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tyres
#1479805
19/03/2014 21:29
19/03/2014 21:29
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sheep
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sheep
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I just picked up some ridiculously cheap 16 inch alloys and need some tyres. Im currently running 195/50/15's now im stepping up to 16's do i go for do i go for 205/50/16, 205/45/16 or 205/40/16's.
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Re: tyres
[Re: ]
#1479806
19/03/2014 21:41
19/03/2014 21:41
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Big_Muzzie
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Big_Muzzie
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Re: tyres
[Re: ]
#1479810
19/03/2014 22:03
19/03/2014 22:03
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nismo
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nismo
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Re: tyres
[Re: ]
#1479874
20/03/2014 12:02
20/03/2014 12:02
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PeteP
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His coupe is a 20V so fitting 20VT sizes may not be the best idea for his car as both will increase the gearing by around 6%. 205/45/16 will be closer to the original diameter. Here's a tool to calculate the effects of changes.
16VT and X1/9 1500
We must all do our part for the planet. I unplugged a row of electric cars that nobody was using.
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Re: tyres
[Re: ]
#1479899
20/03/2014 13:57
20/03/2014 13:57
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PeteP
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Same weight, same deflection. Always assuming that all tyres of the same dimensions have similar sidewall stiffness.
Last edited by petep; 20/03/2014 13:59.
16VT and X1/9 1500
We must all do our part for the planet. I unplugged a row of electric cars that nobody was using.
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Re: tyres
[Re: PeteP]
#1479912
20/03/2014 14:38
20/03/2014 14:38
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Joined: Sep 2009
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szkom
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Same weight, same deflection. Always assuming that all tyres of the same dimensions have similar sidewall stiffness. I'm not sure I follow your assumptions, the tyres are different centre diameters and widths. Your calculation was based on an unloaded tyre. As I've stated before you need to consider the tyre loaded. As you've calculated an unloaded difference of 6% I'd be inclined to think Fiat would've kept the effective rolling radius the same between na and turbo.
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Re: tyres
[Re: ]
#1479928
20/03/2014 16:29
20/03/2014 16:29
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Joined: Dec 2005
Posts: 353 Leicester
Dan
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I just picked up some ridiculously cheap 16 inch alloys and need some tyres. Im currently running 195/50/15's now im stepping up to 16's do i go for do i go for 205/50/16, 205/45/16 or 205/40/16's. Surely 195/50 15s are incorrect for the 20vna - that equates to 576mm diameter which is way off the 20vt diameter of around 610. I think the original 20vna tyre size would have been 195/55 15 205/50 or 225/45 as stated here already are the correct sizes for 16" wheels on the coupe.
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Re: tyres
[Re: ]
#1479930
20/03/2014 16:42
20/03/2014 16:42
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GrahamL
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GrahamL
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20v NA had 205/50 R15 as standard, 20vt had 205/50 R16 or 225/45 R16.
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Re: tyres
[Re: szkom]
#1479932
20/03/2014 17:02
20/03/2014 17:02
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PeteP
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[As you've calculated an unloaded difference of 6% I'd be inclined to think Fiat would've kept the effective rolling radius the same between na and turbo. But Fiat don't, it is close, but not identical. Looking into it further it appears that Sheep's car has the wrong size tyres fitted on his present 15" wheels. The tyres Sheep has on his car are 195/50/15. From the handbook figures the 16v runs 195/55/15 which gives a diameter 596mm, the 16vt is 205/50/16 = 611mm a difference of 2% If, as I believe is the case, the correct 20v tyre size is the same as that of the 16v, then I would be fitting 205/50/16 on the new wheels..
16VT and X1/9 1500
We must all do our part for the planet. I unplugged a row of electric cars that nobody was using.
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Re: tyres
[Re: ]
#1479936
20/03/2014 17:15
20/03/2014 17:15
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GrahamL
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GrahamL
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205/45 R16 would be closer and 225/40 R16 would be identical to the original.
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Re: tyres
[Re: PeteP]
#1479939
20/03/2014 17:24
20/03/2014 17:24
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Posts: 5,024 ation
szkom
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But Fiat don't, it is close, but not identical.
But back to my effective rolling radius argument; with the car sat on its tyres I suspect the effective rolling radius to be near identical. As I've said before you can't establish any kind of suitability from working with an unloaded tyre. I don't disagree with your conclusion, I'd use the stock size also. But your method used to determine circumference is flawed.
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Re: tyres
[Re: ]
#1479950
20/03/2014 18:12
20/03/2014 18:12
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Big_Muzzie
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Big_Muzzie
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Sorry sheep didn't see it was an na, drop 5 off the side wall to get as close as possible to the na 15' size.
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Re: tyres
[Re: andyps]
#1479952
20/03/2014 18:50
20/03/2014 18:50
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sheep
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sheep
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If you want a couple of very good 205/45-16 Yokohama AD08Rs I have a pair for sale I'll leave it to others to decide if the size is right. So looks like 205.45 or 215.45.16's. Andy could you pm me a price for your tyres delivered.
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Re: tyres
[Re: szkom]
#1479974
20/03/2014 20:58
20/03/2014 20:58
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Joined: Dec 2005
Posts: 21,524 Aldershot
PeteP
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I don't disagree with your conclusion, I'd use the stock size also. But your method used to determine circumference is flawed. It's not flawed, it is purely geometry on an unloaded tyre which is the only possible constant which can be used. As soon as you fit it to a car, then variables such as weight, sidewall stiffness and tyre pressure come into play, so then the actual rolling circumference will be affected. But at different rates.
16VT and X1/9 1500
We must all do our part for the planet. I unplugged a row of electric cars that nobody was using.
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Re: tyres
[Re: szkom]
#1480009
20/03/2014 23:09
20/03/2014 23:09
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Joined: Dec 2005
Posts: 21,524 Aldershot
PeteP
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What use is knowing the unloaded circumference when considering gearing? Fundamental, because it is the basis from which you can work out loaded circumference. Everything in physics and engineering has to start from a known position, you then bring in additional data when making calculations for varying conditions.
16VT and X1/9 1500
We must all do our part for the planet. I unplugged a row of electric cars that nobody was using.
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Re: tyres
[Re: PeteP]
#1480012
20/03/2014 23:34
20/03/2014 23:34
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sheep
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sheep
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[As you've calculated an unloaded difference of 6% I'd be inclined to think Fiat would've kept the effective rolling radius the same between na and turbo. If, as I believe is the case, the correct 20v tyre size is the same as that of the 16v, then I would be fitting 205/50/16 on the new wheels.. So i have the wrong size tyres on my 15's at present they should be 195/55, so i should go for 205,50,16 then.
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Re: tyres
[Re: ]
#1480028
21/03/2014 08:10
21/03/2014 08:10
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Big_Muzzie
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Big_Muzzie
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Sheep, pop onto kwikfit site and it will tell you the standard size, then knock5 off for the inch increase.
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Re: tyres
[Re: ]
#1480048
21/03/2014 10:53
21/03/2014 10:53
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Joined: Dec 2005
Posts: 8,421 Lightwater, Surrey
DaveG
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So let me get this right: we can't calculate the effective rolling radius, and the unloaded tyre circumference is "an unnecessary datum"...what size do you suggest the OP uses? And what do other people use when wanting to change sizes? The workshop manual states 205/50-15 for the 15" n/a wheels and 205/50-16 for the 16" 20vt wheels, so I think your statement I'd be inclined to think Fiat would've kept the effective rolling radius the same between na and turbo isn't right. And whilst (when changing sizes) we'd all like to keep the "radius" the same* there isn't an infinite choice of sizes, and with some more common sizes being significantly cheaper, this will often sway the buyer's decision to accept a few % difference either way. * Me? IIRC I've got 225/50x17 which eliminates completely the speedo error and fills the arches nicely (but does rub a bit on the wheel arch liners )
1996 Portofino 20vt & 2000 Pearl White Plus 2008 Ferrari F430 & 2017 Fiat 124 Spider
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Re: tyres
[Re: ]
#1480058
21/03/2014 12:25
21/03/2014 12:25
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Joined: Oct 2013
Posts: 1,290 Staffs
Dazvr6
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I thought 225/40/18 was stretching it a bit! Mine will be an inch bigger than the standard size but yours will be nearly 2 inches. Does that not compromise the handling?
Last edited by Dazvr6; 21/03/2014 12:26.
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