<VV> Compact Spare Tire Issue

[Joel McGregor]

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

Folks,
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.
https://www.caranddriver.com/news/a15343476/how-much-does-a-donut-spare-tire-limit-performance-we-find-out-using-a-ford-mustang/
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 disguised.)
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 car.
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.
Doc
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