Bio-mechanics folks: why are my knees sore with short cranks (155mm)

[big_scot_nanny]

Hi, would appreciate any insight, amateur or otherwise!

My normal bikes all have quite high BBs and 175mm cranks. I have long legs and am 6'5" tall.

On my 2022 Reign e-bike (glorious BTW), I installed 155mm cranks to deal with low BB and strikes. That works perfectly (also have bike in 'high' setting).

However, I get really sore knees on the Reign. Not when pedalling, although it is there too, but mostly when doing techy, saddle down, rough downhills. This does not happen on normal bikes, e.g. was out last night for 2 hours on the Hello Dave and I never had my knees grumble once. I cannot got to 175mm cranks on the Reign, that will not work.

Pain is at the front, top of my knees, both knees.

It is not Q factor, that has been discounted as other bikes have a variety from narrow (commuter) to ridiculous (fat bike).

Is it the lower cadence I fall into when riding 'e'?

Is it the heavier bike and having to muscle it around more on the techy bits?

Is it the 40mm closer together stance when riding saddle down?

Something else?

Thoughts appreciated, I really hope the conclusion is that I should have got a Voima with 175mm cranks

 

[Paulquattro]

I feel your pain but short cranks should lessen the pain as the knee is doing less work due too the smaller circumference of the pedals when rotated
As you say thou its not a rotation issue so it may be down to you having to work the heavier bike harder
Not any help really but its a tough diagnosis from the outside looking in

 

[B1rdie]

I would blame on...
the 40mm closer together stance when riding saddle down?
Not only when riding saddle down, but also when pedaling seated, because of the bike fit rule that says that the knee cap should be on top of the pedal axle when pedals are level, so, maybe moving the saddle 2cm backwards could remedy the problem.

 

[irie]

I would suggest comparing your normal bike cockpit with your Reign cockpit.

From your pain location sound like when you're standing on your pedals your reach to the bar/bar height is very different. IYSWIM

 

[RustyIron]

I have long legs and am 6'5" tall.

Pain is at the front, top of my knees, both knees.

Something else?

The more your legs are bent, the more strain is put on the joint. Applying too much power with bent legs will result in pain or injury.

Possible solutions are to raise the saddle, stand while peddling hard, and consciously reduce the amount of power you apply when you're seated on a dropped saddle.

 

[Mikerb]

my guess.............nothing to do with crank length..........everything to do with cockpit fit. Compare your stance, especially in the ready position, with your other bikes. Most likely is reach too long and/or bar height too low.

 

[BigG]

Tried larger pedals? If clipless - tried moving cleat all the way back as deep as you can?

I’m similar height, had similar problems + calves getting really sore on longer downhills. Turned out cranks were not the problem. I have 180mm ones on XC bike, and 160mm on ebike, no problem anymore regardless of bike.
The biggest difference was after changing pedals.

Size 15 shoes…
If riding clipless I could never get the cleats far back enough, it will always end up just a bit behind main finger joints, never close to middle of the foot. so basically I was riding on my tips most of the time, this is really hard on calves and knees.
There are some shoes that allow to move cleat deeper than usual, but still not enough. I even modified some shoes, cut soles, moved cleats further back.

In the end I abandoned idea of riding clippless and just bought largest flats I could find.
Pedaling Innovations Catalyst XL - platorm length is 160mm (note they also have a shorter non-xl version at 143mm).
Normal platform pedal length is ~115 or less.

I cant even describe how good this pedal feels. My feet are actually supported and all the calve/knee problems are gone. Miss clipping in a bit, but flats are fun too.

Wrong foot/cleat position very often causes knee pain.

I’m still on the lookout for size 15 shoes which will allow me to move cleat back enough, but manufacturers put same range of cleat adjustment on size 8 shoes as on size 15., which obviously doesn’t work.

 

[Mikerb]

agreed....I tried clipless just out of curiosity and the worst aspect is that they position your feet too far forward on the pedals. Most of the time with flats my feet are much further forward on the pedal, especially when up on the pedals on downhill runs.

 

[big_scot_nanny]

thanks folks, really appreciate the insights.

Pedals (DMR vaults) are the same on both eeb and normal bike, so I think we maybe can discount that (although with a 40mm stance difference, there is possibly an argument that longer pedals could allow more foot placement flexbility, and whilst my feet are not as big as BigGs (!), size 13 means they are not small either).

I had not thought about the difference in body position. my saddle to pedal height is set the same, but the giant has both lower stack and shorter reach than my other bikes.

Getting out with the measuring tape later, but in the meantime, it’s a glorious morning so I’m off out on the Reign to climb some big hills.

 

[sam.spoons]

Would this help? Mid-Foot Cycling

 

[Mr President]

I run 155mm cranks on my Ebike (Whyte E160) and longer cranks on unassisted bikes. I was also disappointed by suffering knee pain on the Ebike more so than on other bikes. However, I have come to the conclusion that it is nothing to do with pedalling and everything to do with standing on the techy sections. Why? I think it simply that I can do a lot more techy stuff on an Ebike because it helps me back to the top of the hill. ie I'm doing much more standing up than on an unassisted bike.
I just did a 3 day tour on the Ebike without too much pain because it was more xc, whereas if I'm a jumpin' and a slippin' and a slidin' on steep stuff I would have sore knees at the end of each day.
Does this theory tally with your experience?