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Kenova
04-24-2011, 11:40 AM
While browsing through another forum I found someone who was using hollow bolts in the front suspension of a street/strip car.
Just how much strength does a hollow bolt give up when compared to a similar sized and rated solid bolt?

Ken

Twentyover
04-24-2011, 01:07 PM
Depends on the bore size of the bolt.

CarlC
04-24-2011, 05:57 PM
I agree with Greg.

On life-critical parts it's just not worth the weight savings.

67zo6Camaro
04-24-2011, 10:15 PM
From what I know, which is not much.... Most hollow bolt applications use Titanium or 4130 chromoly material. These material have a much higher tensil strength than your common grade eight bolt. Therefore, If you have the money and you are trying to save every oz. possible for some extreme competition, then the hollow bolt may be your option. And, depending on the maker and/or builder of the bolt they do have sizes that will fit most suspension applications and some of these pre made bolts might even have tested spec numbers and certifications to go with them.....$$$$$$$
Cool stuff at any extreme.

Brett

mrn2obelvedere
04-25-2011, 04:49 AM
While the tensile strength rating of different materials certainly will make up for strength deficiencies to some extent, I think the best reason that you would want to avoid hollow fasteners in a suspension application is because for the most part these fasteners will be loaded in shear. Unlike torsional loading (such as sway bars), the center of the cross section is subject to considerable stress in a shear loading scenario. In the shear loading case, the stresses are inversely proportional to the cross sectional area of the part. In other words, as the cross sectional area is reduced, the stress in the part is increased. In the example of the hollow bolt, the removed material in the center of the bolt represents a significant reduction in cross section, and thus a significant increase in stress.

That said, there are a couple of things that you can do if you wanted to use hollow fasteners, but you need to do your homework. One thing you can do is to use dowel pins in the interface of the parts. This way the dowel pins are supporting the shear loads and the fasteners are only subject to tensile loading from tightening. The other thing that you can do is estimate the loading that a fastener will see in service and see if the fastener will be capable of supporting those loads. However, this can get complicated very quickly, as determining the loads can be tricky and also the structural analysis can be difficult, especially considering that the loading is not static (for instance on a shock mount the loads are constantly changing direction).

Randy67
04-25-2011, 05:14 AM
Some here:
https://www.lefthanderchassis.com/v2a/viewproduct_group.asp?idgroup=2512576
Pricing is fairly reasonable, lower than I expected. I personally stick with solid bolts just for piece of mind and easy replacement.

dontlifttoshift
04-25-2011, 07:01 AM
http://www.racebolt.com/

These guys are local to us. It was a divsion of outhouse racing who built some very fast sprint cars. there is some good information on this site.

fishtail8
04-25-2011, 06:00 PM
I'd be saving that stuff for the race cars. The street can be pretty rough on equipment, more so than some laps on a racetrack. I don't think it'd be worth the risk for the minimal weight savings on a street car. Some guys love spending money on the fancy stuff, we used to sell full titanium bolt kits for sprint cars chassis that would save maybe 5lbs overall. There was always that guy who had to have it, funny part was he didn't win much.

Kenova
04-25-2011, 06:15 PM
Thanks for the links. I was having no luck at all trying to find a source for them.
Between the scarcity and price, I won't be using hollow bolts anytime soon. I asked more out of curiosity.
I guess someone could step up a bolt size to alleviate any durability or safety concerns when assembling a suspension, but would that really result in a weight savings?

Ken

mrn2obelvedere
04-26-2011, 04:19 AM
If you are looking for every last ounce of weight savings, you could save a few pounds if you used fasteners like this everywhere you could. I think that if you have to go up a size then the weight savings would be negated quickly. This is why I was saying that you would need to analyze whether or not the hollow fastener would be applicable in each situation.

CarlC
04-26-2011, 07:09 AM
There's also the hassle factor. If you up-size a fastener then you have to find an A-arm bushing, suspension part, etc. that has a similar size. That's way up there on the PITA scale.

Area is area. Since tension and shear is calculated on area there is no difference. In other words, if you need 0.100in^2 of material to provide the neccessary shear strength, you still need that amount of area no matter what the size of the fastener. So, going bigger does not get you anything.

Those that use hollow bolts may point out that they have not broken one. That's because the fastener joint has a big safety factor and the load on the joint has not exceeded the limits of the fastener.

If there is anyplace where a lightweight fastener could be used it's in aerospace. I'm not an airplane mechanic or engineer, but I can't remember ever seeing a hollow bolt on any structural component on any of them that I've stuck my head in.

For me, the best way to save the 10lbs that MAY be able to be saved by using hollow bolts on all of the large fasteners on the car would be to instead loose that same amount of weight that is applied to the drivers seat.

fbody_mike
04-26-2011, 07:17 AM
I have been out of dirt bikes for a long time, but back in the early 90s I can remember a few of the Kawasaki 250s having hollow suspension bolts. I think it was the axle bolts and maybe even the swing arm pivot bolt.

Kenova
04-26-2011, 09:48 AM
For me, the best way to save the 10lbs that MAY be able to be saved by using hollow bolts on all of the large fasteners on the car would be to instead loose that same amount of weight that is applied to the drivers seat.

LOL I agree, and I've started to work on that.

Ken

CarlC
04-27-2011, 04:02 PM
I have been out of dirt bikes for a long time, but back in the early 90s I can remember a few of the Kawasaki 250s having hollow suspension bolts. I think it was the axle bolts and maybe even the swing arm pivot bolt.

Pretty common, as they are on many street bikes that have the single leg swingarm. However, they are very large in diameter, so getting sufficient shear area, and in the case of the single leg swingarm, a moment loading, is pretty easy. The reason that they have such a large diameter is not to make the bolt hollow, but to allow for a large enough bearing. A solid axle bolt with a 40mm bore would be overkill, but is necessary to get a bearing large enough to carry the applied loads.

Bryce
04-28-2011, 05:42 AM
Shear strength is based on the cross sectional area.

The stress concentration is based on spreading the load out over a larger diameter.

johnny68
04-29-2011, 07:17 AM
and dont forget the position of the sun to mars and the gravuitaional pull of the muffler bearing x pie of course
(now guys im just joking around here i is as brightum as um a box um rocks) just trowing some humor in friends
john ( just jealous i dont get all this tec stuff)

parsonsj
04-29-2011, 07:59 AM
Salad bar, baby!

monteboy84
04-29-2011, 10:43 AM
The ONLY place we use hollow bolts on our race car is for the dust caps on the front hubs, where the drive flange would be bolted on if the hubs were used on the rear. IMHO the best rule any racing class can implement is a minimum weight rule, to save people from themselves. If you have a minimum weight rule on the cars, then trying to shave weight off things like critical suspension bolts becomes quite pointless, and things like titanium bolts become an unnecessary expenditure.

-matt

NJSPEEDER
04-30-2011, 11:18 AM
IMHO the best rule any racing class can implement is a minimum weight rule, to save people from themselves. If you have a minimum weight rule on the cars, then trying to shave weight off things like critical suspension bolts becomes quite pointless, and things like titanium bolts become an unnecessary expenditure.

-matt

I have to disagree with this statement.I have worked on many many race cars over the years that were built to the rules of classes with minimum weights and that didn't prevent us from taking advantage of every opportunity to save weight. The reason is simple, if you have to carry weight around why not decide yourself where it is.

I wouldn't consider hollow bolts in the suspension a good plan regardless of material. Saving weight is a great thing but I am not gonna risk my life for it. Save the hollow bolts for holding the fenders, dash, and other no load items in place.

-Tim

exwestracer
05-02-2011, 08:23 AM
We have used them in many double-shear locations on the suspension of asphalt supermodifieds with no problems whatsoever. I'd think twice about loading a hollow bolt in tension or single shear, and beware of drilled bolts out there on the market. They are NOT the same quality as the "Race-Bolt" brand.

grenade inspector
05-26-2011, 06:11 PM
Area is area. Since tension and shear is calculated on area there is no difference. In other words, if you need 0.100in^2 of material to provide the neccessary shear strength, you still need that amount of area no matter what the size of the fastener. So, going bigger does not get you anything.


this only hold true given a constant set of material properties, if you were to step to titanium or chromoly the required area decreases and therefore the complete bolt is no longer *necessary* to retain the same safety factor.

that being said I 100% agree with this:


For me, the best way to save the 10lbs that MAY be able to be saved by using hollow bolts on all of the large fasteners on the car would be to instead loose that same amount of weight that is applied to the drivers seat.


there are a whole lot of things I would be losing before I chased those last few pounds inside my fasteners personally.

jpgolf14
06-26-2012, 07:47 PM
Interesting discussion here. This is an old thread but there is so much funky info going on here, I just couldn't help myself.

Did everyone forget physics 101? All this talk of bolts in shear.

Bolts are designed the provide a clamp up load in a joint. In other words they are designed to squeeze a joint, just like a clamp. This clamp up load (normal force) creates a frictional force in the joint. The frictional force is the normal force * the coefficient of friction, mu. Ff = Fn * mu. The coefficient depends on the friction between the two pieces of the joint. In most cases: steel on steel, aluminum on steel, or aluminum on aluminum. A commonly quoted value for mu for a steel on steel joint is 0.8. That means that the frictional force is 80% of the normal force.

Ok, lets look at Normal force. This is the preload created by the bolt being torqued. Lets consider a bolt with 1/2" thread. 1/2" bolts are typically torqued to around 100ft lbs. 100ft lbs on a 1/2" bolts results in a preload or normal force of about 12,000lbs.

Now Ff = fn*mu = 12000*0.8. The friction force is 9600lbs. This is the amount of force the joint can take before the joint starts to slip. In other words, the joint takes up the first 9600lb of shear force. If the shear force exceeds 9600lbs, the bolt begins to accept some of the shear.

In other words, the bolt sees ZERO shear unless the load on the joint exceeds 9600lbs. Bolted joints are designed specifically such that the bolt sees no shear force. This is standard practice. Bolts in shear is a big no-no.

Now lets consider a heavy 4000lb car taking a turn at 2g's. Incredibly unlikely. Now lets assume all of the weight has transferred to the outside of the car (unlikely) and all of the weight is on the front of the car (unlikely). So we now have 8000lbs pulling on an a-arm which has two 1/2" bolts, each in double shear. Since they are in double shear, each bolt has two interfaces that can each take 9600lbs. The two a-arm bolted joints can take 4*9600 = 38,400lbs of load. Much more than the 8000lbs they are seeing. In fact the 4000lb car would have to be pulling 5g's around a corner in order for the bolts to see any shear load.

That is a pretty good safety factor if you ask me. There are very few applications where bolts take shear. A lot of aircraft structure in designed this way for a worst case scenario, but will never see shear in service.

John

johngross
06-27-2012, 05:01 AM
I understand your logic and don't profess to be an expert, but it seems like the length of the arms could make a great multiplier for the forces acting on the various bolts. Of course, due to the fact that cars aren't breaking all over the streets, it is pretty sure that there is a large enough safety margin on the various strengths of the bolts.

Norm Peterson
06-27-2012, 05:45 AM
The multiplier would involve the heights of the control arm pivot heights. The further you can get them separated, the easier the job is for the bolted connections. But this has implications with respect to geometry and knuckle design, so it's not exactly a "freebie".


Theoretically, most bolted joints should never put the bolts in shear. But that assumes that the fasteners are properly torqued, that none of that torque is "wasted" pulling the rest of the pieces together, and that the joint remains tight at the as-designed clamp load.

I'm thinking it gets really messy if you were to try to truly analyze the situation where bolts are intended to be placed in shear, in holes with the necessary clearance to be able to reliably install them in a production environment (which means a little movement and some sort of impact factor when the clearance gets taken up in one direction or another).


Norm

jpgolf14
06-27-2012, 03:17 PM
Yeah I was referring to the side load in a high g corner. You could consider a max effort stop as well, but who has enough tire to stop the car at 5 g's?

All good assumptions Norm. Of course it is up to the installer to ensure everything fits well. I always use shims so the bolts don't have to pull parts together. In theory the bolt generally stays well torqued. In practice that generally stays true. I use lock wire on critical joints to ensure the bolt doesn't come loose. Of course over time the material you are bolting can "relax" resulting in lower bolt preload. Generally not a big deal with metals though.

I'll bring up the aviation industry as an example, as they are very conservative. For static strength, you must show that the structure will hold with zero effective preload. In other words the joints are basically pinned and not clamped. The fasteners are assumed to be in shear only, no tension. That brings up the challenges mentioned by Norm. How do you manufacture a joint reliably that takes the shear. First you need lots of fasteners to share the shear. Take a look at a metal aircraft, the skin is littered with rivets. Second the fit between the fastener and hole is very tight. Well actually that depends on the fastener. Rivets are great because they can fit loose in the hole. Then when you buck the rivet, they expand to fit the hole. Wonderfully simple. It just sucks when you have to take the assembly apart. The screw type fasteners fit very tight in the drilled hole. It may even be a slight interference fit, I can't recall. Aviation bolts have long shoulders and only threads where they are needed. The shoulder is what fits in the drilled hole. In a typical hardware store bolt, the shoulder is actually slightly smaller than the threads. That is no good if you want a tight fit. So how do you reliably drill and fill over 1 million holes in a commercial aircraft? Well you hire professional mechanics, pay them $100,000 a year, charge $300 million per plane, and finally you do a lot of rework. Got a hole that is slightly to large due to a dull drill bit? Well step the hole up to the next 1/64" and try again. Of course that doesn't work in the auto industry. The margins are to low. You need to keep manufacturing cost down and you can't afford any rework due to the high rate of production. So you design a joint that has proper preload. It is much easier and faster to drill and fill a hole this way. I will add that when designing an aircraft, from a fatigue point of view the FAA requires you to assume that the bolt has lost half its torque in the life of the aircraft (40 yrs). You know how conservative the aviation industry is.

Even with all of that, manufacturers still add a huge safety factor. In my opinion is safety to say that joints hold enough preload over time to ensure that the bolts don't take any shear.

Norm, you wouldn't happen to be an old time thirdgenner? For some reason I recall your name from thirdgen.org

Norm Peterson
06-27-2012, 05:01 PM
Yeah, I've been a member over on thirdgen even longer than here.
[/hijack]


Norm

exwestracer
06-28-2012, 07:35 PM
61330

The "clamp" theory is valid, but we can't always assume people are smart enough to mount critical attachments in double shear. :rolleyes: (Not bashing O/F Racing, my cars are exactly the same, along with every other supermod out there)

rustomatic
06-28-2012, 09:10 PM
Weight in paperclips... I've sheared hollow bolts on bicycle sprockets while riding; that said, as someone mentioned above, motorcycles do use large diameter hollow bolts for wheel mounting, especially on the front (good for mounting axle sliders with a through-bolt). This works, if you can choose the diameter (large) of the bolts you're replacing...

HarleyR
07-03-2012, 03:15 PM
Is that taken in Madera?

Ron Sutton
05-24-2013, 10:33 AM
Yup. That is Madera.

Ray, how did you get that photo? Your info says you're in PA.