with /65 sidewalls is a lot different than 17" with /40 sidewalls.

Also, going from a 195 to a 245 tire, the stock camber spec no longer has any meaning. Camber demands for a tire that much wider are very different. Not that it'd matter much on the street, other than wearing the tire unevenly, but if you were doing something in competition where having that much more tire mattered you'd see quite a bit of difference with the camber set for the tire.

Oddly, I can't think of a performance car that came with 195s, what is it?
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Re: Physics of Car Tires...

Postby asad137 » Wed Jan 23, 2008 11:23 pm UTC

MotorToad wrote:Uhm... No. Tire construction between a 14" with /65 sidewalls is a lot different than 17" with /40 sidewalls.


Doesn't matter -- the sidewalls have nothing to do with it. If the weight of the car is the same, and the inflation pressure is the same, the area of the contact patch is the same (ok, maybe sidewall stiffness plays a very tiny role...but it's very small. How well does a non-run-flat tire sidewall support the car when there's no air in the tire?).

Also, going from a 195 to a 245 tire, the stock camber spec no longer has any meaning. Camber demands for a tire that much wider are very different.


I entirely agree, but I just used that as an example to refute your "10% wider" statement (in case you said something like "oh, but you probably have to run crazy negative camber bla bla bla"). It's unlikely that anyone with the stock tires would use the extreme edge of the spec anyway. FWIW, my tires wear perfectly evenly across the tread.

Oddly, I can't think of a performance car that came with 195s, what is it?


It's not really a performance car, per se -- It's a 1992 Nissan 240SX. But now it has the a better power-to-weight ratio than a JZA80 Supra TT.

Asad

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Re: Physics of Car Tires...

Postby null » Fri Jan 25, 2008 7:06 pm UTC

I recently discovered something about ABS: The reason it works so badly on loose surfaces is because if your brakes lock, a wedge of snow/gravel/cocaine gathers infront of the wheel, slowing you down. If ABS unlocks your brakes, it clears the wedge of crap infront of your wheels...So your wheels roll more freely.

Appologies if you already knew that...at least I learned something!

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Re: Physics of Car Tires...

Postby wst » Sat Jan 26, 2008 1:12 pm UTC

asad137 wrote:
Oddly, I can't think of a performance car that came with 195s, what is it?


It's not really a performance car, per se -- It's a 1992 Nissan 240SX. But now it has the a better power-to-weight ratio than a JZA80 Supra TT.

Asad


Which is why they're such a popular choice for drifting- narrow tyres, and great power/weight ratio. But not as cool as a Supra TT :P
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Re: Physics of Car Tires...

Postby fjman » Sat Jan 26, 2008 2:30 pm UTC

Heh, I registered exclusively to post in this thread :)

ABS: Does not allow faster braking on a uniform surface. A good driver is quite a bit better. Of course, your mom in her car will be greatly helped by the ability to just slam the brakes and get 90% of the stopping force of a good driver. That being said, there's a few reasons why ABS is good stuff:
1- It prevents the wheels from staying locked for any real amount of time. Thus, you can still steer, which is what most drivers need the help with, for accident avoidance situations.
2- A well-designed ABS system will control each wheel individually. A driver, despite their talent, has but one brake pedal, and has to deal with the average braking force available between all four tires. An ABS system can nearly maximize force at each wheel. This is pretty important when you're on a surface that isn't uniform, tiny patches of sand/ice/etc that hit one side of the car or hit wheels with varying amounts of traction loss.

Burnouts: In drag racing, a burnout will only lay rubber on the launch area if you roll through it in a straight line. A Top Fuel or Funny Car (the biiiiig power guys both, 6-8 thousand horsepower machines) will do this. Most sportsman racers will not, and in some classes it's illegal to do a burnout through the starting lights. However, it does get the tires extremely sticky, and the track surface is prepped with an adhesive compound, generally for at least the first 60', if not to half track (660') or for the full track length. The prep is expensive ($650/drum), so some tracks will use different amounts for different events.

If you're using street tires with normal grooves, a burnout is generally not helpful. A quick spin to clean the tires is all that's required, and you absolutely don't want to roll much (do it sitting still). This is because there's a layer of rubber on the track, and the tread blocks will cut into that rubber, creating "marbles" which actually appear as anything from fine dust to pea-sized bits.

I use a "street" tire on my bike. It's a terrible tire for doing much riding on, but it's technically street legal. One of the things that I do with the burnout is to ensure the tire is up to operating temperature, and then roll forwards with less throttle to find the point at which it hooks (grabs traction and jumps forward). Helps to estimate how much available traction will be on the track on that day.

The wrinkling effect that you can clearly see in some of those videos (or nearly any good photo taken of a launching car on slicks) has more than one reason. As the tire wrinkles, the contact patch gets somewhat larger, indeed. Another effect is that it gets shorter, or the radius from the axle to the ground gets shorter. This increases torque transmission to the ground (shorter lever, if you look at it that way), and allows you to run a higher gearing (lower numerically), leaving the car in lower gear for more of the run, increasing average torque at the ground. Additionally on a bike, generally there is no shock, and running low air pressures gives it the ability to handle an occasionally bumpy track surface. Over time, a dragstrip has to be resurfaced at the starting line, becuase the pavement eventually becomes bumpy (wrinkled) from all of the racing done on it. Top Fuel also uses a rigid rear, unsure about Funny Car. In those two classes, a low air pressure in the tire and a soft carcass'd tire allows the tire to grow at high speed (radius becomes longer) which alters the gearing. In TF, this is particularly important as they do not have transmissions (against the rules!) and they're not allowed a choice of gearing (also illegal!), so tire growth allows higher top speeds.

Tires with shorter sidewalls feel more "planted" during cornering, as they allow less lateral movement than a smaller rim with the same total diameter tire would have. However, they do indeed generally weigh a truckload more, until you get into exotic materials for the wheel (bikes routinely use carbon fiber wheels in racing, for example).

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Re: Physics of Car Tires...

Postby wst » Sat Jan 26, 2008 2:59 pm UTC

fjman wrote: In those two classes, a low air pressure in the tire and a soft carcass'd tire allows the tire to grow at high speed (radius becomes longer) which alters the gearing. In TF, this is particularly important as they do not have transmissions (against the rules!) and they're not allowed a choice of gearing (also illegal!), so tire growth allows higher top speeds.


Nice post. I didn't know that drag tyres were designed to balloon at all, but it's a cool effect. They have quite a major part in the race, it seems :D

Tyre Ballooning in an RC car, to really show off the effect :D
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Re: Physics of Car Tires...

Postby dosboot » Sun Jan 27, 2008 4:17 am UTC

I hope I may ask a slightly OT question:

What's the simplest explanation as to why a semi-truck will take a longer distance to stop than a normal car under identical braking conditions? If the frictional force is proportional to the normal force then the two should have the same deceleration.


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