What are the determining factors when a vehicle is designed in terms of how many lugnuts are required for a given wheel? I googled and didn’t really get good answers.
I assume more lugs=heavier duty since the 1500 trucks I sell use six and the 2500HD and beyond trucks always use eight. But what about cars that use four? Or five? Perhaps six?
What in the design/engineering process determines the amount of lugs needed for a given vehicle? Stress factors like weight? The duty of the vehicle in question? Does more lugs always equate to heavier load distribution or something?
The engineering consideration is the clamping force. Each lug produces a certain amount of clamping force at a specific torque. This multiplied by the number of lugs gives the force clamping the wheel to the hub. Add a suitable factor of safety and if you know what clamping force you need, you know how many lugs you need.
More often than not, the marketing department has a bigger influence on the number of lugs than the engineering department.
I always thought the bigger the torque output, the more lug nuts you need to keep the wheels from flying off. It would make sense that a light compact car would need fewer lug nuts than a pick-up truck that would need a lot of torque especially with a payload, or when towing something.
I agree that makes sense and is what I thought, but what are the parameters? Are some six-lug vehicles over-engineered then, some not? Again, this is a curiosity but there’s an engineering answer here somewhere.
The Honda Integra came with 4-lug wheels, in a 4x100mm bolt pattern. When the Integra Type R was introduced in Japan, it came with a 4x4.5in bolt pattern. That was the same bolt pattern as the Accord/Prelude, but the suspension was totally different and the wheels were unique to that model, so it was an added expense. After 2 years, they switched to a 5-lug wheel (5x100mm) for the Type R, while the regular models, up to the SiR (US GS-R), stuck with the 4x100 pattern.
The Type R was not heavier than the other Integra models, nor did it make appreciably more torque. And yet, Honda went through great trouble to muck with the bolt pattern and lug count. Thing is, people regularly add gobs of horsepower to their base-model Integras and beat the snot out of them on racetracks without issue.
This was a long way of saying that I have no idea. I’m sure there’s some analysis done by fancy mechanical engineers, of which I am not, but in practice it seems arbitrary. My WAG is that it has something to do with wheel quality as well. The Miata, for instance, went from 4-lug wheels to 5-lug without any bump in power (between the 2005 Mazdaspeed and the 2007 base model, for instance) or weight, but the '07 had 17" alloys as standard equipment. 5-lug wheels may have allowed them lower tolerances in wheel strength or something. Or it could have just been that all of their cars had 5-lug wheels at that point.
Interesting replies as always. Waiting for a definitive answer. Why does a Corvette have “x” amount of lugs versus a same model year truck? For example.
I think the major issue is that the more distance you have between the nuts, the more leverage the wheel has to wiggle the nuts loose. It’s not the output of the engine per se*, but that heavier duty axles have bigger wheel hubs and so you need more nuts just to go around it and spread the force holding the wheel on evenly.
*after all you can often get the same engine in a 5-lug 1/2 ton or an 8-lug 1 ton truck
There is also the issue of shear force on the lugs - when the vehicle accelerates, the edge of the lug hole is forced against the lug - not enough strength in the lugs, and the lugs get sheared off.
I was raised to hold a 4-lug wheel very strongly against a car - you don’t see 4 lugs on many cars - the Falcon being an infamous example - when, in the 64 1/2 model year, it got new sheet metal and name - Mustang - it still had the 4 lug wheels (or so I was told).
I’m not a mechanical engineer, so take this with a grain of salt, but as far as I know the bolts should be experiencing little to no shear forces.
The purpose of the bolt is to provide a compressive force from the wheel to the mounting plate. The interface between these two surfaces is what provides resistance against the shear forces. The bolts are held in tension, and need to be strong enough (with extra margin) to provide the necessary compression.
It is like holding two blocks together with a thick rubber band. The band is stetchy and provides almost no resistance to shear forces. But the blocks are difficult to slide against each other because of friction. The shear strength of two flat surfaces held together can be very high.
I’d expect that part of the answer here is redundancy. The more lugs, the less you lose (fractionally) if one breaks.
More lugs provide greater clamping force however you generally wouldn’t design a new wheel and tooling for a vehicle, just choose from a few generic patterns, for example a large Ford SUV and a tiny Suzuki 4WD have the same lug pattern
I am reminded of a story (no cite) that when the first motorways were built in the Uk the overhead bridges were very slim. This was one of the benefits of new engineering techniques and materials.
The problem was that they just looked too thin, so the Ministry of Transport had plastic panels added to the sides to make them “look” right.
The parallel with lug nuts, is that the designers will want more of them on trucks that are perceived as ‘rugged’ because that satisfies the buyers expectations. Of course, if they cost 50 cents each, the bean counters will want fewer of them.
Mind if I add on a related question? Over the past 10 or so years there seems to have been a move from lugs on the hub and lug nuts to using bolts that screw into the hub. Any engineering reason for this?
Mechanically, they’re basically identical. However, lug bolts (as opposed to lug nuts) have the advantage that if the threaded part is damaged somehow, you just need to replace a cheap bolt instead of the entire hub.
I’ve only seen this on some German cars. I guess it’s theoretically cheaper in that instead of a stud which has to be pressed into the hub and a separate nut that threads onto it, you have a bolt which does both jobs.
but let me tell you, it’s that much more of a pain in the ass to get the wheel lined up correctly and try to thread in one of those bolts at the same time.
what? studs are pressed into the hub, if one’s damaged you just knock it out with a hammer, then draw in a replacement using a nut and some washers. if you have bolts and the threads on the bolt are damaged, then yeah, you just need a new bolt. but if the threads in the hub are also damaged, you need a new hub.
Dunno, but my general answer from a general engineering viewpoint is that it’s cheaper. To me, this implies that they had difficulty threading the hub before, but now find that it’s cheaper to thread the hub then to add lugs to the hub. Which would suggest a change in the material or the material processing of the hub.
After watching several UK car shows (Wheeler Dealers etc.) I noticed that this seems to be the norm there and Europe in general (lug bolts instead of lug nuts). In American lug nuts on hub studs is essentially universal and has been for a very long time.
As an aside, the Peugeot American-market Le Car from the early 80s is the only modern car I’ve ever come across that had only three lug nuts per wheel. Not surprising, in that it was a cheap piece of shit like the later Yugo…
I am a mechanical engineer, and I can confirm this is true. The friction between wheel and hub must resist the engine’s accelerating torque, the brakes’ braking torque, and the vertical load from the weight of the vehicle (the latter of which includes potentially very large forces due to bumps in the road) ; the lug bolts/nuts are just there to ensure that there’s no slippage in that face-to-face joint.
Maybe they’ve got the same engine, so the wheel/hub joints will need to tolerate the same accelerative torque. But the truck is going to be designed for a much higher gross weight, which means greater vertical loading and greater braking loads. Example:
2015 Corvette, GVWR = approx. 3700 pounds
2015 Ford F250, GVWR = 10,000 pounds
the gross weight rating will be based on the weakest link between the car’s body and the road, which might not be the wheel/hub joint. But the manufacturer won’t go to the expense/trouble of making that joint way tougher than it needs to be (unless the stylists insist).
Thanks for the quick answers. I guess it could be a European thing, I only started noticing it when I moved to the UK. It definitely makes switching from winter to summer tyres/wheels on my BMW a pain in the arse.
As for 3 wheel bolts, the Smart ForTwo has had only 3 since introduction. There are probably other cars in the same class with only 3. There’s an old Citroen 2CV down the street from me that has 3 as well.
There’s at least one current American-market car with three lugs – the Smart car. I know the Tata Nano in India also has three lugs. I can’t think of any others.