Ridiculously stupid question about tires

[VLC86]

Ok. so dont judge me but...

I have never been able to grasp this problem.

When you get a brand new tire if has lots of chunky thick tread and a relatively complex looking tread pattern (on an average midrange tire) plus it gives good grip. As it slowly wears down the thread gets less and less and the tread pattern becomes less complex. It begins to smooth out to flat rubber. This technically would INCREASE surface area on the road yes?

NOW.. what I dont get is why gripping performance decreases with less tread when the surface area is increasing? Drag cars obviously use slicks and the higher the surface area the better friction and grip you get.

Do you know what i mean?

this has plagued me for ages so please help me figure it out with your tire related wisdom

[aaronevolution]

The deeper the tread the better handling in the wet? Same reason why slicks are illegal on road cars. So as the tread wears down you may have more grip in the dry but less in the wet? Maybe?

[Blown V6 Hatch]

The problem with tyres getting old is, that even though you end up with more tyre on the road (technically) the rubber is old, hard and worn out hence less grip. When tyres are new, the rubber is new and soft so they grip better.

Drag slicks are a bit different, they get good traction because the rubber is a very soft compound with soft sidewalls and when hot they become very sticky so they bite hard into the track. Now, there are companies making tyres that have a limited tread pattern with the soft compound that get very good traction without being a full slick.

The softer the compound of the tyre is, the better the traction.

[VLC86]

The problem with tyres getting old is, that even though you end up with more tyre on the road (technically) the rubber is old, hard and worn out hence less grip. When tyres are new, the rubber is new and soft so they grip better.

Drag slicks are a bit different, they get good traction because the rubber is a very soft compound with soft sidewalls and when hot they become very sticky so they bite hard into the track. Now, there are companies making tyres that have a limited tread pattern with the soft compound that get very good traction without being a full slick.

The softer the compound of the tyre is, the better the traction.

I was hoping for an answer like this.. with mention on tyre compounds and stuff.
So over time the rubber actually changes i suppose? so it isnt as soft and grippy as new

[Blown V6 Hatch]

Absolutely, the rubber goes hard over time. Once the rubber goes hard, the tyre will have very limited traction ability.

Go and get an old hard pair of tyres from the wreckers and see how much traction they give, they will make the car feel like it's gained 50hp over a new pair

[HarmlessMuffinMan]

Basicly, slicks work because of heat... Normal road tyres aren't heated up like racing car wheels, so need to have ''grip'' on them.

[VLC86]

mmmhmmm

im getting this relief-like feeling because i understand now.. as you can tell I know almost nothing about tyres, but i do nderstand the concept of hard rubber/no grip and soft rubber/good grip

[bogan_vk]

the softer the compound the quicker they wear out too, family car tyres are alot harder compound than performance tyres and last longer but dont grip as good as soft compound when the tread wears on all tyres their ability to channel the water out in the wet decreases and wont grip as well as new tyres.

[DAKSTER]

On this subject, kinda.. super low profile tyres have less flex and are therefore more rigid or 'harder' in my opinion. The extra air in a higher profile tyre provides more cushion, effectively making them 'softer' and therefore get more traction.. in my opinion. Obviously older tyres like 75 or even 80 profiles that we used to have are just too tall, and flex too much, which is like trying to run a slalom in thongs. There's a happy medium somewhere in between.. I'd say maybe 40-50 profile.

Its my personal theory that running 20" wheels instead of say 17" wheels is a performance loss, not a gain. You'll do easier burnouts but you wont go around corners as well lol I am also guessing that the 20" will probably be a softer compound to compensate for less grip, and therefore wear out faster..

I'm probably wrong as usual but it seems like basic physics to me. Happy to be corrected if anyone has actual facts to quote coz i don't....

[FusionX]

Dakster: For cornering/circuit work you want a stiff sidewall, otherwise when you turn you'll just roll the tyre off the rim. While for drag racing you want a soft sidewall (high profile) to get some flex for better traction on launching (there is a bit more to it then that when it comes to tyres... but thats the basics of it sort of). But you are right, most times you are better running a 17" rim then a 20" rim as the 17's are usually a lighter rim. Less rolling weight is an immediate performance increase in both acceleration and braking.

Also in regards to tyre compounds. Semi slicks that are used for circuit work (like Bridgestone RE55S or Advan A048) also have a certain number of heat cycles they go through before the rubber starts to go off. We base most of our at around 8 to 10 heat cycles. After that they get used for practice days and burnouts if they aren't stuffed.

[DAKSTER]

I get the cornering/rolling off rim thing, that was the problem with 75 profile tyres of course. But isn't there a point where the rigidity of the sidewall effectively turns the tyre into a solid object, which slides rather than grips? Not sure if that makes sense, hard to word what I want to say, sorry.

Flex contributes to straight line traction, at least I got that one right

I hadn't thought about weight being an influence, of course it must be in general terms.. but
the 17s would actually be heavier than 20's even with a lighter rim too wouldn't they? More tyre... tyres are heavier than modern rims aren't they?

Not arguing, learning

[ari666]

so no one has mentioned aqua-planing?

[Calaber]

This wasn't a ridiculous question at all. The science of manufacturing tyres is very involved these days.

The tread isn't just designed to look good or give grip. The design of the tread pattern is actually intended to pump the water away from beneath the tread in wet conditions and the amount of water that the most effective tread patterns can dissapate is remarkable. Films have been taken of tyres being driven over a saturated glass surface and you can clearly see how the water is being pumped out to the sides of the tyre.

As the tread wears, the capacity of the tread to pump the water away reduces so that the tyre eventually becomes ineffective in wet conditions. That's when you start to experience aquaplaning, where the tyre actually rides up over the water film and loses contact with the road surface. It can even happen with new tyres if they suddenly encounter a flooded roadway.

As the tread wears, the actual contact surface remains basically the same as it was when new, until the tread is completely worn away. You might then have slightly superior dry road grip but absolutely none in the wet. And as pointed out above, as a tyre ages, it hardens, though this doesn't happen quickly. Usually, a tyre will wear out under normal usage before the compound deteriorates. A point to remember arose on the VB-BK thread recently, where one member said his VB SLE still had its original spare, unused, in the boot. It would have been as hard as hell and absolutely useless on the road today.

[VT-565]

Must also keep in mind the difference in construction between Radial vs. Bias ply tyre
The difference in side walls is very evident once you get a road tyre and a slick off the rim, side by side.
The bias ply allows the bead of the tyre to turn a certain amount through sidewall distortion, whereas a Radial tyre fitted to everyday road cars is much more rigid to cope with a different set of stresses.
Radial Drag tyres are a big thing now and take a different set up to launch properly with big power when compared to Bias ply slicks.

[FusionX]

I get the cornering/rolling off rim thing, that was the problem with 75 profile tyres of course. But isn't there a point where the rigidity of the sidewall effectively turns the tyre into a solid object, which slides rather than grips? Not sure if that makes sense, hard to word what I want to say, sorry.

You are right, you do need some flex in the sidewall otherwise it will slide. I guess what I was trying to say is that a tyre made for circuit work will have a stiffer sidewall then a street or drag tyre due to the loads from corner at high speeds.

I hadn't thought about weight being an influence, of course it must be in general terms.. but
the 17s would actually be heavier than 20's even with a lighter rim too wouldn't they? More tyre... tyres are heavier than modern rims aren't they?

Not arguing, learning

There are definitely gains to be had by running light weight rims as it is less mass to turn. Depends on the construction of the rim and tyre. A lot of these large chrome rims weigh a heap.

[VT-565]

You are right, you do need some flex in the sidewall otherwise it will slide. I guess what I was trying to say is that a tyre made for circuit work will have a stiffer sidewall then a street or drag tyre due to the loads from corner at high speeds.

With less 'give' in the sidewall, there is more emphasis placed purely on the compound and the tread block design (if any)....i think that to say generally "its stiffer" would be incorrect, but there would be a balance obviously....not forgetting that some road tyres have to have pretty heavy wall contruction for corner loading etc...

[VLC86]

Lots of good info in here.
When i first got my car it had old hard thread and I did Aqua-plane, wasn't a fun experience. Makes sense that the tread pattern removes water from under the tyre.

A lot of technology goes into tyre manufacturing today I suppose as there is a lot to take into consideration.

Sent from my Desire HD using Tapatalk

[DAKSTER]

With less 'give' in the sidewall, there is more emphasis placed purely on the compound and the tread block design (if any)....i think that to say generally "its stiffer" would be incorrect, but there would be a balance obviously....not forgetting that some road tyres have to have pretty heavy wall contruction for corner loading etc...

Which brings me back to an earlier statement of mine. Would the 20" tyre, with its tiny rigid sidewalls, therefore necessarily be made of a different compound to compensate for the loss of traction, and therefore wear faster than the 17" where there was more room for compromise?

I hadn't mentioned aquaplaning as I guess I thought that was obvious with slick or worn tyres. It makes me wonder though, remembering I haven't owned 20" tyres.. is the tread depth on a new one as deep as the tread depth on the 17s? If not, then its a double whammy.. less tread depth, softer tyre, equals bald tyre that much quicker?

[VT-565]

Which brings me back to an earlier statement of mine. Would the 20" tyre, with its tiny rigid sidewalls, therefore necessarily be made of a different compound to compensate for the loss of traction, and therefore wear faster than the 17" where there was more room for compromise?

20" tyre doesnt necessarily have small sidewalls
If you're talking street radials, then I cannot imagine the manufacturers changing compounds to make the same model tyre, regardless of size. I see what you're getting at though, but when it comes to motorsport tyres, they probably have a different set of rules as there are different objectives

[DAKSTER]

A 20" wheel, if its going to have anything like the same rolling diameter as an original wheel (which it kinda has to if you want the speedo, steering geometry, and gearing to be correct) must necessarily have smaller sidewalls surely?

Having said that, I was talking to someone at our northside cruise the other night who said that his 20s are about an inch taller than factory and how this has messed up his gearing and speedo. I was surprised to hear that, I would have thought they should all have the same rolling diameter. I know i can put 35" tyres on my cruiser instead of 31" to give me extra diff clearance but that also of course affects the gearing and speedo. That's comparing apples and oranges though, the difference in height in that case is very deliberate and has a specific purpose... the diffs are 2 inches further from the ground and less likely to grab a handful of rock and hang on..

In this case though, even if his 20s are an inch taller than normal, that's only an extra half inch in sidewall height. Its still a rubber band around a jam jar lid. 17" with standard rolling diameter would have an extra 2" of sidewall compared to his 20s even with their extra inch of height?

Again, not arguing, just trying to learn. And yeah, I am strictly talking street wheels here. Obviously racing vs street is also apples and oranges..

[Calaber]

If you fit a 20" rim on a car originally fitted with, say, 17's, and you are trying to retain the same overall diameter to ensure the accuracy of your speedo, the walls have to be pretty rigid because of the lack of depth of the tyre between the road and the rim. If you had soft walls on a 20" tyre, you would destroy the rims and the tyres very quickly on a road car. I put 17's on an XC Barina a few years back with 225/45's I think. It looked terrific, steered like a roller skate and rode like a truck. It originally came with 14's with about 185/65 rubber.

Another point that is often missed is that super wide tyres can have less wet weather grip than narrower tyres, because of the wider contact patch distributing the car's overall weight over a larger area, thereby reducing the pressure per square inch on the tyres and reducing their ability to press down on the road in wet conditions. The larger contact patch actually helps the car to aquaplane sooner than the narrower tyres did.

[DAKSTER]

I agree, the sidewalls are necessarily more rigid. Its not just about the tyre size or type either.. if they are taller or wider than the original, its going to upset the steering and suspension geometry too.

Also agree on the wider tyre here too. A perfect example of this is my Landcruiser on 31" muddies. In mud, they are great. Drive down a dirt highway at 100km on them though, the car will wander all over the place, as it 'aquaplanes' on the loose gravel instead of water. Take them off and stick some standard skinny offroad tyres, and the car will steer straight and true.

Interestingly though, if you change the wide all terrain tyres for a tyre of the same size with a street car type tread, it also improves the steering dramatically, both on and off road (though it will still wander a bit) and a street tread will have more road contact than a muddie, so that does that disprove the aquaplane effect we have just proven?

Remember Sol, oils aint oils lol All any of this proves is that there are a bunch of factors, and the car needs to be set up specifically to suit both whatever wheel/tyre combo you put on it and the purpose you are using them for.

I gather from all of this though, unless I intend taking my Berlina circuit racing with specialist soft compound tyres, I am going to get better handling, traction and braking out of a 17" combo than I would from a 20" combo. Does that make sense?

[VS 5.0]

I remember Top Gear Australia tested a VE HSV around their track on two seperate runs, one with 19s & one with 20s. The run with 20s was the slower of the two runs. Tyres too rigid and not having enough grip ?

[Cheap6]

Possibly just that the suspension is optimised for the tyre with greater flex.

Most people here will have some idea of the concept of camber - where the top of the tyre leans in towards (or out away from in much older cars) the centre of the car more than the bottom.

The purpose of designing in camber is to correct for the tyre distorting as load is applied to it during cornering.

A classic model of tyre behaviour is to take a pencil eraser, tilt it relative to the surface of (say) a desk then, while maintaining the tilt, try to push it across the desk such that the surface adjacent to the desk flattens onto it. You can also see what happens to the surface of the eraser if you apply the push and don't tilt it (it lifts up and away from the desk on the trailing edge).

A low profile tyre under load will have less tendency to pull the tyre surface off the road, as will a wider tyre - try holding the eraser closer to the desk top or applying the push with the eraser oriented so that it has a thicker dimension across the direction of push.

(Just to be clear: the individual tread blocks will also behave this way but I'm referring to the tyre as a whole.)

For every tyre and cornering force there will be an optimum angle at which the tyre produces the highest level of grip. A particular suspension design may be optimised for a particular tyre and be compromised on something else.

If you put low profile tyres on a car with suspension and steering designed to compensate for the the distortion of higher profile tyres it will perform worse than it could. (Tilt the eraser more.) Maybe that is what happened with the VE?

That will certainly apply to fitting very low profile tyres to a Commodore designed to use a 65 or 70 series tyre.

I don't think that the effect of feel can be discounted either. It may simply be that the driver was less able to feel the limit of grip for the stiffer tyre.

[old mate]

I agree, the sidewalls are necessarily more rigid. Its not just about the tyre size or type either.. if they are taller or wider than the original, its going to upset the steering and suspension geometry too.

Also agree on the wider tyre here too. A perfect example of this is my Landcruiser on 31" muddies. In mud, they are great. Drive down a dirt highway at 100km on them though, the car will wander all over the place, as it 'aquaplanes' on the loose gravel instead of water. Take them off and stick some standard skinny offroad tyres, and the car will steer straight and true.
Interestingly though, if you change the wide all terrain tyres for a tyre of the same size with a street car type tread, it also improves the steering dramatically, both on and off road, and a street tread will have more road contact than a muddie, so that disproves the aquaplane effect we have just proven.

Remember Sol, oils aint oils lol All any of this proves is that there are a bunch of factors, and the car needs to be set up specifically to suit both whatever wheel/tyre combo you put on it and the purpose you are using them for.

I gather from all of this though, unless I intend taking my Berlina circuit racing with specialist soft compound tyres, I am going to get better handling, traction and braking out of a 17" combo than I would from a 20" combo. Does that make sense?

your comment about muddies v's road tyres and how they move around needs sorting out.

its not the width of the tyre that is causing it to move around at speed, its the actual design of the tread on the tyre. a mud tyre will firstly have alot more rolling resistance than a road tyre, but the way the tread is designed (and the depth of it) means that it will squirm around on the surface. as there is more voids in the tread, the rubber is able to move around alot more than if there were hardly any voids. many mud tyres are actually worse off than say an all terrain or road terrain on wet roads, just because of the way the vehicle moves around on the rubber.

this squirming on the tread blocks is more noticable on harder packed surfaces such as the dirt highway you describe than in mud etc. now when you go to something super agressive like a simex centipede they move around a shitload because of the agressive tread block pattern. but this pattern makes it work in wet clay where they need to throw out the mud as easy as possible.

but back to the 17 v 20 inch argument. a 17 inch wheel and tyre will perform much better than a big 20 inch wheel and tyre in nearly everything. if you had 2 rims of the same design with one in 20 inch and one in 17 inch, the 17 incher will be lighter. if you were to grab the tyres for these wheels the 20 inch tyre would probably be heavier also.

so now once you fit them up to your car, the 20 inch wheel will be heavier. this will mean you have more unsprung weight which will have a negative impact on suspension control. also, rotational inertia will be increased with an increase in wheel weight. its basically like putting a heavier flywheel on the back of your engine. heavier wheels will mean that acceleration is slower and braking takes longer. the braking part is a main issue, especially when you're at a track.

now when you go to smaller sidewalls, they will usually be stiffer. stiff sidewalls can be a good thing, up to a point. the way the sidewall flexes helps a driver feel what the car is doing. most 20 inch tyres that come in a package deal are utter crap and the rubber is quite hard and have stiff sidewalls. this will give you not alot of feedback, and once you start to get close to the limit of traction it will be really hard to get any good feel from the car. i wouldnt be surprised if the shit chinese 20 inch tyres have stiffer sidewalls than a race slick. infact i'd be pretty sure of it.

cars arent my specialty, but alot of stuff from road racing motorcycles translates over so hopefully this gives you a bit on an idea.