The OEM Wheel Bolts and Their Design: The Bean-Counters Strike Back!

Fiat 500s that Do it Naturally.
User avatar
TheAverageEngineer
Posts: 23
Joined: Thu Sep 20, 2018 10:32 am
Location: Davenport, IA
Your Car's Year / Make / Model: 2015 Fiat 500 Abarth

The OEM Wheel Bolts and Their Design: The Bean-Counters Strike Back!

Post by TheAverageEngineer » Sat Feb 23, 2019 10:00 pm

I’ve previously gone over other subtle details of the wheel attachment method for the (modern) Fiat 500, including the use of wheel bolts, the locating studs, as well as the choice and a non-exhaustive history of the 4x98 bolt pattern.

Another topic of interest, which I can offer some comments on from an engineer’s perspective, is regarding the specific details of the OEM wheel bolts.

Disclosure: My commentary/rambling is based on my own reasoning of the technical requirements for the product from the standpoint of the original manufacturer. I am drawing this based on my personal experience as a design engineer for a major manufacturer of off-highway equipment, and although the specific functional aspects of the products at my work are quite different from the automotive industry, the basic engineering and economic principles seem to be mostly the same, based on discussions with friends who do work in engineering in the automotive industry.

The commentary is not based on any direct information or experience from Fiat Group/FCA product engineering or engineers. In areas where my comments are complete speculation (pulling it out of thin air), I will make note of this.


My Wheel Bolts Have Stripped and I Can’t Get the F***ing Things Off To Change My Wheels!!!
This is a relatively common complaint I have seen in various Fiat groups. Unfortunately, this problem is not unique to the Fiat 500, as earlier generations of Chrysler products (pre-FCA) suffer from the same problem. Even worse, and on a much larger scale, Ford products have been suffering from this same problem for quite some time (maybe as long as 30 years).

What happens on our cars is that the heads of the wheel bolts swell in size to the point that a normal 17mm socket does not fully seat. Because of this, there is now a much-reduced length of engagement between the socket and the bolt head, and it becomes extremely easy to round off the corners of the bolt head. At this point, it now becomes a cursing match and some destructive means are often required to remove the offending bolt.

I can speak from some experience on this situation, as I had exactly this thing occur to me about 2 years ago at an autocross event, while going through the process of changing from my road tires to my competition tires.
OneOfTheseIsNotLikeTheOther.jpg
OneOfTheseIsNotLikeTheOther.jpg (140.52 KiB) Viewed 4461 times
To understand why this problem happens, I must detail the overall design of our OEM wheel bolts.

The OEM Design
If you look at one of our OEM wheel bolts closely, when not installed, you will notice that there are two visually distinct regions on the bolt. The threaded shank of the bolt and the conical taper have a rather dull gray color to them. The head of the bolt has a somewhat shiny finish which looks nice. People like their cars to look nice, and if the car has nice wheels, we also like even the wheel bolts or nuts to look nice. This is the reason why the head of the bolt has a different finish from the shank.

OK great, but how is this achieved? In the case of our wheel bolts, this is accomplished by a two-piece wheel bolt. There is a light and thin gauge metal cap, most likely stainless steel, which is stamped onto the headed end. This cap will have good corrosion resistance, and will hold a nice appearance for a long time.
TwoPieces.jpg
TwoPieces.jpg (81.7 KiB) Viewed 4461 times
However, this design is not perfect, and in my mind there are three potential problems with it.

Moisture Ingress
In a perfect world, our shiny stainless steel cap which is stamped over the head of the bolt would also fit perfectly. The surface of the bolt would be perfectly smooth, the underside of the cap would be perfectly smooth, and the cap would fit with 100% contact across all six sides of the hex.

In reality, industrial processes can only be so perfect and there will inevitably be variations and tolerances to deal with. There will be little gaps between the cap and the bolt even in the best cases. If you look closely at the bottom side, there is even a clear gap between the end of the cap material and the upper end of the conical taper of the bolt.

Over time, this gap will be exploited by air and moisture. The material of the cap will take quite some time to succumb to corrosion. However, the bolt itself, or the coating on the bolt material, will be affected by the corrosion sooner. Even a minor amount of corrosion here is a problem, because the corrosion is trapped between the bolt and the cap. What happens is that the corrosion itself expands, and in a process known as “rust jacking” this places stress onto the cap, which causes it to expand as well.
ThatGapTho.jpg
ThatGapTho.jpg (60.55 KiB) Viewed 4461 times
Differential Rates of Thermal Expansion
Thermal expansion (or in reverse, contraction) is the process of a material increasing in size in response to an increase in temperature. Different materials will often have different rates of thermal expansion, which is to say, some materials will expand faster than others.

Differential rates of thermal expansion is a serious issue which can occur in many situations where different materials are required to be in contact with each other. Some common examples would be aluminum cylinder heads on cast iron engine blocks, and steel bolts in aluminum gearbox housings. Both of these examples also include major changes in temperature, which makes it particularly important to design and size components carefully in order to account for thermal expansion.

For our two-piece wheel bolt, normally we should not have quite as critical of a situation as steel bolts in aluminum gearbox housings. However, our bolts will have to face somewhat elevated temperatures as a result of heat transfer coming from the brakes. Because the rate of thermal expansion for stainless steel is about 50% higher than it is for “carbon” steel, it is very possible that the cap of the bolt expands faster than the bolt itself, which could ever so slightly increase the gap between the two materials for some of the time of operation, allowing easier access of air and moisture to the interface.

Material Strength
From various sources that I have read in the past, the base steel material of the OEM wheel bolts is in accordance to ISO Class 10.9. What exactly this entails is not necessarily important for this discussion, but to summarize, Class 10.9 is a good high-strength bolt material, particularly for such a critical function as keeping the wheels attached to a car. To elaborate, our OEM wheel bolt torque specification is at 75 ft-lbs (102 Nm), and it is using an M12x1.25 fine thread pitch. From my experience at work, the nominal torque for an M12x1.75 course thread bolt (85% of proof load) will be on the order of 120 ft-lbs (160 Nm), and a fine thread fastener will normally be able to hold a little bit higher torque.

“OK, you’re losing me here…”

What this means is that the actual bolt material is good for quite a bit more than what is specified to be used. This is a good thing for bolt longevity and safety factor for a critical component. The problem, though, is the pretty shiny cap is not of a similarly high material strength. In fact, it isn’t even close. If the cap is indeed stainless steel as I suspect, the gap between the cap strength and the actual bolt strength is immense.

For comparison sake, a 304 stainless steel (very common grade in many industrial applications) has a yield strength of about 31,000 psi (215 MPa) at the lower end. A bolt made in accordance to ISO Class 10.9 specifications will have a yield strength of approximately 130,000 psi (900 MPa). In short, the yield strength of the steel in the bolt is more than four times that of the cap material.

Now consider the tools. The wrenches and sockets we normally use in a shop setting will be made from some sort of heat-treated steel in order to minimize wear and prolong their useful life. Normally, a good tool should have a higher material strength than what it working against, so a good wrench or socket will have a higher strength than our Class 10.9 bolt.

In summary, in a battle between the wheel bolt steel, the shiny pretty cap, and the 800-pound gorilla at the local tire shop ham-fisting their impact gun at four ugga-duggas, the shiny pretty cap is going to lose.

On my OEM bolts, even with the care I normally take during removal and installation of the wheels, not using any power tools above a little ¼” drive Makita impact driver, and always using a torque wrench set at 75 ft-lbs, there is clear and distinct wear on all sides of the caps. This is even true on a few replacement bolts I purchased after my incident, which were not used for very long and still fit nicely with a 17mm socket.

In my opinion, even there wasn’t a problem with swelling of the caps, the actual material of the caps is not especially durable and there will come a time that the corners will get rounded off, and one will need to resort to destructive means to remove the bolt.

So Why Do They Do It?
Recall my comment from earlier: People like pretty shiny things. People will pay for shiny pretty things. Companies that produce products know this. We humans are petty and vain, and we want our expensive purchases like cars to look nice.

Traditionally, for many steel components on a car that we want to look nice and aren’t painted, chrome plating was very common. Chrome plating is an excellent choice from the perspective of aesthetics as well as corrosion resistance. However, the industrial processes to create chrome plating are often complicated, tedious, environmentally hazardous, or a combination of all of these. As a result, over time it has become more and more expensive to do chrome plating.

Also refer to my title for this topic: “The Bean-Counters Strike Back!” In any major manufacturer of products, you will have folks who deal with issues of product cost and the actual value to customers. Turns out, most people don’t care what process is used to make the heads of their wheel bolts look shiny and pretty, just that they are that way. Further, I think it is a safe bet to assume most Fiat 500 buyers are not doing their own maintenance on the car, as is the case with most vehicles produced today.

As a result, the conclusion from a Value Analyst is that most customers place value on the physical appearance of the wheel bolts, but is not overly concerned with the longevity or issues with corrosion, because that is a problem that their local Studio or other service shop will deal with.

So therefore, the Value Analyst has proposed a part to cost reduce, and inevitably some poor engineer(s) will get tasked with figuring out how to accomplish this. Which leads us back to our two-piece bolt, with a shiny cap stamped onto a steel bolt. The coating on the bolt itself does have decent corrosion resistance, but not quite as much as chrome plating would have.

I don’t know the actual manufacturing costs to make these bolts, but I can make a guess at it based on the retail price. When I bought a few replacement OEM bolts at my local Studio, they were going for about $10 a piece. For a lot of automotive and heavy machinery parts in the Western world, the service parts markup on it might be 5 to 10 times that of the price the OEM paid for them. This would put our bolt at a cost of $1 or $2 each purchased by FCA from the bolt manufacturer. For our purposes let’s average them and say it’s $1.50 each. In the grand scheme of bolts, this is actually quite expensive but understandable considering the application. Therefore, it stands to reason that a chrome plated bolt at the quality standards demanded for this use and by a major auto manufacturer must be noticeably more expensive than $1.50 each, in order to justify the change.

Let’s make up a number and say that the bolt manufacturer is saving FCA $0.50 per bolt to do them in the two-piece design instead of chrome plating. On our cars, with 16 wheel bolts in total, this would amount to a total savings of $8 per car. At the production rate of only the North American market cars this would not be an incredibly high savings (about $80k per year), but almost certainly this bolt will be used on many other platforms and/or markets which produce at higher volumes. If you think of it in terms of millions of cars per year (and at least 16 bolts per car), now you’re considering tens of millions of dollars in savings.

Conclusions
As time goes on, these sorts of situations are only going to become more common. The cost of doing business at every level and scale of business is only getting higher, for many different reasons. Companies want to save money wherever they can to ensure their own financial health (profitability), particularly if most customers don’t really notice or care about the changes they are making. From this perspective, the decision to use a two-piece wheel bolt makes sense to me.

However, the two-piece wheel bolt is still a pain in the ass for end users who do their own maintenance, and especially for folks who remove and reinstall wheels more frequently than normal owners would. If you are in this category, it would be wise to consider switching to a high-quality one-piece steel bolt, especially if your socket is starting to fit rather tight to your OEM bolts.

User avatar
TheAverageEngineer
Posts: 23
Joined: Thu Sep 20, 2018 10:32 am
Location: Davenport, IA
Your Car's Year / Make / Model: 2015 Fiat 500 Abarth

Re: The OEM Wheel Bolts and Their Design: The Bean-Counters Strike Back!

Post by TheAverageEngineer » Sun Feb 24, 2019 7:31 pm

Additional Information
Out of some discussion on the Facebook group, it was noted that on 500s where the wheel bolts would be covered by a wheel center cap (like below), there may instead be a traditional one-piece bolt only made of steel. In this case, it is likely you will not experience the sort of problems that the two-piece wheel bolts have.

Image

User avatar
texanbrit
Posts: 755
Joined: Wed Sep 19, 2018 11:01 pm
Location: Conroe, TX
Your Car's Year / Make / Model: 2013 500c Abarth
Your Instagram (Optional): paul_rogerson_tx
Contact:

Re: The OEM Wheel Bolts and Their Design: The Bean-Counters Strike Back!

Post by texanbrit » Mon Feb 25, 2019 4:03 am

So the folks that paid for the upgraded 17s got the suspect bolts!
2013 Abarth 500c (NGen Turbo'd) - Isabella
2012 500 Pop (broken, engine rebuild on hold) - Popabella
2014 500L (Daily workhorse) - The Hulk
2013 Abarth 500 (broken engine, current project) - Abby

User avatar
TheAverageEngineer
Posts: 23
Joined: Thu Sep 20, 2018 10:32 am
Location: Davenport, IA
Your Car's Year / Make / Model: 2015 Fiat 500 Abarth

Re: The OEM Wheel Bolts and Their Design: The Bean-Counters Strike Back!

Post by TheAverageEngineer » Mon Feb 25, 2019 7:55 pm

texanbrit wrote: Mon Feb 25, 2019 4:03 am So the folks that paid for the upgraded 17s got the suspect bolts!
And the folks with the newer "snowflake" 16" wheels too!

Return to “Tech - Fiat 500 Pop/Sport 2012-2018”