Fiat Coupe Club UK
205/50 tyres versus 225/45 tyres
Posted By: Anonymous
205/50 tyres versus 225/45 tyres - 20/07/2006 18:36
Now then...
Obviously I'd like to fit the 225 tyres for improved sports car aesthetics, but are there any real advantages to fitting the wider-tyred option on the coop?
More un-sprung weight, vaguer steering, wider contact patch, more drag, worse mpg - on the face of it, it's not a good idea but may give me marginally more grip in corners (hardly noticeable in road-driving, surely).
Any comment on which fitment size is better?
Thanks!
Posted By: Anonymous
Re: 205/50 tyres versus 225/45 tyres - 20/07/2006 18:47
I want to buy a set of 17" wheels and when I do so I'm going for 215/40
Posted By: Anonymous
Re: 205/50 tyres versus 225/45 tyres - 20/07/2006 18:49
205/50 is better IMO!
Im doing track days usually, so i used both and i did notice any deference only the 225 are much expensive..
cheers..
Posted By: Anonymous
Re: 205/50 tyres versus 225/45 tyres - 20/07/2006 18:58
They both give you the same sized contact patch, Nigel can lecture for... ohhh... minutes about it
. The contact patch of the 205's is going to be a more elongate shape - potentially better for traction and the 225's will be (obviously) wider, fractionally increasing cornering grip. You will spend more to get the 225's and I always ran them cos I likes me bends, the looks and most importantly they give you better rim protection for those oh-so vulnerable alloys. You'll tramline less with 205's potentially though.
Posted By: Anonymous
Re: 205/50 tyres versus 225/45 tyres - 20/07/2006 19:16
I'm going to 215/35/18..... is this advisable?
Ross
Posted By: Anonymous
Re: 205/50 tyres versus 225/45 tyres - 20/07/2006 19:21
Quote:
They both give you the same sized contact patch, Nigel can lecture for... ohhh... minutes about it . The contact patch of the 205's is going to be a more elongate shape - potentially better for traction and the 225's will be (obviously) wider, fractionally increasing cornering grip. You will spend more to get the 225's and I always ran them cos I likes me bends, the looks and most importantly they give you better rim protection for those oh-so vulnerable alloys. You'll tramline less with 205's potentially though.
Second everything Matt has said 
Joe
Posted By: Trappy
Re: 205/50 tyres versus 225/45 tyres - 20/07/2006 20:23
lol, i vaguely remember the contact patch debate. Wasn't interested in it at the time, but i'm bored now.
So, same rolling diameter... but one is wider. Call me stupid but WTF is that about!
Posted By: Anonymous
Re: 205/50 tyres versus 225/45 tyres - 20/07/2006 20:29
The tyre is wider side to side, on the 225 tyres. But with the same weight of the Coupe and the same tyre pressures the amount of rubber on the road must be the same.
So with a wider tyre, the front to back distance of the contact patch will be less.
So good for corners and less so for standing starts.
Posted By: MCMike
Re: 205/50 tyres versus 225/45 tyres - 20/07/2006 22:44
I don't agree with this, you're going to tell me that if I run 4 spacesavers inflated to 39 psi front, 33 psi back the amount of rubber on the road is the same ?
Posted By: Anonymous
Re: 205/50 tyres versus 225/45 tyres - 20/07/2006 22:59
yes it probably would be, because I imagine a space saver at 39 psi would be hopelessly flat when trying to carry the 400 kg per front wheel of the coupe. I dread to think what pressure they actually run to stay in shape - hence the limited speed/mileage before they fail, or overheat and THEN fail. I would guess they run in the region of 60-65 psi.
So you'd still get your footprint (very long and thin, like a bike tyre), but it'd be meaningless because the sidewalls would be so buckled and unstable it would make the car undriveable.
Posted By: Anonymous
Re: 205/50 tyres versus 225/45 tyres - 20/07/2006 23:08
Yep!
haha
I don't mean to be rude, well yep and no, the amount of rubber is also determined by the shape of the rim, so in your case the amount of rubber would not be the same, the rim would not allow the rubber to deform enough to allow the same contact patch.
however with 205/225 tyres we use the same rims,...then the rim can allow the same size contact patch to be the same. The same weight of the car pressing down has an equal and opposite force pressing up.
Joe
Posted By: Anonymous
Re: 205/50 tyres versus 225/45 tyres - 20/07/2006 23:41
So how come its recognized that using a larger tyre 205 -> 225 uses more fuel?
Ross
Posted By: Flea
Re: 205/50 tyres versus 225/45 tyres - 21/07/2006 00:10
From my experience of F1 tyres in both sizes the 225/45 give more grip in a straight line. One thing that should be considered is the stiffness of the sidewall as this can vary between tyres and will effect the contact patch pressure for pressure.
Posted By: Nigel
Re: 205/50 tyres versus 225/45 tyres - 21/07/2006 01:54
I can't be bothered......
Posted By: Anonymous
Re: 205/50 tyres versus 225/45 tyres - 21/07/2006 02:07
I still don't agree witrh the contact patch being the same either. I won't go into my theory though as it is boring.. Otherwise why would F1 cars run fat tyres.. (or they used to). I have 225 on my car by the way and the car handles pretty well.
Posted By: Anonymous
Re: 205/50 tyres versus 225/45 tyres - 21/07/2006 02:07
Space-savers? Brilliant things!
We did a magazine feature 5 years ago where we bolted 4 x space-savers onto an Evo 6, pumped them up to 50psi and went flat-out at Bruntingthorpe Proving Ground, lobbing the car sideways as we went.
It was like driving on marbles and quite, quite hilarious until two of them erupted at 90mph and the exploding rubber took out the right-hand side skirt.
Which would have been hilarious, except it was my car!
Posted By: Begbie
Re: 205/50 tyres versus 225/45 tyres - 21/07/2006 04:19
Quote:
I can't be bothered......
Did you know the lexus soarer can pull 1.5G on road tyres?
Posted By: Jimbo
Re: 205/50 tyres versus 225/45 tyres - 21/07/2006 04:58
Ah the dullset tones of [***]... I miss him
Posted By: Jimbo
Re: 205/50 tyres versus 225/45 tyres - 21/07/2006 04:58
lol, I cant say [***] !!!
Posted By: Jimbo
Re: 205/50 tyres versus 225/45 tyres - 21/07/2006 04:59
M y c r o f t
Posted By: Anonymous
Re: 205/50 tyres versus 225/45 tyres - 21/07/2006 05:29
Hang on a bit - are you lot saying that a 225 tyre has the same contact patch as a space saver..? Sounds like a load of old boll*cks to me
Seriously, I can probably imagine that some of you lot know a great deal about such things (with is to be both admired and vilified), but how on Earth can a 2 foot wide tyre have the same contact patch as a 1 foot wide tyre (these are imaginery tyres
).
A wider tyre has more grip, it has more grip because there is more of it touching the road... surely???????????????
And to prove this, I want someone to drive over some paint & measure the resulting paint marks on the tarmac, I can guarantee the 225 tyre will leave a bigger splodge of paint than the 205. End of. Don't argue. It's the law. I am the Daddy. Back of the net & smell my cheese. Sorted
Posted By: Anonymous
Re: 205/50 tyres versus 225/45 tyres - 21/07/2006 05:37
Oh & Nigel, if you come on here with some clever science, I'm gonna have a word with the big man with the white beard 'up there', & he's gonna dish out some special hail stones just for the Rugeley area
Posted By: MCMike
Re: 205/50 tyres versus 225/45 tyres - 21/07/2006 14:55
Quote:
A wider tyre has more grip, it has more grip because there is more of it touching the road... surely???????????????
..exactly - that's why sports cars tend to be fitted with wide tyres !!!
Posted By: Anonymous
Re: 205/50 tyres versus 225/45 tyres - 21/07/2006 15:01
if you guys go on with this , Nigel's gonna come
thumpin' in and squash us .....
Posted By: Anonymous
Re: 205/50 tyres versus 225/45 tyres - 21/07/2006 15:44
Posted By: MCMike
Re: 205/50 tyres versus 225/45 tyres - 21/07/2006 16:09
Ok - interesting thread on Pistonheads...
Thread Taken from the above thread...
'The longer the contact patch is, the more gradually this break away occurs. If you shorten the contact patch, the break away occurs more abruptly but you get more absolute grip at the peak. This is because there is less variation in sideways distortion between the front and back of the contact patch, more of the contact patch reaches maximum grip and starts to slide at the same point. Having a shorter contact patch also means you get less self-aligning torque so there is less feedback through the steering about how close the tyres are to breaking away.
When you fit wider tyres, what you're doing is making the contact patch wider and shorter for the same tyre pressure. This means you get a more abrupt breakaway but more grip right on the limit. The disadvantage is more expensive tyres, more tramlining and steering kickback, more wind and rolling resistance and noise, less grip in slippery conditions, a more abrupt breakaway to catch out the unwary driver and less warning through the steering about how close the tyres are to breaking away.
This probably explains why manufacturers tend to put wide tyres on high performance cars and narrower ones on ordinary family saloons.'
Posted By: Anonymous
Re: 205/50 tyres versus 225/45 tyres - 21/07/2006 16:18
now,
thats something that makes sense , for once.
.
Posted By: Trappy
Re: 205/50 tyres versus 225/45 tyres - 21/07/2006 18:29
Quote:
This probably explains why manufacturers tend to put wide tyres on high performance cars and narrower ones on ordinary family saloons.'
So why do all BMWs regardless of engine size seem to have 335/35/18 wheels/tyres then?
Makes them all look stupid I think...
Posted By: Anonymous
Re: 205/50 tyres versus 225/45 tyres - 21/07/2006 21:46
One way of increasing the contact patch is by increasing the wheel circumferance... A bigger wheel will have more area touching the road. Think about that..
Posted By: Anonymous
Re: 205/50 tyres versus 225/45 tyres - 21/07/2006 21:55
At the same tyre pressure, the size of the contact patch would not change.
Quote:
Warning - long!
The school physics books tell you that the coefficient of friction is a constant, but in reality this is an oversimplification. In the case of tyres, the coefficient of friction drops off as you increase the weight on the wheel. This is why you can tune a car's handling by adjusting the anti-roll bar, and also why you *can't* tune with anti-roll bars if the load on the tyres is too small (the vehicle is overtyred).
Because of this, the bigger the contact patch the more grip you can get. In a drag race, dropping the tyre pressure increases the contact patch area and increases grip. Even on road bikes you will see people dropping the tyre pressure to almost nothing for the absolute maximum grip down the strip.
BUT, when you look at lateral grip (side force) other factors start to matter. The tyre develops side force because of the slip angle between the tyre and the road. This slip angle means the tread is being pulled sideways by the road surface. At the front of the contact patch the deflection is relatively small. As you move back along the contact patch the deflection increases steadily. At some point, the sideways forces in the tyre exceed the friction between the tread and the road and the tread starts to slip relative to the road. When the tread is slipping like this it produces less grip on the road. Now imagine increasing the slip angle and imagine what effect this has on the side force. As the slip angle increases the sideways deflection builds up quicker so the front of the contact patch works harder. But more and more of the back of the contact patch is sliding and losing grip. At some point you reach a maximum point where more slip angle means less side force because you are losing more grip at the rear of the contact patch that you are gaining at the front. This is often referred to as 'breaking away' where you ask the tyre for more grip and end up getting less.
The longer the contact patch is, the more gradually this break away occurs. If you shorten the contact patch, the break away occurs more abruptly but you get more absolute grip at the peak. This is because there is less variation in sideways distortion between the front and back of the contact patch, more of the contact patch reaches maximum grip and starts to slide at the same point. Having a shorter contact patch also means you get less self-aligning torque so there is less feedback through the steering about how close the tyres are to breaking away.
When you fit wider tyres, what you're doing is making the contact patch wider and shorter for the same tyre pressure. This means you get a more abrupt breakaway but more grip right on the limit. The disadvantage is more expensive tyres, more tramlining and steering kickback, more wind and rolling resistance and noise, less grip in slippery conditions, a more abrupt breakaway to catch out the unwary driver and less warning through the steering about how close the tyres are to breaking away.
This probably explains why manufacturers tend to put wide tyres on high performance cars and narrower ones on ordinary family saloons.
Joe
Posted By: Anonymous
Re: 205/50 tyres versus 225/45 tyres - 21/07/2006 21:58
Just be happy we don't use a 335 wide tyre, like the Ultima GTR, as they're £670 a pair
Posted By: Nigel
Re: 205/50 tyres versus 225/45 tyres - 22/07/2006 03:29
The reason that spacesavers offer no grip is because they're inflated to 60psi and made from rubber more akin to plastic than the usual compounds associated with road tyres.
Europa - a 2 foot wide tyre WILL have the same contact patch area as a 1 foot wide tyre, as long as the pressures are the same.
For simplicity's sake, I've not made any calculations, but it goes like this:
2 foot wide tyre - contact patch is (predictably) 2 feet wide and say 3 inches long.
I foot wide tyre - contact patch is 1 foot wide (with me so far) and is absolutely DEFINITELY going to be 6" long
Therefore, both tyres have contact patch areas of 72 square inches.
The handling of the two tyres would be very different. Now get this - neither will have more grip than the other - they'll both have the same - they MUST have.
And I still haven't had my hailstone dents sorted, so you can incur His wrath and I don't care.
So there
Posted By: Anonymous
Re: 205/50 tyres versus 225/45 tyres - 22/07/2006 04:45
we love you Nigel ... even though you're daft.
Posted By: Anonymous
Re: 205/50 tyres versus 225/45 tyres - 22/07/2006 05:59
i agree with you but i dont think that 225 will make that difference vs the 205 on the same rim...its very small difference!!
i don’t think that you can even feel it on road and its not worthy to pay all that difference in cost!!
cheers..
