C
On Jun 23, 8:34 pm, cycledogg <[email protected]> wrote:
> Hello,
> I did a search on this question thinking it may have been talked
> about already, but didn't find any results. It's a simple question as
> to building a time trial bike. Should a longer crank arm length (175)
> be used or keep the current size I am using on my road bikes of 172.5?
> I understand that a longer arm may give me better leverage to turn the
> crank but only a 2.5 mm difference is not going to be a factor. I am
> going to use larger rings (55x42). Thanks to all that give helpful
> answers.
> Cheers,
> Rick in Tennessee
Dear Rick,
An email asked me about this, probably because I posted some simple
numbers a long time ago, so I'm cross-posting to RBT.
Short answer, probably no perceptible difference.
A longer crank lowers the overall gearing, but 175/172.5 = 1.0145,
less than a single tooth on your front ring (55/54 = 1.0185).
In terms of fit, most riders from 5'4" to 6'4" use 160 to 180 mm
cranks. This means that their crank length varies less
(proportionally) than their height.
76/64 = 1.1875
180/160 = 1.1250
In other words, the riders probably adapt to the crank length.
In previous threads, some posters have claimed exquisite sensitivity
to 172.5 mm versus 175 mm (or versus 170 mm) cranks, saying that the
longer crank makes their knees hurt.
Other posters have admitted riding with the "wrong" size cranks (175
mm instead of 172.5) or even with mismatched cranks (172.5 on one side
and 175 on the other), which is hardly surprising, given that most
people have somewhat insensitive legs of slightly different lengths.
Assuming that your legs are fully extended at the bottom of the pedal
cycle, the larger crank means that you raise your feet 2.5 mm higher
with the 175 mm crank and wave them that tiny distance further forward
(and backward). So you could hunker down a tenth of an inch further
with the smaller crank.
You could experiment by having someone swap a few cranks and seeing if
you notice the difference.
Cheers,
Carl Fogel
> Hello,
> I did a search on this question thinking it may have been talked
> about already, but didn't find any results. It's a simple question as
> to building a time trial bike. Should a longer crank arm length (175)
> be used or keep the current size I am using on my road bikes of 172.5?
> I understand that a longer arm may give me better leverage to turn the
> crank but only a 2.5 mm difference is not going to be a factor. I am
> going to use larger rings (55x42). Thanks to all that give helpful
> answers.
> Cheers,
> Rick in Tennessee
Dear Rick,
An email asked me about this, probably because I posted some simple
numbers a long time ago, so I'm cross-posting to RBT.
Short answer, probably no perceptible difference.
A longer crank lowers the overall gearing, but 175/172.5 = 1.0145,
less than a single tooth on your front ring (55/54 = 1.0185).
In terms of fit, most riders from 5'4" to 6'4" use 160 to 180 mm
cranks. This means that their crank length varies less
(proportionally) than their height.
76/64 = 1.1875
180/160 = 1.1250
In other words, the riders probably adapt to the crank length.
In previous threads, some posters have claimed exquisite sensitivity
to 172.5 mm versus 175 mm (or versus 170 mm) cranks, saying that the
longer crank makes their knees hurt.
Other posters have admitted riding with the "wrong" size cranks (175
mm instead of 172.5) or even with mismatched cranks (172.5 on one side
and 175 on the other), which is hardly surprising, given that most
people have somewhat insensitive legs of slightly different lengths.
Assuming that your legs are fully extended at the bottom of the pedal
cycle, the larger crank means that you raise your feet 2.5 mm higher
with the 175 mm crank and wave them that tiny distance further forward
(and backward). So you could hunker down a tenth of an inch further
with the smaller crank.
You could experiment by having someone swap a few cranks and seeing if
you notice the difference.
Cheers,
Carl Fogel