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Re: Running 18's
[Re: Broombeastie]
#1252530
23/08/2011 15:41
23/08/2011 15:41
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Joined: Dec 2005
Posts: 8,427 Lightwater, Surrey
DaveG
Club Treasurer Member 311
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Club Treasurer Member 311
Je suis un Coupé
Joined: Dec 2005
Posts: 8,427
Lightwater, Surrey
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Precisely. I run larger diameter tyres on 17" wheels and the speedo is accurate (vs. @GPS) to 1-2mph up to 90. OE wheel/tyre makes speedo read too high, at least on early Coops, maybe later speedos were changed?
1996 Portofino 20vt & 2000 Pearl White Plus 2008 Ferrari F430 & 2017 Fiat 124 Spider
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Re: Running 18's
[Re: Broombeastie]
#1252543
23/08/2011 16:13
23/08/2011 16:13
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Joined: Jan 2006
Posts: 3,034 Sweden
Per
I need some sleep
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I need some sleep
Joined: Jan 2006
Posts: 3,034
Sweden
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All speedos are set slightly too high, on all cars of all brands. Don't remember exactly why but that's how it is. 215/35/18 is a correct rolling radius for the standard wheel on a coupe with 205/50/16 or 225/45/16 No, it's 225/35-18. These are 0.5% bigger diameter than the original 20VT 205/50-16, which is the closest you get with 18".
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Re: Running 18's
[Re: Per]
#1252574
23/08/2011 18:09
23/08/2011 18:09
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nismo
Unregistered
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nismo
Unregistered
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no 215/35/18 is the same rolling radius as a 205/50/16 or 225/45/16 standard coupe wheel and tyre - check for yourself - http://www.rtecshop.com/?p=calculatorStock Tyre: 205/50x16 New Tyre: 215/35x18 Sidewall: 4.0 in .... Sidewall: 3.0 in Radius: 12.0 in .... Radius: 12.0 in Diameter: 24.0 in .... Diameter: 24.0 in Circumference: 75.4 in .... Circumference: 75.4 in Revs/mile: 840.3 .... Revs/mile: 840.3 -------------------------------------------------------------------------------- Your speedo will be 100% accurate! no its 225/35-18.These are 0.5% bigger diameter than the original 20VT 205/50-16, which is the closest you get with 18". as you say yourself its 0.5% bigger so it must be wrong
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Re: Running 18's
[Re: ]
#1252712
23/08/2011 22:12
23/08/2011 22:12
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Joined: Sep 2009
Posts: 5,038 ation
szkom
Club member 2000
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Club member 2000
Forum is my life
Joined: Sep 2009
Posts: 5,038
ation
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no 215/35/18 is the same rolling radius as a 205/50/16 or 225/45/16 standard coupe wheel and tyre - check for yourself - http://www.rtecshop.com/?p=calculatorStock Tyre: 205/50x16 New Tyre: 215/35x18 Sidewall: 4.0 in .... Sidewall: 3.0 in Radius: 12.0 in .... Radius: 12.0 in Diameter: 24.0 in .... Diameter: 24.0 in Circumference: 75.4 in .... Circumference: 75.4 in Revs/mile: 840.3 .... Revs/mile: 840.3 -------------------------------------------------------------------------------- Your speedo will be 100% accurate! no its 225/35-18.These are 0.5% bigger diameter than the original 20VT 205/50-16, which is the closest you get with 18". as you say yourself its 0.5% bigger so it must be wrong Calculating the circumference will not give you a figure of the percentage difference. It's all about the amount the tyre deforms when it contacts the ground. Measure from the ground to the wheel centre and note the measurement. Try it again with different tyre pressures and you will get different figures. These are the numbers you need to use for calculating your effective rolling radius so you can compare. In short it's suck it and see. These tyre calculators are nonsense. There are far too many variables to get anything more than a guess at the percentage difference.
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Re: Running 18's
[Re: Broombeastie]
#1252808
24/08/2011 08:41
24/08/2011 08:41
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nismo
Unregistered
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nismo
Unregistered
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so maths is wrong then i'v seen and read it all now
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Re: Running 18's
[Re: ]
#1252904
24/08/2011 14:11
24/08/2011 14:11
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Joined: Jan 2006
Posts: 3,034 Sweden
Per
I need some sleep
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I need some sleep
Joined: Jan 2006
Posts: 3,034
Sweden
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as you say yourself its 0.5% bigger so it must be wrong Well the 215/35-18 is 0.6% smaller than 205/50-16 so it must be more wrong then..? Check for yourself: http://www.algonet.se/~leffmans/dack/dack.htm Also, most 18" rims are 7.5-8.5" wide which makes a 215mm wide tire a little narrow. Also #2, on your 18" wheels you'd better off with a little more tirewall/comfort than less... Also #3, 225mm looks better, offers more grip and is what 20VT Plus has.
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Re: Running 18's
[Re: Broombeastie]
#1253018
24/08/2011 19:36
24/08/2011 19:36
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THE_G
Unregistered
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THE_G
Unregistered
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what about 19 inch rims
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Re: Running 18's
[Re: Broombeastie]
#1253022
24/08/2011 19:39
24/08/2011 19:39
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Joined: Dec 2005
Posts: 2,168 London
EDz
Competition Level
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Competition Level
Joined: Dec 2005
Posts: 2,168
London
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I think the 225 35s are best option My Speedo matches gps pretty close (maybe a mile out) So they must help make it more Accurate
e46 M3 330D Sport
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Re: Running 18's
[Re: Per]
#1253328
25/08/2011 12:41
25/08/2011 12:41
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Joined: Dec 2005
Posts: 17,368 Staffordshire
Nigel
Forum veteran
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Forum veteran
Joined: Dec 2005
Posts: 17,368
Staffordshire
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If it affected acceleration it must be due to different gearing (bigger wheels). The extra weight wont be noticable that way. You're kidding... Heavier wheels are a brilliant way to slow down a car - rotational inertia is a huge drain on acceleration Increasing the wheel diameter almost always ends up with a corresponding increase in weight, which has an immediate adverse effect on acceleration. Think of your wheels as a pair of extra flywheels - we all know that a lightened flywheel helps throttle response and acceleration, so it follows that heavier rotational items in the drivetrain will have the opposite effect. Why do you think F1 cars have such small wheels? I know that they use tyre deflection as part of the suspension, but if 20" wheels and ultra-low profile tyres gave more grip, they would use them (and then find a way of making the mechanical suspension work around it). Ultimately, a small wheel and tall tyre is easier to accelerate and brake The primary reason for me having 17" wheels on my car is that they are lighter than the stock wheel and thus aid performance. The secondary reason is better handling due to a lower sidewall and less unsprung weight and the final reason was aesthetics.
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Re: Running 18's
[Re: szkom]
#1254140
27/08/2011 13:15
27/08/2011 13:15
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NineOneSix
Unregistered
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NineOneSix
Unregistered
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Your maths is fine. I think you misunderstand. If you have a calculated radius of 12 inches ,when the tyre is unloaded, it will change as soon as you put the load of a car on it. Like I said measure it. Measure from the top of the tyre to the wheel centre and then measure from the ground to the wheel centre. You will produce two different radius measurements. The later of the two is the one you use to calculate the effective rolling radius.
Remember the above mentioned phenomena varies based on tyre, pressure, relative conditions and rotational speed. It is also more pronounced on a higher aspect ratio tyre, like the stock wheels. So once again tyre calculators are nonsense.
Doesn't matter how much the tyre deforms, the circumference does change. Rolling radius will change but the RPM of the wheel does not.
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Re: Running 18's
[Re: ]
#1254254
27/08/2011 20:14
27/08/2011 20:14
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Joined: Sep 2009
Posts: 5,038 ation
szkom
Club member 2000
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Club member 2000
Forum is my life
Joined: Sep 2009
Posts: 5,038
ation
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Your maths is fine. I think you misunderstand. If you have a calculated radius of 12 inches ,when the tyre is unloaded, it will change as soon as you put the load of a car on it. Like I said measure it. Measure from the top of the tyre to the wheel centre and then measure from the ground to the wheel centre. You will produce two different radius measurements. The later of the two is the one you use to calculate the effective rolling radius.
Remember the above mentioned phenomena varies based on tyre, pressure, relative conditions and rotational speed. It is also more pronounced on a higher aspect ratio tyre, like the stock wheels. So once again tyre calculators are nonsense.
Doesn't matter how much the tyre deforms, the circumference does change. Rolling radius will change but the RPM of the wheel does not. Not quite. The circumference of the tyre remains constant. What I'm saying is the deformed tyre will produce a smaller radius (effectively) and therefore produces a smaller effective rolling radius and thus cover less distance per rpm . I think I may produce a how to guide with some diagrams to help explain this.
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