smart51
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| posted on 5/3/10 at 11:52 AM |
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Wider tyres - more grip or not?
There's an argument on another forum about wider tyres. The standard rims support 165 to 185 tyres and after market rims bigger and bigger.
Opinion is in 2 camps. Those who believe that more rubber gives more grip and those who believe that it makes no difference. So who is right? For
info, the car in question is a 700kg open top 2 seater with a front mid engine - rear drive layout.
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mangogrooveworkshop
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| posted on 5/3/10 at 12:00 PM |
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Bigger tyres give less pounds per square inch of grip.....
F1 cars use them cause they have a ton of downforce.....
A simple test is to get the wife to put a trainer shoe on one foot and a stilleto on the other....
get her to stand on your hand ....or back if you like it that way and see which gives more pain edit that I meant pressure
[Edited on 5-3-10 by mangogrooveworkshop]
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turbodisplay
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| posted on 5/3/10 at 12:01 PM |
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More grip in the dry, less in the wet.
Darren
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NOTE:This user is registered as a LocostBuilders trader and may offer commercial services to other users
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brianthemagical
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| posted on 5/3/10 at 12:09 PM |
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Theoretically it'll be the same due to Friction formulae, the main difference then being wear rates and heat. This doesnt take into account the
grip due to deformation of the tyre, conforming to the road surface, thus it's more than likely that a wider tyre gives more grip.
[Edited on 5/3/10 by brianthemagical]
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mcerd1
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| posted on 5/3/10 at 12:12 PM |
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quote:
Originally posted by mangogrooveworkshop
Bigger tyres give less pounds per square inch of grip.....
yes, but limited by the coeficent of friction that you get between the rubber and the road surface (material dependant properties)
so say you've got a tyre/surface combination that gives you a static Cof = 0.7, then you'll only get max. 70% of your weight as static
'grip'
(although in reality is more complex as the tyre also deforms and there is always some slip - so area also comes into it too)
thats why 'sticky' sports/track tyres give you more grip or why you'll get better grip on some roads/tracks than others (in the dry
at least)
its all about having the optimum sized 'contact patch' for the type of tyre and the conditions
thats why rally cars use big wide tyres on dry tarmac stages and very skinny ones on snow/ice - but they also employ experts to test all the possibles
and pick the most suitable one on the day
throw in the deformation, dynamic friction, heat, wear........ and wet roads is a whole other story too
trial and error will probably give you the best/quickest answer (hopefuly other people have done it before - but I guess thats why you asked here in
the first place  )
[Edited on 5/3/10 by mcerd1]
[Edited on 5/3/10 by mcerd1]
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scudderfish
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| posted on 5/3/10 at 12:55 PM |
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The Elise S2 came with skinny front tyres as Lotus said it gave better front end grip. Lots of owners didn't believe them, fitted wider rubber
because it looked better and understeered off at the next roundabout.
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britishtrident
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| posted on 5/3/10 at 12:58 PM |
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All sorts of considerations to do with tyre construction and tyre compound come into play. To put it simply if the wider tyre wasn't designed
for a light weight car it might improve grip a little but definitely won't improve the handling especially in the wet.
As a general rule the best handling comes from using tyres that are relatively narrow for the wheel rim width.
[I] What use our work, Bennet, if we cannot care for those we love? .
― From BBC TV/Amazon's Ripper Street.
[/I]
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procomp
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| posted on 5/3/10 at 01:21 PM |
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Hi
As above far too many things to take into consideration As an out and out overall view.
IRS- Live axle are we even talking the rear or front also in general. Whats the camber gain control situation. whats the initial problems that they
are trying to improve on by going wider in the first place. What Dia are we talking........ There is no one fits all answer.
Cheers Matt
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craig1410
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| posted on 5/3/10 at 02:02 PM |
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One way to look at it is to consider that the corner weight of the car is more or less constant and therefore what you are changing by going for wider
tyres is the "shape" of the contact patch, not the overall size. This is because, if the tyre pressure is, say, 30psi and you have 300lb
corner weight then the contact patch will be approximately 10 square inches. This isn't precise because the tyre carcass will provide some
mechanical support but it's not far off.
So, for a narrower tyre the contact patch will be longer and narrower, for a wider tyre it will be shorter and wider. That is assuming the same tyre
pressures. However, in my experience the tyre pressure of a low profile tyre tends to be higher (perhaps to help avoid the rim hitting the ground on
potholes) so that would mean a smaller contact patch and therefore less rubber on the road. However, this smaller patch would have more pressure on it
as the corner weight is the same and therefore grip should be the same.
As has been mentioned though, it is not as simple as this due to various dynamic mechanisms at play and of course heat effects. I would expect wider
tyres to be slightly better under lateral loads but not as good at braking and traction. You should however get less rolling resistance with a wider
tyre due to the shorter contact patch (ie. the tyre is more round and less flat at the bottom) and for similar reasons you will get less heat buildup
in a wider tyre - good for the track, perhaps not so good for the road.
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speedyxjs
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| posted on 5/3/10 at 02:06 PM |
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I fitted wider tyres to my Jag (225 to 245) as i was struggling for grip in the wet (half throttle would spin the tyres) with the new rubber i only
lose traction at full throttle.
It could also be down to the thread pattern though
How long can i resist the temptation to drop a V8 in?
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britishtrident
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| posted on 5/3/10 at 04:21 PM |
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quote:
Originally posted by speedyxjs
I fitted wider tyres to my Jag (225 to 245) as i was struggling for grip in the wet (half throttle would spin the tyres) with the new rubber i only
lose traction at full throttle.
It could also be down to the thread pattern though
The Jag however has more than enough weight to get 245 rubber working, Jag were one of the first manufacturers to start using what were then
considered very wide low profile tyres on the first generation XJ6 over 40 years ago.
[I] What use our work, Bennet, if we cannot care for those we love? .
― From BBC TV/Amazon's Ripper Street.
[/I]
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iank
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| posted on 5/3/10 at 04:34 PM |
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700kg is very light for a modern tin-top, is it a midget/spitfire?
<IMO> Most lightweight cars (kit or tin-top) are fitted with tyres much wider than optimum, probably based on people experience with much
heavier tin-tops.
A lot of classic mini's seem to do very well on 165 section rubber and are around the 700kg mark.
Finally don't forget a wider wheel/tyre is going to be somewhat heaver so there's a handling trade-off to be made even if wider would
improve dry grip.
</IMO>
[Edited on 5/3/10 by iank]
--
Never argue with an idiot. They drag you down to their level, then beat you with experience.
Anonymous
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smart51
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| posted on 5/3/10 at 05:17 PM |
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The car is a Suzuki Cappuccino and came with 165/65/14 tyres as standard. The factory fit tyre is no longer made and neither is the Eagle F1, which
was said to be the best tyre for the car.
The stock rim size suits 165-185 section tyres and at 4.7kg is so light that people want to stick with them, though 13" or 15" rims would
give a better choice of tyre.
For this discussion, I'm assuming that you've chosen the brand and model of tyre you want and are deciding whether to have 165, 175 or 185
width.
I have 175/65/14 on mine just because it makes the speedo read closer to the true speed. 165/65 reads quite a bit under.
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Confused but excited.
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| posted on 5/3/10 at 06:49 PM |
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Friction is independent of surface area. So in a perfect world, no.
However when you have large amounts of torque being transmitted to the road or large cornering forces trying to push the car sideways, then a wider
tyre will not shear as much as a narrow one, so therefore more grip. Not so much between the tyre and the road as between the tyre contact surface and
the rest of the tyre.
IMHO of course.
[Edited on 5/3/10 by Confused but excited.]
Tell them about the bent treacle edges!
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MikeRJ
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| posted on 5/3/10 at 08:21 PM |
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quote:
Originally posted by scudderfish
The Elise S2 came with skinny front tyres as Lotus said it gave better front end grip. Lots of owners didn't believe them, fitted wider rubber
because it looked better and understeered off at the next roundabout.
'Twas the other way around. They fitted narrower tyres to promote more understeer so people wouldn't get into trouble with the typical
mid-engined-handling-on-the-limit problems. Lots of owners fitted wider tyres...
I can remember a Top Gear episode where Clarkson moaned about the narrow tyres as he found it nearly impossible to provoke a slide compared to the S1.
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MikeRJ
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| posted on 5/3/10 at 08:24 PM |
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quote:
Originally posted by Confused but excited.
Friction is independent of surface area.
The coefficient of friction of rubber/tarmac is rather load dependant i.e. reduces with increasing load.
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Badger_McLetcher
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| posted on 6/3/10 at 02:10 AM |
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Basically cornering force is given (in a perfect world) by:
Mass * Gravity * friction coefficient = cornering force
Of course when you look at this you realise that in effect the cornering G achieved is theoretically the same value of the grip coefficient (which is
about 0.7 for a good road tyre IIRC). The coefficient of friction has a maximum value of 1.
Of course this doesn't factor in many other conditions, such as the rubber of the tyre interlocking with the surface of the road (which is one
of the main reasons how the grip coefficient can be over 1, and why softer tyres tend to grip more).
On the original subject, a larger contact patch would lead to more interlocking.
[Edited on 6/3/10 by Badger_McLetcher]
If disfunction is a function, then I must be some kind of genius.
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britishtrident
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| posted on 6/3/10 at 09:33 AM |
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If you look back to 1960s & 1970s Road & Track road test on the orignal Lotus Elan, Plus2S and Europa you will see lateral g figures of 0.9g
recorded on the skid pan test. In those days Lotus left the factory on very ordinary narrow Dunlop SP Sport or Goddyear G800 tyres, The baby Elan
had 145"x13" tryres as standard the Elan Sprint 155"x13" tyres and even the Plus2S/130 only left the factory with
165x13" tyres.
Most of the very wide tyres available are designed for much heavier cars than a Seven like vehicle, this implies hard compounds and much stiffer
tyre carcass all have depressed crowns. If insufficient weight is loaded on the tyre the depressed crown will not be in full contact with the road
and not contribute to grip.
[Edited on 6/3/10 by britishtrident]
[I] What use our work, Bennet, if we cannot care for those we love? .
― From BBC TV/Amazon's Ripper Street.
[/I]
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Badger_McLetcher
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| posted on 6/3/10 at 10:46 AM |
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quote:
Originally posted by britishtrident
If you look back to 1960s & 1970s Road & Track road test on the orignal Lotus Elan, Plus2S and Europa you will see lateral g figures of 0.9g
recorded on the skid pan test. In those days Lotus left the factory on very ordinary narrow Dunlop SP Sport or Goddyear G800 tyres, The baby Elan
had 145"x13" tryres as standard the Elan Sprint 155"x13" tyres and even the Plus2S/130 only left the factory with
165x13" tyres.
Most of the very wide tyres available are designed for much heavier cars than a Seven like vehicle, this implies hard compounds and much stiffer
tyre carcass all have depressed crowns. If insufficient weight is loaded on the tyre the depressed crown will not be in full contact with the road
and not contribute to grip.
[Edited on 6/3/10 by britishtrident]
You won't find me arguing with that, as obviously the degree of interlocking varies with force pushing down as well as softness of the tyre.
The exact interactions of the tyre with the road are well above my paygrade! (Especially considering I'm a student lol)
If disfunction is a function, then I must be some kind of genius.
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smart51
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| posted on 22/3/10 at 09:49 AM |
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Here is an interesting article on tyres that says tyre width has no real effect on
grip. It also says, though, that tyre pressures have no effect on grip. I could tune the MNR's understeer / oversteer with small changes in
tyre pressure quite easily, so I'm not sure I agree with everything it says.
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David Jenkins
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| posted on 22/3/10 at 12:30 PM |
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There is also the issue of how grip is lost - allegedly (this is also a huge matter of debate).
A view went about a while back that wide tyres give more grip, but when they finally lose the plot then they go big time - one minute grip, next
minute none.
I don't have the experience to comment one way or the other - I can only say that my Locost with its 13x5.5" high-profile tyres is utterly
predictable, in that when it gets to the limit it lets me know in a polite way by progressively losing grip. This gives me a fair chance to take some
corrective action...
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Ivan
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| posted on 22/3/10 at 08:21 PM |
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quote:
Originally posted by smart51
Here is an interesting article on tyres that says tyre width has no real effect on
grip. It also says, though, that tyre pressures have no effect on grip. I could tune the MNR's understeer / oversteer with small changes in
tyre pressure quite easily, so I'm not sure I agree with everything it says.
I think this is a perfect example of where theory is misleading. The tunability you get from pressure variation arises from the changes pressure
causes in tread distortion when cornering which effectively changes the coefficient of friction - i.e. a heavily distorted tread due to sidewall
distortion in an under inflated tyre will lower the coefficient of friction and also affect the rate of a tyre's response to steering input
through tread squirm.
So yes pressure does affect grip by changing both the coefficient of friction and reaction of the tyre to side forces (steering)
[Edited on 22/3/10 by Ivan]
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MikeRJ
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| posted on 22/3/10 at 09:44 PM |
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quote:
Originally posted by smart51
Here is an interesting article on tyres that says tyre width has no real effect on
grip.
Probably written by someone with an O Level/GSCE in Physics who assumes all materials act in an ideal and predictable way. Rubber does not,
it's coefficient of friction is not constant.
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craig1410
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| posted on 22/3/10 at 10:26 PM |
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One of the reasons that grip is lost when a typical road car is fitted with wider tyres is because the suspension geometry isn't able to
maintain good camber control in the corners. This is especially the case with the long suspension travel most road cars have. Sports cars tend to have
less travel and thus camber can be controlled more easily.
As the car corners, the outer suspension is compressed and the body rolls. Depending on geometry the compression will result in camber change and if
this doesn't match the body roll then the tyre will become less and less square to the road surface and grip will be lost. This is less of an
issue with the narrower tyre where the contact patch is longer and narrower. Also, the stiffer sidewalls of the low profile, wider tyres will tend to
resist distortion which is what I believe leads to sudden breakaway. The narrower tyres will suffer less from camber change to begin with but will
tend to flex and lose grip gradually rather than snap away.
Personally, I'd think that a nice square contact patch or perhaps one which is just slightly wider than square is probably about right for most
purposes.
Anyone know of any good books on tyre engineering from a motorsport perspective?
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