Will 170 mm crank arm fit the frame of V20c?

[aminowhey]

The questions:
(1) Why 165 mm is a standard for V20c?
(2) Will 170 mm fit the frame?

Hi riders and tech specialists,
I am looking forward to buying a V20c frameset – the goal is to ride brevets with comfortable speed and comfort. The area I live in has a mix of flats and mountains, the goal is to be comfortable with long 10% grade and at the same time not to be crazy with the speed, I am fine with 40 kph on flats, important to to have relaxed heart rate.

Idea:
I thought to mix Shimano’s road, gravel and mountain components to have the widest gear range.
• Cassette: CS-M5100-11 (11-42T) with a Tanpan Wolftooth RD optimization device
• Crankset: FC-RX-600-11 (46-30T) or FC-RX810-2 (48-31T) from GRX

The problem:
GRX 2-11 speed have crank arms of >170 mm long while standard components of V20c show crank arm’s length of 165 mm.

The questions:
(1) Why 165 mm is a standard?
(2) Will 170 mm fit the frame?

Thank you, riders!

 

[cruzKurt]

I don't see why not, I have 145mm on mine, but because there is a moving bottom bracket, you could go as long as you like on the cranks IMO.

 

[ak-tux]

The questions:
(1) Why 165 mm is a standard?
(2) Will 170 mm fit the frame?

There is a general concept that recumbent cycling is best with shorter cranks than what would typically be on upright bicycles. 165mm cranks are the shortest commonly available and commercially produced cranks. Hence the choice.

Other people install custom cranks that can be as short as 150mm. There is a formula here: http://www.recumbents.com/wisil/misc/crank_angle.asp

That said, however, 170mm will work just fine on your cruzbike. (I have installed 170mm cranks on my S40).

My S40 came with 165mm cranks as well, out of the box. I changed to 170mm becuase I had to match a left-only crank based power meter I had that was 170mm, among other reasons.

Main differences I noticed between the 165mm and 170mm was the cadence, torque and leg extension. With the 165mm cranks my average cadence shot up from approx 90rpm to about 100rpm! But the torque was slightly reduced. It made it feel like it was harder to push the same gear.

 

[Always-Learnin]

Crank length is part of a proper bike fit which is unique to each individual. On a Cruzbike, with it's high degree of fit flexibility, finding the perfect fit sometimes requires shorter cranks. The standard 172mm crank length is just that, a magic number arrived at by supposedly taking the 'average' crank length of most riders...which obviously doesn't work for everyone. It also provides a convenient 'consumer standard' which eliminates the need for major bike companies to keep varied crank lengths on hand and in stock. The Cruzbike 165mm standard seeks to provide a better option for recumbent riders. As others, I have 145mm cranks on both my CBs but am moving to 150mm cranks for a better 'fit' for me. Here's a link for shortened cranks if you are interested: https://bikesmithdesign.com/ Many on this Forum site have used Mark Stonich's services.

 

[ratz]

The trend towards short cranks is to prevent knee bend.

With the boom design, you have to play with the Boom total length and boom forward-backward positioning.

In the end you have to manage these variables:
1) Your feet have to reach the full extension of the pedal
2) Your knees have to not hit the flat of the handle bars when all the way back

Crank length affects those two the most

The auxiliary impact from the crank length
3) Knee bend past 90 degrees. Long cranks contribute to that as a side effect of fixing 1 & 2. On a recumbent, the ability to brace your back against the seat and push means you can overload the knee joint if it's bent past 90 degrees. Long term, that can cause injury
4) If you need the handlebars farther back for knee clearance, then you get more steering leverage and more arms slack; that means twitchier steering (longer tiller relative to fulcrum; 5mm means a lot of leverage change)

The 165 standard is because those are easy to get cheap and prevent problems. The long term riders around here with the best fitting bikes tend to ride 155/150, making dialing in the bike that much easier. Jim P is 6'5" ish and has 155's and I'm 5'10 on 150; Larry is 5'7" I think last I ask him he preferred custom 145's????

Rotor Cranks are the best option for short cranks they make 150, 155, 160, 165 and have ready inventory you can get them with round or oval rings and the price is really good for top notch quality.

Once you have the bike setup, you still have to tune it, and video is the best way; get someone to film 60fps while you ride. Then you can figure out what to fix. The following bike is set up with 150mm cranks, and the handlebars are in the optimal position for arms. Stationary it all looks great, on the road; problem. The rider is a toe pointer causing an unextended leg and over bent knees. I will need to make the boom longer probably 7-10mm. Because the cranks are short I can do that without messing up the handlebar location. If the cranks were long, I could fix the leg extended position, but I would not get the knee bend fixed. It doesn't seem obvious, infact it probably seems counter-intuitive; until you do it. Also need to shim the right shoe with a 1mm cleat shim because I can see in the video that the toe pointing is not equal on both sides due to one leg being longer.

 

[ratz]

arrived at by supposedly taking the 'average' crank

Some skinny hipped frenchman in the 50's. Regression to the mean has made that now the rarity not the norm

 

[aminowhey]

oh, thank you very much, what a great community!

update: I just figured out is that the 165 mm GRX 46/30 11 speed crank exists for FC-RX-600-11 (46-30 T) but not for FC-RX810-2 (48-31 T)

GRX FC-RX600 2x11-speed Crank 46/30

For gravel, road and cyclocross use: Shimano GRX 2x11-speed FC-RX600 Crank 46/30T All versions feature a 2.5mm wider chain line that gives more space in the frame design and wider tyres. GRX 1-speed cranks minimize the complexity of...

www.bike-discount.de

46/30 should be fine, I am for the distance and long road journeys, not for the speed record!

thank you, will go with 165