16

You know sometimes you read something and think "What?"

Well, would you be happy to drive a 900 BHP car in a competition with lug nuts missing? Apparently NASCAR has introduced a rule change so that teams no longer have to fit all of the lug nuts after a tyre change. This may see some cars on the track with just three of five lug nuts fitted.

Furthermore, one of the drivers for next season, Tony Stewart, has just been handed down a $35,000 fine for expressing that opinion that this may be dangerous.

The apparent reason for the rule change is that they hope to reduce the number of mechanics in the box during a pit stop.

I have my own opinion on why this is the wrong thing to do. In the early 1980's, Lancia engineers, working as service crew on the Lancia 037 Group B car, devised a wrench which was essentially multiple equal length extension bars with magnetic sockets on the end. The "drive" wrench would run in the opposite direction to what was required and turn a central cog to drive each socket at the same time. This meant all lug nuts could be removed and refitted by a single mechanic.

Multiple lug nut wrench

The obvious alternative to this is an F1 style single centre nut on a large spindle.

NASCAR appear to have ignored this technical solution and have simply changed the rules to allow what I would personally call incorrectly fitted wheels.

Now, I've personally used a car with one nut removed from each wheel after someone tried to steal my wheels, removed all but the locking nut from one wheel and obviously took the nuts with them. This was only done to get the car home where the missing nuts were replaced.

I've also seen the damage that can be done to a wheel with an incorrectly fitted retaining nut as the Viper did one complete lap of Silverstone with an incorrectly fitted front wheel. When it returned to the pits, the wheel was full of swarf and significantly damaged.

So, my question is this; would you consider it safe to use a car with missing wheel nuts / lug nuts?

(Anyone involved in NASCAR professionally probably shouldn't answer this question for fear they receive a fine too).

EDIT

As suggested, to clarify what I mean by "is it safe?"

I wondered if there are any engineering calculations that can be applied to different nut configurations? Is there likely to be increased failures running in the reduced lug nut configuration? Have there been any case studies into this?

I don't just want an opinion of "Oh, I think it'll be fine." or "Oh, I don't think it's a good idea."

  • 5
    That is one of the coolest tools that I've ever seen. That said, this is an opinion question as written. If you changed into something like "what are the likely failure modes of a four nut wheel running with only three?" Or something similar, it would approach answerability. – Bob Cross Apr 22 '16 at 15:23
  • 1
    Three nuts is safe as long as they make up as equal a triangle as possible (i.e. they are on different sides of the centre axis of the axle/hub) if they are all on the same side of the centre axis you may have issues. Citroen AXs from the 90s only came with three nuts for the wheel unless you got the GT, then you got 4 nuts per wheel. – Mauro Apr 22 '16 at 15:33
  • 1
    @Mauro I wonder why the performance model had four though. Also, I don't know if there is anything in the new rules to say the must be fitted as a "triangle" if they accept three nuts in any of the five holes. – Steve Matthews Apr 22 '16 at 15:35
  • 2
    If they are only required to install 3 lug nuts, why not just build the cars with 3 studs/nuts? (joking!) "I m wearing 3 pieces of flair! if you want me to wear 5, then make the minimum 5!" – rpmerf Apr 22 '16 at 15:42
  • 1
    I've wondered how they choose the number of lug nuts. There was a similar thing on 80's dodge cars. They were using 4 lug, then they moved to 5 lug. Some trucks get 5, others 6, 3/4 tons usually get 8. Wondering how much the torque on the lug nuts effects the number of nuts. wheel diameter, vehicle weight, etc, Personally, I would consider 3 lugs on a 5 lug car to be 'limp mode' – rpmerf Apr 22 '16 at 15:51
4

Quite simply, no it is not safe. If a car manufacturers specifications say 5 studs, that's because they've done their homework on the forces involved. Think of the worst case: braking hard while turning sharply on a rough surface. The mass of the car, acting at an acute angle to the rim, comes down on the wheel lugs with a g-force proportional to the deceleration x mass. If the wheel is at the point where the two missing studs are aligned vertically, all this force is borne by the single lug left that side of the wheel's axle - a recipe for stripped threads, wobbling wheels and in a race: injuries or worse.

This 'decision' is lethal.

In mathematical terms, the wheel now has 3/5 of the strength, with 5/3 of the load applied to each stud. Not safe at all.

  • 3
    Your load case fails to consider the dynamic nature of the wheel spinning at thousands of RPMs, the movement of suspension, flex of other components, etc. it's a gross oversimplification that does not really apply. You also cannot definitively say, without knowing more about the design or the actual load cases seen, that 3 studs or even 2 studs is unsafe in this particular design. The comparison of NASCAR to production cars is also invalid, as you do not know the design differences. Simple example: NASCAR may be using 350ksi maraging steel studs, vs production cars using 150ksi HSLA studs. – CBRF23 May 10 '16 at 1:20
5

Engineers typically design parts to withstand a greater load than they are actually expected to see. This is known as a "factor of safety" and assures the parts won't fail when extenuating circumstances are encountered where the product is stressed beyond the design calculations considered. Given that NASCAR is a very specific type of racing with (assumingly) well understood load cases and tons of telemetry data, it's reasonable to assume they can run a lower FOS than you (the amateur without millions to spend on R&D and without hundred of engineers and data analysts on staff) or the typical manufacturers (who must account for conditions of use by all sorts of people, for all sorts of purposes, in all sorts of environments, all over the world) would normally allow.

For example, let's say a single stud has a shear strength rating of 160ksi. If we have five studs, and we assume a perfectly even distribution of load (for simplicity's sake we will consider only the studs and only the torque of wheels, which we will again simply assume applies only shear load) than our five stud pattern can handle up to 800ksi total load before failing.

Now, if we knew the designer targeted an FOS of 4, then that means the design is capable of handling 4 times the normal expected load, so if we divide 800ksi by 4, we can determine the expected total normal load is 200ksi.

Theoretically then, we should, under normal expected conditions, be able to "safely" handle that load with only 2 studs (200/2 is 100 which is less than 160).

Finally, a few things to consider before you throw away half your lug nuts :P

For one, we (you and I) don't know what the original loads are or what the designed FOS number is - it's not something in the spec sheets, and you don't know if it was 1.5, 2, 4, or 6 - so without doing all the calculations ourselves to determine the normal loads, and knowing all about the design, we can't objectively say whether or not 1 or 5 studs is "safe."

Second, race cars quite often run at a much MUCH lower FOS than production cars. This is what you call "pushing the limits" and part of the reason why stuff breaks on race cars a lot more frequently and a lot differently than on production cars. So where a normal production car might run an FOS of 5, a race car might run at 1.5 - allowing them to remove more material and save weight (or in this case time) with the trade-off being less margin of error and more risk of breaking something.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.