Best way to repair my steel torque arm

C

cwah

100 MW
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hello

I have my bike with variable regen and needed a strong torque arm preventing any movement.

I've used 2 mild steel torque arm of 8mm with 6mm stainless bolt and it lasted well for a year or so.

But now it has split in half for one:
MVIMG-20190706-120832.jpg


And the stainless steel bolt split in 2 on the other side:
MVIMG-20190706-125532.jpg


I'm hesitating between 2 options to repair this:
1. Buy 5mm thick mild steel angle bracket: https://www.ebay.co.uk/itm/Mild-Steel-Angle-Equal-Unequal-Various-Sizes-Lengths-Angle-Iron/291739479215?var=590761552991
That way the angle bracket should be stronger against lateral force. Is mild steel good? Shall I get cold steel?

2. Buy a welding machine and spot weld back the steel bar in the area it split. But I don't know which machine to buy? Any idea?

Thank you
 
Thicker steel. Part of why I used a thick angle iron to make a similar TA. So there would be some meat on both sides of the holes I drilled.
 
The damage is from insufficient material. Why not making them better, either bigger or using better material.

Welding more material on a broken part is ugly, temporary repair.
 
MadRhino said:
The damage is from insufficient material. Why not making them better, either bigger or using better material.

Welding more material on a broken part is ugly, temporary repair.
Click to expand...

What better material should it be?
 
dogman dan said:
Thicker steel. Part of why I used a thick angle iron to make a similar TA. So there would be some meat on both sides of the holes I drilled.
Click to expand...

25x25x5mm mild steel bracket ok?
https://www.ebay.co.uk/itm/Mild-Steel-Angle-Equal-Unequal-Various-Sizes-Lengths-Angle-Iron/291739479215?var=590761552991

I can't get any longer and I need some space to tighten the motor nut
 
cwah said:
MadRhino said:
The damage is from insufficient material. Why not making them better, either bigger or using better material.

Welding more material on a broken part is ugly, temporary repair.
Click to expand...

What better material should it be?
Click to expand...

Harder steel grade
Titanium

Easy to find better, easy to make bigger. What is important is that you improve the next, from this experience.
 
Your torque arm is only as strong as it's weakest point. You used an 8mm bar with a 6mm hole in it. 8 - 6 = 2. So you had a 2mm torque arm.
Secondly, you used Mild steel. This is a job for 4130 chromoly. 50% stronger, twice as hard, able to deform twice as much before cracking. And 4130 is still easy to work with using basic tools. no special bits, files, or saw blades needed.
Using 4130, you might be able to remake that part using 12mm bar, but I would go thicker in depth, the direction the bolts are going through. So 12mm wide, but 24mm deep. 1/2" x 1" would be the standard equivalent, and easier to find.

Those bolts are going to be a problem too. A2-70 is only fit for securing fenders, access panels, or signs on poles. It's really not a structural bolt. Stainless isn't generally used for structural parts unless it's massively oversized. You need to be using a normal bolt of at least 8.8, maybe a 10.9 depending on your design and desired failsafe mode.
 
Don’t tell us that nobody sells flatstock in UK.
Take it from the scrapyard, cut it off any old part that’s big enough, or buy a fresh new piece of flatstock that can be found in UK as well as Burundi.
 
It looks like there was a lot of corrosion going on that may have contributed to the failure. Better material would help. Chromoly is good or any kind of stainless would be better than mild steel. The basic design looks good. Thicker would help.
 
thanks guys.

I just wonder where to buy chromoly. It's easy to get Stainless or Mild steel. But stainless is such a pain to drill I decided to use mild steel.... Chromoly I got almost nothing on ebay.
https://www.ebay.co.uk/sch/i.html?_odkw=chromoly&_osacat=0&_from=R40&_trksid=p2045573.m570.l1313.TR2.TRC0.A0.H0.Xchromoly+bar.TRS0&_nkw=chromoly+bar&_sacat=0
 
Not Cromoly bolts, just grade 8.8 or higher standard metric bolts. 8.8 is equal to grade 5 is standard sizes, 10.9 is equal to grade 8 in American standard.

4130 is the USA standard, and common all over the english speaking world, but the European standard for the same steel is called 25CrMo4. You may have more luck finding it by that name. I found no barstock on Ebay for either of those names, but Ebay is dying. there is less and less on it these days. Your best bet is to check with a local machine shop to see where they would get 4130 / 25CrMo4, or check with a local metal supplier. it's a super common steel, used in just about everything.
 
fechter said:
It looks like there was a lot of corrosion going on that may have contributed to the failure. Better material would help. Chromoly is good or any kind of stainless would be better than mild steel. The basic design looks good. Thicker would help.
Click to expand...

If by "basic design" you mean "design approach", then OK. But these torque arms have a basic design flaw in that the holes drilled were too large for the 8 mm stock used. I guess it all depends on what you mean by "design."

I'd consider making the shape one piece like a dropout and only using one clamping bolt at the bottom of the axle slot.
 
It would be easier on the bolts if you use long shank ones that don't thread into the other piece (like the bottom one) but have nuts on the ends. Then it causes less cracks in the threads from not getting levered back and forth on a tiny area of the bolt. You want to size the shank length to as long as possible without sticking out the back.
download (4).jpeg
 
Drunkskunk said:
Not Cromoly bolts, just grade 8.8 or higher standard metric bolts. 8.8 is equal to grade 5 is standard sizes, 10.9 is equal to grade 8 in American standard.

4130 is the USA standard, and common all over the english speaking world, but the European standard for the same steel is called 25CrMo4. You may have more luck finding it by that name. I found no barstock on Ebay for either of those names, but Ebay is dying. there is less and less on it these days. Your best bet is to check with a local machine shop to see where they would get 4130 / 25CrMo4, or check with a local metal supplier. it's a super common steel, used in just about everything.
Click to expand...

Thanks. Do you mean something like this? It says 8.8
https://www.ebay.co.uk/itm/M6-6mm-BLACK-FULLY-THREADED-SET-SCREWS-HIGH-TENSILE-GRADE-8-8-HEXAGON-HEX-BOLTS/122438700558?hash=item1c81ea4e0e:m:mv9Y280RH_FFuLoMU7Mu6GQ

Or this? It says 12.6 so maybe better although made in china?
https://www.ebay.co.uk/itm/M6-6mm-High-Tensile-12-9-Hex-Head-Bolts-Hexagon-Screws-Hex-Tap-Bolts/322376322354?hash=item4b0f204d32:m:mHlPJ8RpnN_Icj53nuT6_Ng
 
I have cut my first dropout plates off a flat crowbar more than 10yrs ago, and they are still riding hard. At the scrap yard, buy a suspension blade off an old truck. You will have enough to make 100 torque arms for a few buck. Hard steel is everywhere.
 
Drunkskunk said:
Your torque arm is only as strong as it's weakest point. You used an 8mm bar with a 6mm hole in it. 8 - 6 = 2. So you had a 2mm torque arm.
Secondly, you used Mild steel. This is a job for 4130 chromoly. 50% stronger, twice as hard, able to deform twice as much before cracking. And 4130 is still easy to work with using basic tools. no special bits, files, or saw blades needed.
Using 4130, you might be able to remake that part using 12mm bar, but I would go thicker in depth, the direction the bolts are going through. So 12mm wide, but 24mm deep. 1/2" x 1" would be the standard equivalent, and easier to find.

Those bolts are going to be a problem too. A2-70 is only fit for securing fenders, access panels, or signs on poles. It's really not a structural bolt. Stainless isn't generally used for structural parts unless it's massively oversized. You need to be using a normal bolt of at least 8.8, maybe a 10.9 depending on your design and desired failsafe mode.
Click to expand...
OP, listen to this guy. I own a machine shop in the US. The failure point was where the material is far too thin to last long at this job. There are higher quality plated bolts available if you know where to look. Here in the US an easy source is McMaster-Carr. Perhaps there is an equivalent in your area. Good luck!
 
I have no idea where to find these part. I don't know where to go for scrap yard.

I think I'll stick to buying online. So this 25x25x5mm:
https://www.ebay.co.uk/itm/Mild-Steel-Angle-Equal-Unequal-Various-Sizes-Lengths-Angle-Iron/291739479215?var=590761552991

Clamped with this:
https://www.ebay.co.uk/itm/M6-6mm-High-Tensile-12-9-Hex-Head-Bolts-Hexagon-Screws-Hex-Tap-Bolts/322376322354?hash=item4b0f204d32:m:mHlPJ8RpnN_Icj53nuT6_Ng

Is it the best I can do with getting part online?
 
cwah said:
Is it the best I can do with getting part online?
Click to expand...

Probably not.

https://www.aircraftspruce.eu/metals---plastics/steel.html

Google search is your friend.
 
Thank you very much. couldn't find it.

So I get this 1.2" x 1" 1 feet?
https://www.aircraftspruce.eu/4130-rect-tube-1-2x1x-065-1ft.htm

And include these bolts?
https://www.ebay.co.uk/itm/M6-6mm-High-Tensile-12-9-Hex-Head-Bolts-Hexagon-Screws-Hex-Tap-Bolts/322376322354?hash=item4b0f204d32:m:mHlPJ8RpnN_Icj53nuT6_Ng
 
I've used two mild steel torque arm of 8mm with 6mm stainless [https://www.classicmetallic.com/product/hex-bolts/]bolt [/https://www.classicmetallic.com/product/hex-bolts/] a few years back. I am still using the same.
 
katrina said:
I've used two mild steel torque arm of 8mm with 6mm stainless [https://www.classicmetallic.com/product/hex-bolts/]bolt [/https://www.classicmetallic.com/product/hex-bolts/] a few years back. I am still using the same.
Click to expand...

Thanks. How do you order there? Is it just contacting them? so it's a custom order? May be expensive?
 
Best way to repair my steel torque arm
Click to expand...

1/4" thick FLAT BAR STEEL
by
4-8" long x 1-1/2" wide

https://www.canadiantire.ca/en/pdp/steelworks-weldable-steel-hot-rolled-4-x-1-4-in-0616141p.html#srp
https://www.princessauto.com/en/detail/3-16-x-2-in-hot-rolled-flat-steel/A-p8583528e

+
Hose clamps!









:wink:
 
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