# Adverse Effects from running different tire sizes (ver. Haldex)



## Dyzee (Mar 19, 2005)

Hey,
Just wondering if there is a big issue running different overall diameters front and rear on the AWD TT's.
I realize running different diameters is not ideal, but can it hurt anything?
Tried running some searches but didn't come up with anything concrete. http://****************.com/smile/emthdown.gif


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## l88m22vette (Mar 2, 2006)

*Re: Adverse Effects from running different tire sizes (Dyzee)*

Don't do it, you'll screw the AWD...width/AR/etc is all good, so long as the circumferences are the same. Besides, a TT with a stinkbug stance would be awful


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## Dyzee (Mar 19, 2005)

*Re: Adverse Effects from running different tire sizes (l88m22vette)*


_Quote, originally posted by *l88m22vette* »_Don't do it, you'll screw the AWD..

Please elaborate. What breaks on it?
Have people tried this and actually broken something?
I'm just trying to get some concrete info.

Due to my tire stretch my tires are a bit different in rolling diameter.


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## roadyTT (Mar 28, 2006)

*Re: Adverse Effects from running different tire sizes (Dyzee)*

Per Haldex (yes, it's a company) the rolling diameter of all four wheels must be within 4%. Anything outside of that parameter will hurt your system.
Concrete evidence - Go search the forum for things like "Is my quattro broken?" and "All of my wheels don't spin!". A common factor is mismatched wheel diameters
Go with different rolling diameters and pretty soon you'll have all the benefits of a FWD vehicle with the added weight of a Quattro system that doesn't work. Don't forget to post something to the effect of "Is my Haldex controller burnt out?"


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## Dyzee (Mar 19, 2005)

*Re: Adverse Effects from running different tire sizes (roadyTT)*


_Quote, originally posted by *roadyTT* »_Per Haldex (yes, it's a company) the rolling diameter of all four wheels must be within 4%. Anything outside of that parameter will hurt your system.
Concrete evidence - Go search the forum for things like "Is my quattro broken?" and "All of my wheels don't spin!". A common factor is mismatched wheel diameters
Go with different rolling diameters and pretty soon you'll have all the benefits of a FWD vehicle with the added weight of a Quattro system that doesn't work. Don't forget to post something to the effect of "Is my Haldex controller burnt out?"


That's exactly what I needed to know. I just measured my overall diameters, did the math(Not even 2% difference) and I am still within spec. so it should not affect my AWD system. 
I really just wanted to confirm before I did anything stupid


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## storx (Dec 6, 2006)

if you run different size tires.. the car will have an hopping effect going down the road... becuase the haldex is constantly engaging and disengaging the rear wheels...


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## Dyzee (Mar 19, 2005)

*Re: (storx)*


_Quote, originally posted by *storx* »_if you run different size tires.. the car will have an hopping effect going down the road... becuase the haldex is constantly engaging and disengaging the rear wheels... 

Is there a way to solve that without having to change the tires?

(Note: I am asking for people who may search this in the future)


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## roadyTT (Mar 28, 2006)

*Re: (Dyzee)*

Unplug your Haldex controller.
Downside....no AWD


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## concubinedub (Jul 6, 2005)

*Re: (storx)*


_Quote, originally posted by *storx* »_if you run different size tires.. the car will have an hopping effect going down the road... becuase the haldex is constantly engaging and disengaging the rear wheels... 


wrong.
i am running 225/40/18 in the back and 215/40/18 in the front.
there is no hopping, no nothing. only thing is rubbing from being low. the car doesn't handle any better/any less with the different sized tires on. obviously as previous stated you need to stay within a small percentage of change. IE: wouldnt run 275/40/18 with 245/40/18, as for one that is ugly and two that is just ridiculous and anyone with a brain would realize that will fubar your drivetrain.
and yes, my AWD still works. http://****************.com/smile/emthup.gif 


_Modified by concubinedub at 1:35 PM 5-26-2008_


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## tmvw (Jan 14, 2008)

*Re: Adverse Effects from running different tire sizes (roadyTT)*


_Quote, originally posted by *roadyTT* »_Per Haldex (yes, it's a company) the rolling diameter of all four wheels must be within 4%. Anything outside of that parameter will hurt your system.


finally someone who knows what they are talking about. 
i was just ABOUT to post that the overall rolling diameter of the wheels and tires needs to be 4% or less difference http://****************.com/smile/emthup.gif 
search the forums using keywords: staggered haldex and you will find plenty of info to support the 4% claim. (even roadyTT said he confirmed with Haldex)



_Modified by tmvw at 4:26 PM 5-26-2008_


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## Dyzee (Mar 19, 2005)

*Re: Adverse Effects from running different tire sizes (tmvw)*


_Quote, originally posted by *tmvw* »_
finally someone who knows what they are talking about. 
i was just ABOUT to post that the overall rolling diameter of the wheels and tires needs to be 4% or less difference http://****************.com/smile/emthup.gif 
search the forums using keywords: staggered haldex and you will find plenty of info to support the 4% claim. (even roadyTT said he confirmed with Haldex)
_Modified by tmvw at 4:26 PM 5-26-2008_


Found it:
----------------------
Haldex wrote:
Hi Franz,
Yes, we do compensate for tiresizes up to 4%. This since we wread the
wheelspeed and the driveshaft and can understand if you have a minispare,
weared down tires or different sizes.
Best Regards,
Mats Johansson
Haldex Traction AB
Detroit
-----Original Message-----
To: [email protected]
Sent: 11/27/2004 4:29 PM
Subject: Haldex Online Message
que: Does the Haldex System compensate for different wheel diameter
between fron and rear?
I am looking to run a staggered setup on my VWR32 and want to know if i
have to run 100% equal or if i can use slight diameter differences
Thank you very much.
Franz Schmitt
Brookfield, WI
--------------------


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## tmvw (Jan 14, 2008)

*Re: Adverse Effects from running different tire sizes (Dyzee)*


_Quote, originally posted by *Dyzee* »_

Found it:
----------------------
Haldex wrote:
Hi Franz,
Yes, we do compensate for tiresizes up to 4%. This since we wread the
wheelspeed and the driveshaft and can understand if you have a minispare,
weared down tires or different sizes.
Best Regards,
Mats Johansson
Haldex Traction AB
Detroit
-----Original Message-----
To: [email protected]
Sent: 11/27/2004 4:29 PM
Subject: Haldex Online Message
que: Does the Haldex System compensate for different wheel diameter
between fron and rear?
I am looking to run a staggered setup on my VWR32 and want to know if i
have to run 100% equal or if i can use slight diameter differences
Thank you very much.
Franz Schmitt
Brookfield, WI
--------------------

well then, there you go, even confirmed by Haldex themselves http://****************.com/smile/emthup.gif


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## Krissrock (Sep 10, 2005)

concubinedub ----- you're sizes are only 1.24% different in diameter...hence why you don't have a problem


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## turbott920 (Nov 13, 2007)

*Re: (concubinedub)*


_Quote, originally posted by *concubinedub* »_
wrong.
IE: wouldnt run 275/40/18 with 245/40/18, as for one that is ugly and two that is just ridiculous and anyone with a brain would realize that will fubar your drivetrain.


how is running a 30mm width difference in tire size gonna fubar the drivetrain??? dont ppl do that all the time...I did with no issues. front 235/35/19 backs 265/30/19









_Quote, originally posted by *Krissrock* »_concubinedub ----- you're sizes are only 1.24% different in diameter...hence why you don't have a problem

Also did I miss where concubinedub stated what width wheels he is running? otherwise Krissrock, how did you figure out the diameter difference of the tires he is running? only thing I saw was width difference.


_Modified by turbott920 at 10:27 PM 5-26-2008_


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## tmvw (Jan 14, 2008)

*Re: (turbott920)*


_Quote, originally posted by *turbott920* »_
how is running a 30mm width difference in tire size gonna fubar the drivetrain??? dont ppl do that all the time...I did with no issues. front 235/35/19 backs 265/30/19








Also did I miss where concubinedub stated what width wheels he is running? otherwise Krissrock, how did you figure out the diameter difference of the tires he is running? only thing I saw was width difference.



notice how she stated both 40 series tires. your 235/35 vs 265/30 is only .8% difference in rolling diameter.
245/40/18 vs 275/40/18 is 3.7% difference, which would still put you below the 4% threshold. 
just wondering, do you know how tire sizes work/equate to?
wheel width has nothing to do with rolling diameter http://****************.com/smile/emthup.gif


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## concubinedub (Jul 6, 2005)

*Re: (Krissrock)*


_Quote, originally posted by *Krissrock* »_concubinedub ----- you're sizes are only 1.24% different in diameter...hence why you don't have a problem

yes, but the whole question was in regards to different sized tires in general, and i responded to the person who said a general broad statement . http://****************.com/smile/emthup.gif 

oh and my wheels are 18x9.5, 18x8.5


_Modified by concubinedub at 8:45 PM 5-26-2008_


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## turbott920 (Nov 13, 2007)

*Re: (tmvw)*


_Quote, originally posted by *tmvw* »_
just wondering, do you know how tire sizes work/equate to?
wheel width has nothing to do with rolling diameter http://****************.com/smile/emthup.gif 

Is this question to me??? I'm pretty sure I was clearly stating that width has nothing to do with daimeter unless are running a staggered setup with a lot of stretch where the sidewall would actually be smaller due to stretch. IE if you had a 17 x 7.5 wheel in front and a 17 x 8.5 wheel in the back but had the same size tire on both you would have a smaller overall diameter on the rear tire due to the stretch. now 245/40/18 and 275/40/18 are the same diameter...245mm width vs. 275mm width and 40mm sidewall with a 18in wheel opening. both 40 and 18 thus same diameter unless there is stretch. so yes, i know how tire sizes work


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## tmvw (Jan 14, 2008)

*Re: (turbott920)*


_Quote, originally posted by *turbott920* »_
Is this question to me??? I'm pretty sure I was clearly stating that width has nothing to do with daimeter unless are running a staggered setup with a lot of stretch where the sidewall would actually be smaller due to stretch. IE if you had a 17 x 7.5 wheel in front and a 17 x 8.5 wheel in the back but had the same size tire on both you would have a smaller overall diameter on the rear tire due to the stretch. now 245/40/18 and 275/40/18 are the same diameter...245mm width vs. 275mm width and 40mm sidewall with a 18in wheel opening. both 40 and 18 thus same diameter unless there is stretch. so yes, i know how tire sizes work

im sorry, but that is incorrect. 
245 = width in mm of the tire
40 = sidewall to width ratio (which is 40% of whatever the width is, in this case 245, which is 98mm) it does NOT mean you have a 40mm sidewall








18 = wheel diameter
sidewall = 40, which is 40% of 245mm (on the 245/40/18) which is 98mm of sidewall height, and on the 275/40, it is 40% of 275mm, which is 110mm of sidewall height. so the 275/40/18 is TALLER than the 245/40/18
stretch doesn't make a tire smaller either. also, a 275/40/18 tire is STILL 275mm in width, with a 40% sidewall which is 110mm, REGARDLESS of how wide the wheel is. physical numbers don't change, and math does NOT lie. so stretch or not, whatever the size the tire is, is whatever the size the tire is.
i think you should learn how to read tire sizes first http://****************.com/smile/emthup.gif 

heres are good links for you to learn: 
http://www.tirerack.com/tires/...id=46
http://www.tirerack.com/tires/...hid=7


_Modified by tmvw at 11:01 PM 5-26-2008_


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## turbott920 (Nov 13, 2007)

*Re: (tmvw)*

ok this pic is a 215/35/19 on a 19 x 9.5 so from what you are saying the diameter would still be the same if it was on a 19x8??? how does that make sense.








I admit I was wrong about how side wall was calculated. guess I was assuming and we all know the saying for that one.










_Modified by turbott920 at 11:08 PM 5-26-2008_


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## [kyle] (Apr 25, 2008)

*Re: (tmvw)*

cal pwns noobs.


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## tmvw (Jan 14, 2008)

*Re: (turbott920)*


_Quote, originally posted by *turbott920* »_ok this pic is a 215/35/19 on a 19 x 9.5 so from what you are saying the diameter would still be the same if it was on a 19x8??? how does that make sense.



yes because the *tread* width is still 215mm in width.... 
please tell me how you can physically change a tires sectional width by stretching? that leads me to believe you don't know what stretching is either








are you talking about rolling diameter or sectional width? if it's the rolling diameter, really, does math escape you THAT much?
215mm w/ 35% sidewall is STILL going to be 215mm w/ a 35% sidewall, regardless of if it's on a 1" wheel or a 100" wheel.








it's simple math...
here's a link for you to easily see how width and sidewall plays on each other
http://www.miata.net/garage/tirecalc.html


_Modified by tmvw at 11:14 PM 5-26-2008_


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## turbott920 (Nov 13, 2007)

*Re: (tmvw)*

I understand the tread width is still the same but how is the overall diameter going to be the same on a stretched tire vs. a none stretched tire of the same diameter???
here is a wheel with the same size tire as my previous post but just narrower. your telling me that this 19x7.5 with 215/35/19 is going to be the same diameter as a 19x9.5 with the same size tire??? don't you think the fact that the tire is being stretched that much is going to decrease the size of the side wall giving it a smaller overall diameter?


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## tmvw (Jan 14, 2008)

*Re: (turbott920)*


_Quote, originally posted by *turbott920* »_I understand the tread width is still the same but how is the overall diameter going to be the same on a stretched tire vs. a none stretched tire of the same diameter???
here is a wheel with the same size tire as my previous post but just narrower. your telling me that this 19x7.5 with 215/35/19 is going to be the same diameter as a 19x9.5 with the same size tire??? don't you think the fact that the tire is being stretched that much is going to decrease the size of the side wall giving it a smaller overall diameter?


because it is. stretching a tire does not change it's physical dimensions. it may *LOOK* like it is, but it's not. 215/35/19 is always going to be 24.9" in rolling diameter, regardless if it's on a 6" wheel or a 10" wheel. stretching a tire, all it does is what the word says, stretching a tire over a wider wheel. 
take an analogy for example: a guy who usually wears XL shirts wears a small size shirt. given the physical properties of the shirt don't change (we all know fabrics stretch and stay stretched, but lets leave that out for now), that shirt is STILL a SMALL isn't it? 
the reason the sidewall *appears* smaller is because it's being stretched on a different axis than if they were not being stretched. the tire still has a 35% ratio to 215mm of a sidewall....


_Modified by tmvw at 11:29 PM 5-26-2008_


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## turbott920 (Nov 13, 2007)

*Re: (tmvw)*

so what you are saying basically is that stretching a small tire on a wide wheel does not change the profile?
I realize the actual sidewall size is still going to be the same (the tire doesn't magically change what size it is from stretching it) but the rolling diameter (what the whole topic is about) would be different. the profile would be lower.


_Modified by turbott920 at 11:36 PM 5-26-2008_


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## tmvw (Jan 14, 2008)

*Re: (turbott920)*


_Quote, originally posted by *turbott920* »_so what you are saying basically is that stretching a small tire on a wide wheel does not change the profile?

precisely. 35% of 215mm = 75.25mm, no? (215 x .35 = 75.25).
it will LOOK different, but there still is 75.25mm of sidewall on that tire.
again, math does not lie. you still will have 75.25mm of sidewall, regardless of what wheel width it is on.


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## turbott920 (Nov 13, 2007)

*Re: (tmvw)*

right, the writing on the side of the tire is still going to be the same...and there is still going to be that amount of sidewall no matter what, but after you stretch the tire the *overall* diameter is going to be slightly smaller. take another look at that first pic of the tire on a 19x9.5 then imagine the wheel shrinking in 2"...the overall diameter would be slightly higher. Correct me if I am wrong but isn't part of the reason people strecth tires is to get a lower profile??


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## tmvw (Jan 14, 2008)

*Re: (turbott920)*


_Quote, originally posted by *turbott920* »_
I realize the actual sidewall size is still going to be the same (the tire doesn't magically change what size it is from stretching it) but the rolling diameter (what the whole topic is about) would be different. the profile would be lower.


no, the rolling diameter would NOT change either. it's really not as hard as you make it out to be. now i do not know the exact equation to calculate rolling diameter by hand, but if i did, i would show you. use that link i showed you... 24.9" rolling diameter on a 215.35.19 wheel, will still be 24.9, on a 8" or a 10" wheel. 
like i said, it will APPEAR/LOOK/SEEM different, but math does NOT lie

_Quote, originally posted by *turbott920* »_Correct me if I am wrong but isn't part of the reason people strecth tires is to get a lower profile??

no, actually that is wrong, too. 
do you know the origin of how stretch came about? 
back in the days, overseas, there were laws on how a cars' tire HAS to be contained it's fenders, but people wanted to have a wider stance, thus using wider wheels, no? well, that wouldn't work if you increased the tire width either, as that would become illegal if it went past the fender. so to keep it legal, they used a narrower tire on a wider wheel, thus creating a wide stance, while being legal.
stretching does NOT lower it's profile. as i said, i wish i had the exact equation on how you calculate rolling diameter by hand... that would be the easiest way to show you how math does not lie lol
a 215/35/19 tire is STILL 24.9 inches rolling diameter, regardless of what width of wheel it is mounted on.

*are you really trying to argue how something LOOKS versus a mathematics reason?*
here's the link that shows you how to calculate rolling diameter:
http://www.tirerack.com/tires/...hid=7
"To figure the outside diameter of a tire, take the sidewall height and multiply by 2,(remember that the diameter is made up of 2 sidewalls, the one above the wheel, and the one below the wheel) and add the diameter of the wheel to get your answer."
"Width x Aspect Ratio = Section Height
Section Height x 2 = Combined Section Height
Combined Section Height + Wheel Diameter = Tire Diameter
Example: 185/60R14 85H or 185/60HR14
185mm x .60=111mm
111mm x 2=222mm
222mm + 355.6mm(14")= 577.6mm or 22.74" "


_Modified by tmvw at 11:57 PM 5-26-2008_


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## turbott920 (Nov 13, 2007)

*Re: (tmvw)*

lol ok...math doesn't lie, I know about the whole fender and tire inside thing...I'm done. not going to argue this anymore. obviously making something like a tire wider than it is supposed to be will make the height of it come down but that must be too hard for you to visualize. basic physics doesn't lie either...a friend and I (Mikcuz on the vortex) just stretched 205's on his 17x8.5 they were previously on his fat fives 17x7.5 this week. you can literally see the difference in diameter at that small of a width change. That's all.
lol that calculation thing you posted does not figure in any kind of stretch or wheel size. that is basically saying you get diameter from measuring from one side to the other. what does that prove???


_Modified by turbott920 at 12:02 AM 5-27-2008_


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## tmvw (Jan 14, 2008)

*Re: (turbott920)*


_Quote, originally posted by *turbott920* »_lol ok...math doesn't lie, I know about the whole fender and tire inside thing...I'm done. not going to argue this anymore. obviously making something like a tire wider than it is supposed to be will make the height of it come down but that must be too hard for you to visualize. basic physics doesn't lie either...a friend and I (Mikcuz on the vortex) just stretched 205's on his 17x8.5 they were previously on his fat fives 17x7.5 this week. you can literally see the difference in diameter at that small of a width change. That's all.

so you're saying by "eyeballing" it, without a scientific reason, you are safe to assume that just because you think it LOOKS smaller, it really is?








really...?
honestly, go use that equation i posted up... 
again, *arguing a VISUALIZATION vs math/science?* (when it's been proven that tire sizes are all mathematics...)
PS: stretching a tire does not make a tire wider either








PPS: i will bet you money if you went up to an engineer at a tire manufacturer and told him what you just told me, he would laugh uncontrollably in your face.


_Modified by tmvw at 12:02 AM 5-27-2008_


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## turbott920 (Nov 13, 2007)

*Re: (tmvw)*


_Quote, originally posted by *tmvw* »_

PS: stretching a tire does not make a tire wider either









_Modified by tmvw at 12:02 AM 5-27-2008_

yeah this tire didn't get any wider







http://www.youtube.com/watch?v...lated
lol obviously the tread will still be the same but the tire will be wider overall...the tread no but the tire is covering more width.
I will physicall measure my friends stretched tires and take detailed pics just for you tomorrow. 



_Modified by turbott920 at 12:23 AM 5-27-2008_


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## tmvw (Jan 14, 2008)

*Re: (turbott920)*

oh and btw, of course the calculations don't take in wheel width, because, guess what? IT DOESN'T MATTER WHAT WHEEL WIDTH IT IS.


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## l88m22vette (Mar 2, 2006)

*Re: (tmvw)*


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## diehlryan (Feb 22, 2007)

*Re: (tmvw)*


_Quote, originally posted by *tmvw* »_oh and btw, of course the calculations don't take in wheel width, because, guess what? IT DOESN'T MATTER WHAT WHEEL WIDTH IT IS.









You make me laugh. Of course math doens't lie but you need to use the right math. GEOMETRY my fried. Think triangles.
A tires sidewall width determines overall diatmeter when that sidewall is 100% vertical as in Example A.
If you take that sidewall width and stretch it causing the sidewall to be at an angle, the vertical dimension CHANGES as in Example B. This change will obviously change the overall rolling diameter.


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## tmvw (Jan 14, 2008)

*Re: (diehlryan)*


_Quote, originally posted by *diehlryan* »_
You make me laugh. Of course math doens't lie but you need to use the right math. GEOMETRY my fried. Think triangles.
A tires sidewall width determines overall diatmeter when that sidewall is 100% vertical as in Example A.
If you take that sidewall width and stretch it causing the sidewall to be at an angle, the vertical dimension CHANGES as in Example B. This change will obviously change the overall rolling diameter.


i am aware... but that is NOT how you calculate rolling diameter when you are given the tire size...











_Modified by tmvw at 9:07 AM 5-27-2008_


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## diehlryan (Feb 22, 2007)

*Re: (tmvw)*


_Quote, originally posted by *tmvw* »_
i am aware... but that is NOT how you calculate rolling diameter when you are given the tire size...









_Modified by tmvw at 9:07 AM 5-27-2008_

yes it is when you are stretching a tire. 
your calculators are flawed because they were meant to be used when putting the recommended tire on a certain wheel width. 
when you use a tire with less width than is recommended for a wheel width you throw all the old calculations out the window.


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## tmvw (Jan 14, 2008)

*Re: (diehlryan)*


_Quote, originally posted by *diehlryan* »_
yes it is when you are stretching a tire. 
your calculators are flawed because they were meant to be used when putting the recommended tire on a certain wheel width. 
when you use a tire with less width than is recommended for a wheel width you throw all the old calculations out the window.

negative. i just found out a claim to my basis: 
"The width of a tire mounted on a narrow rim would be "narrower" than if the same tire was mounted on a wide rim. *NOTE: because the overall diameter of a steel belted radial is determined by the steel belts, there is little, if any, change to the overall diameter of the tire due to differences in rim width.*"
http://www.tirerack.com/tires/...width
but it does look like i may have been wrong about the overall width widening 










_Modified by tmvw at 9:19 AM 5-27-2008_


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## diehlryan (Feb 22, 2007)

it also talks about the overall diameter being measured when the tire is on its appropriate wheel width..


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## tmvw (Jan 14, 2008)

*Re: (diehlryan)*


_Quote, originally posted by *diehlryan* »_it also talks about the overall diameter being measured when the tire is on its appropriate wheel width.. 

and did you not see what i made bold?
"NOTE: because the overall diameter of a steel belted radial is determined by the steel belts, *there is little, if any*, change to the overall diameter of the tire *due to differences in rim width*."
the tire's overall diameter has little to no variance in measurement when mounting on different size widths because it is a predetermined measurement from the steel belts .....










_Modified by tmvw at 9:31 AM 5-27-2008_


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## turbott920 (Nov 13, 2007)

*Re: (tmvw)*


_Quote, originally posted by *tmvw* »_
and did you not see what i made bold?
"NOTE: because the overall diameter of a steel belted radial is determined by the steel belts, *there is little, if any*, change to the overall diameter of the tire *due to differences in rim width*."
the tire's overall diameter has little to no variance in measurement when mounting on different size widths because it is a predetermined measurement from the steel belts .....









_Modified by tmvw at 9:31 AM 5-27-2008_

lol this quote is from tire rack...of course if you stay within the allowed wheel widths the manufactuerer makes the tire for there will be little to no variance in diameter size but we are talking a stretch outside the normal scope of the tire. so if tire rack says there will possibly be little difference with a recommended wheel what do you think will happen when you go past that??? more change in diameter. DING DING!!!
this is perfect


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## tmvw (Jan 14, 2008)

*Re: (turbott920)*

no. the recommended widths is not for the sidewall angles, it is for the ability of that tire to stay beaded to the wheel at said stretch...
edit: i'm done, either way. no use in us arguing over this non sense stuff. i have provided more than enough math reasoning http://****************.com/smile/emthup.gif 

_Quote, originally posted by *zeroluxxx* »_note: i'm not a tire expert, so i'm not claiming it's one way or another for certain, because, i just plain dont know, i'm only stating my opinion. and i think it's fair to say no one else currently arguing this can defiantly answer this either. bottom line, were enthusiasts, not experts.
with that said, i understand where calvin is coming from, and to be honest, i dont think it relies as heavily on math as we think. i think it has to do more with the construction of a tire. we all know there are steel reinforcing belts that are sandwiched into the tire (along with more rubber and various other materials to provide strength). so, for the tires outside diameter to dramatically change, this would require the steel belts (and all of the other reinforcing elemets) to either expand, or contract a noticeable amount. something that i don't see happening.
i understand that the belts are flexible to absorb road impacts, duh, but there is an obvious difference between flex and expansion/contraction. for example, its like comparing a gold bracelet to a rubber band. the bracelet will deform/flex, but the actual overall diameter (if constrained, say, around a wheel) will not change. where as, the rubber band will both flex, and expand/contract because of it's construction.
what i'm getting at is a tire is built for strength (steel belts), and while it will flex/deform, the actual outer tread area is not going to expand/contract because the steel belts aren't gonna stretch/shorten.
as mentioned earlier, the sidewall is used to make up the difference between different wheel widths. when a tire is on a proper sized wheel, the sidewall is normally bowed out, meaning, there is an obvious surplus of material. stretching the tire uses this surplus material to make up the additional distance to the bead on the wider wheel. dont you think the softer, pliable sidewall will conform and change shape and size before the reinforced, multi-ply, steel laden tread face will? i can't see how varying the sidewall angle would make a length of steel belt shrink (or numerous belts for that matter). if your diameter is getting smaller, you have to do something with the material that comprises that diameter, correct? if it can't shrink, as steel doesn't, how will it change? same thing if we were arguing tire expansion. the rigid steel in the belts would have to expand to allow this. this is why i say it has more to do with tire construction then math.
so, IMO, i have to agree that there is little to no change in the actual diameter once stretched. and as i mentioned before, unless we can get an actual tire engineer/expert to chime in on this little discussion, we're all just speculating.




_Modified by tmvw at 4:34 PM 5-27-2008_


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## diehlryan (Feb 22, 2007)

*Re: (tmvw)*


_Quote, originally posted by *tmvw* »_no. the recommended widths is not for the sidewall angles, it is for the ability of that tire to stay beaded to the wheel at said stretch...
edit: i'm done, either way. no use in us arguing over this non sense stuff. i have provided more than enough math reasoning http://****************.com/smile/emthup.gif 

_Modified by tmvw at 12:47 PM 5-27-2008_

no you didn't you linked to tirerack quotes and internet calculators.
I can understand the steel belt claim though.




_Modified by diehlryan at 3:18 PM 5-27-2008_


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## Krissrock (Sep 10, 2005)

*Re: (diehlryan)*

i think what he's trying to say is that no matter how you stretch the tire out, the total circumference of the tread (around the outside) can't change. THat would require the tire to shrink or grow... Either way, stretching is really not recommended for tires. 
and i think you've both brought a lot of info to the table. EIther way, if you're running two different tire sizes, best to make sure they are close in diameter...even if that means taking a measuring tape to them after you've stretched 'em


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## g60vwr (Apr 9, 2000)

*Re: (tmvw)*

Sorry to drag this back up- 
but Im having problems doing the math
225/40/18 = 90mm
265/40/18 = 106mm
Can SomeoneEXPLAIN how I can calculate to see if this fits within the 4%
Thank you!


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## M this 1! (May 17, 2000)

*Re: (g60vwr)*

225/25.4 gives you how wide the tire is in inches (it's in mm right now)
multiply that number by .4 to get the sidewall height.
multiply that by 2, because there are 2 sidewalls to add in when determining the total diameter of the wheel.
then add that to the rim size to get your total diameter.

NOW you can compare it to the other size and then divide the 2 diameters to get the percentage of deviation.

I can not believe that some are not seeing how stretching a tire WOULDN'T shrink its overall diameter. it may be pretty small or negligable but it is physically impossible to not have some change if the tire is stretched. the diagram of the triangles is exactly right. geometry does not lie. a sidewall put at an angle can not as tall if that same height sidewall were verticle. does not matter how your read into it. reading is NOT the same as math


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## g60vwr (Apr 9, 2000)

*Re: (M this 1!)*

ok so now I have 
225/40 18 at 25.088
265/40 18 at 26.344
When I divide the larger into the smaller I get .952
If I do it the other way I get 1.0500
Is either of those correct? It has to be more than that?


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## steve05ram360 (Aug 14, 2006)

*Re: (M this 1!)*

make your life easy... go here
http://www.differentials.com/calc.html
and enter in the info to find the diameters and go from there.


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## g60vwr (Apr 9, 2000)

*Re: (steve05ram360)*

Thanks, but that gives me the same info I just wrote. Im asking now how do I find the % difference between my tires


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## steve05ram360 (Aug 14, 2006)

oops, sorry
you already have it... 4.8% off...


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## steve05ram360 (Aug 14, 2006)

*Re: (steve05ram360)*

also wanted to add that I picked up an AWD dodge dakota that the owner had put on an odd sized spare... took out the transfer case. definitely a no-no to put adversely different sized tires on an awd car/truck...


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## g60vwr (Apr 9, 2000)

*Re: (steve05ram360)*

how did you get the 4.8%?


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## M this 1! (May 17, 2000)

*Re: (g60vwr)*

from your .952 - 1 = .048. The decimal is just put over 2 spots for a printed 4.8%


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## M this 1! (May 17, 2000)

*Re: (M this 1!)*

you need a 265 35 18 in the back to be proper. the 40 series would look puffy and odd. That tire is a 25.3" diameter tire. now you're only 1.1% off.


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## paullee (Feb 3, 2008)

*Re: (g60vwr)*

My arithmetic is a little rusty, but how about:
25.088 ?
------- = -------
26.344 100
thus...
(25.088 x 100) divided by 26.344 = 95.23%
so the difference in diameter between your front tire and your rear tire is 4.77%.


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## 1.8Tabamoura (Jul 29, 2005)

*Re: (turbott920)*


_Quote, originally posted by *turbott920* »_
lol this quote is from tire rack...of course if you stay within the allowed wheel widths the manufactuerer makes the tire for there will be little to no variance in diameter size but we are talking a stretch outside the normal scope of the tire. so if tire rack says there will possibly be little difference with a recommended wheel what do you think will happen when you go past that??? more change in diameter. DING DING!!!
this is perfect









You are completely wrong . the only flexible thing is the sidewall. the diameter of the tire is determinted by the steel rings that don't contract or expand. I used to think the same way you do untill I went to a tire place and they showed me the same tire in two different widths what happens is that the side wall becomes stretched or buldges but keeping the diameter the same.


_Modified by 1.8Tabamoura at 2:59 PM 8-26-2008_


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## turbott920 (Nov 13, 2007)

*Re: (1.8Tabamoura)*

I am willing to bet the tire place you went to didn't put excessive stretch on the tire and that is what I am refering too...Did they need to use a Cheetah or similar to fit the tire? probably not...I have seen this done first hand and you can visually see the difference when putting the stretched tire next to an unstretched tire. I am done with this debate. If geometry is too much for people to comprehend there is nothing I can do about that. /


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## M this 1! (May 17, 2000)

*Re: (turbott920)*

the belts themselve might not flex but what'll happen is you are getting a concave footprint. just look at a normal tire on a normal rim. you can over inflate that tire and get a convex footprint. the wires in the belts did not lengthen but the due to compliancy did grow in peak diameter. any alignment shop has the old ass diagram of what happens to tread if you under/over inflate.


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## g60vwr (Apr 9, 2000)

*Re: (M this 1!)*

Thanks alot for the help on the math







Im actually really happy I learned something http://****************.com/smile/emthup.gif


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## 1.8Tabamoura (Jul 29, 2005)

*Re: (M this 1!)*


_Quote, originally posted by *M this 1!* »_the belts themselve might not flex but what'll happen is you are getting a concave footprint. just look at a normal tire on a normal rim. you can over inflate that tire and get a convex footprint. the wires in the belts did not lengthen but the due to compliancy did grow in peak diameter. any alignment shop has the old ass diagram of what happens to tread if you under/over inflate.

yes , you can get a tire in a convex shape because you can bend the steel belts with the pressure ( note that I said bend and not stretch) thus causing it to peak at the center , but you can't make a concave shape because you got freaking air pressure pushing the tire from the inside. when you stretch the tires the only thing you stretch is the sidewalls which are flexible, the steel belts are freaking steel how can you stretch that?


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## M this 1! (May 17, 2000)

*Re: (1.8Tabamoura)*

if the outer diameter were not able to become smaller, the sidewalls could NOT end up in that picture. your theory of the outer tread diameter not being able to change is physically impossible unless the sidewalls grew............that and only that is where geometry comes into truth. OR the tire would never bead to the rim.


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