Going from Hollow Tech crank to Square Taper

[wehey]

Hiya all.
I have a cranky knee.
When I was running my BBS02 I was able to use 150mm cranks which pretty much sorted my knee pain after cycling.
I have gone back to a hub motor because of BBS02 controller problems and have had to refit my old 170mm hollow tech crank.
Immediately my knee soreness returned.
As far as I can see you can’t buy shorter hollow tech cranks so I would like to convert it to the old style square tapered crank and a crank set which shorter pedal arms which are plentiful on eBay.
I have measured my bottom bracket which is 72.5mm but it’s the spindle length that I am stuck on. I don’t know what size to order.
Are there any measurements I can take off my bike to help me work out what length spindle I need?
thanks

 

[Theodore Voltaire]

The manufacturers kind of take a lot of the guess work out. You can only buy what they've already decided to make. 73mm is a common cartrige size. This one is 73x123

Shimano BB-UN300 Bottom Bracket - English, 73 x 123mm Spindle, Square Taper JIS | eBay

Fits JIS square-taper cranksets. Spindle Interface Type: Square Taper JIS. Shimano BB-UN300 Square Spindle Bottom Brackets. Hollow spindle. BB Shell Width (mm): 73. Spindle Length: 123. Bottom Bracket Shell I.D.: 1.37.

www.ebay.com

 

[wehey]

The manufacturers kind of take a lot of the guess work out. You can only buy what they've already decided to make. 73mm is a common cartrige size. This one is 73x123

Shimano BB-UN300 Bottom Bracket - English, 73 x 123mm Spindle, Square Taper JIS | eBay

Fits JIS square-taper cranksets. Spindle Interface Type: Square Taper JIS. Shimano BB-UN300 Square Spindle Bottom Brackets. Hollow spindle. BB Shell Width (mm): 73. Spindle Length: 123. Bottom Bracket Shell I.D.: 1.37.

www.ebay.com

Yep I understand this but how do know what spindle length to go for?
There are 4 choices here

 

[Theodore Voltaire]

I guess it won't matter as long as the spindle is wider than the chain stay. I would err on the wide side. You just don't want your crank arms hitting the stay.

 

[amberwolf]

There's a few pages and videos out there on shortening existing cranks, if you're interested in that kind of solution to keep your existing BB/etc stuff.

shorten cranks - Google Search

 

[Chalo]

Are there any measurements I can take off my bike to help me work out what length spindle I need?

If knee pain is a factor, you would most likely be better off using the lowest Q factor (width across pedals) you can. This might be limited by the chainring contacting the frame, or by the crankarms contacting highly flared chainstays. The shorter the BB spindle, the lower the Q factor. The more "square"/unflared the crank arms, the lower the Q factor.

But BBS02's Q factor is extremely wide, so if you got along with that, maybe you're not sensitive to wide Q.

 

[Wattman]

Sorry to disagree with Chalo, but Q-factor should be in accordance with hip width. Wide hipped people tend to do better with higher Q-factor aka BBSO2/BBSHD. More common Hollow Tech Q-factors are in the 150mm range and suit more average or narrow hipped riders. I ride with a wider Q-factor on all my bikes because of my hip dimension. Also, wider hipped riders tend to be more comfortable on a wider saddle due to wider sit bone spacing.
Its basic geometry and not complicated. If you had no knee pain with wider BBS02 Q-factor, then a wider Q-factor may agree with your anatomy.
I also ride with shorter cranks on all my bikes but mostly for hip impingement more than any knee pain. One can also ride with a more aggressive hip angle aka hip projection forward with shorter cranks to ride a bit more aero. Many very good riders ride short cranks and years ago I contacted Sheldon Brown and had a wonderful conversation on the subject as he was a big short crank fan.
Also, you don't have to default to square taper for short cranks. You can stick with Hollow Tech and go shorter crankarm length. Here is a single chainring at 152mm:

Suntour 1x ZERON 30T Aluminum Crankset w/ bottom bracket hollowtech 2 100% authe | eBay

If you care to respond with your overall physiology....are you wide through the hips like many bigger, more muscled men are, then a wider Q-factor may work well for you. So it isn't just about crank length as Chalo said, even though the opposite of what he wrote is true about Q-factor as it relates to knee pain.
If you want to learn about bike fit, a subject many in particular aren't well versed on within the ebike community, perform a search for a brilliant Aussie by the name of Neill on youtube.
He has a Q-factor video that supports what I wrote. Here it is:

Neill is among about three of the smartest bike fitters on the web. A subject I am an expert on if you have any questions.
Btw, Neill is very slight and narrow hipped and rides a very narrow Q-factor but he does a lot of fittings and suggests pedal spacers on many bigger riders as his video explains. Neill is brilliant and a lovely guy.

 

[wehey]

Sorry to disagree with Chalo, but Q-factor should be in accordance with hip width. Wide hipped people tend to do better with higher Q-factor aka BBSO2/BBSHD. More common Hollow Tech Q-factors are in the 150mm range and suit more average or narrow hipped riders. I ride with a wider Q-factor on all my bikes because of my hip dimension. Also, wider hipped riders tend to be more comfortable on a wider saddle due to wider sit bone spacing.
Its basic geometry and not complicated. If you had no knee pain with wider BBS02 Q-factor, then a wider Q-factor may agree with your anatomy.
I also ride with shorter cranks on all my bikes but mostly for hip impingement more than any knee pain. One can also ride with a more aggressive hip angle aka hip projection forward with shorter cranks to ride a bit more aero. Many very good riders ride short cranks and years ago I contacted Sheldon Brown and had a wonderful conversation on the subject as he was a big short crank fan.
Also, you don't have to default to square taper for short cranks. You can stick with Hollow Tech and go shorter crankarm length. Here is a single chainring at 152mm:

Suntour 1x ZERON 30T Aluminum Crankset w/ bottom bracket hollowtech 2 100% authe | eBay

If you care to respond with your overall physiology....are you wide through the hips like many bigger, more muscled men are, then a wider Q-factor may work well for you. So it isn't just about crank length as Chalo said, even though the opposite of what he wrote is true about Q-factor as it relates to knee pain.
If you want to learn about bike fit, a subject many in particular aren't well versed on within the ebike community, perform a search for a brilliant Aussie by the name of Neill on youtube.
He has a Q-factor video that supports what I wrote. Here it is:

Neill is among about three of the smartest bike fitters on the web. A subject I am an expert on if you have any questions.
Btw, Neill is very slight and narrow hipped and rides a very narrow Q-factor but he does a lot of fittings and suggests pedal spacers on many bigger riders as his video explains. Neill is brilliant and a lovely guy.

Thanks for the reply. I have had a bad knee before BBS02, during BBS02 and after BBS02. It’s not cycling related. The shorter the crank arm the leas my knee has to bend and the less sore it becomes.
im really dont want to get into all this Q factor stuff. I just want to sort my cranks out so I can get out and ride

 

[Wattman]

Thanks for the reply. I have had a bad knee before BBS02, during BBS02 and after BBS02. It’s not cycling related. The shorter the crank arm the leas my knee has to bend and the less sore it becomes.
im really dont want to get into all this Q factor stuff. I just want to sort my cranks out so I can get out and ride

Yes, you are directionally correct about wanting shorter cranks with knee pain because the knee will subtend a less acute angle at the top of the pedal stroke with shorter cranks. Also keep in mind if you go shorter cranks, you want to fractionally increase seat height because the bottom of the pedal stroke is higher.
And Chalo is correct to consider Q-factor because the wrong Q-factor can sideload the knee joint and knee joints don't like to be side loaded more than once every second with normal cadence. Bike fit is 3D geometry as it turns out although many problems can be solved in profile aka 2D.
If you really get into the weeds of bike fit, people even consider things like foot varus/valgus. Even leg length differences manifest issues with knees and hips and even saddle pain due many times to rider rotation (yaw) looking down on the rider from above in the vertical plane. Fit is a 3D puzzle.

Ebikes are great for dodgy knees was we all age....I am a senior and for example can't ride a road bike with as aggressive a position as I did when young. Good luck with your new ebike fit.

 

[wehey]

Yes, you are directionally correct about wanting shorter cranks with knee pain because the knee will subtend a less acute angle at the top of the pedal stroke with shorter cranks. Also keep in mind if you go shorter cranks, you want to fractionally increase seat height because the bottom of the pedal stroke is higher.
And Chalo is correct to consider Q-factor because the wrong Q-factor can sideload the knee joint and knee joints don't like to be side loaded more than once every second with normal cadence. Bike fit is 3D geometry as it turns out although many problems can be solved in profile aka 2D.
If you really get into the weeds of bike fit, people even consider things like foot varus/valgus. Even leg length differences manifest issues with knees and hips and even saddle pain due many times to rider rotation (yaw) looking down on the rider from above in the vertical plane. Fit is a 3D puzzle.

Ebikes are great for dodgy knees was we all age....I am a senior and for example can't ride a road bike with as aggressive a position as I did when young. Good luck with your new ebike fit.

Yep, I originally went to 130mm cranks with the BBS02 whilst the knee healed but the saddle was so high that I could barely touch the ground Lol. Changed to 150mm as the knee got better which was pretty good unless I cycled every day.
changing back to 170mm is doable but I get soreness after the ride which lasts overnight but better the next day.
i would be happy with 150mm
I’ve just ordered some 150mm cranks off eBay for £17 and will have to play around with spindle lengths until I find the right one. Incan order off Amazon then return any that don’t work

 

[Wattman]

Absolutely wehey....good stuff. Crank length is a hugely important element in cycling. I run 160mm length Lekkie cranks on two separate BBSHD bikes and a world of difference compared to stock 170mm Bafang BBxxx cranks. If I had any position at Bafang, I would insist crank length be 160mm stock..or a hint shorter. With a motor in particular, a rider doesn't need the leverage of longer cranks. An odd nominal choice by Bafang. Lekkie gets it and therefore get my business. Lekkie cranks are forged versus cast and their square taper stands up better as well. Further, Lekkie cranks have equivalent crank arm offset for a more symmetrical rider about the top tube centerline. Stock Bafang cranks have less offset on the left side of the motor and a slightly reduced Q-factor as a result.
Let us know how your crank change works out and share your riding observations of rear hub motor versus BBS02 which at a little under $500 is still a great little motor. Are you running a torque sensor with your rear hub motor?....or cadence sensing like your BBS02?

 

[Chalo]

Sorry to disagree with Chalo, but Q-factor should be in accordance with hip width. Wide hipped people tend to do better with higher Q-factor aka BBSO2/BBSHD.

That's true, and at 6'8" and large build, I do best with wide Q too. But by odds, given most people's bodies and current trends in cranks design, it's much more likely that Q, or what we used to call "tread width" before Keith Bontrager coined the currently used term, is wider than optimal.

I use flat pedals without toeclips, and when I ride in a bike with narrow Q, my feet tend to wander outwards from the centers of the pedals. With a too-wide crank you don't have the option of migrating your feet inwards.

Short cranks are easy, because kids' bikes exist. Mini and Junior class BMX racing, and recumbent specialty suppliers, mean there are lots of high quality cranks to choose from. Long cranks that I favor are not so easy to find. I use decades-old cranks in 190, 196, 205, 215, and 225mm. If I needed to replace them, it would most likely require some custom metalwork.

Check out Hostel Shoppe for good versatile cranks in short sizes.

Cranks

Established in 1974, Hostel Shoppe leads the way in providing recumbent trikes and bikes to cyclists across the nation since the early 90's.

hostelshoppe.com