The Broken Fiat Club

Not sure the precise best subsection to include this in because I don't know if we have a "general" tech area, but some discussion on the Farceberks lately prompted me to think of how to technically explain the seemingly weird interface of the wheels to the wheel hub of the 500 along with many other Fiat Group/FCA products.

In this mini-series (OK, it will be a few posts) I would like to present the following subjects:

• Section 1: Who in their right mind chose wheel bolts over wheel studs?

• Section 2: Why do we have those dumb little pointy bits coming out of the brake rotor, and why are they made of cheese?

• Section 3: Why the f**k does Fiat still use 4x98?

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.

I encourage folks who have their own commentary, or even better/more accurate information, to contribute to this thread! I will link good commentary by others in this original post. For folks who might have additional questions or items you would like me to comment on, please post them and I will do my best to address it. Such discussions will also be linked here in the original post.

Wheel Bolts vs. Wheel Studs
I won't go into the extremely basic explanation of what wheel bolts or wheel studs look like, but rather the more technical aspects dealing with the comparison of bolts and studs.

Overall, this really seems to be a matter of preference and perhaps historical practice and “design inertia” that has been going on for some time. In terms of major manufacturers, American and Japanese cars still typically use wheel studs as far as I am aware. Today, many European manufacturers are using wheel bolts.

But what are the benefits and drawbacks of each? Perhaps reviewing this we can arrive at a theory on why it would be a reasonable decision to select wheel bolts for the Fiat 500, from the perspective of the original manufacturer.

Wheel Studs
Generally from an OEM, wheel studs will be of the following configuration:

• Large, thin, round flange head: Provides a stop during assembly, and prevents the stud from being pulled straight out of the wheel hub under high axial loading

• Straight knurling/serrated neck: Provides an anti-rotation feature for the stud by digging into the diameter of the holes in the wheel hub through interference/press fit

• Threaded shank: The thing the nut threads onto

From the perspective of just the wheel hub, the wheel, and the hardware that connects the two, I can break down the following steps of manufacture which are specific to this wheel attachment method:

Stud

• Raw material is cut to length

• Round head shape is formed

• Knurling/serrations cut/rolled into the neck

• Threads cut/rolled into the shank

Nut

• Raw material is forged into the rough hex shape

• Threads cut into the inside diameter

Wheel Hub

• Round holes drilled in the wheel hub

Assembly

• Studs are pressed into wheel hub

• Wheel is hung onto hub, piloting over the studs

• Nuts are installed onto the wheel studs and then tightened down

Wheel Stud Advantages

• The wheel studs help to align both the brake rotor and the wheel into the correct position for assembly

• During disassembly/reassembly, it is probably easier for an operator, owner, or technician to see whether the nut is correctly aligned with the threads on the studs

• Removing and replacing a failed wheel stud is possible, without necessarily requiring a new wheel hub (low cost of repair)

Wheel Stud Disadvantages

• Higher part count compared to wheel bolts (more cost)

• Manufacturing of the individual component parts and the assembly process requires more steps compared to wheel bolts (more cost)

Wheel Bolts
This is about as simple as you can make it. The bolt has threads, and there are holes in the wheel hub that also have threads. The bolt runs down into the threaded holes in the wheel hub. In effect, the wheel hub is acting as the nut for the bolt. When the joint is fully seated, the thread engagement between the wheel hub and the bolts is providing the resistance to pull-out from high axial loads.

The steps of manufacture specific to the wheel bolt attachment method are as follows:

Bolt

• Raw material is cut to length

• Hex head shape (typical) is formed

• Threads rolled/cut

Hub

• Round holes drilled

• Threads tapped

• (Note: With modern tools it is possible to drill and tap with a single cutting tool, combining both of these into a single step)

Assembly

• Wheel hung onto the hub

• Holes in the wheel aligned with holes in the hub

• Bolts installed into the hub and tightened down

Wheel Bolt Advantages

• Low part count = low cost

• Fewer steps to produce the parts = low cost

Wheel Bolt Disadvantages

• Can be easier to cause damage to the thread engagement, due to poorer visibility of the interface

• Damage to the threads on the “nut” (wheel hub) will at minimum require repair, often require replacing the wheel hub (higher cost of repair)

• No self-aligning ability for the brake rotor and wheel

Conclusions
My assessment on the use of wheel bolts on the Fiat 500 platform specifically is going to come down to cost. Because both wheel studs and wheel bolts functionally can work just as well, the simpler and lower cost wheel bolt option seems to make the most sense to me for a high-volume, low-cost budget car, which the Fiat 500 fundamentally is.

Rotor/Wheel Locating Pins – Why?
You remember the point I made above with having to align the holes in the wheel with the holes in the hub when using wheel bolts? Now imagine you’re an assembly line operator and you have to do that on every single car down the line. Now imagine you’re on that line and there are 90 cars coming down the assembly line every hour.*

Simply put, from a production perspective they cannot wait around while the line operator fiddles with aligning the brake rotor holes and wheel holes with the holes in the hub on every wheel.

Enter the locating pin/stud. These thread into the wheel hub after the brake rotor is installed, keeping the brake rotor holes aligned with the hub holes. At the same time, the protrusions will help to align the wheel, by fitting into drilled holes and counterbores on the back of the wheels.

After the car is built, the locating pins aren't strictly necessary. They do still fulfill the original role of helping to align the wheels and rotors for assembly while the vehicle is being serviced, but you can go without them if you want.

But why are they made of cheese?
It is a common issue that the locating pins get damaged or broken when being removed, especially after some years and with development of rust. Although it is not functionally a problem if they break off during service later in the car’s life, it is understandably annoying. My explanation of the design choices here will not make such a situation any less annoying, but it may help for understanding why they did it in the first place:

• First, the actual amount of loading that the locating pins take during installation is very low. To serve the basic function, the pins need not be tightened to particularly much torque, either. The pins will be tightened down enough so that they fully seat to the brake rotor, and don’t wobble/vibrate around too much.

• Second, the duration in which the true use of the locating pins is required is very short, and even under such circumstances the loading is still quite low.

As a result, there is no real design requirement for the locating pins to be manufactured out of a particularly high grade of fastener steel. And further, since adding the locating pins is primarily a benefit for ease of manufacturing, any additional strength beyond that which is required to fulfill the original purpose is unnecessary cost.

Conclusion
The locating pins are made out of "cheese-grade" steel because they are only really required during vehicle assembly and don’t see particularly high loading.

*Note: The figure of 90 cars per hour is some rough math based on figures showing 1.5 million 500s have been sold globally from launch until 2015. Compared to this total number, the volume sold in North America is almost a statistical error, with the vast majority of units having been produced in Tychy, Poland. Going with 8 years of production through 2015, a guess at 200 production work days per year, and a workday of 10 hours. This calculates out to about 187,000 cars per year, about 930 cars per day, and about 90 cars per hour … just making nice round numbers. To put this in perspective, this would mean that at any given work station on the line, they have about 40 seconds to complete their job before the next car shows up.

Very through write up, but why 4x98? The metric system loves the nice even multiples of 100. Why wouldn't the engineers?

KyleAllOneWord wrote: ↑Thu Nov 22, 2018 2:08 pm Very through write up, but why 4x98? The metric system loves the nice even multiples of 100. Why wouldn't the engineers?

That is a fantastic question! And I very much think the same about multiples of 100.

At the moment I have not figured out or found a compelling design reason for this choice, but I have some speculation which is coming in my next post.

Why 4x98?
Although I haven’t yet discovered specifically which was the first car to use 4x98, perhaps the strongest “blame” as to why it has survived so long can be cast upon these two:




The original 600 was launched in 1955, and produced in some form or another by Fiat until 1969. The second 500, and probably the best well-known globally, was introduced in 1957 and produced until 1975. Just the 500 and 600 together account for a total of about 9 million cars. While these cars were still being produced, additional new cars also using 4x98 were introduced by Fiat, including the 850, 124, 128, 127, X1/9, 132, 126, and the 131 (eventually accounting for another 20 million cars).

During this time, the 4x98 also migrated to use in other brands such as Lancia and Alfa Romeo. Lancia was purchased by Fiat in 1969, and in the 1970s and well into the 1980s Alfa Romeo was struggling financially (later purchased by Fiat in 1986), and perhaps to save costs they picked up use of 4x98 hubs and wheels from Fiat Group or suppliers to Fiat Group? (Pure speculation)

From non-exhaustive research, at least 74 different car platforms just within Fiat Group have used 4x98 from as early as 1955 continuing in an unbroken chain through today. The usage will increase even further if you include license-built, reverse-engineered, and badge-engineered cars which have been built all over the world, especially in the former Eastern Bloc (see AutoVAZ/Lada and Zastava/Yugo for examples).

So really, 4x98 is a wheel bolt pattern that Fiat has been using on its products for over 60 years, and most likely, they have not seen a compelling reason to change.

But why 4x"98"?
Pure speculation: Perhaps it was meant to be deliberately different from 4x100? From anecdotal evidence of an autocross friend, the 4x100mm bolt pattern was in use in Italy during the 1950s on Alfa Romeo products - the example car was a 1956 Giulietta Spider. At such time Alfa would have been a competitor to Fiat, as this was long before the merger where Fiat took control of the struggling Alfa Romeo.

The engineer in me sees 100mm as a nice round number, when working within the SI system of measurements, to pick for a wheel bolt circle that would be used on compact car platforms. 98mm certainly doesn’t even nicely line up with a conversion from English units; the closest would be 3 7/8” = 98.425mm.

Perhaps Fiat used 98mm instead to prevent people trying to switch wheels between the two competitor companies?

Why 4x98 today?
From the perspective of Fiat Group/FCA as an original equipment manufacturer (OEM), they will be making their own wheels or contracting suppliers to produce unique wheels for them, specifically to fit their cars and align with the overall commercial and engineering goals relating to cost, durability, and styling.

The frequent question I see is on why today Fiat does not use 4x100? Understandably this would make the work of car modifiers and tuners much easier, because 4x100 is a significantly more common bolt pattern used around the world. However, the market for aftermarket wheels has approximately zero impact on an OEM. Fiat Group/FCA will not profit or benefit directly in any manner if, for example, a guy in America is able to directly bolt on a set of 4x100 Enkei RPF1 wheels to his Fiat. They profit from the original sale of the vehicle, and in the sale of OE replacement parts and accessories, which will have already been designed for 4x98.

"Design inertia" is a common reason for particular engineering decisions surviving long after the original reason for adoption. Especially in the case of high volume production, validated designs will have been in place for a long time, machine tooling will have been created and paid for, and vast quantities of parts will already be out on the market in service parts depots, dealerships, and other parts suppliers. Engineers, engineering management, and value analysts ("bean counters") together will often not have a compelling reason that it should be different. "If it ain't broke, don't fix it."