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Location:
Brisbane
Registered:
October 2004
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wider rear tyre = more understear???
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Tue, 23 August 2005 04:27
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just had a chat with the local beaurepairs "wise old man" and he says that if i were to fit wider rear tyres to my car (all other things being equal) the car would understeer more... so i questioned him, if you take a corner at 60 (coasting), and the car is just pushing limits, but still holding line, then whack some wider tyres on the rear and take the same corner at the same speed, same conditions etc. it would understeer, and he assured me it would... now... i can't see this, from what i can see, the co-efficient of friction on the front hasn't changed, the cornering forces haven't changed... why on earth would the front suddenly be down on traction??? i can understand that the front would be the first to let go, but when your not on the throttle... how would it make a difference?!?!?!
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Location:
Sydney
Registered:
May 2003
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Re: wider rear tyre = more understear???
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Tue, 23 August 2005 04:42
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simple physics :0
the grip loading of the rear tyre will push the car foward more due to the better grip hence presenting more of a load for the fronts to handle hence inducing understeer
you will find that when questioned on a simple level it makes sense 
if you were to go for tyres that grip less on the rear that you will induce oversteer before understeer more frequently than normal
best bet is to increase both fronts and rears or use better qaulity fronts for better grip
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Location:
Brisbane
Registered:
May 2002
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Re: wider rear tyre = more understear???
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Tue, 23 August 2005 04:43
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I think he's smoking crack. Yes the front end will probably let go first, but only because the rear end now has more grip. That's not the same as saying wider rear tyres promotes understeer.
It's worth noting that most people who work at tyre shops are skilled at selling/fitting tyres and little else.
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Location:
Brisbane
Registered:
October 2004
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Re: wider rear tyre = more understear???
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Tue, 23 August 2005 04:48
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| 4DaDrift wrote on Tue, 23 August 2005 14:42 |
simple physics :0
the grip loading of the rear tyre will push the car foward more due to the better grip hence presenting more of a load for the fronts to handle hence inducing understeer
you will find that when questioned on a simple level it makes sense
if you were to go for tyres that grip less on the rear that you will induce oversteer before understeer more frequently than normal
best bet is to increase both fronts and rears or use better qaulity fronts for better grip
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perhaps, though the cornering forces are acting across the car, from one side to the other, therefore, in simple physics, the front and rear can be considered independant bodies...
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Location:
Perth
Registered:
October 2003
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Re: wider rear tyre = more understear???
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Tue, 23 August 2005 04:49
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They are skilled at f*&king your nuts up with a rattle gun, thats about it.
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Location:
Cabramatta, NSW
Registered:
May 2002
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Re: wider rear tyre = more understear???
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Tue, 23 August 2005 05:17
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There is the whole "less weight per area" debate...
Depends on how much wider you're going I suppose.
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Location:
Hobart, Tas
Registered:
May 2002
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Re: wider rear tyre = more understear???
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Tue, 23 August 2005 05:37
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| Norbie wrote on Tue, 23 August 2005 14:43 |
I think he's smoking crack. Yes the front end will probably let go first, but only because the rear end now has more grip. That's not the same as saying wider rear tyres promotes understeer.
It's worth noting that most people who work at tyre shops are skilled at selling/fitting tyres and little else.
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If the front end will let go first because the rear has more grip, then you have by definition increased understeer. The rear slip angle has decreased compared to the front.
BTW I think everyone should read "Car suspension at work: theory & practice of steering, handling & roadholding" by Jeff Daniels.
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Location:
Hobart, Tas
Registered:
May 2002
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Re: wider rear tyre = more understear???
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Tue, 23 August 2005 05:39
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[quote title=hickoz_bro wrote on Tue, 23 August 2005 14:48perhaps, though the cornering forces are acting across the car, from one side to the other, therefore, in simple physics, the front and rear can be considered independant bodies...[/quote]
No they can't.
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I supported Toymods
Location:
Epping, Sydney
Registered:
May 2002
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Re: wider rear tyre = more understear???
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Tue, 23 August 2005 05:39
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understeer has increased, yes. but the point at which the car lets go has not come any earlier - just at the point where the back would let go, instead it grips. shortly after that, one would assume the front will let go.
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Location:
Brisbane
Registered:
May 2002
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Re: wider rear tyre = more understear???
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Tue, 23 August 2005 05:50
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| Stefan wrote on Tue, 23 August 2005 15:37 |
If the front end will let go first because the rear has more grip, then you have by definition increased understeer.
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No, you have decreased oversteer.
But now we're just arguing semantics.
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Location:
Hobart, Tas
Registered:
May 2002
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Re: wider rear tyre = more understear???
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Tue, 23 August 2005 05:57
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| Norbie wrote on Tue, 23 August 2005 15:50 |
But now we're just arguing semantics.
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Yes. Need to consider slip angle changes through a corner, really.
Another intersting thing to consider is the way the 'grip' curves (for want of a better word mid-afternoon, yawn) for wider tyres work. They may have 'more grip' earlier on, but this tends to taper off a lot more quickly. Or something.
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I supported Toymods
Location:
Epping, Sydney
Registered:
May 2002
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Re: wider rear tyre = more understear???
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Tue, 23 August 2005 05:59
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true - they hold on better for longer, but when they let go they let go a lot quicker - not so good if you're not an experienced driver.
which is why people should improve handling as well as grip (and understand the difference)
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Toymods Vice President
Location:
Sydney
Registered:
May 2002
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Re: wider rear tyre = more understear???
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Tue, 23 August 2005 06:35
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| hickoz_bro wrote on Tue, 23 August 2005 14:27 |
i questioned him, if you take a corner at 60 (coasting), and the car is just pushing limits, but still holding line, then whack some wider tyres on the rear and take the same corner at the same speed, same conditions etc. it would understeer, and he assured me it would
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This is the part that brings about the confusion. Of course increasing the amount of rear grip without changing the amount of front grip will cause the car to be more likely to understeer on the limit. The problem is that you've been told it will also "reduce the limit" so to speak, or make the front tyres slide with the wide rear tyres when they wouldn't have with the old rears.
There may be some slight reduction in the conditions when understeer is felt, this is due to there not being a point when you'd have all four tyres slipping so the feeling is that the front tyres lose traction earlier.
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Location:
Melbourne, Australia
Registered:
March 2005
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Location:
Brisbane
Registered:
May 2002
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Re: wider rear tyre = more understear???
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Tue, 23 August 2005 07:35
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| Stefan wrote on Tue, 23 August 2005 15:57 |
| Norbie wrote on Tue, 23 August 2005 15:50 |
But now we're just arguing semantics.
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Yes. Need to consider slip angle changes through a corner, really.
Another intersting thing to consider is the way the 'grip' curves (for want of a better word mid-afternoon, yawn) for wider tyres work. They may have 'more grip' earlier on, but this tends to taper off a lot more quickly. Or something.
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Yep, I was under the understanding that it was all about slip angles
Cheers
Wilbo
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: wider rear tyre = more understear???
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Tue, 23 August 2005 07:42
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i fail to see that with equally good quality tyres, assuming that neither one is too big or too small for the car, that going up a small amount in width will actually increase or reduce rear grip, by much..
ultimate grip is a function of the normal force and the friction coefficient of the tyres. (and of course angle of tyres to road etc)
the friction coefficient (depending on road condition) _does_ increase a little with increasing normal force, since you are pushing the tyres into the roads irregularities more....
if you have same compound tyres, the desired slip angle will be very similar for both tyres..
unless we are talking about slicks, where there is not linear friction coefficient, and there is much more chemical bonding of tyre to road (hance faster material loss). in that case, a wider tyre results in more grip for the same normal force..
but for road tyres, you are mostly relying on friction, and friction increases with normal force, or force per unit area (until such time as the rubber cannot get more grip from being pushed into the road surface)... so i would hesitantly say that a wider tyre, given all other identical conditions, would give you _less_ ultimate grip...
of course, once the tyres are sliding, we are back to similar co-efficients for both tyres and possibly some more chemical grip as the tyre heats up at the road surface from slipping...
anyhoo, maybe i'm wrong, but it's a different way to think about it...
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: wider rear tyre = more understear???
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Tue, 23 August 2005 07:44
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oh, and then there is contact patch shape... wider tyre = wider shorter contact patch = less stable?? and then there is changing length of the patch by diameter change and tyre pressure and and and....
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Registered:
July 2005
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Re: wider rear tyre = more understear???
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Tue, 23 August 2005 07:45
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all this slip angle chimstry wizardly crap is bullshit
increasing rear width section over the front tyres does NOTHING to the car. a car itself DOES NOT exhibit ANY form of over or understeer, heck, it doesnt move on its own at all.
it all depends on the DRIVER
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Banned by his request
Location:
moved to tamworth
Registered:
July 2002
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Re: wider rear tyre = more understear???
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Tue, 23 August 2005 10:48
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depending on the geometery having a wider rear track will pronounce understeer.having a narrow rear will do the opposite.as street cars have nearly no aerodynamic aids its all to do with the geometery.but most of the time with small changes,drivers cant tell what is faster,just what they prefur.
mick
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: wider rear tyre = more understear???
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Tue, 23 August 2005 10:53
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if you have a locked rear, and wider track it will try to understeer more of course, but will it do so with an open diff?, and if you are just free-wheeeling around a corner (no throttle or brake) then you will get more weight transfer to outside wheel at the front than the back, so wouldn't it decrease understeer?
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Banned by his request
Location:
moved to tamworth
Registered:
July 2002
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Re: wider rear tyre = more understear???
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Tue, 23 August 2005 10:57
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lol there are no narrow supercars now are there!using different tracks improves on the avalible geometery capabilities.it all about making a care netrual in handling!
mick
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Location:
Brisbane
Registered:
May 2002
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Re: wider rear tyre = more understear???
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Tue, 23 August 2005 13:56
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| oldcorollas wrote on Tue, 23 August 2005 17:42 |
anyhoo, maybe i'm wrong, but it's a different way to think about it...
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You're not wrong, in theory.
In practice though, wider tyres are manufactured with softer/stickier compounds. They can get away with this because the extra surface area of a larger tyre means it's less prone to overheating. That's why wider tyres almost always means better grip.
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Banned by his request
Location:
moved to tamworth
Registered:
July 2002
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Re: wider rear tyre = more understear???
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Tue, 23 August 2005 20:29
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poor old diameter always gets missed when people talk grip!
mick
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Location:
Brisbane
Registered:
May 2002
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Re: wider rear tyre = more understear???
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Tue, 23 August 2005 23:52
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| oldcorollas wrote on Wed, 24 August 2005 00:31 |
hrm.... so if you have sayyy.. Falken Azenis in 215 and 235 widths.. everythign else being equal, the 235's will more grip because a) more chemical stickiness, and b) the soft compund already alloys tyres to deform into the road?
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Yes, that is correct. Don't assume the rubber is exactly the same just because it's the same brand and model!
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: wider rear tyre = more understear???
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Tue, 23 August 2005 23:54
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| kingmick wrote on Wed, 24 August 2005 06:29 |
poor old diameter always gets missed when people talk grip!
mick
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oi fuggya, i mentioned a few posts up 
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Location:
Wollongong
Registered:
November 2004
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Re: wider rear tyre = more understear???
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Wed, 24 August 2005 07:32
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Wider rear tyres does actually upset your rear suspension
Think about how the camber and everything works as the suspension is pushed up. With the inside patch now further in, the forces on the suspension act differently.
Effectively, it makes it harder for the rear suspension to push in (makes the rear suspension "stiffer")
When you stiffen rear the rear suspension, it makes understeer easier to induce.
Therefore, wider tyres will create a "stiffer" suspension, but too most drivers, this "stiffer" suspension is unable to be felt really. But it is one of the reasons why the back end stays in line alot better when you give it a hit around the corner. The front end doesn't have "as much" grip, and the rear has more. Which actually makes the car feel "tighter" to drive.
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Location:
Brisbane
Registered:
May 2002
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Re: wider rear tyre = more understear???
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Wed, 24 August 2005 23:44
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Actually firmer rear suspension promotes oversteer, not understeer.
I'm not sure about wider tyres making the suspension harder though...
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Location:
Brisbane, QLD
Registered:
August 2005
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Re: wider rear tyre = more understear???
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Thu, 25 August 2005 04:59
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i say buy a couple of space savers from the wreckers, chuck em on ur rear... if u get noticeable oversteer, then wider tyres would give u understeer
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Location:
Adelaide
Registered:
April 2005
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Re: wider rear tyre = more understear???
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Thu, 25 August 2005 10:22
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Not a pro. but i think when youre going hard round a corner the whole car is sliding a very tiny bit . but if the back end is gripping better then it will seem like the front is sliding more because the cars grip will be less balanced. maybe thats what he meant bout more understeer
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Location:
Brisbane
Registered:
October 2004
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Re: wider rear tyre = more understear???
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Thu, 25 August 2005 22:31
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wow... i think everyone's been smoking a bit of grandad's secret green grass... 
lets see... bigger rear wheel = higher co-efficient of friction
front has remained unchanged, thus will slip at the same point. rear well slip later...
though increased speed will see understeer conditions...under the same speed as prior to fitting larger rear tyre, it'll be just the same...
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Banned by his request
Location:
moved to tamworth
Registered:
July 2002
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I supported Toymods
Location:
Epping, Sydney
Registered:
May 2002
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Re: wider rear tyre = more understear???
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Fri, 26 August 2005 08:04
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| bahnugget wrote on Thu, 25 August 2005 14:59 |
i say buy a couple of space savers from the wreckers, chuck em on ur rear... if u get noticeable oversteer, then wider tyres would give u understeer
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that's the most stupid thing I've read all day. and that includes a few posts by bubbles.
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: wider rear tyre = more understear???
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Fri, 26 August 2005 10:04
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| hickoz_bro wrote on Fri, 26 August 2005 08:31 |
lets see... bigger rear wheel = higher co-efficient of friction
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thats not necessarily true 
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Location:
Brisbane
Registered:
October 2004
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Re: wider rear tyre = more understear???
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Fri, 26 August 2005 10:23
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it is if all else is equal... same tyre construction will give a larger contact patch... same compound = same co-efficient of friction per unit of area... so the larger tyre has a larger contact patch... thus larger co-efficient of friction. unless there's some fundamental law i'm not getting
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Location:
Brisbane
Registered:
October 2004
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Re: wider rear tyre = more understear???
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Fri, 26 August 2005 21:08
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firstly tribology is the study of lubrication...
secondly, true that co-efficient of friction is a material property, but when coupled with a dimension... say area then we can use and relate to it easier... eg, density is also a material property, but when coupled with volume, we have mass...
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Banned by his request
Location:
moved to tamworth
Registered:
July 2002
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Re: wider rear tyre = more understear???
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Fri, 26 August 2005 22:00
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co-efficient of friction defenition.my tires sticky to the roady,my tires no stickey to the roady very much.lol
mick
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: wider rear tyre = more understear???
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Sat, 27 August 2005 06:19
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| hickoz_bro wrote on Sat, 27 August 2005 07:08 |
firstly tribology is the study of lubrication...
secondly, true that co-efficient of friction is a material property, but when coupled with a dimension... say area then we can use and relate to it easier... eg, density is also a material property, but when coupled with volume, we have mass...
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tribology is actually more about surface... topography... and how it relates to friction and wear.. it's the topography of the hard surface (road) combined with the elastic deformation of the tyres that is of interest to us...(mostly)
the lubrication part of tribology is about overcoming friction due to said topography
so if you had tyres that have a coefficient of friction of 0.8, doubling the size does not make it 1.6...
the normal force acting on the surface, multiplied by the friction coefficient, gives the force required to make the object slide.
N x cf = SF thats how the soefficient is DEFINED.. area is contained in the normal force term..
ie N = load / area....
doubling the contact patch HALVES the normal force, and the friction coefficient remains unchanged, as does the force required to make the tyre slide (ie grip)... theoretically..
which is why i said that (theoretically) increasing tyre size, with all else being equal, will not change ultimate grip.
[Updated on: Sat, 27 August 2005 06:20]
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: wider rear tyre = more understear???
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Sat, 27 August 2005 06:25
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on a idetrack note, this looks interesting
http://www.asmeconferences.org/WTC05/TechnicalProg ramOverview.cfm#73
TK10 tribology of internal combustion engines...
heh heh, a bit of in in joke is that tribology is such a simple area of study (load a sample on a spinning disc, and measure the drag force), that they had to "lube it up" to make it attractive to research funding
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Location:
Brisbane
Registered:
October 2004
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Re: wider rear tyre = more understear???
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Mon, 29 August 2005 13:36
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| oldcorollas wrote on Sat, 27 August 2005 16:19 |
so if you had tyres that have a coefficient of friction of 0.8, doubling the size does not make it 1.6...
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correct... firstly 'cause you can't have a co-efficient of friction exceeding 1, with out some aditional assistance... eg glue... secondly, the relationship between area and the co-efficient would, i presume, be non-linear...as for the rest of it... i'm too tired atm to put up a valid argument...
the point in questions with out all the techno-babble, is that increasing the rear traction won't NOTICABLY detract from the front...
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Location:
Perth W.A
Registered:
September 2004
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Re: wider rear tyre = more understear???
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Thu, 01 September 2005 08:17
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ive recently changed the tires on the supra and i have on the front 225's with Bridgestone RE01s(expensive, very) and 235 maxxsis(not as expensive) on the back.
Around the track ive found the wider tyres on the rear have caused a longer amount of time understeering before the onset of oversteer around a corner..
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Location:
brisbane
Registered:
September 2005
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Re: wider rear tyre = more understear???
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Sun, 25 September 2005 11:30
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ok firstly dragging an old past up but just thought id share my 2 cents
secondly, ive read a few bits and peices on all this oversteer, understeer, cars goin where u try to point them thing. i think one of them was bill sherwoods, fwd vs. rwd vs. 4wd...i think
but from what i can recall and interpret from what i have read is that when car is "just pushing limits, but still holding line" as said in first post i remember that each tire will be sliding at an angle of about 7 degrees, so to do this turn with wider tyres (which in every sensible application will have more grip) then the front will be sliding and the rears not sliding, hence understeer. this is prob what the old guy from the tyre shop who was smokin grandads special green grass was getting at.
if i was u put bigger tyres on the front...easier to get it sideways and better handling under standard conditions as far as i can see, if not bigger all round just for more grip all over. but i think the dude is right
oh yeah one other thing i read, in rwd cars, the reason why they pitch inwards when u put ur foot down a little half way through a corner is because your tyres start to slide a little more, move outwards, and point the car further round the corner, opposite for fwd, hence fwd drifting is a ridiculous concept, so yeah wider tyres not gonna slide as much=more understeer. but who knows how much by
~andrew~
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