Welcome to the North American Subaru Impreza Owners Club Thursday August 28, 2014
Home Forums WikiNASIOC Products Store Modifications Upgrade Garage
Click here to visit TireRack
Brakes & Suspension Forum sponsored by The Tire Rack

Losing traction? Need new tires?
Click here to visit the NASIOC Upgrade Garage...
Here you can view your subscribed threads, work with private messages and edit your profile and preferences Home Registration is free! Visit the NASIOC Store NASIOC Rules Search Find other members Frequently Asked Questions Calendar Archive NASIOC Upgrade Garage Logout
Go Back   NASIOC > NASIOC Technical > Brakes, Steering & Suspension

Welcome to NASIOC - The world's largest online community for Subaru enthusiasts!
Welcome to the NASIOC.com Subaru forum.

You are currently viewing our forum as a guest, which gives you limited access to view most discussions and access our other features. By joining our community, free of charge, you will have access to post topics, communicate privately with other members (PM), respond to polls, upload content and access many other special features. Registration is free, fast and simple, so please join our community today!

If you have any problems with the registration process or your account login, please contact us.
* Registered users of the site do not see these ads.
Reply
 
Thread Tools Display Modes
Old 01-06-2001, 10:50 AM   #1
Jon Bogert
Scooby Specialist
 
Member#: 1133
Join Date: Mar 2000
Location: SE PA
Vehicle:
09 GTR, 02 996 C4S
95 993 C4, 71 911, 04 STI

Post Wider tires aren't always... wider

I was down in the basement poking around this morning and I noticed that the summer tires from the Subaru looked almost as wide at the summer tires from the Audi, when they should be 2cm narrower. Then I noticed the RE92s, which were much narrower than the S-02s even though they're the same size and the same manufacturer.

So I got out the ruler. Following is the tread width (not section width) of each tire, measured from the edge of the feathering on the tread.

BS RE040 225/45-ZR17 (Audi OEM) 7.5" wheels - 7.5"
BS S-02PP 205/55-ZR16 7" wheels - 7.3"
BS RE92 205/55-VR16 unmounted - 6.5"
BS LM-22 205/55-HR16 7" wheels - 6.8"
Nokian NRW 205/55-HR16 7" wheels - 6.9"

Interesting stuff. Maybe Tirerack should list treadwidth as well as size (section width) as there seems to be considerable variation.
* Registered users of the site do not see these ads.
Jon Bogert is offline   Reply With Quote
Old 01-06-2001, 10:53 AM   #2
Gambit
Scooby Specialist
 
Member#: 102
Join Date: Jul 1999
Location: The meeting place
Post

I wonder if tier pressure affects the width? <shrug> I too wonder. Especially when I walk on hte parkin glots and I see some Sunfire GTs and Cavaliers with identical 205/55/16s which look much wider than our Subaru stock tires of similar size.
Gambit is offline   Reply With Quote
Old 01-06-2001, 11:12 AM   #3
ColinL
Scooby Guru
 
Member#: 114
Join Date: Jul 1999
Chapter/Region: MWSOC
Location: Wichita, KS and Whoring, OT
Vehicle:
'03 Evo, Rice White
'01 Erion CBR 929

Post

Once you think you have it narrowed down to a few tires that you'll purchase, you can go directly to the manufacturer's website and find their published specs.

This was how I decided that a 225/50-16 Yokohama AVS S1 would *probably* fit. (Turns out it might, if the car were the same ride height and more stiffly sprung. Oh well, the fit with minor alterations to the rear fender.)

The tire size detail for Yoko is a java applet so I can't link directly to it, but go here and click on Sizes. You can see that the tread width of the 225/50-16 is 7.9" versus 7" for the 205/55-16, but the 205 was mounted on a 6.5" rim for measuring and the 225 on a 7" rim. Hard to estimate what difference that made, but let's guess maybe .25".

Then you can measure the clearance you have at the strut, and measure the clearance at the fender, and make some educated guesses as to how it will fit. (You can jack one of the rear wheels to full bump if you want to get a better idea of fender clearance.)
ColinL is offline   Reply With Quote
Old 01-06-2001, 07:25 PM   #4
Gary (gg)
Scooby Newbie
 
Member#: 179
Join Date: Aug 1999
Location: Wichita, KS
Vehicle:
98 2.5RS
Rally Blue

Post

I ran into the same problem last year. I checked the BFG spec on the G-Force R1's and the 225/50R16's were listed at 8.8" section width and the 245/45R16's were listed at 9.2" on the section width, both had the same diameter at 24.6". When they were delivered, the 225's were spot on whereas the 245's were closer to 9.75" in section width. I chose to go ahead and mount the 245's after contacting several who ran 225 Kuhmo V700's that had no problems in the front with clearance. After running that combo, 245's in the front and 225's in the rear, there is more clearance in the front than the rear.
Gary (gg) is offline   Reply With Quote
Old 01-07-2001, 11:13 AM   #5
Pilot
Scooby Specialist
 
Member#: 513
Join Date: Nov 1999
Post

FYI TireRack now has a page for every tires withteh tire Specs listed on it. You can get them by going first to the tire's description and clicking on specs.
Pilot is offline   Reply With Quote
Old 01-07-2001, 03:04 PM   #6
Jon Bogert
Scooby Specialist
 
Member#: 1133
Join Date: Mar 2000
Location: SE PA
Vehicle:
09 GTR, 02 996 C4S
95 993 C4, 71 911, 04 STI

Smile

Thanks Pilot. Of course, their numbers indicate they're measuring something other than "rubber on the road" which is what I attempted to do. Their tread width measurement looks like it's based on holding a rigid ruler up to the tread and only measuring what touches.
Jon Bogert is offline   Reply With Quote
Old 01-07-2001, 04:40 PM   #7
Gary (gg)
Scooby Newbie
 
Member#: 179
Join Date: Aug 1999
Location: Wichita, KS
Vehicle:
98 2.5RS
Rally Blue

Post

The ratio of tread width to section width would kind of lead to determine if a tire is square or round shouldered.
Gary (gg) is offline   Reply With Quote
Old 01-07-2001, 04:56 PM   #8
Aspen
Scooby Specialist
 
Member#: 145
Join Date: Aug 1999
Chapter/Region: MWSOC
Location: The Windy City
Vehicle:
Banned to
Housework for Summer

Post

Two words: Contact Patch
Aspen is offline   Reply With Quote
Old 01-07-2001, 07:20 PM   #9
AKGC8
Scooby Specialist
 
Member#: 1305
Join Date: Apr 2000
Chapter/Region: AKIC
Vehicle:
2000 Impreza

Post

I read a large article about a year ago in a BMW mag that went into great detail about tire width. Basically, wider isn't necissarily better. Even when you get a wider tire, you still have the same amount of rubber touching the ground. Thats because the tire patch is detirmined by the PSI of the tire, not the width. When you make the tire wider without changing PSI, you still have the same amount of weight pushing down on it. A wider tire just makes the patch shorter and wider. Overall, the amount of rubber on the ground remains the same though. One bad thing about wider tires is your car looses some of its tendency to return to a straight line of travel when you release the wheel in mid turn. I'll have to try to dig up that article some time because I can't remember if they mentioned if changing the shape of the tire patch effected traction at all.
AKGC8 is offline   Reply With Quote
Old 01-07-2001, 08:18 PM   #10
Pilot
Scooby Specialist
 
Member#: 513
Join Date: Nov 1999
Post

Eby,
Actually a while back we hashed this question out in the old tech forum.

Anyway a wider contact patch is better for cornering and response, a narrower contact patch is better for stability.

As for PSI and weight dictating the shape of the cotact patch, as we learned this is mostly true but not entirely.

Lets say you ave a 205/55/16 and a 205/50/16, theoretically the two tires should have the same contact patch area at the same pressure on the same car, actually the 205/55/16 contact patch is marginally (insignificantly) larger because it is slightly longer and the same width as the 205/50/16 contact patch, this is due to the tires carcass not being uniformly elastic (it stretches but only to a certain extent, and in certain dimensions.
Pilot is offline   Reply With Quote
Old 01-07-2001, 08:28 PM   #11
ColinL
Scooby Guru
 
Member#: 114
Join Date: Jul 1999
Chapter/Region: MWSOC
Location: Wichita, KS and Whoring, OT
Vehicle:
'03 Evo, Rice White
'01 Erion CBR 929

Talking

That was a brief and very good summary of that multi-page debate, Aspen.

I am still surprised at the number of folks that incorrectly believed the same thing Eby was quoting from the BMW mag. If bigger tires (width or diameter) didn't have a larger contact patch, why would anyone bother? Why would some drag racing classes be limited by width?
ColinL is offline   Reply With Quote
Old 01-07-2001, 09:36 PM   #12
Aspen
Scooby Specialist
 
Member#: 145
Join Date: Aug 1999
Chapter/Region: MWSOC
Location: The Windy City
Vehicle:
Banned to
Housework for Summer

Post

I agree with ColinL
Aspen is offline   Reply With Quote
Old 01-07-2001, 10:43 PM   #13
AKGC8
Scooby Specialist
 
Member#: 1305
Join Date: Apr 2000
Chapter/Region: AKIC
Vehicle:
2000 Impreza

Post

Uh, I read through that whole thread and it didn't look like any of you came to any real conclusion. Any conclusion that you did come to was based on your own assumptions and theories, so are not necissarily correct.

Drag racing tires are specially designed to run at low PSI and create a larger contact patch. They also have a special sidewall that twists and creates a larger contact patch when under acceleration. A wider tire has a greater ability to create a large contact patch. However, that is only a factor when comparing two different width tires where there is such a difference that the smaller couldn't possibly flatten to the same area patch. On road tires with the relatively small differences in width that can be fit to a given vehicle, it won't really effect the contact patch area, just shape.
AKGC8 is offline   Reply With Quote
Old 01-08-2001, 06:14 AM   #14
ColinL
Scooby Guru
 
Member#: 114
Join Date: Jul 1999
Chapter/Region: MWSOC
Location: Wichita, KS and Whoring, OT
Vehicle:
'03 Evo, Rice White
'01 Erion CBR 929

Post

Eby, you can fit anything from the smallest you can find (maybe a 165/85-15) all the way to a 225/50-16 on an Impreza. Inflate both tires to 32psi, and which has the larger contact patch?

Since there is equal pressure in both, (and let's say the car weighs the same), then surely the laws for physics say that the contact patch is the same.

Wrong! The simple answer does not account for the weight of the vehicle being borne by the tire carcass and NOT air pressure.

Geez.
ColinL is offline   Reply With Quote
Old 01-08-2001, 09:37 AM   #15
AKGC8
Scooby Specialist
 
Member#: 1305
Join Date: Apr 2000
Chapter/Region: AKIC
Vehicle:
2000 Impreza

Post

Well apparently that is one of the assumptions that you made. The tire doesn't support the weight of the car. It acts more like a shock in a suspension system. If you have a stiffer sidewall it doesn't mean that it will support more of the weight of the car. It just means that when you hit a bump it won't compress as fast as a tire with a softer sidewall. Sort of like the difference in a suspension system when you have your adjustable shocks set to hard or soft. When resting though, it will still compress to about the same area as a similar size tire with a softer sidewall. The tire itself does not support the weight of the vehicle.
AKGC8 is offline   Reply With Quote
Old 01-08-2001, 11:06 AM   #16
ColinL
Scooby Guru
 
Member#: 114
Join Date: Jul 1999
Chapter/Region: MWSOC
Location: Wichita, KS and Whoring, OT
Vehicle:
'03 Evo, Rice White
'01 Erion CBR 929

Thumbs down

So if the tires are not supporting the weight of the vehicle then what is? Last time I checked, most cars had but four things touching the ground and they are commonly known as tires.

I am NOT talking about the tire as a component of the suspension, which of course it is. I am referring to the fact that the tire is touching the ground, NOTHING ELSE IS, and something is supporting the acceleration due to gravity. You can calculate the force, calculate the air pressure, and calculate the displacement (size of the contact patch) the tires would have IF they were totally elastic. Obviously they are not, as the carcass is semi-rigid.

This is ridiculous. I'm not sure what previous posts you read but I am starting to suspect is ISN'T the one Pilot and I are referring to.

[This message has been edited by ColinL (edited January 08, 2001).]
ColinL is offline   Reply With Quote
Old 01-08-2001, 02:50 PM   #17
AKGC8
Scooby Specialist
 
Member#: 1305
Join Date: Apr 2000
Chapter/Region: AKIC
Vehicle:
2000 Impreza

Post

A tire still has to compress to a certain area to support the load of the vehicle. The tire itself(ie, not the air) does not support the vehicle. It will compress untill the area of the tire patch and PSI combine to counteract the weight of the vehicle that is pushing down on that wheel. I think your trying to say that not all tires will compress to the same point. That would imply that at some point the tire itself(not the air) is supporting the vehicle, which it is not. Its the pressure of the air that is supporting the vehicle, and that is why it must compress to a specific point so that the vehicle can be supported.

The only way to increase tire patch is to decrease the beginning PSI, or increase the size of the tire to such a point where there is already enough tire area touching the ground that it can bear the load at a lower PSI. In all other cases it will compress to a specific area and PSI.

[This message has been edited by Eby (edited January 08, 2001).]
AKGC8 is offline   Reply With Quote
Old 01-08-2001, 07:58 PM   #18
ColinL
Scooby Guru
 
Member#: 114
Join Date: Jul 1999
Chapter/Region: MWSOC
Location: Wichita, KS and Whoring, OT
Vehicle:
'03 Evo, Rice White
'01 Erion CBR 929

Post

I can't believe I'm getting in this debate. Anyway...

Quote:
or increase the size of the tire to such a point where there is already enough tire area touching the ground that it can bear the load at a lower PSI
So explain why different tires of the same size might have significantly different load ratings.

For example, the 205/55-16 Bridgestone Potenza RE92 v-rated with a load rating of 89 and the Yokohama AVS Sport with a rating of 93. At 32psi do they have the same contact patch? How about the 225/50-16 AVS Sport with a rating of 92? How about the 245/45-16 with a rating of 94? All at 32psi, and all are very close in diameter.
ColinL is offline   Reply With Quote
Old 01-08-2001, 08:38 PM   #19
AKGC8
Scooby Specialist
 
Member#: 1305
Join Date: Apr 2000
Chapter/Region: AKIC
Vehicle:
2000 Impreza

Post

Thats an easy one. They have a different load rating because as you apply more weight the psi inside the tire increases. The tires with higher load ratings are designed to withstand the higher psi of a heavier load.
AKGC8 is offline   Reply With Quote
Old 01-08-2001, 10:21 PM   #20
Aspen
Scooby Specialist
 
Member#: 145
Join Date: Aug 1999
Chapter/Region: MWSOC
Location: The Windy City
Vehicle:
Banned to
Housework for Summer

Post

Alright boys, time to duke it out at the tracks.

Eby,
"They have a different load rating because as you apply more weight the psi inside the tire increases."

Maybe I didn't understand this statement correctly but are you saying that if I measure the pressure of tires before someone gets in and after they are in the car, it will measure differently??? I'm unclear.
Aspen is offline   Reply With Quote
Old 01-08-2001, 10:26 PM   #21
8Complex

Moderator
 
Member#: 922
Join Date: Feb 2000
Chapter/Region: MWSOC
Location: Schaumburg, IL
Vehicle:
04 FXT
Red

Post

Well, as the weight of the car increases and the tire deflects, the area inside the tire will decrease and raise the pressure. Makes sense to me, but I doubt that a person's weight would make much of a difference at all.
8Complex is offline   Reply With Quote
Old 01-08-2001, 10:43 PM   #22
Aspen
Scooby Specialist
 
Member#: 145
Join Date: Aug 1999
Chapter/Region: MWSOC
Location: The Windy City
Vehicle:
Banned to
Housework for Summer

Post

How bout 4 people and some luggage???
Aspen is offline   Reply With Quote
Old 01-08-2001, 10:57 PM   #23
P.K. Motorsports
Scooby Specialist
 
Member#: 1110
Join Date: Mar 2000
Location: USA
Vehicle:
1999 RS
Silver

Post

Oh... my... God... has... this... been... done... before...

BUT

Load ratings can obviously be influenced by MANY factors other than the width of the tire, not insignificantly the contruction of the tire. That was really lame, Colin.

Eby is right. A weak 150-pound man like me can easily squash a tire mounted on a rim with atmoshperic air pressure inside down to the rim. A 3000 pound car puts an average of 5 times as much force on each tire.

The tire is only strong inasmuch as it prevents the air from rushing outwards. A thick balloon (like a whoopie cushion with the neck tied tightly shut) is strong in an elastic sense. You might put all your weight on it and it wouldn't break. Still, it rubber itself is not structurally strong at all, it gains all its strength from holding the air in.
P.K. Motorsports is offline   Reply With Quote
Old 01-09-2001, 12:21 AM   #24
Gary (gg)
Scooby Newbie
 
Member#: 179
Join Date: Aug 1999
Location: Wichita, KS
Vehicle:
98 2.5RS
Rally Blue

Post

I sense Colin and Pilot getting into an another tire debate. I can't wait.
Gary (gg) is offline   Reply With Quote
Old 01-09-2001, 01:28 AM   #25
AKGC8
Scooby Specialist
 
Member#: 1305
Join Date: Apr 2000
Chapter/Region: AKIC
Vehicle:
2000 Impreza

Post

Well I've been doing some more thinking, and I believe colinl is on the right side of this arguement. However, not for any of the reasons that have been stated here. It has nothing to do with elasticity or whatever. A wider tire will have a larger tire patch because it does not have the same PSI of a smaller tire once weight is applied. I overlooked this fact before, but when I started talking about how changing weight effects the PSI of the tire it all became clear. If you have two tires with equal PSI, but different sizes they won't have the same PSI once the weight of the car is applied. The larger tire has more area inside and would require more compression to equal the PSI of the smaller tire(with weight applied). Compressing it that much would make the tire patch larger than the smaller tire. It won't actually compress to the same PSI of the smaller tire though. It will compress to the point where the area of the tire patch and the PSI's match up to support the car. Because the PSI's are going to be less on the larger tire, the area of the tire patch has to be more.

It doesn't really have anything to do with the width of the tire though. A low profile wide tire could have a smaller tire patch than a high profile thin tire. Its all to do with the area inside of the tire.
AKGC8 is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Wider tires on stock WRX rims? chump_driver Brakes, Steering & Suspension 1 05-27-2001 10:21 AM
Alignment on wider tires??? Ben Brakes, Steering & Suspension 3 04-13-2001 05:55 PM
Alignment with wider tires? Ben Legacy Forum 4 04-11-2001 11:47 PM
22b suspension mods to use wider tires Jessie James Brakes, Steering & Suspension 4 03-29-2001 06:45 PM
running wider tires/cutting spring perches? Thug Technical Forum Archive 3 06-07-2000 02:31 PM


All times are GMT -4. The time now is 07:37 AM.


Powered by vBulletin® Version 3.7.0
Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
Powered by Searchlight © 2014 Axivo Inc.
Copyright ©1999 - 2014, North American Subaru Impreza Owners Club, Inc.