<VV> Compact Spare Tire Issue

Jim Becker mr.jebecker at gmail.com
Tue Sep 3 11:45:54 EDT 2019

My wife had a Cadillac Catera which had an unusual configuration for the 
spare.  I no longer remember the tire sizes, but google provided me some 
believable numbers.  The regular tires were 225/55R16.  The spare was a 
conventional tire but in a smaller size (205/65R15), lower speed rating, 
and on a funky stamped steel wheel.  As I recall, the tire brand didn't even 
match the other four.  I never tied it, but I don't think a 225/55R16 would 
have fit into the tire well.  I guessed that some earlier (Europe only) use 
of that body shell probably came with something close to 205/65R15s and they 
didn't bother reshaping the tire well.  The car came with most, if not all 
of the compact spare tire warnings.  I think that cars would have been 
reasonably safe with four of those spare tires installed.  The compact spare 
in the CD test seems as much like this as it does the usual compact spare.

On the apparent improved performance on some of CD's tests, it probably 
reflects lack of precision or lack of repeatability of the tests themselves. 
If they repeated the tests several times without changing the tires, there 
would probably be a scatter shot of results.  It invites some healthy 
skepticism of their "head-to-head" car comparisons where one car stops 5 
feet quicker or does .03 better in lateral acceleration.

Jim Becker

-----Original Message----- 
From: Joel McGregor via VirtualVairs
Sent: Monday, September 02, 2019 9:04 PM
To: virtualvairs at corvair.org
Subject: Re:  Compact Spare Tire Issue

The mini spare in that test was huge compared to most I've seen. 155 width 
and they said is was a very soft compound which wore quickly.
They also claimed with the standard tires skidpad was .88g and with the mini 
spare on the inside rear it was .91g.  Suspect.
They claimed braking distance was better with the mini on the front vs the 
rear.  Also suspect for a front engine car.
>From these results I would have to conclude that the spare in the test was 
made with a racing type compound which fits their quote from Ford.
All that test really showed is that that model of Mustang comes with a high 
performance spare unlike any I've seen on regular cars.

Joel McGregor

-------- Original message --------
From: roboman91324--- via VirtualVairs <virtualvairs at corvair.org>
Date: 9/2/19 8:24 PM (GMT-06:00)
To: virtualvairs at corvair.org, corvairjack at me.com
Subject: <VV> Compact Spare Tire Issue

I want to thank Jack Penard on two fronts. First, apparently he found the 
January, 2017 Car and Driver article he referenced previously and he sent me 
the link.  Apparently, I was not as persistent in my search as I could have 
been.  Second, he made the point that he was sending the link to me 
personally and not copying it to VV in order to save me some embarrassment. 
Jack, I very much appreciate the thought but as anyone who knows me can tell 
you, I don't mind a little embarrassment.  It happens all too often.
Considering that this is a safety issue, I feel it is important to share the 
link with anyone interested.  Here is the link to the article.  I will 
assess the content further on.
OK, now for the assessment.  Despite the redundancy, I will copy my previous 
list of concerns and include commentary in parenthesis.
1. The tread depth on the mini is minuscule compared to a regular tire. 
Therefore, the tread life would be much less.  (I don't blame the 
designers/manufacturers for including minimal tread on these tires because 
they are intended for very limited use.)  The depth of tread is barely at 
the wear limit where you would be subject to an officer issuing a ticket if 
you had a standard tire.  Is this as safe as a normal tire?  This issue is 
aggravated by the smaller contact patch between the tire and the road.  A 
smaller contact patch will wear faster than a larger patch with the same 
weight, speeds, etc. in use.  (C&D did not test this aspect of performance. 
A smaller and narrower tire with harder rubber and higher pressure will wear 
out much quicker.  This is especially the case with shallower tread.)
2.  The rubber composition of the mini is harder than a standard tire.  In 
addition, the tire pressure is higher.  I believe this is because they want 
the mini to retain pressure over many years without attention.  A harder and 
stiffer tire will have poor braking and handling characteristics.  As in 
item 1 above, a tire with higher pressure and a harder substance will have a 
smaller contact patch.  (On this aspect of my concerns, I am pleasantly 
surprised. The diminished performance in taking curves and acceleration were 
minimal to the point that the majority of drivers would not notice.  Braking 
was worse by only a couple of feet.  I suspect these data points are what 
misled you (Jack) to conclude that mini spares are acceptable replacements 
for normal tires.  Mea culpa.  However, your (Jack's) stated use of mini 
spares is that they are adequate for use at all four corners of a vehicle. 
If all tires are minis, there wouldn't be three other high performance tires 
to save the day.  I
wish C&D had done a test with four spares.  Logic dictates that the handling 
results would have been horrifying.)
3.  Due to both 1 and 2 above.  The already poor characteristics of the mini 
tire in normal use are far, far worse in the rain or snow.  While many areas 
of the country do not have rain and or snow year round most do have a rainy 
season as in Southern California.  I do not expect that people living in 
these areas will refrain from driving during this season or will choose to 
change their tires to go driving when the roads are wet.  Deeper treads and 
a more compliant tire are critical for safe operation under these 
conditions.  (C&D did not test the performance in wet conditions.  This 
would have been an easy test to perform.  Harder rubber and minimal tread 
depth will have a negative impact on performance in this area.  However, 
with three high performance tires remaining on the vehicle to take up the 
slack, the dangerous characteristics of the mini spare may have been 
4.  To save weight, both the wheel rim and tire are made of much thinner 
material.  The metal rim is probably made from a less dense material though 
I am not certain of this.  This makes them much weaker if you were to hit a 
rock, pot hole or curb.  Further, with even moderate handling efforts, the 
more pronounced flexing of both metal and rubber could easily cause 
unexpected handling characteristics.  Even standard wheels are subject to 
deformation but are designed to stay as safe as is reasonably possible under 
expected conditions.  (C&D did not test the tires with regard to curb 
scuffing and road hazards that are a common concern everywhere off the 
track.  As regards flexing, they did not test that either but because the 
G-force results are minimally degraded, this may not be a concern.  However, 
the test vehicle had only one spare on it.  Again, the three high 
performance tires took up the slack.)

5.  The perimeter and side walls of the mini/compact tires are very thin 
with fewer plies than you would normally have in even the cheapest of tires. 
This is one reason why they are inflated to almost double that of standard 
tires.  Mini spare tires do not need to meet the government's standards for 
normal use.  This is why they are approved only, I repeat ONLY, for very 
limited and low speed use.  In fact, the government does not require that a 
spare be included with any car.  Many new cars sold today do not have spares 
of any kind as standard equipment. The only reason any are included is 
because it is traditional.  The reason compact spares are even minimally 
safe under strictly limited conditions is because manufacturers want to 
avoid lawsuits.  (There was no comment in the article regarding wall and 
tread plies.  Will there be long term concerns with de-lamination or other 
degrading effects after thousands of miles?  This remains a major concern 
for everyday use of these tires.)

6.  If you have a limited slip differential, the use of a compact spare on 
one side of that axle is of great concern.  Limited slip differentials are 
designed to slip minimally when you drive around a corner.  With a smaller 
diameter wheel on one side, that differential is slipping constantly.  It 
will be destroyed in short order.  Your warranty will not cover this.  All 
wheel drive vehicles will have this problem whichever corner has the mini 
wheel.  For this reason, it is far, far more important for you to use a mini 
wheel only for a very short distance with one of these vehicles.  Of course, 
if you have a compact wheel of the same size at all four corners, this is 
not an issue.  (The article mentioned performance issues with the limited 
slip differential when a mini spare was used on the rear.  However, they 
neglected to discuss the expected damage to the clutches and gearing in the 
differential due to prolonged use of mismatched diameter tires.  Further, in 
wet conditions or snow
, the mini spare may very well be constantly spinning on the surface.)
In conclusion: The one shock I got from the article is the surprisingly good 
performance on curves as well as acceleration and braking when only one 
compact tire is used.  No doubt a great deal of design and development 
effort went into these tires before they were released upon the public. 
They achieved the goal of emulating the performance of a normal tire when 
used in a strictly temporary, low speed and limited environmental 
application.  I will have a little more confidence when one and only one 
mini spare is on a car I ride in.  Of course, this confidence would only 
exist if the car did not have the spare on for an extended duration, at 
higher speed or in wet conditions.  If these conditions exist, it will be a 
white knuckle experience for me.  If a vehicle has mini spares on all four 
corners, I will not ride in it unless I was stranded in Death Valley or some 
similar dire circumstance.
Jack, I now understand how you misinterpreted the C&D "test."  Considering 
the very clever opening paragraphs of the article, it seems apparent that 
their intent was to grab attention with an alleged revelation.  They did not 
design the test that might deliver results that didn't deliver a "WOW" 
factor.  I wonder how many other individuals have likewise misinterpreted 
the article and are happily driving around in complete ignorance of the 
danger they are in.  Shame on C&D for issuing this misleading article.  At 
best, it should be classified as severely incomplete.
Jack, we know each other and I like you.  I would consider it a personal 
loss and a failure if there was an announcement at a forthcoming chapter 
meeting that you were injured or worse in a car accident that might be 
attributed to your use of mini spares.  I know you are an intractable 
individual but I implore you to bite the bullet and buy real tires for your 
Again, thanks for the link to the C&D article.  If you can find the Motor 
Trend article I would love to read it.  Perhaps it addresses aspects of mini 
spare performance that C&D did not.  In one of my previous posts, I stated 
that the article(s) you read were limited in scope and you gleaned from them 
what you wanted to believe.  It seems that is the case.
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