GRX with 165mm cranks

[The Ghost]

I have my GRX (810-2) equipped bike arriving soon and it has 172.5mm cranks.

 

If I want to change them for 165mm cranks, what are my options and are there important things to consider in terms of compatibility?

 

 

I see on the Shimano website that 170mm arm length is listed as the shortest option whereas for the GRX 600-11 lists a 165mm option, would those cranks be compatible?

 

Thanks

[King_Crispy]

Unless you're a midget, why would you want to go that short?

[buckstopper]

I'm sure they would be compatible. I would love to get my hands on a 165, I'm 177cm with an 85 inseam. If they were available for testing I'd try even shorter

[Jewbacca]

Unless you're a midget, why would you want to go that short?

Quite a few of the pro's ride 165, I remember Froome riding even shorter on his TT bike.

 

I ride 170mm on all my bikes and would love 165 on my road/gravel bike if I was building from the ground up.

 

It also allows shorter people a better saddle position as the legs don't need to extend as far nor do the knees come up as high, so as far as I remember, making it easier to maintain a very even pedal stroke while fatigued as well as being able to change position on the bike/get more aero etc without as much effect on comfort and control.

 

Not sure if I have worded that properly. Basically, depending on how you use it, longer isn't always better.

 

I'm also convinced many many people are riding bikes way too big for them.

[buckstopper]

Who decided crank lengths should be what they are, when, and on what basis?

[Underachiever]

I'm sure they would be compatible. I would love to get my hands on a 165, I'm 177cm with an 85 inseam. If they were available for testing I'd try even shorter

I have no idea how the current rules of thumb were developed.

 

For what it is worth...

 

I'm 176cm, 84 inseam and I've been riding 175mm cranks since 1983.  Competitively for many years, then funrider, then racing vets since again from 2014. 

 

I feel most comfortable with an 85 to 95 cadence on the road.  Slightly higher on the MTB on a very steep section.

 

I've ridden 165 or 170 on the track many moons ago, the only reason was to eliminate pedal strikes on the steep embankment.

 

My gravel bike came with 172.5mm cranks, purely because it was set up as a Cx bike, is that is the Cx std for some reason.

 

MTB std is 175, I think.

 

You will have a optimum crank length for a certain length leg not to be prone to injuries I would guess, but in theory the longer the arm the more torque you can exert.  So you can push bigger gears and go faster.

 

Shorter cranks for more spinning and less torque??

[Jewbacca]

I have no idea how the current rules of thumb were developed.

 

For what it is worth...

 

I'm 176cm, 84 inseam and I've been riding 175mm cranks since 1983.  Competitively for many years, then funrider, then racing vets since again from 2014. 

 

I feel most comfortable with an 85 to 95 cadence on the road.  Slightly higher on the MTB on a very steep section.

 

I've ridden 165 or 170 on the track many moons ago, the only reason was to eliminate pedal strikes on the steep embankment.

 

My gravel bike came with 172.5mm cranks, purely because it was set up as a Cx bike, is that is the Cx std for some reason.

 

MTB std is 175, I think.

 

You will have a optimum crank length for a certain length leg not to be prone to injuries I would guess, but in theory the longer the arm the more torque you can exert.  So you can push bigger gears and go faster.

 

Shorter cranks for more spinning and less torque??

Sky did a massive test about this using science and stuff and I think they rubbished the idea.

 

The numbers centred around power vs effort all pointed to shorter cranks being better for sustained efforts. (obviously up to a point before someone says why not use 15mm cranks!)

 

I'm not having a go at you, but what you have said is one of the main reasons cycling is where it is. People have taken 1970 'theory' and kept applying it. Only in the last 10 years have we thrown the out dated rule book out the window and look at the advances?

 

I think you will find that a lot of what we have been taught to believe is really just the industry and it's consumers failing to move forward. 

[Eldron]

Quite a few of the pro's ride 165, I remember Froome riding even shorter on his TT bike.

 

I ride 170mm on all my bikes and would love 165 on my road/gravel bike if I was building from the ground up.

 

It also allows shorter people a better saddle position as the legs don't need to extend as far nor do the knees come up as high, so as far as I remember, making it easier to maintain a very even pedal stroke while fatigued as well as being able to change position on the bike/get more aero etc without as much effect on comfort and control.

 

Not sure if I have worded that properly. Basically, depending on how you use it, longer isn't always better.

 

I'm also convinced many many people are riding bikes way too big for them.

 

I've been having fun with crank length  lately - here is what I have found/learnt.

 

Why? There isn't enough length range in cranks. The difference between my tallest customer's inseam and shortest is around 18%. The difference in commonly available cranks is 5mm or 2.85% based on 170-175mm crank range. Various tests and PhD projects suggest 175mm crank is about right for those taller than 6 foot so anyone under that is theoretically riding cranks too long...

 

One error on your post is saddle position. The saddle height from top to middle of crank is always fixed - if you change the crank length you adjust the saddle to height to compensate. The extension is always the same - the contraction is where most short people have a problem and it manifests as hip pain from impingement.

 

The same PhD project above suggested there was little to no difference in oxygen use/exertion/energy use etc when crank length size was changed. It was purely for comfort/hip pain.

 

Aero is an interesting one - technically shorter cranks means the saddle goes up so it actually makes you less aero. Typically though hip impingement/pain stops you getting into that nice flat back aero position so yes shorter cranks get you into a flatter more aero position. TT positions are a pain in the....hip with really tight hip angles.

[Jewbacca]

I've been having fun with crank length  lately - here is what I have found/learnt.

 

Why? There isn't enough length range in cranks. The difference between my tallest customer's inseam and shortest is around 18%. The difference in commonly available cranks is 5mm or 2.85% based on 170-175mm crank range. Various tests and PhD projects suggest 175mm crank is about right for those taller than 6 foot so anyone under that is theoretically riding cranks too long...

 

One error on your post is saddle position. The saddle height from top to middle of crank is always fixed - if you change the crank length you adjust the saddle to height to compensate. The extension is always the same - the contraction is where most short people have a problem and it manifests as hip pain from impingement.

 

The same PhD project above suggested there was little to no difference in oxygen use/exertion/energy use etc when crank length size was changed. It was purely for comfort/hip pain.

 

Aero is an interesting one - technically shorter cranks means the saddle goes up so it actually makes you less aero. Typically though hip impingement/pain stops you getting into that nice flat back aero position so yes shorter cranks get you into a flatter more aero position. TT positions are a pain in the....hip with really tight hip angles.

As far as I understood the 'science', the 'saddle position' I am talking about for shorter peoples was that on the low and the high you are less extended/compacted, making it more comfortable?

 

But ja, either way, I took from it that most people are riding cranks too long for them because it is 'the norm'. I even know short peoples who swapped out their 170mm cranks on their trail bike for 175mm because 'I'm not a midget'.

 

I snapped those up with glee!

[Me rida my bicycle]

I have been loving my 165's ever since I got them they are now on my 3rd bike. I am 175cm short legs 80cm and used these⬇️ to work out the correct size.

There's no loss in power, I spin much easier and works lekker in technical terrain. O and less stress on joints or so the fisio told me.

[shaper]

Here an article from a uk bike fitter on crank lengths, with reference links for further reading.

 

https://bikedynamics.co.uk/FitGuidecranks.htm

 

TT/Tri often have shorter cranks for comfort, aero and open hip angle to reduce restriction at the top of the pedal stroke when the rider is down on the extensions 

 

https://www.trainingpeaks.com/blog/the-benefits-of-reducing-your-crank-length/

[Eldron]

As far as I understood the 'science', the 'saddle position' I am talking about for shorter peoples was that on the low and the high you are less extended/compacted, making it more comfortable?

 

But ja, either way, I took from it that most people are riding cranks too long for them because it is 'the norm'. I even know short peoples who swapped out their 170mm cranks on their trail bike for 175mm because 'I'm not a midget'.

 

I snapped those up with glee!

 

I think you're understanding it wrong. The "pedal circle" is smaller with shorter cranks but the bottom of the all the crank circles is always the same - the "extended leg position" should always the same. Changing just the crank length (or seat depth or cycling shoe sole thickness or pedal body height or even cycling shorts) changes the overall leg extension and must be compensated for in saddle height.

 

Edit: You set up saddle height like the diagram on the left changing cranks - not the right.

 

 

Edited May 18, 2020 by Eldron

[Eldron]

For those interested in the science - listen to this guy: https://blog.bikefit.com/bf-podcast-ep-9-does-crank-length-matter-with-dr-jim-martin/

[buckstopper]

I changed from 172 to 170 on my road bike and from 175 to 170 on the mtb. Better clearance, hip impingement disappeared. Easier at higher cadence, more open hip angle for improved aero on road bike. Agree with Jewbacca for sustained efforts. What about cleat position, saddle height, fore aft adj, who said the knee should be over the pedal etcetera I just love this sport.

[The Ghost]

Unless you're a midget, why would you want to go that short?

 

What others have said -

https://www.roadbikerider.com/are-crankarms-that-are-too-long-causing-your-knee-pain/

 

I’ve been riding this demo bike for a couple of weeks, and I’m now resting a knee with pain I’ve never had before.

[The Ghost]

But will the 600 series arms fit onto the 800 series chainrings?