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Changing cranksets

Old 03-02-23, 03:28 PM
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RJC1811
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Changing cranksets

I have a Giant Defy 1 Disc with 172.5mm crank arms. I had a total knee replacement about 8 weeks ago. I知 getting my range of motion back gradually but still can稚 spin freely. There is pressure at the top of the stroke on my replaced knee. I知 thinking of going to 165mm cranks to help my knee a little. The side benefit is it may improve my pedaling in general. I have short legs and long torso. I would like opinions on this side of the equation.

Now my second question is swap it for an equal crankset. I currently have a Shimano RS500 and the newer version is the RS510, very similar. The other choice I am looking at is a power2max NGeco crankset. Looks like an easy swap and would give me a power meter. It痴 more money but part of it is offset because I would be buying a new crankset anyways. Your thoughts?

Thanks,
Bob
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Old 03-02-23, 10:28 PM
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Pretty sure you will want to know what model of Bottom Bracket your bike uses.

If it is Shimano Hollowtech, great, super easy ... Shimano press-fit probably a straight swap of cranks but i don't know .... if it is an odd size you might have to get a special (SRAM compatible) BB (22-4 mm.) I hope you have a Shimano BB.


Too tired to do much research.
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Old 03-02-23, 11:02 PM
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As Maelochs sort of hints at, make sure you know which bottom bracket to get with the crank. A quick search on your bike suggests its a pressfit but I didn't see what standard. The upgraded shimano crank will probably be a direct swap and I'm sure the other crank has a bb that will adapt the crank to your bike but, know exactly which BB you will need to fit the crank you're looking at if changing to something different. Personally I'm cheap and don't need incredible accuracy so I'm looking at the new sram force crankset with integrated powermeter, but I'm also not sure I need the extra data.
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Old 03-02-23, 11:04 PM
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You might want to get a full service fitting like retul or whatever you have in your area, so they can take into account of your current physical limitations. Might need to make adjustments to cleats, saddle height and position, etc. I would probably do that before getting new crank. I had one done after my crash a few years back when I was having a lot of numb hands that I was not having before. Discovered that due to the tightness in my shoulder, my reach was not as far as it was before the crash, and I was shifting my body forward, which ended up with me putting more pressure on my hands that I was not doing before. A couple of simple adjustments and it made all the difference. Once I was able to work my should dexterity out and improve my movement, I was able to micro adjust back to where my original settings were.

It was worth the cost in my opinion.
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Old 03-03-23, 07:26 AM
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Hey all, thanks for the replies! I have a Shimano press fit BB71 bottom bracket. Any of the options I’m looking at for crankset will work with that.

100% on the bike fit, I have already talked to a couple local bike shops about it. My knee replacement isn’t anywhere close to being fully healed so too soon for a fit. My plan is to work up to where I can spin freely on the trainer then add resistance and get some hours/miles in to establish my new norm. I feel like I know enough about my body and bike fit to get it close enough for this stage. After that I would go for the expert bike fit to get it dialed right in.

I’m 65 with somewhat tight hips and shorter legs. From what I’ve read, there’s no downside to going to shorter cranks for most people. I think the benefit for me would be less force on my knees and possibly a more stable pelvis. I wouldn’t be approaching my hip flex limits. Of course all of that is getting better. As part of my knee rehab I have started a whole fitness workout including stretching my hips and hamstrings.

I was looking to see if others agreed with me as far as no downside to the shorter cranks. The only possible downside would be for a powerful sprinter and that is not me. Then the choice to spend $130 on a Shimano rs510 165mm crankset or $560 on the Power2max ngeco 165mm crankset with power. I would also be interested in any other suggestions anyone would have.

thanks again,
Bob
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Old 03-03-23, 08:52 AM
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I've seen quite a few videos on shorter cranks, and for the most part, a lot of the fitters discussion on crank length have mentioned that shorter can be an improvement in many situations, but not all. I know I generally have ridden 172.5, but I have recently found the 170's a tad more comfortable as I get up in age. I am also approaching 60, and also have shorter legs and arms, but my middle is a little long. Makes getting clothes a pain as everything needs to be altered to fit correctly. I've been doing yoga in recent years to help with the stretching that I have always hated doing. Have you talked to the fitters about the 165?? What were their thoughts on it?

However, all that being said, I would probably just get the Shimano crank, and if you wanted power, I might just look at pedals, especially if you are not positive if the 165 is going to be the long term fix
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Old 03-03-23, 09:03 AM
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I'd definitely confirm that 165mm works and solves your problem before investing time/money into a new crankset. I think most fitters could help give you direction on this for minimal cost.
I know a few people who have tried shorter crankarms. I'd say the reaction is mixed. Some swear by it and others have gone back to more traditional lengths.
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Old 03-03-23, 09:41 AM
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My old bike has 170’s and I never noticed the difference when I went to the new bike 6years ago. Currently I have my old bike on the trainer because the 170’s made a difference. That said, in the last week I have been pushing hard to gain range of motion so I have been dropping the saddle to make my knee flex more. I felt I was making progress all week but hurt my knee yesterday. My weekly visit with the PT is in a half hour so I’ll see what he says.

Fitters are all over the place. I had one tell me to just do a shorter crank arm on the left side (my replaced knee) and not change the crank. That seems like it might temporarily solve one issue and cause much more down the line. I won’t be going to them for a fit. Others have said 2.5mm difference to a 170 won’t be noticeable and 165mm will get me the relief I’m looking for but not hamper performance.
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Old 03-03-23, 10:24 AM
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Oh if you are dropping the saddle that would explain the additional pain. How far did you drop it? Sounds like you might need to go back to where it was and then move it in smaller increments. I assume you are working with a PT or had been, I would talk to them about some recommendations on alternative exercises to gain range.
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Old 03-03-23, 02:27 PM
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Yes, I probably went too far too fast. They tell you to watch out for post surgery blues. I have done fine but have had a few episodes of frustration where I didn’t feel I was making the progress I should be. This past Sunday/Monday was one of those episodes. My PT had told me the prior week I might be getting near the end of gaining flexion, he said 115 degrees might be it. I got angry and worked like a dog all week, pushing to do more each day. Culminating in the sharp pain yesterday. The good news is that the PT said I didn’t damage anything and I added 7 degrees in a week, up to 122. That’s even with the soreness and swelling I caused yesterday. So I’m happy at that number. This week I’ll back off a little and let the knee calm down but not lose any of that motion. I can’t wait to start riding again!
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Old 03-04-23, 01:56 PM
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The OP might want to consider going to smaller chainrings and spin a higher cadence.
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Old 03-04-23, 04:34 PM
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I think 8 weeks is pretty soon to be changing your bike to accommodate your knee.
I also disagree that you are reaching the end of the line for increased ROM.
I had my knee replaced 13 months ago, and my ROM took a good 4 months to max out.
The knee still feels different from the other one, but I can now do pretty much whatever I want, although I was told not to run.

I have heard many people say it takes 12-18 months to really get to "end state".
Hang in there!
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Old 03-04-23, 05:33 PM
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I had both knees replaced in 2017, at age 64-65. I went back to cycling in 2018 after almost 8 years off. I've logged over 24,000 miles since then. FWIW, I'm turning 70 this June. I simply ask your saddle height, since crank length is usually related to leg length. I have an 83cm cycling inseam and 72.5cm saddle height. I've used 170-175mm cranks over many years. I even tried 175mm after my knee replacements, but decided 170mm was best.

I have a little issue with my right knee at the top of the stroke, but I think it means that I need to work harder at maintaining my range of motion, instead of crutching the problem with shorter cranks.

My doctor said I had only 6 weeks to get my range of motion established. I had one leg that was hard to bend and the other wouldn't straighten. I worked for 16 weeks to get my right leg to fully straighten. That same leg got tight at the top of the stroke, last fall. I went to an orthopaedic surgeon who drew a fluid sample and did a blood test to rule out infection and took an X-ray to compare with my old ones. All I needed was some regular ice application after a 50 mile ride.


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Old 03-07-23, 10:16 PM
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Thanks for more replies. I have been working on fitness and my knee hard, hence the infrequent replies. I’ll address a few things.

I already use a compact crankset 50/34 and have a big cassette 11-32 and I do try to spin more than hammer.

I appreciate the encouragement! I have found some articles and a lot of comments about ROM improving far outside that 6-12 week window. This has been a good week. Still making progress on the knee, lifting weights and walking longer distances.

I do have shorter legs. I’m 175cm and have a 78.7cm inseam. It’s hard to accurately measure femur and tibia length but feel I have shorter femurs. There are not many chairs or couch’s that I can sit fully against the back with my feet touching the floor. I’m too tired to run down and measure my saddle height. I have examined my riding position in depth. It does seem that with a comfortable and appropriate seat height I have fairly severe hip and knee angles. I did have a bit fit 6 years ago and modified 4 years ago by 5he same fitter when I bought a new saddle.

I'm still convinced that I would do better with shorter cranks. Everything I have read has said there are almost no downsides with the possible exception of a pro level sprinter. For most it will smooth the pedal stroke, increase cadence, open the hips and take the load off the knees. That all sounds great to me. The reality for me is I won’t know for weeks, maybe months until my knee if further along in the healing process. We will see.
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Old 03-09-23, 09:29 AM
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With your leg length, it certainly won't hurt to try 165mm crank arms.
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Old 03-09-23, 12:09 PM
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if you use flats, you might try stacking pedal height as a test fit before cranking out the money for the parts.
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Old 03-09-23, 05:06 PM
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Originally Posted by Troul View Post
if you use flats, you might try stacking pedal height as a test fit before cranking out the money for the parts.
I use clipless pedals but beyond that it wouldn稚 work. It would just be the same as lowering my seat the equivalent amount. With shorter cranks the pedals are closer together creating a smaller circle when pedaled. Adding a riser would keep the circumference the same, just Xcm higher. Thanks for the thoughts.
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