to all those engineering gurus, help me out!

[blizzard335]

Heres the situation

My Professor has asked me to write up a short article on "Why does a racing car have a single lug nut, where as a normal road car has 4 or 5?"

Im stumped. I want an engineering point of view, ie; strength, design, materials, maybe some physics (mass moment of inertia, etc...)no calculations involved, just theories.

ive never thought of it other than time advantage (faster pit stops, LOL), but he doesnt want that. and safety issues dont count either, 5 nuts > 1 nut, LOL

help me out guys.

thanx

[AccordWarrior]

Gah, good question...I always assumed it was a time thing.

Take a look at the sheer strength of the single lug versus 5 smaller ones. I'm assuming it's also a weight issue that if you move more of the weight of the wheel into the center you're lowering...blah, I used to know all about that, but it's been almost 5 years since I had an engineering physics class.

But I think by locating the lugnut in the center, you're moving the stress to just the center, or something along those lines.

[blizzard335]

Gah, good question...I always assumed it was a time thing.

Take a look at the sheer strength of the single lug versus 5 smaller ones. I'm assuming it's also a weight issue that if you move more of the weight of the wheel into the center you're lowering...blah, I used to know all about that, but it's been almost 5 years since I had an engineering physics class.

But I think by locating the lugnut in the center, you're moving the stress to just the center, or something along those lines.

thanx man

i was already thinking in the same lines, reducing the moment inertia, but to what extent?

less sheering force?

weight is negligible, i think.

[steelbluesleepR]

i cannot speak for all race teams, but UMR's FSAE team's wheels are only held on by the "jesus nut" that is torqued down to about 60 lb/ft. the nut holds it onto five cone-shaped pins that actually send the torque and power from the engine to the wheels. the advantage, i believe, is the center bolt gets tightened even more by the power of the engine, as one side is reverse threaded (lefty-tighty). another advantage would be the speed issue, as it takes less time to change one huge nut than 5, as well as the huge one is harder to shear.

[AccordWarrior]

thanx man

i was already thinking in the same lines, reducing the moment inertia, but to what extent?

less sheering force?

weight is negligible, i think.

Maybe this is nonsense, but I was thinking in my head...with enough torque on the wheels, you could in theory snap the studs off. If you go to a bigger wheel stud, you'd need more force to snap it. Plus you'd be working with distributing the force over the total round stud, rather than 5 separate smaller studs.

Maybe I"m just amkign this up though, because I'm not 100% sure how a single lug works. I think it's definitely related to breaking wheel studs somehow though.

[blizzard335]

i cannot speak for all race teams, but UMR's FSAE team's wheels are only held on by the "jesus nut" that is torqued down to about 60 lb/ft. the nut holds it onto five cone-shaped pins that actually send the torque and power from the engine to the wheels. the advantage, i believe, is the center bolt gets tightened even more by the power of the engine, as one side is reverse threaded (lefty-tighty). another advantage would be the speed issue, as it takes less time to change one huge nut than 5, as well as the huge one is harder to shear.

ive heard of 'jesus nut' but havent actually known how it actually works.

thanx man

is tha carrera GT using the same nuts as the racing cars?

like accordwarrior said, with the same torque acting on a single stud, its suspect to more force, but since the stud is dead centre, there is no moment force applied right? M = F x d. theres always torque from the shaft, tho

[NAiL05]

Wanna research a single lug car? Go look at the porsche carrera gt. And as steel said when your racing every second counts. So a single nut is easier to change and easier to fit onto rather than 4-5 lugs just for a wheel. The CGT also has the same thread pattern as the jesus nut. The nuts are color coded to which side they go on.

[blizzard335]

Wanna research a single lug car? Go look at the porsche carrera gt. And as steel said when your racing every second counts. So a single nut is easier to change and easier to fit onto rather than 4-5 lugs just for a wheel. The CGT also has the same thread pattern as the jesus nut. The nuts are color coded to which side they go on.

i cant put the time advantage issue as a discussion.

yeah, im searching around teh intrenetz, but all im getting is lug nuts, racing nuts and car nuts.

i'll try jesus nut

[owequitit]

One of the primary reasons is to save time.

So it is pretty lame from your professor's standpoint to not allow that as an argument.

If it were me, I would tell him that.

One of the biggest jobs of a racing engineer is to save time where possible, because the more time a car spends in the pits, the smaller the odds of victory.

Modern racing is usually determined by pit stops or lack thereof...anybody who is an avid racing fan knows that any time that can be saved in the pits is crucial.

So time savings from the standpoint of the guy who actually exists in the real world and not in some classroom (i.e. a REAL engineer, not some accredited blow hard), is absolutely CRITICAL.

The Jesus nut saves time because you have to replace 1 nut and not five. They are also very large, which makes them easy to work with in a hurry without having to have super fingers.

Have you ever dropped a lug nut and had to fiddle with it? That is time a race team may not have in certain situations. In endurance racing, it might not break the bank, but in a 2 hour sprint that occurs at 200MPH, 1 second puts you another 300' behind where you could have been.

4 seconds, and you have lost 1/4 mile.

You lose 1 mile every 18 seconds.

Another good example of racing to save time is to look at NASCAR.

Their rules don't allow a Jesus Nut, so they HAVE to use 5 lug nuts.

To avoid the time penalty associated with changing 5 lug nuts, they glue the lug nuts to the wheels, so that when they have to put them on the car, they throw the wheel up there, and then hit the lugs. No fighting, no maneuvering, no hassle.

Almost as quick as a Jesus nut, but not quite.

So as you can see, not ALL engineering priorites have to do with saving weight, or adding strength etc. If you can hit more than 1 bird with one stone at a time, then great. But it doesn't always happen.

As far as weight savings go, it might be a secondary benefit, but my guess would be that the primary "engineering" advantage would be increased brake surface area.

Sanctioned bodies like Formula 1 often have regulations that prohibit wheels bigger than a certain size.

In the case of F1, it is 15".

Now if you think about how much brake I have to cram in the center of a 15" wheel to stop an 800+HP car from 200+MPH numerous times per lap, it becomes clear that the more swept brake area (the part rubbed by the pads) you have, the better off you are.

If we can utilize a Jesus nut to make the hub smaller, thus allowing more brake surface area, we are that much ahead of the game.

Realistically, cramming 11.2" diameter rotors into a 15" wheel is pushing it with a conventional hub. Make smaller calipers, and wheels with more clearance, and you might get 12".

But what if we then figured out a way to get the hub smaller so that we increase swept area to an equivalent of 14" or 15" rotors? That is a lot more surface area to put pads on...and we still legally utilize our 15" wheels.

Plus, if we cram more swept area into less diameter, we also have the added benefit of reducing unsprung weight for a given diameter, which aids in better handling.

The Jesus nut allows us to shrink the hub for a couple of reasons.

1) By having multiple lugnuts, we have to have enough room between them to get our fingers in there, and be able to manipulate them. This makes the hub bigger and heavier.

2) By spacing the lugnuts out, we have multiple load paths. That means that the forces that are acting upon the wheel are being transferred into our hub at more points.

That means that we need to have sufficient material at more points further out in order to maintain integrity.

This increases hub size and weight.

We also have to have sufficient material on the wheel to maintain strength with multiple load paths. This adds to the weight of the wheel. This might be exaggerated even more due to the materials used to forge wheels. Magnesium and aluminum are a lot lighter than steel, but they are also not as strong, so for a given load, we need relatively more material...

This adds weight. And not just weight, but unsprung weight, which is the worst possible scenario on a car.

3) All space that is occupied behind the wheel by a hub, can't be used by something useful, such as rotors, calipers, or pads, or cooling air.

And there is the thought process of an engineer in the field. They killed 3 birds with 1 stone.

They saved weight.

They saved time.

And they increased braking surface area.

All while maintaining structural strength.

It may seem like splitting hairs, but in F1 dominance is determined by hairs, so they are important.

Jesus Nuts are fairly common in aviation, because even more than in racing weight is enemy #1.

P.S. The McLaren F1 also has wheels held on by Jesus Nuts.

[steelbluesleepR]

jesus nut is also what they call the nut that holds the rotor on top of a helicopter.

[NAiL05]

An ineffective way nascar keeps lugs is they put a lil deal on there hip where they can pull it off if they drop one instead of having to scrambling for it. Really nifty if you ask me. Tripped me out when i saw that lol.

[cjv998]

This adds weight. And not just weight, but unsprung weight, which is the worst possible scenario on a car.

And not only is it unsprung weight, but it's also rotational mass, so the situation is even worse the further out the lug nuts are (once again, this is splitting hairs in most cases).


Nice post as usual, by the way. That's a great point about increasing the area swept by the calipers, and I hadn't thought of that when I first read the thread. When you mentioned gluing the lug nuts to the wheels in Nascar, do they simply an adhesive with low bonding strength (i.e. Elmer's glue as opposed to Super Glue) so the bond breaks easily when they go to torque the lug nut down, or is it more complicated than that?

Also, maybe it's something about being much harder (read: virtually impossible, given only rotational forces that are lined up with the wheel's direction of rotation...although you can still strip the threads) to snap a single central stud vs. 5 non-centrally mounted studs?

Torque is radius x force, therefore by decreasing the radius (in this case, distance from the center of the hub), the torque on the stud is decreased (brought to virtually zero in this case), right?

[lokuputha]

Time is the main reason 1 center nut is used rather than 4/5/6 separate ones.

b/c time is such an issue, 1 center nut simplifies the assembly, thus reducing the number of errors and variances that may be caused in the racing environment.

this leads to reduced chance of torque variance on the nuts, thus avoiding hub and rotor, warping and imbalance; thus, in turn prolonging part durability and performance consistency.

For the purposes of your report, you could talk about the ultimate engineering rule: K.I.S.S.

have fun.

[steelbluesleepR]

When you mentioned gluing the lug nuts to the wheels in Nascar, do they simply an adhesive with low bonding strength (i.e. Elmer's glue as opposed to Super Glue) so the bond breaks easily when they go to torque the lug nut down, or is it more complicated than that?

Also, maybe it's something about being much harder (read: virtually impossible, given only rotational forces that are lined up with the wheel's direction of rotation...although you can still strip the threads) to snap a single central stud vs. 5 non-centrally mounted studs?

Torque is radius x force, therefore by decreasing the radius (in this case, distance from the center of the hub), the torque on the stud is decreased (brought to virtually zero in this case), right?

ya, they jsut use a glue that breaks easily, i believe.

there is very little force sent through the center nut. there are 5 studs that stick out of the hub and into holes in the wheel. these transfer the force to the wheel, while the center lug just holds the wheel on. they are cone shaped so that a slight misalignment wont keep the wheel from being mounted, it just aligns it perfectly.

[owequitit]

And not only is it unsprung weight, but it's also rotational mass, so the situation is even worse the further out the lug nuts are (once again, this is splitting hairs in most cases).


Nice post as usual, by the way. That's a great point about increasing the area swept by the calipers, and I hadn't thought of that when I first read the thread. When you mentioned gluing the lug nuts to the wheels in Nascar, do they simply an adhesive with low bonding strength (i.e. Elmer's glue as opposed to Super Glue) so the bond breaks easily when they go to torque the lug nut down, or is it more complicated than that?

Also, maybe it's something about being much harder (read: virtually impossible, given only rotational forces that are lined up with the wheel's direction of rotation...although you can still strip the threads) to snap a single central stud vs. 5 non-centrally mounted studs?

Torque is radius x force, therefore by decreasing the radius (in this case, distance from the center of the hub), the torque on the stud is decreased (brought to virtually zero in this case), right?

I would imagine they use something fairly high strength like super glue. You don't want them coming off when you are throwing them around during a pitstop.

If you think about the strength of a few drops of superglue, vs a Big 1/2 drive impact wrench trying to tighten a lug nut, which do you suppose will win...

I think they use a jig to put the wheel on, and then slip the lugs over a non threaded post. Glue it, and viola, they are lined up.

As far as F1, yes the torque would be less, and since there is effectively NO torque moment on the shaft (which you said), I am sure it helps with that too.

Although, with the size of those nuts, and threads, they are probably under stressed.

But honestly, I think more than anything it is for extra brake area and a time savings.

I thought of another area where it saves time. If you have ever changed a tire, you know that you have to work the tire around to get all 4 or 5 holes lined up.

With one central shaft and nut, you simply align the center and BOOM, it is on.