Cranklength query

[jimbut]

Why the need for different crank lengths ? Is it related to foot size ?
As an addendum ( is that the right word ) to that. My left foot is rotated outwards slightly more than the right ( actually it`s prob the leg that is rotated ) and as a conseq. my left heel rubs the crank ( right at the spindle ). The left shoe is severely scraped at the inner heel and I cannot use overshoes as a hole quickly wears in the left shoe. What might be a fix for all this. A new foot / leg is not on the cards. My feet are on the small size ( UK 7.5 ) and my cranks are 170mm. Ta for all replies.

 

[jimboalee]

170mm is the most popular because they suit an 'Average' inside leg measurement.
The length is usually 5.5 x inside leg of your trousers.

My trousers are 32" waist, 31" inside leg Burton's tailored.

5.5 x 31 = 170.5, rounded to 170.

Strangely, but not entirely surprising, Professionals go for longer cranks, because Nm = N x m. Torque = Newtons force ( kg x 9.81 ) x meters radius.

When I got to ride Tom Simpson's bike in *********, Nottinghamshire, he had 175 cranks, and it did feel weird.

 

[cyclenic]

I'm not a technical gal, does crank length mean the measurement between your bum on the seat and your foot on the pedal?

 

[HLaB]

I'm not a technical gal, does crank length mean the measurement between your bum on the seat and your foot on the pedal?

No its the dimension between your bottom bracket/ axle and the pedal spindle; its the length of the pedal arm or crank.

 

[Mr Pig]

my left heel rubs the crank. What might be a fix for all this.

Do you use clipless pedals, clips or Power Grips?

 

[jimboalee]

More lost cycle terminology.

175mm cranks were 7" cranks.

7" was the 'adults' usual. More recent chainsets were reduced to 170mm for two reasons. 1/ better fitted the average Japanese and 2/ less metal - less cost.

6" was the 'youths' and 5" was 'juniors' as on my Moulton Mini.

 

[02GF74]

The length of crank is related to the length of bones in your leg - I don't recall what it is but google will tell ya.

Having said that, most mountain bike cranks tend to be longer for the extra leverage, usually 175 mm.

Your heel rubbing could be helped a bit by setting the cleats on your shoe to "peersuade" your foot to point outwards t the heel. Do this by little increments since your leg is set into a somewhat unnatrual position and trying to force it stright my cause injury in the joint. Also when cycling, think about your leg position and try to adjust it.

 

[Saddle bum]

The original british standard crank was 6-3/4 in, this actually equates to 171.4mm. So if you use 170mm cranks, you are riding around on shorter cranks than previous.

At 5ft9in (1.76m) I now ride 172.5 cranks. I have been ridng them for about 5 years. They are a better choice than 170s as far as I am concerned. They only time I actually noticed the difference was going up hills - more power.

 

[jimboalee]

More power because kW = ( Torque Nm x RPM) / 9459

 

[jimbut]

Ta for ALL comments. I use clipless pedals. I`ve recently adjusted the left cleat ( moved it in nearer the bike, thereby moving foot outwards ). This obviously sort of unbalances the riding slightly ( more leverage with the left foot ). The left heel now just misses the crank ( prob about 2mm in it ). Position feels not too odd but there is still no way an overshoe could be accomodated on the left shoe. Ah well, live wi it I suppose ! I`ll be upgrading this yoke soon ( if not splashing out on a new ( Felt F3 ??? ). I`ll perhaps try 172mm cranks.

 

[Mr Pig]

Have you tried moving the seat etc?

 

[sloe]

OTOH the shorter the crank the quicker a cadence you can sustain.

 

[PapaZita]

In my case, a rotated foot was corrected by a pair of Specialized shoes with decent arch support and some wedges under the cleats. I'm much more comfortable now. I'd suggest that a decent bike fit session would be a sensible investment, and will help to identify and correct the underlying cause of your wonky foot.

PZ.

 

[jimboalee]

Check Ribble website for availablity.

You might find 172.5mm are only available on more expensive sets.

 

[mr Mag00]

after being away from road riding for ages i have put a 172.5 mm and 38 front chainset on my road bike as i get stronger i want to put back the old 175 and 43? chainset. is this a good idea?