JB Weld for attaching cantilever posts?
03-31-19, 02:35 PM
#1
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Thread Starter
JB Weld for attaching cantilever posts?
I am working on an older Trek 620. The front cantilever posts were cut off. I have sourced a set of threaded posts from Surly, I believe it was. After removing the rest of the post, drilling and tapping the bosses I am sure there isn't enough thread to secure the posts in the bosses. What do you think about JB Weld for this application?
I would remove pain from the rear of the boss, clean with acetone and reinforce the rear of the post with the JB Weld. Sort of like an epoxy nut. An actual nut would not work due to lack of clearance.
03-31-19, 02:57 PM
#2
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I think you'd be better off finding an actual nut to thread in to.
JB Weld is not the solution.
At issue is how levers work.
Basically, you squeeze the brake lever with 50 pounds of force, that gets multiplied 2:1 at the cantilever, so you have 100 pounds at the brake shoe. But that pivots off the 1/4 inch base of the stud resulting in 400 pounds at the plane the interfaces with. Where the Cantilever levers from is midway up the stud. About a half inch to 3/4 inches away...another lever. Best case scenario is 800 pounds trying to twist the stud out of the frame. On a 1/4 inch base. So you need a (800x4) tinsile strength of 3200 pounds minimum just to keep it put. Let alone any safety margin. What about a panic stop?
The half a thread you have and JB Weld isn't gonna cut it.
*Disclaimer--Back of the napkin calculations only. Off the top of my head. So internet engineers out there: do your own numbers & correct me. Please.
JB Weld is not the solution.
At issue is how levers work.
Basically, you squeeze the brake lever with 50 pounds of force, that gets multiplied 2:1 at the cantilever, so you have 100 pounds at the brake shoe. But that pivots off the 1/4 inch base of the stud resulting in 400 pounds at the plane the interfaces with. Where the Cantilever levers from is midway up the stud. About a half inch to 3/4 inches away...another lever. Best case scenario is 800 pounds trying to twist the stud out of the frame. On a 1/4 inch base. So you need a (800x4) tinsile strength of 3200 pounds minimum just to keep it put. Let alone any safety margin. What about a panic stop?
The half a thread you have and JB Weld isn't gonna cut it.
*Disclaimer--Back of the napkin calculations only. Off the top of my head. So internet engineers out there: do your own numbers & correct me. Please.
Last edited by base2; 03-31-19 at 03:27 PM.
03-31-19, 04:11 PM
#3
Banned
A Frame builder can remove the remains of the post & braze on Whole New ones , then you get it repainted..
# 1 you need to burn off the paint to get clean bare metal ...
then .. the post can be brazed to the boss ..
when building, using those 1st we brazed the post to the base,
its just swaged* on in producing it ,, then the whole assembly is brazed on the fork/stays
* swaging is like cold riveting ..
...
# 1 you need to burn off the paint to get clean bare metal ...
then .. the post can be brazed to the boss ..
when building, using those 1st we brazed the post to the base,
its just swaged* on in producing it ,, then the whole assembly is brazed on the fork/stays
* swaging is like cold riveting ..
...
Last edited by fietsbob; 04-01-19 at 11:19 AM.
03-31-19, 04:39 PM
#4
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take it to a welding shop and have them drop a couple tack welds on the side
04-01-19, 10:53 AM
#5
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No. I wouldn't trust JB Weld for that. Get the posts replaced, or use a caliper brake.
04-01-19, 12:42 PM
#6
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Thread Starter
Thanks for the advice! I will go with plan B for now which is regular brakes. I am starting to not like cantilevers much!
04-01-19, 01:47 PM
#8
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Given the choice, I'd much rather have a long reach caliper than a canti. I recently used JB Weld for the first time, it's pretty cool stuff. I used it to make replacement bushings for my lawnmower wheels. I don't think I would use it for anything critical.
04-01-19, 06:55 PM
#9
Senior Member
This winter I experimented with a cable stop on the chain stay for the rear derailleur. It held for a week and then pulled off the stay. Nope, JB weld ain't gonna work.
04-01-19, 09:23 PM
#10
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Here's a rule: Never ever trust an adhesive alone to hold any structural or functional part of a bicycle together. Ever. For any purpose.
Steel (including welds) has a yield strength from 30,000psi (for lowly 1018) to 63000 psi (for chrome moly) to 200,000 to 350,000 psi (maraging steel, like reynolds 931). Aluminum yield strength is about 35,000 psi. Ti is about 160,000 psi. CF is about 180,000 psi. So frame materials range in yield strength from 30 to 350 thousand psi.
Epoxy has a shear strength of 3500psi. IF you have perfectly prepared surfaces that conform to prevent any large voids that fill with epoxy. Pretty much any bike material will be stronger than JBWeld by a factor of 10.
So, do you really want to trust your brakes to epoxy?
In a production setting, with low viscosity epoxy and proper vacuum clamping, the right epoxy can give very good strength. But for repair (to my mind) there is a limit to its use. Probably the thing if you are gluing CF cloth over a small dent in a CF stay. Or for parts that are otherwise fastened together. I'd never trust it for much of anything else that's structural.
Steel (including welds) has a yield strength from 30,000psi (for lowly 1018) to 63000 psi (for chrome moly) to 200,000 to 350,000 psi (maraging steel, like reynolds 931). Aluminum yield strength is about 35,000 psi. Ti is about 160,000 psi. CF is about 180,000 psi. So frame materials range in yield strength from 30 to 350 thousand psi.
Epoxy has a shear strength of 3500psi. IF you have perfectly prepared surfaces that conform to prevent any large voids that fill with epoxy. Pretty much any bike material will be stronger than JBWeld by a factor of 10.
So, do you really want to trust your brakes to epoxy?
In a production setting, with low viscosity epoxy and proper vacuum clamping, the right epoxy can give very good strength. But for repair (to my mind) there is a limit to its use. Probably the thing if you are gluing CF cloth over a small dent in a CF stay. Or for parts that are otherwise fastened together. I'd never trust it for much of anything else that's structural.
04-02-19, 04:51 AM
#11
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Many years ago I knew a kid who attached the steering wheel of his El Camino to the column with JB Weld. No way I'd ever ride in it. Thankfully, the car didn't last long because the frame was cracked and he never thought to JB that.
04-02-19, 05:32 AM
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A Frame builder can remove the remains of the post & braze on Whole New ones , then you get it repainted..
# 1 you need to burn off the paint to get clean bare metal ...
then .. the post can be brazed to the boss ..
when building, using those 1st we brazed the post to the base,
its just swaged* on in producing it ,, then the whole assembly is brazed on the fork/stays
* swaging is like cold riveting ..
...
# 1 you need to burn off the paint to get clean bare metal ...
then .. the post can be brazed to the boss ..
when building, using those 1st we brazed the post to the base,
its just swaged* on in producing it ,, then the whole assembly is brazed on the fork/stays
* swaging is like cold riveting ..
...
04-02-19, 08:36 AM
#13
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Here's a rule: Never ever trust an adhesive alone to hold any structural or functional part of a bicycle together. Ever. For any purpose.
Steel (including welds) has a yield strength from 30,000psi (for lowly 1018) to 63000 psi (for chrome moly) to 200,000 to 350,000 psi (maraging steel, like reynolds 931). Aluminum yield strength is about 35,000 psi. Ti is about 160,000 psi. CF is about 180,000 psi. So frame materials range in yield strength from 30 to 350 thousand psi.
Epoxy has a shear strength of 3500psi. IF you have perfectly prepared surfaces that conform to prevent any large voids that fill with epoxy. Pretty much any bike material will be stronger than JBWeld by a factor of 10.
So, do you really want to trust your brakes to epoxy?
In a production setting, with low viscosity epoxy and proper vacuum clamping, the right epoxy can give very good strength. But for repair (to my mind) there is a limit to its use. Probably the thing if you are gluing CF cloth over a small dent in a CF stay. Or for parts that are otherwise fastened together. I'd never trust it for much of anything else that's structural.
Steel (including welds) has a yield strength from 30,000psi (for lowly 1018) to 63000 psi (for chrome moly) to 200,000 to 350,000 psi (maraging steel, like reynolds 931). Aluminum yield strength is about 35,000 psi. Ti is about 160,000 psi. CF is about 180,000 psi. So frame materials range in yield strength from 30 to 350 thousand psi.
Epoxy has a shear strength of 3500psi. IF you have perfectly prepared surfaces that conform to prevent any large voids that fill with epoxy. Pretty much any bike material will be stronger than JBWeld by a factor of 10.
So, do you really want to trust your brakes to epoxy?
In a production setting, with low viscosity epoxy and proper vacuum clamping, the right epoxy can give very good strength. But for repair (to my mind) there is a limit to its use. Probably the thing if you are gluing CF cloth over a small dent in a CF stay. Or for parts that are otherwise fastened together. I'd never trust it for much of anything else that's structural.
04-02-19, 05:19 PM
#14
Senior Member
If you value your life and limbs I would recommend that you not use JB Weld for this purpose. JB Weld is great epoxy when used within its limits...maybe even the best there is; however, this application is outside those limits in my opinion.
04-02-19, 06:45 PM
#15
Senior Member
Thread Starter
Here's a rule: Never ever trust an adhesive alone to hold any structural or functional part of a bicycle together. Ever. For any purpose.
Steel (including welds) has a yield strength from 30,000psi (for lowly 1018) to 63000 psi (for chrome moly) to 200,000 to 350,000 psi (maraging steel, like reynolds 931). Aluminum yield strength is about 35,000 psi. Ti is about 160,000 psi. CF is about 180,000 psi. So frame materials range in yield strength from 30 to 350 thousand psi.
Epoxy has a shear strength of 3500psi. IF you have perfectly prepared surfaces that conform to prevent any large voids that fill with epoxy. Pretty much any bike material will be stronger than JBWeld by a factor of 10.
So, do you really want to trust your brakes to epoxy?
In a production setting, with low viscosity epoxy and proper vacuum clamping, the right epoxy can give very good strength. But for repair (to my mind) there is a limit to its use. Probably the thing if you are gluing CF cloth over a small dent in a CF stay. Or for parts that are otherwise fastened together. I'd never trust it for much of anything else that's structural.
Steel (including welds) has a yield strength from 30,000psi (for lowly 1018) to 63000 psi (for chrome moly) to 200,000 to 350,000 psi (maraging steel, like reynolds 931). Aluminum yield strength is about 35,000 psi. Ti is about 160,000 psi. CF is about 180,000 psi. So frame materials range in yield strength from 30 to 350 thousand psi.
Epoxy has a shear strength of 3500psi. IF you have perfectly prepared surfaces that conform to prevent any large voids that fill with epoxy. Pretty much any bike material will be stronger than JBWeld by a factor of 10.
So, do you really want to trust your brakes to epoxy?
In a production setting, with low viscosity epoxy and proper vacuum clamping, the right epoxy can give very good strength. But for repair (to my mind) there is a limit to its use. Probably the thing if you are gluing CF cloth over a small dent in a CF stay. Or for parts that are otherwise fastened together. I'd never trust it for much of anything else that's structural.
04-02-19, 07:32 PM
#17
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04-03-19, 11:32 AM
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I see that now. If he doesn't want to get someone to weld/tack/braze something I have another idea.
Find a nut that fits the thread of the canti posts. File and grind the nut until it. Fits under the existing supports but not enough to make let the nut turn. At this point, just tighten the posts in.
Finding a metric nut shouldn't be too hard but it will need to be ground down quite a bit to fit in there.
The whole fork mount set up looks a little weak, even if there were plenty of threads. If it was my fork and I was dead set against replacing it, I'd also add an old school brake arch. They're available on eBay for under $5 (for the slow boat from China).
Find a nut that fits the thread of the canti posts. File and grind the nut until it. Fits under the existing supports but not enough to make let the nut turn. At this point, just tighten the posts in.
Finding a metric nut shouldn't be too hard but it will need to be ground down quite a bit to fit in there.
The whole fork mount set up looks a little weak, even if there were plenty of threads. If it was my fork and I was dead set against replacing it, I'd also add an old school brake arch. They're available on eBay for under $5 (for the slow boat from China).
04-03-19, 11:41 AM
#21
Banned
In the 80's were the Moots Mounts , a brake boss biting into the metal that can replace the whole thing....
but 30 years have passed..
but 30 years have passed..
04-04-19, 09:03 PM
#22
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04-06-19, 04:54 AM
#24
Senior Member
Here is a thread that describes what I did in a similar brake mount on the rear of an old Ross MTB, which I piggybacked onto the original author's post.
It works fine. I look at that repair as a bit of a 'barnyard' fix, since it's a bike I paid $5 for and ride only a few weeks a year when visiting my parents. If I were to do it again and had time on my hands, I'd probably fabricate an aluminum block that fits in the 'box' with the appropriate thread tapped into the right location.
In my case, I had to grind the nylock nut and the back of the threaded shaft because it bottomed out on the seatstays.
My father has a limited toolset, but did have a motorized grinding wheel. If not for that, I have no idea what I would have done. I'm sure fabbing up something from aluminum would have been a fast and cheap solution from a competent machinist (which I am not).
The 620 is pretty nice, so I think it's worth spending $10 and a couple hours of your time on it.
edit: I should add that the braking forces on the rear are TOWARDS the seatstays and if anything goes wrong the penalties are smaller than in the front, where the wheel tries to rip the brake stud off when you apply it. I would still try one of these solutions.
It works fine. I look at that repair as a bit of a 'barnyard' fix, since it's a bike I paid $5 for and ride only a few weeks a year when visiting my parents. If I were to do it again and had time on my hands, I'd probably fabricate an aluminum block that fits in the 'box' with the appropriate thread tapped into the right location.
In my case, I had to grind the nylock nut and the back of the threaded shaft because it bottomed out on the seatstays.
My father has a limited toolset, but did have a motorized grinding wheel. If not for that, I have no idea what I would have done. I'm sure fabbing up something from aluminum would have been a fast and cheap solution from a competent machinist (which I am not).
The 620 is pretty nice, so I think it's worth spending $10 and a couple hours of your time on it.
edit: I should add that the braking forces on the rear are TOWARDS the seatstays and if anything goes wrong the penalties are smaller than in the front, where the wheel tries to rip the brake stud off when you apply it. I would still try one of these solutions.
Last edited by DiegoFrogs; 04-06-19 at 04:58 AM.
04-08-19, 12:20 PM
#25
Senior Member
Thread Starter
Here is a thread that describes what I did in a similar brake mount on the rear of an old Ross MTB, which I piggybacked onto the original author's post.
It works fine. I look at that repair as a bit of a 'barnyard' fix, since it's a bike I paid $5 for and ride only a few weeks a year when visiting my parents. If I were to do it again and had time on my hands, I'd probably fabricate an aluminum block that fits in the 'box' with the appropriate thread tapped into the right location.
In my case, I had to grind the nylock nut and the back of the threaded shaft because it bottomed out on the seatstays.
My father has a limited toolset, but did have a motorized grinding wheel. If not for that, I have no idea what I would have done. I'm sure fabbing up something from aluminum would have been a fast and cheap solution from a competent machinist (which I am not).
The 620 is pretty nice, so I think it's worth spending $10 and a couple hours of your time on it.
edit: I should add that the braking forces on the rear are TOWARDS the seatstays and if anything goes wrong the penalties are smaller than in the front, where the wheel tries to rip the brake stud off when you apply it. I would still try one of these solutions.
It works fine. I look at that repair as a bit of a 'barnyard' fix, since it's a bike I paid $5 for and ride only a few weeks a year when visiting my parents. If I were to do it again and had time on my hands, I'd probably fabricate an aluminum block that fits in the 'box' with the appropriate thread tapped into the right location.
In my case, I had to grind the nylock nut and the back of the threaded shaft because it bottomed out on the seatstays.
My father has a limited toolset, but did have a motorized grinding wheel. If not for that, I have no idea what I would have done. I'm sure fabbing up something from aluminum would have been a fast and cheap solution from a competent machinist (which I am not).
The 620 is pretty nice, so I think it's worth spending $10 and a couple hours of your time on it.
edit: I should add that the braking forces on the rear are TOWARDS the seatstays and if anything goes wrong the penalties are smaller than in the front, where the wheel tries to rip the brake stud off when you apply it. I would still try one of these solutions.
