Technical Handbrake cable stretching, for the bored!

[atom0007]

I have read about this issue in a few posts now: if one handbrake cable snaps, the other might stretch so it is worth changing it too.

I have done a quick calculation to see if this is true:



This shows that for a 1mm diameter steel wire, you would need to supply a tensile load of 1000N (appromately 100kg equivalent) in order to stretch the wire by 0.6%. For a handbrake cable of 1m length, this is a 6mm stretch.

For several reasons, I think that the cable does not stretch:
1. A typical cable is much bigger than 1mm
2. I doubt the handbrake applies 100kg load to the cable
3. The plastic deformation point onset is something like 1/2 tonne of load, for a 1mm wire! In other words, any extension actually achieved would be a reversible extension, rather than an irreversible stretch.

The better reason to switch a second cable is that the cross sectional area is likely to me tiny due to corrosion. This results in the ultimate strength of the cable being achieved by the handbreak, and ping!

Anyway, that's the theory....how about the practice?

 

[Dcthebear]

Take into account corrosion though! That cable tested was new yes? Years of salt water corrosion will weaken the cable.

 

[atom0007]

What I'm saying is that if you've managed to stretch your cable, you'll be at the point of the cable having a tiny cross section due to corrosion anyway.

In other words, if you have actually managed to stretch your cable, it will be so thin from rust that it will need changed anyway.

Rust is the killer, and the reason for change, not irreversible cable stretch due to having the load on one cable only.

 

[ycming]

I was just searching for drum brake calculation on google and this came up. although its an old post but an interesting post.

Typical test force for handbrake would be approx 400N, measured from center of grip. Although it is far from 1000N that was mentioned by atom007, but you do have mechanical advantage for handbrake lever. Anyone measured the distances so we can work out the ratio to get a better picture?

Also fatigue I guess... but that's something I need to have a good read as I have no idea how to work that out... :bang:

 

[s and b]

I was just searching for drum brake calculation on google and this came up. although its an old post but an interesting post.

Typical test force for handbrake would be approx 400N, measured from center of grip. Although it is far from 1000N that was mentioned by atom007, but you do have mechanical advantage for handbrake lever. Anyone measured the distances so we can work out the ratio to get a better picture?

Also fatigue I guess... but that's something I need to have a good read as I have no idea how to work that out... :bang:

and back in the room

 

[ycming]

Aye... I did some calculation on a random handbrake lever today, the lever ratio is 3.87. Therefore at test force of 400N that equates to 1548N a bit more than we thought!

 

[atom0007]

Can't believe I wrote that. Quite Interesting....to a degree. For the bored.

What I'm saying is that if you've managed to stretch your cable, you'll be at the point of the cable having a tiny cross section due to corrosion anyway.

1. Your 1.5kN would certainly stretch a 1mm wire, but (even for "standard" steel), this would still be in the linear regime, i.e. a completely reversible deformation on removal of load.

2. The handbrake cable is certainly bigger than 1mm in cross section. This reduces the stress to way less than the yield point.

With regards to fatigue, as I understand it, this would certainly reduce the Yield Point.

I can say from direct experience on my broken cable that it looked like corrosion thinning the cross section that resulted in fracture, rather than fatigue. But you'd have to look a lot closer than one case to see.

As I said, that's the theory. I'm still waiting for someone to blow this out of the water with a handbrake cable showing necking.

And out of the room, for a few months....