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Bicycle bottom bracket spindle material choice.

painly

painly

Member
What I have come up with for suitable materials are 4340 or 4140, or in stainless, 416. But to get the properties desirable, these materials need heat treatment. This is the conundrum I am faced with, without a furnace. My question is whether or not there is a machinable, on kiddy machines, metal for my purpose that I can get away without hardening post-machining?

The Spindle and where it broke for me. This spindle is from the tongsheng tsdz2 ebike motor and is notorious for breaking under heavy use, however, these kits rarely see heavy use, so no one has produced an aftermarket alternative. I assume this is originally made of c40? or something similar. They are also produced with a poor design prone to stress risers as you can see; where mine broke. This axle has 2 degree tapers on either end and other than that, which I am half sure I can pull off, is a rather simple exercise in machining without the need for special tools...other than a furnace.
DSC_1908.JPG
stock68-73mmaxle_300x300.jpg


Thanks for any advice!

Barrett
 
Since you are using sealed bearings (I didn't see races in your picture), consider 7075T6 It has a tensile strength of 75000 PSI... it looks like your diameter is around .600 to .700. I've used 70705 cranks in bicycle racing and hard use hill climbing to good effect. Easy machining and no heat treat, but good strength.
 
Maybe that is why it broke: it is too hard. Perhaps they just hardened it and did not temper it to toughen it up. Also, as you say (and it is clearly visible), broke right in the snap ring groove.

The new part would need round bottom grooves to minimize stress risers. I would try it with 4140 or even O1. No heat treating.
 
Johnwa said:
There’s a good discussion on this issue
https://endless-sphere.com/forums/viewtopic.php?t=98361

Have you tried the manufacturer, I found them quite helpful on the issue I had. No parts required though.
Click to expand...

Sorry for no dimensions, I couldn't tell you length off the top of my head, maybe 150mm, the diameter is 15mm.

Hi John

I have emailed them direct and didn't receive a response. This kit had a hall sensor go faulty around 100km in as well. I couldn't have told you what a hall sensor was before this motor...but I ended up diagnosing and fixing with some honeywells that have been performing well for about 600 km now. I LOVE this thing overall, and that's why I want to put the work into future-proofing it a bit with a sturdier spindle. I also like the idea of being able to fabricate a longer spindle and bb sleeve to retrofit to a fat bike. I love the fact it is torque sensing; a must on the trails.

So no response from the company, but I did get a reply from psw power where I purchased. They sent me another spindle, and I promptly rounded the corners on the clip grooves, and added an extra outboard bearing on the non-drive side; where mine snapped (reducing the downward leverage effect). It is holding strong at about 100km...BUT! I have a small tear on my PCL due to the first break, which could have been waaaay worse. I am now dealing with a ganglion cyst that has formed due to this small tear and now 6 months later I still have limited mobility. I am worried about this happening again, and if it is any indication of how easy it was to machine those grooves, it will happen again. This part machined almost like cast iron.

That brings me here!

I think I will try a version, a more ductile version, with an unhardened material first. Twisting or bending isn't my concern, I really just want to ride worry-free without it breaking. The idea of o1 is intriguing as I can get it readily in town.
 
It kind of depends. If they have heat treated to provide requisite strength & you are constrained by the dimensions, that puts some limits to what you can replicate. If they HT to provide partial hardness to outer bearing areas for example, that's a different matter.

Just Googling (ie these may not be accurate). In hot rolled & normalized state:
4140: tensile = 148 ksi, yield = 95ksi
4340: tensile = 185ksi, yield=125 ksi
So 4340 is strong stuff & these values will increase with proper HT.

I machined my radial engine crankshaft from 1144SP (stress proof): tensile = 108 & yield = 90 ksi. Lower than either of the 4xxx series. But its claim to fame is nil post machining distortion when it stress relieves, especially non-symetrical shapes like a crankshaft. Your part looks pretty cylindrical so 4140 would be a better test choice assuming its more readily available.

Even if you have a HT oven, you should expect slight distortion & maybe surface scale so typically its turned slightly oversized, HT, then finish ground (special machine) or hard-turned (special lathe inserts) after after that. If you think the part broke because of groove stress riser or improper heat treatment (ie, not related to the basic material strength) then 1144SP might be a material to look at. It machines beautifully.

ps you can get O1 in many sizes at KBC Canada for reasonable shipping if your local provider is expensive
 
PeterT said:
It kind of depends. If they have heat treated to provide requisite strength & you are constrained by the dimensions, that puts some limits to what you can replicate. If they HT to provide partial hardness to outer bearing areas for example, that's a different matter.

Just Googling (ie these may not be accurate). In hot rolled & normalized state:
4140: tensile = 148 ksi, yield = 95ksi
4340: tensile = 185ksi, yield=125 ksi
So 4340 is strong stuff & these values will increase with proper HT.

I machined my radial engine crankshaft from 1144SP (stress proof): tensile = 108 & yield = 90 ksi. Lower than either of the 4xxx series. But its claim to fame is nil post machining distortion when it stress relieves, especially non-symetrical shapes like a crankshaft. Your part looks pretty cylindrical so 4140 would be a better test choice assuming its more readily available.

Even if you have a HT oven, you should expect slight distortion & maybe surface scale so typically its turned slightly oversized, HT, then finish ground (special machine) or hard-turned (special lathe inserts) after after that. If you think the part broke because of groove stress riser or improper heat treatment (ie, not related to the basic material strength) then 1144SP might be a material to look at. It machines beautifully.

ps you can get O1 in many sizes at KBC Canada for reasonable shipping if your local provider is expensive
Click to expand...

Thanks for your analysis and heads up about KBC, Peter.

being confined to dimensions is an issue and one of the reasons modern mountain bikes use different BB spindle designs. I cannot retrofit a different design without serious rework of the internal cadence gear. So I am going to stick with this 15mm. I feel the original part was over hardened as there was absolutely no twist before it broke. The stress risers are there, but the depth of the groove is ~.5mm - 30 or so times smaller than the overall dia.

I am beginning to like the idea of either o-1 or 4140

Barrett
 
I have been looking a bit more into it...here is where I am.

Dia is 3/4in to allow for cadence drive teeth machining.
Locally - steelmet supply.

o1 in 3ft sections = $83.94
4140 htsr per ft = $6.75 (don't carry normalized)

metal pros had good selection and pricing, but with cutting and shipping 2 ft of 4140 was $~80 , that's at ~$7 per ft!

KBC
o1 in 3ft section = $43.18 and what I saw was $10 for shipping = $53.18

but now that I have an account it won't let me add anything to my cart...or give me prices. I need to phone...

But it's not all about the money...

I started digging for numbers for o1 in an unhardened state and could not find anything concrete. I came up with one data sheet that mentioned 28 ksi? doesn't sound right. I then started looking up 4140 ht machinability, and it appears that it can be done and I end up with a tough part purchased locally. I would machine the diameter with carbide, and then do tapers and the notched cadence section with a fly cutter taking my time. What I worry about however is the ends that need threading. I will be using a fine thread. Does anyone have experience hand tapping 4140 ht? Does machining this stuff in the above-mentioned manner seem doable. Youtube would have me thinking yes.

Thanks for any input!
 
I made a new wheel axle for my road bike a few years ago. I used 4140 htsr and don't recall much difficulty in drilling, turning and threading it. I would have used carbide for turning and HSS for threading and drilling. So it can be done with tools you likely have.
 
Brian Ross said:
I made a new wheel axle for my road bike a few years ago. I used 4140 htsr and don't recall much difficulty in drilling, turning and threading it. I would have used carbide for turning and HSS for threading and drilling. So it can be done with tools you likely have.
Click to expand...

Thanks for the real-world example, Brian. Was this replacing a through type hollow axle? A friend of mine has broken two of the stock 7xxx aluminum ones on his enduro bike.
 
Not sure how those carbon steels would be from corrosion standpoint? Do you give them a clear coat or are they mostly internal & lubricated anyways?

Another nice materials is 303 stainless. Avoid the others 3xx alloys unless you have some experience, it likes to work harden machining & tapping

1613713094487.webp
 
PeterT said:
Not sure how those carbon steels would be from corrosion standpoint? Do you give them a clear coat or are they mostly internal & lubricated anyways?

Another nice materials is 303 stainless. Avoid the others 3xx alloys unless you have some experience, it likes to work harden machining & tapping

View attachment 13383
Click to expand...

Thanks Peter,

The area can be prone to corrosion. Usually the metals used are blackened in some way to prevent this. That was my original thinking with 416. I'm not sure if it would be a bike problem but it would be nice insurance to not have to worry about it. This 303 looks interesting. I will see if I can get it locally! I see the Yield of 303 is a lot better than 416 @ 276 mpa in annealed state.
 
Yes, it was a through hole type for a Campagnolo hub. I couldn't find anything locally to replace it because it is an odd combination of a metric shaft size with imperial threads. So I picked up some stock and made it. I've probably ridden about 3000 km on it, so it seems to be ok.
 
painly said:
Thanks Peter,

The area can be prone to corrosion. Usually the metals used are blackened in some way to prevent this. That was my original thinking with 416. I'm not sure if it would be a bike problem but it would be nice insurance to not have to worry about it. This 303 looks interesting. I will see if I can get it locally! I see the Yield of 303 is a lot better than 416 @ 276 mpa in annealed state.
Click to expand...

That blackening is normally done on guns(blueing) but doesn't offer any corrosion protection, but does add a coating to help hold a film of oil. I posted an article(in links) on fasteners put out by NASA that discusses that process.

Cheers

Wayne
 
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Crankit said:
That blackening is normally done on guns(blueing) but doesn't offer any corrosion protection, I posted an article(in links) on fasteners put out by NASA that discusses that process.

Cheers

Wayne
Click to expand...
Yes even hot bath bluing only offers a layer of oxide to hold oil, less so with cold blue solutions, multiple coatings can build up this by carding (steel wool buffing ) between applications. If anyone needs a small amount I'll send some to you saves buying a bottle. But then again it's inexpensive and adds to a projects appearance too. just 2 cents
 
PeterT said:
Where do you buy it inexpensively?
I find the small bottles are spendy on a $/volume basis. I'm still working on the pint kit I got from Kens Gems (same stuff) & I think that was + $40
I thought KBC sold the solution only but all I see is the kit with sealer
https://www.kbctools.com/itemdetail/?itemCode=1-958-45910

And then there's Travers (shock!)
https://www.traverscanada.com/abc-blackener-kit/p/71-223-807/
Click to expand...

Peter, what you linked to is for non-ferrous metals? Are you looking for gun bluing?
 
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