Wrong crank length on new bike

noodleman

philthy3 wrote:

wavefront wrote:

philthy3 wrote:

For a distance of 50km, the 170mm cranks would need to go through 4681 revolutions or 499930.8cm. The 172.5mm cranks would go through 4613 revolutions or 499587.9cm. So, less revolutions for the longer cranks, because of the greater turning circle.

Ok, maths sorted, unfortunately logic still a little flawed :?

If you're saying riding 50km on a bike with a smaller crank length will require less pedal revolutions then perhaps mount your bike on your turbo, and closely try and observe this for real.

If you're saying your feet may physically move slightly less in a rotational distance with a smaller crank length for a given distance travelled by the bike then yes, but how does that matter?

The guy who does bike fits at Sigma Sport talks a lot of sense. Forward to 3.35

https://m.youtube.com/watch?v=pKccqlqoJGQ

I'm saying shorter cranks will require more revolutions if you read it.

I fully support the view that shorter cranks matter for shorter riders or those who prefer a high cadence. I rode with the standard 172.5mm cranks for a long time, suffering with knee pain and hip problems. Moving to 165mm cranks has saved those issues. As I posted earlier, there will be those who can't tell the difference and pooh-pooh the very notion of it being any help. But there are those of us can tell the difference and do find it a help.

The OP was asking if 2.5mm is a noticeable amount. Maybe some folk will notice it (I cant) but your statement seems based on your experience of a 7.5mm difference.

wavefront

philthy3 wrote:

I'm saying shorter cranks will require more revolutions if you read it.

Put your bike on a turbo, sit by the side of it, spin the crank, try out your theory. Observe. Observe hard. Think about it. Then come back and please edit the above.

Thank you.

(I agree with your philosophy of trying and being able to notice different crank lengths. Some will. Some won't).

figbat

There's no question that your feet will travel further with a longer crank, for a given speed and gearing, which will mean your muscles will work over a slightly longer stroke and your joints will flex slightly further - this may add up to extra fatigue or may be countered by the additional torque you can generate. Some notice it, others don't.

But as others above have noted, cadence is not affected by crank length for a given speed and gearing.

I would be more concerned about how it all works for me in real life than how close it is to some idealised fitment diagram.

fenix

philthy3 wrote:

I'm saying shorter cranks will require more revolutions if you read it.

Doesn't matter where I push on the crank. 1 revolution still takes me the same distance....

StillGoing

Fenix wrote:

philthy3 wrote:

I'm saying shorter cranks will require more revolutions if you read it.

Doesn't matter where I push on the crank. 1 revolution still takes me the same distance....

Draw a circle with a 34.5cm diameter and then lengthen the circumference out. Is it longer than the circumference of the smaller circle with a 34cm diameter? Of course it is, roughly 16cm longer. Your feet would therefore have to travel at a higher speed to achieve 100rpm cadence. 100rpm is 100rpm no matter what the length.

davis

philthy3 wrote:

Fenix wrote:

philthy3 wrote:

I'm saying shorter cranks will require more revolutions if you read it.

Doesn't matter where I push on the crank. 1 revolution still takes me the same distance....

Draw a circle with a 34.5cm diameter and then lengthen the circumference out. Is it longer than the circumference of the smaller circle with a 34cm diameter? Of course it is, roughly 16cm longer. Your feet would therefore have to travel at a higher speed to achieve 100rpm cadence. 100rpm is 100rpm no matter what the length.

Are you genuinely convinced you're making rational sense, or are you perhaps flame-baiting?

daniel_b

philthy3 wrote:

Fenix wrote:

philthy3 wrote:

I'm saying shorter cranks will require more revolutions if you read it.

Doesn't matter where I push on the crank. 1 revolution still takes me the same distance....

Draw a circle with a 34.5cm diameter and then lengthen the circumference out. Is it longer than the circumference of the smaller circle with a 34cm diameter? Of course it is, roughly 16cm longer. Your feet would therefore have to travel at a higher speed to achieve 100rpm cadence. 100rpm is 100rpm no matter what the length.

But that is not the same as revolutions (As per your first qoute above) - there is no difference in the number of revolutions.
The physical distance the foot travels will differ, as the length of the cranks are different, but the revolutions are identical.

Imho the physical distance the foot travels is neither here nor there, it's all about how it feels to the rider, and the respective amount of leverage they need to generate to turn the cranks.

wavefront

philthy3 wrote:

Draw a circle with a 34.5cm diameter and then lengthen the circumference out. Is it longer than the circumference of the smaller circle with a 34cm diameter? Of course it is, roughly 16cm longer. Your feet would therefore have to travel at a higher speed to achieve 100rpm cadence. 100rpm is 100rpm no matter what the length.

Indeed, apart from the missing decimal place.

Glad you realise that you won't need any or less pedal revolutions to travel 50km on the bike, no matter the crank length ("100 rpm is 100 rpm no matter what the length"). I was a bit worried you wouldn't get there

kingpinsam

What really annoys me is that I mine say 170mm on them, but I got the micrometer out and the left one is 170.0024mm and the right is 169.9998mm, it means I can't ride the bike more than 20m without being thrown violently from the saddle.

svetty

wavefront wrote:

philthy3 wrote:

Draw a circle with a 34.5cm diameter and then lengthen the circumference out. Is it longer than the circumference of the smaller circle with a 34cm diameter? Of course it is, roughly 16cm longer. Your feet would therefore have to travel at a higher speed to achieve 100rpm cadence. 100rpm is 100rpm no matter what the length.

Indeed, apart from the missing decimal place.

Glad you realise that you won't need any or less pedal revolutions to travel 50km on the bike, no matter the crank length ("100 rpm is 100 rpm no matter what the length"). I was a bit worried you wouldn't get there

I've had the same debate with 'Phil' some months back......

EBEB

I’m 6’4” most bikes have 175s, but my TT bike had 172.5. Could not tell the difference at all.

TBH, you are going to get a much bigger difference by how much you flex your ankles and how you time it.

fenix

And also how you sit on the saddle. I know I'm not in the same place every time.

StillGoing

Daniel B wrote:

philthy3 wrote:

Fenix wrote:

philthy3 wrote:

I'm saying shorter cranks will require more revolutions if you read it.

Doesn't matter where I push on the crank. 1 revolution still takes me the same distance....

Draw a circle with a 34.5cm diameter and then lengthen the circumference out. Is it longer than the circumference of the smaller circle with a 34cm diameter? Of course it is, roughly 16cm longer. Your feet would therefore have to travel at a higher speed to achieve 100rpm cadence. 100rpm is 100rpm no matter what the length.

But that is not the same as revolutions (As per your first qoute above) - there is no difference in the number of revolutions.
The physical distance the foot travels will differ, as the length of the cranks are different, but the revolutions are identical.

Imho the physical distance the foot travels is neither here nor there, it's all about how it feels to the rider, and the respective amount of leverage they need to generate to turn the cranks.

I've never said anything any different than 100rpm is 100rpm? The point I make is not about the distance the foot has to travel with longer cranks, but more the speed the feet will go to maintain 100rpm is lower with the shorter cranks because of the smaller length of the circumference. The main benefit of shorter cranks is the increased room for the hips at the top of the pedal cycle.

Some won't see that and because they don't notice it rubbish the idea that anyone else would benefit or tell the difference.

fenix

philthy3 wrote:

I'm saying shorter cranks will require more revolutions if you read it.

daniel_b

Fenix wrote:

philthy3 wrote:

I'm saying shorter cranks will require more revolutions if you read it.

Gotta love the qoute system.



Good luck to the OP on getting a resolution he is happy with.

cld531c

I would email wiggle. In the meantime try it on the turbo to see if YOU notice a difference/issue. That is all that matters.

I bought the De Rosa idol in 47cm from Wiggle which came with 170 cranks so am surpised that they would put longer ones on a 48cm.
When I asked wiggle before I bought it they had no idea of crank length or bar width but said if I wasnt happy I could return it. TBH if it had come with longer (or shorter) cranks I would have done (but Im glad it didnt).

Good luck - pretty bike so it would be a shame to lose out

StillGoing

Fenix wrote:

philthy3 wrote:

I'm saying shorter cranks will require more revolutions if you read it.

For the distance was the context the quote was made. If you ride 1 mile, you will turn the pedals more times with shorter cranks than you would with longer cranks with the exact same gearing. Presumably that is beyond your logic? :roll:

daniel_b

It's like riders on a Velodrome - if you ride on the sprinters line, it's the shortest distance around the track, if you are up near the hoardings, it's a lot of metres more, BUT you still complete one revolution of the track - which is equal in the above scenario to one revolution of the pedals.

The rider at the top will physically travel further, and that is parallelled by the foot physically tracing a bigger distance in the air through each revolution with longer cranks.

Personally I think this is a pretty good wind up though - I'd give it 7/10.

cld531c

philthy3 wrote:

Fenix wrote:

philthy3 wrote:

I'm saying shorter cranks will require more revolutions if you read it.

For the distance was the context the quote was made. If you ride 1 mile, you will turn the pedals more times with shorter cranks than you would with longer cranks with the exact same gearing. Presumably that is beyond your logic? :roll:

Beyond everyones I think. Same gear and wheel size = same number of revolutions irrespective of crank size.

fenix

Philthy - look at it this way - one revolution of the crank gives a fixed distance travelled.

Now you could drill the cranks and install pedals half way down the crank if you wanted to.

One revolution of those pedals still takes you exactly the same distance as before.

noodleman

philthy3 wrote:

Fenix wrote:

philthy3 wrote:

I'm saying shorter cranks will require more revolutions if you read it.

For the distance was the context the quote was made. If you ride 1 mile, you will turn the pedals more times with shorter cranks than you would with longer cranks with the exact same gearing. Presumably that is beyond your logic? :roll:

You really need to think that through. Even if there was no crank arm on the chainset and you just painted a dot where the arm used to be, that dot would still only do the same revolution through a complete turn as a dot painted on the end of a 160, 170, 172.5 length arm. How could it do less???

fenix

I think he's getting confused with the distance that the feet would travel in a circle rather than the distance the bike goes forward.

wavefront

philthy3 wrote:

Fenix wrote:

philthy3 wrote:

I'm saying shorter cranks will require more revolutions if you read it.

For the distance was the context the quote was made. If you ride 1 mile, you will turn the pedals more times with shorter cranks than you would with longer cranks with the exact same gearing. Presumably that is beyond your logic? :roll:

No.

mercia_man

Fenix wrote:

Philthy - look at it this way - one revolution of the crank gives a fixed distance travelled.

Now you could drill the cranks and install pedals half way down the crank if you wanted to.

One revolution of those pedals still takes you exactly the same distance as before.

Philthy - this sums it up perfectly. No need to quote complex formulae at great length. Think of a penny farthing with direct drive and 72 inch wheel. One complete revolution will result in the bike moving forward 72 inches - accordingly that bike has a 72 inch gear. Having longer or shorter cranks will make no difference to the distance covered by that wheel in one revolution. You turn the cranks once, the wheel will do one revolution.

Having said that, I agree with you that there is a real benefit for shorter riders having shorter cranks. My wife being one example. Expert bike designer Mike Burrows is a huge believer in short cranks for everyone. He is a fan of 140 mm cranks.

Of course, none of this is much help in answering the poor old OP's question.

DeVlaeminck

Anyway check if there is an official spec and if it says 170 see if they will exchange or replace. Fwiw I can tell the difference between 175 172.5 and 170 so I don't think you are being unreasonable with a new bike but worst case you may need to take a hit and buy some of the preferred length and recoup some of that selling the old ones.

stevie63

I'm another shorter rider and 165mm work much better for me than the 172.5 I had used before. For example if in the drops on the longer cranks my knees would make contact with my stomach. This also caused toe overlap with the front wheel.

pilot_pete

How does the length of your spade handle affect the depth of the hole you dig for yourself? Anyone....? :oops:

PP

svetty

Pilot Pete wrote:

How does the length of your spade handle affect the depth of the hole you dig for yourself? Anyone....? :oops:

PP

However long the handle, it wouldn't be the sharpest spade in the box

on-yer-bike

Get the cranks you want. Don't compromise on a bike like this. You will always be thinking about it if you don't.

blueturtle

Hi all, thanks for all the input re. crank length . I have decided with much deliberation to send the De Rosa back to Wiggle. Yes I could have kept it and swapped out the chainset ,but why spend £3.5k+ on something that was not right in the first place.I don't mind changing the stem length or the saddle if needs be but I had to draw the line somewhere and changing the crankset was a step to far. Incidently I e-mailed Wiggle and the came back with the reply,quote " I can advise that the length of the cranks is not set by the frame size rather it takes into account the geometry of the frame as a whole" unquote. That's a new one on me! Methinks after this episode there is something to be said about going to your local bike shop,cost you more but at least you can see what you are buying before handing over cash.Thanks again for all your advice.