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Drilling 1/2" holes in Bafang cranks to reduce weight


1 kW
Dec 17, 2021
Just got a set of 170 mm Bafang cranks. weighs 438 g (0.97 lbs). If I drill a bunch of 1/2" diameter holes spaced 1/2" apart along it's length, I may be able to reduce the weight by about 0.2 lbs. That should not weaken it right? Anyone done that?
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Bad idea - modified regurgitated cast cranks are a sure road to bloody asphalt pie.
Not worth it, dude. There's better places to lose weight on a bike. Weakening structural integrity should be last on the list.
"Drillium" is the term of art that applies to what you're talking about. It was a '70s fad that resulted in predictable calamities, because cyclists aren't engineers. Heck, cycle manufacturers usually aren't engineers. It's still thinly practiced by component "tuners" from Germany and elsewhere who scrape all the "excess" material from bike parts to slightly reduce their weight and totally eliminate any safety factor or fault tolerance they have.

The fact that you are working with a heavy scabby old bike, but want to poke speed holes into a safety critical part makes me think you should develop a better overall perspective about this stuff.
If I drill a bunch of 1/2" diameter holes spaced 1/2" apart along it's length, I may be able to reduce the weight by about 0.2 lbs.
You're wanting to cut 0.2 lbs. weight? My brother in-law did way better than that with the keto diet. Not that difficult, except when travelling, he says.
That should not weaken it right?
There is an element of 'magical thinking' in this, in the sense that you look at the part and think that it is homogeneous, evenly and perfectly composed, and also that your own changes are the same.

It probably isn't - it's just as likely to have unresolved stresses and unevenly mixed materials in it. In some cases, it may be surface-hardened by design, even if that is done properly. That means a fragile rigid shell supported by a more flexible, possibly tougher core, and you're planning to break the shell.

As well, expect a good chance of introducing micro-cracks in the edges of the holes you make, depending on how you do this and what further work you do. Cracks are stress concentrators, multiplying the load just in a small area.

You are losing your safety margin. That's not what you are thinking of, but the real part doesn't do what you are thinking it will, it does what it does.

Now, think of stomping hard on the pedal and having the arm snap, and your leg sliding along the broken arm (it will likely break at one of the holes and it will also likely be pointing up when you are stomping on it) at that full power.

It's probably your calf, so you may not bleed out before rescue arrives, but you aren't likely to be riding much, and you still have the crash that's about to happen (what situation were you in that you were stomping the pedal?).

@99t4 is your best advice here.
Oh 'CMON MAN ! just go ahead and mock him period.
"cast cranks" ? No metallurgist OH PAPI !
"drilling crank arms" Hey Marty Cheer him on how far do you think he'll get?
Neddy Ever "Ever had a crank break on you?
you should Drillium scabby old tolerance not
Remove the motor and battery and it will really lighten up the bike, maybe by a third, plus you have a greater variety lighter cranks to choose from. At that point you can solve most weight issues with money, and not have to deal with the heavy e-stuff.
OK thanks for the warning. No drilling.
Installed the Bafang cranks (w/o drilling them) on my TSDZ2 which reduced the Q factor from 210 mm to 180 mm.

Looks like people are having a field day with jokes lol. How many of you have broken a crank? Nicobie has but hasn't told us how.

From wikipedia:
"Aluminum cranks may be cast, hot forged or cold forged ("cold" in this context means the billet from which the crank is to be made is heated to a specified temperature well below the melting point, not room temperature). Cold forging gives the metal additional strength, and the cranks can therefore be made lighter without increasing the risk of breakage. Shimano "Hollowtech" aluminum cranks are made by forging the main arms around a hard steel insert which is then withdrawn, leaving an internal void to save weight."

The Bafang cranks are $9.20 ea shipped. https://www.aliexpress.us/item/3256...t_main.16.1d2d18025Axc6m&gatewayAdapt=glo2usa It's nicely finished, with no casting marks. I am guessing it's cold forged since cost wise, that would be lowest to manufacture.

It would be interesting to put a drilled and undrilled Bafang crank on a test stand and observe at what load they actually break.
How many of you have broken a crank?
Me?... never. And that's the point.

Unlike steel, many aluminum alloys have nearly zero and unpredictable fatigue limits. Which ultimately means... when it breaks, it does so catastrophically... and without warning.
Looks like people are having a field day with jokes lol. How many of you have broken a crank? Nicobie has but hasn't told us how.
Seems hard to believe but yes, crank breakage is not uncommon. To the point of recent class-action lawsuits against big-name manufacturers.

Here is some reading material for you:

Depends on your risk tolerance. I believe almost everyone would agree that drilling will not make the cranks stronger. I also believe that many/most people will agree that drilling will make the cranks weaker. How much is up for debate, but a few folks here are expressing that potentially increasing the risk of failure can have devastating effects on the rider. Maybe it’s worth the risk, maybe not. My testicles keep saying it’s not, but others may have a higher risk tolerance.
The only bicycle related injury I have ever seen that resulted in hospital trips was from a broken crank. I am not the journaled elder around here, but I grew up with a parent that was an insurance adjuster, and heard a *LOT* of horror stories, I had busted a crank while trying to shoot between a partially closed gate and ended up face smashing the gate and then eating a lil dirt. low speed no biggy, Then I hear the story of the guy that is just biking home, half off the seat to put more pressure down busts a crank, veers hard right, hits a parked car ricochets off that into the side of a concrete barrier wall, ended up tumbled off the wall and braking a bunch of ribs, collar bone, something in his left ankle and his jaw. Tried to put a claim on the car that he hit (parked and vacant). which is how my old man heard about it. I got a trip to the bike shop to get a top end crank set for my bike. Only time he ever volunteered money for my bike...
I broke a crank arm on a fairly high end Trek MTB years ago. The interesting aspect (to me) is it broke at the top of a five mile 1800' ascent after I had reached the top and was "cooling off". Accordingly I wasn't hurt since I was pedaling so easily. I have a background in metallurgy and detected an internal flaw in the structure that probably precipitated the failure.