What is best way to cut a BB down in size?

[g725s]

I would like to cut a 100mm bottom bracket down to around 72mm so that I could use a 68-73 BBSHD that I own. Also it would allow for a more precise Chainline. And no, I dont plan on converting the bike back.

I've looked for posts by others that might have cut down a bottom bracket, but my searches return no results.

I don't have any special tools to do this. So I was thinking that first I would scribe each end of the BB as precisely as possible. Then I was thinking it would be good to put a hose-clamp on the end of the BB as a guide and then using a hacksaw and then cutting the BB as best I can.

I do know a friend that owns welding fabrication shop. And was planning on running my procedure past him. But maybe he might have a better way to best cut the BB down in size with specialised tools.

I should note that cutting the BB down to 72mm would mean cutting right to the edge of existing welds where an oval tube that connects the BB to chainstays (it's an Electra cruiser).

Ideas anyone?

 

[nutnspecial]

https://endless-sphere.com/forums/viewtopic.php?f=3&t=83690&p=1225946#p1225946

The above thread came to mind.

 

[g725s]

https://endless-sphere.com/forums/viewtopic.php?f=3&t=83690&p=1225946#p1225946

The above thread came to mind.

Oh hey, will you look at that

Yeah I asked that, but I was looking for possibly a thread where someone shows doing it and how they actually did it.

Have you seen someone post anytime where they actually cut the BB down in size?

 

[nutnspecial]

No I have not.

Previous thread is good context to include for other posters trying to help on this interesting topic, though the topic question was already answered pretty completely imo.

I don't see how you can even get that down to 72 before removing important welds? If not just take it to a pro shop for opinion. Beyond hacksaws and angle grinders, a mill would be a pro option. I would use a combo of the former though, diy, should it not destroy the butted tube welds on BB.

 

[Samd]

I would use a hacksaw and reface it. Check the thread depth.


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[g725s]

No I have not.

Previous thread is good context to include for other posters trying to help on this interesting topic, though the topic question was already answered pretty completely imo.

I don't see how you can even get that down to 72 before removing important welds? If not just take it to a pro shop for opinion. Beyond hacksaws and angle grinders, a mill would be a pro option. I would use a combo of the former though, diy, should it not destroy the butted tube welds on BB.

I can't be the only one to want to cut down a BB so as to install a BBSHD or some of the prior less powerful mid-drives can I?

And NO, measured precisely with my calipers it is 71.25mm, maybe I could go 71mm taking literally a hair off of the weld, and really just a hair.

The BBSHD that I have is 68-73, so it should work. I need to remove it from my current build and measure it, but I think that it will fit up to 73mm BB's

Looking up "facing tool" I found this, but I won't be buying this: http://www.parktool.com/blog/repair-help/bottom-bracket-tapping-threading-chasing-and-facing#article-section-3

$330... https://www.amazon.com/Park-Tool-Bottom-Bracket-Facing/dp/B00781XTKC

 

[spinningmagnets]

You might consider building up the drive that you don't want (68mm) on a bike that already fits it, and then sell it. Then buy a drive that just slips in the frame you like. That being said...

There is a tool called a facing tool. It not great for removing a lot of material, but if you hack off the majority (while keeping the BB a hair fat) then you can use the facing tool to shave off the last few mm.

 

[g725s]

You might consider building up the drive that you don't want (68mm) on a bike that already fits it, and then sell it. Then buy a drive that just slips in the frame you like. That being said...

There is a tool called a facing tool. It not great for removing a lot of material, but if you hack off the majority (while keeping the BB a hair fat) then you can use the facing tool to shave off the last few mm.

Spinningmagnets, You also have this frame (Electra Lux Fat 7D) http://www.electrabike.com/bikes/cruiser-lux-fat-tire-7d/mens. And you know as it is now you are limited on what chainrings you can use(really just the Lekkie with a spacer and your current steel Bafang chainring) and still have a decent chainline in the lowest gear. And with your current stock heavy metal chainring it is still not as close as stock as far as best chainline. You said it is 1/4" from the tire, or width of the chain itself(which is over 1/4" at 7.21mm), my stock setup is 1/8" from the tire.

From my calculations if I cut it down I could use a Chainring Adapter (either the Bafang Alloy or California Ebike chainring adapter) with a spacer and that would open up a lot of different chainring options from 42t up.

I do have one question and another measurement I'd like you to take from your bike for me...How close do your crank arms come to the chainstay's? The way I calculate the crank arms, if I cut the BB down and use a 68mm BBSHD, I'd have about 1/2" clearance of the end of the crank arms hitting the chainring tubing. And I guess since you used the 120mm BBSHD in your 100mm BB you could only accurately measure the chainring side as you did not put any spacers on that side right? As I'd be taking a tad over 1/2" off the BB on either side the crank arms will be closer than yours by about that much respectively at least on the chainring side anyway, as you had to put nearly 3/4" of spacers on the other side right? https://www.electricbike.com/bbshd-installation-basics/

 

[Samd]

If its just a middrive sliding inside you're overanalysing it.


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[Ykick]

I haven’t cut a BB before but have done similar workpiece using hacksaw and hose clamp as guide. Worked fine if careful but I’d probably just sell my current drive and buy one which slips in without any mod. Yes, I’m lazy….

 

[g725s]

If its just a middrive sliding inside you're overanalysing it.

So it's better not to analyse it and just have a bad chainline?

 

[Samd]

Chainline was already determined IIRC.
If so get the saw out.


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[g725s]

Chainline was already determined IIRC.
If so get the saw out

Who determined exact chainline? I think I have some idea now but was not exactly sure when I posted this. And it took some analyzing, sorry about that
Hmm, cut first and analyze mistskes after

 

[Dogboy1200]

I cut down a 100mm BB to 73mm on a steel frame. I did it to mount a 68mm BBS02 into 20 in fat bike for my 5 year old. I had the 02 in my bike but upgraded to the HD. To cut it I just marked both sides where I wanted to cut, then only cutting one side I cut close to the line with an angle grinder. Then I finished to the line with a flat file and kept measuring multiple points around the cut edge to the uncut edge on the other side to ensure parallelism. Then repeated the process for the other side. It turned out well. I was able to use the HPC chian ring adapter I had that had given a terrible chian line on my bike but a great chian line on his. Also I used the short and offset cranks that came with the bike. I just cut the sprocket off the right crank arm. The bike was a Walmart mongoose King Kong. I disabled the PAS and set the throttle to by displays command so it gives him 9 levels of power that he's allowed to adjust as he feels comfortable (I have the controller parameters set at 10 amps max) I control the top speed in the advanced settings to about 10-15 mph depending on where we are riding. I had to make some mods to get the seat low enough and also put the steering stem on backwards to make it fit him better.

 

[g725s]

I cut down a 100mm BB to 73mm on a steel frame.

Thanks for posting that. Good work. I can only imagine his smile on his first ride!

 

[robocam]

My Trek HiFi Pro 29 came with a BB9? bottom bracket. I cut it down to 68mm and installed an FSA BB30 to BSA adapter. I had to trim 1mm off the diameter of the adapter so that it would fit the smaller opening of the BB95. I wonder if I'm the only one around with a BB95 to 68mm conversion =)

I just measured so that the end result would be 68mm and set the Dremel guide to the appropriate depth and removed the same amount of material from both sides using a cutting disc. The key is to go very slowly and spray it every few seconds with some water. Go around in circles many times. Don't cut straight through in one pass. As you cut, the disc will wear, so you want to minimize any influence that has on your cut, or it may come out uneven. I feel lucky that it came out exactly 68mm.

https://www.dremel.com/en/products/-/show-product/accessories/565-multipurpose-cutting-kit

I would like to cut a 100mm bottom bracket down to around 72mm so that I could use a 68-73 BBSHD that I own. Also it would allow for a more precise Chainline. And no, I dont plan on converting the bike back.

I've looked for posts by others that might have cut down a bottom bracket, but my searches return no results.

I don't have any special tools to do this. So I was thinking that first I would scribe each end of the BB as precisely as possible. Then I was thinking it would be good to put a hose-clamp on the end of the BB as a guide and then using a hacksaw and then cutting the BB as best I can.

I do know a friend that owns welding fabrication shop. And was planning on running my procedure past him. But maybe he might have a better way to best cut the BB down in size with specialised tools.

I should note that cutting the BB down to 72mm would mean cutting right to the edge of existing welds where an oval tube that connects the BB to chainstays (it's an Electra cruiser).

Ideas anyone?

 

[Samd]

Chainline was already determined IIRC.
If so get the saw out

Who determined exact chainline? I think I have some idea now but was not exactly sure when I posted this. And it took some analyzing, sorry about that
Hmm, cut first and analyze mistskes after

No but there's so much tolerance in the BBS if it looked like it lined up before then I wouldn't lose too much time planning to do it. I'd do it.

 

[g725s]

No but there's so much tolerance in the BBS if it looked like it lined up before then I wouldn't lose too much time planning to do it. I'd do it.

Sam, it is obvious I like to be more positive of my calculations than you, before I cut into a $600 bike and risk not coming out perfectly. I don't have as many BB modifications under my belt like you do where I can just eyeball it and expect it to come out perfectly. Please post some of the BB modifications you've done just by eyeballing it

 

[Samd]

A lot of the early carbon bikes needed it, even my Commencal meta 55 carbon. I was really nervous doing that given what they cost (3500 GBP RRP).

A lot of other carbon bikes require rebuilding with epoxy, kevlar and aramid to get the drive swung up. Its the ideal time to cut the sides flat.


Finished.


I think what you are missing is that the internal radius of the drive flange and the variability in fitment of what is essentially a loose slide fit drive is really imprecise. I didn't mean my comments to discourage. I meant quite the opposite and I am sorry if that didn't make sense.

 

[robocam]

How can you rebuild that area to the same structural integrity after cutting it like that?

...A lot of other carbon bikes require rebuilding with epoxy, kevlar and aramid to get the drive swung up. Its the ideal time to cut the sides flat...

 

[g725s]

I can't believe you would cut up a carbon frame or any frame like that. Those are structural components! A downtube is under high tension. Weakening it like that seems like a recipie for disaster.

No one here is cutting a carbon downtube. My Bottom Bracket(BB) is aluminium. One poster above cut a chromoly BB. And the other was aluminium BB. No mention of carbon here.

 

[robocam]

This looks like something that could affect a frame's structural integrity.

I can't believe you would cut up a carbon frame or any frame like that. Those are structural components! A downtube is under high tension. Weakening it like that seems like a recipie for disaster.

No one here is cutting a carbon downtube. My Bottom Bracket(BB) is aluminium. One poster above cut a chromoly BB. And the other was aluminium BB. No mention of carbon here.

 

[Samd]

No the purpose is not the carbon cutting. That aluminium is cut.

As a Mech Eng, that area is in tension robo. Its easy to make stronger with composites. I do maybe a dozen high end frames a year, FWIW.

G9275s, how much is the chainline out by now (give or take a few mm?)

 

[robocam]

Have you had any frame failures due to this type of modification? I assume these bikes are ridden hard off road since you need to swing the motor up. What does that area look like after you've reinforced it?

...Its easy to make stronger with composites. I do maybe a dozen high end frames a year, FWIW...

 

[Chalo]

How can you rebuild that area to the same structural integrity after cutting it like that?

It won't have the same structural integrity. Maybe it doesn't need to have that much integrity, or maybe it does. Once you start cutting your frame, you're on your own and you assume responsibility for what happens.

This is the way we learn what we can get away with. At my size (6'8" and over 340 lbs) I am already pushing the envelope for normal bicycles, so I'll let others do this kind of research. I'm already doing my own kind of research when I ride bikes intended for someone half my size.

For years I've watched motorized bike guys drill holes in their frames in places where I expect them to have substantial stresses, but in practice it doesn't turn out as disastrous as I would have guessed.

Anytime you see bikes with curved or shaped tubes, you can assume that they've been overbuilt for style purposes. A bike that has been overbuilt can be compromised somewhat and still have enough integrity to do a typical bike's job.