Effect of drilling holes through screws?

If I bore small diameter holes though the stainless steel / titanium / aluminium screws used on my bicycles, e.g. using EDM, then..

Aside from saving a bit of weight what impact would it have on the strength of the fastners?

Would it make them a bit more resilient against typical fastening twisting forces?

TIA

I would think that drilling things like brake levers and changing steel bolts to titanium would save more weight with less hassle.

The start of a very unhealthy obsession?

Cranks are really over-engineered and heavy. Why not start there?

I over drilled one once and….

Why would you want to?

Is this to lighten up your aero seatpack?

I can’t see retrospectively drilling some holes in your bolts is going to anything but create some interesting stressrisers.

You can probably buy a hollow bolt that is designed through materials and treatment to be as strong as typical bolt found on a pushy.

But the only bolt I’d consider replacing with a bolt with a hole in it is probably a chainring bolt and you already get them with holes in them and in alloy.

Would it make them a bit more resilient against typical fastening twisting forces?

No. You can’t generally make thing more resilient by removing metal. In particular you’d introduce the possibility that the walls of the tube you’ve created would buckle under the twisting force.

Google “Drillium” & No.

Google Drillium

Oh, woof.
*(thinks) – my TRP Spyres should work with any cable-actuated road levers.

If you would like to reduce the torque/load capacity of the fastener reducing its cross sectional area will be a good start…

I can drill carbon yes?

Good luck drilling right through a stainless screw.

Aside from saving a bit of weight what impact would it have on the strength of the fastners?

Not much that you’d notice* for most of the fasteners. It would reduce the sheer strength, so I wouldn’t do it to your brake bolts.

*Until you try to undo them and they are a bit stuck, at which point you’ll twist the head off leaving the rest behind. Of course the food news is that they are pre-drilled to accept the extractor

If you want hollow screws, just buy vented cap heads (for vac aplications).

Like these: https://www.accu.co.uk/562-vented-screws

They aren’t cheap. We use them at work in certain locations that are pumped down to low pressures.

Just no.

Conversely if one was to fill in the hole in the middle of one’s nuts would it make them strongerer and more resistant to being twisted?

What’s the risk to reward ratio on a 0.01g lighter calliper bolt?

As a one time Drillium on everything bodger, I can also recommend Choppium.

Choppium is cutting your seatpost so that no more than the necessary for minimum insertion is left, cutting your quill stem again for minimum insertion, reducing your bars so that there’s no surplus length past the edge of your palm. You can add a spot of drillium to the last 2″ of the ends of the bars, but don’t go wild – it’s not nice if they break. 🙂

And more relevant to the OP, cutting every screw so that only the minimum amount of necessary threads are left. That’ll probably save as much weight as hollow.

I’m surprised we don’t see much Drillium these days. We used to have to do it with handcranked drills and it could take several nights just to do a chainring, an electric drill would make short work of the job.

I used to work with a guy, machinist but also part-time 250cc motorbike racer – UK champion and eventually got a works Aprillia ride. He was forever tinkering with his bike – lightening parts, make steel stuff in alloy. Decided that alloy brake calliper retainer bolts would be a great idea. Next race, off the grid pronto and first into a tight corner – big handful of brake, calliper fails and he breaks his collar bone.

nuff said!

Vented screws are cool but will be heavier than ti and alloy screws and as said above, cut them to their minimums. I built an 18lb mountain bike years ago, it was pretty pants. Great up hills but I think we need a bit of heft to settle the bike. It weighs in around 22 ish now and so much better. We built young James an Attitude which was down at that weight which worked well for him as a light weight ten year old back then.

As above if you minimise seat post, fork tube, cable length, chain length, these are good but how far do you go ie a smaller cassette which means shorter derailleur and chain but can you ride up your local hills, do you shorten bars, cranks, pedals (all of which impact on performance), light weight wheels which means narrow… go for really lightweight tyres but the tread and therefore grip reduces significantly – I’ve been there and one or two bikes in the shed still have aspects of this approach- they are the least ridden and I have ‘fattening up’ plans for both.

Ref above and the motor bikes, as a lad I lightened my Suzzi X7 by drilling and it would flex in a light wind, didn’t handle much better. But we do have an 86kg 250cc race bike with 62-65bhp – that’s 750bhp per tonne pilotless.

Lionheart

…As above if you minimise seat post, fork tube, cable length, chain length, these are good but how far do you go…

What I learned from Drillium and Choppium was have nothing heavier than it need be, and nothing lighter than it should be.

The bike has to be rideable not just barely usable.

epicyclo – Proper race car and race bike philosophy there!

Have you considered reaming your steerer tube?

What I learned from Drillium and Choppium was have nothing heavier than it need be, and nothing lighter than it should be.

Did you work for Lotus? The ‘adding lightness’ philosophy is you take one tube at a time out of the spacefrane chassis, when it buckles pu the last one you removed back in and you have the perfect balance of strength and lightness.

It would reduce the sheer strength, so I wouldn’t do it to your brake bolts.

Bolts aren’t intended to be loaded in sheer, including brake rotor bolts. The bolts clamp the rotor against the hub hard enough that the friction between the rotor and hub prevents the rotor from slipping.

Because the bolts are loaded in tension along their axis, removing metal will cause a proportional increase in the stress on the bolt. If you remove 50% of the metal, then the remaining metal will be under twice as much stress.

Bolts aren’t intended to be loaded in sheer, i

Shear strength is a fundamental part of specing bolts. Screws didn’t used to be good for resisting shear but the newer screws used to replace nails in building are, which is what they are allowed.
Shear strength of the bolt is also a function of its cross sectional area and crucial to being able to tighten the bolt sufficiently to create the tension required and also to being able to loosen the bolt without shearing the head off

While the friction between the hub and rotor is largely responsible for resisting the shear forces some will inevitably transfer to the bolt.

I was thinking more of the caliper bolts, which will have to resist quite a lot of shear, on account of the mounting post having bugger all surface area in contact to create friction.

Shear strength of the bolt is also a function of its cross sectional area and crucial to being able to tighten the bolt sufficiently to create the tension required and also to being able to loosen the bolt without shearing the head off

The point there is that the torsional strength will be a function of the distribution of material, so a larger diameter tube will be torsionally stiffer than a solid bar of the same cross-sectional area. However, the tensile strength and pure shear (using a screw as a peg to hang stuff off a wall, for example) will basically just be a function of the cross-sectional area. Of course, the wall thickness has to be sufficient to resist crumpling. I suspect that’s what the OP is thinking of.

But basically, drilling out bolts on a bike to save weight is not a good idea. The weight savings will be tiny, so just not worth it unless you are seriously competing for world cup podiums.

Thanks for the thoughts especially the ones related to mechanical engineering.

I thought it was possible a hollow shaft could be stronger in torsion than a solid shaft because it has two, rather than one surface.

There is some math at this quora webpage + which indicates that, as torsion is concentrated at the outside diameter and tends to zero at the center, then removing material from the center has a relatively small effect on torsional strength.

Following the math there, for a shaft with 8mm diameter (M8 screw), cutting out a 2mm diameter hole reduces torsional strength by 0.004% whilst removing 6% of mass.

However, I have a feeling that the math there is just showing torsional stiffness, and thus is an oversimplification of actual torsional strength before failure under load. If taking into account material elasticity then doesn’t having a 2nd internal surface to share the stress under elastic deformation help make the shaft stronger under torsional load?

Appreciate thoughts or leads, my Mech. Eng. is flimsy as I studied Elec. Eng.

I’ve also had way too many bolts snap/break on bike, often not great spec so if drilling/modifying then would ideally be with better bolts. A lot of mine, I now replace with stainless so though corrosion free, they are heavier 😐

Seen far too many stainless bolts break on the machinery I used to work on(retired fitter/ex motor mech)
Depends on conditions and quality, stainless will rust/corrode

To the OP
A quick experiment you could do is to find the tourque required for a brake calliper bolt. Drill out a spare and then do a torque test(not on a frame/fork)on a drilled out bolt v non drilled, see which one fails first

Did you work for Lotus? The ‘adding lightness’ philosophy is you take one tube at a time out of the spacefrane chassis, when it buckles pu the last one you removed back in and you have the perfect balance of strength and lightness.

True but an integral part of that philosophy was Colin Chapmans view that there was basically a limitless supply of wannabe racing drivers and therefore the driver was essentially an expendable component…

cromolyolly

Did you work for Lotus?

I learned all this sort of stuff as a teenager madly experimenting on old bikes. Had plenty time and no money. Learned about stress risers too, but didn’t know it had a name at the time, but I used to lightly countersink my Drillium.

There wasn’t much I didn’t try – usually badly. Probably fortunately for me because when I moved on to making motorbikes go faster my bodging skills were applied with more discretion (and I’d read P E Irving and Judge by then). 🙂

A tip for the weight weenies out there, don’t try to externally double butt a straight gauge tube with handheld emery cloth tape unless you have accurate callipers (or heavy padding for your gentleman’s accessories).

As for stainless steel screws, as Trekster says, I don’t trust them in critical parts like brake mounts. Having owned a boat, I learned about crevice corrosion – the part looks ok but it’s been attacked from within.

A big crap in the morning is way more effective at weight loss.

The point there is that the torsional strength will be a function of the distribution of material, so a larger diameter tube will be torsionally stiffer than a solid bar of the same cross-sectional area

What do you mean by “of the same cross sectional area”? That the hollow tube would have to have a wall thickness equal to 1/2 the diameter of the bolt?

I seem to recall that torsional force created shear stress and that stress increased linearly across the cross sectional radius. So there was more on the outside, certainly.

Colin Chapmans view that there was basically a limitless supply of wannabe racing drivers and therefore the driver was essentially an expendable component…

True. Seems he wasn’t too far off base, given how they sold.

However, I have a feeling that the math there is just showing torsional stiffness, a

You have to take thread pitch into account because it affects both cross sectional area and tensile stress due to how effectively it is held in place??? Or something like that which is dimly swirling around my head.

A quick experiment you could do is to find the tourque required for a brake calliper bolt.

All the boots of that type I’ve seen that failed, you could see the outline of the hollowed out hexagin from the head in the material, so something strange happened to them during manufacture. If you got one of those, drilling it out may not affect it much……

What do you mean by “of the same cross sectional area”?

The same amount of metal, so if you took a solid bar and used it to make a tube, the cross-sectional area of the metal would be the same even though the outer diameter of the tube would be greater. The tensile strength will be the same because there’s the same amount of metal to resist tension, but the torsional strength and stiffness of the tube will be greater because the material is distributed differently.