P
Pete Biggs
Guest
Below is a copy of an article posted to another forum. It is all correct and reasonable?
thanks ~PB
=================================
The only difference that there can ever be between two brakes operating from the same lever with the
same clearance is that due to progression. This is the amount that the mechanical advantage changes
as the brake moves through it's travel.
The simplest way this is done is to move the pivots so that the radius between the pad and pivot is
at about 45 degrees to the rim face. This is what V brakes (and cantis) do with their pivots on the
forks. Cantis would also be progressive like V brakes if it weren't for the straddle cable
undermining it all again. However even V brakes only exhibit about 15% improvement over a neutral
mechanism due to progression, the rest of their performance is due to the increased cable travel.
The ultimate force available is proportional to the square of the cable travel, so with double the
travel of a drop bar lever you could get up to four times the braking. In practice you wouldn't need
that much so I doubt that V brakes are optimised for maximum force. Either way, the 15% improvement
from progression looks pretty insignificant. All of this argument ignores friction, if you include
the effect of friction you gain even more with extra cable travel, because cable tension, and hence
friction is reduced.
Cable stretch: All materials stretch/deform by a amount proportional to the load applied, whilst the
load remains within the elastic limit. Beyond the elastic limit the structure takes a permanent set.
The stretch on the cable is well within the elastic limit, but with 1.3m of cable and only 10mm of
travel, it doesn't take much force to soak up half the travel. The front cable is half the length
and so requires twice the force to stretch the same amount of course. The amount of stretch (S) is:
S=FL/(AE) where L is the unstretched length F is the force applied A is the cross sectional area of
the cable and E is the Youngs Modulus for stainless steel, a factor that defines how rigid a
material is.
thanks ~PB
=================================
The only difference that there can ever be between two brakes operating from the same lever with the
same clearance is that due to progression. This is the amount that the mechanical advantage changes
as the brake moves through it's travel.
The simplest way this is done is to move the pivots so that the radius between the pad and pivot is
at about 45 degrees to the rim face. This is what V brakes (and cantis) do with their pivots on the
forks. Cantis would also be progressive like V brakes if it weren't for the straddle cable
undermining it all again. However even V brakes only exhibit about 15% improvement over a neutral
mechanism due to progression, the rest of their performance is due to the increased cable travel.
The ultimate force available is proportional to the square of the cable travel, so with double the
travel of a drop bar lever you could get up to four times the braking. In practice you wouldn't need
that much so I doubt that V brakes are optimised for maximum force. Either way, the 15% improvement
from progression looks pretty insignificant. All of this argument ignores friction, if you include
the effect of friction you gain even more with extra cable travel, because cable tension, and hence
friction is reduced.
Cable stretch: All materials stretch/deform by a amount proportional to the load applied, whilst the
load remains within the elastic limit. Beyond the elastic limit the structure takes a permanent set.
The stretch on the cable is well within the elastic limit, but with 1.3m of cable and only 10mm of
travel, it doesn't take much force to soak up half the travel. The front cable is half the length
and so requires twice the force to stretch the same amount of course. The amount of stretch (S) is:
S=FL/(AE) where L is the unstretched length F is the force applied A is the cross sectional area of
the cable and E is the Youngs Modulus for stainless steel, a factor that defines how rigid a
material is.