T.R | Title | User | Personal Name | Date | Lines |
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1528.1 | Sticktion rather than friction ? | UNTADA::LEWIS | Laax Crap Skier | Wed Aug 21 1991 10:55 | 31 |
1528.2 | | FORTY2::PALKA | | Wed Aug 21 1991 12:17 | 30 |
| Actually the amount of tyre surface in contact with the road does not
change much with wider tyres. It is determined largely by the pressure
of the air in the tyre (approximately you have area=weight/pressure
- there is some inaccuracy because the pressure is not even due to the
forces involved in distorting the natural shape of the tyre).
Wider tyres have a shorter and wider patch in contact with the road
than narrow tyres (at the same pressure). If you increase the pressure
in the tyre it becomes more 'round', and the patch in contact with the
road gets smaller.
Also there are actually 2 different coefficients of friction. One is
valid for situations where the two objects are not slipping and the
other for situations where the objects are slipping. The 'slipping'
coefficient is less than the 'non slipping' one. Naturally you want to
be driving in the 'non slipping' case.
None of the above answers why wider tyres are used. Wide tyres give you
2 things
1) Low profile
This results in a smaller sidewall, and hence less distortion
when cornering.
2) More total tyre area
This may result in less stress and heat build up in the tyre -
the force to accelerate the car is comunicated via the tyre. If
you have a more powerful engine you probably need to spread the
load over a greater amount of rubber to avoid overheating the
tyre. It may also increase the number of miles you get from the
tyre.
Andrew
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1528.4 | Never mind the quality - feel the width | UNTADA::LEWIS | Laax Crap Skier | Wed Aug 21 1991 13:01 | 47 |
1528.5 | Not sure how it was resolved though! :^) | NEWOA::SAXBY | Trailing Edge Technology | Wed Aug 21 1991 13:06 | 4 |
|
This narrow and wide tyre argument has been explored before.
Mark
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1528.7 | It's all Tribology | CRATE::WATSON | C is its own virus | Wed Aug 21 1991 13:29 | 11 |
1528.8 | Confusion | DCOPST::BRIANH::NAYLOR | Tigers fly, Spiders roar! | Wed Aug 21 1991 17:09 | 12 |
| Friction of itself is not related to area. Basically there are two measurements
to worry about -
1 is the "coefficient of friction" which is the resistance to motion
between two surfaces. This has nothing to do with area at all.
2 is the "total resistance to motion", which is the coefficient of
friction multiplied by the area (roughly speaking) This is what
most people refer to (incorrectly) as 'friction'.
The fomulae for the above are really very simple (O-level stuff in my day, and
long forgotten!). The application of them in the real world is something else.
If you want lots of "sticktion", simply put, you want a lot of contact area,
ie fat tyres with lots of footprint. However, as was mentioned, it gets
really complicated by temperature and fluid dynamics ......
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1528.10 | | GWYNED::BURTON | | Thu Aug 22 1991 16:13 | 42 |
| The physics of tyres is highly complex and I do not profess to be an expert by
any means. I would however like to give my observations on wide vs narrow
tyres since I have driven on both types on the same vehicle.
o The amount of traction you get from a tyre depends on the frictional force
generated by the tyre and the road. This is determined by a combination of
factors. Tyre width, inflation, tread design, and hardness/softness of the
rubber are the major factors influencing friction from the tyre's perspective.
I will not discuss the road since we have no control over that. For maximum
traction going straight, the goal is to hit some optimal window of stress
(pounds per square inch) on the road. I do not know what this stress window
is. Too little stress and the tyres become airborne as speed increases, too
much and the tyres start spinning.
o Obviously a car must do more than go in a straight line so you can't have
giant marshmallow tyres on a tiny car. To corner properly, the tyres must
again apply an optimal amount of stress to the road. Cornering is
complicated in that the tyre tends to roll sideways due to lateral stress
being applied. Rolling removes some of the area in contact with the road,
the stress dynamically increases, and the tyre loses traction. Suspension
geometry and sidewall height/stiffness determine how much roll the tyre will
have and hence how much traction the tyre will have on turns. A suspension
and tyres appropriate for maximum performance on curves will give a stiff,
harsh ride so tradeoffs must be made.
o As you go faster, the amount of stress applied to the roads should increase
to keep the car from wandering in the straights and sliding on the turns.
Formula-one cars accomplish this by the use of wings. A front and rear wing
applies more downward force on the tyres the faster the car goes.
o To go well in rain and snow means that the tyres must channel the
precipitation away from the contact area so the tyres can see that optimal
stress level with the road. Optimal traction on wet roads requires a much
higher stress window than on dry roads. Narrow tyres are preferred over
wide ones for wet surfaces.
o A street tyre must be designed for all types of road surfaces, weather
conditions, speeds, and turns. It is a compromise in every area in that it
does nothing great but everything well.
Jim
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1528.11 | | DCOPST::BRIANH::NAYLOR | Tigers fly, Spiders roar! | Thu Aug 22 1991 19:14 | 29 |
| Very interesting, but I'll still stay off Michelins on wet roads :^)
Lets face it, how many of us actually care very much about a 1% difference in
sticking force, really? Derek is certainly the only one who has a critical
need for real grip uinder all sorts of different conditions. The rest of us
drive on compromises - fairly good in the dry and on corners, but suboptimised
to attain some form of grip in the wet. As I said, I've never had a Michelin
that I would trust on wet roads (I've had X's and MXV's) so it boils down to
my driving requirements and my driving techniques and abilities. I've had to
slow down recently (55 limit and all that) so my current requirements aren't
as stringent as they would be in the UK, perhaps.
The Alfa is shod with Firestones - we use it predominantly in the dry, the tyres
are great on dry roads, and fair in the wet, and they were cheap! They are
185/70 x 14. The last set did almost 50,000 miles before wearing out.
The Audi has Bridgestones, fat ones! Don't know the size off hand, but it must
be something like 205/70 x 14 They're fine in the dry and I haven't had a
chance to check in the wet yet.
The Ford has it's OE Yokohamas. Thin things for a small car. They slide
very easily in the dry, and I don't know what they're like in the wet either.
These tyres will probably be the first to be changed!
The best tyre/car combo I ever had was my Fiat 124 on Semperits. What grip!
The E-type was shod with Dunlops, tyres I also normally avoid but at 200 quid
a time, they stayed on! That car was totally impossible in the wet, but it
did have over 300 horses pumping away .....
Oh yes, the plane is shod one side with a McCreary Air Hawk and the other with
a Goodyear. They are 6.00 x 6, 6 ply. The nose wheel is 4.00 x 5 Goodyear.
With luck, and good maintenance, they should last about 10 years .....
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1528.12 | The bigger the tires, the better!!! | CTOAVX::KWOLEK | | Tue Sep 10 1991 00:15 | 33 |
| We had the same question come up awhile back in US carbuffs. I
answered it there and didn't get too many negative comments so I'll try
again. The first thing to understand is that even when the car is
moving forward, the contact area that touches the road (unless the tire
is spinning) is stationary to the road.
Now, if you look very close at that contact area, you will find two
irregular surfaces that are "bonded" to each other. Someone mentioned
the viscous (or fluid like) tire sinking into the solid irregular road.
This is how you get traction. If both surfaces were smooth, there
would be no traction, such as on ice. The larger the bonding area, the
stronger the bond and the more power you need to break the bond.
Hence, a larger tire of the same compound will have more traction than
a small one.
Consider two boards and a handful of nails. If I drive in one nail
and apply a strong enough lateral force, the nail will shear and the
boards fall apart. Drive in twenty nails and it will require a much
greater force. This is effectly increasing the contact area.
Back to the tire, instead of nails, you have all the little pieces
of rubber grabbing the road. The larger the contact area, the larger
the bond.
As far a classic physics goes, all the formulae work. But the
surfaces are smooth and not bonded together. In the lab we used blocks
of wood with weights on top. And as far as we could measure, the
surface area did not make a difference.
One more point. When the drag racers spin their tires, they are
actually melting them and the thin layer of molten rubber is a better
bond than a cold tire. However, even with the extra traction, if too
much power is applied, all the bonding is broke and the wheel spins
freely.
Regards, John
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1528.13 | bigger or different. | BAHTAT::DODD | gone to Helen's land | Thu Sep 12 1991 15:44 | 8 |
| Dunno about drag racers but heavy haulage railway locos are now built
with "creep control". more tractive force can be achieved if the wheels
are allowed to slip ever so slightly. This has got nothing to do with
bonding as it is steel on steel. Clearly uncontrolled wheelspin is a
bad thing.
My physics is not up to explaining this.
Andrew
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