Replacement Tyres

Richard George · Post by **Richard George** » Mon Sep 24, 2001 4:43 pm

Hi

I have a 1994 1800 fitted with the Version 2 rims and Dunlop Monza 200
185/60 x 14 tyres.

I am now up for replacing all four tyres.

My usual tyre supplier has suggested fitting American General tyres and also
going up to 195/60 x 14.

Can I have some feedback please ??

What are your comments about going on to 195/60 x 14 size tyres ?? Would
it put the speedo reading out ?

As far as breed of tyre goes I have no pets - all I am looking for is a tyre
that is quiet, has good handleling at a reasonable cost. (I am not a speed
demon but do appreciate the car's spirit)

Breeds I have looked at are Dunlop, Goodyear, General, and top of the range,
Falkin.

Our other car, BMW E30 323i, is fitted with Falkin 195/65 x 14 tyres.

As I am not subscribed to the Elist would you please reply to me at
denric@xtra.co.nz

Thank you

Richard George
Mem No 21106
Wanganui

Scott Elder · Post by **Scott Elder** » Mon Sep 24, 2001 6:14 pm

the fatter the tyres the better. more surface area = more traction.
Generally speaking, it's cheaper to get mid-range "fat" tyres than it is to
get top-range thinner ones with a better overall design for better handling
etc.

Gary Morrison · Post by **Gary Morrison** » Tue Sep 25, 2001 12:35 pm

Correction. Fatter tyres = same surface area. The change is that the contact
patch changes direction, narrow tyre contact patch is fore-aft, fat tyre
contact patch is sideways. More traction in corners, less for straightline
braking etc.

carl.halvorsen · Post by **carl.halvorsen** » Wed Sep 26, 2001 6:09 am

Thats right Gary. I was waiting for someone to explain that. It is a common
misconception about wide tires = more surface area.

Carl

D. M. Warrington · Post by **D. M. Warrington** » Wed Sep 26, 2001 8:43 am

Semi-correction. Yes, it's true that the contact patch remains the same area
but changes direction, becoming more sideways as the width of the tyre
increases. But a more sideways patch doesn't mean more sideways force
available for cornering and less for straight line acceleration and braking.
Although the details of what's going on between tyre and road in the contact
patch can become quite complicated, the 'friction circle' picture is pretty
close to accurate - i.e. the total force the tyre can apply to the road
before sliding is essentially the same in all directions.

This isn't to say that different choices of wheels and tyres will have no
effect on handling - there are the 'stickiness' and wear characteristics of
varying rubber compounds, the water dispersal and noise properties of tread
patterns, the total weight of wheel and tyre, the softness/hardness of the
tyre sidewall (which has the effect of an extra spring) at a given inflation
pressure and so on. But for a given tyre, loaded with a given weight, the
maximum force is pretty much the same in all directions.

Miata magazine had a nice description of the friction circle in a mid-90's
issue - I can find the reference if anyone's interested. Another quite nice
reference is a series of articles on the Physics of Racing by Brian Beckman,
available on the web at
www.miata.net/sport/Physics/
- the second article is about tyre friction in general and the effects of
weight transfer, and the seventh about the friction circle, which underlines
the key point that the more of the tyre's maximum force you are using for
straight line acceleration or braking, the less is left for cornering.

Which is a neat opening to plug my line of work, and say that it's all
Physics, folks!

Cheers,
Don
---------------------------------

At 06:09 AM 9/26/01 +1200, you wrote:
[...]

Shane O'Dea · Post by **Shane O'Dea** » Wed Sep 26, 2001 4:02 pm

Surely if the tire is the same diameter the surface area touching the
road must increase for a wider tire?

Shane

[/quote][/quote][/quote][/quote][/quote][/quote][/quote][/quote]
On 26/09/01, 08:43:27, "D. M. Warrington" <dmw@physics.otago.ac.nz> wrote
regarding Re: Replacement Tyres :

Semi-correction. Yes, it's true that the contact patch remains the same

area

but changes direction, becoming more sideways as the width of the tyre
increases. But a more sideways patch doesn't mean more sideways force
available for cornering and less for straight line acceleration and

braking.

Although the details of what's going on between tyre and road in the

contact

patch can become quite complicated, the 'friction circle' picture is pretty
close to accurate - i.e. the total force the tyre can apply to the road
before sliding is essentially the same in all directions.

This isn't to say that different choices of wheels and tyres will have no
effect on handling - there are the 'stickiness' and wear characteristics of
varying rubber compounds, the water dispersal and noise properties of tread
patterns, the total weight of wheel and tyre, the softness/hardness of the
tyre sidewall (which has the effect of an extra spring) at a given

inflation

pressure and so on. But for a given tyre, loaded with a given weight, the
maximum force is pretty much the same in all directions.

Miata magazine had a nice description of the friction circle in a mid-90's
issue - I can find the reference if anyone's interested. Another quite nice
reference is a series of articles on the Physics of Racing by Brian

Beckman,

available on the web at
www.miata.net/sport/Physics/
- the second article is about tyre friction in general and the effects of
weight transfer, and the seventh about the friction circle, which

underlines

the key point that the more of the tyre's maximum force you are using for
straight line acceleration or braking, the less is left for cornering.

Which is a neat opening to plug my line of work, and say that it's all
Physics, folks!

Cheers,
Don
---------------------------------

At 06:09 AM 9/26/01 +1200, you wrote:
Thats right Gary. I was waiting for someone to explain that. It is a

common
[...]

Gary Morrison · Post by **Gary Morrison** » Wed Sep 26, 2001 5:37 pm

Oops, Thanks Don, you're righter than me!

D. M. Warrington · Post by **D. M. Warrington** » Thu Sep 27, 2001 8:56 am

At 04:02 AM 9/26/01 GMT, you wrote:

Surely if the tire is the same diameter the surface area touching the
road must increase for a wider tire?

Shane

I agree that that sounds logical at first sight, but it's not so.

If you imagine gently lowering your MX5 on to the road with a crane, the
tires which are initially circular gradually deform, getting a larger and
larger flattened bit on the bottom - the contact patch. The force the tyre
is pushing down on the road with is roughly equal to the air pressure in the
tire times the area of the contact patch. [The fact that the pressure times
the area gives the force is more apparent from the 'old' unit of pressure,
as 'pounds per square inch'.] The road reacts to this downward push of the
air in the tyres by obligingly pushing up on the car with an exactly equal
and opposite force, reducing the amount of the car's weight that the crane
needs to support. When the area of the contact patch has increased to the
point that this upward force is equal to the car's weight, the process stops
- the car's weight is now fully supported by the road and the crane cable is
slack.

So for a car with even weight distribution, the contact patch area of each
tyre is roughly one quarter of the weight of the car divided by the air
pressure in the tyre. If you use wider tyres, the fore/aft length of the
contact patch will be smaller, but the area will be the same. As the weight
carried by a given wheel changes (more weight on the rear wheels when
accelerating, more on the outside wheels when cornering) the contact patch
area of that tyre will change to compensate - more weight will result in a
larger contact area.
This picture ignores the part of the support carried by the springiness of
the rubber casing of the tyre, but for a correctly inflated normal tyre,
it's the air inside that does most of the pushing, so it is roughly right.

Sorry if the above sounds too much like a lecture - it's an occupational hazard.
Cheers,
Don

D. M. Warrington · Post by **D. M. Warrington** » Thu Sep 27, 2001 9:15 am

Oops! Sorry, Fletch. Composed and sent my longwinded reply before I read
yours. Could have saved myself the effort.

Don
-----------------

[...]

Shane O'Dea · Post by **Shane O'Dea** » Thu Sep 27, 2001 9:35 am

Thanks Don.

So carrying this one step further...

...more downforce created by aerodynamic means would increase the size of
the contact patch as speed increases?

Shane

[/quote][/quote][/quote][/quote][/quote][/quote][/quote][/quote]
On 27/09/01, 08:56:16, "D. M. Warrington" <dmw@physics.otago.ac.nz> wrote
regarding Re: Replacement Tyres :

At 04:02 AM 9/26/01 GMT, you wrote:
Surely if the tire is the same diameter the surface area touching the
road must increase for a wider tire?

Shane

I agree that that sounds logical at first sight, but it's not so.

If you imagine gently lowering your MX5 on to the road with a crane, the
tires which are initially circular gradually deform, getting a larger and
larger flattened bit on the bottom - the contact patch. The force the tyre
is pushing down on the road with is roughly equal to the air pressure in

the

tire times the area of the contact patch. [The fact that the pressure times
the area gives the force is more apparent from the 'old' unit of pressure,
as 'pounds per square inch'.] The road reacts to this downward push of the
air in the tyres by obligingly pushing up on the car with an exactly equal
and opposite force, reducing the amount of the car's weight that the crane
needs to support. When the area of the contact patch has increased to the
point that this upward force is equal to the car's weight, the process

stops

- the car's weight is now fully supported by the road and the crane cable

is

slack.

So for a car with even weight distribution, the contact patch area of each
tyre is roughly one quarter of the weight of the car divided by the air
pressure in the tyre. If you use wider tyres, the fore/aft length of the
contact patch will be smaller, but the area will be the same. As the weight
carried by a given wheel changes (more weight on the rear wheels when
accelerating, more on the outside wheels when cornering) the contact patch
area of that tyre will change to compensate - more weight will result in a
larger contact area.
This picture ignores the part of the support carried by the springiness of
the rubber casing of the tyre, but for a correctly inflated normal tyre,
it's the air inside that does most of the pushing, so it is roughly

right.

Sorry if the above sounds too much like a lecture - it's an occupational

hazard.
[...]

D. M. Warrington · Post by **D. M. Warrington** » Thu Sep 27, 2001 2:54 pm

At 09:35 PM 9/26/01 GMT, you wrote:

Thanks Don.

So carrying this one step further...

...more downforce created by aerodynamic means would increase the size of
the contact patch as speed increases?

Shane

Yes, the extra downforce at higher speeds from aerodynamics (wings etc) will
increase the size of the contact patch.

However the chief point is not so much that the contact patch is bigger, but
that the greater downforce means that more horizontal friction force of the
tyre on the road is available, and this means more grip for cornering faster
or for accelerating/braking without loss of traction. (In rough
approximation, the maximum available sideways or fore/aft friction force is
proportional to the downforce.)

Of course another way of increasing the downforce (to increase the available
friction) would be simply to add weight to the car. Add a couple of bags of
rocks in the passenger seat. But in this case the extra weight means that
ALL the extra horizontal force it makes available is needed just to produce
the SAME linear or cornering acceleration of the beefier car, so you don't
gain anything. In contrast, the aerodynamic downforce from wings makes more
friction force available without increasing the weight of the car, allowing
faster cornering speed and greater acceleration/braking.

Cheers,
Don
----------------------------

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