195/65/15 on front 205/60/15 on rear

Where the power meets the road.

Moderators: Helpinators, Moderators

FrmRgz2Rchz
First Gear
Posts: 148
Joined: Wed Oct 16, 2002 10:50 pm
Location: Hilliard, OH

195/65/15 on front 205/60/15 on rear

Post by FrmRgz2Rchz »

How is my center diff handling the small difference in tire size?
Andy Rose
1993 Subaru Legacy Sport Sedan
2003 Subaru Impreza WRX
2004 Subaru Impreza WRX STI
1999 Hyundai Accent
ciper
Knowledgeable
Knowledgeable
Posts: 4388
Joined: Tue Oct 15, 2002 8:16 pm
Location: SFCA

Post by ciper »

7mm or larger circumference difference, way out of specified limits. If you have driven for very long like this Id expect your center differential to be very weak. Already its only an 8kg unit with tons of mileage, its probably dead.

Put your car on jack stands and use the parking brake while in gear to see how easy it is to stop the rears.

What made you do this in the first place? At least move the two larger tires to the same side of the vehicle!
Last edited by ciper on Tue Feb 10, 2004 1:50 am, edited 1 time in total.
FrmRgz2Rchz
First Gear
Posts: 148
Joined: Wed Oct 16, 2002 10:50 pm
Location: Hilliard, OH

Post by FrmRgz2Rchz »

I busted my two front tires last weekend and needed to get back on the road. I couldn't find 2 used 205-60-15, and didn't want to buy new ones.

Won't it mess up the front diff if I have two different sizes on the front.
Andy Rose
1993 Subaru Legacy Sport Sedan
2003 Subaru Impreza WRX
2004 Subaru Impreza WRX STI
1999 Hyundai Accent
ericS2the6
First Gear
Posts: 217
Joined: Sat Oct 19, 2002 5:36 am

Post by ericS2the6 »

yeah put them on the same side like ciper said.

everytime u turn the inside wheels are traveling less than the outside. youd just be doing this all the time. your diffs are designed for this
evolutionmovement
Knowledgeable
Knowledgeable
Posts: 9809
Joined: Mon Jun 16, 2003 11:20 pm
Location: Beverly, MA

Post by evolutionmovement »

Yes, to your car it would just seem like you're mostly travelling in a circle rather than straight.

Steve
Midnight in a Perfect World on Amazon or order anywhere. The first book in a quartet chronicling the rise of a man from angry criminal to philanthropist. Midnight... is a distopic noirish novel featuring 'Duchess', a modified 1990 Subaru Legacy wagon.
FrmRgz2Rchz
First Gear
Posts: 148
Joined: Wed Oct 16, 2002 10:50 pm
Location: Hilliard, OH

Post by FrmRgz2Rchz »

I thought the limited slip in the front tried to make both front wheels spin at the same time. If the right and left tires are different sizes, won't this be hurting the diff.

I really don't see how the slightly smaller tires on the back is hurting my center diff all that much. I have full tread on the 205's and the 195's are almost bald. The guy at the tire place said they are the same size and when I looked up the difference on the net, I found that they are about 7mm bigger than the 205's that I had on the car.

I figured that the center diff would think that the back tires are having slight traction problems and do whatever it normally does when this happens. Is this difference in tire diamater causing it to overheat, or what really is happening?
Andy Rose
1993 Subaru Legacy Sport Sedan
2003 Subaru Impreza WRX
2004 Subaru Impreza WRX STI
1999 Hyundai Accent
LegacyT
Fourth Gear
Posts: 1220
Joined: Mon Oct 21, 2002 9:53 pm
Location: Ottawa, Ontario, Canada, Now Sioux Lookout

Post by LegacyT »

Arent 195/65R15's and 205/60R15's virtually identical in overall diameter. Just that the 205's in the rear are 10 millimeters wider. I don't think its hurting anything as long as the diameter of front and rear tires are the same, in this case they are. Heh, are you goin for the dragster/ musclecar look with the fatties in the rear? :D

Mark,
1991 Turbo Sedan, Aspen White 5MT, Sold RIP
1994 Turbo Sedan, Crimson Pearl 5MT, from British Columbia-no rust!
FrmRgz2Rchz
First Gear
Posts: 148
Joined: Wed Oct 16, 2002 10:50 pm
Location: Hilliard, OH

Post by FrmRgz2Rchz »

I don't think the naked eye can tell a difference, but the 195's are about 7mm bigger in diameter. I would guess with the loss of tread in the front and full tread in the rear, they are about 4 or 5mm bigger. I have to drive on them until I get my summer tires in, so I hope it doesn't cause too much damage.
Andy Rose
1993 Subaru Legacy Sport Sedan
2003 Subaru Impreza WRX
2004 Subaru Impreza WRX STI
1999 Hyundai Accent
ciper
Knowledgeable
Knowledgeable
Posts: 4388
Joined: Tue Oct 15, 2002 8:16 pm
Location: SFCA

Post by ciper »

PUT THEM ON THE SAME SIDE

The differential has a fluid. You are constantly shearing that fluid between the plates. It will wear down. Its not a servicable unit so you will need a replacement eventually.

Lets say the 205s are 635mm and the 195s are 640mm. If you put one 635mm and one 640mm on the front differential the sum is 1275. Do the same on the rear and the speeds between the two are EQUAL, meaning your center diff works like normal.

Now the way you have it is two 635 on the front for 1270 and two 640 on the rear for 1280, meaning your center differential is constant being worn. PLUS your rear wheels are always pushing the car, causing odd loading on the car that could effect handling.

You could even put on 205 on the front left and one 205 on the rear right to save the differential.

"The guy at the tire place said they are the same size"
Ohh, he must know more than we do since he works on cars every day :roll:

Do you have ABS?
Last edited by ciper on Tue Feb 10, 2004 12:19 am, edited 1 time in total.
boostjunkie
Knowledgeable
Knowledgeable
Posts: 1770
Joined: Mon Dec 16, 2002 3:20 pm
Location: Owings Mills, MD
Contact:

Post by boostjunkie »

Consider yourself lucky that you don't have a rear lsd. Then you'd need THREE tires of the same size!! I had that problem this winter when I busted up one of my winter wheels.

The STi guys are just plain fuxored in this situation :twisted:

"Four new tires, please." :shock:
[url=http://www.angelfire.com/md3/91turbolegacy/images/On_the_Lawn.jpg]1991 Legacy Turbo (RIP)[/url]

[url=http://www.angelfire.com/md3/91turbolegacy/images/Summer_Car_Wash3.jpg]2000 Celica GT-S[/url]
vrg3
Vikash
Posts: 12517
Joined: Sun Nov 03, 2002 2:13 am
Location: USA, OH, Cleveland (sometimes visiting DC though)
Contact:

Post by vrg3 »

Three of the same size and one a different size would never be permissible on an AWD Subaru.

With a rear and center LSD and an open front diff, you have to have two identical diameter rear tires and have the front tires average out to the same diameter as the rears.
"Just reading vrg3's convoluted, information-packed posts made me feel better all over again." -- subyluvr2212
boostjunkie
Knowledgeable
Knowledgeable
Posts: 1770
Joined: Mon Dec 16, 2002 3:20 pm
Location: Owings Mills, MD
Contact:

Post by boostjunkie »

vrg3, could you explain the reasoning for this? Now I'm confused.
[url=http://www.angelfire.com/md3/91turbolegacy/images/On_the_Lawn.jpg]1991 Legacy Turbo (RIP)[/url]

[url=http://www.angelfire.com/md3/91turbolegacy/images/Summer_Car_Wash3.jpg]2000 Celica GT-S[/url]
vrg3
Vikash
Posts: 12517
Joined: Sun Nov 03, 2002 2:13 am
Location: USA, OH, Cleveland (sometimes visiting DC though)
Contact:

Post by vrg3 »

Well, the rear left and rear right tires have to be the same speed so they don't upset the rear LSD, right?

And the front and rear axles have to be the same speed so they don't upset the center LSD...

The rear axle will be moving at a speed dependent on the rear tires. The front axle has to move this same speed. So, the front tires have to average out to the same size as the rears.
"Just reading vrg3's convoluted, information-packed posts made me feel better all over again." -- subyluvr2212
ciper
Knowledgeable
Knowledgeable
Posts: 4388
Joined: Tue Oct 15, 2002 8:16 pm
Location: SFCA

Post by ciper »

Lets say you had 3 of 205s are 635mm and one the 195s are 640mm.

You would have 1270 total on the rear (rear diff is okay) and 1275 on the front, meaning the center diff outputs are unequal.
boostjunkie
Knowledgeable
Knowledgeable
Posts: 1770
Joined: Mon Dec 16, 2002 3:20 pm
Location: Owings Mills, MD
Contact:

Post by boostjunkie »

But isn't that within the 7mm allowed variation in rolling diameter?

I'm basing it off of this quote. I don't know the specified limits :( :

"7mm or larger diameter difference, way out of specified limits."
[url=http://www.angelfire.com/md3/91turbolegacy/images/On_the_Lawn.jpg]1991 Legacy Turbo (RIP)[/url]

[url=http://www.angelfire.com/md3/91turbolegacy/images/Summer_Car_Wash3.jpg]2000 Celica GT-S[/url]
ciper
Knowledgeable
Knowledgeable
Posts: 4388
Joined: Tue Oct 15, 2002 8:16 pm
Location: SFCA

Post by ciper »

http://forums.s2ki.com/forums/showthrea ... genumber=1

I cant find the good thread on nabisco but I think the calculation was a max difference of 4mm?
vrg3
Vikash
Posts: 12517
Joined: Sun Nov 03, 2002 2:13 am
Location: USA, OH, Cleveland (sometimes visiting DC though)
Contact:

Post by vrg3 »

boostjunkie - Are you sure that quote wasn't talking about circumference as opposed to diameter?
"Just reading vrg3's convoluted, information-packed posts made me feel better all over again." -- subyluvr2212
ciper
Knowledgeable
Knowledgeable
Posts: 4388
Joined: Tue Oct 15, 2002 8:16 pm
Location: SFCA

Post by ciper »

Sorry, my mistake. I didnt mean diameter in the post, outside distance.
LaureltheQueen
Spelling Nazi
Spelling Nazi
Posts: 4644
Joined: Fri Aug 29, 2003 7:14 am
Location: Lynnwood, WA
Contact:

Post by LaureltheQueen »

ciper
Knowledgeable
Knowledgeable
Posts: 4388
Joined: Tue Oct 15, 2002 8:16 pm
Location: SFCA

Post by ciper »

The miata one isnt good, you have to be connected to the net and run java to use it. http://www.angelfire.com/rock3/duzy/tire.htm
LaureltheQueen
Spelling Nazi
Spelling Nazi
Posts: 4644
Joined: Fri Aug 29, 2003 7:14 am
Location: Lynnwood, WA
Contact:

Post by LaureltheQueen »

as long as you're connected, and run java, there's no problem with it. :)
vrg3
Vikash
Posts: 12517
Joined: Sun Nov 03, 2002 2:13 am
Location: USA, OH, Cleveland (sometimes visiting DC though)
Contact:

Post by vrg3 »

You don't even have to be running Windows! :)
"Just reading vrg3's convoluted, information-packed posts made me feel better all over again." -- subyluvr2212
ciper
Knowledgeable
Knowledgeable
Posts: 4388
Joined: Tue Oct 15, 2002 8:16 pm
Location: SFCA

Post by ciper »

It doesnt do offset calulcations.
musketeerracing
First Gear
Posts: 114
Joined: Thu Jan 08, 2004 4:37 pm

Calculations

Post by musketeerracing »

Just for fun(!) I did the math and get 634.5mm circumference for the 195s and 627 for the 205s. That 17.5mm difference translates into one extra revolution for the 205s for every 84 revolutions of the 195s, or 1576 revolutions per km for the 195s, and 1595 for the 205s, or 19 extra revolutions per km for the 205s.

Now we need some specs for the viscous diff, which I don't have. Anyone? What can it handle?

At 30km/h (20mph) the difference isn't too wacky perhaps - (happily for the math) about 600 extra revolutions per hour, 10 per minute, 0.167 per second.

At 120km/h (about 70mph) the difference seen by the diff is greater: 2400 per hour, 40 per minute, and (now we're getting to the point) 0.6 per second.

Purely subjectively, I think that more than half a revolution per second is getting into the range where the viscous fluid thickens, and so the diff is being wrenched. But maybe not. Anyone?

ACP
ciper
Knowledgeable
Knowledgeable
Posts: 4388
Joined: Tue Oct 15, 2002 8:16 pm
Location: SFCA

Post by ciper »

The person with mismatched tires needs to jack the car up and use a measuring tape to measure the tires. Three tires of the same size of different models (even the same brand) aren't going to be the same.

I feel most of you dont have experience with many Legacy. Ive looked at so many high mileage (with the intent to purchase for myself or others) to know that even the SAME tires on the car that weren't rotated are enough to damage the center differential. Especially on automatics that are rendered into FWD because the plates are so worn.
Post Reply