Hollow Bolts

[Kenova]

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]

Depends on the bore size of the bolt.

[CarlC]

I agree with Greg.

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

[67zo6Camaro]

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]

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]

Some here:
https://www.lefthanderchassis.com/v2...dgroup=2512576
Pricing is fairly reasonable, lower than I expected. I personally stick with solid bolts just for piece of mind and easy replacement.

[dontlifttoshift]

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]

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]

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]

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]

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]

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]

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]

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]

Shear strength is based on the cross sectional area.

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

[johnny68]

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]

Salad bar, baby!

[monteboy84]

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]

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]

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.