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mack_turtle

looking for a crankset

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Posted (edited)

170mm arms, must work with a 24mm (not 24/22 SRAM), 110 BCD or something more modern. (the BB compatibility is non-negotiable.)

I have an old 105 crankset on my gravel bike but with the 130BCD on it, chainring options are limited. I want to try an oval ring but most of the options available in a 38–42t size that would be useful to me don't come in a 130BCD option, probably because the spider is too big for an oval to fit.

Edited by mack_turtle

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I have an XT 96BCD but it's 175 arms. Any specific reason you want 170-172.5?

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Posted (edited)
On 4/8/2021 at 8:33 AM, Chief said:

I have an XT 96BCD but it's 175 arms. Any specific reason you want 170-172.5?

good question. I have 172.5mm now and I couldn't tell you why longer would be bad. If you ask 10 bike fitters what size cranks one should use, you'll get a dozen answers. I'm 5'9" and I don't know if it would make a different. this is on a singlespeed gravel bike. I think most likely, 170mm is the ticket for me.

edit: I should be able to make a "mtn" crankset fit in my BB with some 24mm spacers on the spindle.

Edited by mack_turtle

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correction: I can make a mtn crankset fit on my bike with the help of 5m worth of 24mm ID spacers. preferably 2.5mm on each side. that would probably be a good move, because I am starting to think that the narrow stance on my road cranks is hurting my knees on longer rides, and switching to a mtn crankset would add at least 20mm to my stance width.

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On 4/8/2021 at 9:17 AM, mack_turtle said:

good question. I have 172.5mm now and I couldn't tell you why longer would be bad. If you ask 10 bike fitters what size cranks one should use, you'll get a dozen answers. I'm 5'9" and I don't know if it would make a different.

I had 170mm on my old road bike.  A fitter convinced me I needed 172.5.  Not long after, my orthopedist recommended to change my bike fit in order to minimize my knee angle due to some knee and hip arthritis.  Shorter cranks would help in that regard, but the angle change from 2.5mm has got to be tiny.

Edited by AntonioGG
one comma makes all the difference

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I should hold off on the crankset. I need to figure out a few things first, and I am willing to experiment on my own to do this:

  • saddle position. I think I have this down, but some camera work and a digital goniometer will give me more confidence.
  • ideal crank arm length: I'd hate to buy a 175mm crank and then realize that it's way off and I have mutant legs that would be better off with shorter ones.
  • q-factor: part of my issue is aching knees on the lateral side (outside). from what I've read, that's a sign that my knee is wiggling left-right through my pedal stroke because my cranks are too long or the stance width is off to a high enough degree to hurt over many miles. cleat/ insole wedges can help if applied correctly. wider cranks or pedals with wider spindles to widen your stance might be the answer. the cranks on my mtb are 175mm wide, plus the center of a SPD pedal places my cleats at at 285mm apart. on my road cranks, the same setup puts my cleats at 255, 30mm narrower! I would think that would have an effect on one's pedal stroke.

After I feel confident with the biomechanics of my fit, I can consider if I need a different chainring interface. that seems far less important now.

Edited by mack_turtle
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I should measure my gravel bike and my old road bike.  On my road rides I used to get terrible pain on the outside of my feet.  I have varus wedges (1° left and 0.5° right side) and that helps, but I had never considered Q-factor until recently, when I gave my old XX crankset to @Shinerider and he noticed the wear on one of the cranks from my foot rubbing on it and suggested pedal spacers.  I bet I find my old road bike is much narrower than my MTBs.

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32 minutes ago, AntonioGG said:

 I bet I find my old road bike is much narrower than my MTBs.

I've been reading about this. standard clipless pedals are around 55mm from the crank to the center where the cleat goes, varying about 5mm. you can get wider pedals with longer axles, but that's usually overlooked in the industry.

road cranks are typically right around 145mm wide, so a pair of 55mm pedals will space your cleats at 255mm with little room for adjustment.

mtn cranks are usually 170-175mm wide, so your stance is more like 285mm.

so if you put the same pedals on your road and mtn/hybrid/touring bike and ride in the same shoes, you have to pedal with a stance that is 25–30mm different! that's a big jump and certain to cause some problems. there are other factors, of course, and road/mtn style riding are often very different. individual bodies can adapt to different widths to some degree, but not every rider has the same level of adaptability. the industry is just starting to address this with pedals that have a wider stance. my goal is to find a stance that works for my body and apply it to both bikes.

I suspect that the gravel bike with it's narrow cranks would fit better if it was a bit wider, but that's a vague theory that needs testing. I just know that the outside of my knees hurt after long rides on the gravel bike and that is making things un-fun.

Edited by mack_turtle
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On 4/13/2021 at 1:14 PM, AntonioGG said:

I had 170mm on my old road bike.  A fitter convinced me I needed 172.5.  Not long after, my orthopedist recommended to change my bike fit in order to minimize my knee angle due to some knee and hip arthritis.  Shorter cranks would help in that regard, but the angle change from 2.5mm has got to be tiny.

I think I've wrapped my head around this somewhat now as well. your saddle height determines both how far your leg extends away from you at the bottom of your pedal stroke, and how high it comes up at the top. you can optimize your saddle height for the bottom of the stroke, but if you're the height of a normal human and did that with 250mm long cranks, your knee would hit you in the face at the top of your pedal stroke. this exaggeration is only to illustrate that cranks can be too long. most likely, most road bikes are coming with 172.5s to save manufacturers money. it's likely that seating pedaling would cause me to over-bend my knee enough to be uncomfortable at the top of my pedal stroke if they are too long (175 or longer in my case), or I'd have to raise my saddle so much that I would not get a powerful extension at the bottom. pushing a bit of a hard gear on a singlespeed could make this worse.

Edited by mack_turtle

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(I'm recording this for my own benefit more than anyone else's.)

quick phone camera video and some measuring with an app called Angleus tells me that my knee flexes to about 145° at the bottom of my pedal stoke, which is healthy but I could stand to lower the saddle a tiny bit. at the top of the stroke, the knee angle cinches in to 75°, which is on the tight side. If I raise my saddle, I'll be straining to reach the bottom of the pedal stroke and hurting my knees, but the top of the stroke will be better. if I lower the saddle, the bottom of the stroke might still be healthy but the top will get even more pinched. this is why crank length matters. I am now in the market for a 170mm crankset.

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and to tack onto the q-factor (in which the Q stands for "quack" because it's a measure of how much your feet stick out like a duck, which is a bit misleading), I'm looking into the actual width of my Anterior Superior Iliac Spine or ASIS. My chiro has x-rays that should be able to give that number with some accuracy. if your bike's q-factor is wider or narrower than your ASIS, that would result in some lateral movement in the hip and knee because the leg can't pump up and down like a non-pivoting piston, but rather has to move at an angle.

https://coachendurancesports.com/do-your-knees-angle-in-or-out-while-cycling-this-might-be-why/

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I'm piggy-backing onto your thread here because I think you found out the reason why I get the really bad hotfoot on my old road bike.

Both MTB and road versions of Shimano SPD puts the center in the same spot.

But my road bike Q-factor is ~156mm while my MTB QF is 180mm.  I can ride my MTB for 12+ hours but my road bike at 4+ hours was painful.  I'll check my gravel bike soon too.

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I've had 170,172.5, and 175 cranks on my road bikes. I had no knee, hip or foot problems with any of them. What I did notice, and this is on a road bike is that a longer crank length made climbing hills easier. The 175 cranks I have were on my last FS and I bought them because that is what was available at the time when I built it. My new frame is boost and those cranks didn't work without spacing them asymmetrically. I bought a set of 170 boost cranks for the new bike and don't notice any difference. Please don't think I'm trying to sell you on a set of cranks I'm not if you can use them fine if not fine also. Let me finish by saying don't fall into analysis paralysis, much like the bike fit debate I've found over the years that if something is wrong with how your bike fits you'll know it if you have no issues don't go looking for them.

Either way good luck.

Edited by Chief
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10 minutes ago, Chief said:

 if you have no issues don't go looking for them.

Wisdom!!!

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3 hours ago, Chief said:

I've had 170,172.5, and 175 cranks on my road bikes. I had no knee, hip or foot problems with any of them.

Why not 190mm cranks? If you could ignore pedal strikes on the ground, would 250mm cranks not be better? Certainly there's a functional limit for knee articulation going both directions, straightening and bending.

not looking for issues here. having trouble walking for a few days after a long ride is a problem that found me. I am having knee problems. my muscles are barely sore after riding, by my knee joints hurt. On longer rides, I have to limp home with agony in my knees. any observer can see my knees spaying wildly out at the top of my pedal stroke, which might indicate a low saddle. but if I put my saddle any higher, i get a different kind of painful movement at the bottom of my pedal stroke. after many years and experiments, plus the consensus of two bike fitters and the same advice from most of those goofy fitting calculations, I am certain that my saddle height is dead -on, or at least close enough that it's not the problem.

this started with a desire to try an oval ring, but I decided that if I am going to change the cranks on my bike, I better make darn sure I'm not ignoring the arm length and q-factor that might make a difference. I've read perspectives from everyone from Steve Hogg to John Wirath and the consensus is that cranks really can't be too short (within reason), but they can be too long, and as little as 5mm overall length can make a difference. cranks that are too long by just a few millimeters will restrict your range of motion, but cranks that are too short just make you spin faster and less efficiently. I conclude that there's a happy medium, just like handlebar width and all that stuff about RAD measurements.

that seems to leave crank arm length, cleat positioning, and horizontal spacing related to q-factor and pedal width, as far as bike components go. I also need to train my body to improve hip mobility and balance, but I just want to not disadvantage myself from a skeletal perspective from component choice. no amount of conditioning and technique is going to help if the complex set of levers and pivots that are my bones are set in such a way that they are being forced to move in ways that they did not evolve to move.

Edited by mack_turtle
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28 minutes ago, mack_turtle said:

I also need to train my body to improve hip mobility and balance, but I just want to not disadvantage myself from a skeletal perspective from component choice. no amount of conditioning and technique is going to help if the complex set of levers and pivots that are my bones are set in such a way that they are being forced to move in ways that they did not evolve to move.

That's something I have worked on quite a bit.  Lots of core including hip stuff, Pistol and Hungarian squats and I do trail running to add something other than the usual cycling motion.  I did all this after having to pull out of a 24hr race with really bad back pain:  diagnosis:  super tight hips/hamstrings/back + weak core.

 

Would love to go back to playing racquet sports too.

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2 hours ago, mack_turtle said:

Why not 190mm cranks? If you could ignore pedal strikes on the ground, would 250mm cranks not be better? Certainly there's a functional limit for knee articulation going both directions, straightening and bending.

 

That's fine I get your point. Not arguing or saying that what you're trying to do is wrong. I'm just saying for me the length doesn't really effect me that I am aware of. 

OK take this for what it's worth. You say you're having knee problems especially that your joints are sore after long rides but your muscles aren't. You say that your knees are splayed out at the top of your pedal stroke. You also say you have trouble walking for a few days after a long ride. I'm no doctor but you may want to investigate whether or not you may have arthritis. I know because these all sound like symptoms my wife is going through.

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Arthritis at 39? FML!

I appreciate that everyone is different. I'm pretty sensitive to how my shoes fit, the angle of my computer monitor, and apparently where the contact point are on my bike.

Edited by mack_turtle
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38 minutes ago, mack_turtle said:

Arthritis at 39? FML!

I was diagnosed right around that age!  I think I know all the PT exercises there are for just about anything.  They do help a lot.

I wonder 500 years after I'm dead if they find my bones, they're going to wonder what was up with that dude.  "this individual led a hard life as evidenced by the wear in his knees and hips as well as titanium plate and screws on his leg, and chewed hard to eat grasses and wore his teeth (grind my teeth...)"  Weird stuff I think about.   I also think with some of my ailments and genes I would not have been one of those that survived paleo times past the age of 40.

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