Compact spare in the rear - Toyota GR86, 86, FR-S and Subaru BRZ Forum & Owners Community

mdm · 07-20-2016, 09:56 AM

The FRS manual says that the compact spare should be only used in the front. If you have a flat in the rear, you should put the donut in the front, and the normal front wheel in the rear.

Why is that? Clearances in the rear? LSD gets confused? Nannies get confused?

Do you guys observe this? Has anyone had problems due to putting the donut in the rear?

Also, has anyone tried to have insurance-covered roadside assistance replace a rear flat properly, i.e., demand that that they do the swap, meaning twice as much work? Did they do it?

Cockatoo · 07-20-2016, 10:07 AM

Good way to say goodbye to your diff....

Sport-Tech · 07-20-2016, 10:19 AM

At least they give you a spare - so many sports and performance cars do not these days.

Dadhawk · 07-20-2016, 10:45 AM

Quote:

Originally Posted by mdm

The FRS manual says that the compact spare should be only used in the front. If you have a flat in the rear, you should put the donut in the front, and the normal front wheel in the rear.

Rear wheel drive combined with two different tire sizes is a recipe for bad news. Ultimately, 99% of people will probably never realize they should do this, but the manual covers Toyota for liability when you go sliding off the road sideways.

mdm · 07-20-2016, 10:47 AM

Quote:

Originally Posted by Cockatoo

Good way to say goodbye to your diff....

OK, but why exactly? From the point of view of the diff, driving on the donut is like driving in a gentle turn all the time. Surely there would be some strain, but how is it worse from driving fast and hard on a winding race track?

Cockatoo · 07-20-2016, 11:28 AM

Quote:

Originally Posted by mdm

OK, but why exactly? From the point of view of the diff, driving on the donut is like driving in a gentle turn all the time. Surely there would be some strain, but how is it worse from driving fast and hard on a winding race track?

If you are driving with any kind if speed with massively different diameter tires on the rear, it is way more stress than "driving in a gentle turn". The diff is working hard to spin those wheels at the same rate, and a lot of heat builds up.

mdm · 07-20-2016, 11:56 AM

Quote:

Originally Posted by Cockatoo

If you are driving with any kind if speed with massively different diameter tires.

By my calculation, the [nominal] difference in diameter between the T135/80D16 spare and the 215/45R17 regular tire is 0.5%.

That's equivalent to the difference caused by a 300-m radius turn. Is that massive?

Or is the actual difference in diameter larger than the nominal difference, e.g. becuase the spare and the regular tire compress very differently?

FX86 · 07-20-2016, 11:58 AM

remove spare tire because weight reduction

Th3rdSun · 07-20-2016, 01:32 PM

If I'm not mistaken,you aren't supposed to put a donut on the driving axle. So if you have a flat on a FWD you should always have the donut on the rear,even if you have a flat in the front.

I'm pretty sure most AWD cars spares are the same size as the OEM size wheel,and of course,Jeeps,SUVs,and the like have full sized spares.

guybo · 07-20-2016, 01:39 PM

It's because of the diff. Certain LSD can be damaged by 2 tires of dissimilar diameter being mounted on the drive axle. This is why it's important to rotate tires on schedule and never install 2 different brands of tires or 2 tires with very different wear on the drive axle.

That said, the manual is being anal about it. You aren't supposed to go more than 50 miles or 50 mph on the donut anyway and you won't blow out the diff.

Don't be a lazy ass. Just put the donut on the front. If you are a lazy ass and call aaa to come change your tire 1) shame on you 2) if you insist that the aaa man install the donut on the front you had BETTER give him a good tip ya ****.

-your friendly AAA man who's sick of changing tires for snotty lazy asses

mdm · 07-20-2016, 02:23 PM

Quote:

Originally Posted by Th3rdSun

So if you have a flat on a FWD you should always have the donut on the rear,even if you have a flat in the front.

I have never seen an FWD manual saying so.

mdm · 07-20-2016, 02:24 PM

Quote:

Originally Posted by guybo

Just put the donut on the front.

I am not saying I wouldn't do so. I am just curious why is this exactly recommended, especially that by my calculations (which might be wrong) driving with a donut is equivalent to a relatively mild turn.

mav1178 · 07-20-2016, 02:35 PM

Quote:

Originally Posted by mdm

OK, but why exactly? From the point of view of the diff, driving on the donut is like driving in a gentle turn all the time. Surely there would be some strain, but how is it worse from driving fast and hard on a winding race track?

Basically it's putting excessive strain and heat on the diff, about the same as if you were to drift on the stock Torsen.

Assuming the average driver and lack of mechanical knowledge, you'd have a premature warranty case that may get denied by corporate due to "abuse"

-alex

extrashaky · 07-20-2016, 02:41 PM

I suspect if you put the donut on the rear, the wheel speed sensors are going to throw a code, freak out completely thinking you're skidding around in a circle and send the traction control into a panic. Even if it doesn't damage the diff, I suspect it would make for an unusual driving experience.

Anybody have a source for Australian parts? The Aussies have an option for a full sized spare that has a different trunk floor and tray. I would love to be able to order those parts at a reasonable cost.

07-20-2016, 09:56 AM	#1
mdm not a twin owner Join Date: Jan 2016 Drives: 2016 Veloster Turbo DCT Location: Northern Virginia, USA Posts: 734 Thanks: 992 Thanked 463 Times in 267 Posts Mentioned: 7 Post(s) Tagged: 2 Thread(s)	Compact spare in the rear The FRS manual says that the compact spare should be only used in the front. If you have a flat in the rear, you should put the donut in the front, and the normal front wheel in the rear. Why is that? Clearances in the rear? LSD gets confused? Nannies get confused? Do you guys observe this? Has anyone had problems due to putting the donut in the rear? Also, has anyone tried to have insurance-covered roadside assistance replace a rear flat properly, i.e., demand that that they do the swap, meaning twice as much work? Did they do it?

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07-20-2016, 10:07 AM	#2
Cockatoo TALLY FT86 Join Date: Mar 2014 Drives: 2014 FR-S Monogram -- SBD Turbo -- Location: Tallahassee, FL Posts: 624 Thanks: 185 Thanked 445 Times in 242 Posts Mentioned: 12 Post(s) Tagged: 1 Thread(s)	Good way to say goodbye to your diff....

07-20-2016, 10:19 AM	#3
Sport-Tech Senior Member Join Date: Oct 2010 Drives: TBD Location: Toronto Posts: 2,583 Thanks: 665 Thanked 685 Times in 386 Posts Mentioned: 2 Post(s) Tagged: 0 Thread(s)	At least they give you a spare - so many sports and performance cars do not these days.

07-20-2016, 11:58 AM	#8
FX86 Banned Join Date: May 2016 Drives: on the wrongside of the road Location: rear floor mats Posts: 2,240 Thanks: 1,084 Thanked 2,087 Times in 1,015 Posts Mentioned: 14 Post(s) Tagged: 1 Thread(s)	remove spare tire because weight reduction

07-20-2016, 01:32 PM	#9
Th3rdSun Senior Member Join Date: Jun 2015 Drives: 2013 FR-S MT Hot Lava Location: Chicagoland Area Posts: 587 Thanks: 156 Thanked 309 Times in 193 Posts Mentioned: 5 Post(s) Tagged: 1 Thread(s)	If I'm not mistaken,you aren't supposed to put a donut on the driving axle. So if you have a flat on a FWD you should always have the donut on the rear,even if you have a flat in the front. I'm pretty sure most AWD cars spares are the same size as the OEM size wheel,and of course,Jeeps,SUVs,and the like have full sized spares.

07-20-2016, 01:39 PM	#10
guybo Huge E85 fan! Join Date: May 2016 Drives: 2016 Scion FRS Location: Tampa, FL, USA Posts: 1,850 Thanks: 539 Thanked 1,010 Times in 605 Posts Mentioned: 32 Post(s) Tagged: 0 Thread(s)	It's because of the diff. Certain LSD can be damaged by 2 tires of dissimilar diameter being mounted on the drive axle. This is why it's important to rotate tires on schedule and never install 2 different brands of tires or 2 tires with very different wear on the drive axle. That said, the manual is being anal about it. You aren't supposed to go more than 50 miles or 50 mph on the donut anyway and you won't blow out the diff. Don't be a lazy ass. Just put the donut on the front. If you are a lazy ass and call aaa to come change your tire 1) shame on you 2) if you insist that the aaa man install the donut on the front you had BETTER give him a good tip ya ****. -your friendly AAA man who's sick of changing tires for snotty lazy asses

07-20-2016, 02:41 PM	#14
extrashaky Senior Member Join Date: Mar 2014 Drives: 2014 BRZ Limited Location: USA Posts: 4,046 Thanks: 1,100 Thanked 5,620 Times in 2,267 Posts Mentioned: 55 Post(s) Tagged: 1 Thread(s)	I suspect if you put the donut on the rear, the wheel speed sensors are going to throw a code, freak out completely thinking you're skidding around in a circle and send the traction control into a panic. Even if it doesn't damage the diff, I suspect it would make for an unusual driving experience. Anybody have a source for Australian parts? The Aussies have an option for a full sized spare that has a different trunk floor and tray. I would love to be able to order those parts at a reasonable cost.

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