Using gear cable as brake cable

[FishFright]

I tried to bodge a gear cable with some FSA TT brake levers that used unusual cables. I few squeezes popped the nipple clean off the end of the cable

 

[Yellow Saddle]

I tried to bodge a gear cable with some FSA TT brake levers that used unusual cables. I few squeezes popped the nipple clean off the end of the cable

Now that is useful to know. Thanks.

 

[Ajax Bay]

I said: "Does a brake cable stretch a bit when applied? Yes, but so little as to have no functional effect (think about how the brake levers operate the cable and the cable operates the calipers). And so, being thinner gauge, the gear cable would stretch too, and maybe a tad more, but, again, so little as to have no functional effect."

I don't know where to start

I had hoped you would know and get stuck in. But I decided I'd try and quantify the approximate difference in stretch if one used a gear cable in place of a brake cable, and whether that would affect the effective operation of the brake calipers.
1) The length of pull on an STI (brake lever) is about 9mm max (ie from 'resting' position to touching the (drop) bar. Proper adjustment means that full strength braking is effected (with my adjustment) at 5mm (ie with 4mm 'spare'). Operating the rear calipers, would a gear cable stretch >4mm more than a brake cable would?
2) Wire ropes/cables do not possess a Young’s Modulus but an ‘apparent’ Modulus of Elasticity can be used and the various generic steel cable types have working Moduli of Elasticity. So using Hooke's Law:
Elastic Extension = FL/EA where F = load applied (N), L = rope length (mm), E = elastic modulus (N/mm2),A = metallic cross section (mm2)
F The 'full' (ie 4 fingers in opposition to the thumb) grip of an adult is in the range 250N-500N. Even from the drop bars (the strongest braking position for the hand (and for other reasons)) the full grip cannot be exerted so, say, a force of 200N can be applied. The mechanical advantage of the levers is 3 (vector distances from axis are 54mm and 18mm) so this implies an applied force of 600N on the cable (ie max cable tension).
L 1300mm is a typical rear cable run for a normal bike (580mm TT c-t-c) and cables under tape on drop bars. Hybrids/MTB cable run will be rather less.
E General E-module of the cable construction
Spiral strand, type 1x7 EM = 126000
A The area of a 1.2mm dia gear cable is 1.11mm2. Brake cable is 1.6mm dia so area = 2.01mm2

3) For 2.0mm2 area (brake) cable that gives an elastic extension of 3.1mm for a 600N load
For 1.1mm2 area (gear) cable that gives an elastic extension of 5.6mm for a 600N load
This means that the difference in elastic extension (stretch) between the two types of cable is 2.5mm.
Conclusion The STI levers have a margin (ie spare travel) of 4mm so using a gear cable, in extremis, would not result in the lever 'bottoming out' on the bars.
Comments
a. And since the main braking effect is using the front brake (which has a much shorter cable run) riders using a gear cable in place of a brake cable in an 'emergency' should maintain an effective braking system.
b. This result may not hold for flat bars - someone else can do the working, if they're so inclined. And off course long run cables (eg for tandems or recombents would result in significantly more stretch with the implicit failure of an effective braking system.
c. I fully appreciate that I have made assumptions to get this result - feel free to challenge those and indeed the calculation generally.
d. If the nipple won't stay on with (say) a 600N force applied (which may be what we should conclude from @FishFright 's experience) then this is academic.

 

[User6179]

I don't know but you can definitely use a bike cable as an accelerator cable on a 1980s mini metro .

 

[Racing roadkill]

I said: "Does a brake cable stretch a bit when applied? Yes, but so little as to have no functional effect (think about how the brake levers operate the cable and the cable operates the calipers). And so, being thinner gauge, the gear cable would stretch too, and maybe a tad more, but, again, so little as to have no functional effect."

I had hoped you would know and get stuck in. But I decided I'd try and quantify the approximate difference in stretch if one used a gear cable in place of a brake cable, and whether that would affect the effective operation of the brake calipers.
1) The length of pull on an STI (brake lever) is about 9mm max (ie from 'resting' position to touching the (drop) bar. Proper adjustment means that full strength braking is effected (with my adjustment) at 5mm (ie with 4mm 'spare'). Operating the rear calipers, would a gear cable stretch >4mm more than a brake cable would?
2) Wire ropes/cables do not possess a Young’s Modulus but an ‘apparent’ Modulus of Elasticity can be used and the various generic steel cable types have working Moduli of Elasticity. So using Hooke's Law:
Elastic Extension = FL/EA where F = load applied (N), L = rope length (mm), E = elastic modulus (N/mm2),A = metallic cross section (mm2)
F The 'full' (ie 4 fingers in opposition to the thumb) grip of an adult is in the range 250N-500N. Even from the drop bars (the strongest braking position for the hand (and for other reasons)) the full grip cannot be exerted so, say, a force of 200N can be applied. The mechanical advantage of the levers is 3 (vector distances from axis are 54mm and 18mm) so this implies an applied force of 600N on the cable (ie max cable tension).
L 1300mm is a typical rear cable run for a normal bike (580mm TT c-t-c) and cables under tape on drop bars. Hybrids/MTB cable run will be rather less.
E General E-module of the cable construction
Spiral strand, type 1x7 EM = 126000
A The area of a 1.2mm dia gear cable is 1.11mm2. Brake cable is 1.6mm dia so area = 2.01mm2

3) For 2.0mm2 area (brake) cable that gives an elastic extension of 3.1mm for a 600N load
For 1.1mm2 area (gear) cable that gives an elastic extension of 5.6mm for a 600N load
This means that the difference in elastic extension (stretch) between the two types of cable is 2.5mm.
Conclusion The STI levers have a margin (ie spare travel) of 4mm so using a gear cable, in extremis, would not result in the lever 'bottoming out' on the bars.
Comments
a. And since the main braking effect is using the front brake (which has a much shorter cable run) riders using a gear cable in place of a brake cable in an 'emergency' should maintain an effective braking system.
b. This result may not hold for flat bars - someone else can do the working, if they're so inclined. And off course long run cables (eg for tandems or recombents would result in significantly more stretch with the implicit failure of an effective braking system.
c. I fully appreciate that I have made assumptions to get this result - feel free to challenge those and indeed the calculation generally.
d. If the nipple won't stay on with (say) a 600N force applied (which may be what we should conclude from @FishFright 's experience) then this is academic.

You're talking to someone who's job this is, and has been for 17 years, and has a PhD in a subject that included a lot of this. I could start tearing what you've just posted to shreds, however.

This picture of a Panda will suffice.

 

[Racing roadkill]

I don't know but you can definitely use a bike cable as an accelerator cable on a 1980s mini metro .

True dat.

 

[S-Express]

You're talking to someone who's job this is, and has been for 17 years, and has a PhD in a subject that included a lot of this. I could start tearing what you've just posted to shreds, however.

However.....we'll just have to take your word for that. Like all your other claims to superior knowledge that you never seem to back-up...

 

[Yellow Saddle]

You're talking to someone who's job this is, and has been for 17 years, and has a PhD in a subject that included a lot of this. I could start tearing what you've just posted to shreds, however.
View attachment 356282
This picture of a Panda will suffice.

Your CV is quite impressive so far:

1) Made a lot of money selling gyroscopes
2) Met the queen.
3) Got a PhD in steel cable physics
4) Black belt n karate.
Edit: And rode with Chris Froome. (Sorry for omitting that RRC, please dont' rip me to shreds).

How do we know all this? You told us of course.

We stand reminded. Next goal: how to string an argument together.

 

[T.M.H.N.E.T]

Your CV is quite impressive so far:

1) Made a lot of money selling gyroscopes
2) Met the queen.
3) Got a PhD in steel cable physics
4) Black belt n karate.

How do we know all this? You told us of course.

We stand reminded. Next goal: how to string an argument together.

You forgot rode with Chris Froome

 

[Yellow Saddle]

You forgot rode with Chris Froome

Fixed that. Thanks.

 

[Tigerbiten]

I hate to say this but I've used a gear cable as a brake cable for over 6 years now.
BUT ............
It's used as a parking/drag brake working off a friction lever onto a disk brake on the back wheel of my recumbent trike.
Front/main brakes are twin hydraulics disks off one lever.
So with the right setup it can be done successfully but I wouldn't like to bodge it.

 

[Ajax Bay]

You're talking to someone who's job this is, and has been for 17 years, and has a PhD in a subject that included a lot of this. I could start tearing what you've just posted to shreds, however.

Late to this, so thanks for the Panda wave (regret significance completely over my head). Have just enjoyed a hundred k in the Devon sunshine: checking out how hard I can squeeze a brake (in Newtons?) when a tractor comes up the narrow 'gravel in the middle' road I'm going down; stopping to chat with a guy rebuilding a cob wall; repairing my first flat in 3 months; ziptying the bridge of my rear mudguard after it failed (fatique; more than 15Mm). I carry a spare bridge but reckoned the ziptie would be quicker.
I'm very keen to see where I've gone wrong (though 'shreds' seems a bit destructive). My work was just trying to see if some calculations based on reasonable assumptions supported what I thought initially (ie that using a gear cable to operate the rear brake would, in an emergency, work). My academic and experimental work in this area was a long time ago and you are therefor far more up to date. I can see that if it's your job, then spending your time on such elasticity trivia might be a bore, but think how others could benefit from your knowledge, if you were prepared to share it (eg on here).